WMO-No. 948

By Yang Xiaoqing

WORLD METEOROLOGICAL ORGANIZATION

OPERATIONAL HYDROLOGY REPORT No. 47

MANUAL ON SEDIMENT MANAGEMENTAND

MEASUREMENT

Secretariat of the World Meteorological Organization – Geneva – Switzerland

The World Meteorological Organization (WMO), of which 187* States and Territories are Members, is a specialized agencyof the United Nations. The purposes of the Organization are:

(a) To facilitate worldwide cooperation in the establishment of networks of stations for the making of meteorologicalobservations as well as hydrological and other geophysical observations related to meteorology, and to promote theestablishment and maintenance of centres charged with the provision of meteorological and related services;

(b) To promote the establishment and maintenance of systems for the rapid exchange of meteorological and related infor-mation;

(c) To promote standardization of meteorological and related observations and to ensure the uniform publication ofobservations and statistics;

(d) To further the application of meteorology to aviation, shipping, water problems, agriculture and other human activi-ties;

(e) To promote activities in operational hydrology and to further close cooperation between Meteorological andHydrological Services; and

(f) To encourage research and training in meteorology and, as appropriate, in related fields and to assist in coordinatingthe international aspects of such research and training.

(Convention of the World Meteorological Organization, Article 2)

The Organization consists of the following:

The World Meteorological Congress, the supreme body of the Organization, brings together the delegates of Membersonce every four years to determine general policies for the fulfilment of the purposes of the Organization, to approve long-term plans, to authorize maximum expenditures for the following financial period, to adopt Technical Regulations relatingto international meteorological and operational hydrological practice, to elect the President and Vice-Presidents of theOrganization and members of the Executive Council and to appoint the Secretary-General;

The Executive Council, composed of 36 directors of national Meteorological or Hydrometeorological Services, meets atleast once a year to review the activities of the Organization and to implement the programmes approved by Congress;

The six regional associations (Africa, Asia, South America, North and Central America, South-West Pacific and Europe),composed of Members, coordinate meteorological and related activities within their respective Regions;

The eight technical commissions, composed of experts designated by Members, study matters within their specificareas of competence (technical commissions have been established for basic systems, instruments and methods of observa-tion, atmospheric sciences, aeronautical meteorology, agricultural meteorology, marine meteorology, hydrology, and climatology);

The Secretariat, headed by the Secretary-General, serves as the administrative, documentation and information centre ofthe Organization. It prepares, edits, produces and distributes the publications of the Organization, carries out the dutiesspecified in the Convention and other Basic Documents and provides secretariat support to the work of the constituentbodies of WMO described above.

________* On 30 November 2003.

THE WORLD METEOROLOGICAL ORGANIZATION

WMO-No. 948

By Yang Xiaoqing

WORLD METEOROLOGICAL ORGANIZATION

OPERATIONAL HYDROLOGY REPORT No. 47

MANUAL ON SEDIMENT MANAGEMENTAND

MEASUREMENT

Secretariat of the World Meteorological Organization – Geneva – Switzerland2003

© 2003, World Meteorological Organization

ISBN: 92-63-10948-6

NOTE

The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the World MeteorologicalOrganization concerning the legal status of any country, territory, city or area, or of its authorities,or concerning the delimitation of its frontiers or boundaries.

Page

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Summary (English, French, Russian and Spanish) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

CHAPTER 1 — ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Impacts of soil erosion on ecology and environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Desertification and degradation of agricultural production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.2 Sediment-related disasters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Impacts of river sedimentation on ecology and environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3.1 River sediment and flood disasters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3.1.1 Conveyance capacity of rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3.1.2 Fluvial process and instability of river channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3.1.3 Safety of training works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3.1.4 Sediment deposits by floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.1.5 Variation of groundwater level and salinity by river sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2 Environment of sediment-laden rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2.1 Deposition in irrigation systems and desertification at irrigation system heads . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2.2 Impacts of river channel shifting on environment and ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Reservoir sedimentation and environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4.1 Loss of reservoir storage capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4.2 Water pollution by reservoir sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.3 Rise of groundwater level and salinity by deposit extension in reservoir backwater regions . . . . . . . . . . . . . . 61.4.4 Problems of downstream reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.4.1 Flood plain collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.4.2 Downstream navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.5 Case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.6 Guanting Reservoir in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.7 Aswan High Dam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5 Utilization of sediment resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CHAPTER 2 — SOIL EROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2 Natural erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.1 Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.2 Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.3 Freeze-thaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.4 Living organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Accelerated erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4 Factors affecting soil erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.1 Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.2 Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.3 Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.4 Soil characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.4.5 Vegetation cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4.6 Human activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.5 Degree and intensity of soil erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.5.1 Soil loss tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.5.2 Soil erosion intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.6 Sediment yield in a basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.7 Monitoring of soil erosion and sediment yield in a basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.7.1 Runoff plots and experiments in the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

CONTENTS

iv CONTENTS

Page

2.7.2 Measurements of soil and water losses on pilot watersheds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.7.3 Measurement with Cs-137 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.7.4 Dynamic monitoring by remote sensing and GIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.8 Prediction of soil erosion and sediment yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.8.1 Prediction of soil erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.8.2 Prediction of sediment yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.8.3 USLE and RUSLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.8.4 Empirical regression statistical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.8.5 Deterministic sediment yield models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.9 Soil erosion control and watershed management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.9.1 Soil and water conservation planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.9.2 Measures for soil and water conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.10 Summary on global soil erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1 Patterns of sediment transport in rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.1 Bed material load and wash load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.2 Bed load, saltation and suspended load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.3 Continuity of sediment movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.4 Relative importance of bed load and suspended load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.2 Bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.1 Incipient motion of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.1.1 Stochastic property of incipient motion of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.1.2 Condition of incipient motion for non-cohesive uniform sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.1.3 Condition for incipient motion of cohesive sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2.2 Bed form and resistance in fluvial streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.2.1 Development of bed forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.2.2 Flow resistance in alluvial streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.2.3 Bed load transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.2.3.1 Transport of uniform bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.2.3.2 Transport of non-uniform bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.3.3 Characteristics of transport of gravel bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.3 Suspended sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.3.1 Mechanism of sediment moving in suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.3.2 Diffusion equation and vertical distribution of suspended sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.3.3 Transport rate of suspended load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.3.4 Non-equilibrium transport of suspended sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.4 Total sediment load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.4.1 Einstein’s bed load function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.4.2 Colby’s method (1964) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.4.3 Bagnold’s work (1966) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.4.4 The Engelund-Hansen formula (1972) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.4.5 The Ackers-White formula (1973) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.4.6 Yang’s approach (1996) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.4.7 Formula of the Wuhan University of Hydraulic and Electric Engineering (WUHEE) . . . . . . . . . . . . . . . . . . . 473.4.8 Estimation of total sediment load including wash load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.4.8.1 Annual sediment load evaluated by the relationship between flow discharge and sediment transport rate . . . . 473.4.8.2 Estimation of sediment load based on factors in river basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.4.8.3 Estimation of sediment yield of a watershed from reservoir deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.5 Hyperconcentrated flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

CHAPTER 4 — FLUVIAL PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.2 Categories of rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.2.1 Mountainous and upland rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.2.2 Plain and piedmont rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

CONTENTS v

Page

4.3 Classification of river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554.3.1 River patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554.3.2 Methods for classification of river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.3.3 Characteristics of rivers with different patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.3.4 Causes for formation of river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.3.5 Transformation of river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.3.6 Critical relationships between different river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.3.6.1 Relationships between longitudinal slope and river patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.6.2 Relationships between longitudinal slope and mean discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.6.3 Relationships between longitudinal slope and maximum discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.6.4 Wandering index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.6.5 Relationships between longitudinal slope, bed sediment and discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.7 Indexes of river stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.7.1 Longitudinal stability of river channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.7.2 Transversal stability of river channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.4 Morphology of rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.4.1 Dominant discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.4.1.1 Determination of dominant discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614.4.1.2 Bankfull discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614.4.1.3 Empirical expression for bankfull discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.4.1.4 Bankfull discharge estimated by recurrence intervals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.4.2 Longitudinal profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.4.2.1 Geometric expressions of longitudinal profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.4.2.2 Empirical relationships between longitudinal slope and watershed factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.4.3 Cross-sectional morphology of rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644.4.3.1 Hydraulic geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.4.3.2 Hydraulic geometry along rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.4.3.3 Relationships between factors of watershed and hydraulic geometry along rivers . . . . . . . . . . . . . . . . . . . . . . 664.4.3.4 Analytic solution of hydraulic geometry along rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.4.3.5 Hydraulic geometry of gravel rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.4.3.6 Hydraulic geometry for canals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.5 Fluvial processes of meandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.5.1 Plane morphology of meandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.5.2 Relationships between meander wavelength and discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.5.3 Relationships between central angle and curvature radius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.5.4 Relationships between meander elements and width of straight (crossing) reaches . . . . . . . . . . . . . . . . . . . . . 704.5.5 Relationships between configurations and cross-sectional geometry of meanders . . . . . . . . . . . . . . . . . . . . . . 704.5.6 Crossings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.5.7 Dynamic line of flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.5.8 Transversal slope of water surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.5.9 Longitudinal slope of water surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.5.10 Transversal circulating flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.5.10.1 Distribution for transversal velocity (radial) of circulating flows (Rozovski, 1957, 1965) . . . . . . . . . . . . . . . . 714.5.10.2 Relative intensity of circulating flows (Xie, 1987) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.5.10.3 Vortex intensity of circulating flow (Xie, 1987) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.5.10.4 Transversal slope of bed surface and distribution of sediment particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.5.11 Sediment transport in meandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.5.11.1 Transport of suspended load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.5.11.2 Bed load transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.5.12 Characteristics of fluvial processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.5.12.1 Collapse of concave banks and growth of convex banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.5.12.2 Migration of meanderings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.5.12.3 Cutoffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.6 Fluvial processes of wandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.6.1 Flow and sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.6.1.1 Characteristics of river flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.6.1.2 Characteristics of sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.6.2 Morphological features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

vi CONTENTS

Page

4.6.2.1 Static features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.6.2.2 Dynamic features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.6.2.3 Node points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.6.3 Channel degradation and aggradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.6.3.1 Characteristics of degradation and aggradation for wandering rivers with high sediment concentration . . . . . 774.6.3.2 Degradation and aggradation for wandering rivers with relative low sediment concentration . . . . . . . . . . . . . 774.6.4 Degradation and aggradation in hyperconcentrated floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.6.4.1 Features of hyperconcentrated floods in the Lower Yellow River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.6.4.2 Flow patterns and transport modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.6.4.3 Features of degradation and aggradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.6.5 Shrinking of river channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.7 Fluvial processes of anabranched rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.7.1 Morphological characteristics of anabranched rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.1.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.1.2 Morphological indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.2 Morphology of cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.3 Ratio of discharge and sediment diversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.4 Fluvial processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.4.1 Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.7.4.2 Channel deformation for different anabranched rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.8 Fluvial processes of straight rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.8.1 Morphological features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.8.2 Features of flow and sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.8.3 Features of fluvial processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.9 Stabilization and rectification of river channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.9.1 Parameters of river training planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.1.1 Determination of design discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.1.2 Determination of channel width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.1.3 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.2 Structures of river training works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.2.1 Structures of training works for moderate and low flow channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.9.2.2 Structures of training works for flood channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.9.2.3 Dredging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.9.3 River training of meandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.9.3.1 Measures of river training for stabilizing river channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.9.3.2 Cutoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.4 River training of wandering rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.5 River training of anabranched rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.5.1 Measures for stabilizing flow diversion ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.5.2 Works of fork-channel blockade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.6 River training of straight rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.7 Regulation of shoal reaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.9.7.1 Parameters for designing navigation courses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

CHAPTER 5 — RESERVOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES . . . . . . . . . . . . . . . 875.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.1.1 Dam construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.1.2 Rate of loss of storage capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.1.3 Sustainable development of reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.1.4 Prediction of reservoir sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.1.5 Issues related to reservoir sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.2 Processes of deposition in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.2.1 Movement of sediment in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.2.2 Basic characteristics of reservoir deposits (Qian, et al., 1987) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.2.2.1 Longitudinal profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.2.2.2 Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.2.2.3 Lateral distribution of deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

CONTENTS vii

Page

5.2.2.4 Spatial distribution of deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915.2.2.5 Headward extension of backwater deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.2.2.6 Physical characteristics of deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.3 Sediment release from reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.3.1 Sediment release during flood detention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.3.2 Density current venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.2.1 Phenomenon and formation of density current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.2.2 Venting of density current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.3 Erosion in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.3.3.1 Retrogressive and progressive erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.3.3.2 Erosion in the fluctuating backwater region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.3.3.3 Empirical method of erosion prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.4 Empirical estimation of long-term deposition in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.4.1 Method of trap efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.4.2 Method of rate of storage capacity loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.4.3 Process of depletion of reservoir storage capacity (lifespan of a reservoir) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.5 Numerical modelling of reservoir sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.5.2 Basic equations (for unit width) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.5.2.1 Continuity equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.5.2.2 Momentum equation of one-dimensional sediment-laden flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.5.2.3 Supplementary equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.6 Reservoir sedimentation management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995.6.1 Universality of reservoir sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995.6.2 Indicators of reservoir sedimentation problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005.6.3 Basic operating rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005.6.4 Sediment design of hydrological projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005.6.4.1 Collection and evaluation of basic data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.6.4.2 Sediment input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.6.4.3 Sediment design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.6.4.4 Prevention of sediment problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.6.4.5 Prediction of the fluvial processes below a project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.6.4.6 Planning for sediment measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.6.5 Methods of reducing sediment input in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.6.5.1 Soil conservation practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.6.6 Overview of remedial measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.1 Drawdown flushing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.2 Reservoir emptying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.3 Lateral erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.4 Siphon dredging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.5 Dredging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.6.6.6 Design of sediment sluicing facilities of reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.7 Fluvial processes below reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1 Fluvial processes below impounding reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1.1 Changes in flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1.2 Drastic reduction in sediment load and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1.3 Erosion below dams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1.4 Armouring of bed sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.7.1.5 Adjustment of longitudinal profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055.7.1.6 Adjustment of cross-sectional shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055.7.1.7 Adjustment of channel pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.7.2 Fluvial processes below detention reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.7.2.1 Changes in flow and sediment regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.7.2.2 Aggravation of deposition below dams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.8 Case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.8.1 Liujiaxia Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075.8.2 Sanmenxia Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075.8.3 Heisonglin Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

viii CONTENTS

Page

5.8.4 Shuicaozi Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.8.5 Guanting Reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1105.8.6 Tarbela Dam Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1105.9 Measurement of erosion and deposition in the reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.9.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.9.1.1 Contour method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.9.1.2 Range-line method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.9.1.3 Composite method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.9.2 Instrumentation for positioning and depth sounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.9.2.1 Depth sounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.9.2.2 Positioning of sounding points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.2.3 Surveying system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.2.4 Positioning by the Global Positioning System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.2.5 Measuring sediment thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.3 Measurement of bed material composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.3.1 Undisturbed sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.9.3.2 Radioisotope density probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.9.3.3 Selection of sampling points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.9.4 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.9.4.1 Computation of reservoir capacity or amount of deposition or erosion in river reaches . . . . . . . . . . . . . . . . . . 1145.9.4.2 Computation of capacity from topographic surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155.9.4.3 Unit weight of sediment deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT . . . . . . . . . . . . . . . . . . . . . . . 1186.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.1.1 Type of sediment load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.1.2 Network for measurement of sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.1.3 Classification of hydrometric stations for sediment measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.1.4 Total load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.1.5 Sedimentation surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.1.6 Parameters to be collected for a complete sediment data set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.2 Measurement of suspended sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.2.1 Method of measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.2.1.1 Measurement of suspended sediment discharge in a vertical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.2.1.2 Measurement of sediment discharge in a cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1216.2.1.3 Sampling for size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236.2.1.4 Frequency and timing of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236.2.2 Computation of sediment discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246.2.3 Measuring devices and instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.2.3.1 Sampler for taking representative samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.2.3.2 Basic requirements for an ideal sampler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.2.3.3 Some developments in mechanical devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.2.3.4 Some developments in the in situ measurement of sediment concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266.2.3.5 Intercomparison of measuring devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266.3 Measurement of bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276.3.1 Direct measurement of bed load discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276.3.1.1 Characteristics of bed load movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276.3.1.2 Frequency of measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1286.3.1.3 Selection of sampling verticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1286.3.2 Indirect method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.3.2.1 Sedimentation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.3.2.2 Dune tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.3.2.3 Tracer method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.3.2.4 Investigation of the lithologic properties of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.3.3 Measuring devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1306.3.3.1 Technical requirements for an ideal bed load sampler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1306.3.3.2 Various kinds of bed load samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

CONTENTS ix

Page

6.3.3.3 New developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316.3.4 Calibration of samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316.3.4.1 Direct field calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316.3.4.2 Laboratory calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.3.5 Computation of bed load discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.4 Measurement of total sediment discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.4.1 Direct methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.4.1.1 Measurement of suspended sediment and bed load discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.4.1.2 Measurement by means of turbulence flume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336.4.1.3 Measurement by sediment accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336.4.2 Computation of total sediment load from measured suspended sediment discharge data at a hydrometric station . . 1336.4.2.1 Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336.4.2.2 The modified Einstein procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346.4.2.3 Correction coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356.4.2.4 Ratio of bed load discharge to suspended-sediment discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356.4.3 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356.5 Laboratory procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366.5.1 Determination of sediment concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366.5.1.1 Evaporation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366.5.1.2 Filtration method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366.5.1.3 Displacement method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1376.5.1.4 Accuracy requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1376.5.2 Size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386.5.2.1 Methods for size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386.5.2.2 Treatment of samples for size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1406.5.2.3 Measurement of physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.6 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.6.1 Data processing for suspended load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.6.1.1 Computation of sediment discharge and cross-sectional average sediment concentration . . . . . . . . . . . . . . . . 1416.6.1.2 Computation of average daily sediment discharge and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.6.1.3 Sediment transport curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1426.6.1.4 Data processing for suspended sediment size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1426.6.2 Data processing for bed load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436.6.3 Examination of processed data and data processing using computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436.7 Assessment of accuracy and reliability in measurement of sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.1 General description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.2 Major factors influencing the reliability of measurement of sediment transport . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.2.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.2.2 Characteristics of measuring sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.2.3 Sampling frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446.7.2.4 In situ measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1456.7.2.5 Measurement of concentration and size analysis in the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1456.7.2.6 Computation method and data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1466.7.3 Major factors influencing the reliability of bed load measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1466.7.4 Analysis of systematic errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1466.7.5 Analysis of random errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1476.8 Summaries and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.8.1 Fundamental concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.8.2 Implementation of measuring programmes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.8.3 Measuring site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.8.4 Measurement of suspended sediment discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.5 Corrections for transport in the unmeasured zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.6 Frequency of measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.7 Sampling apparatus — suspended sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.8 Sampling apparatus — bed sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.9 Computation of total load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.10 Size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.11 Method of size analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

x CONTENTS

Page

6.8.12 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496.8.13 Assessment of accuracy and reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506.8.14 Monitoring for sediment quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

CHAPTER 7 — WATER QUALITY RELATED TO TRANSPORT OF SEDIMENT AND TOXIC MATERIAL . . 1537.1 Effects of sediment and heavy metals on water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1537.1.1 Absorption of heavy metals in sediment particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1537.1.2 Effects of sediment particles absorbing heavy metals on water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547.2 Effects of sediment and toxic organic material on water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1557.2.1 Absorption of toxic organic material on sediment particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1557.2.2 Effects of sediment particles absorbing toxic organic material on water quality . . . . . . . . . . . . . . . . . . . . . . . . 1577.3 Water quality model of sediment and toxic organic material and heavy metal . . . . . . . . . . . . . . . . . . . . . . . . . 157References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

FOREWORD

Water resources are the most indispensable resources for human life. As the primary enhancing agent for the sustainable development ofsocieties and economies, the development and utilization of water resources are becoming more and more important. In the developmentof water resources, sediment and related problems have always presented a great challenge. Increasing attention is being focused on abetter understanding of the processes of erosion and sedimentation and their relationship to the surface runoff component of the hydrolog-ical cycle. To provide a basic understanding of these processes, WMO published Operational Hydrology Report No. 16, Measurement ofRiver Sediments (WMO-No. 561) and Operational Hydrology Report No. 29, Manual on Operational Methods for the Measurement ofSediment Transport (WMO-No. 686). However, there was a great need to provide a manual or report to describe the comprehensiveprocesses of erosion, sediment transportation, fluvial processes and reservoir sedimentation, etc. Therefore, WMO decided to publish anupdated manual on sediment measurement and management.

The tenth session of the Commission for Hydrology (CHy-X) in 1996 requested Ms Yang Xiaoqing (China), the expert on sedimentof the Working Group on Basic Systems of CHy, to undertake the task of preparing a manual on sediment management and measurement.With the support of the Ministry of Water Resources of China, an expert team was organized to undertake the work. Ms Yang Xiaoqing,Dr Long Yuqian, Dr Wan Zhaohui, Dr Zhou Zhide, Messrs Zhou Wenhao, Hua Shaozu, Weng Jianhua, et al., high-level Chinese experts,were included in the team. It is hoped that this Manual will provide a guide for water resources engineers, planners, managers andhydrologists.

The authors of the individual chapters are as follows:Chapter 1: Ms Yang XiaoqingChapter 2: Messrs Hua Shaozu, Liu Xiaoying, Ms Yang Xiaoqing, Wu DeyiChapter 3: Dr Wan ZhaohuiChapter 4: Mr Zhou WenhaoChapter 5: Dr Zhou Zhide, Dr Long YuqianChapter 6: Dr Long Yuqian, Ms Zhu Xiaoyuan, Mr Zhou GangyanChapter 7: Messrs Weng Jianhua, P. Literathy (Hungary)

It is with great pleasure that I express my gratitude to Mr B.J. Stewart (Australia), the Chairperson of the Working Group on BasicSystems of CHy, as well as Messrs G. Leeks (United Kingdom) and G.D. Glysson (United States) for their review and useful recommen-dations and suggestions.

(G.O.P. Obasi)Secretary-General

This report covers a wide range of issues related to sedimenta-tion. Its objectives are to present to readers a basicunderstanding of operational methods of sediment transportmeasurement, and serve as a practical reference in dealing withsedimentation engineering.

Ecological and environmental concerns are increasinglyaffecting the sustainable development of human societies world-wide. In Chapter 1, the impacts of soil erosion and river andreservoir sedimentation on ecologies and environments arediscussed, as are potential benefits of sediment as a resource.

Chapter 2 presents soil erosion in detail, including itsbasic characteristics, monitoring and prediction of erosion andsediment yield in a basin, soil and water conservation, and water-shed management. Finally, an overview of the global issue of soilerosion is presented.

In Chapter 3, the contents of sediment transport in riversare discussed. The basic concepts of patterns of river sedimenttransport form the basis on which to deal with river sediment.They are elucidated concisely and thoroughly. Following this is adiscussion on bed load, suspended load and total sediment load,using authoritative papers. Based on a large amount of data andpapers, mainly developed in China, hyperconcentrated flow isdiscussed briefly at the end of this chapter.

Chapter 4 elaborates on fluvial processes. The mainpoints include classification of patterns of alluvial rivers, fluvialprocesses of each basic river pattern, and stabilization and rectifi-cation of river channels. In this report, the alluvial rivers areclassified into four basic patterns: meandering, wandering,anabranched and straight. In many literatures, three basic river

patterns are differentiated: meandering, braided and straight. Sucha difference may be induced by the large amount of sediment loadtransported by some Chinese rivers.

Reservoirs play a significant role in human society,including flood control, water supply, power generation, irrigation,navigation improvement, recreation, etc. With the passage of time,many reservoirs, particularly those built on sediment-laden rivers,lose a certain percentage of their storage capacity due to sedimen-tation. In Chapter 5, the subject of reservoir sedimentation and itsimpacts on river processes are expanded upon. Depositionprocesses in reservoirs are presented first. Then, methods of esti-mation of long-term deposition in reservoirs, both empirical andnumerical, are briefly discussed. A discussion of reservoirmanagement follows, emphasizing the possibility of preservinglong-term reservoir capacity for permanent usage. Six case studiesshow the reality of reservoir sedimentation problems.

Accurate sediment data are the basis of every aspect ofsediment management and numerical (computer) modelling ofsedimentation. In Chapter 6, operational methods of sedimentmeasurement, including measurements of suspended sediment,bed load and total sediment load, are discussed. Also, laboratoryprocedures, data processing and assessment of accuracy and relia-bility in sediment measurement are presented. Finally, somerecommendations for sediment measurement are given.

Water pollution is an increasingly important issue in manyplaces, particularly in developing countries. In Chapter 7, waterquality related to the transport of sediment and toxic materials, themain source of water pollution, is elucidated briefly. To quantifysuch impact, a water quality model is introduced.

SUMMARY

RÉSUMÉ

Le présent rapport couvre un large éventail de questions relatives à lasédimentation. Il permettra au lecteur de se familiariser avec lesméthodes de mesure des transports solides et servira de référencepratique pour tous les aspects scientifiques et techniques de lasédimentation.

Le développement durable des sociétés humaines est de plusen plus soumis à des impératifs écologiques. Le chapitre 1 traite desincidences de l’érosion des sols et de la sédimentation des coursd'eau et des réservoirs sur l’environnement et les écosystèmes et desavantages que peuvent présenter les sédiments en tant que ressource.

Dans le chapitre 2, les mécanismes d’érosion des sols sontdécrits en détail, on y évoque aussi la surveillance et la prévision del’érosion et des apports solides dans un bassin donné, la conservationdes sols et des eaux et la gestion des bassins versants. Le chapitre setermine par un bilan général de la question de l’érosion des sols.

Le chapitre 3 traite des transports solides dans les coursd’eau et des principes de base qui régissent les mécanismes en jeu.La description de ces processus est à la fois concise et exhaustive.S’appuyant sur des études faisant autorité en la matière, le rapportaborde ensuite la question de la charge de fond, de la charge solide

en suspension et de la charge solide totale. Enfin, le chapitre setermine par une présentation succincte de l’écoulementhyperconcentré, qui a fait l’objet de nombreuses études, enparticulier en Chine.

Le chapitre 4 est consacré aux processus fluviatiles. Parmiles principaux thèmes abordés, on citera la classification des rivièresalluviales, les processus fluviatiles propres à chaque type de rivièreainsi que la stabilisation et la rectification du lit des cours d’eau. Lerapport distingue quatre types principaux de lit fluvial : lit àméandres, lit divaguant, lit anastomosé et lit rectiligne. Or, dans lalittérature scientifique, on ne distingue le plus souvent que troiscatégories : lit à méandres, lit tressé et lit rectiligne. Cette différenceest peut-être due au fait que certains cours d’eau chinois charrientune grande quantité de matières solides.

Les réservoirs revêtent une grande importance pour la luttecontre les inondations, l’approvisionnement en eau, la productiond’énergie, l’irrigation, l’amélioration de la navigation, les loisirs, etc.Avec le temps, de nombreux réservoirs, en particulier ceux qui ontété construits sur des cours d’eau à forte charge solide, ont perdu unepartie de leur capacité de stockage à cause de la sédimentation. La

xiv MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

В настоящем отчете охватывается широкий круг вопросов,

касающихся отложения наносов. Его задачи – представить

читателям основы понимания процесса переноса наносов и

оперативных методов его измерения, а также послужить

практическим справочником в решении прикладных задач,

связанных с наносами.

Обеспокоенности по поводу экологии и окружающей

среды во все возрастающей степени влияют на устойчивое

развитие человеческих сообществ по всему миру. В главе 1

рассматриваются воздействия эрозии почвы, а также

заиления рек и водохранилищ на экологические аспекты и

окружающую среду, так же как и потенциальные выгоды

использования наносов как ресурса.

В главе 2 подробно представлены сведения об эрозии

почвы, включая ее основные характеристики, мониторинг и

предсказание эрозии и твердого стока в бассейне, сохранение

почв и воды, а также регулирование водосборов. И наконец,

представлен общий обзор глобальной проблемы эрозии почв.

В главе 3 обсуждается су ть процесса переноса

наносов в реках. Базовые концепции процесса переноса

речных наносов формируют основу, на которой

рассматриваются проблемы, связанные с речными наносами.

Они разъясняются сжато, но тщательно. В соответствии с

этим, а также с использованием авторитетных работ,

рассматриваются вопросы, касающиеся донных и

взвешенных наносов, а также суммарный твердый сток. На

основе использования большого количества данных и работ,

подготовленных главным образом в Китае, в конце данной

главы кратко рассматриваются потоки с очень большим

содержанием наносов.

Четвертая глава посвящена процессам,

происходящим в реках. Основные вопросы включают:

классификацию типов аллювиальных рек, флювиальные

процессы в реках каждого основного типа, а также

стабилизацию и спрямление речных русел. В настоящем

отчете аллювиальные реки классифицируются по четырем

основным типам: меандрирующие, блуждающие,

разветвляющиеся на рукава и прямые. Во многих

литературных источниках классификация рек проводится по

трем основным типам: меандрирующие, разветвляющиеся на

рукава и прямые. Такое различие может быть вызвано тем,

что некоторые реки в Китае переносят большое количество

наносов.

Водохранилища играют значительную роль в

человеческом обществе, включая регулирование паводков,

водоснабжение, выработку энергии, ирригацию, улучшение

навигации, отдых и т.д. Со временем многие водохранилища,

в особенности те, которые построены на реках, несущих

много наносов, теряют из-за отложения наносов

определенный процент своего полезного объема. В главе 5

подробно излагается вопрос о заилении водохранилищ и его

воздействиях на речные процессы. Сначала представлены

процессы отложения наносов в водохранилищах. Затем

кратко рассматриваются как эмпирические, так и численные

методы оценки отложения наносов в водохранилищах за

длительные периоды времени. Затем следует описание

регулирования водохранилищ с основным вниманием к

возможности долгосрочного сохранения полезного объема

водохранилища с целью его постоянного использования. На

примере шести конкретных исследований показана

реальность проблем заиления водохранилищ.

Точные данные о наносах являются основой каждого

аспекта регулирования стока наносов и численного

(компьютерного) моделирования процесса отложения

наносов. В главе 6 рассматриваются оперативные методы

измерения наносов, включая измерения взвешенных и

донных наносов и общего их количества. Также представлены

применяемые в лабораториях процедуры, обработка данных

и оценка точности и надежности при измерении наносов. И

наконец, приводятся некоторые рекомендации, касающиеся

измерения наносов.

Загрязнение воды становится повсеместно и во все

возрастающей степени важным вопросом, в особенности в

развивающихся странах. В главе 7 кратко освещаются

вопросы качества воды, связанные с переносом наносов и

токсичных материалов, являющихся основным источником

загрязнения воды. Для количественного описания такого

воздействия приводится модель качества воды.

РЕЗЮМЕ

question de la sédimentation des réservoirs et de ses répercussionssur les processus fluviatiles est traitée en détail dans le chapitre 5, quidébute par une description des mécanismes de dépôt dans lesréservoirs, avant d’exposer brièvement les méthodes — empiriqueset numériques — d’estimation de ces dépôts considérés sur delongues périodes. Les auteurs s’intéressent ensuite à la gestion desréservoirs, envisagée dans la perspective de préserver durablementleur capacité. Les problèmes de sédimentation des réservoirs sontillustrés par six études de cas.

La gestion des sédiments et la modélisation numérique dela sédimentation doivent s’appuyer à tous les niveaux sur desdonnées précises. Le chapitre 6 est consacré aux méthodes de

mesure des sédiments, notamment de la charge solide ensuspension, de la charge de fond et de la charge solide totale. Il estaussi question des pratiques de laboratoire, du traitement desdonnées ainsi que de la précision et de la fiabilité des mesuresrelatives aux sédiments. Le chapitre se termine par quelquesrecommandations dans ce domaine.

La pollution de l’eau est un problème qui devient de plus enplus préoccupant, en particulier dans les pays en développement. Lechapitre 7 évoque brièvement la question de la qualité de l’eau dansle contexte du transport de sédiments et de matières toxiques,principale cause de la pollution de l’eau. Un modèle de la qualité del’eau est utilisé pour quantifier les effets de cette pollution.

SUMMARY xv

RESUMEN

Este informe, que abarca gran número de cuestiones relacionadascon la sedimentación, tiene dos objetivos: dar al lector una ideabásica de los métodos utilizados a nivel operativo para la medicióndel transporte de sedimentos, y servir de referencia práctica enmateria de ingeniería de la sedimentación.

Las preocupaciones ecológicas y ambientales inciden cadadía más en el desarrollo sostenible de las sociedades humanas entodo el mundo. En el Capítulo 1 se examinan los efectos para laecología y el medio ambiente de la erosión de los suelos y de lasedimentación en ríos y embalses, así como los posibles beneficiosde los sedimentos aprovechados como recurso.

En el Capítulo 2 se aborda en detalle el tema de la erosiónde los suelos, incluidas sus características básicas, la vigilancia ypredicción de la erosión, el aporte de sólidos en una cuenca, laconservación de suelos y aguas, y el manejo de cuencas. Porúltimo, se presenta un panorama general del tema global de laerosión de los suelos.

En el Capítulo 3 se examina el tema del transporte desedimentos en los ríos. Se examinan de manera concisa y detenidalos conceptos básicos del comportamiento de los sedimentostransportados en los cauces fluviales, que forman la base del análisisdel sedimento de los ríos. Esto va seguido de un análisis del arrastrede fondo, la carga en suspensión y del arrastre total, en la que sehace referencia a documentos de autoridades en la materia. En laúltima sección del capítulo se estudia brevemente el flujohiperconcentrado, sobre la base de un importante volumen deinformación y de documentos técnicos, provenientes mayormentede China.

En el Capítulo 4 se analizan detenidamente los procesosfluviales. Los principales puntos incluyen la clasificación delcomportamiento de los ríos aluviales, los procesos fluvialesasociados con cada comportamiento básico, y la estabilización yrectificación de los canales de los ríos. En ese informe, los ríosaluviales se clasifican en cuatro categorías básicas: sinuosos,tortuosos, divergentes y rectos. Muchos autores emplean unaclasificación basada en tres categorías: sinuosos, trenzados y rectos.

La diferencia puede obedecer a la elevada carga de sedimentos quetransportan algunos ríos de China.

Los embalses desempeñan un papel importante para lasociedad en campos como control de crecidas, suministro de agua,generación de energía hidroeléctrica, riego, mejora de la navegación,recreo, etc. Con el paso del tiempo, en muchos embalses, en especiallos construidos en ríos que arrastran gran volumen de sedimentos, seha observado una cierta reducción de su capacidad dealmacenamiento debido a la sedimentación. En el Capítulo 5, el temade la sedimentación en los embalses y sus efectos sobre los procesosfluviales es objeto de un análisis más detallado. Se presentan primerolos procesos de deposición en los embalses. A continuación seexaminan los métodos, tanto empíricos como numéricos, deestimación de la deposición a largo plazo en los embalses. Másadelante se aborda la cuestión de la gestión de los embalses,haciendo hincapié en la posibilidad de preservar su capacidad a largoplazo para el uso permanente. Seis estudios de caso muestran larealidad de los problemas relacionados con la sedimentación en losembalses.

La exactitud de los datos es esencial para todos los aspectosde la gestión de los sedimentos y de la modelización numérica de lasedimentación con empleo de computadoras. En el capítulo 6 seexaminan los métodos operativos de medición de los sedimentos,incluida la medición de la carga/sedimento en suspensión, delarrastre de fondo y del arrastre total. Asimismo, se presentan losprocedimientos de laboratorio, el procesamiento de datos y laevaluación de la exactitud y fiabilidad de las mediciones de lossedimentos. Por último, se hacen algunas recomendaciones encuanto a la medición de los sedimentos.

La contaminación de las aguas es una cuestión que cadadía cobra mayor importancia en muchos lugares, sobre todo en lospaíses en desarrollo. En el Capítulo 7 se analiza brevemente lacalidad de las aguas en relación con el transporte de sedimentos ymateriales tóxicos, la principal fuente de contaminación de lasaguas. Se introduce un modelo de calidad de las aguas con el fin decuantificar esos efectos.

1.1 INTRODUCTIONSedimentation impacts many aspects of the environment — soilerosion, water quality, water supply, flood control, river regula-tion, reservoir lifespan, groundwater table, irrigation, navigation,fishing, tourism, etc. It has attracted increasing attention from thepublic and engineers in the field. In this Manual, the authors try todescribe the main problems and issues related to river and reser-voir sedimentation to help the reader understand them better.

1.2 IMPACTS OF SOIL EROSION ON ECOLOGYAND ENVIRONMENT

Soil and water conservation is one of the most critical environ-mental issues facing many countries, especially developingcountries. Water is the source of life and soil is the root of exis-tence. Water and soil resources are the most fundamental materialson which people rely for existence and development. The develop-ment of society is determined by its capacity to use its resources.Some of these resources may in time become exhausted or deteri-orate. Soil has been defined by the International Science Societyas ‘a limited and irreplaceable resource’, and the growing degrada-tion and loss of soil means that the expanding population in manyparts of the world is pressing this resource to its limits. In itsabsence, the biosphere environments of man would collapse, withdevastating results for humanity.

Soil and water conservation is a multidisciplinaryapplied science studying soil and water loss and control measures,in order to protect, improve and support rational uses of soil andwater resources in mountainous areas which suffer water and winderosion. Conservation also helps maintain and increase landproductivity.

Soil and water loss causes land resource destruction andreduction in soil fertility, which leads to the deterioration of theenvironment and the loss of ecological balance, causing naturaldisasters and constraining the development of agriculture, conse-quently increasing poverty.

China, for example, is seriously affected by soil erosion.Its total erosion area is 3.67 million km2, being 38.2 per cent ofthe total territory, of which 1.79 million km2 is eroded by waterand 1.88 million km2 by wind. Soil loss is 5 billion tons per year.The land is lost at a rate of about 0.13 million ha per year. In someeroded areas, land destruction and deterioration have even threat-ened people’s existence. The Loess Plateau, one of the mostseriously eroded areas in China, contributes a large amount ofsediment to the Yellow River. According to long-term statistics,1.6 billion tons of sediment are lost annually into the Yellow River.Two thirds of the total sediment is transported by the river insuspension and poured into the near sea and deep sea. The remain-ing one third of the sediment load is deposited in the lowerreaches of the river. As a result, the river bed rises by 8 to 10 cmeach year to create an unfavourable situation in which the riverbed is 4 to 10 m higher than the ground elevation outside thelevee. This has brought flood and drought disasters and poverty,and has greatly threatened the safety of the population. It is alsothe main constraint upon the development of agriculture and the

economy in the river basin. The 1.6 billion tons of sedimentcontain 40 000 tons of nitrogen, phosphate and potash fertilizers(N, P and K fertilizers). In north-east China, 7 million tons of N, Pand K are lost each year due to soil erosion.

The objectives of erosion control are to protect the twomost valuable natural resources, i.e. soil and water, and to preventthe occurrence of the unfavourable consequences of such a loss.

Erosion control measures must be harmonized with agri-cultural production and water resources conservation. Suchmeasures should cover the following aspects:(1) Comprehensive treatment. Soil and water conservation

requires the unified planning of water systems, forests, farm-land, and roads in mountainous and hilly areas, to achieveintegrated management and comprehensive development.

(2) Principal body of construction. Soil and water conservationis trans-sectoral and multidisciplinary. It should insist onadopting a combination of vegetative measures to protectland surfaces, structural measures to reduce and disperserunoff on land surfaces, and tillage measures to prevent soilloss caused by agricultural activity.

(3) Watershed management planning and activities. Theseshould bring ecological, social and economic benefits tostakeholders, so as to ensure sustainable development ofwatershed management.

In a river basin, soil erosion causes the deterioration ofecology and environment and the degradation of agriculturalproduction. Even more seriously, it makes farmland foreveruseless by reducing the fertility and productivity of soil. Sedimentdeposited in river channels raises the water level of floods, andtherefore brings a series of ecological and environmental problemsand aggravates flood disasters, not only by the flood itself but alsoby the sediment carried by the flood. On the other hand, the scour-ing of river channels lowers the water level and causes problemsfor water supply and navigation, and also threatens the safety ofriver training works. Reservoir sites are limited, precious, and notrenewable resources. Reservoir sedimentation reduces the storagecapacity and impacts the functions designed for reservoirs, such aswater supply, flood control, irrigation and power generation.Downstream from reservoirs, scouring of river channels occursand also has a number of negative impacts on ecology and envi-ronment. In this chapter, the impacts of sediment on ecology andenvironment will be introduced.

Sediment in water has two opposite effects on waterquality and environment. On the one hand, sediment particles inwater, especially the fine ones, absorb some pollutants and therebyimprove water quality to a certain degree. On the other hand, sedi-ment also serves as the major pollutant, carrier and storage agentof other pollutants, such as pesticides, residues, absorbed phos-phorus, nitrogen, organic compounds, pathogenic bacteria andviruses, and affects the water purity, transparency and quality. Thedetails of the impacts of sediment on water quality are describedin Chapter 7.

Soil erosion and sedimentation are among the greatest ofthe world’s modern environmental concerns. In many parts of the

CHAPTER 1

ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION

world, soil erosion has not only caused land deterioration andhampered the development of agriculture and industry, but alsoincreased sediment yield from the watershed. Soil erosion thusoften devastates topsoil and causes nutrient loss. Some mountainand hilly regions have become bare areas, causing environmentaldegradation. Emergency events of debris flow, bank collapse andlandslides are often disastrous for people’s lives and property, aswell as infrastructure. The main impacts of soil erosion aredescribed below.

1.2.1 Desertification and degradation of agriculturalproduction

One of the most serious consequences of worldwide soil erosion isdesertification. Population growth in some developing countries,inappropriate land use, deforestation, soil erosion by human activi-ties, and inappropriate water resources utilization cause severe landdesertification. At present, the total area affected by desertificationin the world is 45.61 million km2, accounting for 35 per cent oftotal global land. It accounts for 55 per cent and 75 per cent of thetotal area of Africa and Australia, respectively. Areas affected bydesertification increase by 50 000 to 70 000 km2 annually, includ-ing about 6 million ha of farmland (Zhu, 1992). China is a largecountry, but only one tenth of its land is cultivable. Desertificationhas reached 334 000 km2 and is increasing at an annual rate of1 560 km2. In Mongolia, the total land area is 156.5 million ha,among which 5.0 million ha is already covered with sand.

Erosion damages soil structure, causing the loss of fertil-ity, and consequently reduces agricultural production. Much ofSouth-West Asia, China, India, South-East Asia, North Africa,Central America and Mexico suffer from severe land degradation.In South America, land degradation is most acute on the cultivatedlands of the Andes Mountains. Water and wind erosion hasdamaged some Argentine farmland. In China, the soil-eroded areahas reached 1.79 million km2, accounting for 18.7 per cent of thetotal territory, with an increase of 2 460 km2 per year. In the last50 years, 2.6 million ha of farmland has been lost due to soilerosion. About 5 billion tons of eroded sediment enters rivers,lakes and seas. In India, it is estimated that about 6 billion tons ofsoil is lost each year as a result of sheet erosion. In addition, gullyand ravine erosion damages 8 000 ha of farmland annually.

1.2.2 Sediment-related disastersSediment-related disasters, such as debris flow, landslides andslope collapses, often induce huge damage to people, economiesand the environment. Debris flows exist to some extent in themountainous areas of more than 70 countries. China is a moun-tainous country, of which 69 per cent of the territory is composedof mountains and hills. Owing to a peculiar natural and human-geographic environment, almost all provinces, autonomousregions and municipalities are endangered and troubled by debrisflows, landslides, and other sediment-related disasters. Incompletestatistics show that, in China, there are more than 8 500 debrisflow ravines and 100 000 places susceptible to landslides, whichthreaten the safety of 36 main train lines, 36 per cent of the roadsand more than 200 medium and small cities. Debris flows occurfar more frequently and forcefully than in other countries, andcaused a loss of more than US$ 12 billion in 1990. In 1953, aglacier-induced debris flow occurred at Guxiang Ravine, Bomi,Tibet, with a peak discharge of 28 600 m3 s–1. Taiwan is a moun-tainous area. The characteristics of geography and climate — i.e.

broken rock, steep slope, torrential and concentrated rain, andshort and rapid flow — cause debris flows, landslides and slopecollapses to occur frequently.

Indonesia has about 17 active volcanoes. It experiencesnot only direct disasters due to frequent eruptions, pyroclasticflow and nuce ardente, but also indirect disasters due to secondarylahar caused by rainfall after eruptions have occurred. Many liveshave been lost. Also, huge amounts of volcanic product such asash, sand and gravel are deposited loosely on the slope around acrater during the eruption. According to records, approximately300 million m3 of volcanic product were produced by the eruptionof Mt. Agung in 1963, 22 million m3 by the eruption of Mt.Merupi in 1969 and 53 million m3 by the eruption of Mt.Galunggung in 1982 (Sabo, 1995).

In Japan, mountainous areas account for 74 per cent oftotal territory. Earthquakes, debris flows and volcanic eruptionsoccur often. During the torrential rains of August 1993, total rain-fall exceeded 800 mm. These heavy rains caused a series ofoverbank floods and debris flows in the Kagoshima area, includ-ing one that struck a train. These floods and debris flows causedthe interruption of transportation in the region due to the submer-gence of roads, and seriously interrupted the lives of localresidents by cutting power lines or breaking water supplies. Thesuccessive heavy rains left 141 people dead or missing and about150 000 houses damaged. The total losses, including damage topublic facilities, agriculture, forestry, and fishing was estimated at1 trillion Japanese Yen. Another type of debris flow in Japan iscaused by volcanic eruption. Large amounts of rock, earth, sandare released from volcanic eruptions and loosely pile up on slopes.When heavy rain comes, the volcanic materials form debris flowswith a huge damage capacity. After Mt. Unzen Fugendake eruptedin 1990, a pyroclastic flow occurred in June 1991. Forty-threepeople were reported dead or missing, nine were injured, and 179buildings burned down.

1.3 IMPACTS OF RIVER SEDIMENTATION ONECOLOGY AND ENVIRONMENT

Deposit and scour are common in rivers because of the differencebetween sediment load and the real sediment transportation capac-ity of flow. Deposition in river channels raises the elevation ofriver beds. Consequently, it enhances the water level at the samedischarge, and increases the occurrence and the damage of floods.On the other hand, scour brings some safety problems for rivertraining works, lowers water levels, and therefore affects watersupply and navigation along rivers.

1.3.1 River sediment and flood disastersOwing to serious soil erosion in the river basin, a large amount ofsediment load enters the Yellow River and is deposited in thelower reaches. The river bed rises about 5 to 10 cm annually. Theriver bed below Zhengzhou City, the capital of Henan Province, ishigher than the ambient ground, a so-called suspended river(Figure 1.1), and the river channel serves as the watershed bound-ary of the Haihe and Huaihe Rivers. If the river dikes were tobreak along the lower reaches, the maximum area affected byfloods would be 250 000 km2 north to Tianjin City and south tothe Huaihe River, an area among the most economically devel-oped in China. The maximum population affected by floodswould be 100 million. Such floods have occurred a number oftimes in history.

2 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

1.3.1.1 CONVEYANCE CAPACITY OF RIVERS

The conveyance capacity of a river changes due to deposition andscouring of the river channel. On the Lower Yellow River, the riverchannel is complicated, composed of the main channel and floodplains, and the total width may reach 10 to 20 km. There are0.22 million ha of cultivated farmlands, and about 1.5 millionpeople live on the flood plains. Middle and low floods are artifi-cially constricted within the main channels and 85 per cent of thedeposit is in the main channel. Therefore, the conveyance capacityof the main channel significantly decreases, which is called riverchannel shrinking. From May 1986 to May 1994, the flow areas ofthe main channel of 36 cross-sections on the Lower Yellow Riverdecreased by about 27 per cent. The water stage under the flowdischarge of 3 000 m3 s–1 rises by 0.12 to 0.15 m annually. Thebankfull discharge was reduced to between 2 800 and3 700 m3 s–1. The number of occurrences of flow over flood plainstherefore greatly increased in recent years. In 1996, the flooddischarge was only 7 860 m3 s–1 at Huayuankou Station nearZhengzhou City, which was much less than the flood of 22 300 m3

s–1 in 1958, but the flood stage was 94.73 m, the highest recorded,0.91 m higher than that of 1958. The inundated land on the floodplain was about 250 000 ha, with 1.07 million people affected.The direct loss was about US$ 800 million (Hu, 1996).

1.3.1.2 FLUVIAL PROCESS AND INSTABILITY OF RIVER CHANNEL

The fluvial processes in both planar and longitudinal directionssignificantly affect river behaviour and stability, especially forlarge rivers, which play very important roles in a country’ssustainable development of its economy, ecology and environ-ment. The fluvial processes may cause or aggravate the disasters.The 1998 flood in the Middle Yangtze River of China was a goodexample of this.

The middle reaches of the Yangtze River are a river-lakesystem composed of the Jinjiang River (i.e. the Middle YangtzeRiver), Dongting Lake and other lakes (Figure 1.2). During floods,part of the water is delivered to Dongting Lake through threeconnecting river channel, mitigating the peak flood water passingthrough the Jinjiang River channel. Owing to sediment depositionat the end reaches of the three connecting channels, theirconveyance capacities have greatly decreased. The Lower Jinjiangwas once a typical meandering river, with 12 sharp bends. Twobends, Zhongzhouzi and Shangchewan, were artificially cut off in1967 and 1969, respectively, and the Shatanzi was naturally cutoff. In 1972, the cutoffs of the three bends reduced the river lengthby 81 km. Therefore, the bed slope, flow and sedimentconveyance capacities increased. This reduced the ratio of flowentering Dongting Lake to the remaining flow in the main stream,and caused degradation of the Lower Jinjiang River.Consequently, the scoured sediment deposits flowed downstreamfrom Luoshan to Wuhan City, capital of Hubei Province, andraised the flood stage there. During the flood of 1931, 50.4 percent of the peak flow of 66 700 m3 s–1 was delivered to Dongting

Lake by the three connecting rivers, among them 28.4 per cent(18 970 m3 s–1) by the Ouchi River. However, only 6 000 m3 s–1,10 per cent of the total peak flow discharge, was delivered by theOuchi River in 1998. The flow volume annually delivered toDongting Lake was 146 billion m3 from 1951 to 1958, butdecreased to 69.7 billion m3 from 1981 to 1994, which means thatthe runoff through the Lower Jinjiang increased by 76.3 billionm3, significantly aggravating flood disasters. Although the peakflood of 61 500 m3 s–1 in 1998 was smaller than the 66 800 m3 s–1

in 1954, the flood stages at the stations along the middle reacheswere the highest on record.

Because of lake sedimentation and reclamation throughthe occupation of the lake as farmland due to the pressure of popu-lation growth, the area and storage capacity of Dongting Lakehave been reduced significantly, as shown in Table 1.1. Thisgreatly weakens its regulatory role during floods of the YangtzeRiver. Only about 10 billion m3 of water volume was diverted tothe detention areas during the 1998 floods, compared with 102.3billion m3 in 1954. This is one important reason why the 1998floods created a record high stage.

1.3.1.3 SAFETY OF TRAINING WORKS

Near bridges, groins, and other training works, flow velocity maybe larger than the upstream and downstream flow, due to thereduction of flow width caused by the structures. Scouring of riverchannels in the vicinity of structures is a common phenomenon,and threatens the safety of the structures and training works. If theestimated scour is wrong in the design stage, accidents may occur.

CHAPTER 1 — ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION 3

Figure 1.1 — Suspended river of the Lower Yellow River of China.

Figure 1.2 — The middle reaches of the Yangtze River.

L (km)

Jinjiang R.

1.3.1.4 SEDIMENT DEPOSITS BY FLOODS

Floods carry much higher sediment concentrations than normalflows. Damage estimates of floods should therefore also includeenvironmental deterioration by sediment deposition and the highcost of clearing up the deposition. Along the two banks of theYellow River, there are more than 40 alluvial fans formed byfloods. The sediment of the fans has a high content of fine parti-cles which are easily blown away by wind. Those areas thusbecome desertified. In August 1982, about 400 million m3 of floodwater was diverted to the Dongping detention area, and at thesame time about 5 million m3 of sediment (mostly sand) wasdiverted. Consequently, 425 ha of farmland were lost because ofthe sediment deposition.

1.3.1.5 VARIATION OF GROUNDWATER LEVEL AND SALINITY BY

RIVER SEDIMENTATION

Accumulated river sedimentation raises river water levels. Owingto the recharge of river water to groundwater in the adjacent areas,groundwater levels along river banks may rise and cause farmlandsalinity or other environmental problems. In the Lower YellowRiver, the flow water is generally 3 to 5 m higher than the adjacentground surface. It is estimated that about 49 800 tons of salt isrecharged annually to the groundwater by lateral filtration of riverflow. The groundwater level at 0.5 km from the river channelreaches 0.6 to 0.7 m, and serious salinity occurs along the riverareas.

1.3.2 Environment of sediment-laden rivers1.3.2.1 DEPOSITION IN IRRIGATION SYSTEMS AND

DESERTIFICATION AT IRRIGATION SYSTEM HEADS

The problem of sediment deposition in irrigation systems iscommonly encountered, especially in heavily sediment-ladenrivers. Dredging and clearing up the deposition in irrigation canalsis high-cost and labor-intensive work. There are many factors tobe taken into consideration to prevent, reduce and deal with thesediment entering irrigation systems. Appropriate intake type;settling pool at the head; reasonable design of canal, includingdiverted discharge and sediment concentration; bed slope; sideslope; size of cross-section; material (roughness); operation; andmaintenance are some examples of such factors.

On the Lower Yellow River, a large amount of farmlandrelies heavily on irrigation from the river. In total, there are 128intakes and 1.86 million ha of irrigated land in Henan andShandong provinces. From 1958 to 1990 (stopped from 1962 to

1965), 233.3 billion m3 (8.04 billion m3 per year) of water and3.865 billion tons (133 million tons per year) of sediment werediverted into irrigation systems. From 1981 to 1990, the annualvalues were 11.1 billion m3 and 120 million tons, respectively.Among the 120 million tons of sediment, 33.22 per cent, 35.32 percent, 22.9 per cent, and 8.56 per cent were deposited respectivelyin settling pools, irrigation systems, farmland and drainagesystems. This means that 77.1 per cent of sediment deposition, i.e.92.52 million tons annually and about 3 billion m3 in total, mustbe dredged or dealt with. In 1990, about 50 000 ha of settling poolareas at heads of the irrigation systems were filled up with about1 billion m3 of sediment. Moreover, the deposition in the canalswas dredged out and placed on a narrow belt along the two sidesof the canal. These depositions contain coarse sand, and form sandhills or dunes. It is dry and windy in the winter and springseasons, which causes the local people to suffer disasters due toserious desertification.

1.3.2.2 IMPACTS OF RIVER CHANNEL SHIFTING ON ENVIRONMENT

AND ECOLOGY

In the Yellow River, about 1 billion tons of sediment enter thedelta region annually, most of which deposits in the delta coastalarea and near the sea, creating some new land (average of 20 to30 km2 per year) and extending the river to the sea. Because of thedeposition, the river shifted its channel many times and created theGrand North China Plain. Figures 1.3 and 1.4 show the moderndelta and the change of the river mouth channels since 1855.Owing to the frequent channel shifting, the development of thelocal economy was limited. China has made a great effort to stabi-lize the river mouth (Yang and Zhang, 1998).

1.4 RESERVOIR SEDIMENTATION ANDENVIRONMENT

1.4.1 Loss of reservoir storage capacityReservoir sedimentation and the consequent loss of storage capac-ity affect reservoir benefits, such as flood control, water supply,irrigation, navigation, power generation, fishing and recreation. Inarid and semi-arid regions, reservoir sedimentation problemsbecome most acute where the loss of storage capacity by reservoirsedimentation is above 1 to 2 per cent per year and the lifetime ofmost reservoirs is only 20 to 30 years. The Welbedacht Reservoirin South Africa, completed in 1973 with a 152.2 million m3

storage capacity, lost most of its storage capacity (66 per cent)within the first 13 years of its existence (Rooseboom, 1992).

In India, measurements of reservoir sedimentation indi-cate that the average annual loss in storage capacity of nineimportant reservoirs is between 0.34 and 1.79 per cent. Among 23large reservoirs, the measured rate of storage loss was less than thedesigned rate in only two reservoirs; in other reservoirs, it wasmore than five times larger than the designed rate (Central WaterCommission, 1996).

In Italy, an analysis of 268 reservoirs distributed over thecountry with a mean age of 50 years showed the following loss ofreservoir storage capacity: 1.5 per cent of the reservoirs werecompletely filled by sediment, 4.5 per cent had lost 50 per cent oftheir storage capacity, and 17.5 per cent had lost 20 per cent oftheir storage. The Ichari Reservoir in India silted up to crest levelof the spillway in two years. The Austin Reservoir lost 41.5 percent of its total storage volume from 1893 to 1897, and the damgave way in 1900. The new Lake Austin of the Colorado River in

4 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 1.1Change in area and storage capacity of Dongting Lake

Year Area (km2) Storage capacity (109 m3)

1825 6000

1896 5400

1932 4700

1949 4350 29.3

1954 3910 26.8

1958 3141 22.8

1971 2820 18.8

1977 2740 17.8

1983 2691 17.4

1995 2625 16.7

Texas lost 95.6 per cent of its capacity in 13 years, the HabraReservoir in Algeria 58 per cent in 22 years, and the WuchiehReservoir in Taiwan 98.7 per cent in 35 years. The Indus Rivercarries about 74 billion m3 of water and 300 million tons ofsuspended sediment per year into the Tarbela Reservoir. In the sixyears after its commissioning in 1974, it accumulated about 950million m3 of sediment in the upper 30 km of the delta (Wu, et al.,1996).

The loss of storage capacity in reservoirs in the UnitedStates due to sedimentation accounts for an annual monetary lossof US$ 100 million (Julien, 1994).

The average annual loss of storage capacity for 28reservoirs in Taiwan, China (with original storage capacitiesranging from 0.65 to 708 million m3) is 1.45 per cent.

In China, the Yellow River is a heavily sediment-laden river with an annual sediment load of 1.6 billion tons.As of 1989, the losses caused by reservoir sedimentation hadreached 10.9 billion m3, accounting for 21 per cent of thetotal storage capacity of all reservoirs on the main stem aswell as tributaries. Among them, 2.9 billion m3 were in thereservoirs on the tributaries, accounting for 26 per cent ofthe total.

CHAPTER 1 — ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION 5

Figure 1.4 — Change of the Yellow River mouth channels since 1855.

Figure 1.3 — The modern delta of the Yellow River.

1.4.2 Water pollution by reservoir sedimentationIn the initial stage of reservoir sedimentation, the deposition ofsediment can actually improve the water quality by absorbingpollutants. According to observations carried out at GuantingReservoir, one ton of sediment can absorb 700 g of dissolved lead.Mud deposited on the reservoir floor displays strong adsorption ofarsenic, of which the concentration on the floor is 10 to 100 timeshigher than that in water. Similarly, the concentration of chromiumon reservoir floors is about 20 000 times higher than that in water.Thus, deposited sediment as well as the layer of water near thefloor will be progressively polluted. Pollutants increasingly accu-mulate in the lower part of the reservoir. In time, they become soconcentrated that this part of the storage becomes in itself a sourceof pollution. Phenol in the reservoir’s water has already slightlypolluted the groundwater of Beijing.

1.4.3 Rise of groundwater level and salinity by depositextension in reservoir backwater regions

Sediment deposition in reservoirs extends both downstream andupstream. When sediment goes into a reservoir, it deposits in theupper end of the backwater region first, due to slowing flow veloc-ity. With the development of reservoir sedimentation, thedeposition may extend upward and cause a river bed higher thanthe normal pool of the reservoir, which induces some environmen-tal and ecological problems.

1.4.4 Problems of downstream reservoir1.4.4.1 FLOOD PLAIN COLLAPSE

Since the impounding of Sanmenxia Reservoir on the YellowRiver of China from 1960 to 1964, the flow discharge and sedi-ment transport rate downstream of the reservoir have been greatlychanged. Most of the sediment carried from upstream has beenstored in the reservoir, and the duration of medium floods (4 000to 6 000 m3 s–1) has exceeded 20 days due to reservoir regulation.Total scoured sediment has been as high as 2.31 billion tons, and300 km2 of flood plains have been scoured away by floods, with aloss of 47 000 ha of farmland on the flood plains.

The Danjiangkou Reservoir (DJK) is on the HanjiangRiver, the longest tributary of China’s Yangtze River. The riverdownstream from the dam was originally a wide, shallow andbraided channel with a rapidly shifting thalweg and lots of well-developed unstable mid-channel bars, and was regarded as atypical meandering braided river. Bank erosion was fast becauseof quite high flood peaks, frequent and rapid channel shifting, andlow silt-clay content in the bank material. After the dam wasconstructed in 1959, the river bottom was scoured down and bankerosion slowed. However, after the bed scouring the bed materialwas coarser and had higher resistance than before, which causedthe bank erosion to return. In the 130 km reach immediatelydownstream from the dam in the 1968 to 1981 period, 16.35million tons of sediment was annually scoured and supplied down-stream by bank erosion, accounting for 42.8 per cent of the totalsediment load of the reach. Bank erosion became a major sedi-ment contributor in the reach.

1.4.4.2 DOWNSTREAM NAVIGATION

When a reservoir is built on a river, much of the sediment is storedin the reservoir. The flow released from the reservoir carries muchless sediment than the natural flow, which interrupts the sedimentbalance and results in scouring in downstream reaches and a

lowering of the water level. For a navigable river, this may resultin insufficient water depth during the low flow seasons. Since theGezhouba Dam on the Yangtze River was built in 1981, the down-stream river bed has been scoured and the water level during lowflow at Yichang has been lowered by 1.05 m, reducing the waterdepth downstream, approaching the channel of the Nos. 2 and 3Navigation Locks, to only about 3 m. The designed minimumwater depth for No. 2 Lock (for barge fleets of 10 000 tons) is4.5 m. This affects navigation on the reach.

The Rhine River is the most important navigationchannel in Europe, due to its well-balanced discharge conditions.A number of dams and navigation locks have been constructed inthe Upper Rhine above Iffezheim, Germany, to ensure a safe andefficient navigation channel. Erosion is often observed due to adeficit in bed-load transport caused by the dam impoundment andtrapping of the bed-load supply from upstream reaches and tribu-taries. Downstream from Iffezheim to the Dutch border, some500 km long, is a freely flowing stream regulated by groins, guidedikes and bank revetments, so the morphological changes can onlyoccur in the river bottom. A careful field measurement has indi-cated that on the 500 km of reaches there are nine reaches withalternating aggradation and degradation, as shown in Table 1.2.The total deficit of bed load in the Rhine is about 350 000 tons peryear, 50 000 tons per year in the Upper and Middle Rhine, and300 000 tons per year in the Lower Rhine. The highest bed degra-dation rates of 8 to 9 mm per year have been observed betweenMannheim and Mainz, and 11 mm per year aggradation has alsobeen observed in the mining subsidence in the Karlsruhe andMannheim areas. Finally, 260 000 tons per year of bed load anddredged material have to be artificially transported by barges anddumped back to the river to compensate the bed-load deficit(Dröge, 1992).

On the other hand, reservoir regulation greatly changesthe flow and sediment conditions in the reservoir downstreamreaches. After the construction of the Aswan High Dam (AHD),the flood flows of the Nile River downstream were largely elimi-nated. During the winter closure (December to February), aminimum flow discharge of about 700 m3 s–1 is released for navi-gation. With the small sediment supply and low flow velocity, thethalweg of the low flow channel continuously shifts on the wideand shallow channel and multiple thalweg channels are formed.The water depths are only 1.75 m at Selwa Bahary, 1.4 to 1.65 m

6 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Section Reach Length Balance Width Bed change(km) (km) (103t) (m) (mm per

year)

1 334.0–356.0 22.0 –73 170 –11

2 356.0–426.7 70.7 +207 180 +9

3 426.7–483.5 56.8 –201 220 –9

4 483.5–528.8 45.3 +103 400 +3

5 528.8–660.1 131.3 –129 200 –3

6 660.1–703.6 43.5 +109 240 +6

7 703.6–768.0 64.4 –281 260 -9

8 768.0–800.0 32.0 +178 280 +11

9 800.0–857.5 57.5 –251 300 –8

Sum/ 334.0–857.5 523.5 –338 230average

Table 1.2Mean bed-load in the Rhine River (1981–1990)

at Rozaiquat and 1.35 to 2.1 m at Armant between Aswan andLuxor. The water depths are not sufficient for navigation(Gaweesh and Ahmed, 1995).

1.4.5 Case studiesThe construction of reservoirs, especially large reservoirs, greatlychanges the natural river conditions and causes a number of envi-ronmental and ecological problems related to sedimentation. Onthe one hand, the sediment carried by flow largely deposits in thereservoir because of the reduction of flow velocity, and diminishesthe benefits of the reservoir. On the other hand, the flow releasedfrom the reservoir carries much less sediment than the natural flowand scours the downstream river channel. It may cause watersupply and navigation problems. Engineers and planners shouldpay close attention to these problems in the planning and designstages and try to find available measures or operations to mitigatethe damaging effects of the reservoirs as much as possible. Somecase studies given below are expected to provide experience in thisarea.

1.4.6 Guanting Reservoir in ChinaGuanting Reservoir on the Yongding River in northern China(Figure 1.5) has a storage capacity of 2.27 billion m3, consistingof two parts. One is on the Yongding River with a capacity ofabout 0.91 billion m3 (40 per cent of the total); the other is on theGuishui River, a tributary of the Yongding, with 1.36 billion m3

(60 per cent of the total). Almost all runoff and sediment loadcomes from the Yongding River. By 1998, the total sedimentationin the reservoir had reached 0.646 billion m3, with only about52 million m3 (9 per cent of the total deposit) in the Guishui andmore than 90 per cent in the Yongding. The reservoir sedimenta-tion greatly reduces the functions of the reservoir for flood controland water supply. Moreover, as the deposition delta in theYongding River progressed forward to the dam, a mouth bar at theGuishui River mouth formed and rose to an elevation of 474.4 min 1997, making the storage capacity of 0.254 billion m3 in theGuishui River useless.

On the other hand, the deposition has extended upwardto a point 36 km from the dam where the bed elevation reached507 m, 29 m above the normal pool of the reservoir. It caused theriver, at the confluence of two upstream tributaries, the Sangganand Yang Rivers, to rise by 4.3 m, which is 1.6 m higher than theground levels outside of the levees. A rise of the water level in the

backwater region due to sediment deposition there led to a generalrise in the groundwater table in the riparian region. Contours ofequal rise in the groundwater table occurred in the triangular areabetween the Sanggan and the Yang Rivers. A major part of the areahad a rise in groundwater table of 3 to 4 m, coming to withinabout 1.5 m below the ground surface. This caused extensive landsalinization. In the past, the area subjected to salinization was only533 ha, but it has increased about 14-fold, to 7 333 ha. The annualloss in food production due to waterlogging in the reservoir regionhas been estimated at 25 000 tons. With the deposition in the back-water region progressing upstream to an extent greater thananticipated, some relocated people were again affected by the risein the groundwater table subsequent to the rise in river level,resulting in waterlogging, the collapse of numerous houses andeven the formation of some marshes. The total area affected isover 20 000 ha. Rehabilitation involves both economic and socio-logical problems (Zhang, Jiang and Lin, 1986).

1.4.7 Aswan High DamThe Nile River in Africa is the second largest river in the world,with a total river basin of 2.9 million km2 and a length of6 825 km. The Nile flows through nine countries: the Republic ofTanzania, Burundi, the Democratic Republic of the Congo,Rwanda, Kenya, Uganda, Ethiopia, Sudan and Egypt. It has1 400 km in Egypt, where it empties into the Mediterranean Sea.About 96 per cent of the territory of Egypt is desert, with anannual precipitation of only a few centimetres. The population isconcentrated along the Nile and the river delta. The annual runoffat the dam site is 84.0 billion m3, with a yearly fluctuation of 41.3to 134 billion m3. If the yearly runoff is more than 130 to 140billion m3, a food disaster occurs. However if it is less than 40 to50 billion m3, it causes droughts. The annual sediment load is 316million tons, with a sediment concentration of 3.764 g/l. Floodslike the one in 1878, with a maximum daily runoff of 1.14 billionm3, and droughts like the one lasting nine years (1979 to 1988)can create disastrous situations for the Egyptian people.

The Aswan High Dam (AHD) is on the Lower Nile Riverin southern Egypt. The reservoir is called Lake Nasser, with a totalcapacity of 168 billion m3. The construction of the AHD hasprovided Egypt with comprehensive benefits. The water dischargein a year ranged from 1 000 to 10 000 m3 s–1 before the dam wasconstructed. After the dam was completed, the maximum waterdischarge was limited to 2 500 m3 s–1 and the sediment

CHAPTER 1 — ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION 7

Figure 1.5 — Guanting Reservoir.

concentration was reduced to between 0.03 and 0.1 g/l. The hugestorage capacity of the reservoir successfully controlled the floodsin 1968, 1975 and 1988, and met the irrigation requirements for thenine-year drought from 1979 to 1988. Before the AHD, the riversupplied 4 billion m3 and 48 billion m3 of water to Sudan and Egypt,respectively. The water benefit from reservoir regulation is 22 billionm3, shared by Egypt and Sudan according to the 1959 Nile WatersAgreement. Now, 14.5 billion m3 and 7.5 billion m3 of additionalwater annually goes to Sudan and Egypt, respectively, or 18.5 billionm3 and 55.5 billion m3 in total. Egypt’s agricultural productionincreased 20-fold from 1960 to 1987, and the wheat yield rose from1.1 million tons in 1952 to 4.5 million tons in 1991. The AHD hasalso created other benefits for both Sudan and Egypt, such as powergeneration of about 10 billion kWh per year (53 per cent of the totalelectric power of Egypt in 1977), improvement in the navigationconditions upstream and downstream, development of tourism, and10 000 tons of fish annually.

The AHD also creates some ecological and environmen-tal problems for both the upper and lower reaches. Some of themare related to reservoir sedimentation (Said Rushdi, 1993):(1) Residents moving. The homes of close to 400 000 Nubians

and an array of temples, tombs and fortresses were inundatedforever by the reservoir.

(2) Water loss. Water losses during the 1970 to 1986 period were190.4 billion m3, with an annual loss of 11.2 billion m3 dueto evaporation and seepage of Lake Nasser. Evaporation inthe lake results in a 10 to 15 per cent increase in the totaldissolved solids of water, and affects the water quality.

(3) Salinity of irrigated land. The salt content in the water of theNile is 0.02 per cent and 0.035 per cent, at the dam site andmouth, respectively. The annual irrigation water from theriver is about 40 billion m3, meaning about 12 million tons ofsalt is added to soil and groundwater by filtration. It is esti-mated that about 96 kg of salt are deposited on each feddan(about 0.42 ha) per year. Therefore, an appropriate drainagesystem had to be established. During the initial irrigationperiod, drainage was not given appropriate consideration. Thegroundwater table rose and land salinity occurred. Since the1970s, the Government has paid much attention to drainage,and has taken rational measures to control salinity. By July1992, 87 per cent of the drainage system was completed. Thesalt content in soil has been controlled effectively. The situa-tion has improved greatly, and agricultural production hasincreased by 15 to 30 per cent.

(4) Decline of land fertility. In the past, the silt left by floodsprovided inundated farmland with a large amount of naturaland organic fertilizer. However, the clear water released fromthe reservoir is lacking in such fertilizer, and therefore thefertility of the land has deteriorated.

(5) Degradation and channel shift of the downstream reach ofthe dam. The annual sediment load at the dam site is about134 million tons (ranging from 60 million to 180 milliontons). After the reservoir was put into operation, most of thesediment load deposited in the reservoir. The maximum sedi-ment concentration of the Nile at the dam site before thereservoir construction was about 3 764 mg/l. However, afterthe dam was completed it was reduced to 30 to 100 mg/l.Scouring has occurred in the downstream reaches. At thebeginning, the scouring rate was fast, ranging from 2.2 to3 cm per year in a 478 km-long reach downstream from the

dam, as the riverbed became coarse and the flow conditionsmuch more uniform than before. The river bed in mostdownstream reaches was scoured by 42 to 66 cm on averageuntil the 1980s (Figure 1.6). The maximum local value was 2m. The water level was lowered too, which reduced the waterhead difference between upstream and downstream of someweirs, and therefore produced safety problems. The waterdepth in some reaches was not enough for navigation due tothe water level falling. Another problem caused by the scour-ing of clear water was shifts of the river channel and thecollapse of river banks, as shown in Figure 1.7. In the mid-1980s, stable river channel conditions reached downstreamand the rates of scouring almost stopped. After river channelprotection works were constructed, the lateral shift of theriver channel was limited. On the other hand, about 300 000feddans of old farmland deteriorated because the topsoil wasused as raw material for brick making. Before the AHD,large amounts of sediment provided by the annual floodswere the raw material source for brick-making.

(6) Erosion of coastline. Erosion at the river mouth is found andthe delta area is threatened because the reduced incomingsediment cannot fully supply the amount carried away bytidal flow. The coast line at the mouth of the Rosetta drawsback about 150 m per year. Sand losses are in the order of200 000 tons per year west of the Rosetta mouth and400 000 tons per year west of the Damietta mouth. Theaquifer beneath the northern reach of the delta 15 to 35 kminland from the sea has the same salinity as the sea.

(7) About 82 per cent of irrigation and drainage canal systemsare overgrown with weeds and grass, which increases theroughness of the canal system, reduces the flow conveyancecapacity of the system, impacts the navigation conditions,and increases the loss due to evaporation. Moreover, weedsand grass provide a habitat for some vehicles of diseases.

The Government and people of Egypt have made greatefforts to control and eliminate the negative impacts of the AHDon the ecology and environment, and the benefits of the AHD havemade the reservoir shine with the great splendour of Egypt.

1.5 UTILIZATION OF SEDIMENT RESOURCESRiver sediment brings many problems, as described above.However, it does not always cause trouble and can sometimeseven be utilized as a precious resource. Sediment eroded fromupstream basins normally contains organic manure, fertilizers and

8 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 1.6 — Change in water level below the AHD.

other matter. Farmland irrigated by water with sediment may havehigher production levels because of fertility in the sediment.Sediment may also be diverted to warp and improve lowlands. Onthe Lower Yellow River, by 1990 230 000 ha of lowland had beendeveloped into highly productive farmland by warping, including120 000 ha as paddy fields.

The sediment may also be used as construction materialfor earth embankments and dikes for flood control. It is a goodlocal material, with the advantages of low costs, shorttransportation, and convenience. In some developing countries, thesediment dredged from rivers, lakes or reservoirs is used to makebricks.

REFERENCESCentral Water Commission, 1996: Experience in sedimentation of

Indian reservoirs and current scenario. Proceedings of theInternational Conference on Reservoir Sedimentation, 1996,Volume 1, pp. 53-72.

Dröge, B., 1992: Changes of river morphology by controllederosion and deposition-bed load budget of the River Rhine.Proceedings of the Fifth International Symposium on RiverSedimentation, Karlsruhe.

Egyptian Committee on Large Dams, 1993: Aswan High Dam: Avital achievement fully controlled, Volume 11, Cairo.

Gaweesh, M.T.K., and A.F. Ahmed, 1995: Navigation difficultiesunder controlled flow conditions on the Nile River. TheHydraulics of Water Resources and their Development,

HYDRA 2000, Twenty-sixth IAHR Congress, Volume 4pp. 30-35.

Hu, Yisan (ed.) 1996: Flood Control of the Yellow River. YellowRiver Press (in Chinese).

Julien, P.Y., 1994: Erosion and Sedimentation. CambridgeUniversity Press.

Rooseboom, A., 1992: River sediment problems in South Africa.Proceedings of the Fifth International Symposium on RiverSedimentation, Karlsruhe.

Sabo Technical Centre (STC), 1995: Sabo in Indonesia. Ministryof Public Works, Indonesia, JICA.

Said Rushdi, 1993: The River Nile: Geology, Hydrology andUtilization. Pergamon Press.

Wu, Chian Min, et al., 1996: International Handbook on ReservoirSedimentation. Proceedings of the International Conferenceon Reservoir River Sedimentation, pp. 571-612.

Yang Xiaoqing and Zhang Shiqi, 1998: Fluvial Processes of theYellow River Delta. International Workshop on Aspects andImpacts of a Changing Sediment Regime, Bangkok,Thailand, 16-20 November 1998, p. 149.

Zhang Qishun, Jiang Naisen and Lin Bingnan, 1986:Environmental problems associated with sediment deposi-tion in Guanting Reservoir. International Journal ofSediment Research, Volume 1, Number 1, August 1986,pp. 67–78.

Zhu Zhenda, 1992: Desertification Disasters Prevention andControl Methods in China. Hubei Kexue Press (in Chinese).

CHAPTER 1 — ECOLOGY AND ENVIRONMENT RELATED TO SEDIMENTATION 9

Figure 1.7 — Change of the Nile River channel between 364 and 381.75 km below the AHD.

2.1 INTRODUCTIONErosion is a process in which earth or rock material is loosened ordissolved and removed from any part of the Earth’s surface, and isoften differentiated according to the eroding agent (wind, water,rain-splash) and the source (short, gully, rill, etc.). Soil loss isdefined as the quantity of soil actually removed by erosion from asmall area (Piest and Miller, 1975). Whereas weathering involvesonly the breakdown of rock, erosion additionally entails thedetachment and transport of weathered material from one locationto another, denuding the Earth’s surface and delivering sedimentto the fluvial system by exogenous and geological forces.Exogenous forces include solar radiation, rain and micro-organicactivities, especially the aspects of water, ice and wind, and withhumans as a significant, anthropogenic factor. Geological force isa reference to the Earth’s crustal movement caused by geologicaltectonic movement. According to the agents causing and affectingthe erosion process, erosion can be classified into two major types,natural (normal) erosion and accelerated (abnormal) erosion.

2.2 NATURAL EROSIONIn its broadest sense, natural erosion is a process which refers toerosion that occurs under normal conditions and which leads tothe formation of a normal soil profile of the Earth in its naturalenvironment without human interference. The rate of this erosionis less than that of genetic soil forming. It is caused mainly bynatural exogenous forces such as water, gravity, wind, temperaturevariation and glaciers. Natural erosion includes geological erosion.

Erosion caused by geological factors is defined as theerosion of the Earth’s surface under natural or undisturbed condi-tions (Gottschal, 1975). This process has been occurring sincecontinents emerged from the sea, and includes soil formation aswell as erosion processes. The rate of this erosion, combined withthe complex processes of soil formation, largely determines thetype and distribution of soil on the Earth’s surface.

2.2.1 WaterErosion can be caused by the kinetic energy of raindrops imping-ing on the soil surface and by the mechanical force of surfacerunoff. Surface runoff is caused by heavy rainfall and snow waterfrom spring thaw in the natural or artificial hydrographic network.Erosion caused by water is the most common, widespread andharmful type of soil erosion in the world. The main categories ofthis type of erosion are surface erosion and channel (or gully)erosion.

(1) Surface erosion. Surface erosion is caused byprecipitation and surface runoff. Soil particles are first detached byraindrops, then carried down a sloping surface. Surface erosion isa feature of splash erosion, sheet erosion and rill erosion. The rateof this type of erosion is determined by slope gradient, kineticenergy of raindrops, direction of splash, shear stress among soilparticles and soil structure.

Splash erosion: Splash erosion refers to the destruction ofthe Earth’s surface by raindrops. Soil particles which are detachedand displaced from the soil surface by raindrops are carried and

gathered up by runoff to form a thin mud flow on the land surface,moving from upper parts to lower parts of slopes. This leads to soilerosion during the process of rainfall. Splash erosion destroys soilstructure and blocks the porosity of soil; as a result, it creates theconditions to form runoff on slopes, since rain water cannot perme-ate the soil. Experiments have shown that on moderate slopes, 90per cent of the erosion is caused by splash. Runoff scouring canplay a key role only when the land slope is 9°.

Sheet erosion: Sheet erosion is the weathering away of athin layer of land surface, and is caused by runoff, which isdistributed over the land surface with relatively lower velocities.Sheet erosion generally occurs on gentle slopes close to mountainridges. Sheet erosion more or less removes a thin layer or sheet ofsoil from a gentle sloping land or watershed. It is a rather incon-spicuous type of erosion because the total amount removed in astorm is usually small. However, over a period of years, theamount of eroded sediment can become significant. Sheet erosioninvolves two processes. First, soil particles are detached from thebody of the soil by raindrops. Second, the particles are transportedfrom their original location by surface runoff, which is formedwhen the rate of rainfall exceeds the infiltration rate of water intosoil and water starts to flow over the surface of sloping land. Atthis point, the second erosion transport process takes place. Theflowing water picks up the raindrop-detached particles and carriesthem along. The action of sheet erosion causes the soil mantle tothin, and finally the underlying rock and mineral substrata are laidbare over a large area.

Rill erosion: Rill erosion is the process of a thin layer ofsurface flow accumulating and concentrating in depressions toform rills. In rill erosion, detachment is caused primarily by theenergy of flowing water. According to field measurements of a rill,when the land slope is 5.7 to 40 per cent and the rain intensity is32 to 117 mm per hour, the water depth and runoff velocity are0.28 to 0.99 mm and 5.4 to 32 cm s–1, respectively, and the rillwidth is less than 20 cm. The rill depth is over the cultivation layerand the rill is easily removed by normal tillage operations. Thereis no sharp break marking the end of sheet erosion and the begin-ning of rill erosion. Rills form as soon as surface flow begins. Thenumber of rills that develop in a given area can vary widely,depending mainly on the irregularity of the soil surface and theamount and velocity of runoff. Detachment and transport of soilparticles are greater in rill erosion than in sheet erosion. This isdue to acceleration of the water velocity as it concentrates andmoves in rills.

(2) Channel erosion. Channel erosion cuts deeply intothe soil when ordinary tillage tools cannot smooth the ground. Itoften follows sheet and rill erosion. It occurs on the steeper slopingland, either where runoff from a slope increases sufficiently involume or velocity to cut deep incisions, or where the concentratedwater flows long enough in the same channel. Gullies may developfrom rills which are allowed to go unchecked. Often, they developin natural depressions of the land surface where runoff water accu-mulates. The rate and extent of gully development is closely relatedto the amount and velocity of runoff water. Gully depth ranges

CHAPTER 2

SOIL EROSION

between 30 cm and 2 to 3 m in general, and may sometimes evenreach several dozen metres. Gullies have large dimensions, andtheir development is more complicated. The forms of erosioninclude retrograde or backward erosion, vertical erosion and lateralerosion, together with accompanying landslides and mudflow, etc.Gullies may grow into gorges and canyons, which are usuallymoulded by watercourse erosion.

Channel erosion can be subdivided into shallow gullyerosion, gully erosion, gulch or canyon erosion and watercourseerosion.

Shallow gully erosion: Shallow gully erosion mainlyoccurs on relatively steep slopes, and is the result of the furtherdevelopment of many rills with concentrations of sufficient runoff.The depth of gullies is generally between 0.5 and 1.0 m and thewidth is in excess of their depth, forming shallow cross-sections.Shallow erosion develops to form gully heads and drops, whichare the main features of gully erosion.

Gully erosion: Through the accumulation of large quan-tities of runoff coming from rills and shallow gullies, or throughthe gradual deepening of rills, gully erosion of various sizes andforms comes into being. The first form includes any gully with adepth of between 30 cm and 2 to 3 m. In this form, typical washprevails over marked backward or retrogressive erosion and verti-cal or depth erosion, the erosion curve being compensated bywaterfall erosion. Besides retrogressive and vertical erosion,lateral erosion also appears here, together with accessory land-slides, soil flow and other phenomena. According to the forms oferosion gullies viewed in cross-section, flat, narrow, broad andround gullies are distinguishable. Flat forms occur mostly onshallow soil, or in connection with a specific lithic structure ofslope. In this form, characterized by a V-shaped cross-section,lateral erosion prevails over vertical erosion. Narrow acute formsare created with a narrow V-section, the breadth of the gullyusually being equal to or smaller than its depth.

Gulch erosion: Gulch gullies have a wide bottom and areU-shaped. Here, lateral erosion prevails over depth erosion; activegullies maintain steep or even perpendicular sides (Zachar, 1982).With concentrated runoff cutting the gully bed, retrogressive orheadward erosion, gully bed erosion and lateral erosion are active.As runoff discharge increases, gully erosion develops rapidly byvertical erosion, retrogressive and lateral erosion to make aU-shaped cross-section. The slope of the gulch bed is distin-guished from the original land surface. Its slope upstream of thegully bed is steeper than downstream. Vertical erosion decreases,and retrogressive erosion and lateral erosion collapse are active.Gulch development depends on large quantities of water to supplyenergy for both detaching and transporting the soil. Drops can befound at the gully heads, where the retrogressive erosion will startwith the next rainfall. The retrogressive erosion causes the drophead gradually to increase; as a result, collapse of the lateral slopetakes place due to vertical erosion of the gully bed.

Watercourse erosion: Watercourse or river erosionoccurs where there is a permanent water flow, and usually shows avarying intensity as the flow varies. The smaller the catchmentarea of the watercourse, and the less favourable the conditions ofdischarge, the greater the fluctuation of erosion intensity. Theuppermost branches resemble gullies and therefore constitute atransition between river and gullies. The boundary line betweenthe hydrographic network and gullies remains arbitrary, especiallyin semi-arid and arid regions. According to the prevailing direction

of influence, a distinction can be made between vertical or bottomerosion, which deepens on the profile and compensates the erosioncurve; lateral erosion, which broadens the river bed and may causea change in the flow direction; and retrogressive or retrogradeerosion. From this point of view, gully and river erosion aresimilar, but river erosion changes the surface of the watercourseonly to a small extent and damages only soil. In general, by lateralmovement of the river course as it meanders, the area covered bygullies may considerably increase at the expense of agriculturalland. In gully erosion, the typical action is retrogressive erosion; inriver erosion, it is lateral erosion. In this connection, it is possibleto speak of river erosion of the soil occurring along banks andduring flood conditions. Under the influence of this process,various kinds of undermining action may occur together with slipsand rifts of banks and slopes. During floods, surface wash, gullies,hollows and other forms may also occur (Zachar, 1982).

(3) Gravitational erosion. Gravitational erosion is, asthe name imples, caused mainly by gravitational agents. Its maincharacteristic is the transport of surface materials as part of ajoint action with other exogenous agents, especially watererosion and infiltrated water. The stability of the earth on thesteep slope is maintained by internal soil friction and cohesion,as well as protection of vegetation. This internal friction andcohesive force is decreased when influenced by exogenousagents such as vegetation depletion or raindrop splashing.Consequently, under the influence of gravity, soil and parentmaterials begin to move. Gravitational erosion includesavalanches, landslides, debris slides, cave and hole erosion andvarious kinds of mudflows and debris flows.

Avalanches: Avalanches are a phenomenon of the suddencollapsing, rolling and dropping of rock and earth when they areseparated by cracks. Avalanches usually occur in high mountain-ous areas with steep side slopes, especially in areas of severe rivererosion.

Landslides: Landslides are primarily caused by gravita-tional forces, the result of shear failures along the boundary of themoving mass of soil or rock. However, owing to progressivefailure, landslides can occur at an average shear stress consider-ably less than the peak strength of the soil or rock. Landslidesgenerally occur on slopes of 12 to 32°. Within this range, thelarger the slope gradient, the higher the possibility that gravita-tional force exceeds resistance to movement. Landslides usuallyoccur in strongly weathered rock, and have close relationshipswith faults or shattered zones. Abnormally high water tables alonga fault often cause landslides. A small-scale shattered zone aroundintrusive rock, which forms a good conduit of groundwater, canalso trigger them.

Debris slides: Debris slides are a phenomenon in whichcrushed materials, weathered from rocks and earth on steep slopesand cliffs, slide downward along the slope under the pull ofgravity. On steep slopes, soil and rocks are affected by cold, heat,dryness and humidity. The alternate action of freezing and thawingwill thus cause a decrease of cohesive force and loosening of soiland rock surfaces. Unstable crushed materials with parent rockswill be formed. These crushed materials will go downward underthe action of gravitation during the rainy season.

Cave and hole erosion: Sinkholes, loess caves andnatural loess bridge erosion are forms of erosion in the loessregions. Loess soil is typically loose, porous, homogeneous andeasy to cultivate, and causes erosion. Surface runoff permeates

CHAPTER 2 — SOIL EROSION 11

underground along vertical cracks of the loess soil, and thusresults in underground dissolving and washing, and the binding ofdissolved elements and small particles to deep layers. With conse-quent weakening of the ceiling, the stability of the overlyinglayers is impaired. The final stage creates corridors and caves orsinkholes.

Debris flows: Debris flows are mixed flows of rock, soil,water and air between sediment-laden water flows and landslides.The occurrence of debris flows is mainly connected to thegeomorphologic conditions of a certain gradient in mountain areaswhere, under certain water content conditions, large quantities ofunstable, loose rocks cause rock, soil, water and air materials tostart, collect, mingle and move. Debris flows, the product ofdegraded mountain environment, are one of the main hazards withthe most unexpected consequences among the numerous naturaldisasters in mountainous areas. They are closely related to acertain topography, geomorphology, geological structure and geot-ectonic movement, as well as hydrological and climaticconditions. Usually, areas of debris flow development have thecharacteristics of complicated geological structures, soft and looserock formations, developed joints and fissures, and activecollapses and landslides. The entrapment of heavy rains, glacialand snow melt-waters or rivers or lakes all can trigger the occur-rence of debris flows.

2.2.2 WindErosion caused by wind is a process of detachment, transport anddeposition of soil and sand particles due to air current. This typeof erosion occurs mainly in those areas where there is a lack ofprecipitation together with predominantly high temperatures, i.e.arid regions. Wind affects the soil by desiccating the surface layersand drying up and removing soil particles by deflation. Thestronger the wind, the greater its influence on soil.

2.2.3 Freeze-thawErosion caused by freeze-thaw is a process of mechanical abrasionof soil caused by temperature changes, which occurs predomi-nantly in cold regions where the average temperature is below 0°C.

2.2.4 Living organismsSoil erosion can be caused by living organisms, through phyloge-netic and zoological processes. Phylogenetic processes includesoil destruction by roots. Zoological processes occur whenanimals destroy the soil when searching for food, moving or exca-vating their hiding places on the surface and under the ground.

2.3 ACCELERATED EROSIONAccelerated erosion is defined as the increased rate of erosion overthe normal or geologic erosion, brought about by human activities,such as deforestation, indiscreet reclamation cultivation, overgraz-ing for food fibre and meat, and development of industries. Thisaccelerates normal soil erosion rates caused by water, wind,temperature and gravitational force, etc. The accelerated erosion isin excess of the natural erosion which has brought changes innatural cover and soil conditions. The accelerated erosion rate ishigher than the rate of soil formation, which causes a restructuringof the Earth’s surface by the wash of soil particles and nutrientswhich can no longer be resupplied by the soil formation process.The unfavourable consequences of industrialization and urbaniza-tion processes pose a threat not only to soil, but also to water.

2.4 FACTORS AFFECTING SOIL EROSIONErosion is initiated by natural forces and can be intensified byhuman activities. The erosion process is controlled by the actionand interaction of many factors. The factors affecting soil erosionmay be grouped into two categories: natural factors and humanactivities. The most prominent natural factors include meteorol-ogy, geology, topography, composition of earth surface andvegetation cover. Human activities play both positive and negativeroles in soil erosion, and are the major factors causing modernacceleration erosion. The positive ones include various measuresof soil erosion control and proper comprehensive watershedmanagement. The negative ones include poor or improper landuse, reclamation, construction and urbanization.

2.4.1 MeteorologyMeteorological factors affecting soil erosion are precipitation,wind and snowmelt.

Precipitation: Precipitation includes rainfall, snow, hailand many other types. Rainfall, especially rainstorms, is the mainfactor affecting soil erosion. Main elements of rainfall includeamount, intensity, duration, spectrum of raindrop and fallingvelocity. The most significant characteristic value of rainfall is thekinetic energy of raindrops impacting the soil surface.

Amount of rainfall: In general, soil erosion will increaseto a point with an increase in the amount of rainfall. However, thisis not the only factor. Rainfall intensity and the spectrum of rain-drops, etc. also determine the amount of soil erosion a stormcauses. A storm with an intensity of less than 10 mm/h, the erosivethreshold value, will not result in soil erosion.

Raindrops: Raindrop characteristics include form, size,velocity of falling drops and terminal velocity. In general, smalldrops are in the shape of a circle, and larger drops are oblate. Thediameter of drops ranges from 0.2 to 7 mm.

Mutchler and Young (1975) studied the process of soilsplash erosion by raindrops and found that when the water layeron the land surface was thinner than one fifth the diameter of araindrop, the raindrop had strong erodibility. However, it was alsodetermined that when the water layer exceeded three times thediameter of the raindrop, the erodibility was greatly weakened(Jansson, 1982). The relationship between rain intensity, kineticenergy and erosive force of rain is of most importance for rainerosion. Low intensity rain is mainly composed of small drops,while high intensity rain has at least some much larger drops. Theformulae to calculate the medium diameter of raindrops are asfollows (Zhu, 1992):

Laws and Parson’s equation:

d50 = 2.23I0.182 (2.1)

Hudson (1981):

d50 = 1.63 + 1.33I – 0.33I2 + 0.02I3 (2.2)

where d50 is the medium diameter of raindrop in mm, and I is therain intensity in mm/h.

The relationships between rain intensity and kineticenergy are:

Zhong’s equation:

E = 23.49I0.29 (2.3)

12 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Wischmeier and Smith’s equation:

E = 210 + 89 log I (2.4)

Kinnell’s equation:

E = 29.82 [1 – eρ (0.044π0.214)] (2.5)

Studies have shown that EI30 or EI15, the product ofkinetic energy of rainfall and the maximum rain intensity in 30 or15 minutes, is an appropriate parameter to estimate soil loss.

Snow and glacier: A solid form of precipitation signifi-cant for erosion is snowfall, because in the spring when snowthaws it may cause surface runoff and soil erosion. The runofffrom snowfall is dependent on the physical properties and depth ofthe snow distribution of the snow cover and on thaw processes.

A glacier is a mass of ice predominant in cold regionswhere the annual mean temperature is below 0°C. A specificfeature of glacial erosion is the action of a large mass of icemoving slowly. Furrowing, cutting, ploughing and scouring are themost pronounced forms of glacial erosion.

2.4.2 GeologyThe character of bedrock and tectonic movement has significanteffects on soil erosion.

Rocks susceptible to weathering often suffer strongerosion. Soils weathered on limestone and dolomite formationsare relatively resistant; those on igneous rocks are less so; andthose on various sediments such as sandstone, loam, clay, chalk,flysch formations and loess sediments are least resistant.

New tectonic movement is the most significant cause oferosion changes, affecting the degree of erosion as well as thespeed of gully development. Earthquakes are quick tectonic move-ments that loosen surface materials and produce landslides orcollapses, therefore greatly increasing soil erosion.

Volcanos are geological actions releasing large amountsof loose volcanic material. They not only change topography, butalso plug mountain areas and cause serious debris flows duringhigh-intensity rainstorms, increasing erosion rates greatly.Volcanic rock is easily eroded.

2.4.3 TopographyTopography is the basic factor constituting the natural environ-ment. Erosion is closely related to the types and characteristics oftopography. Topographical characteristics include the gradient,length and direction of slope, which affect erosion through theintensity of runoff formed on it.

(1) Slope gradient. The relationship between gradientand erosive intensity are shown in Table 2.1.

Soil erosion increases first with an increase of gradient,but when the gradient reaches a certain value, erosion no longerincreases with the increase of gradient. The turning value of thegradient is called the critical gradient.

Horton’s formula:

e = K (x0.6 – xc0.6) sin α / tan 0.3 α (2.6)

where e is the depth of eroded soil per unit of time, x the distancefrom the slope top, xc is the critical distance from the slope topwhere no erosion occurs, K is the coefficient depending on the

depth of runoff, erodibility of soil and roughness of groundsurface, and α is the slope angle. As the slope angle increases to56.8°, the value of (sin α / tan 0.3 α) reaches the maximum of0.737 (Jansson, 1982).

According to the analysis of observation data by therunoff plots at the Suide and Lishi soil conservation experimentalstations, in gully hilly loess areas the turning gradient is generally25 to 28° (Chen, et al., 1988).

(2) Slope length. There are different views on theimpacts of slope length on soil erosion. Rose suggested that soilerosion decreases with an increase in slope length because longslopes increase the sediment concentration of flow and thereforemore energy is consumed in sediment transport and less soil iseroded. For flat slopes, erosion is not closely related to slopelength; for steep slopes, erosion is in proportion to slope length.

Some formulae expressing the relationships between soilerosion and slope length follow (Zhu, 1992):

Zingg’s formula:

E = AL1.6 (2.7)

Kernev’s formula:

L < 50 m Rck = Ai0.75 M1.5 L1.5 (kg s–1) (2.8)L = 50–200 m Rck = Ai0.75 M1.5 L1.5 (kg s–1) (2.9)

where E is the mass of eroded soil (t), Rck is the erosion rate inkg s–1, L is the slope length in m, i or θ is the slope angle, and Mis the rain intensity in mm min–1.

(3) Slope shape and direction. Slope shapes can bedivided into straight, convex, concave and compound types. Thestraight slope has an approximately constant slope gradientthroughout; the maximum runoff at highest velocity is concen-trated on the lower part, and erosion intensity is higher on theupper part. The gradient of convex slopes increases along theslope length and the flow disperses down the slope. Convex slopeshave the highest intensity of soil erosion. Concave slopes flattenout toward the bottom of the slope and sediment carried in runoffwater settles as flow velocity decreases. Compound slopes havecombinations of different slopes.

CHAPTER 2 — SOIL EROSION 13

Table 2.1Relationship between gradient and soil erosion

Authors Formulae Coefficient a

Musgrave, 1947 1.35

Zingg, 1940 1.49

Hudson and Juckon, 1971 E ∝ Sa 2.0

Kilinc and Richardson, 1973 1.66

Smith and White, 1947 E ∝ b + cSa 1.33

Meyer and Monke, 1965 E ∝ (S – Sc)a 2.0–2.5

Wischimeir, et al., 1958 E ∝ (0.43 + 0.3S + 0.043S2)

Liu (from Chen, 1988) d = 0.012S1.4 + 0.56

h = 3.47 × 10–3 I2.16 + 0.57Chen, et al., 1988 h = 3.98 × 10–4 I2.44 + 0.2

h = 3.16 × 10–7 I5.35 + 10.5h = 3.02 × I3.18 + 0.55

NOTE: E, W – eroded soil amount (t km–2); h – scouring depth (mm); I, S – gradient

(degree).

Slope direction in the sunlight or in the shade influencesthe soil moisture and temperature. Slope exposure to solar radia-tion on southern and western slopes causes the rapid thaw of snowresulting from differences in day and night temperatures.Consequently, this results in higher surface runoff from the snowthaw, increasing the intensity of soil erosion.

2.4.4 Soil characteristicsErosion is affected by soil characteristics such as infiltration,detachability by raindrops and runoff and susceptibility to rill,gully and channel erosion.

(1) Texture. Erosion due to raindrops is affected by soiltexture. Ekern obtained the relationship between soil particle sizeand relative amount of splashed soil material, as shown inTable 2.2. Jansson demonstrated that filtration is a function of soiltexture, as shown in Table 2.3.

(2) Structure. The contents of clay, organic matter,calcium, magnesium, and free iron oxide contribute to soil aggre-gation. Aggregation increases the number of large pores and thusincreases the soil infiltration rate and reduces runoff. But disinte-gration of soil structure may be caused by both mechanical andnatural agents. Dryness, wetness and freezing-thawing are impor-tant in soil disintegration processes.

Soil profile: In areas where the soil consists of layers ofdifferent textures, the water erosion resistance of the areas isaffected by stratification of the layers. Where a permeable layerrests on an impermeable layer, it may become oversaturated withwater which the lower layer is unable to absorb. This leads to anintensive wash of the permeable layer.

Content of soil moisture: Water always moves undertension in well-drained soil, and an increase of soil moisturemakes soil less porous. This is the reason that soil moisture affectssurface runoff.

Soil porosity: Water moves more readily through poroussoil than dense soil. A dense layer near the surface slows watermovement because of low porosity. The moisture content of theoverlying layer soon begins to increase.

Aggregation and surface sealing: Aggregate formation isdependent on organic matter and the types of bases in the soil.Black soil with 40 per cent aggregates has two to four times the

infiltration rate of loose soil with less than 5 per cent aggregates,according to an investigation conducted in the Loess Plateauregion.

Topsoil depth: Topsoil depth affects soil erodibility. Itsprimary effect is on infiltration. Topsoil allows water infiltration toproceed unrestricted for a time until layers of different porosityare reached. Its second effect is on the organic matter content ofthe surface. If the topsoil is thin and subsoil is ploughed up andmixed with it, the organic matter content is lowered. This resultsin lower aggregate stability and higher erosion. The third effect ison the general fertility of the soil. The deeper the topsoil, thegreater the nitrogen release and, as a consequence, the greater thevegetative cover produced. Erosion losses are less than those froman area of shallow topsoil.

Water-holding capacity: Soil texture largely determineswater-holding capacity. Various textured soils erode differentlybecause of differences in infiltration, percolation and detachabil-ity. Clay, compared with sand, can hold a great deal more water,and a high percentage of available pore space can be filled.Water-holding capacity affects soil erosion through its influenceon detachability of soil by runoff during heavy rains. Sand iseasily detached and washes away readily under a high velocityof runoff, and clays may seal over and be virtually impossible todetach.

(3) Soil erodibility. Bouyoucos suggested that soilerodibility equals (per cent of sand + per cent of silt) / (per cent ofclay). Wischmeier, et al., defined it as (per cent of silt + per centof very fine sand) × (100 – per cent of clay). There are manyerodibility indices in the literatures. Some are expressed in termsof soil texture, some in soil structure, and some in water transmis-sion and aggregation stability or dispersion (Jansson, 1982).

Dispersion rate: This is the weight ratio of sand and clayparticles dispersed during the experiment time period to the totalsand and clay.

Rate of surface aggregation: This is the ratio of surfaceareas of the sediment particles larger than 0.5 mm (cm2 g–1) to thetotal surface areas of aggregate silt and clay particles.

Factor K in the Universal Soil Loss Equation (USLE):(Wischmeier, et al., 1971): The nomogram has been drawn usingfive parameters, i.e. percentage of silt (0.002 to 0.05 mm) and finesand (0.05 to 0.10 mm) in the total, percentage of sand (0.1 to2.0 mm) in the total, content of organic matter, structure and infil-tration. By artificial rainfall, Dumas determined the K value inUSLE as:

1g 1 000 K = 3.4623 – 0.0282 X1 – 0.1695 X2 – 0.0212 X3 (2.10)

where X1 and X2 are the percentages of gravel and organic matter,respectively, and X3 is the equivalent weight of soil moistureretention.

Melton’s formula:

(2.11)

where E is the anti-erosion rate of soil, dz is the soil dispersionrate, Me is the equivalent weight of soil moisture retention,and e is the content of soil colloid. E < 10 means high anti-erosion properties, and 12 < E < 115 means low anti-erosioproperties.

14 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 2.2Relationship between particle size and relative amount of

splashed material

Grain size (mm) Relative amount of splashedmaterial during 5 min (%)

0.84–0.59 30.0

0.42–0.25 77.2

0.25–0.175 100.0

0.10–0.05 61.0

0.05–0.002 21.0

Table 2.3Variation of soil infiltration

Soil texture Infiltration (mm/h)

Clay loam 2.5–5.1

Silt loam 7.6–12.5

Loam 12.7–25.4

Loamy sand 25.4–50.8

Ed M

e

z e=

2.4.5 Vegetation coverVegetation cover protects the soil surface from the direct impactof raindrops and from the effects of wind. It enhances theinfiltration of rainfall into the soil and slows surface runoff,thereby improving the physical, chemical and biologicalproperties of the soil.

2.4.6 Human activitiesSoil erosion is the result of exogenic forces exceeding soilerodibility thresholds. Natural factors are potential effects whilehuman activities are main factors that positively or negatively affecterosion intensity.

(1) Destruction of vegetation cover. Populationincrease has brought about and continues to bring aboutextensive changes in land use. Operations that reduce vegetationcover may induce accelerated erosion. These include cuttingtrees and forest fires, etc.

(2) Cultivation. Cultivation on steep slopes may destroyvegetation and loosen the soil, thus causing serious soil erosion.Different crops provide different degrees of vegetation cover. Asan example, the relative erosion, C, on crop plots and bare soil inWest Africa is compared, as shown in Table 2.4. Cultivationapproaches are of great significance in erosion. Contour plough-ing, strip cropping and terracing reduce erosion significantly(Jansson, 1982).

(3) Overgrazing and burning. Overgrazing and burningare land use practices that leave the soil unprotected. In semi-aridand arid marginal lands, where recovery of vegetation is slow,overgrazing causes low vegetation coverage and major erosion.Burning of grass, bushes and trees is a practice in remote moun-tainous areas, where people live simple lives. Burning beforeintensive rain also increases erosion tremendously.

(4) Mining, road and dam construction, urbanization.Spoil banks resulting from strip mining, particularly in coal mines,are often steep-sided and devoid of vegetation. Construction andexploitation activities may produce a great amount of waste soil,rocks and coal, which may be washed into rivers, accelerating soilerosion. Dam construction may cause sedimentation in upstreamreaches and scouring in downstream reaches. Water withdrawalfrom wells may lower the groundwater table and thus increasegully erosion. Urban expansion involves the construction of roads,pipes, buildings and ground paving. During landscaping and the

construction of urban areas, sediment yield reaches a high peak,then declines as the land ‘heals’, and finally reaches a low, stablevalue.

(5) Land use and tillage. Land use and tillage are typicalanthropogenic factors which affect erosion intensity. The intensityof soil erosion in agricultural soil is significantly affected by theposition and shape of the plot. Observations have shown thaterosion intensity in contour farming is considerably less than thatin plots tilled downslope in straight lines.

2.5 DEGREE AND INTENSITY OF SOIL EROSION2.5.1 Soil loss toleranceAn evaluation of the seriousness of soil erosion needs to take intoaccount how much soil a given specific site is losing currently andthe maximum soil loss tolerable by natural resources. Soil losstolerance is defined as ‘the maximum rate of annual soil erosionthat may occur and still permit a high level of crop productivity tobe obtained economically and indefinitely’ (Schertz, 1983). Somescientists have suggested that soil loss tolerance is in the range oftwo to six tons per acre for various types of soil.

The soil formation rate is an important factor in deter-mining soil loss tolerance. Under natural conditions, the formationof one inch of soil takes 100 to 300 years, while it takes about 100years under farming conditions. An estimate puts the renewal rateat 0.5 tons per acre per year for unconsolidated parent material,and much less for consolidated material. The formation of theweathering surface layer on a base rock of granite requires 10 000to 100 000 thousand years, while a base rock of non-granite needsmuch more time (Margan, 1980).

2.5.2 Soil erosion intensitySoil erosion intensity means that under the action of natural agentsand human activities, the soil eroded due to denudation anddisplacement per unit area and unit time is expressed by the soilerosion modulus. According to the Chinese Standard, erosionintensity is classified as shown in Table 2.5 (Guo, 1998).

2.6 SEDIMENT YIELD IN A BASINWater erosion is the most important type of erosion because runoffis essential to transport the eroded sediment. In the entire processof erosion and transport, soil erosion, soil loss and sediment yieldin a basin are three different but closely related concepts.

Sediment yield is defined as the total sediment outflowfrom a watershed or drainage basin, measurable at a cross-sectionof reference in a specified period of time (Piest and Miller, 1975).In the comprehensive planning of a medium or small watershed, ifthe gross erosion and sediment delivery ratio are known, the sedi-ment yield can be predicted.

CHAPTER 2 — SOIL EROSION 15

Table 2.4Soil erosion on crop plots and bare soil

Type Relative erosion C (%)

Bare soil 100

Dense forest or thick straw mulch 0.1

Savannah and grassland, no grazed crops 1

Late planted with slow development: 1st year 30–80

2nd year 10

Crops with rapid development 10

Maize, sorghum, millet 30–90

Intensive rice (second cycle) 10–20

Cotton, tobacco (second cycle) 50

Ground nuts 40–80

Cassava (first year) 20–80

Palms, coffee, cocoa with crops 10–30

NOTE: C – factor in USLE.

Table 2.5Classification standards of soil erosion intensity

Degree Mean annual erosion Mean lostmodulus (t km–2.a) thickness (mm/a)

Slight < 200, 500, 1 000 < 0.15, 0.37, 0.74

Light 200, 500, 1 000–2 500 0.15, 0.37, 0.74–1.9

Moderate 2 500–5 000 1.9–3.7

Intensive 5 000–8 000 3.7–5.9

Utterly intensive 8 000–15 000 5.9–11.1

Severe >15 000 >11.1

In estimating the gross erosion in a basin, if sheeterosion plays the main role, the Musgrave Equation or USLE canbe used. But if gully erosion is the main type, it should be the sumof slope, gully and river channel erosion. The sediment deliveryratio on the first sub-area of the hilly loess gully of China’sMiddle Yellow River can be close to 1.0, while in the YangtzeRiver Basin it is only about 0.25. The value for 12 small water-sheds with areas of 0.98 to 129 km2 in the Pigeon Roost Creekregion of the United States is in the range of 0.28 to 0.76.

Factors affecting sediment delivery ratio include landuse, vegetation cover, runoff discharge, sediment size, density ofchannel network, relief and catchment area, etc.

2.7 MONITORING OF SOIL EROSION ANDSEDIMENT YIELD IN A BASIN

The measurement should be conducted on three levels, i.e. devel-oping a network of measurement stations in a basin, establishingan observation base in representative and experimental water-sheds, and establishing groups of runoff plots.

2.7.1 Runoff plots and experiments in the laboratoryA runoff plot is an isolated slope used to study the mechanics ofrunoff and sediment yields and the effects of soil conservationmeasures. Experiments in the laboratory can be conducted by arti-ficial rainfall to simulate natural phenomena, including debrisflows and landslides (Meng, 1996).

(1) Plots under natural conditions.Plot size: The smallest plot is only about 1 to 2 m2, and

easy to manufacture and install, especially for the preliminaryexperiments which require large numbers of plots for studying therelative erodibility of various types of soil. In measuring runoff, alonger plot is essential. In the United States, a runoff plot of2 × 22 m is normally used for the study of cultivation and croprotation.

Flume: A flume is installed at the outlet of the plot tomeasure the runoff discharge. One type of measuring flume is anH-shaped flume designed and produced by the United States. Itsrange is 0.0028 to 3.08 m3 s–1. The Parshall flume is anotherpopular tool for measuring the flow discharge on a plot (Hudson,1981).

Tank and divisor: On a small plot, the runoff is led firstinto a collecting tank. But on a large one, a divisor is used todivide the runoff accurately, so as to reduce the tank size. TheGeib divisor with a number of similar rectangular slots is widelyused in the United States. Only the flow passing through themiddle slot is collected and measured. The total flow dischargecan then be calculated by the calibrated ratio. A highly successfulmoving sampler is the Coshocton revolving wheel sampler. This isinstalled under the discharge from a flume, such as an ‘H’ flume,with the water force turning a wheel mounted on a vertical axis. Anarrow slot in the wheel passes the flow on each revolution, and asample is taken.

In China, more than 500 runoff plots of various sizeshave been established since the 1950s. Mini-plots are generallyone to several square metres in size. They are used to study thebasic rules of runoff and soil erosion, such as splash erosion,stability of soil aggregate, the processes of topsoil becomingcrust, soil erosion durability and so on. Common plots are gener-ally 5 × 20 m. They can be used to study the whole progress ofrill and inter-rill erosion. Normal cultivation and relevant

measurements can be carried out on them. The natural large sizeplot is a small natural catchment with an area of several hectaresand includes rills, shallow gullies and even cutting gullies withfarmland, wasteland and forest. They can be applied to study thetransportation of runoff and sediment and the equilibrium ofsedimentation.

(2) Rainfall simulator. The rainfall simulator has twoimportant advantages. It is not restricted by the existence ofnatural rainfall and it can repeat heavy rainstorms to obtaindesired results. In 1950, the application of a rainfall simulatorfound that erodibility is linked to the kinetic energy of raindrops.There are different rainfall simulators such as the non-pressurizeddropper and spray simulator. China has developed an experimentalrainfall simulation device system in the laboratory, and a largeslope surface simulator device in the field, as well as a portablesmall rainfall simulator.

2.7.2 Measurements of soil and water losses on pilotwatersheds

The measurement of soil and water losses on representative orpilot watersheds started in the 1930s, and there were nearly 1 000such watersheds in the world by 1974.

Representative watershed: This should be a naturalwatershed with an area of 10 to 250 km2 in general. Its purpose isto study the mechanics of runoff and sediment yields, to explainthe essences of physical processes of various factors and todevelop a mathematical model of sediment yield in a basin.

Experimental watersheds: These are usually coupledwatersheds used for comparison tests. Two watersheds shouldhave similar topography, relief, soil and vegetation and the areashould not exceed 4 km2 in general. A calibration period isrequired before any soil conservation works are done.Subsequently, soil conservation measures are conducted on onewatershed while the other is kept under natural conditions (Zhu,1992).

Runoff plots are not able to reflect the runoff and soilloss of the whole watershed. It is necessary to establish somemonitoring stations and conduct measurements simultaneously.These measurements include soil moisture, groundwater table,scour and deposit in river channels, sedimentation in small reser-voirs, pools and check dams, as well as the discharge andsediment concentration at the outlet station of the watershed.

The relevant samplers used for the measurements inwatersheds are as follows.

Pumpable automatic sampler: This can take samplesintermittently and put them in order into bottles. This sampler isquite useful for monitoring sediment delivery in small rivers. It iscontrolled by a transducer of water level (Walling, 1981).

C-type wheel sampler: This was developed in 1947 byPomerence and applied to the watersheds near Coshocton, Ohio,United States. Parsons later calibrated and improved it. Thissampler has been used in combination with the ‘H’ flume to takean equal volume sample intermittently.

2.7.3 Measurement method with Cs-137The spatial distribution of soil erosion is essential for the study oflong-term soil erosion. The Cs-137 method is useful for this. Theradioactive micro-particles are the result of a nuclear test in 1954.Cs-137 has a long half-life so there are many Cs-137 samples stillpreserved in the topsoil layer. Cs-137 can be absorbed intensively

16 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

by topsoil and transported, with some soil loss. If the accumulatedCs-137 in a region is measured, then the amount of soil loss ordeposition can be estimated.

The Cs-137 method has been extensively used tomeasure the age of depositions by taking rock-core samples inlakes. It is also a perfect tool to determine the location, size andduration of a deposition belt in a river system. For example, inMaluna Creek, New South Wales, Australia, the deposition rate ofthe alluvial fan was 4 cm per year as determined by the Cs-137method (Walling, 1981).

2.7.4 Dynamic monitoring by remote sensing and GIS(1) Dynamic monitoring by aerial photography. Aerial

photography is a proper means to illustrate and evaluate the topog-raphy, soil, climate and influence of best management practices(BMPs) on erosion. Aerial photography is easy to obtain. Using a35 mm camera with a 135 mm lens at about 300 m above theslope, photographs can be enlarged to 1:2000-scale negatives. Thisscale can provide a clear picture of rill erosion. Erosion rates canbe measured accurately using a sequence of time-lapse, low-alti-tude aerial photographs and photogrammetric procedures (Frazier,et al., 1983).

(2) Investigation of soil erosion by space remotesensing. Satellite remote sensing imaging can provide informationon various factors affecting soil erosion on the ground surface,such as the relief, topography, constitutive material on the ground,vegetation cover and land use. Based on the image vein of thestructure, tone, picture, geometry and topography obtained by thesatellite image, the relationship between the main factors can beanalysed and calibrated in the sample plot. Then the pattern, inten-sity and level of soil erosion can be obtained (Yellow RiverConservancy Commission (YRCC), 1991).

(3) A geographic information system (GIS). By using aGIS, planners can establish the correlation of land cover andtopography with runoff, drainage area and terrain configurationsobtained in various environmental conditions. This approachenables water quality data from various sources to be integratedinto a comprehensive system capable of combining and referenc-ing such diverse data elements as conventional map information,Landsat imagery and tabular data obtained on the ground.

Technologies of the 1980s, including remote sensing andGIS, are attractive because of their capabilities for analysing dataof large and small areas, integrating numerous variables into theevaluation processes, and easily updating databases (Walsh, 1985).

2.8 PREDICTION OF SOIL EROSION ANDSEDIMENT YIELD

2.8.1 Prediction of soil erosionScientific planning and land treatment for soil and water conserva-tion require relationships between erosion-causing factors andthose that help to reduce soil loss. USLE, usually with some modi-fications, is the frequent basis for determining the quantity of soilthat detaches from each small area of a watershed (Foster andWischmeier, 1973).

2.8.2 Prediction of sediment yieldSedimentation is the consequence of a complex natural processinvolving soil detachment, entrainment, transport and deposition.Sediment yield is the amount of sediment transported from adrainage basin. It is a portion of gross erosion (the sum of all

erosion in the watershed). Sediment sources include upland sheet-rill erosion, gullies, river banks, channels, construction sites, spoilbanks and roadsides. Sediment yield from upland sheet-rillerosion sources is usually greater than that from other sources(American Society of Agricultural Engineers (ASAE), 1977).

Sediment yield prediction is needed for many specificpurposes. Simulations are used to extend short-term samplingprogrammes to compose adequate databases. This is frequentlydone to predict sediment storage requirements for the design offlood control structures. Models are used to predict the future water-shed response to various land-use alternatives. This is an integralpart of evaluation of the effectiveness of alternative plans in a basin.Another concern is related to research, because modelling is anordered sequence of steps in time and space, presenting a complexprocess, and information gaps can be identified (ASAE, 1977).

The specific needs for sediment yield prediction are sovaried that no single model could meet them without a great lossof efficiency. The needs generally fall into the categories of lengthof model event time, area to be simulated, and sediment sources.

Event time: In selecting or designing a model, the lengthof event time should be determined. In situations where animalsand plants are affected by high concentrations of sediment andchemicals in public waters, the storm model of sediment concen-tration is required. A single storm simulation is required wheninformation on sediment concentration throughout a storm isneeded. Longer simulation periods may be more useful in consid-ering other problems. For example, estimating quarterly ormonthly sediment yield is desirable for determining seasonal vari-ations of sediment yield. These determinations are required for theselection of land use and management techniques to control sedi-ment yield and runoff. The estimation of average annual sedimentyield is sufficient for the design of reservoirs and conservationstructures and for other concerns with sediment deposition over along period. Prediction of long-term sediment yield trends isrequired for the planning and maintenance of channels. Channelstability depends greatly on sediment yield from upland water-sheds. These types of problems require long-term estimation.

Watershed size: Large watersheds usually need lessmodelling details than small ones. Therefore, in developingmodels, different sediment yield predictors are needed for differ-ent watershed sizes. The contribution of groundwater to runoff isusually higher for a larger watershed than a small one. Sedimentsources are more variable in a large watershed (ASAE, 1977).

Sediment sources: Gullies in a watershed contributequite a large amount of sediment per unit area to gross erosion.Some gullies are sand sources that contribute to the bed materialin channels. This requires bed load transport to be included in themodel.

Urban areas in watersheds present special problemsbecause of their high runoff rates and pollution potentials.Conservation structures create problems in predicting sedimentyield from their drainage areas. The contributions of roadsides andditches often may be ignored. Channels, especially in large water-sheds, may contribute significantly to sediment yield. Sand istransported differently than fine sediment (silt and clay) andpresents special computational needs.

Sand in sediment yield often merits special attentionbecause its deposition causes the most damage. Gully and channelsediment sources are especially important for downstream damageif they contain a large percentage of sand.

CHAPTER 2 — SOIL EROSION 17

Usually, the texture of surface soil is different from thatof sub-soils. This is particularly significant when chemical trans-port is considered. Large amounts of chemicals attach to clay,while small amounts of chemicals attach to sand.

Methods of sediment yield prediction: At present, manysediment yield prediction methods are available and have beenused for various purposes. In general, such methods can begrouped into two categories: those derived from statistical analysis(statistical equations); and deterministic models which includeempirical parametric approaches, using time variant interactionsof physical processes.

(1) Statistical equations. These are usually the equationsrelating sediment yield to one or more factors on watersheds orclimate. The methods are commonly used for measuring averagesediment yields over long time periods. Long-term sediment yieldcan be adequately estimated for a particular watershed, but resultscannot be extrapolated to other watersheds.

(2) Deterministic models. These models introducenumeric values (parameters) to quantify the factors affectingerosion, transport and deposition. These parameters can be derivedempirically or calibrated using fitting techniques. One example ofthe parametric model is the Wischmeier-Smith soil loss equation.Many sediment yield models use the equation as a basis becauseof the widespread availability of the parameters for conditions inthe United States. A modified version of the Wischmeier-Smithequation incorporates the soil detachment-soil transport concept ofMayer and Wischmeier.

Some models use time variant interactions of physicalprocesses. These models are developed using theoretical dynamicequations. They have a structure of hydrologic or hydraulicprocesses, depending on the model objectives. These processes aredefined and linked mathematically using sound theoreticalapproaches. In general, the basic equations are for conservation ofmass, momentum and energy. Certain flow and sediment proper-ties must be evaluated. Among them are time distributions of flowrate, sediment concentration, flow depth, and rates of rill andinter-rill erosion (ASAE, 1977).

A theory of variable and non-point-source areas hasbeen developed. This theory can be used to explain facts suchas the hydrological response on runoff formation. This view-point has provided the basis to establish the idea of partialarea, that is that the runoff yield area is much smaller than thetotal area.

Similarly, for most rivers, most sediment dischargecomes from only a relatively small area in the watersheds. In theNile River basin, the area suffering soil erosion is only 10 to15 per cent of the total area of 2.9 million km2. In the AlbertaRiver basin, more than 90 per cent of sediment yield comes froman area of less than 10 per cent of the total of 430 000 km2. In theYellow River basin of China, the total area of which is750 000 km2, 90 per cent of sediment discharge comes from40 per cent of the total, while 75 per cent of coarse sediment(particle size greater than 0.05 mm) comes from an area of lessthan 15 per cent of the total.

In 1987, a new model Revised USLE (RUSLE) wasdeveloped by the United States Department of Agriculture(USDA). It has several distinguishing features: (1) Data areprocessed by computer; (2) A new erosive factor R, rainfall-runoff, is introduced and its seasonal distribution is related to thecrop rotation system; (3) The factor of soil erodibility varying with

seasons, K, is introduced; (4) The factors of vegetation cover, C, iscalculated using subfactors of land use (PLU), canopy density(CC), ground cover (SC) and ground surface roughness (SR); (5)A factor of gradient and length of slope, LS, representing the rateof rill erosion to inter-rill erosion and different shape of slope, isintroduced; (6) The P value of soil erosion presents the rotationsystem of grass and crop land, contour ploughing and the drainageof the soil subsurface layer.

Recently, with the development of the new model ofwater erosion prediction project (WEPP) for the purpose of replac-ing USLE, USDA has modified the erosion predicting modelsbased on erosive processes. There are three types of WEPPmodels, i.e. cross-section model, watershed model and net andgrid model (Chen and Fei, 1996).

In the 1980s and 1990s, China developed several para-metric or conceptual models of sediment yield for sediment-ladenrivers and high sediment-yield regions, especially for the Yellowand Yangtze Rivers.

2.8.3 USLE and RUSLE(1) Historical review. Zingg’s equation:

X = CS1.4L1.6 or A = CS1.4L0.6 (2.12)

where X is the total soil loss, A is the average soil loss per unitarea, C is the constant, S is the degree of slope, and L is the slopelength.

In the early 1950s, Van Doren and Bartelli proposed theerosion equation A = f (T, S, L, P, K, I, E, R, M), where A is theannual estimate of soil erosion, T is the measured soil loss, S isthe slope, L is the length of slope, P is the practice effectiveness,K is the soil erodibility, I is the intensity and frequency of 30-minutes rainfall, E is the previous erosion, and M is themanagement level (Mayer, 1984).

By 1956, precipitation, soil loss, and related data ofmore than 7 000 plot-years and 500 watershed-years had beenassembled at the National Runoff and Soil Loss Data Center in theUnited States. Between 1956 and 1970, additional data of severalthousand plot-years and watershed-years were added to thedatabank. The resulting USLE was introduced at a series ofregional soil loss prediction workshops from 1959 to 1962. Acomplete presentation of USLE is in USDA AgriculturalHandbook 282, which was revised in 1978 (Mayer, 1984).

(2) USLE and RUSLE. USLE is a comprehensivetechnique to estimate cropland erosion. It considers six majorfactors affecting upland soil erosion, i.e. rainfall erosion, soilerodibility, slope and slope length, cropping, management tech-niques, and measures of soil conservation. Wischmeier clarifiedthe term as follows: The name ‘universal soil-loss equation’ orig-inated as a means of distinguishing this prediction model fromthe highly regionalized models. However, its application islimited to states and countries where information is available forlocal evaluation of the equation’s individual factors. The uses ofUSLE are tremendous. It has become a major tool for estimatingsoil erosion in the United States and many other countries. As istrue for any tool, however, its use is limited to certain purposes,and it can always be improved. The result of one such improve-ment is RUSLE. In RUSLE, the major factors have beenextended, and it is also used to measure the conditions of forestland and roads, etc.

18 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Wischmeier-Smith’s equation:

A = RKLSCP (2.13)

where A is the amount of soil loss per unit area in a specific fieldin t/a, R is the factor of rainfall erodibility, K is the factor of soilerodibility defined as the amount of soil loss under the singlenumber of index EI30, E is the rainfall kinetic energy, I is the 30-minute maximum rainfall intensity, and LS is the factor of slopedegree and slope length:

S > 9% (2.14)

S ≤ 9% (2.15)

where λ is the real slope length in ft, the slope length of a standardplot being 72.6 feet, S is the real slope degree in per cent, the stan-dard value being 9 per cent, C is the factor of vegetation cover andmanagement, and P is the factor of soil conservation measures.

Onstad and Foster (1975): A slope is divided into severalslope segments and the soil detachment on each segment is:

(lb ft–1) (2.16)

(kg m–1) (2.17)

where Ej is the ability of soil detachment on slope segment j, andXj is the slope length of slope segment j.

Wj = 0.5RST + 15Qjq pj1/3 (English system) (2.18)

Wj = aRST + 0.22 (1 – a) Qjq pj1/3 (metric system) (2.19)

where Wj is the energy factor representing the combination ofrainfall energy and runoff energy, a is the coefficient (0–1), Rst isthe factor of rainstorm (EI unit of USLE), Qj is the volume of rain-storm at segment j (in or m3), and qpj is the peak value of the rateof rainstorm runoff on slope segment j (in/h or m3/h).

The accumulated soil detachments on the whole slopeare the sum of all slope segments.

(lb ft–1) (2.20)

(kg m–1) (2.21)

where Txj is the soil transportation per unit width on Xj.If Txj > Ej, soil erosion will occur on the segment; if Txj

< Ej, soil deposition will occur on the segment.The empirical modification of USLE was done by

Mutchler and Murphree. Factor Re was derived by McGregor-Mutchler as:

Re = 0.273 + 0.217 exp (–0.048i) – 0.413 exp (–0.072i) (2.22)

For factor Kc, recent studies show that the soil erodibil-ity in an entire year should be a variable. According to the datafrom Holly Springs, Mississippi, United States, it is:

Kc = 1 + 0.69 cos [(t – 2.2) 2π / 12] (2.23)

where Kc is double the increasing rate of average K values duringthe different periods.

For factor L = (λ/22.13)m, Mutchler and Greer haveobtained m = 0.15 for a slope of 0.2 per cent based on simulatedrainfall data. For steep slopes, Wischmeier derived:

m = 1.2 (sin θ)1/3 (2.24)

Factor S, S = 65.41 sin2 θ + 4.56 sin θ + 0.065 (2.25)

For factor C, Mutchler has recommended a set ofsubfactors, i.e. C1 residual products of land use; C2 the combina-tion of residual stubble; C3 the ploughing intensity; C4 the largeroughness; C5 the influence of vegetation cover.

The recommended RUSLE is:

A = RRcKKcLSC1C2C3C4C5P (2.26)

2.8.4 Empirical regression statistical model

(1) Slope sediment yield modelsThe YRCC:

(2.27)

where Ms is the modulus of slope erosion (t km–2), C is thepercentage of vegetation cover in per cent, P is the rainfall in arainstorm in mm, i is the average rain intensity in a rainstorm inmm min–1, J is the gradient (5), and Pa is the percentage of soilmoisture content before the rainfall in per cent.

North-West Institute of Soil and Water Conservation,China Academy:

Ms = 3.27 × 10–5 (EI30)1.57 J1.06 (2.28)

where E is the kinetic energy of rainfall in kg.m m–2, I30 is themaximum 30-minute rain intensity in mm min–1, and J is thegradient.

Kolnev (Russian Federation):

R = ainCTI0.75L1.50i1.50 (2.29)

where R is the soil loss on slope surface per 1 m width, I is thegradient, L is the length of slope, i the rain intensity, nCT is therunoff coefficient, and ai is the coefficient.

Caroni (Italy):

W = aImbDcHd (2.30)

where W is the soil erosion at the outlet of the plot, Im is themaximum rain intensity, D is the duration of rain intensity> 10 mm/h (Dc) or total rain duration (Dt), H is the total runoff,and a, b, c, and d are coefficients.

CHAPTER 2 — SOIL EROSION 19

EW KCPS j

x xj

j

j j= − −

( )

.( )

. .

185 58

1 51

1 5

EW KCPS

x xjj j

j j= −

( )( )

.–

.

31

1 51

1 5

TW KSCP

xxjj

j=( )

.

.

185 58

1 5

TW KSCP

xxjj

j=( ) .

31

1 5

LSS

= (.

) ( ). .λ

72 6 9

0 3 1 3

LSS S

=+ +

(.

). . .

.

.λ

72 6

0 43 0 30 0 043

6 613

0 32

MC

P i J PS a=51 1

0 151 2 1 5 0 26 0 48.

.. . . .

20 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

(2) Prediction models on gully erosionBeer-Johenson’s equation (data source: loess area of

serious gully erosion in the western part of Idaho, United States):

x1 = 0.01x40.982 x6

–0.044 x80.7954 x14

–0.2473 e–0.036x3 (2.31)

where X1 is the increase of gully surface area during a given timeduration, X3 is the difference between the annual and normalprecipitation, X4 is the index of runoff in inches, X6 is the terracearea basin in acres, X8 is the gully length at the beginning in ft, andX14 is the length between the gully end and the basin divide in ft.

Tompson’s equation:

R = 0.15A0.49 S0.14 P0.74 E1.00 (2.32)

where R is the annual rate of gully head forward (ft), A is the basinarea in acres, S is the gradient of drainage channel in per cent, P isthe annual accumulated rainfall of intensity larger than 0.5 in/24-hr in inches, and E is the percentage of clay content in weight inthe profile of erosive soil.

Estimation of length of erosive gully in the Europeanpart of the former USSR:

(2.33)

where H is the depth of the erosive benchmark in m, Qo is thecross-sectional discharge of the gully in m3 s–1, Vp is the flowvelocity of erosive rock-soil in m s–1, n is the roughness, 0.05, andA is the coefficient between 5 and 10.

(3) Sediment yield model in small watershedYRCC Formula (data source: the loess hilly gully region

in northern Shaanxi Province, China): There are 14 small gullywatersheds with an area range of 0.1 to 187 km2, a gully lengthrange of 0.5 to 24.1 km and a gradient range of 0.017 to 0.27.

(2.34)

where Ws is the total sediment yield from a single storm flood in t,WT is the total amount of flood in a single storm in m3, and L isthe length of major gully channel in km.

North-west Institute of Soil and Water Conservation,China Academy (data source: the small watersheds in loess hillygully regions):

Ms = 0.37M1.15 JKP (2.35)

where Ms is the modulus of sediment yield in a single storm int km–2, M is the modulus of flood volume in m3 km–2, J is themean gradient of the watershed, K is the factor of soil erodibilitypresenting the ratio of the amount of sand and clay to the total,and P is the vegetation coefficient related to the canopy density inthe watershed.

Wang (1997) (data source: the small watersheds of theNanchuan River, western Shanxi Province, China):

LnY = –2.650 + 0.962S + 0.00218L2 + 8.414L0.5

– 4.162L2/3 + 3.252B0.5 – 1.459B2/3 – 2.227T (2.36)+ 2.456T2 – 1.392F2

where Y is the mean annual soil erosion in the watershed in t, L isthe length of the watershed (100 m), S is the soil sort (for loessS = 1, for brown soil in forestry land S = 0), B is the mean widthof watershed in 100 m, T is the ratio terrace area to the total, and Fis the ratio of forestry area to the total (Zhu, 1992).

Anderson’s equation:

log SS = –3.721 + 0.116A + 1.637FQp + 1.244MAq

+ 0.401S + 0.0486Sc + 0.482S/A + 0.028Bc (2.37)– 0.0036Oc + 0.942R + 0.00086Rc

where SS is the mean annual sediment flux in the watershed, A thewatershed area, MAq is the amount of mean annual runoff, FQp isthe peak degree of discharge, S is the gradient of the river, Sc is thecontent of silt and clay, S/A is the rate of surface aggregate, Bc isthe cultivated area where there is a lack of crop cover effect, Oc isthe other cultivated area, R are the roads, and Rc is the recentlycut-off forest area.

Flaxman (data source: 27 watersheds with areas rangingfrom 12 to 54 miles2 in the western United States):

log (Y + 100) = 524.2 – 270.71g (x1 + 100)+ 6.41g (x2 +100) –1.71g (x3 +100) + 4.01g (x4 +100) (2.38)

+ 1g (x5 + 100)

where Y is the modulus of mean annual sediment yield in t mile–2,x1 the ratio of mean annual precipitation in in to the mean annualtemperature in °F, x2 is the mean watershed gradient in per cent, x3is the portion of soil particles larger than 1.0 mm in per cent, x4 isthe index of soil aggregate in per cent, and x5 is the flood dischargestored by soil (1 csm = 0.011 m3 s–1 km–2) (ASAE, 1977).

Bali and Karale (India):

(2.39)

where SI is the index of sediment yield, Ei is the weighted value oferosion intensity unit, Aie is the area of watershed erosive intensityunit in hm2, D is the sediment delivery ratio in per cent, and AW isthe total watershed area in hm2 (Zhu, 1992).

(4) Sediment yield models in large and middle catchmentsBivariate regression on sediment yield. Concerning

precipitation as an independent variable, Langbein and Schummderived two equations by regression theory.

(2.40)

and

(2.41)

where S is the sediment yield in t km–2.a, and P is the effectiveprecipitation in mm (Jansson, 1982).

Fournier found the following relationships:

(2.42)

LHQ

V n Ap

= 0 28 00 667

2 67 2 0 67.

.

. .

WW

Ls

T= 1 16

1 38

0 92.

.

.

SIE A D

AW

i ie= ×∑ 107

SP

P=

⋅

+ ⋅

−

−20 57 10

1 1 47 10

4 2 3

8 3 33

.

.

.

.

SP

P=

⋅

+ ⋅

−

−4 14 10

1 1 47 10

4 2 3

8 3 33

.

.

.

.

YP

P

m= − +49 78 6 14

2

. .

(2.43)

(2.44)

(2.45)

where Y is the suspended sediment yield in t km–2.a, Pm is themean precipitation in the month with maximum rainfall, and P isthe mean annual rainfall. Equation 2.42 is for a region with lowrelief and a precipitation regime of Pm

2/P<20 and Pm2/P>8.1;

Equation 2.43 for a region with low relief and a precipitationregime of Pm

2/P>20; Equation 2.44 is for a region withpronounced relief in all climates except semi-arid, and Equation2.45 is for a region with pronounced relief in a semi-arid climate.The basic data are from 96 drainage basins, each larger than2 000 km2 (Jansson, 1982).

Multiple regression on sediment yield. The variablesinclude five factors: climate, relief, soil, vegetation and land use.

Hindall, for a northern plateau:

Qs = 51.1St–0.72 (2.46)

For a central plain and western part of a glacial erosionmountain area:

Qs = 2.82 • 1010 Qa1.43 Q25

0.43 L–3.29 S–2.26 I–1.52 (2.47)

For a northern ridge of a mountain and low-lying land:

Qs = 4.37 • 10–12 Qa2.63 Q2

–5.83 Q255.92

L–2.52 Rc1.31 Si

–6.33 Fd8.26

(2.48)

For a region without ice-laid drift:

Qs = 197 • 1A–2.38 Qa–3.14 Si

0.19 L4.14 Fd–4.48 D–1.43 S2.01 (2.49)

where Qs is the modulus of mean annual sediment yield int mile–2.a, Qa is the mean discharge in ft3 s–1, Q25 is the flooddischarge with a recurrence interval of 25 years, Q2 is the flooddischarge with recurrence interval of two years, L is the length ofthe major river, in miles, St is the area of lake or marsh, S is theexponent that refers to the soil seepage ability, Si the gradient ofthe main channel in ft mile–1, I is the exponent of rain intensity(24-hour rain with recurrence interval of two years) in inches, Ro

is the modulus of flood volume in ft3 s–1.mile2, Fd is the meanfreeze depth on 28 February in inches, and D is the duration coef-ficient of river channel discharge (10 per cent discharge divided by90 per cent discharge using duration curve).

To estimate the sediment yield reduction on the majortributaries of the Middle Yellow River located in the high andcoarse sediment-yield regions due to soil and water conserva-tion works, YRCC developed the sediment yield model fortributaries.(1) Statistical model on monthly effective rain-sedimentyield: From the 17 tributaries in the middle reaches, the relationship

of the monthly sediment concentration (ρ) to the monthly runoff(W) and maximum sediment concentration (ρm) is:

ρ = ρm

[1 – e–K (w0 – Ws)] (2.50)

Ws = ρW0 (2.51)

where W0 is the base flow without sediment yield, K is the para-meter, and Ws is the monthly sediment yield.(2) Statistical model on annual rain-sediment yield: Therelationship of the annual sediment discharge (Sa) to the annualprecipitation (Pa) is:

Sa = CPad (2.52)

The relationship of the annual sediment discharge andthe effective annual precipitation (Pa1) is:

Sa = aPa1β (2.53)

(2.54)

where P1 and P30 are the maximum 1-day and 30-day rains,respectively, and Pf and Pa are rain in the flood season (June toSeptember) and annual rains, respectively.

The relationship of the annual sediment discharge andeffective annual precipitation (Pa2) is:

Sa = αPa2β (2.55)

Pa2β1P1 + (βm – β1) (P30 – P1) + (βf – βm) (Pf – P30) +(2.56)

(1 – βf) (Pa – Pf)S1 and S30 are maximum one-day and 30-day sediment dischargesrespectively, and Sf is the sediment discharge during the floodseason (Zhang, et al., 1998).

(5) Remote sensing information model for water erosion

(2.57)

where E is the depth of soil erosion in the basin in mm; I is therain intensity in mm min–1, I0 is the threshold value of rain inten-sity for erosion in mm min–1, h is the depth of land surface runoffin mm, ST is the effective depth of soil layer in mm, d is the meandiameter of soil particles, v is the degree of vegetation cover in percent, and C0, C1, C2, C3 are the geographic parameters.

2.8.5 Deterministic sediment yield modelsThe deterministic sediment yield models are developed based onfundamental erosion processes.

Simons, et al., developed a model based on the concep-tion that a basin may be divided into two portions: surface runoffand river channel systems. Four main processes of runoff are:interruption, infiltration, flowing routing and sediment routing(Walling, 1981).

CHAPTER 2 — SOIL EROSION 21

P PP P

Paf

a1 130

30 122= + + +( ) ( )

β β30= = =∑ ∑ ∑( ) / ( ) / ( ) /S

Sn

S

Sn

S

Sn

fi

aim

i

aif

fi

ai

; ; ;where ,

E CI I

Ih

S

deo

o

o

c T c c v=

− −( ) ( )1 2 3

YP

P

m= − +475 4 27 12

2

. .

YP

P

m= − +513 21 52 49

2

. .

YP

P

m= − +737 62 91 78

2

. .

The flowing routing was based on the continuity equa-tion of flow and the approximate momentum equation of kineticwave, as well as a set of resistance functions under differenthydraulic conditions. Sediment routing requires calculating thedetachment amount of soil accounted for by rainfall splash andland runoff, the amount of detachment and transport of wash loadby runoff, and the transported bed load. It is assumed that thedetachment soil amount due to rain splash is a singular exponentfunction of rain intensity. The sediment transportation function isthe combination of the Meyer-Peter and Muller bed-load equationand the Einstein suspended-load equation (Walling, 1981).

The Upland Soil Erosion Model for Unstable Land Flowcan simulate the processes of sediment transportation and reliefevolution. The relief trends to the concave shape which is widelyobserved in the natural environment. The model is applied whenthe land flow erosion is mainly caused by sheet erosion; thekinetic-wave approximation is effective for flow solution, and theslope gradient is less than 25 per cent; or the detachment andtransportation by splash is negligible. The simulation may be real-ized by the continuity equation and momentum equation of soiland water.

In the sediment transport equation, sediment consists ofbed load and suspended load. The bed load may be calculated by:

qb = β (τ – τc)β2 (2.58)

where qb is the bed load discharge per unit width, τ is the bound-ary shearing stress, τ0 is the critical shearing stress, and β1 and β2are the constants.

The suspended sediment discharge may be calculatedby:

(2.59)

where Sq is the suspended sediment discharge per unit width,G = d50/Y, W = Vs /(0.4 U*), Vs is the settling velocity of sediment,U* is the shearing velocity defined as √τ / ρ, ρ is the water density,V is the mean velocity, r = ξ /Y, and ξ is the measurement distancefrom the river bed.

The total sediment discharge is the sum of bed load andsuspended discharges.

qs = qb + Sq (2.60)

The Water Erosion Prediction Project (WEPP) wascarried out by USDA. There are three versions of the WEPP: thehillslope, watershed and grid versions.

The hillslope version can directly replace USLE andRUSLE. Only the function of slope sediment silting is added inthe WEPP.

The watershed version includes the hillslope versioncalculating erosion on slopes. It can be used to predict thesediment yield in watersheds and to calculate sedimenttransportation, siltation and scour in river channels, sheet erosionon terraces, and shallow gully erosion and sedimentation inreservoirs.

The grid version can be used for any geographicalregions which do not correspond to the boundary. These regionsmay be divided into a number of units. The hillslope version canbe used to calculate the erosion for each unit area.

The WEPP is a model consisting of various proceduremodules related to soil erosion.

(1) Module on erosion process. The soil erosion processin the WEPP is divided into three stages: erosion, transport anddeposition. There are two types of erosion: rill erosion and inter-rill erosion. The inter-rill erosion caused by splash and thin-layerflow is the function of the gradient and square of the rain intensity.The rill erosion caused by runoff is the linear function of theshearing force of flow.

(2) Module on hydrologic process. This has severalsub-modules such as meteorology, infiltration and freezing-thawing. The meteorology sub-module consists of volume andduration of rainstorm, ratio of peak rain intensity to mean rainintensity, duration of peak rain, daily maximum and minimumtemperature, wind speed and solar radiation. These meteorologi-cal factors include the duration of runoff, peak runoff coefficient,total runoff including snow-melt, growth amount of vegetation,and resolving ratio in residues and water content in different soillayers. The sub-module of infiltration uses the Green-AmptEquation to describe the rule of infiltration. The freezing-thawingsub-module has been used for frost, snow-melt and snow accu-mulation in soil.

(3) Module on plant growth and residue process: Thismodule is used to estimate the effects of plant and soil residues onsoil erosion.

(4) Module on water use process. Based on the sub-modules of meteorology, plant growth and infiltration, itsimulates the dynamic variation of water content in soil, taking oneday as a time step. It can also estimate the potential or realevapotranspiration.

(5) Module on hydraulic process. Using the data ofrunoff, hydraulic roughness, duration of runoff and peak runoffcoefficients, this module can estimate the process of runoff bydynamic-wave equation.

(6) Module on soil process. This module presents thedynamic variation of soil and its ground characteristics by dailytracing. The concerned variables are natural surface roughness,artificial roughness (height of ridge culture), bulk density of soiland ability of saturation conduction water, soil erodibility andshearing force of critical flow. This module also considers theeffects of cultivation, weathering and aggregate rainfall on theground characteristics (Liu, 1997).

Models for Erosion and Sediment Yield in SmallWatersheds in the Loess Hilly-gullied Regions of ShaanxiProvince, China.

The new ERODE model, combining erosion models andGIS, is useful in planning soil conservation measures and water-shed management. This model is designed to estimate annualrunoff and soil loss. In the Loess Plateau, most rainfall takes theform of storms. The runoff and sediment yield are calculated foreach rainstorm. The model consists of three sub-models:slopeland, gully and slope, and gully.

(1) Slopeland sub-model: This considers only the splasherosion when there is no rill. It considers both rill erosion by flowin the dominant act and the inter-rill erosion by splash erosion. Itincludes two main processes. One is the effect of crust on top soil;

22 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Sq G

G

V

U

r

rdr

rr

rdr

qb

w

ww

G

I

w

G

I

=−

+−

+−

−

∫

∫

11 6 12 5

1

2 51

1

. ( )[( . ) ( )

. ln ( ) ]

*

the other simulates the processes of soil erosion and transportationon rills.

Erosion force of rainfall:

Ek = (P – Z) (28.83 + 13.51gI) (2.61)

where P is the total rainfall in mm, Z is the volume intercepted byvegetation, and I is the mean rain intensity in mm min–1.

Erosion force of runoff:

Ps = 0.001ρgRA sin θ (2.62)

where A is the area in m2, g is the gravity acceleration, R is the runoffin mm, ρ is the water density, and θ is the gradient in degrees.

Splash erosion:

(2.63)

where Di is the splash erosion in kg m–2, J is the soil crust factor (ittakes 0.7 with crust and 1.0 without crust), τ is the shearing forceon top soil, and Cv is the degree of vegetation cover in per cent.

Rill Erosion (Dr):

(2.64)

Sediment transport force by flow (Tc):

Tc = 0.0081Ps1.55 (kg m–2) (2.65)

Soil erosion (SL):

if Di < Tc, then SL = Di (2.66)

if Di > Tc, then SL = Tc (2.67)

if Si + Dr < Tc, then SL = Si + Dr (2.68)

if Si + Dr > Tc, then SL = Tc (2.69)

where SL is the amount of soil erosion, and Si is the amount ofinter-rill erosion, 2.5 kg m–2.

(2) Gully-slope sub-model: Because of the steep slopeof gullies, gravitational erosion occurs often. But the sedimentyield caused by landslides, even in large scale, may not be soenormous. In fact, the collapse of shallow layers often has moreeffect on sediment yield. Another erosion process is cave erosion.

Amount of runoff:

for red clay area: QG = 1.086 • 10–4 PA0.164 I30

1.04 E1.14 (mm) (2.70)

for loess area: QE = 1.29 • 10–4 PA0.225 E1.509 (mm) (2.71)

where PA is the antecedent affecting rainfall, and E is the kineticenergy of rainfall in joule m–2.mm.

Amount of sediment:

for red clay area: SG = 106.57QG1.138 (2.72)

for loess area: SE = 225.2QE1.196 (2.73)

Amount of sediment caused by cave erosion:

Ss = 91.84Rs1.04L0.373J1.02 (2.74)

St = 169.02RT1.04L0.13 (2.75)

ST = St – Ss (2.76)

where Ss is the amount of soil erosion on the topsoil of the cave, St

is the gross sediment yield from the cave in kg, L is the length ofthe cave in m, J is the height of the cave in m, ST is the amount ofnet erosion of the cave in kg, RT is the runoff depth of the cave inmm, and Rs is the runoff depth on the land surface slope in mm.

(3) Sub-model on gully erosion. Sediment delivery ratio(Rsd):

(2.77)

where P is the amount of rainstorm in mm, C is the runoff coeffi-cient, Sm is the sediment concentration during the flood peak inkg m–3 and Ea/E is the proportion of kinetic energy by rainfall ofintensity exceeding 0.15 mm min–1.

2.9 SOIL EROSION CONTROL AND WATERSHEDMANAGEMENT

Soil erosion control is a complex engineering system to promotethe sustainable development of agricultural production and socialeconomies. It concerns a number of aspects such as environment,scientific techniques, economies, societies, policies and regula-tions. Soil erosion control includes planning and management ofsoil and water conservation measures closely related to watershedmanagement.

2.9.1 Soil and water conservation planningSoil and water conservation planning is to control soil erosion andregulate river channels in a certain area. It is based on the situationof soil and water loss, conditions of natural resources and socialeconomy, and the strategic goals of national economic develop-ment following the principle of soil and water conservation andecology.

(1) Categories and tasks of planningThe categories of soil and water conservation may be dividedinto the following: (1) National scale — taking a large riverbasin or large natural region as a unit; (2) Large river scale —taking large rivers with an area ranging from several dozen tohundreds of thousands of km2 as a unit; (3) Large tributary scale— taking an area of several thousand to several tens of thou-sands of km2 as a unit; (4) Small watershed scale — several toseveral hundred km2.

The first two categories require strategic middle- andlong-term planning. The basic tasks are systematically to carry out

CHAPTER 2 — SOIL EROSION 23

€

R P C SE

Esd ma= −0 0277 0 29 0 19 0 59 0 44. ( ). . . .

D Pr sb

= ⋅− −7 9 10

4 8 0 5 8. ( )

. .τ

τ

D JE

eik Cv= −

0 0152 68 0 48

.( . sin . )

τθ

environmental recognition and resource evaluation based on acomprehensive investigation of nature and society. They shouldpredict the economic, ecological, environmental and social bene-fits of soil and water conservation, and coordinate agriculture,forestry and husbandry in planning. The third and fourth cate-gories are the practical planning for middle and short termscovering medium and small scopes. The basic tasks are to producelocal plans on land use, tillage, vegetation and engineeringprojects.

(2) Planning approachesThe land use plan is the core plan. Land evaluation is based onland specifications. The definition of land evaluation in The Sketchon Land Evaluation edited by the Food and AgriculturalOrganization of the United Nations (FAO) is to compare and illus-trate the basic conditions of soil, vegetation, meteorology andother aspects of the land, and to carry out appraisals and compar-isons for the prospective land.

At present, the Land Potential Gradation developed bythe USDA Soil Conservation Service has been widely adoptedunder the recommendation of FAO. The land is divided into eightdegrees based on the limited intensity of crop or grass which hasbeen acted upon by soil. Optimum land use planning usuallyadopts the method of linear planning. Linear algebra is used forresource distribution. That is, with the existing natural resources,resources of social economy and technology, a scientific decisionshould be made to achieve the best social, economic and ecologi-cal benefits.

2.9.2 Measures for soil and water conservation

(1) Cultivation measures for soil andwater conservation

Increasing the roughness of land surface, changing the micro-relief of land slope and improving vegetation cover can fostersoil and water conservation and improve soil texture (Hudson,1981).

Tied ridging: Closely spaced ridges are arranged on theground surface to form a series of rectangular depressions. Whenthe soil becomes saturated and the depressions are full, overflowoccurs and the ridges break. On slope ground, once a ridge isbroken, a small flood is released and bursts the next ridge, storingmore water, and so on down to the end of the slope. This measurehas been used successfully on deep permeable soils of East Africaand in western Gansu Province, China.

Contour cultivation and grass strips: On gentle slopesor where erosion risk does not warrant major earth-movingworks, it may be sufficient to slow surface runoff by carrying outall tillage operations on the contours. Another protection measureinvolves using grass strips when the soil erosion is not severe.Surface runoff moving down the slope is intercepted by the grassstrips, the velocity is slowed, and silt is deposited in the grassstrips.

Ridge and furrow: The ground is tilled into wide parallelridges approximately 10 m wide, with intervening furrows about0.5 m deep. Surface runoff moves across the ridges to the furrows,then down the furrow, which is on a gradient of about 1:400. Thismethod is particularly suitable to large areas of gentle slopingland, but for channel terraces it requires some controlled surfacedrainage.

(2) Engineering measures for soil and water conservation(a) Agricultural arable land. Channel terrace: If surfacerunoff flows down the slope of arable land without any impedi-ment, it not only carries away the soil dislodged by splash erosion,but also scours the soil down. To avoid this, terraces are used tointercept the surface runoff. In some African countries, a broad-based contour ridge has a wide (15 m) and low bank and a shallowchannel with gentle sloping sides; a narrow-based contour ridgehas a steep-sided bank with a width of only 3 to 4 m (Hudson,1981).

Bench terrace: Bench terraces entail converting a steepslope into a series of steps with horizontal or near horizontalledges. To hold up the vertical face, some structures are necessary.Usually these are stone structures, but bricks or timber are alsoused. There are different types of bench terraces, such as outward-sloping, inward-sloping and reverse-sloping ones. It is desirablefor each bench to be as wide as possible for cultivated crops.Small terraces for fruit-trees, coffee plants and vines are equallyeffective and require less earthmoving.

Irrigation terrace: A flat bench terrace has a raised lip atthe outer edge to retain irrigation water. It is extensively used forthe production of rice, and also for tea, fruit trees, and other highvalue crops. For paddy fields, the terraces are level so that eachterrace becomes a shallow pond.

Orchard terrace: If the soil is too shallow or the slope istoo steep, bench terracing may not be practical. In this case, theland may be developed for tree crops by using intermittentterraces, otherwise known as orchard terraces. These are small,level or reverse-slope terraces, each having one line of trees. Theimportant feature of any of these development techniques for steeperosion-prone slopes is that the land between the terraces must beplanted with a vigorous cover crop, such as a creeping legume. InKenya, one type of intermittent terrace is used. The excavated soilis used to build a bank above the ditch with the purpose of catch-ing silt to form a more level terrace.

Terrace systems: Terraces, as mechanical erosion-controlmeasures in slope cropland, are used to alter flow length, providetemporary runoff storage, and reduce slope gradient. Terracesystems can meet water management and erosion-control needsfor intensive slope cropland.(b) Non-arable land. Mechanical protection of forest soils:Mechanical protection is not usually required for natural forests,but commercial planting may well need some protection duringestablishment and after harvesting. Two forms are most common:contour trenches and contour furrows. Both are similar to thestructures used for arable land. Contour trenches are commonlyused in America on steep land from 30 to 75 per cent. Thetrenches are usually built without any gradient in the channel,since the objective is to hold runoff until it infiltrates the soil.Cross-ties are added every 10 to 15 m to further restrict watermovement. Contour furrows are similar in form, but smaller, andare used on gentle slopes up to about 35 per cent. They have asmaller water-holding capacity (Hudson, 1981).

Mechanical controls on grazing land: Poor grazing landhas such low production levels that only very simple and inexpen-sive measures are economically justified. Such measures are notdesigned to control soil movement directly, but to improve thevegetation by reducing runoff and increasing infiltration. Twotypes of structures are used. Pasture furrows are small and havelevel open drains that follow the real contours and are fairly close

24 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

together, like the large channel terraces used on arable land. Theother approach is to form many small surface depressions whichhold and store runoff.

Erosion control on roads: Siting and alignment: Thesiting of a new road can be established efficiently using aerialphotography. The first rule of road siting is to place roads oncrests wherever possible. When it is impossible, the next align-ment is on a gentle gradient fairly close to the real contours.Gradients of the order of 1/100 to 1/500 are desirable for the open-channel drains required along roads. A gradient of 1/100 to 1/20may cause some problems for controlling soil erosion on sidedrains. For a gradient steeper than 1/20 it is usual to adopt azigzag layout or the combination of one reach on gentle slope andsome reaches straight down the slope.

Road drainage: In siting roads, swamp and permanentlywet areas should always be avoided. Roads straight up and downthe steepest of slopes need side drains only to deal with the runofffrom the road surface, and this water can be easily dealt with bymitre drains. A wide shallow cross-section with a gentle side slopewill provide the best hydraulic design, and regular mowing of thecover grass has been shown to be the most effective and cheapestmaintenance.(c) Structures for gully erosion control. Temporary struc-tures: If the objective is to slow down the water and so causedeposition of silt, there is no need for the structures to be water-tight. These are called porous checks.

Wire bolsters: If there is plenty of loose rock availablenearby it can be used to build a loose rock-fill dam anchored inplace by wire netting. Galvanized wire netting of a fairly stoutgauge and two metres or more in width is laid out flat across thegully bed. Loose rock is packed on one half of the width of thenetting, and the other half is wrapped over the stones and laced tothe other edge.

Netting dam: Another use of wire netting is to formsmall check dams, usually near the top end of gullies. Woodenposts are driven into the bed of the gully and used to support astrip of wire netting which forms a low wall across the gully. Lightbrush or straw is piled loosely against the upstream side of thenetting wall.

Brushwood dam: In wooded areas, two types of siltretaining dam are adopted. The brushwood dam uses smallbranches, up to two or three cm in diameter, packed across thedirection of flow. They can be anchored by packing them betweenrows of vertical stakes.

Log dam: When heavier timber is available it can beused for log-piling dams. Two rows of vertical posts are driveninto the bed of the gully, extending up the side to above floodlevel, and then logs are packed in between them. In a wide,shallow river it is best to drive in all the vertical posts to the sameheight above the ground, so that the top of the dam follows thesection of the river bed.

Brick weirs: The shape that gives the best strength-to-weight ratio is an arch weir. A single thickness of brickwork canbe built to a height of 1 to 1.5 m over a circular span of about 2 m.A straight wall of similar size would need three or four times morebrickwork to be of comparable strength.

Permanent structures:Silt-trap dam: A quick positive reduction in sediment

movement can be achieved by building permanent silt trappingdams.

Regulation dam: This is a useful application of perma-nent dams to regulate flash floods, using the leaky bath-tubprinciple. A permanent dam is built with sufficient storage for therunoff from a single storm. The outlet consists of a pipe whichallows the flood water to drain away in one or two days, leavingthe storage reservoir empty for the next storm.

Gully-head dam: This is used when an active gully isdeveloping steadily in an upstream direction and must be stoppedbefore it threatens roads, bridges or similar structures. An effectiveway of controlling the erosive force of runoff over the gully headis to submerge the gully head in the pond of a permanentimpounding dam. The energy of the inrushing water is then dissi-pated as it flows into the pond.

Drop structure: This is built using masonry, bricks, orconcrete to allow the flood runoff to pass over harmlessly. Thecapacity of drop structures is controlled by the size of the inlet. Itacts as a rectangular weir with the flow proportional to the lengthof the weir.

Cabion: The main problem with rigid structures is thatthey cannot adapt to the conditions of surrounding soil. Oneconstruction that can overcome this difficulty is a more sophisti-cated version of the wire netting bolsters. This method wasdeveloped in Italy and uses pre-fabricated rectangular basketscalled cabions. Its main advantage is that there is sufficient flexi-bility for the structure to adjust to settling resulting from scouringthe foundation (Hudson, 1981).

Sediment controlling reservoir: This reservoir can trapsediment (rock, silt and floating material) scoured down by aflood. There are four types of sediment controlling dams: (a)sluicing gate dams; (b) open mouth dams; (c) grid dams; and (d)net dams.

(3) Vegetation measures for soil andwater conservation

Soil and water conservation forests: Any artificial or naturalforests having the function of improving the ecological environ-ment, conserving water resources, preventing soil erosion orregulating the hydrological status of rivers, lakes, and reservoirsare called soil and water conservation forests.

One hectare of forest can store about 300 m3 of rainfall.The forestry canopy intercepts the rainfall, and the layer of with-ered branches and falling leaves absorbs the surface runoff.According to measurements, the canopy can intercept 15 to 40 percent of rainfall, and 1 kg of withered fallen material can absorb 2to 3 kg of water. Also, the permeability of forest soil is 3 to 10times higher than grassland or arable land.

Forests can lower the mean annual temperature, reducethe temperature difference and increase humidity. Each hectare offorests can absorb 192 kg of CO2 in a day. The dust content inforests is 20 to 40 per cent lower than in open country.

Mixed forests of male and female trees, bush and tree orcomplex mixed forests should be arranged according to localconditions.

Grass for soil and water conservation: Planting grass canconserve soil and water and improve the physical and chemicalquality of soil. The functions of grass are to: (a) store water,preserve soil moisture and prevent soil erosion; (b) improve soiland increase its fertility (one hectare of alfalfa can fix 225 kg ofnitrogen in three years, which is about 750 kg of ammoniumnitrate); and (c) provide forage, fertilizer and fuel.

CHAPTER 2 — SOIL EROSION 25

(4) Wind erosion control on cultivated land in arid areasStubble mulching and less ploughing: Stubble mulching is one ofthe most effective methods to prevent wind erosion and conservesoil moisture. It is mainly used for crops of wheat, sorghum, andother millets. The crop is planted directly in the field where the landis covered with stubble, thus using less ploughing for intertill crops.

Contour strip cropping: The strip is perpendicular to thecommon wind direction. It also needs appropriate crop stubble toresist wind erosion.

Wind breaks and forest belts to resist wind: The effi-ciency of this method depends on wind speed and direction, andon the shape, width and spacing of the wind break. If the winddirection is perpendicular to forest belts, the wind speed willdecrease by 70 to 80 per cent. It is usual to take an interval of1 300 ft for forest belts. In areas with high and medium intensivewind erosion, the intervals between belts may be 350 to 450 ft and500 to 650 ft, respectively (Woodruff, et al., 1981).

(5) Erosion and sediment control for surface miningImproper surface mining or waste piles from deep mining willcause serious soil erosion. The key is to have proper planningbefore mining starts. Provisions must be made for the placement ofoverburden, controlling head cutting and sheet erosion, sedimentretention, and land stabilization. Suitable soil should be placed onthe surface to facilitate the growth of new vegetation. Spoil pilesshould be kept away from the system. Land stabilization duringmining and after reclamation must be an integral part of the plan-ning. Settlement basins may be used to trap sediment. In miningprocesses, backfill and reclamation should be carried out simulta-neously. Measures for soil and water conservation should be part ofthese processes. The entire area should be protected by vegetationor using other methods. Drainage systems and settling ponds areadopted to eliminate the impacts of surface mining on water qualityof the adjacent areas. A monitoring system should be established tomonitor the dispensing polymer electrolytes and flocculation ofsuspended sediment (ASAE, 1977).

(6) Erosion and sediment control in urban areasExtension of urban area requires the construction of roads andbuildings on a large scale that disturbs the original environmentand removes a lot of soil, thus causing soil erosion. Erosion and

sediment control in urban areas in Malaysia reduced by 68 to 80per cent the sediment yield from construction sites between 1966and 1974 according to a report by United States GeologicalSurvey (USGS). Much sediment yield comes from urbanconstruction. The report by Wolman and Schick shows that thesediment yield from urbanized or developing areas ranged fromseveral hundred to 55 000 t km–2 per year. Yorke and Herb indi-cated that the annual average sediment yield from cultivated landwas 620 t km–2, while that from construction sites ranged from1 610 to 22 600 t km–2, and the average was 7 330 t km–2.

Desirable practices for erosion and sediment controlinclude the following:

Temporary structural practices: They can be divided intotwo groups: water control and sediment control. Water controlincludes small diversion terraces (dikes), small waterways(swales), and grade stabilization structures. The sediment controlpractices consist of sediment traps for drainage areas smaller than2 ha and sediment basins for drainage areas of 2 to 40 ha.

Permanent structural practices: Diversion, grassedwaterways, level spreaders and subsurface drains are used foragriculture, and storm drain outlet protection, land grading andriprap are used only for urban areas.

Vegetation practices: These include both temporary andpermanent practices for establishing ground surface cover tocontrol soil erosion. They include seeding, sodding, mulching, aswell as criteria on ground cover, vines, shrubs and trees.

Special practices: These include vegetation tidal bankstabilization using original topsoil, protection of tress in urbanareas, seeding strip-mine areas, dune stabilization, dust controland protective material for channel and steep slopes (ASAE,1977).

2.10 SUMMARY ON GLOBAL SOIL EROSIONThe problem of soil erosion has been given more and more atten-tion in the world. The total erosive area in the world is 25 millionkm2, accounting for 16.8 per cent of the total continent area. Onethird to one fourth of the topsoil of arable land suffers fromserious soil erosion. About 60 billion tons of fertilized top soil iseroded and about 17 billion tons of sediment flow into the oceanor sea annually. Fournier pointed out that the maximum sedimentyield occurs in semi-arid regions. Based on an estimation by

26 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 2.6Present status of soil erosion and global trends

Water erosion Wind erosion

Regions Area Annual denudation Annual losses Trend Area Trend(106 hm2) (mm) (106t) (106 hm2)

Africa 227 0.023 201 + 186 +

Asia 441 0.153 1592 + 222 +

South America 123 0.067 603 + 42 –

Central America 46 0.055 758 + 35 –

North America 60 0.055 758 +/– 35 –

Europe 114 0.032 425 +/– 42 +/–

Oceania 83 0.390 293 + 16 +

World 1094 0.079 3872 + 548 +

NOTES: 1. Area after Oldman, 1991–1992; 2. Denudation after Lal, 1994; 3. Soil losses after Walling, 1987; 4. “+” increasing, “–” decreasing.

UNDP, 5 to 7 million km2 of arable land is lost annually due tosoil erosion, and the yearly economic losses reach $10 billion(Zhao, et al., 1997).

The mean annual precipitation and rain intensity are themost important factors affecting water erosion. In tropical regions,intensive downpours can cause much more damage than intemperate climates. In general, water erosion in the regionsbetween latitudes 40° North and 40° South is the most serious inthe world. It includes North America and part of South America,most of Africa, except the dry and desert areas and the equatorialforest, Asia up to 40° N, as well as the dry central areas ofAustralia.

As for wind erosion, the main regions are North America(the Great Plains Dust Bowl), the Sahara and Kalahari deserts inAfrica, north-western China, Central Asia (particularly the steppesof Russia), and central Australia (Hudson, 1981).

Global sediment maps: Different methods have beenused to survey water erosion on a global scale. The map byFournier is a map of suspended sediment yield in a basin arealarger than 2 000 km2. Another world map of erosion rates wascontributed by Strakhov on the basis of the suspended load in 60rivers. Table 2.6 shows the present status of soil erosion and globaltrends (Jansson, 1982).

A total of about 50 million km2 land is in arid, semi-arid, and dry sub-humid regions. In these regions, about 3.1 billionha and 3.1 billion ha of grassland are undergoing medium andserious desertification, respectively; 335 million ha and 170million ha of rainfed cropland are suffering medium and seriousdesertification, respectively; 40 million ha and 13 million ha ofirrigated cropland are being subjected to medium and seriousdesertification, respectively (Wang, 1997).

REFERENCES21st Century United Nations Conference on Environment and

Development, 3–14 June 1992 (Chinese version translatedby China Environment Bureau).

ASAE, 1977: Soil erosion and sedimentation. Proceedings of theNational Symposium on Soil Erosion and Sedimentation byWater, ASAE Publication 4–77, Palmer House, Chicago.

Bennet, H.M., 1939: Soil Conservation. New York-London.Chen Qibo and Fei Xiliang, 1996: The new progress in prediction

of soil erosion. Journal of Chinese Soil and WaterConservation, Number 2.

Chen Yongzong, Jing Ke and Cai Guoqiang, 1988: Modern SoilErosion and Management of the Loess Plateau. SciencePress (in Chinese).

China 21 Century on Population, Environment and DevelopmentWhite Book (Chinese version), Beijing, 1994.

Division of Sediment, Chinese Hydraulic Engineering Society(CHES), 1992: Manual of Sediment. China EnvironmentalScience Press (in Chinese).

Encyclopedia of Chinese Agriculture (Volume on WaterEngineering), 1987 (in Chinese).

Flaxman, E.M., 1963: Channel stability in undisturbed cohesivesoils. Journal of the Hydraulics Division, American Societyof Civil Engineers (ASCE), 89 (Hy2), Proc. 3462, March,pp. 87–96.

Foster, G.R. and W.H. Wischmeier, 1973: Evaluating IrregularSlopes for Soil Loss Pediction. Paper 73–227, ASAE,Lexington.

Frazier, B.E., D.K. McCool and C.F. Engle, 1983: Soil erosion inthe Palouse: an aerial perspective. Journal of Soil and WaterConservation, Volume 38, Number 2.

FAO, 1965: Soil Erosion by Water, Some Measures for its Controlon Cultivated Lands.

Gong Shiyang, 1998: Soil Erosion on the Loess Plateau of theYellow River Basin (in Chinese).

Gottschal, L.C., 1975: Nature of Sedimentation Problems.Guo, Tingfu, et al., (ed.) 1998: Standards for Classification and

Gradation of Soil Erosion. Trade Standard, Ministry ofWater Resources of China (in Chinese).

Holy, M., 1982: Erosion Environment. Technical University ofPrague, Czechoslovakia (translated by Janaa Ondrachova).

Hua Shaozu, 1990: Planning on Soil and Water Conservation.Regional training course on soil erosion and its control.

Hudson, N., 1981: Soil Erosion . Batsford Academic andEducational, London.

Jansson, M.B., 1982: Land Erosion by Water in DifferentClimates. UNGI Report Number 57.

Liu Tungsheng, 1985: Loess in China. Springer Series in PhysicalEnvironment.

Liu Zengwen, 1997: Introduction to the WEPP model on predic-tion of water erosion. Journal of Chinese Soil and WaterConservation, Number 12.

Margan, R.P.C., 1980: Soil Conservation Problems and Prospects.Mayer, L.D., 1984: Evaluation of the Universal Soil Loss

Equation. Journal of Soil and Water Conservation,Volume 39, Number 2.

Meng Qingmei, Hua Shaozu, et al., 1996: Soil and WaterConservation in the Loess Plateau. Yellow River HydraulicPress (in Chinese).

Meyer, L.D., 1984: Evolution of the Universal Soil Loss Equation.Journal of Soil and Water Conservation, Volume 39, Number 2.

Ministry of Water Resources of China, 1998: Standards forClassification and Gradation of Soil Erosion-Trade Standard.

Mutchler, C.K., 1963: Runoff plot design and installation for soilerosion studies. Agricultural Research Service ReportNumber 41–79, USDA, August 1963.

Norman, H., 1981: Soil Conservation. Batsford Academic andEducational Ltd, London.

Onstad, C.A. and G.R. Foster, 1974: Erosion and depositionmodeling on watershed. Scientific Journal Series 8537,Minnesota Agricultural Experiment Station.

Piest, R.F. and C.R. Miller, 1975: Sediment sources and sediment yield.Chapter IV of Sedimentation Engineering, (ed.) V.A. Vanoni.

Schertz, D.L., 1983: The basis for soil loss tolerances. Journal ofSoil and Water Conservation, Volume 38, Number 1.

Sharma, 1998: CTA of UN participatory watershed managementprogramme in Asia. Proceedings of the InternationalSymposium on Comprehensive Watershed Management(ISWM-’98), 7–10 September 1998, Beijing.

Spomer, R.G. and R.L. Mahurin, 1984: Time-lapse remote sensingfor rapid measurement of changing landforms. Journal ofSoil and Water Conservation, Volume 39, Number 6.

Vanoni, V.A. (ed.), 1975: Sediment Engineering. ASCE, New York.Walling, D.E., 1998: Opportunities for using environmental

radionuclides in the study of watershed sediment budgets.Proceedings of the International Symposium onComprehensive Watershed Management (ISWM-’98),7–10 September 1998, Beijing.

CHAPTER 2 — SOIL EROSION 27

Walsh, S.J., 1985: Geographic information systems for naturalresource management. Journal of Soil and WaterConservation, Volume 40, No 2.

Wang, Lixian, 1995: Soil and Water Conservation. Beijing ForestUniversity, June.

Wang Lixian, 1997: Land Degradation in Globe and itsPreventing and Harnessing Measures, Soil and WaterConservation of China (in Chinese).

Williams, J.R. and H.D. Berndt, 1972: Sediment yield computedwith universal equation. Journal of the Hydraulics Division,ASCE, 98 (Hy12), Proc. 9426, December, pp. 2087–2098.

Wu Changwen, Liu Weichang, and Zhan Dingsheng, 1997:Principles and methods of planning of soil and water conser-vation in urban areas. Journal of Chinese Soil and WaterConservation, Number 1.

Xin Shuseng and Jiang Deqi, 1982: An Introduction to Soil andWater Conservation in China. Agricultural Press (inChinese).

YRCC, 1991: Remote Sensing in the Yellow River Basin-SoilErosion (in Chinese).

Zachar, D. 1982: Soil Erosion. Forest Research Institute, Zvolen.Zhang Shengli, et al., 1998: The Reasons and Trends of Changes

in Runoff and Sediment Yield of the Middle Yellow RiverBasin. Yellow River Hydraulic Press (in Chinese).

Zhao Yi and Liang Weilu, 1997: Soil erosion and its control in theUSA. Journal of Chinese Soil and Water Conservation,Number 5 (in Chinese).

Zhu Pengcheng (ed.), 1992: Sediment Manual . ChineseEnvironment Science Press (in Chinese).

28 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

3.1 PATTERNS OF SEDIMENT TRANSPORT INRIVERS

3.1.1 Bed material load and wash loadSediment is classified as either bed load or suspended load accord-ing to the patterns and laws of movement. It can also be classifiedas bed material load and wash load according to the particle size,its origin and effect in fluvial processes.

The ratios of fine to coarse sediment in river bed mate-rial sediment are quite different. Sediment in river beds is often(but not always) composed of much coarser and much less finesediment than moving sediment. There is always an exchangebetween coarse sediment and bed material during transport.Incoming coarse sediment may originate directly from the riverbed of an upstream reach. It is directly supplied from the bedand therefore is called bed material load. In contrast, fine sedi-ment, eroded and washed from upland watersheds, istransported through a channel over a long distance and isscarcely ever deposited in the channel; therefore, it is called“wash load”. Thus, the amount of coarse sediment carried byflow depends on sediment transport capacity and exhibits awell-defined relationship with the flow discharge. In contrast,the concentration of fine sediment depends only on the supplyof sediment from the upstream reach, and no obvious relation-ship with flow discharge is found.

Sediment can be classified as bed material load andwash load, or bed load and suspended load. It should be empha-sized that the two sets of classification of sediment are distinct andshould not be intermingled. Bed material load may move as bothbed load and suspended load, and the same is true for wash load.Of course, wash load is fine and mainly moves as suspended load.It is not correct to identify the bed material load with bed load andwash load with suspended load.

3.1.2 Bed load, saltation and suspended loadIt should be pointed out that only bed material load, not washload, is discussed here.

At low flow, although some sediment moves in suspen-sion, most sediment particles move in the form of sliding, rollingand saltation in a zone close to the bed surface with a thickness of1 to 3 times the particle diameter. Such sediment is called bedload. This zone is called the bed surface layer.

With increasing flow velocity, some particles are caughtby turbulent eddies. Entering the main flow region, these particlesare transported downstream by flow. Sediment supported by turbu-lent eddies and moving downstream in suspension is calledsuspended load.

With a high level of shear stress, however, not only canthe particles enter into motion on the bed surface, but also thosein the subsurface layer of the bed can do so as well. This motionpenetrates further into the bed in response to further increases inshear stress. The velocity of the moving sediment is significantlysmaller in a deeper bed. The sediment that moves in such a wayis called the laminated load.

3.1.3 Continuity of sediment movementSediment motion can be viewed as a continuum even though thesediment is classified in categories such as bed load andsuspended load according to its mode of movement. There arecontinuous exchanges between these loads as well as between thematerial in the bed and that being transported. That is, there is anexchange between suspended load and bed load, and between bedload and bed material. When a large eddy sweeps over the riverbed, a direct exchange between suspended load and bed materialcan occur.

3.1.4 Relative importance of bed load and suspended loadThe relative importance of bed load and suspended load dependson sediment size and flow velocity. For the same composition ofbed sediment, sediment slides, rolls or moves in saltation if flowvelocity is low. As velocity increases, part of the sediment iscarried into the main flow zone and becomes suspended load. Therest remains in the bed surface layer and moves as bed load, butthe thickness of the bed surface layer is augmented. Following stillfurther increases in flow velocity, the suspended load is greater,and it exceeds the bed load. In general, for ordinary river flows,sediment coarser than a certain diameter moves mainly as bedload, and sediment finer than that diameter moves mostly insuspension.

If the critical conditions for sediment incipient motion,the fall in velocity of the sediment and the nature of the turbulenceof flow are known, the patterns of sediment motion in flow can beroughly predicted, as shown in Figure 3.1 (Chien and Wan, 1983).The condition for sediment initiation using the shear velocity as

CHAPTER 3

SEDIMENT TRANSPORT IN RIVERS

Diameter D (mm)

Figure 3.1 — Zoning of sediment movements.1 – Grain Reynolds number U*D/v = 3.5; 2 – Form resistance dominates;

3 – Skin friction dominates; 4 – Fall velocity ω; 5 – Sliding, rolling and saltating;

6 – Threshold shear velocity U*

Fall

velo

city

ωor

thre

shol

d sh

ear

velo

city

U*

(cm

s-1

)

GilbertUS waterwaysExperiment Station White

the main parameter, which will be discussed later, is shown asCOD in the figure. A conclusion from the data by Nikijin is thatthe shear velocity in most zones of flow equals roughly the root-mean-square of the vertical component of the fluctuating velocity,except in the zone close to the boundary. Curve EOF in Figure 3.1represents the fall velocities of sediment of various sizes. CurvesCOD and EOF divide Figure 3.1 into several zones, and each ofthem is characterized by a different kind of sediment movement:1. In the zone below curve DOE, the fall velocity of the sedi-

ment coming from upstream is larger than the verticalcomponent of the fluctuating velocity and, therefore, sedi-ment will settle. Because the shear velocity of flow is lowerthan the critical value for sediment initiation, i.e. U*< U*c,the settled sediment will accumulate on the bed.

2. In the zone between CO and OE, the sediment in flow canremain in suspension because the fall velocity of the sedi-ment is less than the upward component of fluctuatingvelocity. However, the sediment on the bed of the same sizecannot be picked up by the flow because of the influence ofthe laminar sublayer and cohesive forces. One can say forsimplicity that sediment coming from upstream is trans-ported through the river channel without any exchange withthe bed sediment.

3. In the zone between DO and OF, the shear stress of flow isover the threshold value for initiation but the turbulence isnot strong enough for sediment suspension. Sediment movesin this zone as bed load.

4. In the zone above CO and OF, sediment cannot resist move-ment by flow and is likely to be suspended once it begins tomove. Bed load and suspended load coexist in this zone.The higher the shear velocity, the more suspended load therewill be.

3.2 BED LOAD3.2.1 Incipient motion of sediment3.2.1.1 STOCHASTIC PROPERTY OF INCIPIENT MOTION OF

SEDIMENT

Incipient motion is an important critical condition that determineswhich sediment starts to move under the action of flow. If flowintensity exceeds a certain value, sediment particles begin tomove. The flow condition that corresponds to this critical limit iscalled incipience.

Although the flow condition for which the sedimentgrains on the bed start to move is a well-defined physical concept,many difficulties are encountered in determining the actual thresh-old condition for specific cases. A typical bed surface is composedof innumerable sediment grains of various combinations of sizes,shapes, specific gravities, orientations, packing and locations.Besides, water flow also has fluctuation characteristics. Therefore,the forces exerted on sediment grains vary with both time andspace. Thus, even for uniform sediment, the grains do not all startto move or come to rest together. For non-uniform sediment, theconditions are much more complicated. Even for given flowconditions, one cannot define a specific grain size such that largerparticles remain at rest and smaller particles are all in motion.Also, the spatial distribution of sediment movement at a certaininstant is such that grains move at some places and remain at restat others. And at certain locations of the bed, sediment movesduring one time interval, and fails to move during another. Theincipient motion of sediment is clearly a stochastic phenomenon.

Obviously, if the criterion for incipient motion is deter-mined according to any one of two, three or four differentconditions, the results will be quite different.

Dou (1962) used the velocity near the bed as thehydraulic parameter to determine the incipient motion of sedi-ment. According to his analysis, in which the fluctuation of flow isconsidered, three probabilities for incipient motion that corre-spond to Kramer’s criteria (1935) for bed load movement are asfollows:1. Occasional individual motion,

pc1 = p [u0 > uc = uc– + 3σu0

= 2.11uc– ] = 0.00135 (3.1)

2. Sparse motion,pc2 = p [u0 > uc = uc

– + 2σu0= 1.74uc

– ] = 0.0227 (3.2)

3. Strong motion,pc3 = p [u0 > uc = uc

– + σu0= 1.37uc

– ] = 0.159 (3.3)

where uc– is the time average of the critical velocity near the bed,

and σu0is the standard deviation of the velocity fluctuation near

the bed.

3.2.1.2 CONDITION OF INCIPIENT MOTION FOR NON-COHESIVE

UNIFORM SEDIMENT

Here, the simplest case of non-cohesive uniform sediment isdiscussed. The hydraulic parameters for this condition can beexpressed by shear stress (drag force) or average velocity.

(A) Shear stress approach. Early in 1936, starting withthe balance of forces acting on a particle on the bed, Shields(1936) deduced the following function for the incipient motion ofnon-cohesive uniform sediment:

(3.4)

This is the formula Shields used for threshold drag force, where τc

is threshold drag force, γs is unit weight of sediment, and γ and υare unit weight and kinetic viscosity of water, respectively. Theform of the function f in Equation 3.4 must be determined byexperiment. Based on data from experiments by Shields and otherinvestigators, an average curve shown in Figure 3.2 was obtained.

Actually, there were no data for grain Reynolds numberssmaller than 2 when Shields drew the curve. Compared with therelationship between the drag factor and the grain Reynoldsnumber for a settling particle, Shields deduced that in that rangeτc/ (γs – γ) D was proportional to the reciprocal of grain Reynolds

30 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.2 — Condition for incipient motion for non-cohesivesediment (Shields curve and its modification).

τ

γ γ υ0

( )( )

s

fU D

−= ∗

Curve forlaminar flow

number. After Shields’ work, a number of other researchers,including Tixon, Li, White and Mantz studied the incipient motionof sediment. Their results are included in Figure 3.2. A belt for theincipient drag force can be drawn to represent the data. This curvehas the following characteristics:(a) It has a saddle shape. A minimum value of τc/ (γs – γ) D

occurs for Re* of about 10.(b) For Re* smaller than 2, τc/ (γs – γ) D is proportional to Re*

with an exponent of –0.3.(c) If U*D/ ν >10, the incipient drag force increases with the

increase of grain weight. If Re* is larger than 1 000,τc/ (γs – γ) D has a constant value of about 0.045.

(B) Incipient velocity approach. A relationship betweenthe velocity field and shear stress field exists. Therefore, if thedrag force for incipient motion is known, the velocity for incipientmotion can be deduced. For instance, if the logarithmic velocityformula:

(3.5)

is adopted and substituted into Equation 3.4, the latter can betransformed into the following:

(3.6)

where R is hydraulic radius Ks is roughness, and χ is the coeffi-cient. For the belt zone in Figure 3.2 with Re larger than 60, thef(U*D/ν) has a value in the range of 0.03 to 0.06. Hence,

(3.7)

For natural sediment, (γs – γ)/γ may be taken as 1.65, and theformula is then:

(3.8)

Many formulae for the critical velocity take this form. They areslightly different because the structure and coefficients of thevelocity formulae they used are somewhat different.

For instance, the Goncharov (1962) formula is:

(3.9)

and the Levy (1956) formulae are:for R/D90 > 90,

(3.10)

for R/D90 = 10~40,

(3.11)

The Shamov(1952) formula:

(3.12)

where h is the water depth.(C) Comparison of the two approaches. Although the

drag force and velocity for incipient motion provide two differentexpressions for the same phenomenon and can be mutually trans-formed from one to another, they represent two study approachesbased on two different concepts. Each has advantages and disad-vantages. The following discussion is primarily a comparison ofthem.

The incipient motion of sediment is a dynamic process.The force causing sediment motion, in the final analysis, is thedrag force exerted by flow on the particles. In practical applica-tions, an important advantage of the formula for the critical dragforce is that it can be taken as a constant for a particular flowcondition and for a specific grain size, even though it is a functionof the grain Reynolds number. In contrast, the corresponding criti-cal velocity varies with the grain Reynolds number, and it dependsgreatly on the water depth. A serious disadvantage of the dragforce concept is that the slope is included in the formula. Becausethe measurement of slope in rivers requires high precision, theresults obtained are less reliable than those based on the averagevelocity; the latter is measured regularly at hydrological stations.Furthermore, the concept of velocity and water depth is easier forpeople to visualize. Thus, the concept of critical velocity also hasconvenient features.

3.2.1.3 CONDITION FOR INCIPIENT MOTION OF COHESIVE

SEDIMENT

Sediment finer than a critical size becomes harder and harder tomove because of the cohesion between the finer grains.

In the study of critical conditions for the motion of cohe-sive sediment, two different cases arise. One is that ofunconsolidated sediment newly deposited during the naturalprocess of siltation. Another is the cohesive sediment formedduring a long-term process of deposition that has undergone phys-ical and chemical action.

(A) Incipient conditions for newly deposited cohesivesediment. The forces acting on a particle include weight, drag,uplift and cohesive force. Several such semi-empirical equationsare as follows:

Tang’s equation for incipient motion of cohesivesediment:

(3.13)

where γb is the unit weight of sediment on the bed, γb0 is the unitweight of consolidated sediment (= 1.6 g cm–3), and k is aconstant equal to 2.9 × 10–4 g cm–1 The distinguishing feature ofthe formula is that the relative consolidation of the sediment onthe bed is included in the cohesive term.

The Wuhan Institute of Hydraulic and ElectricEngineering (1961) equation for incipient motion of cohesivesediment:

(3.14)

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 31

U UK K

R

s

R

s

= ∗ =5 75 12 27 5 75 12 270. log . . log .χ τ

ρ

χ

U

gD

fU D

K

c

s

R

sγ γ

γ

υ

χ

−= ∗5 75 12 27. ( ) log .

U

gDK

c

s

R

sγ γ

γ

χ

−= ( ~ . ) log .1 1 4 12 27

U

gD K

c R

s

= ( . ~ . ) log .1 28 1 79 12 27χ

U

gD

h

D

c

sγ γ

γ

−= 1 06

8 8

95

. log.

U

gD

R

D

c = 1 412

90

. log

U

gD

R

D

c = +1 04 0 8710

90

. . log

τ γ γγ

γc s

b

b

Dk

D= − +

1

77 53 2

0

10

.. ( )

Uh

DD

h

Dc

s=−

+ ×+

−0 14

70 72

1 2

17 6 6 05 1010

.

.

/

. .γ γ

γ

U

gD

h

D

c =

1 47

1 6

.

/

The Dou (1960) equation for incipient motion of cohe-sive sediment:

(3.15)

where ha is atmospheric pressure in the water column, and d is thethickness of the water molecule, 3 × 10–8 cm.

Detailed studies on the incipient motion of clayey mudwere conducted by Migniot (1968, 1977). Clayey mud belongs tothe category of Bingham fluid. Migniot found that the incipientfriction velocity is closely related to the Bingham shear stress, asshown in Figure 3.3. If τB is less than 15 dyne cm–2, the clayeymud is in a plastic state:

U*c = 0.95τB

1/4 (3.16)

For τB

larger than 15 dyne cm–2, the clayey mudbecomes consolidated,

U*c = 0.50τB

1/2 (3.17)

(B) Incipient motion of consolidated cohesive sedi-ment. The cohesion among clayey grains is quite complicated.Knowledge in this respect is still limited. Up to now, no prop-erty of consolidated cohesive sediment has been found toestabish a good relationship with incipient shear stress or incipi-ent velocity.

3.2.2 Bed form and resistance in fluvial streamsThe bed of an alluvial stream changes with flow conditions. In theFox example in sand bedded rivers, when sediment particles areset in motion, ripples form on the bed. With the change of flowconditions, different bed forms appear, such as dunes, flat bed andsand waves, etc. Different bed forms have different roughnesses ofbed surface, consequently, this changes the resistance to flow andaffects the flow and sediment transport accordingly. Variations ofbed form and resistance are the main characteristics of fluvialstreams, and they should be studied in depth.

3.2.2.1 DEVELOPMENT OF BED FORMS

With increasing flow velocity, bed forms will experience severaldifferent stages, as shown in Figure 3.4.

Soon after some particles are in motion, a few particlesmay gather on the bed and form a small ridge; this ridge graduallymoves downstream and tends to increase in length. Finally, theridges connect with each other and ripples with a regular shapeform, as shown in Figure 3.4(b). The longitudinal cross-sections ofripples are usually not symmetrical. The upstream face is long andhas a gentle slope, and the downstream face is short and steep. Theformer is generally between 2 and 4 times as long as the latter.Ripple height is usually between 0.5 and 2 cm; the highest rippleis not more than 5 cm. The wave length normally does not exceed30 cm, and they are usually within the range of 1 to 15 cm.

With increasing flow velocity, ripples develop further andeventually become dunes (Figure 3.4(c)). Dune size is closely relatedto water depth. Figure 3.5 shows that the heights and lengths varysignificantly in different rivers (Chien and Wan, 1983).

32 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.3 — Relationship between incipient friction velocity of clayeymud and Bingham yield stress (after Migniot).

Figure 3.4 — Various phases of bed form development. Figure 3.5 — Longitudinal profiles of dunes in various rivers.

U

g

h

h

h

h

h

D

c

d

s

a a

a2

26 25 41 6 111 740=−

+ + +γ γ

γ

δ( . . ) ( )

Distance (m)(d) Klaralven River

Distance (m)(c) Mississippi River

Distance (m)(b) Volga River

Distance (m)(a) Nanjing Reach, Yangtze River

Distance (m)(e) Huayuankou Reach, Yellow River

Bingham yield stress τB (dyne cm–2)

Inci

pien

t fri

ctio

n ve

loci

ty (

cm s

–1)

Ele

vatio

n (m

)E

leva

tion

(m)

If the dune reaches a certain height and the flow velocityis then increased further, the dune decays; its wave lengthincreases and its height gradually decreases to the form shown inFigure 3.4(d). With still further increases in velocity, the bedbecomes flat again (Figure 3.4(e)).

The sediment transport rate is quite high in the secondflat bed phase. If the velocity continues to increase, the flowapproaches or becomes supercritical (Froude number of aboutunity or even larger), and the bed forms a sand wave(Figure 3.4(f)). A sand wave is a type of bed configuration that isin phase with the wave on the water surface, and these two wavesinteract strongly. The differences between a sand wave and a duneare as follows. The shape of dune is non-symmetrical, and thestreamlines of the flow separate at the dune peak; in contrast, asand wave is symmetrical, more like a surface wave; the stream-lines are almost parallel to the river bed and no separation occurs.

Sand waves can move either in the same direction as theflow, as do ripples and dunes, or in the opposite direction. Theformer is called a “downstreamward sand wave” and the latter iscalled a “upstreamward sand wave” or “antidune”. Antidunesoften form in shallow flows that are moving at high velocities.Even though the sand wave as a whole profile moves upstream,the movement and transport of every particle is in the direction ofthe flow.

In the development of antidunes, the amplitude of thesurface wave may exceed that of the sand wave by a factor of 1.5to 2. The trough of surface waves can even be below the crest ofthe sand waves (Figure 3.6) (Simons and Richardson, 1960). Inthis instance, the waves on the water surface are unstable andbreak (Figure 3.4).

If the velocity is higher than that for which sand wavesform, the undulating bed resembles that of a mountain stream,with chutes and pools. The flow is supercritical at the chutes andsubcritical in the pools. The transition from supercritical flow tosubcritical flow is achieved through a hydraulic jump(Figure 3.4(h)), and the entire bed form migrates slowly upstream.Severe erosion occurs at the chutes, and the sediment particleseroded from these regions are deposited in the pools. In naturalrivers on plains, the velocity is seldom high enough for thisphenomenon to occur.

In ordinary rivers, the most common bed features areripples and dunes. Sand waves, chutes and pools occur much lessoften. In natural rivers, the process described above may not occurin a normal progression; various types of bed forms can exist atthe same time, and the process of development may differ fromone instance to another.

3.2.2.2 FLOW RESISTANCE IN ALLUVIAL STREAMS

As discussed above, with the change of flow conditions, variousbed configurations form on the bed surface of alluvial streams.During the ripple and dune phases, the flow separates at the crestof the bed form so that the pressure on the downstream andupstream sides differ. The net force thus produced is the formresistance. During the sand wave phase, the undulation of the sandbed is much more pronounced than it is in the ripple and dunephases, but the sand waves have a symmetrical shape with no flowseparation at their crests. Therefore, their form resistance issmaller and the energy loss is less than that for ripples and dunes.The corresponding energy loss is only a little more than that for aplane bed, because the breaking of the sand waves generates astrong local turbulence that dissipates parts of the flow energy. Asa major component of resistance, form resistance changes as theflow conditions change. Hence, the friction factor in an alluvialriver is not just a constant, but varies with flow conditions.

Einstein (1950) suggested that the resistance of an allu-vial stream consists of bed resistance and bank resistance.Furthermore, the bed resistance consists of grain friction and bedform resistance. Although grain friction and bed form resistanceboth act on the bed surface, the ways in which they affect themovement of bed material are different. The formation of bedform resistance is the result of the separation of flow at the peaksof sand waves and the unsymmetrical distribution of pressure onthe stoss and lee faces. The turbulence created by bed form resis-tance occurs mainly in the separation on the lee face, and it occursat some distance from the bed grains. The role of the eddy createdby bed-form resistance on bed load movement is thus not as directas that from the grain friction. The eddy created by the corre-sponding flow potential energy from grains on the channel bedplays a large role in the transportation of bed material for grainfriction only.

A. Einstein’s approach. The bed roughness of an alluvialchannel consists of two parts, namely, grain roughness or skinroughness due to the sediment particle size, and form roughnessdue to the existence of bed forms. According to Einstein, the shearstress or drag force acting along an alluvial bed can be dividedinto two parts, i.e.,

τ = τ' + τ'' (3.18)

= γJ (R'b + Rb

'')

where τ is the total drag force acting along an alluvial bed, τ' andτ'' are the drag force due to grain roughness and form roughness,respectively, γ is the specific weight of water, J is the energy orchannel slope, and R'

b and Rb'' are the hydraulic radii due to grain

roughness and form roughness, respectively.The grain friction denotes the resistance to a two-dimen-

sional flow, which is not affected by side banks, with a plane bed.The grain friction can be described by the following equation:

(3.19)

where R'b is the hydraulic radius due to grain friction, Ks is a

representative roughness, which is taken as D65, the particle sizeof bed material of which 65 per cent by weight is finer, byEinstein, χ is a function of Ks/δ, and δ = 11.6υ/U* the thickness oflaminar sublayer . The relationship between χ and Ks /δ isshown in Figure 3.7.

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 33

Figure 3.6 — Antidune on the verge of breaking (after Simons andRichardson).

U

U (

R '

K)

*

b

s= 5.75log 12.27

χ

U gR Jb*'=

Based on data from 10 rivers in the United States,Einstein and Barbarossa (1952) established a relationship for bedform resistance U/ U*'' = F(Ψ’), as shown in Figure 3.8. Where:

(3.20)

where D35 is the particle size of sediment of which 35 per cent byweight is finer, R 'b the hydraulic radius due to grain friction,

, and R ''b is the hydraulic radius due to bed formresistance. With an increase of flow intensity, i.e. a decrease of ψ ',dunes tend to diminish, and the dune resistance decreasescorrespondingly.

Among the 10 rivers analysed by Einstein andBarbarossa, eight had values of D35 smaller than 0.5 mm, and theother two had D35 values of 0.7 and 1.0 mm. In later experimentswith coarser bed materials, the result departed from the meancurve of the 10 rivers of the United States, as shown in Figure 3.8.The bed form resistance for coarse sand was shown to be smallerthan that for medium and fine sand.

The following procedures are for the computation oftotal hydraulic radius due to grain and form roughness when thewater discharge and bed material are given, or vice versa.1. Assume a value of Rb'.2. Apply Equation 3.19 to determine U by R'b and D65 (= Ks).3. Compute Ψ' using Equation 3.20 and the corresponding

value of U/U*'' from Figure 3.8.

4. Compute U*'' and the corresponding value of Rb''.5. Compute Rb = Rb' + Rb'' and the corresponding channel

cross-sectional area A.6. Verify using the continuity equation Q = UA. If the

computed Q agrees with the given Q, the problem is solved.Otherwise, assume another value of Rb' and repeat the proce-dure until agreement is reached between the computed andthe given Q.

B. Engelund and Hansen’s approach (1972). The rela-tionship between grain friction and total resistance shown inFigure 3.9 is considered to be reliable and is popularly used.

According to Engelund and Hansen, the shear stress ofdrag force acting along an alluvial bed can be divided into twoparts, i.e.,

τ = τ' + τ'' (3.21)

or γhJ = γh (J' + J') (3.22)

where J' and J'' are energy loss or friction slope due to grain fric-tion and that due to bed form, respectively. Engelund thinks thetwo expressions of grain friction are equivalent and interchange-able.

τ' = γhJ' = γh'J (3.23)

Divided by (γs – γ)D, Equation 3.22 turns into:

Θ = Θ' + Θ'' (3.24)

where (3.25)

This parameter is simply the inverse of the Einstein flowparameter, Ψ.

The abscissa is the parameter for the flow intensity dueto grain friction:

(3.26)

And the parameter for the flow intensity related to bed form Θ'' is:

(3.27)

34 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.8 — Relationship between bed form resistance and flow para-meter compared with experiment (after Einstein and Barbarossa).

Figure 3.7 — χχ versus Ks/δδ..Figure 3.9 — Relationship between grain friction and total bedresistance (after Engelund and Hansen).

Θ γ γγ

= −hJ

Ds

Θ γ γγ

' '= −

h J

Ds

Θ γ γγ

Θ Θ""

– '= − =h J

Ds

′′ = ′′U gR Jb*

′ =−

′Ψ γ γ

γs

b

D

R J35

stationary sandwave and flat bed

Missouri River near Fort Randell, S.D.Missouri River near Pierre, S.D.Missouri River near Omaha, Nebr.Elkhorn River near Waterloo, Nebr.Big Sioux River near Akron, IowaPlatte River near Ashland, Nebr.Niobrara River near Butte, Nebr.Salinas River at San Lucas, CaliforniaNacimiento River near Junction, Calif.Salinas River at Paso Robles(easily vegetated)'/U'' *

Per

cent

The lower branch in Figure 3.9 corresponds to the dune phase.

Θ ' = 0.06 + 0.4Θ2 (3.28)

As Θ decreases, Θ ' gradually approaches the constant value of0.06, which corresponds to the condition of incipient motion. IfΘ >0.4,

Θ ' = 0.4Θ2 (3.29)

In contrast, for high transport rates of sediment and with sandwaves forming on the bed, the data fall near the other curve. Forflat bed or for stationary sandwaves without local enlargementloss,

Θ ' = Θ (3.30)

But in the sand wave phase, as a result of the additional energyloss caused by the breakage of the water surface, Θ ' is smallerthan Θ. Engelund was able to express the resistance losses for allphases of bed configuration, except for the ripple phase, in asingle figure.

The following procedure is to determine a stage-discharge relationship by using Figure 3.9.Step 1: Determine J and h from a field survey of slope and

channel cross-section.Step 2: Compute Θ from Equation 3.25 for the given sediment

size D.Step 3: Determine Θ' from Figure 3.9 with Θ from Step 2.Step 4: Compute h' from Equation 3.26.Step 5: Compute U from Equation 3.19. In the case of two

dimensional flow, Rb'= h'. Correspondingly, Rb'' = h'',Rb = h.

Step 6: Determine the channel cross-sectional area A correspond-ing to the h value selected in Step 1.

Step 7: Compute Q = UA. The stage-discharge relationship canbe determined by selecting different h values and repeat-ing the processes.

3.2.3 Bed load transport3.2.3.1 TRANSPORT OF UNIFORM BED LOAD

A number of formulae for bed load transport have been proposedby scientists. These formulae are based on different modes ofmotion and employ different parameters, including shear stressand flow velocity. Several representative ones are briefly intro-duced, as follows.

FORMULAE WITH SHEAR STRESS AS THE MAIN PARAMETER

(a) The Meyer–Peter formula. Based on the data of a greatnumber of experiments, Meyer-Peter (1934, 1948) developedthe following bed load formula by isolating involved para-meters one by one.

(3.31)

where Q (= BhU) is the total discharge through the cross-section,and Qb is the part of the discharge pertaining to the bed:

Qb = BRbU (3.32)

where Kb is a coefficient for bed resistance, Kb' is the roughnesscoefficient due to grain resistance, gb is the rate of bed load trans-port per unit width by dry weight, and a and b are constants. Theformula was calibrated against measured data, as shown in Figure3.10, in order to determine the two constants.

The Meyer-Peter formula is based on a large quantity ofexperimental data. The main variables in the experiments variedwithin the following ranges:

Width of flume: 0.15–2 mFlow depth: 0.01–1.2 mEnergy slope: 0.04–2 per centDensity of sediment: 1.25–4 g cm–3

Diameter of sediment: 0.40–30 mmThe Meyer-Peter formula is more reliable than some

others for rivers carrying coarse sand and gravel. It has been widelyused and the results obtained from it are generally satisfactory.(b) Einstein bed load theory. Einstein noticed the stochastic

nature of bed load motion and combined statistics withmodern fluid mechanics. Applying probability theory andmaking some hypotheses, Einstein (1950) derived a mathe-matical expression for the relationship between the bed loadtransport intensity Φ and the flow parameter Ψ:

(3.33)

where

(3.34)

(3.35)

where the constants, determined through experiments, are asfollows:

1/η0 = 2.0 (3.36)

(3.37)

B*=1/7 (3.38)

Figure 3.11 is a comparison of the function withmeasured data, and it shows that the function represents the dataquite well.(c) The Engelund formula. Engelund and Fredsφe (1976) treated

sediment particles as spheres of diameter D, so that there areapproximately 1/D2 spherical particles in a unit area of thebed surface. For a certain flow intensity, the proportion of

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 35

gQ

Q

K

KhJ a D b

ggb b

bs

s

sb′

= −( ) +

−

3 2

4 4

1 3 2 32 3

/ / //γ γ γ γ γ

γ

Figure 3.10 — Comparison of the Meyer-Peter formula with measureddata.

11

1

1 2

0

0

1−

− −

−−∫ =

+π ψ η

ψ ηe dt

A

A

t

B

B

* /

* / *

*

Φ

Φ

Φ =−

g

gD

b

s sγ

γ

γ γ

12 1

21

3

ψγ γ

γ=

−s

b

D

R J /

A*.

=1

0 023

γ

the particles on the bed surface that are moving is p. Themean velocity of the bed load particles is ub. Hence, the rateof bed load transport gb is given by:

(3.39)

Based on the balance of forces acting on particles moving as bedload, the following equation can be derived:

(3.40)

where Θc is the Θ value at the incipient motion of particles, α is aconstant for a sandy river bed, and α = 9.3. They deduced theconclusion that the proportion of the particles on the bed surfacethat are moving, p, is:

(3.41)

where β is a kinetic frictional coefficient. CombiningEquations 3.39 and 3.40 with Equation 3.41, Engelund andFredsφe obtained their bed load formula:

(3.42)

Based on data from flume experiments, Θc = 0.046, and β = 0.8.(d) The Ackers-White formula. Ackers and White (1973)

collected 1 000 sets of experimental data from previousresearchers. Following Bagnold’s approach, they derived afunctional relationship between dimensionless parameters.Then they conducted a regression analysis with the data todetermine a functional relationship. Their formula includesboth bed load and suspended load. Nevertheless, the formulawas simplified into a bed load formula for natural sandcoarser than 2.5 mm, in the following form:

(3.43)

where (3.44)

(3.45)

where SWb represents the average concentration of sediment loadin weight per unit volume.

BED LOAD FORMULAE WITH VELOCITY AS THE MAIN PARAMETER

In the former USSR, scientists employed the average velocityinstead. These formulae can be rewritten in a more general form asfollows:

gb = gshbSvbu–b (3.46)

where hb stands for the thickness of bed load layer and Svb for thevolume concentration of bed load in the layer.

Different researchers made various assumptions abouthb, Svb and ub, and thus they obtained different formulae. Table 3.1presents three representative ones. The three share much in theirapproach, although they differ in details.

COMPARISON OF BED LOAD FORMULAE

Having thoroughly analysed the bed load formulae presented inthe preceding section, Chien (1980) pointed out that these formu-lae have common properties and give similar results under certainconditions, even though they have different forms.

The comparison is based on the following conditions:First, the channel bed is flat and the characteristic roughness is thesediment diameter. Second, except for the Ackers-White formula,the threshold condition of the initiation of bed load motion istaken as Θc = 0.047.

Figure 3.12 shows a comparison of the Meyer-Peter,Bagnold, Einstein and Yalin formulae. The figure shows that for Ψ > 2, the Bagnold, Einstein and Meyer-Peter formulae areclose together, but the Yalin formula yields smaller values for thebed load transport rate. Figure 3.12 also shows that for low inten-sity of bed load transport, the Φ – Ψ curves slope gently. That is,if Θ << 1, a slight variation in Θ responds to great change in Φ.This trend is more apparent if Θ is close to Θc. In other words, bedload transport is quite sensitive to the flow if the transport inten-sity is low. Figure 3.13 also shows that the bed load formulaediverge for Ψ < 2. The Φ – 1/Ψ curves in this range approachstraight lines on a log-log plot. The Meyer-Peter, Bagnold, Yalin

36 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.11 — Comparison of Einstein bed load function withmeasured data (uniform sediment) (after Einstein).

u

U

s c

*

.= −

α 1 0 7Θ

Θ

p c= −6

πβ( )Θ Θ

gD

Ubs

c c= − −9 3

0 7.

( )( . )*β

γ

ΘΘ Θ Θ Θ

Y 0.025 M

0.171

1.5

= −

MU

g Dh

Ds

=−γ γ

γ

1

3210

YS h

D

Wb

s

=γ

γ

g Dp

Du

b s b=π

γ6

32

Figure 3.12 — A comparison of Meyer-Peter, Einstein, Bagnold andYalin formulae.

Gravel 28.65 2.68Sand 5.20 2.68Brown coal 5.20 1.25 Meyer-Peter

Bary grains 5.20 4.22Sand 0.785 2.68 GilbertPlastic 4.75 × 3.18 × 2.38 1.052 ChienSand D42 – 1.26 2.68Plastic 3.88 1.13

Wilson

Symbol Material Diameter Specific Author(mm) gravity

Meyer-PeterBagnoldYalinEinstein

Φ

Ψ(Θ

)

and Engelund formulae approach lines indicating an exponent of1.5 on the (1/Ψ) term. In contrast, the Einstein bed load functionapproaches the line:

(3.47)

and the exponent is therefore 1. The exponent for the Ackers-White formula is 1.35–1.45, a value that falls between the othertwo values. A serious difficulty arises from the suspension of thematerial with high intensity bed load transport; in this case, onecannot readily separate suspended load from bed load. So far, thedata for high intensities of bed load motion are insufficient, there-fore one cannot conclude which formula is the best one to use.

3.2.3.2 TRANSPORT OF NON-UNIFORM BED LOAD

The foregoing is only for uniform sediment. However, sediment innatural rivers is always non-uniform. Two techniques are used todeal with non-uniform bed load motion. If only the total bed loadtransport rate is required, the bed load formulae introduced insection 3.2.3.1 can be used directly, but a representative diametermust be determined. If instead the transport rates of various diam-eters are required, the mutual effects of the various particle sizesmust be studied.

DETERMINATION OF REPRESENTATIVE DIAMETER FOR

CALCULATING TRANSPORT RATE OF NON-UNIFORM BED LOAD

Einstein found from data measured in both small streams andflume experiments that D35 can be used as the diameter in the bedload formulae. D35 stands for the diameter for which 35 per centof the bed material is finer. Meyer-Peter (1948) suggested anotherform of representative diameter:

(3.48)

where ∆pi stands for the percentage of particles of diameter Di inthe bed material. Chien examined the two representative diame-ters, with the results presented in Figure 3.13.

The results show that Dm is preferable to D35 for lowintensities of bed load motion, but no difference was foundbetween the two for high intensities.

BED LOAD TRANSPORT RATES OF VARIOUS GRAIN SIZES

Many engineering situations require not only calculating the totalbed load transport rate but also the transport rates of the various

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 37

Figure 3.13 — Comparison of measured bed load transport rates fornon-uniform sediment with results calculated using differentrepresentative diameters.

DD p

mi i

=∑ ∆

100

Author u–b hb Svb gb (kg/m/s) Valid range Note

0.2 < D < 0.73 mm, K' and K'' areand 13 < D < 65 mm function of D.

Sharmov 1.02 < h < 3.94 m Therefore the(1959) K'D 0.18 < h < 2.16 m formula includes

0.4 < U < 1.02 m s-1 the square root of0.8 < U < 2.95 m s-1 D instead of D.

0.25 < D < 23 mmLevy (1957) α' (U – Uc) α''D 5 < h/D < 500 ______

1 < U/Uc < 3.5

The coefficient 3is suitable for

0.08 < D < 10 mm river flow and 5Goncharov 10 < h/D < 1 550 for flumes; ς is a(1962) 0.72 < U/Uc < 13.1 coefficient related

to turbulence.

Table 3.1Bed load formulas with velocity as the main parameter

UU D

h

c−

1 2

1 4

.

/

′′

KU

Uc

1 2

3

.

0 95

1 2

1 2

3

1 4

.

.

.

/

DU

U

UU D

h

c

c

⋅ −

2

3

1 4

DU

gD

U UD

hc

( )

⋅ −

/

3 0 5 3 1

1 4

1

1 4

3

3

. – .

.

.

( )( )

+

⋅ −

⋅ −

ζ D

U

U

UU

c

c

αα

α

ζ

4

6

3

2

2

1 1 4

1 1 4

1 4

+

+

⋅

U

UU

U

U

U

c

c

c

.

.

.

α αζ1

1 4

1 4

+

⋅−

( )D

UU

U

c

c

.

.

α k

c

U

U

U1

1 4

3

3−

.

′′′

⋅

αU

gD

D

h

3

1 4/

(a) Dm as therepresenta-tive dameter

(a) Dm as therepresenta-tive dameter

Einstein bed load functionMeyer-Peter formula

Slightly non-uniform natural sandExtremely non-uniform natural sandSlightly non-uniform plastic material

Non-uniform natural sand

ΦΨ

=7 9.

grain sizes of non-uniform sediment. For example, both the finingprocess in the upstream reaches of a dam and an armouring layerin the downstream reaches require such a calculation.

Few researchers have studied the movement of varioussizes of non-uniform sediment because the mutual interactionsbetween the various sizes are complex. Some results of Einstein(1950) and Chien are presented here.

The following formula was obtained by Einstein:

(3.49)

and it is suitable for various groups of grain sizes:

(3.50)

(3.51)

where i0 and ib are the percentage of sediment with a size D in bedload and that in bed material, respectively. Y, a function of∆ = Ks/χ as shown in Figure 3.14, is a correction factor for liftforce, and X is the maximum grain size subject to the hiding effectin a sediment mixture.

Under the conditions of a rough bed, i.e., ∆/δ >1.8:

X = 0.77∆ (3.52)

Under the conditions of a smooth bed, i.e., ∆/δ <1.8:

X = 1.39δ (3.53)

β = log 10.6 is a constant. ξ , a function of D/X as shown inFigure 3.15, is a factor concerning the hiding effect for particlesfiner than X.

The procedure for the computation of bed load of differ-ent grain sizes from Einstein’s bed load transport function is asfollows:Step 1: From the given bed material and flow condition, compute

Ψ* from Equation 3.50. The values of ξ and Y can bedetermined from Figures 3.14 and 3.15. The value of βx

can be determined from Equations 3.51 and 3.52.Step 2: From Figure 3.11, determine Φ*.Step 3: Bed load by weight per unit width of a given size ibgb can

be computed from Equations 3.48 and 3.50.Step 4: Repeat the preceding steps for each size fraction and get

ibgb for each size fraction.Step 5: Sum up the results over the size range for bed material

and get a total bed load.

TRANSPORTATION OF EXTREMELY NON-UNIFORM SEDIMENT

Einstein and Chien (1953) carried out experiments with sedimentswith a wide size range. Their experiments revealed that the bedmaterial was sorted by the flow, with large and small particlesgathering at different places. If sorting occurs, coarse particles arecovered by a layer of fine sediment, so that the coarse sand shield-ing zones are much fewer and the sheltering effect on themovement of fine particles is also less. Such an effect is consid-ered by introducing a factor θ, which is a function of the grainReynolds number, into the lift force. And after some modifica-tions, the flow parameter Ψ* is redefined as:

(3.54)

Details can be found in the references (Einstein andChien, 1953).

3.2.3.3 CHARACTERISTICS OF TRANSPORT OF GRAVEL BED LOAD

FLUCTUATION AND BURSTING

Sediment transport is a stochastic phenomenon, and the stochasticcharacteristics of gravel bed load are even more obvious. Underalmost unchanged flow conditions, the transport rate of gravel bedload might vary within a wide range.

A parameter ∆ξ is used to denote the variation of gravelbed load:

∆ξ = (gbmax – gbmin)/gb (3.55)

where gbmax is the measured temporal value corresponding to a95 per cent frequency of varying gravel bed load, gbmin is themeasured temporal value corresponding to a 5 per cent frequencyof varying gravel bed load, and gb is the average value of varyinggravel bed.

According to field data obtained from gauging stationsalong the Upper Yangtze River, ∆ξ varies within the range of 5 to8. In Tan’s (1983) paper, field data obtained from Inner River,Dujiangyan, were cited. Under conditions with almost constant

38 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.14 — Y – Ks/δδ..

βx

X=

log .10 6

∆

Figure 3.15 — Hiding effect on movement of fine particles in non-uniform sediment.

Ψ Ψ∗ =( )ξ β β

θ

Y X/2

11

1

2

0

0

1

1− =

+−

− −

−∫π ψ η

ψ ηe dt

A

A

t

B

B

* * /

* * / * *

* *

Φ

Φ

Φ Φ

Ψ Ψ

*

*

=

=

i

i

Y

b

X

0

2

2ξ

β

β

Ks/δ

ξ

D/X

discharge, the rates of gravel bed transport varied greatly. Theratio of rates of gravel bed load of two neighbouring measure-ments was taken as an index: more than half of the indices werelarger than 5. The maximum ones reached 500 to 700.Consequently, field data of gravel bed load should be treated care-fully. In order to avoid inaccurate results, a long-term series offrequent measurement may be required.

LATERAL DISTRIBUTION OF GRAVEL BED LOAD DISCHARGE

Owing to the uneven distribution of flow, bed material and incom-ing sediment, the transport of gravel bed load in the lateraldirection is uneven too. In many cases, the transport of gravel bedload is limited to a certain zone within the full width, as shown inFigure 3.16. The region where the transport of gravel occurs issometimes called the belt of gravel transport. The belt of graveltransport changes with variations in flow conditions, as shown inFigure 3.16. The highly three-dimensional characteristics of graveltransport should be noticed while dealing with field data.

ARMOURING PROCESS (YANG, 1997)When the sediment transport capacity of a channel exceeds therate of sediment supply from upstream, the channel may bedegraded. Because of the non-uniformity of the bed material size,finer materials will be transported at a faster rate than the coarsermaterials, and the remaining bed material becomes coarser. This

coarsening process will stop once a layer of coarse materialcompletely covers the streambed and protects the finer materialsbeneath it from being transported. After this process is completed,the streambed is armoured and the coarser layer is called thearmour layer. A definition sketch of armouring is shown inFigure 3.17. From this,

Ya = Y – Yd (3.56)

where Ya is the thickness of the armour layer, Y is the depth fromoriginal streambed to the bottom of the armouring layer, and Yd isthe depth from the original streambed to the top of the armouringlayer or the depth of degradation.

Based on the definition of armouring layer thickness,

Ya = (∆p)Y (3.57)

where ∆p is the decimal percentage of material larger than thearmouring size.

From Equations 3.56 and 3.57,

Yd = Ya (1/∆p – 1) (3.58)

The required armour layer thickness varies with the sizeof the armouring material. Usually, two to three armouring particlediameters or 0.5 ft, whichever is smaller, should be sufficient.

SELECTED EROSION

In his study of incipient motion, Gessler (1971, 1972, 1976)considered the effect of flow fluctuations. In turbulent flow,

τ0 = τ–0 + τ'0 (3.59)

where τ0, τ–0 and τ'0 are the instant, temporal average and fluctuat-ing value of the drag force exerted by the flow on the bed,respectively. If:

τ0 < τc (3.60)

and also (3.61)

sediment cannot be entrained.

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 39

Figure 3.16 — Lateral distribution of gravel transport.

Figure 3.17 — Definition sketch of streambed armouring. Figure 3.18 — q versus τc/τ0.

Ele

vatio

n (m

)4 b

[kg

(s m

-1)]

Dm

in(m

m)

U (

m s

-1)]

τc/τ0

Prob

abili

ty (

q)

€

ττ

ττ

'0

0 01< −c

From numerous flume experiments, Gessler concludedthat the fluctuation of drag force follows a normal error distribu-tion. Thus, the probability of sediment staying on the bed is:

(3.62)

where σ is the standard deviation of τ '0/τ–0; the relationshipbetween q and τc/τ0 is shown in Figure 3.18.

Because the probability of incipient motion of the sedi-ment with grain diameter D is (1-q), the size distribution curves ofthe bed materials washed away and remaining on the bed can bederived. If the maximum and minimum grain diameters of theoriginal bed material are known and the weight percentage ofgrain with diameter of D is p0 (D), the accumulated percentage ofthe sediment with a diameter less than D is:

∫Dmin

Dp0 (D)dD (3.63)

For the armouring layer of the bed after scouring, the frequency ofgrains with diameter D is:

pa (D) = C1qp0dD (3.64)

where the coefficient C1 can be determined by means of thefollowing equation:

∫Dmin

Dmax pa (D)dD = 1 (3.65)

Thus, the sediment size distribution of the armouring layer is:

(3.66)

and the size distribution of the sediment washed out is:

(3.67)

3.3 SUSPENDED SEDIMENT TRANSPORT

3.3.1 Mechanism of sediment moving in suspensionSuspended sediment transportation is closely related to the turbulentbursting phenomenon. The following frames show the observedphenomena and the related mechanism of particle suspension for asmooth bed (Table 3.2). If the low-speed streak of flow near the bedis lifted due to a burst of turbulence, the sediment there is carriedupward. If the fall velocity of a particle is large, the particle willquickly fall back to the bed. Such particles are part of the saltationload. If, in contrast, the fall velocity is small, the sediment can becarried upward along with the low-speed water element until thelatter breaks up; at that moment the sediment has reached its highestposition and begins to settle back down. As the particles fall, someof them, caught in the downward moving part of the high-speedstreak of flow, will return to the near-bed region, while others,caught in an upward-moving eddy, are lifted again. The higher theturbulence intensity and the smaller the particle size, the greater the

portion of the particles that are lifted up. In this way, some sedimentis kept in suspension. However, in the process a continuousexchange occurs between the suspended sediment and the sedimentin the near-bed region.

3.3.2 Diffusion equation and vertical distribution ofsuspended sediment

In turbulent flow, the movement of water elements, and how theychange positions between water layers, also causes sedimentexchanges between the layers. At the same time, sediment parti-cles, because of their greater specific weight, tend to settle andmove toward the bed. As a result, the sediment concentration isgreater near the bed than it is at a point some distance above thebed. Because of this variation in concentration, water elementsmoving upward carry a greater amount of sediment than the waterbodies moving downward. Thus, the exchange between theupward and downward water elements of the same volume resultsin a net transport of sediment in the upward direction. The amountof the upward sediment flux per unit horizontal area is propor-tional to the concentration gradient dSv/dy and is written as–εydSv/dy. The amount of the downward sediment flux per unithorizontal area due to settling is written as ωSv. Here, only thevertical concentration profile of suspended load carried by a two-dimensional flow in a state of equilibrium is studied. Under suchconditions, the upward sediment flux equals the downward sedi-ment flux:

(3.68)

where ω is the fall velocity of sediment particles, Sv is the sedi-ment concentration in volume, and εy is the sediment exchangecoefficient. In order to solve the differential Equation 3.68, onemust determine the vertical distribution of εy. The simplest proce-dure is to assume that it is a constant. The solution is then:

(3.69)

40 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

qp D dD

qp D dD

D

D

D

D

0

0

( )

( )

min

min

max

∫∫

( ) ( )

( ) ( )

min

min

max

1

1

0

0

−

−

∫∫

q p D dD

q p D dD

D

D

D

D

Phenomenon

Sediment particles arelifted up from the bed

Sediment in the near-bed region ispicked up and lifted by the upward

moving low-speed band of flow

Sediment reaches its highestposition as the burst breaks up

Particles are entrained by waterbodies with large momentum,

and swept away

Sediment falls into another eddythat is moving upward

As a high-speed region of the flowreaches the bed, it spreads towardboth sides (in the z direction), and

carries sediment into theneighbouring low-speed region

The highest position isreached by particles

being lifted

Particles start to fall

Some particles fall intothe near bed region

Other particles are liftedup again before entering

the near bed region

Mechanism

Table 3.2Observed phenomena and related mechanism of particle

suspension for a smooth bed

€

ε ωyv

v

dS

dyS+ = 0

qx

dxc

=−

−∞

−∫1

2 2

2

201

σ π σ

τ

τ exp

S

Sev

va

y a y=− −ω ε( ) /

where Sva is a reference concentration of the suspension atdistance a above the bed. The experimental results of Hurst agreewell with the formula in Equation 3.69. Later on, Rouse obtainedthe results shown in Figure 3.19 using a series of grids moving insimple harmonic motion in a cylinder. The figure shows that theexperimental results essentially follow the theoretical curve for allexcept the coarsest particles. Lane and Kalinske made analyses ofdata from a natural river and found that Equation 3.69 also gavesatisfactory results for this practical case. They suggested that εy

could be expressed as follows:

(3.70)

where κ is the Karman constant in the logarithmic formula for thevelocity distribution. If the usual value of κ = 0.4 is taken, then:

εy = 0.067U*h (3.71)

The sediment exchange coefficient is not nearly aconstant, but is a function of position in space. From the theory ofturbulent flow, the diffusion coefficient is equivalent to themomentum exchange coefficient εm, and it is related to the veloc-ity gradient in the following way:

(3.72)

For simplicity, one assumes that:

εy = εm (3.73)

For two-dimensional flow, the shear stress is linearly distributedalong the depth, so that:

(3.74)

where τ0 stands for the shear stress at the bed. For a logarithmvelocity profile:

(3.75)

Differentiating, one obtains:

(3.76)

Then, substituting Equations 3.74 and 3.76 into Equation 3.72yields:

(3.77)

By integrating this expression and taking the average, one obtainsEquation 3.70.

The substitution of Equation 3.77 into Equation 3.68yields:

(3.78)

after integration, this gives the vertical concentration profile ofsuspended load:

(3.79)

where (3.80)

For a dune-covered bed, and in the absence of moreexperimental data for U*, Einstein suggested that U* can bereplaced by the shear velocity relevant to grain frictionU'

*= (R

b'gJ)0.5.

The exponent z in the expression for suspended loadaffects the distribution of the sediment concentration. Figure 3.20compares the relative vertical distributions of suspended loadconcentration obtained from Equation 3.79. The figure shows thata smaller value of z results in a more uniform distribution. Thus,the height of the suspension is also a function of z. In the case ofz = 5, the amount of sediment carried in suspension is very small;

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 41

Figure 3.19 — Vertical distribution of sediment concentration forvarious particle sizes in a sediment mixture (Rouse experiments withuniform stirring) (after Rouse).

ετ

ρm

du

dy

=

τ τ=

0 1 –

y

h

du

dy

U

y= ∗

κ

1

ε ε κy m U yh y

h= =

−*

u

U

y

y*

ln=

1

0κ

κ ωU yh y

h

dS

dySv

v*

−+ = 0

S

S

h y

y

a

h a

v

va

z

=−

−

zU

=ω

κ *

Figure 3.20 — Relative distribution of suspended load obtained fromthe diffusion theory (after Rouse).

εκ

y

U h= *

6

Sv/Sva

ωω

=

−(y

a)

yε

LnSv

Sva y

(y a)

ω =

−ε

1/4 mm

1/8 mm

1/16 mm

1/32 mm

the discharge ratio of suspended load to bed load is then 1:4,according to an estimation based on the Einstein method. Fromthe practical point of view,

(3.81)

can be taken as the threshold value for sediment suspension.However, various researchers have used other threshold values.Bagnold (1966) used the value 3 and Engelund (1965) the value 2;these values yield ratios of suspended load to bed load of 2:1 and0.9:1, respectively.

Since Equation 3.79 was derived analytically in the1930s, a number of studies have been conducted to test the diffu-sion theory against field observations and laboratory data. Theverification has two aspects: whether the formula structure iscorrect, and whether the analytical expression for the exponent z isvalid. The conclusion is as follows. The formula structure is essen-tially correct; but there is a certain deviation between themeasured exponent z1 and the analytical expression z.

3.3.3 Transport rate of suspended loadIf vertical profiles of both the concentration Svy and the velocity uy

are known, the discharge of suspended load passing through across-section of unit area at y per unit time is uySvy; integration ofuySvy over the depth yields the discharge of suspended sedimentper unit width. In practical applications, one difficulty remainsbecause the diffusion theory gives only a relative quantity of sedi-ment concentration. From Equation 3.79, the concentration at anyposition remains unknown unless Sa, the concentration at thereference position at distance a above the bed, is known.

Another difficulty is that the upper and lower limits forthe integration need to be determined. The simplest way is to inte-grate from the bed to the free surface to get the total sedimentdischarge. But both velocity and sediment concentration approachinfinity at y = 0 according to the logarithm velocity distributionformula and Equation 3.79.

Here the Einstein (1950) method of dealing with thesetwo difficulties is introduced. According to Einstein’s concept, theregion near the bed is called the bed layer. In the bed layer, sedimentparticles move as bed load by sliding, rolling or saltating. The lawof bed load motion is completely different from that of suspendedload. Since the bed load motion is dominant in the bed layer, i.e.,the layer below the suspension region and above the bed, theextension of the concentration distribution for the suspended loadto the near-bed region is not theoretically feasible.

If a in Equation 3.79 denotes the thickness of the bedlayer, then the suspended sediment discharge per unit width can beexpressed as:

gs = γs ∫ahSvyuydy (3.82)

If (3.83)

is used, then after substituting the logarithm velocity distributionformula and Equation 3.79 into Equation 3.82 and simplifying,one obtains:

(3.84)

where

(3.85)

(3.86)

Clearly, I1 and I2 are functions of A and z, and their values can beobtained by numerical integration with the results shown inFigures 3.21 and 3.22.

Einstein’s equations can be applied to compute thesuspended load discharge for given flow and sediment with thefollowing procedure.

42 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Aa

h=

g U S ah

I Is s va= ⋅ +

11 6 2 30330 2

1 2. . log.

*γ∆

IA

A

y

ydy

IA

A

y

yydy

z

z

z

A

z

z

z

A

1

11

2

11

0 2161

1

0 2161

1

=−

−

=−

−

−

−

∫

∫

.( )

.( )

ln

Figure 3.21 — Relationship of I1 and A for suspended sedimentdischarge with z as a parameter (after Einstein).

Figure 3.22 — Relationship of I2 and A for suspended sedimentdischarge with z as a parameter (after Einstein).

A

ω

κU*

= 5

I 1I 2

Step 1: Compute a = 2D, U* = (ghJ)1/2

Step 2: Compute ∆ = Ks/χ, where Ks= D and χ can be obtainedfrom Figure 3.7

Step 3: Compute a = 2D, A = a/hStep 4: Compute z = ω/(κU*)Step 5: Get I1 from Figure 3.22 and I2 from Figure 3.23Step 6: Compute Sva = ibgb/(11.6 × 2DU*) (details of determin-

ing Sva are discussed in section 3.4)Step 7: Compute gs from Equation 3.84

3.3.4 Non-equilibrium transport of suspended sedimentThe vertical concentration distribution of suspended load forsteady uniform flow is treated in the preceding section. Thissection treats the special case of non-equilibrium sediment trans-port in which the distribution of concentration varies in thestreamwise direction even though the flow of water is steady anduniform. Typical examples of such a transport are the degradationprocess induced by clear water erosion downstream of a newlybuilt dam and the aggradation process in a settling basin.

For simplicity, the following approximations are introduced.1. Sediment motion is steady:

∂Sv / ∂t = 0 (3.87)

2. The streamwise variation of the sediment exchange coef-ficient is negligible:

∂εx / ∂x = 0 (3.88)

3. The second derivative of sediment concentration withrespect to x is negligible compared to that in the y direction.

∂2Sv / ∂x2 << ∂2Sv / ∂y2 (3.89)

For these conditions, the diffusion equation of sedimenttransport becomes:

(3.90)

For uniform sediment, the equation of non-equilibriumsediment transport is the solution to this differential equation withsuitable boundary conditions.

The recovery of sediment concentration along the flowdirection by scouring is discussed first. If the variation of sedimentexchange coefficient with elevation can be neglected and its depth-averaged value is used, then Equation 3.90 can be furthersimplified,

(3.91)

Hou, et al. studied conditions when the velocity profileat inflow was uniform, and they defined the boundary conditionsas follows:1. Free surface condition. At the free surface, y = h, theupward transport by turbulent diffusion is the same as that due tosediment settling, so that no sediment crosses the free surface.

(3.92)

2. Channel bed condition. The sediment concentration atthe bed approaches the saturation value Sv0 within a relative shortdistance. Thus, at y = 0,

Sv = Sv0 (3.93)

3. At the entrance to the section, x = 0,

Sv = Sv0 f (y) (3.94)

If the inflow water is clear, then f (y) = 0.The boundary conditions and the process of recovery of

sediment concentration in the direction of flow are shown inFigure 3.23. The objective is to determine the sediment concentra-tion distribution Sv (x,y) throughout the flow field.

For these conditions, the solution to the differentialequation has the form:

(3.95)

where

(3.96)

and the coefficient βn can be calculated from:

(3.97)

The depth-averaged concentration can be obtained fromthe integral of Equation 3.95 with respect to y over the depth h.

An example of the recovery of sediment concentrationresulting from clear water erosion as calculated from Equation3.95 is shown for flow with a slope of J = 0.0001, depth h = 2.4 m,mean velocity U = 1.9 m s–1, and particle sizes of 0.04 mm and0.1 mm. The computed results, shown in Figure 3.24, indicate thatthe distance required for the recovery of concentration from clearwater to the saturation state is generally not long if the sediment isuniform and the streamwise variation of sediment size gradationcaused by clear water erosion is negligible. In the example shown

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 43

uS

xy

S

y y

S

y

S

y

v v y v v∂

∂ε

∂

∂

∂ε

∂

∂

∂ω

∂

∂= + +

2

2

uS

xy

S

y

S

y

v v v∂

∂ε

∂

∂ω

∂

∂= +

2

2

ε∂

∂ωy

vv

S

yS+ = 0

S x y S y xpy

x

UA

x

Uy

v vy y

yn

y nn

n

( , ) exp

exp exp sin

= −

−

−

−

−

∑

=

∞

0

2

1

2 2

2

4

ωε

ωε

ωε

ε ββ

A

h

hf y

y

yydy

n

n

y n y n

y

y n

n

h

=

+ +

−−

+

( ) ⋅

∫

4

1

2

2

4

1

2 4

22 2 2

0

βω

ε β

ω

ε β

ε ω

ω ε β

ωε

βexp sin

tan βε β

ωn

y nh = −

2

Figure 3.23 — Variation of sediment concentration in a channel with amovable bed starting with clear water at the point of inflow.

S/Sv0 S/Sv0

(a) (b)

in Figure 3.24, the concentration recovers 89 per cent of the satu-rated value within a distance of 800 m.

In the foregoing discussion, the sediment was supposedto be uniform. Hence, the bed material does not change duringdegradation, i.e. the sediment-carrying capacity of the flow doesnot vary along the river course unless the cross-section of the flowchanges. For this condition, the recovery distance is the distanceover which the streambed is scoured. The studies conducted byvarious authors confirm that this distance is usually not long. Innature, however, the bed material is composed of sediment withmixed particle sizes. Because the flow can carry fine particlesmore readily than coarse ones, most of the fine sediment is carriedaway while the coarse sediment stays in place. The result is calledthe armouring of the bed, and it causes a decrease in the sediment-carrying capacity. This phenomenon starts upstream andprogresses downstream. For this reason, the distance for the sedi-ment concentration to recover differs from that for erosion.Although the former is rather short, the latter distance is quite long(Chien, et al., 1986).

The analysis of deposition is quite similar to that of therecovery of sediment concentration; only the boundary and initialconditions are different. As an example, Zhang’s (1980) paper canbe referred to; details will not be discussed here.

3.4 TOTAL SEDIMENT LOADThe total sediment load should include both bed load andsuspended load. In the previous paragraphs, relationships andcharacteristics of bed load and suspended load are discussed. Thesum of the amount of bed load and suspended load is the total bedmaterial load that can be transported for a given flow and in givenboundary conditions. The characteristics of bed material load aredifferent from those of wash load. Consequently, formulae andmethods for calculating the bed material load and the wash loadare also different. Only sediment discharge in the form of bedmaterial load can be calculated on the basis of mechanics. Thiswill be discussed first.

3.4.1 Einstein’s bed load functionEinstein’s (1950) bed load function provides a method for comput-ing the bed material load, and considers bed material, bed loadand suspended load in combination. For the sake of convenience,one can assume that the transition from bed load to suspendedload occurs entirely at one elevation, i.e. below a given elevationbed load movement prevails, and above this, suspension prevails.The results of flume experiments reveal that unless the movementof sediment is quite intense, this critical elevation is about twograin diameters above the river bed. Einstein’s formulae for thesediment carried as bed load and that carried as suspended loadare given as follows:

For bed load:

(3.98)

where (3.99)

(3.100)

For suspended load:

isgs = 11.6U*Sva (PI1 + I2) γs (3.101)

where (3.102)

(3.103)

(3.104)

(3.105)

(3.106)

The quantities i0, ib and is are the portions of sedimentwith median diameter D in the bed material, bed load, andsuspended load respectively; gb refers to the sediment dischargeof bed load and gs to the suspended load by weight per unitwidth.

The next question is how to determine sediment concen-tration at the interface between the two (at the elevation a = 2D); itis to be used as the specific reference concentration Sva inEquation 3.101. The mean sediment concentration (expressed inpercent by volume) in the bed surface layer is:

(3.107)

where ub– denotes the mean velocity of bed load movement.

If the sediment concentration at the top of the bedsurface layer is proportional to the mean value of sedimentconcentration in the layer, and it is proportional to the frictionvelocity, then:

(3.108)

The coefficient ζ has been shown in artificial flumeexperiments to be the reciprocal of the well known constant 11.6.Thus, the above expression can be rewritten as:

ibgb = 11.6SuaU*αγs (3.109)

44 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

€

Ph

K Xs=

1

0 43430 2

.log( .

/)

IA

A

y

ydy

z

zz

A

1

1

1

0 2161

1=

−−−

∫.( )

( )

IA

A

y

yydy

z

zz

A

2

1

1

0 2161

1=

−−−

∫.( )

( ) ln

Aa

h=

zkU

=ω

*

i g

Du

b b

b s2 γ

Si g

DUua

b b

s

= ξγ2 *

Figure 3.24 — Recovery of sediment concentration by clear watererosion along a channel.

11

1

2

1

1

0

0

− =+

−

− −

−

∫π

φ

φψ η

ψ η

eA

A

t

B

B

dt* * /

* * /

* *

* *

φγ

γ γ*

/ /( ) ( )=

−

i

i gD

b

s0

1 23

1 21

ψ ξβ β

θ

γ γ

γ* '

( / )=

−Y

D

R J

s

b

2

Substituting it into Equation 3.101, one obtains:

isgb = ibgb (PI1 + I2) (3.110)

If gT denotes the total discharge of bed material expressed byweight per unit width, including both bed load and suspendedload, and iT denotes the portion of sediment with diameter D inbed material load, then:

iTgT = ibgb (1 + PI1 + I2) (3.111)

This is Einstein’s formula for the sediment transport capacity asbed material load, in which the term ibgb can be deduced from theEinstein bed load function. If sand waves exist on the bed surface,the term U* should be replaced by U**' = (R

b'gJ)0.5. Details of the

computation method and procedure can be found in the originalworks of Einstein.

3.4.2 Colby’s method (1964)Einstein’s procedure is complicated and laborious for practicaluse. Guided by Einstein’s theory, a few methods for calculatingsediment transport were established using field data observed athydrometric stations. Among these formulae, the Colby method,the modified Einstein procedure (Colby and Hembree, 1955) andthe Toffaletti formula (1969) have been widely used in westerncountries.

The Colby method is suitable for rivers with beds ofmedium to fine sand. The sediment transport capacity of a riverdepends mainly on three factors: velocity, flow depth and sedi-ment diameter (or fall velocity). Instead of using regressionanalysis or an empirical curve fitting to express the effects of thesefactors on sediment transport capacity, Colby developed a set ofgraphs shown in Figure 3.25. Altogether 24 curves are included,and they correspond to values of h varying by factors of 1 000 andto various values of median diameter. The curves in Figure 3.25are for a temperature of 60°F, D50 = 0.2 to 0.3 mm, and for flowswith a negligible amount of fine silt and clay. If the conditions arenot such, then the sediment transport found on the chart should bemultiplied by a correction factor:

1 + (k1k2 – 1) 0.01k3 (3.112)

where k1, k2 and k3 are correction coefficients for temperature,content of fine silt and clay and median diameter of bed material,respectively, as shown in Figure 3.26. Colby’s method is based onmeasured data and consequently it cannot be used for a designedpurpose.

3.4.3 Bagnold’s work (1966)Bagnold’s formulae for sediment transport capacity for both bedload and suspended load, which are in submerged weight, are asfollows:

(3.113)

(3.114)

where eb is the efficiency of bed load movement. Then, the trans-port rate of total bed material load by submerged weight is:

(3.115)

He verified Equation 3.115 using various flume data forwhich D was within the range of 0.11 to 5 mm, with satisfactoryresults.

3.4.4 The Engelund-Hansen formula (1972)The Engelund-Hansen formula is broadly recognized as one of themost reliable formulae. They applied Bagnold’s stream powerconcept and the similarity principle to obtain a sediment transportformula:

fΦT = 0.4Θ5/2 (3.116)

where f = 8ghJ / U2 (3.117)

Φ = gT [γs (γs – γ) gD3]–1/2 (3.118)

Θ = τ / (γs – γ) D (3.119)

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 45

Figure 3.25 —Work chart for the relationship for sediment transportcapacity (after Colby).

Figure 3.26 — Correction factors (after Colby).

g Ue

bb'

tan= τ

α0

g UU

s'

.= 0 01 0τω

g Ue U

Tb' = +τα ω0 0 001(

tan. )

Clay concentration (ppm)

Mean velocity (m s–1)

h = 0.03 m h = 0.3 m h = 3 m h = 30 m

mm

mm

mm

mm

mm

0.1

mm

0.2

mm

0.3

mm

0.2

mm

where g is the gravitational acceleration; h is the water depth, U*is the average flow velocity; g is the total sediment discharge byweight per unit width, γs and γ are the specific weights of sedi-ment and water, respectively, D is the median particle diameter,and τ is the shear stress along the bed.

Strictly speaking, Equation 3.116 should be applied toflows with dune beds in accordance with the similarity principle.Data from flume experiments show that the Engelund-Hansenformula fits well not only for the dune-covered bed configuration,but also for one with antidunes. If the mean velocity of sedimentmovement is taken to be proportional not to the friction velocityU

*, but to the friction velocity U**

′ relevant to grain resistance onthe bed surface, then the final expression for the Engelund-Hansensediment transport capacity formula takes the form:

(3.120)

and it is shown by the dotted line in Figure 3.27.

If Θ is small, fφT ~ φ2

If Θ is large, fφT ~ φ3

3.4.5 The Ackers-White formula (1973)Based on Bagnold’s river power concept, Ackers and Whiteapplied dimensional analysis to express the transport rate of sedi-ment in terms of some dimensionless parameter. They used adimensionless parameter X to divide all sediment into threegroups: coarse, fine and medium,

(3.121)

If X > 60, the sediment is coarse, and the value corre-sponds to D > 2.5 mm for natural sediment; if X < 1, the sedimentis fine, and it corresponds to D < 0.04 mm for natural sediment; ifX is in between, 1 ≤ X ≤ 60, the sediment is in the transitionalregion between the two for natural sediment.

They postulated that only part of the shear stress on thechannel beds is effective in causing the movement of coarse sedi-ment, while in the case of fine sediment, suspended loadmovement predominates, and the total shear stress is effective incausing sediment movement. They suggested the mobility numberfor sediment as follows:

(3.122)

where n = 0 for coarse sedimentn = 1 for fine sedimentn = f(X) for sediment in the transition region.

The parameter of sediment mobility M is no differentfrom the parameter of flow intensity Θ, which is referred tofrequently in the preceding sections and chapters.

In establishing the formula for sediment transport capac-ity, Ackers and White also adopted the Bagnold concept that the

intensity of sediment transport is related to the power provided byflow. They assumed the efficiency of sediment transport to beproportional to M. By combining the efficiency in M, they attainedthe following parameter for sediment transport:

(3.123)

where SwT is the sediment concentration in percentage by weightfor a water column above a unit element of the bed surface. Froma large amount of flume data, they found this parameter of sedi-ment transport to be a function of M and X. By analysing 1 000sets of flume data, they obtained the final expression as:

(3.124)

The condition of incipient motion of sediment is M = A,where for coarse sediment:

n = 0A = 0.17c = 0.025m = 1.5

For sediment in the transition region:

n = 1 – 0.56 log X (3.125)

(3.126)

log c = 2.86 log X – (log X)2 – 353 (3.127)

(3.128)

If fine sediment exhibits any effect of cohesion amongthe particles, the above-mentioned formulae are not applicable.Figure 3.28 is a graphical representation of Equation 3.124.

3.4.6 Yang’s approach (1996)Yang defined the unit power as the velocity-slope product. Hisapproach considers that the rate of work carried out by a unit of

46 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 3.27 — Comparison of Engelund-Hansen formula againstflume data (after Engelund and Hansen).

f TΦ Θ Θ= +0 3 0 152 2

. .

X

g

vD

s

=

−

γ γ

γ2

13

MU

gD

U

h

D

n

s

n=−

−* [

log( )

]γ γ

γ32

10

1

γγ

γ

=S h

D

U

U

wT

s

n( )*

γ = −c (M

A

n1)

Ripple Dune

AX

= +0 23

0 14.

.

mX

= +9 66

1 34.

.

water in transporting sediment must be directly related to the rateof work available to a unit weight of water. Thus, the total sedimentconcentration or total bed material load St must be directly related tounit river power. Using dimensional analysis and considering that acritical unit of river power UcrJ is required at incipient motion, hefound the best form of expressing the total bed material load:

(3.129)

where I1 and I2 are dimensionless parameters reflecting the flowand sediment characteristics U*, v, ω and D. Running a multipleregression analysis for 463 sets of laboratory data, he obtained thefinal expression, as follows:

(3.130)

where St is the total sediment concentration in ppm by weight.The critical dimensionless unit of river power UcrJ/ω is

the product of the dimensionless critical velocity Ucr/ω and theenergy slope J, where:

(3.131)

(3.132)

As the rate of sediment transport increases, the need toinclude incipient motion criteria in a sediment transport equationdecreases. For sediment concentrations higher than about 100 ppmby weight, Yang introduced the following unit river power equation:

(3.133)

3.4.7 Formula of the Wuhan University of Hydraulic andElectric Engineering (WUHEE)

For rivers flowing over alluvial plains, suspended load predomi-nates, and the bed load is generally negligible. In such cases, the

suspended sediment transport capacity is approximately equal tothe total transport capacity. Among such formulae, the formulamost widely used in China is that developed at WUHEE:

(3.134)

A similar formula is the Velikanov formula:

(3.135)

The principle parameter in these formulae is the productof U2/gh and U/ω. In Figure 3.29, a comparison of Equation 3.134with field and laboratory data displays some scatter.

3.4.8 Estimation of total sediment load includingwash load

The sediment transport capacity formulae presented in the preced-ing sections that were established on the basis of mechanicsshould be used to compute only the sediment discharge in theform of bed channel-derived load. For the wash load, the relation-ship between sediment transport rate and flow rate is based onfactors related to the common background of the watershed. Sucha relationship can be established only from data observed in thefield, including: (i) data of sediment load measurement at hydro-metric stations; (ii) information of sediment yield in drainagebasins; and (iii) measurements of sediment deposits in reservoirs.The following section contains a discussion of the nature of suchdata and methods for processing the three kinds of data.

3.4.8.1 ANNUAL SEDIMENT LOAD EVALUATED BY THE

RELATIONSHIP BETWEEN FLOW DISCHARGE AND

SEDIMENT TRANSPORT RATE

Regular measurements of flow discharge and sediment samplingare the routine work of hydrometric stations. In most cases, dataare obtained for sediment transport rates related to various flowrates. By means of the relationship between flow discharge andsediment transport rate and the frequency curve for flow, one canevaluate the total sediment load at a given hydrometric station.However, this approach is affected by three procedural difficulties.

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 47

log )S I I lUJ U J

tcr= + −1 2 og (

ω ω

log . . log . log

( . . log . log )

*

*

SD U

D U UJ U J

t

cr

= − −

+ − −−

5 435 0 286 0 457

1 799 0 409 0 314

ων ωων ω ω

) log (

log . . log . log

( . . log – . log )

*

*

SD U

D U UJ

t = − −

+ −

5 165 0 153 0 297

1 780 0 360 0 480

ω

ν ω

ω

ν ω ω) log (

UcrU D

U D

ων

ν=

−+ ≤ <

2 5

0 06

1 2 70.

log( * ) .

. *0.066 for

U U D* *.ω ν

= ≤2 05 70 for

S kU

ghvm

m= ( )

3

ω

S kU

ghvm = ( )

3

ω

Figure 3.29 — Comparison of Equation 3.134 with observed data.

Figure 3.28 — Ackers-White formula for sediment transport capacity(after Ackers and White).

Yangtze River

Yellow River

People’s Canal

Qingtong irrigation area

Sanmenxia Reservoir

Guanting Reservoir

Flume data by WUHEE

Sedi

men

t con

cent

ratio

n, S

m(k

g m

–3)

U3/(ghω)

(1) The observed field data usually do not include the measure-ment of bed load, and the portion of suspended load near the bedsurface is not easy to measure, so the measured data do not fullyreflect the total sediment load carried by the flow. (2) In somerivers, the measured data points are widely scattered; thus it isdifficult to establish a relationship of flow discharge versus sedi-ment transport rate by conventional methods of curve fitting. (3)Fewer data for sediment transport rate are available than for flowdischarge, and data series may be too short to be representative ofaverage conditions. Efforts made to find rational solutions to thesedifficulties are discussed as follows.

A. Evaluation of total sediment load based on measurement ofsuspended load

Because of its size, a suspended load sampler is designed toexclude the main zone of bed load transport close to the bed. Also,the bed load is well outside the scope of suspended load sampling.For the wash load, which is mainly composed of fine sedimentthat is uniformly vertically distributed, the mean sediment concen-tration obtained by conventional sampling methods shouldrepresent satisfactorily the true mean value. However, for the bedmaterial load, especially particles coarser than fine sand, a consid-erable part is concentrated near the streambed, and it is notincluded in the results of suspended sediment sampling. How toestimate the unmeasured sediment load is a major concern.Samples are taken at points and by depth-integration devices, andthe necessary corrections for these two methods are different.Chien and Wan (1956) proposed a correction method for pointsamples, and Chien (1953) proposed a correction method fordepth-integrating samples. Details are not discussed here, but onecan refer to Chien and Wan (1983).

B. Method of establishing a relationship for discharge-sedimenttransport rate from scattered data points

If the wash load in the drainage area is large, and both regionalfactors (such as vegetation cover, topography and soils, etc.) andthe rainfall distribution are strongly non-uniform, the data pointsfor the measured sediment transport rate plotted against measureddischarge usually display a wide band of scatter. If the relationshipis established by following the trend of the data, considerable errorwill result in the computation of the annual sediment load usingthat relationship and the corresponding frequency curve for riverdischarge.

The wide scatter of the data points can result from twocircumstances. First, owing to the large spatial differences, runoffformed in different areas may lead to quite different sedimentconcentrations, sometimes high, and other times low. Second, thescatter may be due to temporal differences in runoff. For example,in early spring there is a high volume of runoff because of meltingice and snow; in summer and autumn, heavy rainstorms causefloods. The sediment concentrations for these two cases differgreatly. In some drainage areas, both conditions occur and thesituation is then even more complicated. In addition, heavily sedi-ment-laden rivers, because of the self-regulation of the channel,are characterized by the ‘more sediment may be released if moresediment is supplied’ phenomenon. Such a situation can enhancethe extreme scatter of data points in a plot of sediment transportrate against water discharge.

In analyses of hydrological data, one can sometimesdetermine the concrete causes of the scatter of data points. One

can then calculate a set of relationships for discharge vs. sedimenttransport rate and the corresponding discharge frequency curve forthe specific events of runoff originating from different sourceareas or occurring in different seasons. The total sediment trans-port rates for given time periods are then evaluated separately. Anexample is illustrated in Chien and Wan (1983).

3.4.8.2 ESTIMATION OF SEDIMENT LOAD BASED ON FACTORS IN

RIVER BASINS

If soil erosion is the source of sediment, the amount of sedimentconveyed in the river system is naturally related to the variousfactors that affect soil erosion in the watershed. If such relation-ships can be shown graphically or expressed by mathematicalequations, the amount of sediment originating from the watershedand conveyed into the river can be deduced from the characteris-tics of river basin factors; such a process can be useful if there is alack of field data.

In practical applications, two approaches are possible.The first is to establish a direct relationship for the sediment loadconveyed into the river expressed in terms of the characteristics ofthe given watershed, and based on measured data from the hydro-metric network. The second is to estimate the amount of soileroded from the ground surface, and then to estimate how much ofthat material can be carried into the river (see Chapter 1).

Anderson (1951) analysed measured data for 29 water-sheds in Oregon, United States (watershed areas ranging from 145to 18 850 km2), to establish a relationship between suspended loadand various regional characteristics of watersheds that had hydro-metric stations (he assumed that the bed load was negligible). Theincluded factors comprise a set that appears to be quite complete.The standards relating to the measurement, units and physicalinterpretation of these factors are given in Table 3.3.

3.4.8.3 ESTIMATION OF SEDIMENT YIELD OF A WATERSHED FROM

RESERVOIR DEPOSITION

If a large reservoir is constructed in a river, all of the sedimentload from the upstream areas will be intercepted by the reservoir.Thus, measuring the amount of deposition in the reservoir is a reli-able way to assess the sediment yield of the drainage area.

If, on the contrary, the storage capacity of the reservoiris not large relative to the volume of runoff, then part of the sedi-ment load may be carried downstream. Thus, the sediment yieldbased on deposition in the reservoir must include the efficiencyof the reservoir in trapping sediment. Figure 3.30 shows the rela-tionship between the sediment outflow to inflow ratio duringflood events and the characteristics of the reservoir, with sedi-ment size and concentration as additional parameters (Xia, Hanand Jiao, 1980), where V is the storage volume of the reservoir,Qi is the inflow and Q0 is the outflow. The abscissa of thediagram, VQi/Q0

2, has the dimension of time; it reflects the timeof flood detention in the reservoir. In addition, the efficiency ofsediment release is also related to sediment size and sedimentconcentration. Fine sediment can be released much more easilythan coarse sediment. If the fine sediment concentration exceeds50 kg m–3, the fall velocity of the sediment is less, and moresediment is released.

In addition to the above approaches, physical andmathematical models have been used recently to study theformulation and confluence of runoff, including the concept ofsediment yield.

48 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

3.5 HYPERCONCENTRATED FLOWIn an ordinary sediment-laden flow, sediment is carried by theflow and it has little effect on flow behaviour. Therefore such aneffect can be ignored. In hyperconcentrated flow, however, theexistence of large amounts of solid particles remarkably influ-ences or changes fluid properties and flow behaviour. In suchcases, the above-mentioned influence or change must be consid-ered. In many cases of hyperconcentrated flow, sediment togetherwith water, forming a pseudo-one-phase fluid, moves as its ownentity, and the sediment can no longer be considered as materialcarried by water.

The existence of hyperconcentrated flow cannot bejudged simply by concentration alone. Grain size compositionand mineral content of sediment play a very important role. Asregards the Yellow River where the incoming sediment hassimilar mineral content and grain size composition, flow with aconcentration higher than 200 kg m–3 can be considered ashyperconcentrated.

In a natural environment, debris flows, turbidity flowsalong the sea bottom and hyperconcentrated density flows can beconsidered as hyperconcentrated flow. Hyperconcentratedhydrotransport is a kind of hyperconcentrated flow in industry.

In hyperconcentrated flow in the Yellow River basin,the size composition of the suspended sediment exhibits some

distinct features: at a given gauging station, the higher theconcentration, the coarser the suspended sediment (Chien andWan, 1986).

When the total concentration exceeds a certain value,clay content no longer increases with concentration, but rathermaintains a certain value. The persistence of fine-material

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 49

Figure 3.30 — Relationship between efficiency of sediment release andcharacteristics of reservoir and sediment load.

Factors in watershed Symbol Unit Average value Range Physical meaning

Average annual MAq m3 km–2 0.325 0.0114–0.0817 Magnitude of runoffFlow runoff steepness FOp – 3.56 1.98–.30 Intensity of runoff

of flood discharge

The content of Representing the source ofSoil silt and clay in SC per cent 23.0 19.1–22.0 suspended load easily suspended

the surface soil and carried away by the flowlayer (15 cm)

per cent/ It reflects the permeability andAggregate ratio* B (cm2 g–1) 1.37 0.56-3.84 ability of the soil to withstand

erosion

Geography Area of watershed A km2 2.00 145–18 850 —River gradient J m km–1 172 40–286 Average gradient of surface soil in

watershed

Road R per cent 0.3 0.05–0.6 Road construction includes watersoil erosion

Forest cut within RC per cent in 6.0 0–30.4 Cutting down forests destroyslast ten years ten years protection provided by forest

Cultivated land BC per cent 4.0 0–22 Some erosion will be greatlyLand utilization with thin cover reduced if land surface is covered

by plants

Cultivated land OC per cent 12 0–48 As abovedifferent from BC

Cultivated land =(BC + OC) × A C km2 20.7 0–173.5 —

Eroded bank EB m 5 180 62 500 Soil resulting from bank erosiondirectly enters the river

Table 3.3Principal factors affecting sediment yield in western Oregon, United States (after Anderson)

* Definition of soil aggregate ratio B and the technique for measuring the term B are given in reference (Anderson, 1951).

(s)R

elea

se e

ffic

ienc

y

Si > 50 kg m–3

Si > 50 kg m–3

concentration is a general characteristic not only of the hypercon-centrated flow in a river system, but also of the hyperconcentratedlahar-runoff flow.

The features noted above are important in the transportof hyperconcentrated flow. A certain amount of fine particles forman intricate network of the floc structure which effectively reducesthe fall velocity of coarse particles, thereby ensuring a high sedi-ment transport capacity. When the concentration rises beyond acertain limit, further increases in concentration will only make thesediment composition coarser. The clay content does not increasewith the high concentration, thereby ensuring the flow will nottransform into a laminar one which requires a much larger slope tobe kept in motion.

It is well known that clear water is a Newtonian fluidwith a viscosity of m. Water with a low concentration of sedimentremains Newtonian fluid, but the viscosity increases with increas-ing concentration. As sediment concentration exceeds a certainvalue, particularly for sediment containing clay particles, thewater-sediment mixture no longer behaves as a Newtonian fluid.The critical concentration Sv0 varies according to the size compo-sition and mineral composition of sediment as well as the waterquality.

Data from rheological measurements indicate that mosthyperconcentrated flows can be described as Bingham fluid. Themixture of water and finer granular particles carried by debrisflows can also be described as Bingham fluid. The rheologicalequation of Bingham fluid is:

(3.136)

where τB and η are called the Bingham yield stress and the coeffi-cient of rigidity, respectively.

Bingham yield stress and rigidity vary with the sizecomposition and mineral composition of sediment and the sedi-ment concentration. The higher the content of fine particles, thelarger the Bingham yield stress and the rigidity. They increaserapidly when sediment concentration increases. Researchers havedrawn up various empirical formulae for describing the relation-ship between rheological parameters and sediment concentrations.Exponential formulae are widely used, such as:

τB = KSvm (3.137)

In many cases m = 3 is adopted, but in some cases m issmaller in a low concentration region but larger in a high concen-tration region.

Fall velocity of sediment particles is an important para-meter in sediment transport. The fall velocity of sediment particlesin hyperconcentrated flow may be reduced many times due to theincrease in viscosity, the backflow caused by other settling parti-cles and a reduction in the effective weight, etc.

The most widely adopted formula of the gross fall veloc-ity for uniform discrete sediment particles is suggested byRichardson and Zaki (1954):

(3.138)

Chien (1980) suggested that the exponent m is a functionof the grain Reynolds number.

(3.139)

There are several patterns of hyperconcentrated flow.A. Neutrally buoyant load motion. If a flow carries enough clay

material, the mixture may exhibit strong yield strength, andmost sediment in the flow will belong to neutrally buoyant load.Mud flow in the Loess Plateau is an example of such flow.

B. Neutrally buoyant load motion + suspended load + bed load.A part of fine sediment moves as neutrally buoyant load,while coarse sediment is transported as suspended load andbed load.

C. Suspended load + bed load motion. If a hyperconcentratedflow carries very little clay material, sediment moves mainlyas suspended load along with a small part of bed load.

D. Laminated load motion and neutrally buoyant load + lami-nated load. If the energy slope of a flow is sufficiently highand there is only cohesive material available, laminated loadmotion may develop. Water debris flow is essentially a lami-nated load motion. On the other hand, in viscous debris flowgravel, cobbles and big stones may move as laminated loadand sand and silt may be neutrally buoyant load.

For hyperconcentrated flow, laminar flow or turbulentflow may occur, depending on relevant conditions.

If the concentration is high enough, laminar flow mightappear in small rivers or canals. Considering Bingham fluidflowing in an open channel with slope J, a theoretical velocityprofile is obtained as follows:

(3.140)

Equation 3.140 can be rewritten as:

(3.141)

where γm is the specific weight of the mixture, H is the depth, andy is the distance from the bed; up is the maximum velocity in theprofile and equal to the velocity of the plug zone. In the plug zone,y > H – τB/γmJ, where the shear stress is smaller than the Binghamyield stress, there is no relative motion between layers, and thewhole fluid moves as an entity with velocity up, as shown inFigure 3.31.

(3.142)

A hyperconcentrated flow transforms into turbulent flowif the Reynolds number is large. The flow begins to develop intoturbulence at Rem = 2 000 and develops fully into turbulence ifRem >10 000. The flow is in a transitional region if Rem = 2 000 to10 000.

(3.143)

In a fully developed turbulent flow, the velocity distribu-tion still follows the logarithmic formula, but the velocity gradient

50 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

τ τ η= +B

du

dy

ω

ω0

1= −( )Svm

m fD

= ( )ω

ν0

uy

HJ yJ y HJ

m m BB

m

= − − ≤ ≤ −2

2 2 0η

γ γ ττ

γ( ),

u u

u

yJ

HJy H

J

p

p

m

m B

B

m

−= −

−≤ ≤ −( ) ,1 0

2γ

γ τ

τ

γ

u uJ

HJ

HJ

y Hpm B

m

B

m

= = − − < ≤γ

η

τ

γ

τ

γ2

2( ) ,

RHU

H

U

em

Bm

=

+

4

12

ρ

ητ

η( )

or the Karman constant κ is different from that of clear water aflow. The κ constant varies with concentration, as shown inFigure 3.32.

In the case of a pipe flow, the Reynolds number shouldbe modified as:

(3.144)

and the f vs. Re1 relationship shifts a little from the f vs. Rem curvefor open channel flow.

As mentioned above, in hyperconcentrated flow, the fallvelocity of sediment particles is reduced quite substantially.Consequently, in such a flow, sediment is easier to transport. If allthe sediment particles belong to a neutrally buoyant load, the flowcan be maintained, provided the potential energy of the flow issufficient for overcoming the resistance. If not all the sedimentparticles belong to a neutrally buoyant load, due to the reductionof their fall velocities, coarse particles are easier to transport in

clay slurry than in clear water. Wan and Sheng (1978) found thatin a region of high concentration, the relationship S – U3/gHω0,which is used to describe sediment carrying capacity has a reversetendency and has a hook-like outline, as shown in Figure 3.33.Here, S is the average concentration of a flow under equilibriumconditions, and ω0 is the fall velocity of a single particle in still,clear water.

In the region of high concentration (about S >200 kg m–3) more sediment can be carried by flow, with evenweaker intensity. In other words, high concentration does notrequire high flow intensity to be carried. This is a very usefulconcept.

The reason for the reverse tendency of the S – U3/gHω0

relationship is the obvious reduction in the fall velocity at highconcentrations, particularly at high concentrations of fine parti-cles. If the reduction in fall velocity due to concentration has beentaken into consideration, the hyperconcentrated flow follows thesame law as that followed by an ordinary sediment-laden flow.

REFERENCESAckers, P. and W.R. White, 1973: Sediment transport: new

approach and analysis. Journal of the Hydraulics Division,Proceedings. ASCE, 99 (Hy11), pp. 2014-2060.

Agricultural Research Service, USDA 1975: Present and prospec-tive technology for predicting sediment yields and sources.Proceedings of the Sediment Yield Workshop, USDASedimentation Lab., p. 285.

Anderson, H.W., 1951: Physical characteristics of soils related toerosion. Journal of Soil and Water Conservation,pp. 129-133.

Bagnold, R.A., 1966: An approach to the sediment problem fromgeneral physics. USGS, Professional Paper 422-I, p. 37.

Boyce, R.C., 1975: Sediment routing with sediment-deliveryratios. Present and Prospective Technology for PredictingSediment Yields and Sources, Agricultural Research Service,USDA, pp. 61–73.

Cao, Ruxuan, 1975: Discussion on sediment-carrying capacity ofhyperconcentrated flow. Selected Papers of the Symposiumon Sediment Problems on the Yellow River, Volume 2,pp. 249-258 (in Chinese).

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 51

Figure 3.32 — Variation of constant κ with sediment concentration.

RUR

R

U

em

B2

4

12

3

=

+

ρ

ητ

η( )

Figure 3.33 — Sediment carrying capacity S – U3/gHw0.

Figure 3.31 — Velocity distribution of a laminar flow.

x

S (k

g m

–3)

€

( ) .U

Hw

3

0

0 92

S (k

g m

–3)

Chien, Ning, 1980: Comparison of bed load formulae. Journal ofHydraulic Engineering, Number 4, pp. 1-11 (in Chinese).

Chien, Ning, 1980: Preliminary study on the mechanism of hyperconcentrated flow in the north-west region of China. Selected Papers of the Symposium on SedimentProblems on the Yellow River, Volume 4, pp. 244-267 (inChinese).

Chien, Ning and Zhaohui Wan, 1983: Mechanics of SedimentMotion. Science Press.

Chien, Ning, Ren Zhang and Zhide Zhou, 1986: FluvialProcesses. Science Press (in Chinese).

Chien, Ning and Zhaohui Wan, 1986: A Critical Review of theResearch on Hyperconcentrated Flow in China. InternationalResearch and Training Center on Erosion and Sedimentation,p. 42.

Colby, B.R., 1964: Discharge of sands and mean-velocity relation-ships in sand bed streams. Professional Paper 462-A, USGS,p. 47.

Colby, B.R. and C.H. Hembree, 1955: Computation of total sedi-ment discharge, Niobrara River near Cody, Nebraska. WaterSupply Paper 1357, USGS.

Dou, Guoren, 1960: On threshold velocity of sediment particles.Journal of Hydraulic Engineering, April, pp. 22-31 (inChinese).

Dou, Guoren, 1962: Theory on threshold motion of sedimentparticles. Scientia Sinica, Volume 11, Number 7, pp. 999-1032 (in Russian).

Einstein, H.A., 1950: The bed load function for sediment transportin open channel flows, Technical Bulletin 1026, p. 71,USDA.

Einstein, H.A. and N.L. Barbarossa, 1952: River channel rough-ness. Transactions, ASCE, Volume 117, pp. 1121-1146.

Einstein, H.A. and Ning Chien, 1953: Transport of sedimentmixtures with large ranges of grain sizes. Sediment SeriesNumber 2, Missouri River Division, U.S. Army Corps ofEngineers, p. 49.

Engelund, F., 1965: A criterion for the occurrence of suspendedload. La Houille Blanche, Number 6, p. 607.

Engelund, F. and E. Hansen, 1972: A Monograph on SedimentTransport in Alluvial Streams. Teknisk Forlag, Copenhagen,p. 62.

Englund, F. and J. Fredsφe, 1976: A sediment transport model forstraight alluvial channels. Nordic Hydrology, Volume 7,pp. 293-306.

Fei, X., 1982: Viscosity of the fluid with hyperconcentration-coef-ficient of rigidity. Journal of Hydraulic Engineering,Number 3, p. 57-63 (in Chinese).

Fei, X., 1983: Grain composition and flow properties of heavily-concentrated suspension. Proceedings of the InternationalSymposium on River Sedimentation, pp. 307-308 (inChinese).

Foster, G.R. (ed.): Soil Erosion: Prediction and Control. SpecialPublication Number 21, Soil Conservation Society ofAmerica, 1977, p. 393.

Gessler, J., 1970: Self-stabilizing tendencies of sediment mixtureswith large range of grain sizes. Journal of the Waterways andHarbor Division, ASCE, 96 (WW2).

Gessler, J., 1971: Critical shear stress for sediment mixtures.Proceedings of the Fourteenth Congress. InternationalAssociation for Hydraulic Research, Volume 3, pp. 1-8.

Gessler, J., 1976: Stochastic aspects of incipient motion onriverbeds. Stochastic Approach to Water Resources ,Volume 2, (ed.) H.W. Shen, p. 26.

Goncharov, B.N., 1962: River Dynamics. Hydrology andMetrology Press, Leningrad, pp. 226-236 and p. 252 (inRussian).

Goncharov, V.N., 1962: Basic River Dynamics. Hydro-Meteorological Press, Leningrad (in Russian).

Gong, Shiyang and Guishu Xiong, 1980: The origin and transportof sediment of the Yellow River. Proceedings of theInternational Symposium on River Sedimentation, Volume 1,pp. 43-52 (in Chinese).

Jiang, Zhongshan and Wenjin Song: Sediment yield in smallwatersheds in the Gullied-Hilly Loess areas along the middlereaches of the Yellow River. Proceedings of the InternationalSymposium on River Sedimentation, 1980, Volume 1, pp. 63-72 (in Chinese).

Kramer, H., 1935: Sand mixtures and sand movement in fluvialmodels. Transactions, ASCE, Volume 100, pp.798-838.

Levy, E.E., 1956: River Mechanics. National Energy Press,Moscow (in Russian).

Levy, I.I., 1957: River Dynamics. National Energy ResourcesPress, Moscow, pp.127-140.

Li, Changhua and Meixiu Sun, 1964: Criteria for threshold shearstress and ripple formation. Proceedings of the NanjingHydraulic Research Institute (River and Habour Division)(in Chinese).

Meyer-Peter, E., H. Favre and H.A. Einstein, 1934: NeuereVersuchsresultate uber den Geschiebetrieb. SchweizBauzeitung, Volume 103, Number 12, pp. 147-150.

Meyer-Peter, E. and R. Muller, 1948: Formula for bed load trans-port. Proceedings of the Second Meeting. InternationalAssociation for Hydraulic Research, Volume 6.

Migniot, C., 1968: Study on physical properties of fine sediment(silt) and their properties under flow dynamics. La HouilleBlanche, Volume 7 (in French).

Migniot, C., 1977: Effects of flow, wave and wind on sediment. LaHouille Blanche, Number 1 (in French).

Renfro, G. W., 1975: Use of erosion equations and sediment-delivery ratios for predicting sediment yield. Present and Prospective Technology for Predicting Sediment Yields and Sources, Agricultural Research Service, USDA,pp. 33-45.

Richardson, J.F. and W.N. Zaki, 1954: Sedimentation and fluidiza-tion, Pt. 1. Transactions, Institute of Chemical Engineers,Volume 32, pp. 35-53.

Rouse, H., 1938: Experiments on the mechanics of sedimentsuspension. Proceedings of the Fifth International Congressof Applied Mechanics, pp. 550-554.

Scott, K.M. and R.L. Dinehart, 1985: Sediment transport anddeposit characteristics of hyperconcentrated streamflowevolved from lahar at Mount St. Helens. Proceedings of theInternational Workshop on Flow at Hyperconcentrations ofSediment, International Research and Training Center onErosion and Sedimentation, pp. 3-2-33.

Shamov, G.E., 1952: Formulae for determining near-bed velocityand bed load discharge. Proceedings of the Soviet NationalHydrology Institute, Volume 36 (in Russian).

Sharmov, G.I., 1959: River Sedimentation. Hydrology andMetrology Press, Leningrad, pp. 84-93.

52 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Shields, A., 1936: Anwendung der Aechlichkeitsmechanik und derTurbulenzforschung auf die Geschiebewegung, PreussischeVersuchsanstalt fur Wasserbau und Schiffbau, Berlin.

Simons, D.B. and E.V. Richardson, 1960: Resistance of flow inalluvial channels. Journal of the Hydraulics Division, ASCE,86 (Hy5), pp. 73-99.

Tang, Cunben, 1964: Law of sediment threshold. Journal ofHydraulic Engineering (in Chinese).

Tan, Ying, 1983: Inttermittent surges of gravel transport in rivers.Proceedings of the Second International Symposium onRiver Sedimentation, Water Resources and Electric PowerPress.

Tixon, L.J., 1948: Etude des conditions dans lesquelles les partic-ules solides sont transportées dans les courants à lit mobiles.Proceedings, Association internationale des scienceshydrologiques, Volume 1, Oslo, pp. 293-310.

Toffaletti, F.B., 1969: Definitive computations of sand discharge inrivers. Journal of the Hydraulics Division. ASCE, 95 (Hy1),pp. 225-246.

Yang, Chih-Ted, 1973: Incipient motion and sediment transport.Journal of the Hydraulics Division, ASCE, 99 (Hy10),pp. 1679-1704.

Yang, C.T., 1996: Sediment Transport — Theory and Practice.McGraw-Hill.

Yang, Wenhai and Wenlin Zhao, 1983: An experimental study ofthe resistance to flow with hyperconcentration in rough

flumes. Proceedings of the Second International Symposiumon River Sedimentation, pp. 45-55.

Wan, Zhaohui and Shoubai Sheng, 1978: Phenomena of hyper-concentrated flow on the stem and tributaries of the YellowRiver. Selected papers from the Symposium on SedimentProblems on the Yellow River, Volume 1, pp.141-158 (inChinese).

Wan, Zhaohui, Qingsong Li, Tison Yang and Jungfeng Tian, 1998:Incipient motion of fine particles under large water depth.Proceedings, InterCoh 98.

White, C.M., 1940: Equilibrium of grains on bed of stream.Proceedings, Royal Society of London, Series A,Volume 174, pp. 322-334.

White, S.J., 1970: Plain bed thresholds for fine grained sediments.Nature, Volume 228, Number 5267, pp. 152-153.

Wuhan University of Hydraulic and Electric Engineering(WUHEE), 1961: River Dynamics. China Industrial Press (inChinese).

Xia, Zhenhuan, Qiwei Han and Enze Jiao, 1980: The long-termcapacity of a reservoir. Proceedings of the InternationalSymposium on River Sedimentation. Volume 2, pp. 753-762(in Chinese).

Zhang, Qishun, 1981: Study of sediment-diffusion mechanism foropen-channel flow and its application. Chinese Journal ofSediment Research, Volume 1, pp. 37-52 (in Chinese).

CHAPTER 3 — SEDIMENT TRANSPORT IN RIVERS 53

4.1 INTRODUCTIONFluvial processes, broadly speaking, involve the study of entirehistorical processes of formation and evolution of various parts ofa river valley from its origin to estuary and belong to thegeomorphologic category. But in a narrower sense, fluvialprocesses relate to river changes that occur owing to naturalconditions or human activities and belong to the category of riverdynamics. The latter is more spectacular from an engineering pointof view.

The fluvial processes of alluvial rivers are the result ofthe interaction of flow, sediment and channel bed. The channelbed influences the current structure and sediment movement, andthe flow and sediment transport enhance changes in the channelbed. They are interdependent and condition each other. Becausethe flow and sediment transport are ever changing, the fluvialprocesses are quite complicated, which can benefit humans or leadto disasters, so rivers should be monitored. River regulation andtraining works must take into account the characteristics of fluvialprocesses of rivers so that the river training works can help riversto do what they would do naturally rather than force them into anunnatural situation, which would ultimately lead to failure. Fluvialprocesses are quite different from one river to another and rivertraining measures are also multifarious. The purpose of thischapter is to introduce the main aspects of fluvial processes, rivertraining and river sediment management, including categories ofrivers, classification of river patterns, river morphology, fluvialprocesses for rivers with different patterns, and the operationalmeasures of channel stabilization and rectification, so as to meetplanning and design requirements for river regulation and rivertraining works.

4.2 CATEGORIES OF RIVERSAccording to their geometrical position, rivers can be divided intotwo major types: mountainous rivers and plain rivers. The upperreaches of large rivers are always the mountainous or uplandrivers, while the lower reaches are plain rivers.

4.2.1 Mountainous and upland riversMountainous and upland rivers have the following features:(1) The flood peak rises rapidly and falls sharply, and the

maximum discharge might be hundreds or thousands oftimes higher than the minimum. In South China, most moun-tainous rivers have a sediment concentration of less than1 kg m–3 in flood seasons. However, for the rivers in theLoess Plateau in North China, the maximum sedimentconcentration might be over 1 000 kg m–3, and debris floodsoften occur in the mountainous rivers in southwest China.

(2) Under the effects of the geological structure and flowactions, well-developed terraces exist along both sides ofsuch rivers, but there is no wide flood plain. Diluvial fansand mouth bars often occur at the outfalls of their tributaries.

(3) The longitudinal profile is steep, the torrents wind throughshallow shoals, and the channel bed manifests itself risingand falling along the river.

(4) The valley cross-section is V- or U-shaped (Figure 4.1).(5) The river bed is composed of base rock and gravel. When

earthquakes occur, landslides, mountain slides and rapid beddeformation take place, and the channel may often beblocked. Dammed and falling water is formed upstream anddownstream of the block.

4.2.2 Plain and piedmont riversThe features of plain rivers can be described as follows.(1) These rivers have large catchment areas and smooth flood

hydrographs. The ratio of the maximum to minimumdischarge at the Yichang Station on the Middle YangtzeRiver in China is only 26, and 10 at Bahadurabad Station onthe Brahmaputra River in Bangladesh. However, for riverswith less runoff and concentrated rainstorms, such as theLower Yellow River, the flood peak still rises and fallsrapidly. The average ratio of maximum to minimumdischarge at Huayuankou Station on the Lower Yellow Riveris as high as 446.

CHAPTER 4

FLUVIAL PROCESSES

Figure 4.1 — Morphology of mountainous river valleys on Maohu Reach of the Beipanjiang River (China) (a) V-shaped valley,(b) U-shaped valley, ∇ 1 high water level.

(a) (b)

(2) The incoming sediment load is determined by thecharacteristics of the river basin. For example, the long-term annual incoming sediment load at Yichang Stationamounts to 0.521 × 109t, with an average sedimentconcentration of 1.18 kg m–3, while for the Yellow Riverflowing through the Loess Plateau, a seriously erodedregion, the long-term annual incoming sediment load is1.62 × 109t, with an average sediment concentration of 37.6kg m–3 at Shanxian Station.

(3) The river valleys have deep alluvial layers with thicknessesof tens or hundreds of metres. The channel beds arecomposed of loose sediment deposits which can be easilyeroded.

(4) The fully developed valleys have a main channel and wideflood plains (Figure 4.2).

(5) The longitudinal profile is even and smooth. The channelslope of the Middle and Lower Yangtze River is0.1–0.027‰, the Lower Yellow River is 0.1–0.2‰, and

channel slopes of the Upper, Middle, and LowerBrahmaputra River are 0.086–0.071‰, 0.072–0.047‰, and0.038–0.034‰, respectively. The annual runoff and sedimentload for the major rivers in the world are listed in Table 4.1(Chien and Dai, 1980; Sedimentation Committee, 1992;China–Bangladesh Joint Expert Team, 1991).

4.3 CLASSIFICATION OF RIVER PATTERNS4.3.1 River patternsIn China, rivers are often categorized in four basic patternsaccording to their static and dynamic characteristics.(1) Straight: Straight rivers are usually relatively short reaches

having negligible sinuosity at the bankfull stage. At lowstages, there are sand bars on both sides of the stream, andthe thalweg meanders in a sinuous path along the bars(Figure 4.3 (a)). The alternate sand bars move downstreamand the thalweg also shifts simultaneously. Long, straightrivers rarely occur naturally, and are often engineered.

CHAPTER 4 — FLUVIAL PROCESSES 55

1, 2, 3 — Flood, middle and low flow; 4 — Valley slopes; 5 — Flood plains; 6 —Lips of flood plains; 7 — Side bar; 8 — Levees; 9 — Sediment deposit; 10 — Original rock bed.

Figure 4.2 — Morphology of plain river valleys.

Table 4.1Annual runoff and sediment load of some rivers in the world

State River Station Drainage Annual runoff Annual Average sedimentarea (km2) (109 m3) sediment load concentration

(109t) (kg/m3)

Bangladesh Brahmaputra River Bahadurabad 535 000 618 0.499 0.81

Bangladesh Ganges Harding Bridge 963 000 344 0.196 0.57

Pakistan India River Kodli 969 000 175 0.435 2.49

Burma Irrawaddy River Polom 430 000 427 0.299 0.70

Viet Nam Red River Hanoi 119 000 123 0.130 1.06

United States Mississippi Rver Estuary 322 000 561 0.312 0.56

United States Missouri River Herman 1 370 000 61.6 0.218 3.54

United States Colorado River Grand Canyon 356 000 5.6 0.182 11.67

Brazil Amazon River Estuary 5 770 000 5 710 0.363 0.06

Egypt Nile River Gfla 2 978 000 89.2 0.111 1.25

China Yellow River Shanxian 688 384 43.2 1.62 37.6

China Yangtze River Datong 1 700 000 921.1 0.478 0.52

China Pearl River Wuzhou 329 725 227 0.0718 0.32

China Yongding River Guanting 50 800 1.40 0.081 57.8

(2) Meandering: Meandering rivers consist of a series of bendsof alternate curvature connected by straight crossings(Figure 4.3 (b)), and the slopes are usually relatively flat.The natural meandering channels are unstable, with bankcaving at the downstream part of concave bands. There aredeep pools in the bends and high velocities along the outerconcave banks. The depth at crossings is relatively shallowcompared to the depth at bends.

(3) Wandering: The river channels of wandering rivers are wideand shallow and divided by numerous unstable mid-bars.

They have a braided appearance at low flow, but all the barsare inundated or destroyed at the flood stage. The banks arepoorly defined and unstable, and the main stream frequentlyand rapidly shifts from one side to the other. The subsidiarychannels are also unstable and often change in flood seasons(Figure 4.3 (c)).

(4) Anabranched or branched: The appearance of anabranchedrivers is similar to that of branched rivers, but the mid-sandbars are higher and more stable, and some of them becomethe islands lived on and cultivated by local people, and can

56 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.2Classification of river patterns by different authors

River patternAuthor

Meandering Non-meandering

Leopold (USA) Meandering Straight Braided

Rosinski (Russian Meandering Periodic widening WanderingFederation)

Contragies (Russian Free Non-freeFederation) meandering meandering Single channel Branched

Xie (China) Meandering Straight Branched Wandering

Fang (China) Meandering Mid-island Shifting

Chien (China) Meandering Straight Anabranched Wandering

Lane, Chang (USA) Meandering Straight Steep slope braided Mild-slope braided

Simons (USA) Meandering Straight Braided

Ling (China) Stable Unstable Straightening- Stable Shiftingmeandering meandering meandering branched branched

Figure 4.3 — River patterns.

(a) Straight (Guankou Reach, Xishui River, China)

(b) Meandering (Chencun Reach, Weihe River, China)

(c) Braided (Wandering) (Huayuankou Reach, Yellow River)

(d) Branched (Maanshan Reach, Yangtze River) 1, 2, 3, islands

be inundated only by extraordinary floods. The channels ofanabranched rivers are divided by stable and high islandsinto more than two branches. One is the main channel andthe others are subsidiary channels. The main channel and thesubsidiary channels are also relatively stable, but can bechanged under some flow and sediment transport conditions(Figure 4.3 (d)).

4.3.2 Methods for classification of river patternsA prerequisite for the systematic study of fluvial processes is toclassify the river patterns according to the plan morphology (staticcondition) and the features of evolution (dynamic condition) of theriver. However, until now, there has been no unified method usedfor such classification. For example, Leopold and Wolman (1957)classified rivers into the categories of meandering, straight andbraided according to the plane morphology of the rivers. Fang(1964) classified rivers as mid-island, meandering and shiftingbased on the coefficient variation of peak flood discharge (Cv);the ratio of incoming sediment concentration to sediment carryingcapacity, and the ratio of the maximum width of water surfaceduring floods to the width of the channel. Chien, et al. (1987) andXie, et al. (1987) stressed the static and dynamic features of riversand classified rivers as straight, anabranched or branched, mean-dering and wandering. The static features of rivers denote theplanform, configuration, mega-bedform, and topography of riverchannels. Dynamic features include scope and intensity of maincurrent shifting, migration of the main channel, strength of deposi-tion and erosion in the main channel and of the banks, etc. Table4.2 shows the classification of river patterns suggested by differentauthors (Chang, 1988; Ling, 1963; Xie, 1980; Simons, 1979;Rosinski, 1950).

Leopold’s classification is much more simple andgeneralized. However, according to the experience of Chineseriver scientists, if the Middle and Lower Yangzte River and theLower Yellow River were classified into the same river pattern, thebraided pattern, this would be quite inappropriate and theclassification of river patterns would lose its significance. TheMiddle and Lower Yangzte River, having 41 branched reacheswith a total length of 817 km, has high lands, high mid-bars, andits channels are relatively stable, while the Lower Yellow Riverhas its wandering main current with large shifting scopes in atransversal direction, and a changeable and unpredictableconfiguration over a length of 275 km, with a wide channel bedand dense and scattered mid-bars. These two rivers, in fact, reflecttwo different river patterns with different fluvial processes.Therefore, Chinese scientists prefer to classify the two rivers intothe anabranched and wandering categories, respectively, ratherthan the braided category. As for piedmont rivers with a largeslope, coarse bed sediment, low mid-bars and stable filaments inlow water periods, but with apparent deformation of channel inhigh water periods, they are still classified in the wanderingcategory.

4.3.3 Characteristics of rivers with different patternsSometimes a river has the features of two river patterns. Forexample, the Brahmaputra River at the India-Bangladesh borderhas the appearance of a braided pattern. It is a wandering river, andwhile the dynamic features of the river in Bangladesh are those ofa wandering river, the stable islands occupied by local people are3–4 m above the low water level, and some of them have existed

for more than 100 years. It is thus a wandering-anabranched river(Zhou and Chen, 1998). The upper part of the Lower Yellow Riveris a typical wandering river, and its lower part is a typicalmeandering river.

4.3.4 Causes for formation of river patternsThe pattern of a river is determined by the characteristics of itswatershed, i.e. (i) incoming runoff and its hydrograph; (ii)incoming sediment load and its hydrograph, and size distributionof sediment; and (iii) boundary conditions such as the topogra-phy of the valley, geological structure, sediment particles, andsoil composition of the channel and banks. For most alluvialrivers, boundary conditions play a significant role in the forma-tion of river patterns. If the boundary, including channel bed andbanks, is composed of sand or silt, a wandering river such as theLower Yellow occurs. When the channel bed is composed ofsand and silt and the banks have some clay or sandy clay, ameandering river such as the Jingjiang Reach of the MiddleYangtze occurs. This conclusion was also proved by the experi-ments conducted by Ying (1965) and Schumm (1972). Theirexperiments were carried out on channel beds with uniformslopes, and the channel beds and banks were composed of sand.When bed materials were added, the wandering river ultimatelyoccurred. Once the bed materials and clay or white bole wereadded, the clay or white bole settled on the banks, and then themeandering river occurred.

In addition to the boundary conditions, supplementaryfactors, such as the sedimentation of river bed, scope of dischargevariation, floods features and geographic conditions, etc., also ledto subsidiary effects on the formation of river patterns. Chien(1987) gave the summarization as shown in Table 4.3.

4.3.5 Transformation of river patternsThe pattern of a river defined by the definite conditions of its riverbasin can be transformed when remarkable changes occur in thenatural conditions in the river basin, or after large-scale humanactivities. For example, the Murrumbidgee River in Australia wasa typical wandering river in ancient times, when it had a dryclimate, less runoff, worse vegetation, more sediment load andless clay and silt in its bed material. However, the meanderingriver was later transformed, because the climate became wet, rain-fall increased, vegetation grew and the incoming sediment loaddecreased (Schumm, 1968). The Missouri River in the UnitedStates was a meandering river in the 19th century. The vegetationand forests on the banks and floodplains were destroyed by floods,especially the 1881 flood, and the river gradually widened andstraightened (Schumm, 1971). If a reservoir is put into operation,the river pattern in the reach downstream of the reservoir ischanged. For example, after the Sanmenxia Reservoir on theYellow River was impounded in the early 1960s, the number ofregulated low floods increased, and the channel bed downstreamof the reservoir was eroded by the clear water released from thereservoir. The wandering reach of the Lower Yellow River thustended to be transformed into a single, meandering channel.

4.3.6 Critical relationships between different river patternsThere are some critical conditions in distinguishing betweendifferent river patterns. If a factor of a river pattern such as thelongitudinal slope is close to a critical value, a small change of thefactor may result in a great change in the river pattern.

CHAPTER 4 — FLUVIAL PROCESSES 57

58 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.3Conditions for different river patterns

Condition Wandering river Branched river Meandering river Straight river

Loose particles Materials lying Two-layer structures Banks composed ofComposition of with low-erosion- between wandering having erosion more clay or havingband material resistance and meandering rivers resistance on both more vegetation

on both side banks side banks

Side banks controlledNode point control by node points with short

Boundary lies at entrance or intervals or wide distri-condition Node point – exit of branch, – bution of exposed bedrock

and transversal on both side banks causedfree shifting by geological tectonicis restricted movement

Standing of down-Water level stream water level

withstanding – – in flood season –benefits maintenanceof meandering river

Incoming Relatively large Low incoming bed Low incoming bedsediment load amount of material load with a material load with a –

from watershed incoming bed certain amount of certain amount ofmaterial load wash load wash load

AccumulationEquilibrium of in past.

Incoming longitudinal Channel Longitudinal erosion Longitudinal erosionsediment erosion and aggradation and deposition and depositioncondition deposition is beneficial to are basically in are basically in –

formation of equilibrium equilibriumwandering

Channel deposition Weakened erosionin moderate and low in flood season

Yearly erosion water encourages – and weakened –and deposition the development of deposition in

a wandering river non-flood season

Small ranges of Small ranges ofRange of Large range discharge variation discharge variation

Incoming discharge of discharge and coefficient and coefficient –runoff variation variation variation of variation ofcondition flood discharge flood discharge

Rising and Sharp rising and Slow rising and Slow rising andfalling of flood falling of flood falling of flood falling of flood –

Slope of straight riverson estuarine is

small, but that havingexposed bedrock or dense

Slope of valley Steep slope Smooth slope Smooth slope vegetation on both sidesof banks could be formed

and developed undervarious slopes

On middle and Straight river with exposedOn alluvial fan On middle and lower part of bedrock or dense vegeta-out of gorge or lower part of alluvial plain tion on both sides of banks

Geographical site upper part of alluvial plain withstood by main could be formed andalluvial plain river or lake in developed under different

flood season geographic sites

4.3.6.1 RELATIONSHIPS BETWEEN LONGITUDINAL SLOPE AND

RIVER PATTERNS

The geological site of a river plays a great role in the formation ofits river pattern. A river flowing out of a gorge, with a steep slope,would easily develop into a braided-wandering river, while a riveron a plain, with a smooth slope, would be a sinuous (meandering)river. An empirical relation was established by Chien and Zhou(1965).

S = 0.01Qn–0.44 (4.1)

where Qn is the bankfull discharge in m3 s–1, and S is the longitu-dinal slope in 1/10 000. The rivers in the region above the S-Q linebelong to the wandering pattern, while those in the region belowthe line belong to the meandering pattern. For rivers of the samesize, the rivers develop from a meandering to a wandering patternas the slope increases.

4.3.6.2 RELATIONSHIPS BETWEEN LONGITUDINAL SLOPE AND

MEAN DISCHARGE

Lane (1957) established two relationships between longitudinalslope and mean discharge:

S = 0.0041Qm–0.25 (4.2)

S = 0.0007Qm–0.25 (4.3)

where Qm is the average annual discharge in m3 s–1, and S is thelongitudinal slope. The rivers in the region above Equation 4.2belong to the wandering pattern, while those below Equation 4.3are meandering. The rivers in the region between Equations 4.2and 4.3 are in transition from meandering to wandering.

4.3.6.3 RELATIONSHIPS BETWEEN LONGITUDINAL SLOPE AND

MAXIMUM DISCHARGE

Romashen (1977) analysed the data of valley slopes and averagemaximum discharge from 250 reaches of the rivers in the formerUSSR and divided the rivers into branched, un-shaped meander-ing, and meandering patterns. He concluded that the criticalcondition between branched and un-shaped meandering rivers is:

Qmax S = 1.4 (4.4)

and the critical condition between un-shaped meandering andmeandering rivers is:

Qmax S = 0.35 (4.5)

where Qmax is the maximum discharge in m3 s–1, and S is thelongitudinal slope. According to Romashen’s analysis, branchedrivers could also be divided into the channel bed branched pattern(corresponding to wandering) and the floodplain branched pattern(corresponding to branched). A channel bed branched river has asteep valley slope and low discharge, while the flood plainbranched river has a mild valley slope and large discharge.

4.3.6.4 WANDERING INDEX

Chien, et al. (1965) analysed the data from 21 stations on theYangtze River, the Yellow River and other plain rivers in China,and found the following wandering index:

(4.6)

where ∆Q is the rising range of flood discharge in m3 s–1, Qn isthe bankfull discharge in m3 s–1, T the duration of floods in days,d is the depth under bankfull discharge in m, S is the slope, D35 isthe grain size of bed material for 35 per cent finer in mm, Qmaxand Qmin are the maximum and minimum daily discharges in theflood season in m3 s–1, b is the channel width under bankfulldischarge in m, and bmax is the surface width under the historicalhighest water level including width of flood plains in m. Theranges of available data are: Q, 242–92 600 m3 s–1; Qn, 35–58 500m3 s–1; d, 0.32–17.0 m; b, 85–3 010 m; D50, 0.06–0.32 mm; andC, 1.7–1 010 kg m–3. Θ > 5 is a wandering (braided) river, Θ < 2is a non-wandering river, and Θ = 2–5 is a transitional river.

4.3.6.5 RELATIONSHIPS BETWEEN LONGITUDINAL SLOPE, BED

SEDIMENT AND DISCHARGE

Based on the theory of flow power, Chang (1988) established fourregions by three critical straight lines for the reactions amonglongitudinal slope, bed sediment and discharge.

Critical straight line 1: Sc/d1/2 = 0.00238Q–0.51 (4.7)

Critical straight line 2: S/d1/2 = 0.05Q–0.55 (4.8)

Critical straight line 3: S/d1/2 = 0.047Q–0.51 (4.9)

where Sc is the critical slope corresponding to bed load, d is themedium size of bed material in mm, Q is the bankfull discharge incfs, and S is the longitudinal slope. If the unit of discharge is inm3 s–1, the coefficient in Equation 4.7 is 0.000386; in Equation4.8 it is 0.00704; and in Equation 4.9 it is 0.00763.

The rivers in the region between critical straight lines1 and 2 are meandering or straight; the rivers in the regionbetween critical straight lines 2 and 3 are straight or braided, andthe rivers in the region above critical straight line 3 are mild slopebraided and steep slope braided, which are separated by a hypo-thetical straight line.

4.3.7 Indexes of river stabilityThe characteristics of alluvial processes are determined by theadded conditions of the river basin. The index of river stabilityis a mark to express the local, temporal and relative variation ofthe river channel when the incoming runoff and sediment loadfrom the watershed change over time. The stability of a riverand its equilibrium are two different concepts. The latterdenotes that, as a whole, no erosion or deposition occurs in theriver when the incoming sediment load carried by the flow fromupstream reach is equal to the sediment carrying capacity of theflow. Obviously, a stable river is not often a river in equilib-rium. Similarly, an equilibrium river is not necessarily a stableriver (Chien, 1958).

The index of river stability can be divided into indices oflongitudinal (river channel) stability and transversal (river bank)stability.

4.3.7.1 LONGITUDINAL STABILITY OF RIVER CHANNELS

Longitudinal stability denotes the variability of channel bed due toaggradation and degradation of the bed along the river. It depends

CHAPTER 4 — FLUVIAL PROCESSES 59

Θ∆

=−

+(

.)( ) ( ) ( ) ( )

. max min

max min

. . max .Q

TQ

ds

D

Q Q

Q Q

b

d

b

bn0 5 35

0 6 0 6 0 45 0 3

on the erodibility of bed sediment and flow intensity, and can beexpressed by the Rohkin and Chien numbers.(1) Rohkin Number. The longitudinal stability of a riverchannel depends on the ratio of the tractive force acting on a sedi-ment particle by flow to the resistance force against the motion ofthe particle, and can be expressed by the Rohkin Number, i.e.:

Φ = d/s (4.10)

where d is the grain size of bed material (d35 or d50 in mm), and Sis the channel slope in 1/1 000. The larger the parameter, the morestable the channel.(2) The Chien Number (1958, 1987). The stability of analluvial river is determined by the incoming runoff and sedimentload from its river basin. For a quasi-equilibrium river, the sedi-ment carrying capacity is equal to the incoming bed material loadand depends on the incoming flow and boundary conditions. Therelative stability can thus be expressed by the hydraulic parameterof sediment carrying capacity, i.e.,

K = D/dS (4.11)

where D is the grain size of bed material (d35 or d50 in mm), d isthe depth under dominant discharge in m, and S is the longitudinalslope in 1/1 000. The larger the parameter K, the more stable thechannel. The parameters of Φ and K of the Yangtze and YellowRivers are listed in Table 4.4.

Obviously, the physical meanings of the above twonumbers are the same, although they are derived using differentapproaches.

4.3.7.2 TRANSVERSAL STABILITY OF RIVER CHANNELS

(1) The Altounin Number (1962). The transversal stabilityof a channel is related to the stability of the banks. The mainfactors affecting transversal stability are flow direction, erodibilityof band soil and elevation difference between floodplain and mainchannel. It is highly complicated and has not been fully studied,but some indirect relationships have been obtained.

Ψ = Q0.5/S0.2b (4.12)

where Q is the dominant discharge in m3 h–1; b is the channelwidth under the dominant discharge in m, and S is the slope underthe dominant discharge in m km–1. The larger the parameter Ψ,the more stable the channel.

(2) The Xie Number (1987). The transversal stability of anatural channel is related to the channel banks and can beexpressed as follows:

C = b/B (4.13)

where b is the channel width for low water in m, and B is thechannel width under the dominant discharge in m (see section4.3.1). The larger the parameter C, the narrower the main channel,thus, the more stable the banks. The parameters of Ψ and C for theYangtze River and the Yellow River are listed in Table 4.5.

4.4 MORPHOLOGY OF RIVERSUnder the effects of flow action over a long period, an alluvialriver may be in a quasi-equilibrium state through the self-adjustingaction of the channel. Some functional relationships exist betweenthe river morphology, including cross-sectional geometry andlongitudinal profile, and river basin factors. These relationshipsare called hydraulic geometry equations, i.e.,

b = F1 (Q, Go, Do); d = F2 (Q, Go, Do); s = F3 (Q, Go, Do) (4.14)

where b is the channel width, d is the channel depth, s is the longi-tudinal slope of the river, Q is the incoming water discharge andits hydrograph from the upper reach; Go is the incoming sedimentload and its hydrograph from the upper reach, and Do is the sizecomposition of incoming sediment load.

For the incoming sediment load with different grainsizes, only the bed material load has an effect on channel forma-tion. Once the incoming bed material load settles down, itbecomes the material composing the channel boundary and alsoplays an important role in channel stability and cross-section form.Therefore, Equation 4.14 can be transformed as follows:

b = f1 (Q, G, D); d = f2 (Q, G, D); s = f3 (Q, G, D) (4.15)

where G is the incoming bed material load and its hydrograph, andD is the boundary conditions, including the composition ofchannel bed and banks.

Because morphological relationships depict the relation-ship between rivers suited to the conditions of incoming runoffand sediment load and the channel boundary, they have becomethe basis of the hydraulic computation of alluvial rivers, predictionof fluvial processes and river training, etc. and have had the mostsignificance in river engineering.

4.4.1 Dominant dischargeThe dominant discharge is such a discharge that its channel-forming effects are equivalent to the comprehensive actions

60 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

River Reach and river pattern Φ K

Yangtze River Jingjiang, meandering 2.9–4.1 0.27–0.33Wuhan, branched 6.7–7.8 0.39–0.52Nanjing, branched 7 0.35

Yellow River Upstream of Gaocun,wandering river; 0.31–0.47 0.18–0.21From Gaocun to 0.42–0.54 0.17Taochengpu,transitional reach

Brahmaputra Noonkaw-Aricha, 2.8–7.9 0.65–1.81River wandering-branched

Table 4.4ΦΦ and K

River Reach and river pattern Ψ C

Yangtze River Jingjiang, 0.87–1.56 0.67–0.77meandering river

Upstream of Gaocun, 0.18–0.45 0.09–0.17Yellow River wandering river;

From Gaocun to 0.48–0.75 0.17–0.20Taochengpu

Table 4.5ΨΨ and C

produced by discharge according to a long-term discharge hydro-graph. Dominant discharge has the greatest influence on themolding of river channels. Although the highest flood has greatpotential, it cannot play a maximum role in channel formationbecause of its short time period. Low water flow has a long dura-tion, but it cannot play the maximum role either because it has asmall volume of discharge. Therefore, dominant discharge is arather large discharge, instead of the maximum flood discharge.

4.4.1.1 DETERMINATION OF DOMINANT DISCHARGE

(1) Makaviev’s method (1955) — The effect ofdischarge on channel formation depends on its sediment-carryingcapacity and time duration. The sediment-carrying capacity can beexpressed by the product of Qm and S, where Q is the discharge; mis an exponential; and S is the longitudinal slope. Let the frequencyof occurrence of the discharge be p. The discharge correspondingto the maximum QmSp has the maximum effect on channel forma-tion and can be adopted as the dominant discharge. Procedures fordetermining the dominant discharge are listed as follows:(i) Divide the long-term measured hydrograph at a cross-section

on the studied reach into a number of discharge grades.(ii) Calculate the occurrence frequency of each discharge grade.(iii) Draw the discharge-slope relationship and determine the

mean slope corresponding to each discharge grade.(iv) Compute the product of QmSp for each graded discharge.

Draw the relationship between flow discharge and sedimentdischarge on a logarithmic paper, where the exponential m isthe slope of the curve line. Generally, for plain rivers, m isequal to 2.

(v) Draw the QmSp-Q relationship.(vi) Find the maximum QmSp. The discharge corresponding to

the maximum QmSp is the dominant discharge.As shown in Figure 4.4, there are two peak values of

QmSp. The discharge corresponding to the first peak of QmSp isanalogous to long-term average maximum flood discharge with anoccurrence frequency of 0.5–6.5 and 3 per cent on average, and itswater level corresponds to the bankfull water level. It is called thefirst dominant discharge. The discharge corresponding to thesecond peak of QmSp is slightly higher than the long-term averagedischarge with an occurrence frequency of 17.5–44.5 per cent and30 per cent on average, and its water level corresponds to theelevation of the point bar. It is called the second dominantdischarge.

Generally, the first dominant discharge is used as thedominant discharge to determine the river morphology of thechannel for moderate discharges. The second dominant dischargemoulds the channel of low discharge and is applied to the regula-tion of navigation course.

(2) Chien’s method (1987) — The channel-formingeffect of a discharge depends on its corresponding sedimentdischarge and time duration. As shown in Figure 4.5, whendrawing the curves of sediment discharge (curve A), frequency(curve B) and the product of sediment discharge and frequency(curve C) for various grades of discharge, the discharge corre-sponding to the maximum value on curve C is the dominantdischarge.

Benson and Thomas (1966) calculated the dominantdischarge with the same method based on the data from nine riversin the United States. The results indicate that the occurrencefrequency of the dominant discharge is 7.6–19.5 per cent, with theaverage of 12.4 per cent. In their calculation, the sedimentdischarge is only for the suspended load.

(3) Chikurimora’s method (1969) — This method usedthe weighted sediment discharge to determine the dominantdischarge, i.e.,

(4.16)

where Qd is the dominant discharge in m3 s–1, Qi is the dischargeof i grade in m3 s–1, Qsi is the sediment discharge correspondingto the discharge of i grade in t s–1, and n is the number of dividedgrades.

4.4.1.2 BANKFULL DISCHARGE

The field and experimental data indicate that the velocity in riverchannels increases with the rising of water levels. The effect onchannel formation is greatest when the water level is at the eleva-tion of floodplains. The flow disperses and the effect on channelformation is weakened when the water level rises further. Andrews(1980) also concluded that bankfull discharge corresponds to thedischarge when sediment transport is the strongest. It is thusreasonable that bankfull discharge can be used as the dominantdischarge. When determining the bankfull discharge for a riverreach, the reach should be of sufficient length, and some cross-sections and their corresponding water levels should be measuredin the reach so as to avoid shortcomings caused by using data fromonly one or two cross-sections. Bankfull discharge is determineddirectly according to the water level corresponding to the eleva-tions of the floodplains along the reach. This method is similar to

CHAPTER 4 — FLUVIAL PROCESSES 61

Figure 4.4 — Relationship between QmSp and Q. Figure 4.5 — Determination of dominant discharge.

€

QQ Q

Qd

i

nsi i

i

nsi

=∑

∑=

=

1

1

Dis

char

ge (

m3

s–1 )

(A)

Sedi

men

t dis

char

ge

(B)

Freq

uenc

y

(C) A

×B

that proposed by Leopold (1964). For determining the bankfullstage, Riley (1972) also suggested measuring the width-depth ratioof the cross-section at different water levels. The width-depth ratiodecreased with an increasing water level, and reincreased with anincreasing water level when the flow was over the flood plains.Riley concluded that the water level corresponding to the turningpoint of the relationship between the width-depth ratio and waterlevel was the bankfull stage.

It should be pointed out that under some conditions, forexample when the channel cross-section is not regular, the naturallevees along the flood plains are higher than the flood plains, orthe mountainous or upland rivers have no flood plains, etc., theaccurate determination of the bankfull stage is somewhat difficult.

4.4.1.3 EMPIRICAL EXPRESSION FOR BANKFULL DISCHARGE

(1) Expression of the Institute of Hydraulic Research ofthe YRCC (1978):

Qd = 7.7Qf0.85 + 90Qf

1/3 (4.17)

where Qd is the dominant discharge (bankfull discharge) in m3 s–1,and Qf is the long-term average discharge in flood seasons inm3 s–1.

(2) Hey’s expression (1975):

Qb = 1.06A0.8 (4.18)

where Qb is the bankfull discharge in m3 s–1, and A is the area ofthe watershed (km2).

(3) Williams’ expression (1978):

Qb = 4.0A1.21S0.28 (4.19)

where Qb is the bankfull discharge in m3 s–1, A is the wetted areaof cross-section at bankfull stage in m2, and S is the slope of thewater surface.

(4) Relationship between bankfull discharge and annualaverage discharge. Based on the data published by Shumm (1968)and Carlton (1965), Chang (1979) established the relationshipbetween bankfull discharge and annual average discharge, asshown in Figure 4.6.

4.4.1.4 BANKFULL DISCHARGE ESTIMATED BY RECURRENCE

INTERVALS

(1) Leopold (1964) found that the recurrence interval of bankfulldischarge was 1.5 years, based on the data from 13 stationsin the eastern United States.

(2) Nixon (1959) concluded that bankfull discharge had anaverage frequency of 0.6 per cent based on data from riversin England and Wales.

(3) Pickup (1976) found that there were two bankfull dischargesbased on data from the intermittent river in the CumberlandRiver basin in Australia. One corresponded to a flood with arecurrence interval of 20 years and played a role in theformation of river banks, size and shape of the main channel.Another corresponded to the floods occurring 3 to 5 times ayear, which determined channel width and the slope of lowwater.

(4) Emmett (1975) found that the recurrence intervals of bank-full discharge was 1.5 years.

(5) Chien (1987) concluded that the use of a discharge with arecurrence frequency was reasonable. He made a simplifica-tion for the computation of the dominant discharge. A floodwith an interval of 1.5 years might be roughly applied as thedominant discharge if there were not enough data.

62 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.6Coefficient ββ for different material

Material Calcareous limerock Limestone Dolomite Apos and stone

β 0.017 0.010 0.008 0.003

Figure 4.7 — Longitudinal profile of the Lower Yellow River,Figure 4.6 — Relationship between bankfull and annual averagedischarge (after Chang, 1979).

Annual average discharge (cfs)

Ban

kful

l dis

char

ge (

cfs)

Dis

tanc

e fr

om g

orge

out

let (

km)

Water surface differences betweengorge outlet and stations (m)

4.4.2 Longitudinal profilesThe longitudinal profile is the result of long-term actions betweenflow and channel bed. The shape of the longitudinal profiledepends on the incoming runoff and sediment load from its water-shed and the geology of the channel, while the elevation of thelongitudinal profile is controlled by the downstream base oferosion.

Seen in detail, a longitudinal profile of a natural river isa smooth curve, and can be categorized into three types: sunken,protruding and straight. For most plain rivers, the longitudinalprofiles are sunken, but for some mountainous rivers in the frontreach, they are protruding. The longitudinal profiles for bothmountainous and plain rivers are fluctuated and are saw-toothedbecause of different geology, channel widths, and the existence ofpools, crossings and sand bars.

4.4.2.1 GEOMETRIC EXPRESSIONS OF LONGITUDINAL PROFILES

The longitudinal profile can be expressed by exponential orsemi-logarithmic curves when there are no break points alongthe river.(1) Sternberg’s expression (Chien, et al., 1987). If there areno different materials coming from the tributaries, the bed materi-als of an alluvial river become finer and finer because they sufferfrom wear and tear when moving along the river. The relationshipbetween the weight of bed material and the distance it moves canbe expressed as follows:

W = Woe–β1L (4.20)

where Wo is the weight of bed material at the beginning cross-section, W is the weight of bed material after passing a distance L,and β is the wear coefficient of the particle. Because the weight ofa sediment particle is directly proportional to its diameter,Equation 4.20 can be transformed into:

D = Doe–β1L (4.21)

where Do is the grain size of bed material at the beginning cross-section, D is the grain size of bed material after travelling adistance L, and β is the wear of the coefficient particle as shown inTable 4.6 (Sedimentation Committee, 1992).(2) Shulits’ expression (1941). The channel slope is directlyproportional to the grain size of bed material, so:

S = Soe–α1L (4.22)

where So is the channel longitudinal slope at the beginning cross-section, S is the channel longitudinal slope at a position with adistance of L from the beginning cross-section, L is the distance,and α is the change of the coefficient slope, which is related to thematerials of channel bed and banks. Based on the data from thereach downstream of Otowi on the Rio Grande River, the followingexpressions are obtained (Sedimentation Committee, 1992).

D50 = 0.47e–0.0059L (4.23)

S = 0.022e–0.0092L (4.24)

where D50 is the median size of bed material in mm, and L is thedistance in m.

(3) The Yivanov Expression (1951)

h = H (l/L)n (4.25)

where h is the elevation at a certain position, H is the elevation atthe origin of a river, L is the horizontal distance from the origin tothe estuary, l is the distance from the position to the estuary, and nis the morphological exponent.(4) The Chien Expression (1965). The longitudinal profileof the Lower Yellow River can be expressed as follows:

F = 2.45L0.60 (4.26)

where F is the difference of water surface from the outlet of theriver gorge to a gauge station in m, and L is the distance from theoutlet to the station in km (Figure 4.7).

4.4.2.2 EMPIRICAL RELATIONSHIPS BETWEEN LONGITUDINAL

SLOPE AND WATERSHED FACTORS

(1) Relationships between longitudinal slope and thecomposition of the channel bed. From the viewpoint of sedimenttransport equilibrium, the coarser the sediment in the channel, thesteeper the channel slope. The opposite is also true. When bedsediment becomes finer, the channel slope becomes smoother.(a) For the channel slope of main streams and tributaries of the

Middle and Lower Yellow River (Chien, 1987):

S = 41D501.3 (4.27)

(b) For the slope of the Jingjiang Reach of the Yangtze River(YRWRC, 1959):

S = 25D502.38 (4.28)

(c) The slope of the rivers in the Central Asian part of the formerUSSR can be expressed as follows (Altwunin, 1957):

S = 0.85D501.10 (4.29)

where S is the longitudinal slope in 1/10 000, and D50 is themedian size of bed material in mm.(2) Relationsihps between longitudinal slope and dischargeor watershed area. Since sediment discharge is proportional to ahigh power of flow discharge, the relationship between slope anddischarge reflects to some extent the relationship between slopeand the incoming runoff and sediment load from the river basin. Ifthere are not enough measured data, the watershed area can beused instead of the discharge.(a) The slope for rivers in Siberia, Russia (Makkaveev, 1959):

(4.30)

(b)The slope for rivers in China (Li, 1965):

(4.31)

(c) The slope for rivers in the eastern United States (Hack,1957):

CHAPTER 4 — FLUVIAL PROCESSES 63

SQ

=2500 43.

SQ

=20 9

0 27

..

(4.32)

(d) The slope for the mainstream and tributaries of the YellowRiver (Chien, 1965):

(4.33)

where Q is the dominant discharge (Equation 4.30) or bankfulldischarge (Equation 4.31) in m3 s–1, A is the watershed area inkm2, D50 is the median size of bed sediment in mm, and S is theslope in 1/10 000.(3) Relationships between longitudinal slope and factors ofsediment.(a) The slope of rivers in China (in laboratory) (Li, 1965):

(4.34)

where Qn is the bankfull discharge in m3 s–1, C is the sedimentconcentration of bed material load corresponding to bankfulldischarge in kg m–3, and D50 is the medium size of bed sedimentin mm.(b) Longitudinal slope of the Lower Weihe River (a main tribu-

tary of the Yellow River) (North-west Institute, 1962)

(4.35)

where n is the Manning coefficient; w is the mean fall velocity incm s–1, C is the sediment concentration in kg m–3, Q is the domi-nant discharge in m3 s–1, and D50 is the median size of bedsediment in mm.(c) Longitudinal slope for some rivers in England

Hey (1982) obtained the following expression based onthe data from 25 stable crossings on the River Wye (a gravel-bedriver):

(4.36)

where S is the longitudinal slope, Qn is the bankfull discharge inm3 s–1, D50 is the median size of bed sediment in mm, and Gb isthe sediment discharge of bed load under bankfull discharge inm3 s–1.

Hey (1982) extended his data to 66 stable reaches on theRivers Wye, Severn and Tweed, and obtained the followingexpression:

(4.37)

in which the bankfull discharge was calculated using the correc-tion Colebook formula and the sediment discharge of bed loadwas calculated using the Meyer-Peter formula.(4) Theoretical solution of longitudinal slope for alluvialrivers and estuaries.

Dou (1964) deduced an equation of slope from thetheory of minimum activity for alluvial rivers and estuaries.

(4.38)

After simplification,

(4.39)

where S is the slope, Q is the long-term average discharge, C is thelong-term average sediment concentration, Vos, Vob are the haltingvelocity of suspended load and bed material, respectively, α is therelative stability of the channel banks and channel bed, αbank———αbed(when the stability of the channel banks is close to the stability ofthe channel bed, α = 1.0); β the coefficient of tidal wave (β =1 + 0.35

∆HH , where ∆H is the tidal difference, H is the mean depth

under moderate tidal stage) and for the general estuary and non-tidal estuary β = 1, K is the parameter, K = 0.055 γsη (√g/c)6,where γs is the specific weight of the sediment particle; g isgravity acceleration; c is the Chezy coefficient, η is the ratio ofbottom velocity to mean velocity, and δ is the ratio of averagesediment concentration to near-bed sediment concentration undersaturation. Values for αbank and αbed for different materials arelisted in Table 4.7.

4.4.3 Cross-sectional morphology of riversThe cross-sectional morphology of rivers can be divided into twocategories, the morphology of cross-sections at a station, andcross-sectional morphology along rivers. The morphology of across-section at a station means the changes in sizes of the cross-section in a short reach or at a cross-section under a differentdischarge. It reflects the changes in geometry of the wetted cross-section. The cross-sectional morphology along rivers implies thechanges in channel geometry of different rivers, or in the upperand lower reaches of the same river, caused by different incomingrunoff and sediment load and conditions of channel boundaries.The data at different cross-sections of different rivers or differentreaches of the same river are unified by the bankfull discharge orthe discharge corresponding to a certain occurrence frequency.They reflect the changes in the channel geometry of the rivers.The two cross-sectional morphologies cannot be obscured,because the changes in bed sediment and slopes along the river aresubstantially greater than those at a cross-section.

64 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.7Indexes of soil stability, αbank and αbed

Material composing banksand channel bed αbank αbed

(grain size in mm)

Coarse sand 2.5–2.0Medium-coarse sand 2.0–1.5Medium sand (2.0-1.0) 1.5–1.2Fine sand (1.0–0.5) 1.1–0.9Silty sand (0.5–0.25) 1.0–0.8Silty clay (0.25–0.10) 1.0–0.8Mild clay (0.10–0.05) 1.7–1.13Clay (0.05–0.01) 2.2–1.8Heavy clay 2.5–2.3

Sn w C

Q D=

+2620

0 385

3 5 1 28 1 28

0 35750

. . .

..

SC

QD

n

=

45.5

0.591 2

50

/

S Q D Gn b= −1 02

0 52501 10 0 11

.. . .

S Q D Gn b= −0 679

5 3500 97 0 13

.. . .

Sg V C

K V Q g x

V C V Q

k

x

k V Q

gV C

os

o

os ob

ob

os

= +∂

∂

+∂

∂

1 150 43

0 807

22 4 4 4

4 2 22 9

8 9

2 2 2 2

2 22 9

2 2

21 3

. ( ).

( )

. ( )

//

/

/

ηβ

α δ

α

β

α

β

Sg V C

k V Q

os

ob

= 1 152

4 4 4

4 2 22 9

. { }/η

α

SD

A=

2 96

0 34

50

.

SD

A=

60 50

0 6.

4.4.3.1 HYDRAULIC GEOMETRY

Leopold (1953) concluded that natural rivers in a state of equilib-rium have simple exponential relationships between the width,depth, flow velocity and discharge, like those on the graded canalsin India and Pakistan. These relationships are called the hydraulicgeometry of rivers, and are expressed as follows:

b = α1Q β1; d = α2Q β2; v = α3Q β3 (4.40)

According to the continuity law of flow:

β1 + β2 + β3 = 1 (4.41)and

α1α2α3 = 1 (4.42)

The above expressions are simplified and special examples. Infact, the coefficients α1, α2, α3 and exponentials β1, β2, β3 are vari-ables in the relationships both for cross-sections and along therivers because of the influences of other factors, in addition tothose of the discharge. The exponential in the relationships of rivermorphology are listed in Table 4.8 (Chien, et al., 1987).

Chien (1987) suggested that the exponential for thehydraulic geometry of rivers, β1, β2, β3, can be adopted as 0.14,0.43 and 0.43 respectively, on average, as shown in Table 4.9.

4.4.3.2 HYDRAULIC GEOMETRY ALONG RIVERS

(1) Changes of exponential. The exponentials in Leopold’sexpressions for the hydraulic geometry along rivers are changeable

for different rivers, as shown in Table 4.8. The bankfull dischargeshould be used in these expressions, but the exponentials aredifferent for different frequencies of dominant discharges(Table 4.10) (Chien, et al., 1987).(2) Ratio of width to depth of cross-sections. Ratios ofwidth to depth are used to express the shapes of cross-sections,either the narrow-deep type or the wide-shallow type. Based onthe plain rivers in the former USSR, the National Institute ofHydrology Research suggested the following expression (Xie,1987):

(4.43)

where b and d are the average width and depth, respectively, ofcross-sections in a reach corresponding to the bankfull discharge

CHAPTER 4 — FLUVIAL PROCESSES 65

Table 4.8Exponential in relationships of hydraulic geometry for rivers throughout the world

Country River Cross-sectional relationship Longitudinal relationship

β1 β2 β3 β1 β2 β3 Q

6 small wandering rivers in north China 0.48 0.35Meandering in the Lower Yellow RiverBend reach 0.16 0.30

China Straight reach 0.28 0.18Jingjiang Reach of the Yangtze River 0.08 0.46 0.46(average of 3 cross-sections)

Rivers in middle and west regions 0.26 0.40 0.34 0.50 0.40 0.10 Qm

Intermittent rivers in semi-arid regions 0.29 0.36 0.34 0.50 0.30 0.2016 rivers in Central Pennsylvania Stale 0.55 0.36 0.09 Q2.33

United States Brandywine Creek in eastern United States 0.4 0.41 0.55 0.42 0.45 0.13 Qn

Data from 158 stations 0.12 0.45 0.43White River in Washington State 0.38 0.33 0.27Small river on beaches affected by tide 0.09 0.13 0.78 0.76 0.20 0.04

discharge whenvelocity is maximum

27 rivers in England and Wales 0.49 0.27 0.24 Qn

23 gravel rivers in England and Wales 0.45 0.40 0.15 Qn

United Kingdom 17 rivers in low land in England 0.53 0.40 0.07 Qn

17 rivers in high land 0.52 0.32 0.16 Qn

3 small rivers in southern England 0.13 0.42 0.44

Rivers with gravel-composed banks 0.50 0.415 0.085 Qn

and channel bedCanada 20 gravel rivers 0.527 0.333 0.140 Q2

12 sand rivers 0.53 0.32 0.15 Qn

10 stations on the Rhine River in Europe 0.13 0.41 0.43

NOTE: Qn is the bankfull discharge; Qm the annual average discharge; and Qa is the discharge corresponding to the occurrence interval of a years.

€

b

d= ξ

Table 4.9Average ββ1, ββ2 and ββ3

Sources of data β1 β2 β3

Average value of data at 206cross-sections on the River Ryton in 0.16 0.43 0.42the United Kingdom

Average value of data at 158 stations 0.12 0.45 0.43Data from 10 stations on the Rhine 0.13 0.41 0.43River in Europe

On average 0.14 0.43 0.43

in m, and ζ is the geometrical coefficient. For a gravel channel bedζ = 1.4, for a coarse sand bed ζ = 2.75, and for a fine sand bedζ = 5.5. For the rivers in China, ζ is small for meandering riversand larger for wandering (braided) rivers, as shown in Table 4.11.This expression is widely used in China because of its simplestructure.

(3) Altounin’s expression (Xie, 1987):

(4.44)

where b and d are the average width and depth of a cross-sectioncorresponding to the bankfull discharge respectively, m is theexponential and η is the morphological coefficient. Values for mand η are listed in Table 4.12.

4.4.3.3 RELATIONSHIPS BETWEEN FACTORS OF WATERSHED AND

HYDRAULIC GEOMETRY ALONG RIVERS

(1) Effects of sediment composition. Based on the data from90 rivers with areas of 4.4–147 000 km2 and an annual averagedischarge of 0.57–146 m3 s–1, Schumm (1960), obtained thefollowing expression:

(4.45)

(4.46)

where b and d are the width and depth, respectively, and ib and iware the percentage of silt and clay in both the banks and channelbed. Later, Schumm (1968) further obtained the following expres-sion based on the data from the rivers in plains of the UnitedStates and the Murumbidgee River and paleochannels in Australia.

b = 43.7Qm0.38M–0.39 (4.47)

d = 0.51Qm0.29M0.342 (4.48)

where Qm, is the annual average discharge (all units are in m ands), and

(4.49)

In the formation of alluvial rivers, the size of a cross-section isdetermined mainly by the discharge, and the shape of a cross-section, wide or deep, is determined by the composition of bed andbank materials.(2) Effects of materials composing the channel boundary.Ferguson (1973) used the percentage of silt-clay in bank materialsMw as a parameter and found the following expression based onSchumm’s (1968) data.

(4.50)

where Q2.33 is the discharge corresponding to an occurrence of2.33 years. The stronger the erosion-resistance of bank materials,the smaller the channel width.

Bray (1982) used the data from 70 gravel rivers inCanada and found the following expression:

(4.51)

where Q2 is the discharge corresponding to an occurrence intervalof 2 years in m3 s–1. Coefficients α relating to bank materials areshown in Table 4.13.

Chien (1963) found the ratio of stable width to depthunder the dominant discharge as follows:

(4.52)

66 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.13Relationship between αα and bank material

Bank material α

Sand and medium gravel (d > 64 mm) 19.3

Sand and gravel (d < 64 mm) 20.1

Gravel covered by silt 15.2

Silt and clay 14.1

b

duc= 42 5λ .

DominantLocation River discharge β1 β2 β3

(m3 s–1)

United States Brandywine Q50% 0.34 0.45 0.32Q15% 0.38 0.42 0.32Q2% 0.45 0.43 0.17Qn 0.42 0.45 0.05

Puerto Rico Manati River Q70% 0.46 0.27 0.27Basin Q50% 0.44 0.30 0.35

Q30% 0.46 0.32 0.25

United Bolindin River Q50% 0.46 0.16 0.38Kingdom Basin Q15% 0.54 0.23 0.23

Q2 0.61 0.31 0.08

Table 4.10Different exponentials in Leopold’s expressions for different

dominant discharges

NOTE: Q6% is the discharge corresponding to an occurrence frequency of 6 per

cent; and Qn the bankfull discharge.

b

d M

Mi b i d

b d

b w

=

=+

+

255

2

2

1 08.

M

Mi b i d

b d

b w=+

+

2

2

Reach ζ

Yangtze River Meandering Lower Jingiang River 2.55–2.70Meandering Upper Jingiang River 2.67–3.27Branched, downstream of Chenlingji 3.42–3.63

Hanjiang River Meandering 2.0Yellow River Wandering, upstream of Gaocun 19.0–32.0

Transitional, downstream of Gaocun 8.6–12.4

Table 4.11Geometrical coefficient for rivers in China (Sedimentation

Committee, 1992)

Table 4.12m and ηη in Altwunin’s expression

Reach η M

Mountain reaches 10–16Mountain foot reaches 9–10 0.8–1.0Middle reaches of rivers 5–9

Lower reaches of rivers Silt bank 3–4Sand bank 8–10 0.5–0.8

b

dM

0 760 64

34 6

..

.= −

b Q Mw= −33 1 2 33

0 58 0 66. .

. .

b

dQ= α 2

0 2.

b

d

m

= η

where λuc is the ratio of threshold velocities of bed and bankmaterials when water depth equals 1 m.(3) Effects of incoming sediment load. Considering therelative erodibility of bed and bank materials for more than 60rivers in China and abroad, Yu (1982) established the followingexpressions.

(4.53)

(4.54)

(4.55)

where b and d are the average width and depth under long-termaverage discharge, Qm is the long-term average discharge, D50 isthe median size of bed material in mm, d50 is the median size ofsuspended load in mm, C is the long-term sediment concentrationin kg m–3, and m is the stability index equal to the side-slope coef-ficient of the bank lying between the historical low stage and thelong-term annual water stage. The data range for long-termaverage discharge is 3.6–28 000 m3 s–1; for long-term averagesediment concentration, 0.08–179 kg m–3; the median size ofsuspended load is 0.017–0.077 mm; the median size of bed sedi-ment is 0.025–13.5 mm.

Taking the channel slope reflecting the incoming sedi-ment load, Altwunin (1957) obtained the following expression forrivers in Central Asia, in the USSR.

b = AQ0.5S–0.2 (4.56)

where Q is the dominant discharge, S is the channel slope, and A isthe coefficient of the stable width of the channel, which is relatedto river patterns and ranges from 0.75 to 1.70.

Velikanov’s non-dimensional expression (1958):

(4.57)

(4.58)

where Q is the dominant discharge in m3 s–1, b and d are theaverage width and depth corresponding to the dominant dischargein a reach in m, S is the longitudinal slope of the channel; D50 isthe median size of bed material in m, A1 and A2 are the empiricalcoefficients, and Z1 and Z2 are the empirical exponentials. A1, A2and Z1, Z2 are listed in Table 4.14.

4.4.3.4 ANALYTIC SOLUTION OF HYDRAULIC GEOMETRY ALONG

RIVERS

Under the equilibrium state of sediment transportation, thehydraulic geometry along rivers can be obtained by the analyticsolutions of the following equations:

Equation of flow continuity Q = bdu (4.59)

Equation of flow resistance (4.60)

Equation of sediment carrying (4.61)

Since there are four unknowns, a supplementary equation must beadded. The solutions reflect the relationship between the factor ofwatershed and hydraulic geometry. However, the coefficient andexponents in these solutions should be calibrated with measureddata, and some revision is needed according to the calibration.(1) Xie’s expressions (1980). Xie introduced Equation 4.43as the supplementary equation, and obtained the following rela-tionships.

(4.62)

(4.63)

(4.64)

(4.65)

where C is the sediment carrying capacity, W is the falling veloc-ity of suspended load, K and m are the coefficient and exponentialin the equation of sediment-carrying capacity of flow, n is theManning coefficient, ζ is the morphological coefficient inEquation 4.43, and Q is the discharge.(2) Parker’s expressions. According to the shear stress distri-bution on the cross-sectional boundary of gravel rivers and bedload-carrying capacity of flow, Parker obtained the follow solu-tions (1978).

(4.66)

(4.67)

(4.68)

where b is the width of the water surface, d is the average depth ofthe main channel, Qn is the bankfull discharge, G is the amount ofbed load transported under the bankfull discharge, and D50 is themedian size of the sediment composition along the channelboundary.

CHAPTER 4 — FLUVIAL PROCESSES 67

Table 4.14A1, A2 and Z1, Z2

River A1 A2 Z1 Z2

Jingjiang Reach of the Yangtze River 1.16 0.16 0.39 0.38

Wandering reach for rivers innorthern China, and small rivers 15.6 0.27 0.39 0.33in models

Rivers in the former USSR 5.60 0.29 0.40 0.35

b

DA

Q

D gD S

d

DA

Q

D gD S

Z

Z

501

502

50

502

502

50

1

2

=

=

[ ]

[ ]

Un

d S=1 2 3 1 2/ /

Cu

gdwm= k (

3

)

bk

g

Q

Cm

W

dk

g

Q

C W

ug

kC W Q

sg S n

k

m

m

m

m

m

m

=

=

=

=

0 2 0 8

0 2

0 6

0 2 0 2

0 1

0 1 0 6

0 3

0 1 0 1

0 3

0 3 0 60 2 0 3 0 1

0 73 0 4 0 2

0 73

. / .

.

.

. .

. /

. .

.

. / .

.

. / .. / . .

. . .

. /

( / )

( )

( )

(

ζ

ζ

ζ

CC W

Q

m0 73 0 73

0 2

. / .

.)

b

D

Q

D gD

G

D gD

d

D

Q

D gD

G

D gD

SQ

D gD

n

s s

n

s s

n

s

50

6

502

50

0 296

502

50

1 296

50

6

502

50

1 075

502

50

1 075

4

502

50

1 062

3 09 10

3 56 10

1 37 10

= ×− −

= ×− −

= ×−

−

−

−

. ( ) ( )

. ( ) ( )

. ( ) (

. .

. .

.

γ γ

γ

γ γ

γ

γ γ

γ

γ γ

γ

γ γ

γ

GG

D gDs502

50

1 062

γ γ

γ

−)

.

b Qm

DC d

d Qm

DC d

b

dQ

m

DC d

m

m

m

=

=

=

− −

− − −

− −

3 5

0 26

13 5

0 5

50

0 28 0 1350

0 10

0 40

50

0 18 0 1150

0 08

0 10

50

0 46 0 0250

0 02

. ( )

. ( )

. ( )

. . . .

. . . .

. . . .

(3) Dou’s hypothesis of minimum activity. Dou (1964)supposed that river channels would mould their cross-sections tothe state wherein channel activity is at a minimum, and establishedthe following expression as the minimum activity parameter.

(4.69)

where Q2 is the discharge with an occurrence interval of twoyears, Qm is the long-term average discharge, λα is the ratio ofstability indices of bank and bed materials, αband/αbed, and αbandand αbed are the stability indices for bank and bed materialsrespectively (in Table 4.7), U is the average velocity in the cross-section, Uob is the stop velocity of bed sediment, and b and d arethe width and depth of the channel. The sediment-carrying capac-ity of flow is expressed as:

(4.70)

where U is the average velocity in the cross-section, d is theaverage depth of flow, and Uos is the velocity of suspended sedi-ment. Dou (1964) obtained the following solutions.

(4.71)

(4.72)

(4.73)

4.4.3.5 HYDRAULIC GEOMETRY OF GRAVEL RIVERS

(1) Kellerhall’s expressions (1967). Kellerhall’s empiricalexpressions for quasi-equilibrium gravel rivers are as follows:

b = 1.8Q0.5 (4.74)

d– = 0.116Q0.4ks

–0.12 (4.75)

where b is the width of water surface in ft, Q is the bankfulldischarge in ft3 s–1, d

– is the average depth in ft, and ks is theNikuradse roughness of sand particles in ft.(2) Parker’s expressions (1979). Parker supposed that theshear stress of the bankfull discharge exceeds the critical shearstress by 20 per cent, and obtained the following expressions.

b* = 4.4Q0.5* (4.76)

d* = 0.253(Q*)0.415 (4.77)

S = 0.223(Q*)–0.41 (4.78)

where b* is the non-dimensional width of water surface,b* = b/d50; b is the width of water surface, d50 is the median sizeof bed sediment, Q* is the non-dimensional bankfull discharge,

Q* = Q/ (γs–1 gd550)1/2, Q is the bankfull discharge, γs is the

specific weight of bed sediment, d* is the non-dimensional depth,d* = d–/d50, and d– is the average depth.

4.4.3.6 HYDRAULIC GEOMETRY FOR CANALS

(1) Lacey’s expressions (Chang, 1988)

Flow resistance: U = 1.15(f d–)1/2 (4.79)

(4.80)

where U is the average velocity in ft s–1, d– is the average depth inft; d– = A/b, A is the discharge area in ft2, b is the width of watersurface in ft, R is the hydraulic radius in ft, S is the canal slope, f isthe Lacey silting coefficient, and Nd is the absolute roughness.

Hydraulic geometry: f = 1.6D501/2 (4.81)

Nd = 0.0225f1/4 (4.82)

b = 2.67Q1/2 (4.83)

where b is the width of water surface in ft, and Q is the discharge,in cfs.

Canal slope: (4.84)

When Q and D50 are given, the width, depth and slope can beobtained. As regards the ranges of application of Lacey’s method,the median size of bed sediment is 0.15–0.4 mm, and discharge is5–5 000 ft3 s–1. The canal bed is composed of sand and the sideslope is composed of cohesive material.(2) Blench’s expressions (Chang, 1988).

Channel bed factor: Fb = U2/d (4.85)

where U is the average velocity in ft s–1, and d is the flow depth inft.

Side slope factor: Fs = U3/b– (4.86)

where b– is the average width, b– = A/d in ft, A is the discharge areain ft2, and d is the flow depth in ft.

Flow resistance: (4.87)

where υ is the coefficient of kinetic viscosity, and C is the concen-tration of suspended sediment in ppm.

Empirical value of bed and side slope factors are:

Fs = 1.9d1/2 (4.88)

Fs = 0.1 for light cohesive side slope; Fs = 0.2 formedium cohesive side slope; Fs = 0.3 for high cohesive side slope.

From Equations (4.85), (4.86) and (4.87):

(4.89)

(4.90)

68 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

U

gdS

C Ub2

1 43 63 1

2330= +. ( )( )

/

ν

C kU

gdUos

=3

bgu CQ

k u

dk U Q

gU c

bd

g U c Q

k U

os m

ob

ob m

os

os m

ob

=

=

=

1 33

0 81

1 65

3

8 81 9

2 81 3

4 4 4 2

4 14 141 9

. ( )

. ( )

. ( )

/

/

/

λ

λ

λ

α

α

α

UNa

d R S=1 346 1 4 1 2 1 2. / / /

Sf

Q=

5 3

1 61830

/

/

bF Q

F

dF Q

F

b

s

s

b

=

=

( )

( )

/

/

1 2

21 3

KQ

Q

U

U

b

dn

m ob

= +2 20 15[( ) . ]

λα

(4.91)

If discharge, sediment concentration, grain size and theviscosity of the slope material are given, the size of a stable canalcan be determined. This method is suitable for a sandy canal withthe side slope composed of cohesive material.

(3) Simons and Albertson’s expressions.Formula of quasi-equilibrium width:

P = K1Q0.5 (4.92)

b– = 0.9p = 0.9k1Q0.5 (4.93)

b– = 0.92b – 2.0 (4.94)

Formula of canal depth:

R = K2Q0.36 (4.95)

d = 1.21R (R < 7ft) (4.96)

d – 2 + 0.93R (R≥7ft) (4.97)

Formula of flow resistance:

U = K3 (R2S)m (4.98)

(4.99)

where P is the wetted perimeter, b– is the average width, d is thecanal depth, K1 is the coefficient related to canal types, R is thehydraulic radius, K2 is the coefficient related to canal types, m isan exponential, K3 and K4 are the coefficients, and S is the longitu-dinal slope. All units are in the English system.

The above morphological formulae were estimatedbased on the data for a sandy canal with medium and fine bedsediment and for a cohesive canal with the bed sediment finer thansandy, coarse sand gravel canals. For a type 4 canal, the mediumsize of bed sediment is 20–82 mm.

Simons and Albertson divided canals into 5 types: (a)sandy bed and side sandy slope; (b) sandy bed and cohesive sideslope; (c) cohesive bed and cohesive side slope; (d) coarse parti-cles without viscosity; (e) the same as (b), but with a highsediment transport and a sediment concentration of 2 000–8 000 ppm. The coefficients are listed in Table 4.15.

4.5 FLUVIAL PROCESSES OF MEANDERINGRIVERS

4.5.1 Plane morphology of meandering riversMeandering rivers consist of a series of bends of alternate curva-tures connected by straight crossing reaches. The terms used todescribe stable meanders are defined in Figure 4.8.

Essential elements of meandering rivers include:meandering wave length (Lm); meandering belt width (Tm)(Hm); curvature radius (R); width of straight reach (crossing)(B); length of curve line (s); Central angle (θ); and length ofcrossing (L).

4.5.2 Relationships between meander wavelength anddischarge

Based on the data from natural rivers and small rivers in laboratories,the wavelength and discharge have the following relationship:

Lm = kQm (4.100)

where Lm is the meander wave length, Q is the discharge coeffi-cient, and k and the exponential m vary according to the results ofthe different authors listed in Table 4.16.

For some rivers in the United States (Chien, et al., 1987):

Lm = 0.935Qm0.8M–0.74 (4.101)

where Qm is the annual average discharge in m3 s–1, and M is thecontent of silt-clay in the bed and bank materials.

4.5.3 Relationships between central angle and curvatureradius(1) Lacey’s formula (Sedimentation Committee, 1992)

(4.102)

where R is the curvature radius, ϕ is the central angle in radians,and Q is the discharge.

CHAPTER 4 — FLUVIAL PROCESSES 69

U

gdSk

Ub2

40 37= ( )

.

γ

Table 4.15Coefficients for various types of canals

Coefficient Type of canal

K1 3.5 2.6 2.2 1.75 1.7

K2 0.52 0.44 0.37 0.23 0.34

K3 13.9 16.0 – 17.9 16.0

K4 0.33 0.54 0.87 – –

m 0.33 0.33 – 0.29 0.29

Figure 4.8 — Morphological elements of meandering rivers.

Table 4.16Coefficient k and exponent m

Author Source of data K m Q

Chien Rivers in India, the 50 0.5 Bankfull discharge(1965) United States, China

and from model

Dury Sinuous valleys of 54.3 0.5 Long-term(1964) some rivers in average maximum

the world discharge

Carlson 156 0.46 Annual average(1965) discharge

RQ

=0 5.

ϕ

S F Fc

gQb s= + ×

( ) ( ) / . ( )

56

12

112

163 63 1

2330γ

Tm or Hm: meandering belt widthR: curvature radiusB: width of straight reachS: length of curve lineQ: central angle

(2) Formula for the Lower Yellow River (YRCC, 1985)

For the reach upstream of Gaocun: (4.103)

For the reach downstream of Gaocun: (4.104)

(3) Formula for the Middle and Lower Yangtze River(YRWRC, 1959):

(4.105)

where Q–max is the long-term average maximum discharge in m3 s–1,R is the curvature radius in m, and ϕ is the central angle in radians.

(4) Ouyang’s formula (1983):

R = 48.1 (QS1/2)0.83 (4.106)

where Q is the bankfull discharge, S is the slope for the bankfulldischarge, and R is the curvature radius in m.

(5) Chien’s formula (1987):

R = kQn0.5S–0.25ϕ–1.3 (4.107)

where Qn is the bankfull discharge in m3 s–1, S is the slope in1/10 000, and ϕ is the central angle in radian. For the Yellow andYongding Rivers, K = 10, for the Jingjiang River (part of theMiddle Yangtze River) and Nanyunhe River, K = 3. The aboveformula was confirmed by Velikanov based on the data from plainrivers in the former USSR (1958).

4.5.4 Relationships between meander elements and widthof straight (crossing) reaches(1) Relationships between curvature radius and width of

straight reach (crossing):

R = KRB (4.108)

where R is the curvature radius, and B is the width of the crossingreach. The coefficients of KR are listed in Table 4.17.

(2) Relationship between wave length and the width ofstraight reach:

Lm = KLB (4.109)

(3) Relationship between meander belt width and thewidth of straight reaches:

Tm = KTB (4.110)

(4) Relationship between the length and the width ofstraight reach:

I = KIB (4.111)

Coefficients of KL, KT and KI are listed in Table 4.18.

4.5.5 Relationships between configurations and cross-sectional geometry of meanders

Chitale’s empirical expression (1970). Based on the data from 42rivers, Chitale obtained:

(4.112)

(4.113)

where D is the average size of bed sediment, Hm is the meander bedwidth, B and h are the width and depth of flow, respectively, s is thelength of the curve line, and S is the slope in 1/10 000 (all units oflength are in m).

4.5.6 CrossingsCrossing sections are located between bends of reverse curvature.In alluvial rivers, crossing sections are approximately rectangular,in contrast to triangular sections in bends. The water surface slopethrough crossings is usually flat at high stages, resulting in deposi-tion in the crossings. At low stages, the water surface slope overcrossings becomes relatively steep. For the relatively stable cross-ing of the Arkansas River prior to canalization, the maximumdepth in crossings was a function of channel width (Peterson,1986).

4.5.7 Dynamic line of flowThe transversal distribution of velocity in the cross-section of abend is not uniform and there is always a maximum velocityalong the water surface of the cross-section. The dynamic line offlow is a line along the river that connects the locations on thewater surface where the vertical average velocities are themaximum. It is also called the main current line. The linebecomes sinuous and flows along the concave sides of bends inlow waters, and passes straight through the centre part of thewater surface in high waters.

(1) Chang’s expression (1983). Based on the measureddata from the Jingjiang Reach of the Yangtze River, the relation-ship between curvature radius of the flow dynamic line (R) and thecurvature radius of bend (R0) was expressed as follows:

(4.114)

where √b/d is the average cross-sectional geometry, Q is thedischarge, and S is the slope in 1/10 000.

(2) YRWRC expression (1971). Based on the data fromthe Yangtze River, the following expression was obtained:

70 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

R R Qd Sb

d= 0 26 0

0 73 0 72 23

12 0 23

. ( ) ( ). . .

Table 4.18Coefficient of KL KT and KI

Sources of data KL KT KI

Rivers in China, the United States andFrance, and in laboratories (Chien, 1987) 12 4.3 9–15

Carlston, 1965 5.8 1–3

€

s Bh

DH

S

H Bh

Dh

S

m

m B

=

=

− − −

− − −

0 917

36 3

0 065 0 077 0 052

0 471 0 050 0 453

. ( ) ( )

. ( ) ( )

. . .

/. . .

Table 4.17Coefficients KR

Author River KR

Chien, et al., Rivers in China, the United States1987 and France, and in laboratories 3

YRCC, 1985 Lower Yellow River 2–6

YRCC, 1987 Yangtze River 3.5 < KR < 5–10

R =4500

185ϕ

R =3220

185ϕ

RQ

=−

330

0 73

1 15max

.

.ϕ

(4.115)

where R is the curvature radius of flow dynamic line in m, R0 isthe curvature radius of channel bend in m, Q is the discharge inm3 s–1; g is the gravity acceleration in m s–2, and A is thedischarge cross-section in m2.

(3) Chang’s theoretical expression (1982):

(4.116)

where R is the curvature radius of flow dynamic line in m, R0 isthe curvature radius of channel bend in m; Q is the discharge inm3 s–1, g is the acceleration of gravity in m s–2, A is the dischargecross-sectional area in m2, φ is the central angle of bend inradians, and S is the slope of flow dynamic line in 1/10 000.

4.5.8 Transversal slope of water surfaceWhen water flows through the channel bend, the elevation of thewater surface on the concave side is always higher than that on theconvex side. The difference between surface elevations on bothsides and the transversal slope can be expressed as follows (Chien,1987).

(4.117)

(4.118)

where ∆h is the difference between water elevations on concaveand convex sides in m, b is the width of water surface at the cross-section in m, V is the average velocity in the cross-section inm s–1, R is the curvature radius of dynamic line of flow in m, andg is the acceleration velocity of gravity in m s–2.

4.5.9 Longitudinal slope of water surfaceUnder the influence of transversal slope of the water surface, thetransversal distribution of longitudinal slope of water surface isnot uniform. The maximum slope appears where the circulatingflow is developed. As shown in Table 4.19, the maximum longitu-dinal slope of water surface occurs at the top of a bend. Thelongitudinal slope upstream of the top is smaller than that down-stream of the bend. In the reach upstream of the top, the

longitudinal slope at the convex side is larger than that at theconcave side. The opposite occurs in the downstream reach of thetop (Chien, et al., 1987).

4.5.10 Transversal circulating flowsUnder the action of the transversal slope, i.e., the difference insurface elevations between concave and convex sides, a circulating(spiral) flow will form with the surface flow towards the concavebank and the bottom flow towards the convex bank. The structureof the circulating flow is complicated in natural rivers. In additionto the main circulating flow caused by the transversal slope, sub-circulating flows also occur under the local action of meanders(Figure 4.9) (Zhang, 1980).

4.5.10.1 DISTRIBUTION FOR TRANSVERSAL VELOCITY (RADIAL) OF

CIRCULATING FLOWS (ROZOVSKI, 1957, 1965)

For a smooth bed surface: (4.119)

For a rough bed surface: (4.120)

The relationship between F1(η), F2(η) and η is shown inFigure 4.10.when k = 0.5, C ≥ 50 (Xie, 1987),

(4.121)

where VZ is the transversal velocity at the position with a distanceZ above channel bed, K is the Karman constant (in smooth andregular-shaped bends, K = 0.5, and for natural rivers,K = 0.3–0.55), U is the vertical-average value of longitudinalvelocities along depth in m s–1, ∆ is the water depth at a vertical

CHAPTER 4 — FLUVIAL PROCESSES 71

∆

∆

hV

g

b

R

Sh

b

V

gRZ

=

= =

α

α

2

2

Figure 4.9 — Transversal velocities and circulating flows in theLaijiapu Reach of the Yangtze River.

€

V dUk R

Fg

kCF

VdU

k rF

g

kcF

Z i

z

= −

= − + +

2 2

2 1

1 2

2 0 8 1

[ ( ) ( )]

{ ( ) [ ( ) . ( ln ]}/

η η

η η η

V Ud

RZ = −6 2 1( )η

Site of cross-section Water surface slope Water surface slopeat concave side at convex side

(1/10 000) (1/10 000)

Inlet of bend –0.007 0.424

From inlet to top 0.019 0.079of bend

Top of bend 0.0849 2.40

From top to outlet 0.530 0.21of bend

Outlet of bend 0.700 0.797

Table 4.19Longitudinal slope of water surface at Laijiapu of the

Jingjiang River

R RQgA= 0 053 0

0 35. ( )

.

RSg

RQA=

10

23

ϕ( )

Figure 4.10 — Relationship between F1 (η), F2 (η) and η.

line, R is the curvature radius of the position at the vertical line,η = Z/∆, and C is the Chezy coefficient.

4.5.10.2 RELATIVE INTENSITY OF CIRCULATING FLOWS (XIE, 1987)The transversal velocity VZ in Equations 4.119, 4.120 and 4.121 at apoint above the channel bed with a distance of Z can be consideredas the intensity of circulating flow at that point. The ratio of Vz tothe corresponding average longitudinal velocity along the verticalU, Vz/U, is called the relative intensity of the circulating flow, fromEquation 4.121:

(4.122)

η = 0.01 near the channel bed, and η = 0.99 near the watersurface. Hence:

(4.123)

4.5.10.3 VORTEX INTENSITY OF CIRCULATING FLOW (XIE, 1987)The ratio of the transversal velocity Vz to the longitudinal velocityVx at a certain point is called the vortex intensity, i.e.:

(4.124)

when η = 0.01 near the channel bed and η = 0.99 near the watersurface, then:

(4.125)

4.5.10.4 TRANSVERSAL SLOPE OF BED SURFACE AND DISTRIBUTION

OF SEDIMENT PARTICLES

Under the action of circulating flow and channel bed, the transver-sal transport of sediment particles occurs and the transversal slopeof the channel bed surface is thereby formed. Because of thecomplicated exchanges of sediment between the transported parti-cles and bed sediment, the distribution of bed sediment alsobecomes non-uniform. Coarse particles appear near the thalwegline. Chang (1988) introduced some advanced results on the trans-versal slope of bed surface.

(1) The Falcon-Ascanio-Kennedy expression (1983).Based on an equilibrium of the radial component of flow actingforce and the component of float weight of sediment particles ontransversal slope, the following expression is obtained:

τor = Zb (1 – λ) (ρ – ρ) g sin β (4.126)

where τor is the radial component of boundary shear stress, Zb isthe thickness of the bed surface layer, λ is air voids of bed surfacelayer, ρs, ρ are specific weights of sediment and water, β is thetransversal slope (dip angle) of the channel bed surface, and g isacceleration of gravity.

(4.127)

where D is the depth of flow, r is the radius of curvature, U is the verti-cal average of longitudinal velocity, and m is the parameter, m = 1/f1/2.

(4.128)

where d = d50, the median size of bed sediment, U* (r) is the shearvelocity at position r, U* (r) = U/ (f/8)1/2, and U*c is the criticalshear velocity of moving bed sediment particles.

According to the Shields shear stress:

τ*c = τc /(ρs – ρ) gd, (4.129)

(4.130)

where τ*c is critical shear stress.Putting Equations 4.127, 4.128 and 4.130 into 4.126, the

longitudinal slope of the bed surface can be expressed as follows:

(4.131)

where St is the longitudinal slope of the channel bed surface, andFd is the density Froude Number.

(4.132)

Equation 4.133 was proved by flume experimental data.If U and the transversal changes of sediment particles are given,the transversal slope of the bed surface can be obtained by integralof Equation 4.133. The average velocity for a vertical line is:

(4.133)

where U(r) is the average velocity for a vertical line correspondingto the radius of y, Sc is the longitudinal slope at the central linewith the radius of rc; f is friction in the Darcy-Weisbach formula,and D(r) is the depth corresponding to the radius of r.

Putting Equation 4.133 into Equation 4.131 and inte-grating, then:

(4.134)

It is a slight protruding line.(2) The Englund-Bridge expression. Considering the

equilibrium of acting force caused by spiral currents, bottomcurrents, gravity and friction on sediment particles on the transver-sal slope of a bed surface and in the longitudinal direction,Englund (1974) obtained:

(4.135)

where β is the transversal slope of bed surface, θ is the angle ofrepose, tan θ is the coefficient of dynamic friction, and δ is the

72 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

τ ρor

m

m m

D

rU=

+

+

1

2

2

( )

Z dU r

Ub

c

= *

*

( )

U c gdcs

c*/

*/

( ) ( )= =−τ

ρ

ρ ρ

ρτ1 2 1 2

SD

rF

f

ft d

c= ≈+

− +sin

( ) ( )

( )

*/ /

/β

τ

λ

8 1

1 1 2

1 2 1 2

1 2) (

FU

gd

ds

=−

( )/ρ ρ

ρ1 2

1 1 1 1 8

1

1

2

8

1 2 1 2 1 2 1 2

1 2

1 2

1 21 2

D D r r

f

f

S g

fg d

c c

c

c c

s

/ / / /*

/

/

//

( )( )

[ ]

− = −−

+

−

τ

λ

τ

ρ ρ

ρ

V

V g

kc

z

x=

−

+ +

6 2 1

1 1

( )

( ln )

η

η

V

V

d

R

z

x

= −( . – . )10 1 5 23

V

U

d

R

z = −( . – . )0 588 5 88

V

U

d

R

z = −6 2 1( )η

U r Sr

r

gD r

fc

c( ) [( )

]/= 8

1 2

tantan

tanδ

β

θ=

intersection angle between the direction of the bottom current andthe longitudinal flow direction.

Bridge (1977), based on the equilibrium of transversaltractive force and the component of gravitational force acting on aparticle on the transversal slope of the bed surface, furtherobtained the following expression:

(4.136)

(4.137)

where τ0 is the shear stress on the longitudinal bed surface, and dis the grain size of sediment particles on the transversal slope ofthe bed surface.

For fully developed flow:

(4.138)

(4.139)

If the water depth D is known, the corresponding grainsize of the sediment, d, on the transversal slope of bed surface canbe obtained. Here, d is the grain size of sediment particles at anypoint on the transversal slope of the bed surface, r is the radius,and Sc is the longitudinal slope corresponding to the radius rc onthe central line.

(4.140)

where S is the longitudinal slope corresponding toradius r.

(3) The Odgaard expression (1981, 1982, 1984).Odgaard’s method is a revision of the Falcon-Ascanio-Kennedyexpression:

(4.141)

where α = a/V, a is the projective area of a spheroid after stan-dardization, V is the volume of the spheroid, S = ρs/ρ specificweight of the sediment; dcr is the diameter of sediment particles inthe critical state of moving, and m' is the reciprocal of the velocityexponential of grain roughness.

From the Shields critical shear stress:

(4.142)

From Equation 4.133 (4.143)

From Strickler’s formula, the Manning’s roughness coef-ficient n is proportional to 1/6 power of grain size d, in m;N = d1/6/21.1.

(4.144)

Odgaard supposed that the transversal bed surface was ina straight line and that the Shields critical shear stress τ*c wasproportional to –2/3 power of d:

(4.145)

Putting Equation 4.145 into Equation 4.144

(4.146)

In application of Odgaard’s expression, firstly m' shouldbe calculated by Equation 4.142, and then the transversal slope ofthe bed surface can be calculated by Equation 4.141, if averagedepth velocity and grain size on the slope are given.

4.5.11 Sediment transport in meandering rivers4.5.11.1 TRANSPORT OF SUSPENDED LOAD

In general, the distribution of suspended load is not uniform alongthe depth. The sediment concentration is higher and the grain sizeis coarser near the channel bed. In a bend reach, because of theinfluences of spiral flow, water with high concentrations andcoarse particles is concentrated along the convex bank, and thatwith low sediment concentrations and fine sediment particles is inthe concave bank. The distribution of sediment concentrationthrough the depth near the concave side is also more uniform. In astraight (crossing) reach, the distribution of sediment concentra-tion along depth is uniform, and the transversal distribution ofvertical average concentration corresponds to the transversaldistribution of vertical average velocity.

The transversal sediment discharge caused by circulating(spiral) flow can be shown in the expression by Xie (1987):

(4.147)

where gsn is the transversal sediment discharge per unit width; d isthe flow depth; R is the radius of the curvature, ηa = a/h, a is thethickness of the bed surface layer, and gs

— is the average longitudi-nal sediment discharge per unit width.

(4.148)

(4.149)

z is the exponential in the sediment concentration distribution.

4.5.11.2 BED LOAD TRANSPORTATION

The transport of bed load in meandering rivers is characterized bythe following two phenomena (Xie, 1987):

(1) According to experimental data, the sedimentparticles eroded from the concave bank of a bend are carried byflow and partly deposited at the crossing and convex bank of thenext bend. The remaining particles are further carried and depositedat the downstream crossings and convex banks of downstream

CHAPTER 4 — FLUVIAL PROCESSES 73

tantan

( )

( ) tan

βτ δ

ρ ρ

τ

ρ ρ φ

=−

=−

3

2

3

2

0

0

dg

dg

s

s

τ ρτ

ρ τ

ρ ρ φ

0

3

2

=

=−

gDSr

dDS

r

cc

c c

s( ) tan

S Sr

rc

c=

sin[( ) ]

'

' ( ' )/β =

−

+

+

3

2 1

1

2

2

1 2

d D

r

U

S gd

m

m mcr

m KU

S gdcr c

'[( ) ]*

/=

− 11 2τ

U

U

m

m

D

D

r

rc c c

c='

'( )( )

U

U

d

d

D

D

r

rc

r

c

c= (') ( )( )

/ /1 6 1 2

d

d

D

D

r

rc

c

1

5 3 3 2= ( ) ( )/ /

U

U

D

D

r

rc c

c= ( ) ( )/ /7 18 1 4

g gd

R JJsn s

an=

−6

1

1

η

J d

J d

a

a

z

a

z

a

1

1

1

1

10 001

2 11

0 01

=−

=

=−

=

∫

∫

( ) , .

( – )( ) , .

ηη

η η

η ηη

η η

η

η

bends. However, when the circulating flow is strong, the sedimentparticles eroded from the concave bank are carried directly to theopposite convex bank and settle there. The former is called same-side transporting of sediment and the latter, different-sidetransporting. For meandering rivers, same-side transporting ofsediment is more common than different-side transporting.

(2) Bedload particles often move in a transporting beltalong the river instead of spreading all over the channel bed. Thetransporting belt is situated near the point bars of convex banks.

The transversal transport of bed load caused by spiralflow is controlled by the transversal slope of the bed surface.Ikeda (1982) conducted a wind tunnel experiment with sand parti-cles of 0.26 and 0.42 mm and estimated:

(4.150)

where q*' is the dimensionless transversal bed load discharge perunit width, τ* is the dimensionless shear stress or Shields shearstress, and τ*c is the critical Shields stress.

(4.151)

where qb' is the transversal bed load discharge, and S = ρs/ρ, ρs, ρis the specific weight of sand and flow.

Parker (1984) considered the effects of the transversalslope of the bed surface and the spiral flow, and estimated:

(4.152)

where qb, q* are the longitudinal bed load discharge; CL is thecoefficient of lifting force; CD is the coefficient of tractive force; δis the angle between bottom velocity and longitudinal velocity,and Φ is the angle of repose.

4.5.12 Characteristics of fluvial processes4.5.12.1 COLLAPSE OF CONCAVE BANKS AND GROWTH OF

CONVEX BANKS

Generally, meandering rivers are in the equilibrium state of sedi-ment transport. Under the action of spiral flow, the sedimentdeposited at convex banks is mainly from erosion of the concaveside. As a result, the channel has a continuous migration over theyears. Figure 4.11 shows the transversal migration of the cross-section at the apex of the Laijiapu Bend in the Yangtze River. Theriver channel migrated rightward a distance of one km in tenyears. Some examples of the rate of bank collapse for riversthroughout the world are listed in Table 4.20 (Chien, et al., 1987).

4.5.12.2 MIGRATION OF MEANDERINGS

The shear stresses acting on the bank and channel bed reach amaximum at the position downstream of the apex of the bend, andthe eroded sediment particles deposit at the convex bank, causingthe point bar to develop. With the collapse of the concave bankand the growth of the convex bank, the channel bend, as a whole,gradually migrates downstream. During the migration process, theoutside of the bend is changed, but the centre part of the crossingmay remain basically unchanged. Therefore, the adjacent bendsmove around a fixed point, and an S-shaped meander may form(Figure 4.12).

4.5.12.3 CUTOFFS

As the S-shaped bend develops, the apex of the two adjacentbends located on the same side come closer, and the difference ofwater surface at both ends of the neck becomes larger. Once theoverbank flood occurs, the neck may be scoured. A new channelmay be formed, widened and deepened, and the old bendway maybecome separated from the river by deposition, surviving as anoxbow lake. This phenomenon is called natural cutoff.Subsequently, the channel upstream of the cutoff is erodedbecause of the steep slope, and the channel downstream of thecutoff is deposited because of the lower slope. If the pilot channelis not protected from erosion, a new meander (bend) is formedagain (Figure 4.13).

4.6 FLUVIAL PROCESSES OF WANDERING RIVERSThe Lower Yellow River is a notorious wandering river. Its specialfluvial processes are used to describe the outstanding features ofwandering rivers.

74 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 4.12 — Changes in S-shaped bends.

q

c c

* *

*

*

*

.'

tan. [ ( )]

β

τ

τ

τ

τ0 0085 1

0 5−

qq

S gd

b* /'

'

[( ) ]=

− 13 1 2

q

q

q

q

C Cb

b

L D c' 'tan

( / ) tan

tan( ) tan*

*

*

*

/= = −+

δφ

φ

τ

τβ

1 1 2

Figure 4.11 — Migration of the apex of the cross-section at theLaijiapu Bend.

(a) Accumulative erosion at concave bank and deposition at convexbank

(b) Changes of cross-sections at top of bend1. Accumulative erosion at concave bank2. Accumulative deposition at convex bank3. Accumulative difference of deposition and erosion

4.6.1 Flow and sediment transport4.6.1.1 CHARACTERISTICS OF RIVER FLOW

Wandering rivers have steep slopes, small water depths and highflow velocities in wide and shallow channels with fragmented,

broken and disorganized channel beds. For example, in theHuayuankou Reach of the Lower Yellow River, which is a typicalwandering reach, the channel slope is 0.0002–0.00025, the waterdepth is only 1 to 3 m, and the velocity is higher than 3 m s–1.Special water surface phenomena, corresponding to bed formssuch as dunes and anti-dunes etc., often occur because the Froudenumbers of its flow are far greater than those in ordinary alluvialrivers. The Brahmaputra River in Bangladesh is a wanderingbranched river. Although its channel slope is smoother than that ofthe Lower Yellow River, its velocity is also high because of itslarge volume of discharge. Similar flow surface phenomena alsooccur in that river (Zhou, 1998, 1995).

4.6.1.2 CHARACTERISTICS OF SEDIMENT TRANSPORT

In China, all the wandering rivers carry huge amounts of sedi-ment load. For example, the long-term average sedimentconcentration is 27.3 kg m–3 at Huayuankou Station on theLower Yellow River, and 44.2 kg m–3 at Sanjiadian Station onthe Yongding River. Sediment concentration and sedimentdischarge vary substantially at the same flow discharge. The

CHAPTER 4 — FLUVIAL PROCESSES 75

Area of Width of Annual Rate of bank Date ofCountry River watershed river (m) discharge collapse survey Remarks

(km2) (m3 s–1) (m/a)

Yangtze Jingjiang Reach max. 88.4 1949–1967 MeanderingRiver av. 30.0

Jiujiang estuary Max. 200 BranchedMin. 2.5

China Av. 48.7Yellow Railway bridge 470 WanderingRiver Tongbadou

Tongbadou-Gaocun 409Gaocun Sunkou 178 Transition

Rheidol River 179 1.75 1951–1971United Kingdom Endrick River 98 25 6.9 0.5 1986–1957

Tyfi River 633 2.65 1905–1971

Mississippi River 23 1722–197114.9–40.5 1963–1970

Ohio River 0.36 1807–1958White River 6 042 66.2 0.67 1937–1968

United States Downstream of 1020–1400 1/3 width of 1879–1954 MeanderingMissouri River flood plainLittle Missouri River 91.5 16 1.7–7.0 More than

100 yearsDes Moines River 6.6 1880–1970

Pembina River 64 19.2 3.35 1910–1956Canada Beatton River 16 000 370 225 0.48 1250 years Meandering

Australia Torrens River 78 5–10 0.58 1960–1963

Poland Wisloka River 22.5 8–11 1970–1972

Comprehensive max. 100 1897–1958Former USSR statistics av. 10–15

Obi River 1 434 0–15 1897–1958

Sweden Klaralven River 5 420– 120 650 0.23 1800–185011 820 0.32 1850–1950 Meandering

1.6 1950–1956

Czechoslovakia Hernad River 5 400 50–60 10–30 5–10 1937–1972

Bangladesh Brahmaputra River 934 990 6 000– 1 898 6–275 1952–196313 000

Table 4.20Rate of river bank collapse (Chien, et al., 1987)

Figure 4.13 — Changes in the Nianziwan Bend on the Yangtze Riverafter cutoff.

sediment-carrying capacity for bed material load is determinedby both the flow intensity and incoming sediment concentra-tion. Meanwhile, when the sediment concentration of incomingrunoff is rather high, the sediment-carrying capacity for bedmaterial load is also high along the river. The more incomingsediment there is, the more sediment is sluiced. If the incomingsediment concentration is taken as a parameter, the relation-ships between sediment discharge, incoming sedimentconcentration and flow discharge at the stations on the LowerYellow River may be expressed as follows (SedimentationCommittee, 1992).

(4.153)

where Qs is the sediment discharge for bed material load in t s–1,Q is the flow discharge in m3 s–1, S0 is the sediment concentrationfor bed material load at the upper neighbour station in kg m–3, Kis the coefficient of sediment transport, and α and β are the expo-nents. For the stations on the Lower Yellow River, α = 1.1–1.3,β = 0.7–0.9, and K is determined by degradation or aggradation atan earlier stage.

4.6.2 Morphological features4.6.2.1 STATIC FEATURES

The static features of wandering rivers include the following:(1) There are dense mid-bars, branches and scattered flows inthe channel. (2) The channel configurations are more smoothand straight, with a sinuous coefficient (total length ofbranches)/(length of channel) of 1–1.3, which is smaller thanthat of meandering rivers (1.5–2.5). (3) The channel beds arewide and shallow. The maximum width of the wandering reachof the Lower Yellow River is more than 10 km, and b d–1 is20–40, which is 10 times as large as that of the meanderingreach of the Yangtze River.

4.6.2.2 DYNAMIC FEATURES

The dynamic behaviour of wandering rivers may be described asfollows: (1) The mid-bars move quickly and the river bed can beeasily eroded and deposited. (2) The positions of main currentschange constantly. Sometimes, the position of the main currentcan change completely during a flood. (3) The range of maincurrent shifting is large and the shifting rate is high. For example,the main current has migrated 6 km in 24 hours in the LowerYellow River. (4) There are two types of migration of the mainchannel — gradual shifting and sudden shifting in channel evolu-tion. Gradual shifting often occurs in flood-rising periods, andsudden shifting occurs in flood-falling periods.

4.6.2.3 NODE POINTS

In the wandering reach of the Lower Yellow River, the riverconfiguration is chequered longitudinally with wide and narrowchannels. The wide channel contains dispersed flows, dense mid-bars, disordered branches, a scattered platform, and a strongshifting of the main current. The narrow channel contains rela-tively concentrated flows, less sand bars, and a weak migration ofthe main channel. The narrow channel is called the node point(Chien and Zhou, 1965).

The node points play an important role in controllingthe wandering of the main current and the changes ofconfiguration.

There are two types of node points. The first is called thegrade 1 node point, which has a fixed position and two supportbases on both sides of the channel, and plays a role in controllingthe configuration above medium water level. The second is calledthe grade 2 node point, which has an unfixed position and onesupport base on one side of the channel. It can control the configu-ration below the median water level.

The conditions for forming the grade 1 node point on theLower Yellow River are: (1) Man-made controlling works on bothsides of the river channel (Figure 4.14 (b)); (2) Cliff or vulnerablespots on one side of the channel, and a clay boundary on anotherside (Figure 4.14 (a), (c), (d) and (e)).

The grade 2 node points are shown in Figure 4.15. Theirsupport bases are often embankments or high banks on one side ofthe channel, and the other side is a low bank or side bar. The posi-tion of grade 2 node points may migrate along the reach when thedischarge changes.

76 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 4.14 — Grade 1 node point on the Lower Yellow River.

Q kQ Ss = α β0

(a) Convex cliff of Mongshan (b) Man-made structure control

(c) Vulnerable spot and unerodible bank (d) Vulnerable spot revetment (e) Convex vulnerable spot

Figure 4.15 —Grade 2 node point on the Lower Yellow River.

(a) 26 March 1959, Q = 774 m3 s–1

(b) 3 April 1959, Q = 2870 m3 s–1

(c) 15 August 1959, Q = 3800 m3 s–1

Grade 1 node points have the following features in plainmorphology (Chien, et al., 1965):

B2 = 3.82B1– 1.45 (4.154)

B2 = 0.34L – 0.31 (4.155)

where B2 is the shifting range of the main channel in a wide reach,B1 is the shifting range of the main channel in a narrow reach, andL is the length of the wide reach.

4.6.3 Channel degradation and aggradation4.6.3.1 CHARACTERISTICS OF DEGRADATION AND AGGRADATION

FOR WANDERING RIVERS WITH HIGH SEDIMENT

CONCENTRATION

The Lower Yellow River is a remarkable example of a river with alarge amount of sediment load. Its features of channel degradationand aggradation may be described as follows (Chien, et al., 1965,1987).

(1) The Lower Yellow River is characterized by seriousaggradation with an annual amount of siltation of 0.4 × 109t. Thechannel bed has risen by 7 to 10 cm/yr in past years and long-termaccumulation has resulted in the river becoming a suspended riverhaving flood plains 3 to 5 m above the ground outside theembankments. Ninety per cent of the siltation is in the wanderingreach.

(2) The channel aggradation of the Lower Yellow Rivercan be classified into two types, namely streamwise depositionand retrogressive deposition. The main cause of streamwise depo-sition is the huge amount of incoming sediment load from thewatershed and the insufficient sediment carrying capacity of theflow. The deposition develops from upstream to downstreamreaches resulting in a decrease in the sediment concentration andgrain sizes of suspended load along the river. Retrogressive depo-sition is caused by the raised local datum of an estuary, caused byestuarine deposition. The range of retrogressive deposition is 200to 300 km from the river mouth of the Lower Yellow River, whileall the deposition occurring in the wandering reach is streamwisedeposition (Zhou, 1982).

(3) Deposition occurs mainly in flood seasons, whichaccount for 70 per cent of annual deposition. During the floodseason, about 90 per cent of deposition is caused by floods. Mostdeposits are silted on the flood plains, and the main channel is in astate of erosion during the floods. According to measured datafrom six overbank floods in the period from 1950 to 1960, thetotal amount of deposition, including the deposition on floodplains and the erosion in the main channel, was 1.65 × 109t.

Under the conditions of medium and low flows, deposi-tion always occurs in the main channel. The depositions on floodplains during floods and in the main channel in medium and lowflows are restricted, which results in the parallel raising of theflood plains and the main channel.

(4) During a flood, the channel bed is eroded in therising stage and aggraded in the falling stage. The intensity oferosion and aggradation in floods may be expressed as follows(Sedimentation Committee, 1992):

(4.156)

where ∆G is the intensity of channel degradation and aggradationduring a flood in t day–1, “–” marks degradation, and “+” marksaggradation, S/Q is the coefficient of incoming sediment load inkg.s m–6, and S and Q are the average sediment concentrations inkg m–3 and average discharge in m3 s–1 during the flood, respec-tively. In the Lower Yellow River, if S/Q ≥ 0.015, both the mainchannel and the flood plains are in deposition, and if S/Q < 0.015,the main channel suffers from erosion and the flood plains are indeposition.

4.6.3.2 DEGRADATION AND AGGRADATION FOR

WANDERING RIVERS WITH RELATIVE LOW

SEDIMENT CONCENTRATION

As mentioned above, the Brahmaputra River in Bangladesh is awandering-branched river with a long-term average sedimentconcentration of 0.81 kg m–3. The features of degradation andaggradation for the river can be summarized as follows (Zhou,1998).

(1) The degradation and aggradation of the river ismainly caused by the transport of bed load and the coarse particlesof suspended load near the channel bed. The river is nearly inequilibrium, with an average deposited thickness of 0.01 m in thepast one hundred years.

(2) The main form of channel degradation and aggrada-tion is the growth and decline of the main channel and branches.No obvious raising of the surface of islands and side bars is found.There is no retrogressive deposition because the estuary has noextension.

(3) Erosion and deposition are affected by suddenevents in the upper reaches. For example, following the greatearthquake in the 1950s in Upper Assam, India, the YalutsangpoRiver and the Upper Brahmaputra River caused earth and debris toslip into the river, which rose 3 m at Dibrugarh in five years. From1950 to 1957, the channel bed rose by 0.5 to 2.4 m in a reach of168 km of the river in India. Deposits in the upper reaches havebeen carried into the Lower Brahmaputra River in Bangladeshsince the late 1970s, and this has resulted in a gradual aggradationof the downstream channel.

4.6.4 Degradation and aggradation in hyperconcentratedfloods

4.6.4.1 FEATURES OF HYPERCONCENTRATED FLOODS IN THE

LOWER YELLOW RIVER

The hyperconcentrated floods coming from the Loess Plateau inthe Middle Yellow River basin have a peak discharge of 4 000 to8 000 m3 s–1 and a sediment concentration higher than400 kg m–3, the highest being 911 kg m–3 after regulation by theSanmenxia Reservoir. On average, the size distribution ofsuspended load of the hyperconcentrated floods is as follows.Sediment size smaller than 0.025 mm accounts for 50 per cent,0.025–0.05 mm, 24 per cent and coarser than 0.05 mm, 26 percent. The average median size has a relationship with themaximum sediment concentration in a flood, as expressed below(Zhou, 1998):

D50 = 0.000027 × S + 0.0139 (4.157)

where D50 is the average median size of suspended load in a floodin mm, and S is the maximum sediment concentration in a flood inkg m–3.

CHAPTER 4 — FLUVIAL PROCESSES 77

∆G Q SQ

SQ= −137 0 33

2 0 75[ . ( ) ]

.

4.6.4.2 FLOW PATTERNS AND TRANSPORT MODES

The flow pattern of hyperconcentrated flow can be classified intothe laminar and turbulent flows. Large amounts of fine suspendedsediment restrain the development of turbulence. When the sedi-ment concentration, especially for the fine sediment, reaches acertain level, the turbulent flow is easily trensferred into thelaminar. The transport modes of the hyperconcentrated flow canalso be classified into the pseudo-homogeneous and heteroge-neous (two phases) flows based on the vertical distribution of theconcentration according to the field data. The effective Reynold’snumbers of the hyperconcentrated floods in the Lower YellowRiver are higher than the critical Reynold’s number, and the verti-cal distribution of sediment concentrations is not uniform. All thehyperconcentrated floods in the Lower Yellow River thus belongto the turbulent and heterogeneous two-phase flow (Zhou, 1982,1995, 1998, Zhao, et al., 1998).

4.6.4.3 FEATURES OF DEGRADATION AND AGGRADATION

(1) Because the high viscosity of hyperconcentratedflow causes a decrease in the falling velocity of sediment parti-cles, the sediment carried by the hyperconcentrated flow may betransported over a relatively long distance. However, high levelsof aggradation occur simultaneously when the flow passesthrough the Lower Yellow River. According to measured data, thedeposition caused the hyperconcentrated floods on averageaccounts for 55 per cent of the incoming sediment load and 75per cent of the total deposition in the Lower Yellow River, ofwhich 86 per cent is deposited in the wandering reach of the river(Zhou, 1995).

(2) During aggradation, the sediment for various grainsizes settles and results in the decrease of their sedimentconcentration in the wandering reach. However, coarse sedimentparticles settle easily, and their deposition accounts for a largerportion of the incoming sediment, while the deposition of finesediment particles accounts for a lesser portion of the incomingsediment. Therefore, the suspended sediment carried by hyper-concentrated floods becomes finer and finer along the river(Zhou, 1998).

(3) The wandering reach of the Lower Yellow River hasa wide and shallow channel. The serious deposition of the hyper-concentrated flood mainly occurs on the low flood plains besidethe main channel, causing the main channel to narrow. On theother hand, the flood flow is forced to be concentrated in the mainchannel, and leads to erosion. Under certain conditions the mainchannel can be sustantially cut down to form the so-called highflood plains and deep main channel. During the hyperconcentratedfloods in 1977, on the upper part of the wandering reach the widthof the main channel decreased from 3 000 m to 400 m, and themaximum elevation difference between the flood plains and themain channel reached 6 m at Huayuankou Station due to sharpcutting. The preconditions for forming such high flood plains anddeep main channels include (Zhou, 1983, 1998) the following: (1)the flood peak discharge should be over 5 000 to 6 000 m3 s–1; (2)the sediment concentration should be higher than 400 kg m–3; and(3) the flood peak discharge and the maximum concentrationshould occur nearly at the same time. These three preconditionsshould be satisfied simultaneously. Furthermore, even though sucha cross-section is shaped, it is unstable and may easily be erodedby the wandering flow, and recover the original wide and shallowcross-section.

4.6.5 Shrinking of river channelSince the 1980s, under the combined influences of climatechange, increases in water supply in rural and urban areas, and thecompletion of large reservoirs in the upper reach, etc., incomingrunoff and sediment load in the Lower Yellow River havedecreased by 34 and 48 per cent, respectively. No flow in thedownstream channel occurs for three to four months in the dryseason every year. Although the total aggradation has decreased inthe Lower Yellow River, 85 per cent of the deposition has accumu-lated in the main channel. As a comparison, the deposition in themain channel was 23 per cent in the 1950s. As a result, the widthof the main channel in the wandering reach decreased from1 000–1 500 m to 800–1 000 m, with the minimum width beingonly 600 m. The flood-conveying capacity of the main channel hasalso dramatically decreased, which causes the flood control condi-tions to worsen. If the runoff increases, the question of whetherthe channel can be enlarged to the width of the 1950s will remaina problem.

4.7 FLUVIAL PROCESSES OF ANABRANCHEDRIVERS

The branches are characterized by stable islands. The riverchannel is divided by the islands into two or more stable branches.There are 41 branched reaches with a total length of 817 km onthe Middle and Lower Yangtze River from Chenglingji toJiangying, over a stretch of 1 120 km.

78 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 4.16 — Types of anabranched rivers.

(a) Straight

(c) Goose-head

Tianxingzhou branched reach Moerzhou branched reach(b) Slightly sinuous

Nanyang IslandTieban Island

4.7.1 Morphological characteristics of anabranched rivers4.7.1.1 CLASSIFICATION

According to their shape, anabranched rivers can be classified intothree subtypes (Xie, 1987).

(1) Straight anabranched rivers. Each branch is rela-tively straight. The sinuous index is 1.0 to 1.2 and the branches aresymmetrical (Figure 4.16 (a)).

(2) Slightly sinuous anabranched rivers. The outlines ofthese anabranched rivers are slightly sinuous; but at least onebranch should have a sinuous index of 1.2 to 1.5. Most rivers havetwo simple branches, but some have three multi-branches(Figure 4.16 (b)).

(3) Goose-head-shaped anabranched rivers. At least onebranch has a sinuous index larger than 1.5. Most rivers have twoor more islands to divide the channel into a multi-branched onewith three or more branches (Figure 4.16 (c)).

4.7.1.2 MORPHOLOGICAL INDICES

The plain morphology of anabranched rivers can be expressed asfollows (Sedimentation Committee, 1992):

(1) Coefficient of branches K1

(4.158)

(2) Index of branches K2

(4.159)

The K2 of a branched river must be larger than 1.5.

(3) Widened ratio K3

(4.160)

(4) Length-width ratio K4

(4.161)

(5) Length-width ratio of island K5

(4.162)

4.7.2 Morphology of cross-sectionsIf there are two branches, let b0, d0, and S0 be the width, depth andslope of the single channel before bifurcation, b1, d1, S1 and b2,d2, S2 represent the width, depth and slope of the two branches,and suppose that the roughness and morphological relationshipremain unchanged, then (Chien, et al., 1987),

(4.163)

(4.164)

(4.165)

(4.166)

Let m = Q1/Q0 (4.167)

Q2 = (1 – m) Q2 (4.168)

Thus, (4.169)

(4.170)

since S1 > S0, S2 > S0, m < 1; hence d1 < d0 and d2 < d0Putting Equations 4.169 and 4.170 into Equation 4.163,

(4.171)

4.7.3 Ratio of discharge and sediment diversionTaking two branches as an example, the ratio of discharge diver-sion of the main branch can be expressed as follows (Xie, 1987).

(4.172)

where m and n represent the main subbranch and branch, respec-tively.

(4.173)

where Am and An are the discharge area of the main subbranch andbranch, respectively, Lm and Ln are the length of the main branchand subbranch, respectively, and nm and nn are the roughness ofthe main subbranch and branch, respectively. The ratio of sedi-ment diversion can be expressed as follows:

(4.174)

where S is the average sediment concentration in kg m–3.If Sm/Sn = Ks,

(4.175)

4.7.4 Fluvial processes

4.7.4.1 MAIN FEATURES

The main feature of the fluvial processes of anabranched rivers isthe growth and decline of the main channel and branches. Themain channel might be transformed into the branch, and thebranch also might be transformed into the main channel becauseof changes in water and sediment diversion. During transforma-tion, the original main channel is silted and raised, while theoriginal branch is scoured, and descends.

4.7.4.2 CHANNEL DEFORMATION FOR DIFFERENT ANABRANCHED

RIVERS

(1) The fluvial processes of straight anabranched riversare the alternate distribution of pools and side bars and theirparallel shifting downstream. If the flow conditions and theentrance of the branches are changed, the main channel is trans-formed into a branch and the branch may be transformed into themain channel.

CHAPTER 4 — FLUVIAL PROCESSES 79

bd

bd

bd

0

0

1

1

3

3= = =ζ

Q n d S

Q n d S

Q n d S

0 0

113

0

12

1 1

113

1

12

2 2

113

2

12

1

1

1

=

=

=

ζ

ζ

ζ

dSS m d

dSS m d

10

1

322

311

0

20

2

322

311

01

=

= −

( )

( ) ( )

ζ mm m

m m n nQn Sn

Qm Sm

Q SQ S Q S

=+

=+

11

ξ mm

m

sm

n

n

Kn

=−

+1

K1 =Total length of branches

length of central line of channel

K2 =×2 total length of islands and mid –bars in branched reach

Length of central line of channel

K3 =Maximum width (including width of islands)Width in narrow reach upstream of branches

K4 =Length of branched reach

Maximum width of branched reach

K5 =Length of island

Maximum width of island

ηmm

m n

QQ Q= +

ηmn

m

n

m

m

n

m

n

A

A

d

d

L

L

n

n

=

+

1

12 3 1 2

( ) ( ) ( )/ /

b b bSS m

SS m

B B b

1 2 00

1

311

611 0

2

311

611

1 2 0

1+ = + −

+ >

[( ) ( ) ( ) ]

( )

(2) For a slightly sinuous anabranched river, the mainchannel is often located on the concave side. When the concavebank is eroded and shifts to a hard boundary, the main channelbecomes stable, and the branches are also stable. Such a stablesituation might last more than 100 years.

(3) Goose-head-shaped anabranched rivers mostly areformed from slightly-sinuous anabranched rivers. For example, inthe Luxikou Reach of the Yangtze River, the left branch continu-ously developed through erosion in the floods of 1926, 1931 and1933, and a typical goose-head-shaped branch was formed in1934. An unstable goose-head-shaped and multi-anabranchedreach was finally formed after the island was cut by flood flows(Figure 4.17).

4.8 FLUVIAL PROCESSES OF STRAIGHT RIVERS

4.8.1 Morphological featuresIn alluvial rivers, straight rivers have straight outlines with a rela-tively short length, such as the straight reach between two bendsof meandering rivers or the single straight river between twobranched reaches. Straight rivers have the following main features.

(1) Alternate side bars. Alternate side bars cause themain current line to be sinuous. The size of side bars depends onthe size of river channels (Xie, 1987).

b = 0.57B (4.176)

L = 2.8B (4.177)

where b is the width of the side bar, B is the width of the channel,and L is the length of the side bar. All the parameters are under thebankfull discharge.

(2) Side bars alternate with pools along the river. Therelationship between the distance of pools and river width can beexpressed as follows (Chien, 1987).

Lp = 6B (4.178)

where Lp is the distance between the pools and B is the width ofthe river. The expression coincides with that of the meander. Thisimplies that straight rivers, in essence, have the generality ofmeandering rivers.

(3) Riffles and pools occur alternately along thethalweg. In low water seasons, sprays can be found on the surfacedownstream near the riffles.

4.8.2 Features of flow and sediment transportStraight rivers have pools and crossings with a sinuous maincurrent. Straight rivers also have circulating flows, but the flowintensity is weaker than that of meandering rivers. The sedimenttransport rate of bed load on crossings is lower than that in pools.Obvious sorting of sediment particles can be found. The coarseparticles are concentrated on crossings and the sediment composi-tion in pools is fine. Sediment sorting also exists vertically on thecrossings. The coarse sediment is located near the surface, whilefine sediment is situated in the deep layers.

4.8.3 Features of fluvial processes(1) The migration of alternate side bars downstream and

the corresponding shifting of pools are the major characteristics ofthe fluvial processes of straight rivers. Therefore, the river, includ-ing the side bars, pools and crossings, as a whole moves somedistance downstream after a certain time period.

(2) The river channel is widened periodically. Whenthe side bars move down, the erosive banks on both sides arecovered by the side bars. Correspondingly, the formerly coveredbanks are exposed and re-eroded by the flow. Thus, the banklines recede, causing the channel to be gradually widened.Then, the wide side bars are cut off by the flow and becomemid-bars or islands. Once one branch is blocked, the islandconnects with the bank and the channel becomes narrowed onceagain.

4.9 STABILIZATION AND RECTIFICATION OFRIVER CHANNELS

The training of alluvial rivers can be classified into long distanceregulation and local regulation. Long distance regulation is aimedat flood control and navigation, while local regulation aims toprevent banks from collapsing, to stabilize the intake of waterdiversion and enforce the channels upstream and downstream ofbridges, etc.

80 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 4.17 — Luxikou anabranched reach on the Yangtze River.

Luxikou

4.9.1 Parameters of river training planningThe main parameters of river training include design dischargeand planning of channel width and channel alignment.

4.9.1.1 DETERMINATION OF DESIGN DISCHARGE

(1) Design discharge of flood channels (Xie, 1987). Themain purposes of training flood channels are to raise the flood-carrying capacity of the channel, prevent important embankmentsfrom collapsing, and guarantee flood control safety. Design flooddischarge is determined by the recurrence intervals of floods. InChina, the recurrence of river training works in the most importantregion is 1 to 0.33 per cent, in important regions it is 2 per centand in general regions it is 10 to 5 per cent. But most rivers haverecurrence intervals of 5 per cent. The recurrence interval ofdesign flood for rivers varies from country to country dependingon the economy of the country.

(2) Design discharge of low flow channels. The purposesof training low flow channels are to ensure the conditions ofnavigation and water diversion and to stabilize the location ofdiversion intakes. Two methods can be used to determine designdischarge:(a) The discharge is determined using the water level, which is

in accordance with a guarantee modulus from the long-termdaily water levels. The guarantee modulus of navigation inChina is 90–95 per cent;

(b) The low discharge corresponding to the historical lowestwater level or the long-term average low water level is takenfor the design discharge for the low flow channel.

(3) Design discharge of moderate flow channel. Thefloods of alluvial rivers are conveyed mainly by the moderate flowchannel that is moulded by the dominant discharge. If the moder-ate channel of a river is controlled, the training of its flood channeland low flow channel can be easily resolved. The determination ofdominant discharge is illustrated in section 4.4.1.

4.9.1.2 DETERMINATION OF CHANNEL WIDTH

The channel width of river training is the surface width of thestraight channel (crossing) corresponding to the bankfulldischarge. Two methods can be used to determine the trainingwidth of the channel:(a) The morphological relationships in section 4.4.3 can be used

to determine the channel width under the bankfull discharge.Coefficients and exponents in these relationships should bedetermined with field data from the trained river;

(b) Statistical method. Analysing the data from typical riverswith the same river pattern and a channel width correspond-ing to the bankfull discharge may be used for the trainedriver.

4.9.1.3 ALIGNMENT

In order to minimize damage caused by the stream on stabilizationand rectification structures, the river channel should be shaped inan alignment consisting of a series of easy bends with the flowdirected from one bend to the next one downstream. In the LowerYellow River, the principles of river regulation are mainly forflood control, but proper consideration is given to the protection offloodplains, as well as diversion for irrigation and improvementsfor navigation. The aim of river regulation is to stabilize thechannel for moderate floods through effective measures, becausemoderate floods often threaten vulnerable sections. Therefore, the

dominant discharge is adopted as the design flood of the moderatechannel regulation.

The alignment of river training of the Lower YellowRiver is determined by the following relationships according tofield data of the Lower Yellow River (Xu, 1983).

R = 3250/Φ2.2 (4.179)

R = (2 – 4) B (4.180)

L = (2 – 5) B (4.181)

where R is the radius of the bend, L is the length of straightstretch, and B is the channel width in the straight stretch (all in m).There are two types of alignment in the Lower Yellow River: (a)alignment with successive bends; and (b) alignment with sharpbends and long straight stretches (Figure 4.18).

4.9.2 Structures of river training works4.9.2.1 STRUCTURES OF TRAINING WORKS FOR MODERATE AND

LOW FLOW CHANNELS

Structures of training works on moderate and low flow channelsmainly include long and short groins and revetment.

(1) Groins. Groins extend out from the bank into theflow. Long or short groins are used to cut off side channels andchutes, concentrate a braided river into a single channel, concen-trate a channel to increase depth, realign a river reach, preventbank erosion and protect structures along banks and near bridgesand utility crossings. Groins are aligned either at an angle orperpendicular to the flow. Experience indicates that groins alignedeither at right angles to the bank or in a slightly downstream direc-tion are more effective than groins angled upstream from the bankline.

The length of the groin depends on its location (in acrossing, at a bend, across an old channel, etc.). The length of longgroins on the Lower Yellow River is 100 to 200 m, the longestbeing 3 km, and short groins are 10 to 20 m long.

Groin spacing is usually 1.5 to 6 times the groin length,but 1.5 to 2.0 times the groin length gives the best defined channelfor navigation. Groin spacing is equal to groin length on theYellow River. Figure 4.19 shows the dike system on the rivers.

On the rivers in the United States, spur dikes, pile dikes,pile dikes filled with stone, dikes and fencing dikes are widelyused, but earth-rock groin structures are used on the Yellow,Yangtze and other rivers.

CHAPTER 4 — FLUVIAL PROCESSES 81

Figure 4.18 — Typical alignment of river training of the Lower YellowRiver.

Vulnerable spot atPenglou

Constraint atMazhangzhuang

Vulnerable spot atYingfang

(2) Revetments. A revetment is used to stabilize concavebanks or to protect eroding bank lines of flood plains. In theUnited States, various types of revetment are used, such as stan-dard revetments with mattresses on the stream banks, standardtrench-fill revetments on stream banks, pile revetments with stonefill and stone-fill revetments, etc., but in China, revetments useearth-stone structures and standard revetments with a mattress, etc.

According to the needs of channel stabilization andrectification, groins and revetments can be used simultaneouslyfor protecting banks or flood plains from erosion.

4.9.2.2 STRUCTURES OF TRAINING WORKS FOR FLOOD CHANNELS

The structures of training works on flood channels includeembankments and bank protecting works. Embankments shouldbe parallel with the direction of flood flow and the moderate flowshould be taken into consideration. In China, the flood peakdischarge with a recurrence interval of 0.33 to 1 per cent should betaken as the design discharge for the most important embank-ments. The recurrence interval of important embankments is 2 percent, and for general embankments this figure is 10 to 5 per cent.The top elevation and space of embankments are determined bythe hydraulic computation of water surface profile correspondingto the design recurrence intervals. The design recurrence intervalsof embankments vary from country to country because they arerelated to national flood defence policies.

The top width of the embankment depends on the pathof filtration and traffic requirements in flood seasons. The sideslope of the embankment is related to the soil properties of theembankment, rates of rising and falling of water levels duringfloods, the duration of floods, wind waves and filtration, etc. If theembankment is composed of loam or sand loam and is higher than

5 m, and the flood duration is long, a side slope coefficient of 2.5to 3.0 can be adopted. The Jingjiang embankment on the YangtzeRiver is 10 m high and the flood duration is 1 to 3 months. Theside slope coefficient of its upstream slope is 2.5 to 3, and that ofthe downstream slope is 3 to 6.3.

The bank protecting works in the flood channels are thesame as those in moderate and low flow channels.

4.9.2.3 DREDGING

Dredging is widely used in the improvement and maintenance ofnavigation conditions in rivers and harbours. In recent years,dredging has also been used in desilting reservoir sedimentation,strengthening embankments, and forming and improving farm-land.

The major problems associated with the disposal ofdredged material are: (i) ensuring the availability of a sufficientdisposal area for initial and future maintenance dredging within areasonable (economically feasible) distance of the dredging opera-tions; and (ii) the potential adverse environmental effectsassociated with the disposal of dredged material, including anincrease in turbidity, the resuspension of contaminated sedimentand a decrease in dissolved oxygen.

Costs and the potential environmental impacts are funda-mental considerations when evaluating alternative dredging anddisposal methods and disposal sites, and many factors must beconsidered in developing dredging operations, including:(a) Determining the quality of the material to be dredged

initially and the frequency and quantity of future mainte-nance dredging;

(b) Sampling to determine the physical and chemical propertiesof the material to be dredged to ensure that an appropriatetype of dredge is used, to assess dredged production rates sothat time and cost estimates are realistic, and to identify anypollutants in the material to be dredged;

(c) Selecting an appropriate dredge type and size, disposalmethod and disposal area to ensure environment protection;

(d) Identifying adequate disposal areas for both initial and futuremaintenance dredging considering the physical properties ofthe dredged material;

(e) Long-term management of disposal sites for a maximumstorage volume and beneficial use after the sites are filled(Peterson, 1986).

4.9.3 River training of meandering rivers4.9.3.1 MEASURES OF RIVER TRAINING FOR STABILIZING RIVER

CHANNELS

In order to stabilize the channels, bank protecting works are usedto prevent the successive collapse of banks on the concave side.There are three types of bank protecting works which arecommonly used.(1) Smooth bank protecting works. The anti-erosion materials or

matters are bedded directly on the banks or channel beds.(2) Groins. Groins or spur-dikes are used to direct the flow.(3) Combined use of smooth bank protecting works and groins

and spur-dikes. The combined works are often used toprotect the banks on a long reach.

Meanwhile, ecologically acceptable designs, e.g.preserving or recreating meander bends and the range of geomor-phological and flow environments for habitat improvementpurposes etc., should be taken into consideration.

82 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 4.19 — Dike systems (after Peterson, 1986).

4.9.3.2 CUTOFF

(1) Natural cutoff can develop on a meandering river,as the neck between two neighbouring bends becomes thinner.Once overbank flow occurs, the neck can be cut off and a newchannel connecting the two bends may form. Natural cutoffsoccurred 33 times between 1700 and 1870 on the MississippiRiver (Xie, 1987).

(2) Artificial cutoff. A limited pilot channel of a rela-tively small cross-sectional area is excavated to connect the long,looping bends, enabling the excavated channel to be developedand enlarged to full channel dimensions by the flow.

The length of the designed pilot channel is determinedby the cutoff ratio (length of original river channel / length of pilotchannel). The optimized cutoff ratio is 3 to 9, while the ratio of theexcavated cross-sectional area and the original cross-sectional areais 1/5 to 1/30, and the side slope of the excavated channel is 1:2 to1:3, according to data from rivers in China (Xie, 1987).

The cross-sections of the pilot channel should be madeas deep as possible in order to increase the channel’s flow velocityand erosive capacity.

On navigation rivers, the pilot channel should bedesigned according to the navigation standard. The excavatedwidth and depth should meet the needs of navigation so as toensure that navigation is unimpeded after excavation.

The entrance location of the pilot channel should bedesigned in accordance with the configuration of meanders andthe geological structure of channel bends. The newly developedchannel should be protected by bank protecting works to avoidthe renewed development of long, looping bends. On theMississippi River a dike system had to be built in 1975 in order tomaintain the navigation depth.

4.9.4 River training of wandering riversThe regulation of wandering rivers is mainly aimed at control-ling the main current, rectifying the plan configuration,transforming the wide-shallow and scattered channels into asmooth, stable and single channel in order to increase floodconveying capacity, and improving the conditions of floodcontrol. For wandering rivers with a high sediment concentra-tion, river control is much more complicated. Soil and waterconservation works in upstream eroded areas and reservoirs onthe main stems and tributaries should be constructed so as toadjust the conditions of incoming runoff and sediment load, inaddition to the training works on the rivers.

On the Lower Yellow River, the river training worksconsist of the embankment, works at vulnerable spots andconstraint works. At vulnerable spots there are spur dikes andgroin systems, and bank protecting works are constructed alongthe surface of the embankment where the flow often attacks inorder to protect the embankment and the banks. The constraintworks consisting of long groins with short spur-dike systems andrevetments are constructed along the embankment and flood plainsto protect the banks and flood plains, to control the main currentand to form a stable channel of moderate flow.

The length of the groin is two thirds of the practicallength. The angle between the flow and the groin is 30 to 45°. Theratio of groin space to groin length is 0.8 to 1.04. After regulation,the wandering range in the wandering reach of the Lower YellowRiver has decreased from 2 200 to 1 600 m, and the area of theflood plains has increased by 9 000 ha. In the transitional reach,

the maximum wandering range has been reduced from 5 400 to1 400 m, with the average range from 1 800 to 5 600 m. The waterdepth at bankfull discharge has increased from 1.47–2.37 m to2.05–3.73 m. All the changes in the wandering of the maincurrent, the shape of the cross-section and the curvature indicatethat the river channel has the tendency to be transformed into ameandering river in the transitional reach (Xu, 1983).

4.9.5 River training of anabranched riversThe aim of river training in anabranched rivers is to stabilize theratio of flow diversion, or to improve the conditions of water andsediment transport in the main and fork channels.

4.9.5.1 MEASURES FOR STABILIZING FLOW DIVERSION RATIO

For the stabilization of flow diversion ratio, the plan configurationof the anabranched river should be stabilized. Therefore, thecontrol works at the upstream node point, fish-mouth works at thehead of the island and bank protecting works at the entrance of thebranched reach should be constructed to fix the inlet flow andisland of the river.

4.9.5.2 WORKS OF FORK-CHANNEL BLOCKADE

On multi-branched reaches, measures for blocking fork-channelsand strengthening the main channel should be adopted to meet therequirements relating to navigation and water supply for industryand agriculture. A chute dike can be used to block the fork channelon medium and minor rivers. On rivers with high sedimentconcentrations, fence dikes and other infiltrated dikes are used,which can easily be blocked by turbid water.

4.9.6 River training of straight riversThe aim of river training on straight rivers is to fix the alternatepoint bars so as to stabilize the straight channels. Dikes with anupstream direction and submerged dikes can be used to reinforcethe point bars. Low dike systems have been used on the RhineRiver with favourable results.

4.9.7 Regulation of shoal reachesThe purpose of the regulation of shoal reaches is to improvenavigation conditions. The main training measures include: (1) theconstruction of river training works to constrain the water flow, fixthe upstream and downstream point bars, block the subsidiarybranches, stabilize the bank lines and maintain the size of thenavigation course, and (2) dredging the navigation course todecrease deposition and maintain the scale of the navigationcourse.

4.9.7.1 PARAMETERS FOR DESIGNING NAVIGATION COURSES

(1) Assurance rate of navigation. The assurance rate ofnavigation should be determined according to the state standard.For example, the navigation standard for natural rivers in China islisted in Table 4.21.

(2) Size of navigation courses.(a) Minimum water depth.

dmin = t + ∆d (4.182)

where dmin is the minimum water depth in the navigation course, tis the maximum draught of allowance ships, and ∆d is the addi-tional depth.

CHAPTER 4 — FLUVIAL PROCESSES 83

(b) Minimum width.For double line course

bmin = 2 (b + b1) (4.183)

where bmin is the minimum width of the navigation course; b isthe maximum width of allowance fleets, and b1 is the distancebetween ships or between ship and bank. The width of the naviga-tion course should be 5 times larger than the ship width.(c) Curvature radius.

Rmin = (3 – 6) c (4.184)

where Rmin is the minimum curvature radius, and c is themaximum length of a fleet.

The length of a straight reach should be two times thatof the maximum length of a fleet. The determination of the controlline of the navigation course should refer to the data from riversthat have similar conditions of hydrology, geology and navigation.

The conditions of navigable rivers include: (i) singlechannel and no bifurcations in low water periods; (ii) smooth banklines, uniform curvatures, and appropriate length of crossingreaches; (iii) no obvious difference between the depth in pools andon the shoals of crossing reaches; (iv) uniform water surfaceslope; (v) symmetrical cross-sections approximate to parabola oncrossing reaches; (vi) no criss-cross or short criss-cross betweenupstream and downstream pools. A physical model is significantfor studying the regulation of important shoals and crossingreaches.

REFERENCESAndrews, E.D., 1980: Effective and bankfull discharge of streams

in the Yampa River basin, Colorado and Wyoming. Journalof Hydrology, Volume 46, pp. 311-330.

Altwunin, C.T. and Y.A. Bozonov, 1957: Protective Constructionsof Rivers. Water Conservancy Press (in Chinese).

Altwunin, C.T., 1962: Regulated Rivers. Finance Press.Benson, M.A. and D.M. Thomas, 1966: A dominant discharge.

Bulletin of the International Association of ScientificHydrology, Volume 11, Number 2, pp. 76-80.

Bray, D.T., 1982: Regime Equations for Gravel-bed Rivers. (ed.)R.D. Hey, et al., pp. 517-552.

Bridge, J.S., 1977: Flow, bed topography, grain size and sedimen-tary structure in open channel bends: a three-dimensionalmodel. Earth Surface Processes and Landforms, pp. 401-416.

Carlston, C.W., 1965: The relation of free meander geometry tostream discharge and its geomorphic implications. AmericanJournal of Science, Volume 263, pp. 860-885.

Chai Tingsheng, 1963: Study on geometrical relations of theMiddle and Lower Yangtze River. Compilation of ResearchReports, Nanjing Hydraulic Research Institute.

China-Bangladesh Joint Expert Team, 1991: Study Report onFlood Control and River Training Project on theBrahmaputra River in Bangladesh. Volumes 1 and 2.

Chang, H.H., 1979: Geometry of rivers in regime. Journal of theHydraulics Division, ASCE, 105 (Hy6).

Chang, H.H., 1988: Fluvial Processes in River Engineering. JohnWiley and Sons.

Chang Jingdou and Sun Hancheng, 1983: Influences of changes ofbend flow conditions on formation of river patterns of Upperand Lower Jingjiang River. Journal of Sediment Research,Volume 1.

Chang Ruijing, 1961: Dynamics of River Flows. China IndustrialPress.

Chang Zhitang, Ling Wanquan and Shen Yongjian, 1982: Studieson Dynamic Axis Line of Flow in Natural MeanderingRivers. Compilation of papers of Changjiang River ScienceInstitute.

Chien Ning, 1958: Discussion on stability index of alluvial rivers.Journal of Geography, Volume 24, Number 2, pp.128-144.

Chien Ning and Dai Dingzhong, 1980: The problems of river sedi-mentation and the present status of its research in China.Proceedings of the International Symposium on RiverSedimentation, Volume 1, Guanghua Press.

Chien Ning, Zhang Ren and Zhou Zhide, 1987: Fluvial Processes.Science Press (in Chinese).

Chien Ning and Zhou Wenhao, 1965: Fluvial Processes of theLower Yellow River. Science Press (in Chinese).

Chikurimora, 1969: Calculation of dominated discharge.Proceedings of the Tenth Congress, IAHR, Volume 5.

Chitale, S.V., 1970: River channel patterns. Journal of theHyraulics Division, ASCE 96 (Hy1), pp. 201-222.

Dou Guoren, 1964: Channel morphology of alluvial rivers andtidal estuaries. Journal of Hydraulic Engineering, pp. 1-13(in Chinese).

Dury, G.H., 1964: Principles of under fit streams, ProfessionalPaper Number 352-A, USGS.

Emmett, W.W., 1975: The channels and waters of the Upper SalmonRiver, Idaho. Professional Paper Number 870-1, USGS.

Englund, F., 1974: Flow and topography in channel bends.Journal of the Hyraulics Division, ASCE, 100 (Hy11),pp. 1631-1648.

Falcon-Ascanio, M. and J.F. Kennedy, 1983: Flow in alluvial rivercurves. Journal of Fluid Mechanics, Volume 133, pp. 1-16.

Fang Zongdai, 1964: Classification of river patterns and its appli-cations to river training. Journal of Hydraulic Engineering(in Chinese).

Ferguson, R.I., 1973: Channel Pattern and Sediment Type Area.Volume 5, pp. 38-41.

Friedman, G.M. and J.E. Sander, 1978: Principles ofSedimentology. John Wiley and Sons.

Hack, J.T., 1957: Studies on longitudinal stream profiles in Virginiaand Maryland. Professional Paper Number 294-B, USGS.

84 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 4.21Navigation standard for natural rivers

Grade of navigation course 1 2 3 4 5 6 7

Minimum depth at shoals (m) 73.2 2.5–3.0 1.8–2.3 1.5–1.8 1.2–1.5 1.0–1.2 0.8–1.0

Assurance rate of navigation (%) 98–99 93–97 90–95 85–95 80–93 80–90 75–90

Hey, R.D., 1975: Design discharge for natural channels. Science,Technology and Environment Management.

Hey, R.D., et al., (ed.), 1982: Gravel-bed Rivers. John Wiley andSons, pp. 5-13 and pp. 553-580.

Huang Sheng and Zhong Xiujuan, 1986: The classification andprocess of estuary in China. Proceedings of the ThirdInternational Symposium on River Sedimentation, Universityof Mississippi, pp. 501-506.

Ikeda, S., 1982: Bed load transport on side slope. Journal ofHydraulic Engineering, ASCE, 108 (Hy11), pp. 1369-1373.

Institute of Hydraulic Research of the YRCC, 1978: Preliminaryanalysis of effective volume of reservoirs on rivers carryinghuge sediment load. Collections of Reservoir Sediment.

Kellerhal, R., 1967: Stable channels with gravel-paved beds.Journal of the Waterways and Harbor Division, ASCE, 93pp. 63-64.

Lane, E.W., 1957: A study of the shape of channels formed bynatural streams flowing in erodible material.Sediment Series Number 0, Missouri River Division, U.S.Army Corps of Engineers.

Leopold, L.B. and T. Maddock, 1953: The hydraulic geometry ofstream channels and some physiographic implications.Professional Paper Number 252, USGS.

Leopold, L.B. and M.G. Wolman, 1957: River channel patterns:braid, meandering and straight. Professional Paper Number282-B, USGS, pp. 45-62.

Leopold, L.B, M.G. Wolman and J.P. Miller, 1964: FluvialProcesses in Geomorphology. W.H. Freeman.

Li Baoru, 1965: Calculation of longitudinal slopes and longitudi-nal profiles. Journal of Yellow River (in Chinese).

Ling Chenkun, 1963: Categorization of river patterns. Journal ofNanjing University. Nanjing University Press.

Makkaveev, N.E., 1955: River Channel and Erosion in itsWatershed. Academy of the USSR Press.

Makkaveev, N.E, 1959: Morphological characteristics of riversystems. Fluvial Processes of Plain Rivers (in Russian).

Nixon, M.A., 1959: A study of the bankfull stages in rivers inEngland and Wales. Proceedings of the Institute of CivilEngineers, Paper Number 6322.

North-west Institute of Hydraulic Research, 1962: Analyses andstudies on river bed morphology at stations on the Middleand Lower Weihe River. Compilations of River Regulationsof the Middle and Lower Weihe River.

Odgaard, A.J., 1981: Transversal bed slope in alluvial channelbends. Journal of the Hydraulics Division, ASCE, 107(Hy12), pp. 1677-1694.

Odgaard, A.J., 1982: Bed characteristics in alluvial channel bends.Journal of the Hydraulics Division, ASCE, 108 (Hy11),pp. 1268-1281.

Odgaard, A.J., 1984: Grain distribution of river bed armor layers.Journal of the Hydraulics Division, ASCE, 110 (Hy9),pp. 1267-1272.

Ouyang Lutai, 1983: Preliminary discussion on evolution andstability of meanders. Journal of Sediment Research.

Parker, G., 1978: Self-formed straight rivers with equilibriumbanks and mobile beds (Part 2. The gravel river). Journal ofFluid Mechanics, Vo1. 89, Part 1, pp. 127-146.

Parker, G., 1979: Hydraulic geometry of active gravel rivers.Journal of the Hydraulics Division, ASCE, 105 (Hy9)pp. 1185-1201.

Parker, G., 1984: Discussion of lateral bed load transport on sideslope, by S. Ikeda. Journal of Hydraulic Engineering, ASCE,110 (2), pp. 197-199.

Peterson, M.S., 1986: River Engineering. Prentice-Hall,Englewood Cliffs.

Pickup, G. and R.F. Warner, 1976: Effects of hydrologic regime onmagnitude and frequency of dominant discharge. Journal ofHydrology, Volume 29, Number 1/2, pp. 51-76.

Riley, S.J., 1972: A comparison of morphometric measures ofbankfull. Journal of Hydrology, Volume 17, pp. 23-31.

Rohkin, F.M., 1948: River Mechanics, Hydraulic Problems ofNatural Rivers.

Romashen, V.V., 1977: Categories of Fluvial Processes.Translations of Geomorphology (21): Rivers. Institute ofGeography, the Academy of Science of China, pp. 1-12.

Rosinski, K.U., 1950: River Channels. Academy of Science of theUSSR (in Russian).

Rozovski, Y.A., 1965: Circulating flow and its relations to watersurface. Proceedings of Fluvial Processes, China SciencePress (translated, in Chinese).

Schumm, S.A., 1960: The shape of alluvial channels inrelation to sediment type. Professional Paper Number 352-B,USGS.

Schumm, S.A., 1968: River adjustment of altered hydrologicregimen-Murrumbidgee River and plane channels, Australia.Professional Paper Number 598, USGS.

Schumm, S.A., 1971: Fluvial Geomorphology: ChannelAdjustment and River Metamorphosis in River Mechanics.(ed.) H. W. Shen, Volume 1.

Schumm, S.A. and H.R. Khan, 1972: Experimental study ofchannel patterns. Bulletin, Volume 83, Number 6, GeologicalSociety of America, pp. 1755-1770.

Scientific Research Institute of the Yangtze River, 1959:Characteristics of Jingjiang River. Journal of SedimentResearch.

Sedimentation Committee, CHES, 1992: Handbook of RiverSedimentation. China Environmental Press.

Shulits, S., 1941: Rational equation of river bed profile transac-tion. AGU, Volume 22, Number 1941.

Simons, D.B., 1979, River and Channel Morphology, Modelingof River. (ed.) Hsieh Wen Shen, John Wiley and Sons.

The Yellow River Conservancy Committee, 1985: Training YellowRiver Press.

Velikanov, M.A., 1958: Fluvial Processes. Moscow Press.Williams, G.P., 1978: Bankfull discharge of rivers. Water

Resources Research, Volume 14, Number 6, pp. 1141-1154.Xie Jianheng, 1980: River Sedimentation Engineering. Water

Conservancy Press, pp. 235-311 (in Chinese).Xie Jianheng, Ding Junsong and Wang Yunhui, 1987: Fluvial

Processes and River Training. Water Conservancy andElectripower Press.

Xu Fuling, Guo Tiying, Hu Yisan, et al., 1983: Training means forthe Lower Yellow Reach of the Yellow River and their effect.Proceedings of the Second International Symposium of RiverSedimentation, Nanjing.

Yangtze River Water Resources Committee (YRWRC), 1959:Study on characteristics of the Jingjiang River. Journal ofSedimentation Research, Volume 4.

Yangtze River Water Resources Committee (YRWRC), 1971:River Training of Meandering River Cut-off. p. 97.

CHAPTER 4 — FLUVIAL PROCESSES 85

Yellow River Conservancy Committee (YRCC), 1981: River regu-lation. Yellow River Press.

Ying Xueliang, 1965: Causes for the formation of a meanderingriver and experiment. Journal of Geology, Volume 31,Number 4, pp. 287-303.

Yivanov, Y.V., 1951: Quantitative characteristics for longitudinalprofiles of rivers. Journal of Geographical Information,Volume 82, Number 7 (in Russian).

Yu Jun, 1982: Studies and applications of geometrical formulae ofplain rivers. Renminchangjiang, Volume 3.

Zhang Ruijing, and Xie Baoling, 1980: Studies on fluvialprocesses of meandering rivers. Proceedings of theInternational Symposium on River Sedimentation, Volume 1,Beijing, pp. 427-436.

Zhao Yan, Zhou Wenhao, Fei Xiangjun, et al., 1998: Basic laws offluvial processes of the Lower Yellow River. Yellow RiverPress.

Zhou Wenhao and Chen Jianguo, 1998: River morphology andchannel stabilization of the Brahmaputra River inBangladesh. International Journal of Sediment Research,Volume 13, Number 4, pp. 44-58.

Zhou Wenhao and Chen Jianguo, 1998: Behaviours of sedimenttransport for different grain sizes in hyperconcentrated

floods of the Lower Yellow River. Proceedings of theSeventh International Symposium on River Sedimentation,Hong Kong.

Zhou Wenhao and Fan Zhao, 1982: Changes of longitudinalprofile of the Lower Yellow River. Journal of SedimentResearch, Volume 4 (in Chinese).

Zhou Wenhao and Fon Zhao, 1983: Influences of hyperconcen-trated floods on degradation and aggradation of the LowerYellow River. Proceedings of the China Institute of WaterResources and Hydropower Research, Volume 11.

Zhou Wenhao, Zeng Qinghua, et al., 1982: Characteristics offluvial processes for hyperconcentrated flow in the YellowRiver. Proceedings of the Second International Symposiumof River Sedimentation, pp. 618-626.

Zhou Wenhao, Zeng Qinghua, et al., 1995: Sediment transportbehaviours for different size groups in the Lower YellowRiver. International Sediment Research, Volume 10,Number 3, pp. 51-68.

Zhou Wenhao and Zhao Huaxia, 1995: Characteristics for channeldeformation of the Brahmaputra River in Bangladesh.Proceedings of Advances in Hydro-sciences andEngineering, Volume II, March, pp. 1967-1974.

86 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

5.1 INTRODUCTIONReservoirs are built for many purposes, including flood

control, water supply (for agriculture, industry, and urban usage),power generation, navigation and recreation, etc. As rivers carrysediment load, whether in large or small amounts, reservoirsedimentation occurs simultaneously with the impounding ofwater. Meanwhile, the river channel downstream of the reservoirexperiences modifications induced by the changes in flow andsediment regimes. Those changes upstream and downstream ofdams lead to intensive changes in environment, ecology and rivermorphology, affecting engineering projects along the river, etc. Itis necessary to predict such changes. Making full use of thebenefits of reservoirs and developing appropriate measures tomitigate the side effects of dam construction are a necessity for thesustainable development of reservoirs.

5.1.1 Dam constructionAccording to the International Committee on Large Dams, largedams (higher than 15 m) numbered 5 000 in 1950, and by 1985the number had increased to more than 36 000, half of which werein China. In recent years, however, the rate of construction hasdecreased because very few good dam sites remain to be exploitedin a way that is both economically and environmentally sustain-able in developed countries.

Furthermore, many reservoirs have been filling withsediment, which depletes their storage capacity, and many haveexceeded their life expectancy. Some of them are to bedecommissioned.

Although there has been a decline in dam constructionin recent years, 292 dams higher than 60 m were still underconstruction in 1994, including 68 in China, 48 in Japan and 37 inTurkey.

The total storage capacity of reservoirs in the world hasbeen estimated by various sources. One estimation is 4 000 to6 000 billion m3, and another is 5 per cent of the total runoff in theworld (38 830 billion m3), i.e. 2 000 billion m3. The percentagesof runoff regulated by reservoirs in each continent of the world arelisted in Table 5.1 (Beaumont, 1978). With the exception of SouthAmerica and Oceania, the percentages for the rest of the conti-nents range from 14 to 21 per cent.

In China, dam construction has boomed since 1949,when the People’s Republic of China was founded. As of 1995there were 86 000 dams, 18 000 of which were large dams. Thereare 358 large reservoirs (larger than 100 million m3), with a totalstorage capacity of 300 billion m3, which accounts for more thantwo thirds of the total reservoir storage capacity of the country.

5.1.2 Rate of loss of storage capacityThe rate of loss of storage capacity depends on the sedi-

ment yield of the river on which a reservoir is built, themorphologic factors of the reservoir and the operational scheme ofa project, etc. In various regions, the rates of loss of reservoircapacity are quite different.

Globally, the overall annual loss rate of reservoir storagecapacity is estimated at 1 to 2 per cent of the total storage capacity.In China in 1989, 232 large and medium-sized reservoirs had a totalloss of 11.5 billion m3, accounting for 14.3 per cent of the totalcapacity of 80.4 billion m3. Tables 5.2 (Qian, 1994) and 5.3 (Qian,et al., 1987) list some loss rates of storage capacity in China. Thedifferences among the various reservoirs are quite significant.

At the end of the 1950s, an investigation was conductedon the situation of sedimentation in 1 100 reservoirs in the UnitedStates. Data from 66 representative reservoirs are listed inTable 5.4 (Gottschalk, 1964).

CHAPTER 5

RESERVOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES

Table 5.1Percentage of regulated runoff (%)

Africa North America Europe Asia* Oceania South America

21.0 20.6 15.1 14.0 6.1 4.1

* Not including China

Table 5.2Annual loss rate of storage capacity in some provinces in China (%)

Shaanxi Shanxi Gansu Inner Mongolia Ningxia Hebei Shandong Hubei

3.02 2.9 2.4 2.1 2.0 1.1 0.44 0.20

Table 5.3Total capacity loss of reservoirs in China

Reservoir River Dam height Design Percentage(m) capacity of loss (%)

(106 m3)

Qingtongxia Yellow 42.7 605 93.0

Yanguoxia Yellow 57 220 74.6

Gongzui Dadu 88 310 71.0

Sanmenxia Yellow 106 3 760 39.0

Guanting Yongding 45 2 270 24.3

Naodehai Liuhe 41.5 196 19.5

Fengjiashan Qianhe 73 389 5.9

Danjiangkou Hanjiang 110 16 050 3.9

The rate of loss of reservoir capacity is closely related tothe rate of erosion of the watershed above the reservoir. Table 5.5provides evidence of such a situation (YRCC, 1993).

5.1.3 Sustainable development of reservoirsThe concept of sustainable water management presumes sociallyacceptable, ecologically sound, economically justifiable and tech-nically feasible projects. It has strong ethical connotations, such asenvironment protection, respect of future generations and equitywithin our generation. The concept of sustainable developmenthas to be made operational: whatever the definition used for theterm ‘sustainable’, to make the definition operational, one mustlist all the consequences of each possible decision, assess theirlikelihood, and plan the optimum value system that will be used inthe future to evaluate these consequences.

Applied to reservoirs, the operational concept ofsustainable management presumes the extension of the useful lifeof reservoirs to a reasonable maximum. In order to attain this goal,appropriate decisions should be made at each phase of thereservoir’s life cycle, including the planning, design,implementation and operational stages. Once the end approaches,the storage reservoir should be decommissioned with the leastpossible harm to the affected society. In short, prolonging theirlifespan is a key issue for the sustainable development ofreservoirs.

How to preserve the long-term capacity of reservoirs isthe most important issue. Three basic methods of sediment controlfor reservoirs are as follows: (1) decreasing the amount of sediment

that enters a reservoir by reducing sediment erosion from thewatershed upstream of the reservoir or by intercepting the sedimentbefore it enters the reservoir; (2) sluicing sediment-laden flows todecrease the amount of sediment that deposits in the reservoir; and(3) removing the deposited sediment by flushing, dredging, and/orsyphoning, etc.

5.1.4 Prediction of reservoir sedimentationNowadays, the prediction of reservoir sedimentation is mainlybased on mathematical modelling, although empirical methods arestill in use.

The major drawback of sedimentation models remainsthe uncertainty of sediment transport computations and of the esti-mation of river channel resistance. These are the basic researchtopics of lasting priority in sedimentation engineering.

5.1.5 Issues related to reservoir sedimentationThe construction of a dam in a river valley causes changes in theflow regime, which consequently leads to a significant change insediment regime and a transformation of fluvial processes. Threeriver reaches should be studied in this respect, namely the reser-voir itself, the upstream reach and the reach below the dam.

The impacts of reservoir sedimentation manifest them-selves in many areas, such as the environment, ecology, the safetyof the project, the economy, and society in general. These impactsare discussed in Chapter 1.

5.2 PROCESSES OF DEPOSITION IN RESERVOIRS5.2.1 Movement of sediment in reservoirsSediment movement mainly depends on water flow. In a reservoir,there are two main patterns of flow motion, namely backwaterflow and quasi-uniform flow. Under the conditions of backwaterflow, the water depth increases longitudinally, and the flow veloc-ity decreases accordingly. Sediment transport may have twopatterns. The first pattern is sediment transport under open channelflow, where sediment particles diffuse to the whole section. As theflow velocity decreases longitudinally, deposition takes place; thisis called backwater deposition. The second pattern is sedimenttransport by density current, which is formed by a heavy sedimentload with fine particles, which dives into the bottom of the reser-voir and moves along the channel bed toward the dam. Thesediment transport under quasi-uniform flow is similar to that ofnatural rivers. When the incoming sediment load is different fromthe sediment transport capacity of the flow, longitudinal deposi-tion or erosion will occur. In summary, the sediment transportpatterns in reservoirs may be classified as follows:(1) Sediment transport under quasi-uniform open channel flow;(2) Sediment transport under backwater flow:

(i) Sediment transport under open channel flow;(ii) Sediment transport by density current.

5.2.2 Basic characteristics of reservoir deposits(Qian, et al., 1987)

5.2.2.1 LONGITUDINAL PROFILES

There are three different shapes of longitudinal profiles of depositsin reservoirs, namely delta, wedge and narrow band. The geomet-ric shape of reservoir deposits depends on: (i) the composition anddiameter of the incoming sediment load; (ii) the amount of incom-ing load relative to the storage capacity; and (iii) the geometry andoperational mode of the reservoir.

88 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 5.4Reservoir sedimentation in various regions in the United States

Number of Loss of storage Annual lossRegion reservoirs storage rate

capacity (%)

North-east 3 24.7 0.82South-east 10 18.6 0.81Middle West 11 14.0 0.85Middle south 12 8.8 0.51North Great Plains 9 9.6 1.28South-west 15 15.7 0.53North-west 6 7.0 0.30Total 66 15.6 0.71

Table 5.5Loss rate of reservoir capacity in a 30-year period in the

Yellow River basin

Rate of erosion Rate of loss of Annual rate of(t km–2.a) total capacity (%) loss (%)

20 000–30 000 52.6 1.7515 000–20 000 51.2 1.7110 000–15 000 41.1 1.375 000–10 000 43.1 1.442 000–5 000 41.1 1.371 000–2 000 20.1 0.67

500–1 000 15.4 0.51200–500 14.0 0.47100–200 11.7 0.39

<100 3.8 0.13

A delta forms in most impounding (storage) reservoirs inwhich the ratio of the storage capacity, V, to the incoming annualrunoff, W, is large; the pool level is frequently kept at high eleva-tions, and the incoming sediment load is comparatively coarse(Figure 5.1).

A wedge forms in gorge-type reservoirs in which V/W issmall, incoming sediment is comparatively fine, and the pool levelfrequently fluctuates. Sediment will soon reach the dam site, asshown in Figure 5.2.

A narrow band may form in some of gorge-type reser-voirs in which V/W is large, the incoming sediment iscomparatively fine and the pool level fluctuates frequently. Thisshape of deposit is caused by the large fluctuations in pool level(Figure 5.3).

Several rules of thumb have been developed to differen-tiate between the various shapes of deposits in reservoirs.

Jiao: Delta V/Ws > 2, ∆H/Ho < 0.15Wedge V/Ws < 2, ∆H/ Ho > 0.15

where V is the average storage capacity in a time interval, ∆T, inm3, Ws is the incoming sediment load in ∆T, in m3, ∆H is theamplitude of pool level in ∆T, in m3, and Ho is the average waterdepth above the discharging outlet in ∆T (Jiao, 1980).

Another rule of thumb:Delta SV/Q > 108 ∆H/Ho < 0.1Band 0.25 × 108 < SV/Q < 108 0.1< ∆H/Ho < 1Wedge SV/Q < 0.25 × 108 ∆H/Ho > 0.1

where S is the sediment concentration in kg m–3, and Q is thedischarge in m3 s–1.

Luo used only one parameter, Ws/γs'VDelta 0.78–1.75Band 1.1–3.98Wedge 4.38–5.2

where γs is the unit weight of deposits in t m–3 (Luo, 1977).

5.2.2.2 DELTA

(1) Longitudinal profile. The longitudinal profile of a deltacan be divided into several reaches: tail reach, top-set reach, foresetreach, and bottom-set reach.(i) Tail reach: This is a transition reach between the natural

stream and the delta proper. The flow, after entering thebackwater zone created by the construction of the dam,begins to deposit part of its sediment load. The bed becomesprogressively flatter and finer in composition along the rivercourse. Following the rise of the top-set in the reach immedi-ately below, the tail reach will extend upstream at a slowrate. The tail reach is usually of limited length for mostreservoirs, especially those built on mountain streams. Thecharacteristics of tail reach may be summarized as follows:the reach has super-saturated sediment transport, a selectivedeposition of sediment particles, a broad and shallow cross-section, and a wandering river reach.

(ii) Top-set reach: The top-set reach of the delta represents areach essentially in equilibrium. The selective process of thebed material in the direction of flow is no longer perceptible.Almost all the incoming load is able to move through thisreach and deposit on the foreset of the delta, making thedelta advance. This advancing of the delta causes the back-water to rise further. This, in turn, disrupts the temporarybalance maintained in the preceding stage and brings aboutfurther deposition. The top-set bed will gradually rise as aconsequence of the advance of the delta. However, whenrising, the bed profile remains essentially parallel. Flow andsediment transport in a state of equilibrium are the maincharacteristics of the top-set reach.

(iii) Foreset reach: The water depth abruptly increasesdownstream from the pivot point of the delta, and onceagain selective settling of sediment particles occurs. Thebed in this reach is formed under circumstances similar tothose of the free settling of particles in a settling basin. Theslope of the foreset is slightly less than the angle of reposeof the sediment particles in still water. If density current isformed and moves along the bottom of the reservoir, theforeset will be modified and maintained with a muchsmaller slope.

The main characteristics of the foreset reach are therapid increase in water depth, the drastic decrease in flowvelocity, the selective deposition of sediment particles, andthe advance of the delta.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 89

Figure 5.1 — Deltatic deposits in Guanting Reservoir, China.

Figure 5.2 — Wedge-shaped deposit in Bajiazui Reservoir, China.

Figure 5.3 — Narrow band deposit in Fengman Reservoir, China.

135 115 95 75 55 35 15 0Distance from dam (km)

Bed

ele

vatio

n (m

)

250

240

230

220

210

200

190

(iv) Bottom-set reach: Materials brought to the bottom-set reachare fine, usually those carried by the density current. The bedslope is quite flat. Fine deposits and flat slopes are the mainfeatures of the bottom-set reach.

(2) Quasi-equilibrium state. The establishment of thequasi-equilibrium state in a reservoir starts at the top-set. The rela-tionship between the top-set slope, J, and the original slope, Jo, isshown in Figure 5.4, in which data from 45 reservoirs areincluded. Three straight lines represent:

Line 1 J = Jo

Line 2 J = 0.5 Jo

Line 3 J = 0.2 Jo

Most of the points are close to Line 2. This means thatthe equilibrium slope of the top-set is smaller than the original bedslope. The changes in slope imply that in addition to the rivergradient, the composition of bed material may also play an activerole in accomplishing the readjustment of the new river channel.In a reservoir, part of the incoming wash load may turn into bedmaterial, and consequently the deposit will be finer than the origi-nal bed material. Figure 5.5 shows the relationship between D/Do

and J/Jo, where Do is the D50 of the original river bed, and D is theD50 of the deposit of the top-set. The finer the deposit of the top-set, the flatter the equilibrium slope of the top-set.

The quasi-equilibrium state is reached under the adjust-ment of all factors related to the formation of a river channel,among which the bed slope and the composition of bed materialmay be of most prominence.

There are many empirical expressions to determine theequilibrium slope of the top-set.

By definition, an equilibrium slope is in dynamic equi-librium, i.e. there is no obvious deformation over a comparativelylong period. On the slope, the flow is uniform. For suspended loadand bed load, the equilibrium slopes are different.

The governing factors of an equilibrium slope are: (1)dominant discharge; (2) river roughness; (3) river bed composi-tion; (4) sediment transport capacity: either suspended load or bedload; (5) river channel morphology: hydraulic geometry. In addi-tion, some engineers believe that a raising of the base level mayhave some effect.

Analytical or empirical approaches can be used to esti-mate the equilibrium slope. For an analytical approach, fourconditions must be fulfilled when an equilibrium state is reached:(1) uniform flow; (2) flow continuity; (3) sediment transport insaturation; (4) channel morphology in shape. There are four equa-tions for solving four unknowns.

(i) For suspended load:

(5.1)

Q = BhV (5.2)

(5.3)

or (5.4)

(5.5)

(5.6)

(5.7)

(ii) For bed load, one should use a bed load transportformula, e.g. Meyer-Peter and Muller formulae, or others.

(5.8)

where Qb is the bed load discharge in t s–1, and γs and γ are thespecific weights of sediment and water, respectively in t m–3, andDb is the diameter of bed load in m;

(5.9)

(5.10)

Adopting B as a constant, one obtains:

(5.11)

90 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.5 — Relationship between D/Do and J/Jo.

Figure 5.4 — Relationship between the topset slope and original slope.

€

Vn

R J=1 2 3 1 2/ /

ρ*( )= K

V

ghW

m3

Jn C W g

K Q

m

m=

2 0 4 0 73 0 73 0 73

0 73 0 2

. * . / . .

. / .

ρ

B AQ

J=

0 5

0 2

.

.

Jn A gW

K Q

m

m=

20 11 5 11 25 33 35 33

25 33 10 44

/ / * / /

/ /

( )ρ

B

hC B A

Q

J

= =0 5

0 2

.

.

( ) . ( ) . ( )

( ) ( )

/

/ /

K

KhJ D

g

Q

B

s

rb

s b

sγ γ γ γ

γ γγ

= − +

−

0 047 0 125 1 3

2 3 2 3

Kns =1

KD

r =26

901 6/

Jg

Q

BD

K

Kn

Q

B

s

s

b sb

s

r

b=

−+

−[(

.) . ]

( ) ( )

//

/ / /

0 1250 047 10

1 22 3

15 7 6 7 6 7

γγ γ

γγ γ

γ0 0.5 1.0

J/Jo

1.0

0.5

0

D/D

o

10–4 10–3 10–2 10–1 10

Original bed slope (Jo)

10–1

10–2

10–3

10–4

Top-

set s

lope

(J)

For an empirical approach, there are various expressionsbased on field data.(i) For suspended load:

(a) IWHR (11 reservoirs)

(5.12)

where q is the unit discharge in the flood season in m3 s–1, ρ is themean sediment concentration in the flood season in kg m–3, and ωis the mean settling velocity of suspended load in cm s–1.

(b) Li (based on rivers and models) (Shaanxi Institute andTsinghua University, 1979)

(5.13)

where Q is the bankfull discharge in m3 s–1, ρ is the mean concen-tration of bed material load in flood season in kg m–3, and D50 isthe D50 of bed material, in mm.

(c) Establishing a relationship between J and Jo, originalbed slope, for example:

(5.14)

where d50 is the d50 of incoming sediment load in mm, D50 is theD50 of original bed material in mm, H is the raising of the baselevel in m, and V is reservoir capacity relevant to H in m3.(ii) For bed load:

(5.15)

where Q is the mean annual discharge in m3 s–1.

5.2.2.3 LATERAL DISTRIBUTION OF DEPOSITS

The lateral distribution of deposits depends on the location of thecross-section, the operational mode of the reservoir and the sedi-ment concentration of the river, etc.

For reservoirs built on sediment-laden rivers, the parallelraising of the near-dam cross-sections in impounding reservoirs istypical, as in the Guanting and Sanmenxia (1960–1964)Reservoirs.

In reservoirs with drawdown in flood seasons, high floodplains and deep main channels occur. The size of the main channeldepends on the discharging capacity of the outlets, as inSanmenxia (1964–1973), Naodehai and Heisonglin Reservoirs.

The flood plains rise in elevation and no surface erosion occurs;only banks might collapse.

In the fluctuating backwater region in reservoirs built onclear rivers, deposition mainly takes place in the main channel, butin the permanent backwater region, parallel raising of the channelbed may take place.

5.2.2.4 SPATIAL DISTRIBUTION OF DEPOSITS

Understanding spatial distribution is useful for determining thedepletion of each part of the storage capacity, which is the basisfor planning the future operation of reservoirs.

Nowadays, analytical methods are commonly applied tosolve this problem by using computer sediment models. However,there are still many empirical methods in usage. One of them isthe empirical area-reduction method, developed by Borland andMiller (Borland and Miller, 1960) based on field data from 30reservoirs in the United States. In Figure 5.6 there are four curvesrepresenting four types of reservoir morphology with variousdistributions of sediment.

Type I Lake m = 3.5–4.5Type II Flood plain-foothill m = 2.5–3.5Type III Hill m = 1.5–2.5Type IV Gorge m = 1.0–1.5

where m is the exponent in the expression V = Nhm, h is the waterdepth at the dam site, and V is the storage capacity at h.

Based on Table 5.6, the weighted class of a reservoir isselected. Where a choice of two types is given, sediment particlesize is used to determine which to choose according to Table 5.7.

For the user’s convenience, a working diagram isplotted, as shown in Figure 5.7. The steps of the empirical areareduction method are as follows: (1) Determine sediment inflow;(2) Select the design curve; (3) Compute new zero-capacity at thedam site: use the basic expression F = (Vs – Vo)/HAo to computepo using Figure 5.9, where Vs is the total sediment deposition, Vo

is the reservoir capacity at each elevation h, H-original is the depth

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 91

Jq

= × −1 28 10 40 6

0 305. ( ).

.ρω

JQ

D= 0 00455 0 550

0 59. [( ) ]. .ρ

J Jd

D HVo/ . ( ) ( ). .=19 5150

50

0 1 0 15

J

JHQJ

oo= −0 79 0 17. ( ) .

Figure 5.6— Relative distribution of deposits in reservoirs.

Table 5.7Effect of sediment particle size

Predominant particle size Type

Sand or coarser I

Silt II

Clay III

Table 5.6Selection of the weighted class of a reservoir

Reservoir operation Operational Shape Weightedclass class class

I I ISediment submerged II I or II

III II

II I I or IIModerate drawdown II II

III II or III

III I IIConsiderable drawdown II II or III

III III

Normally empty IV All IV

Perc

enta

ge o

f re

serv

oir

dept

h

Percentage of sediment deposited

of the reservoir normal pool, Ao is the reservoir area at a givenelevation, p = (h – hmin)/H, hmin is the original bottom elevation,and ho = poH + hmin; (4) Distribute the sediment:(i) Compute a, relative sediment area at each relative depth, p:

Type I a = 5.047p1.85(1-p)0.36

Type II a = 2.487p0.57(1-p)0.41

Type III a = 16.97p1.15(1-p)2.32

Type IV a = 1.486p–0.25(1-p)1.34

(ii) Compute Ao/ao, area correction factor (relevant to po);

(iii) Compute the area at each elevation occupied by sediment(aAo);

(iv) Compute the sediment volume for each stage incrementabove the new zero-capacity elevation: Vs = 0.5 (A1 + A2) H

(v) Compute the revised area and capacity curves.This method is suitable for large impounding reservoirs.

5.2.2.5 HEADWARD EXTENSION OF BACKWATER DEPOSITION

The location of the terminal of the backwater region is not fixed; itshifts to and fro. However, the long-term trend is headward exten-sion.

In pace with the advancing and rising of a delta, thebackwater will extend upstream, which, in turn, causes depositionto propagate upstream. In certain circumstances, the headwardextension of backwater deposition may develop to a grand scale,hampering the drainage and flood control of riparian lands.

As regards the location of the terminal of the backwaterregion in Sanmenxia Reservoir, for 18 years the location of theterminal in the main channel shifted upstream discontinuously, asin 1964, 1966, 1970, and 1977 it was pushed downstream byfloods. Only the terminal on the flood plain extended upwardcontinuously.

In Figure 5.8, an empirical relationship is established todetermine the extent of headward extension of backwater deposition.

5.2.2.6 PHYSICAL CHARACTERISTICS OF DEPOSITS

(1) Longitudinal distribution of deposit diameter. Theincoming coarse sediment almost all deposits in the tail reach; atthe entrance of the top-set the bed material rapidly becomes finer.On the top-set the bed material is almost uniform, and is muchfiner than the original bed material. At the entrance of the foresetbed, the material becomes finer once again. At the bottom set, thedeposit of density current is mainly within the range of 0.002 to0.003 mm. A turning point exists in most of the curves, which liesat the location of 60 to 80 per cent of the length of the backwaterfrom the dam site.

(2) Unit weight of deposits. The unit weight of depositsis mainly determined by the initial unit weight, the operationalmode of the reservoir, and the consolidation rate of the deposits.(i) Initial unit weight of deposits. Figure 5.9 shows the initial

unit weight of deposits of different particle sizes. Han, et al.presented an expression for the initial unit weight ofdeposits, as follows (Han, et al., 1981).

For D ≤ 1 mm:

(5.16)

92 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.7 — Relative depth versus relative area of deposition.

Figure 5.9 — Relationship between initial unit weight of deposit andsediment size.Figure 5.8 — Relationship between ∆∆H and S/QJ.

h

H—

a

Diameter (mm)

Uni

t wee

ight

(1

m–1

)

γδin

D

D=

+1 41

43. ( )

For D > 1 mm:

(5.17)

where δ is the thickness of the film water, δ = 4*10–4, D1 is thecritical diameter, D1 = 1 mm, and δin is in t m–3.

Lara and Pemberton analysed 1 300 samples of reser-voir deposits in the United States and gave a measure of theeffect of the operational mode of reservoirs on the initial unitweight of deposits, as listed in Table 5.8 (Lara and Pemberton,1965).

The initial unit weight of a mixture may be calculated bythe following expression:

γsin = acPc + amPm + asPs (5.18)

where ac, am and as are the initial unit weights for clay, silt andsand, respectively (Table 5.8), and Pc, Pm and Ps are the percent-ages of clay, silt and sand in the mixture, respectively.(ii) The effect of duration of deposition. Miller developed an

approximate expression for determining the average unitweight of a deposited mixture in T years, as follows(Miller, 1953):

(5.19)

where γin is the average unit weight after T years of reservoir oper-ation; γso is the initial unit weight, and k is the constant related tothe operational mode of the reservoir and sediment size; its valuesare given in Table 5.9.(iii) Long-term unit weight of deposits. This can be determined

using Table 5.10.

5.3 SEDIMENT RELEASE FROM RESERVOIRS5.3.1 Sediment release during flood detentionFor reservoirs with serious deposition, it is necessary to knowhow the situation will develop after a flood. During a flood,water may be discharged from the reservoir at some low-leveloutlets or spillways, but flood detention occurs when theincoming flow discharge is larger than the outgoing flowdischarge. Under such circumstances part of the incomingsediment load deposits in the reservoir and the rest sluices out ofthe reservoir.

The discharging efficiency (= 1 – trap efficiency) in aperiod of time (e.g. during a flood), η, is a function of the size ofthe incoming sediment load, the duration of the sediment parti-cles in the reservoir, the characteristics of the reservoir and theratio of the incoming water discharge to the outgoing waterdischarge, etc.

(5.20)

where V is the storage capacity below the highest pool levelduring a flood, Qi and Qo are the average inflow and outflowdischarges, respectively, and ω is the mean settling velocity ofthe suspended load.

Based on data from several Chinese reservoirs, anempirical diagram (Figure 5.10) is obtained for determining thedischarge efficiency.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 93

γinD D

D= − −

−1 89 0 47 0 095 1

1. . exp( . )

Table 5.9Value of k

Operational mode k (for metric units)

Clay Silt Sand

1 256 91 0

2 135 29 0

3 0 0 0

Table 5.8Initial unit weight of sediment

Initial unit weight (kg m–3)Operational mode of reservoirs

ac Clay < 0.004 mm) am Silt (0.004–0.062 mm) as Sand (0.062–2.0 mm)

Sediment always or nearly submerged 416 1 120 1 550

Moderate to considerable reservoir drawdown 561 1 140 1 550

Empty reservoir 641 1 150 1 550

River bed sediment 961 1 170 1 550

Table 5.10Long-term unit weight

Sediment Size (mm) Unit weight (t m–3)

Clay < 0.005 0.8–1.2

Silt 0.005–0.05 1.0–1.3

Medium and fine sand 0.01–0.5 1.3–1.5

Coarse sand and fine gravel 0.5–10 1.4–1.8

Medium gravel > 10 1.7–2.1

NOTE: Operational mode: 1-deposits submerged under water for long-term period

2-pool level drops in medium or large-scale

3-long term dry reservoir.

γ γst so kT

TL nT= +

−−0 434

11. ( )

ηω

= fVQ

Q

Bi

o

( , , )1 1 1

2

Figure 5.10 — Discharge efficiency.

D50 (mm) S (kg m–3)

For small and medium-sized reservoirs, the amount ofdeposition during a flood may be determined by the followingexpression, which is based on data from reservoirs built on sedi-ment-laden rivers in north-west China. The incoming sediment isloess with particles of 0.008 to 0.0375 mm. The duration of theflood peak is less than 2 days, and the detention period is from 1to 6 days.

η = ηw1.5 (5.21)

where ηw is the water release efficiency, ηw = Wo/Wi, Wo is theoutgoing flow during the detention period, and Wi is the incomingflow during the detention period.

5.3.2 Density current venting5.3.2.1 PHENOMENON AND FORMATION OF DENSITY CURRENT

When two fluids with a similar state but slightly different densitiesmove in relation to each other, a density current may form.

When a turbid sediment-laden flow enters a clear waterreservoir, a density current may form if the turbid flow has enoughvelocity and fine particles. The density current moves along thereservoir bed towards the dam. Under favourable conditions the

density current may reach the dam. If the bottom outlet is openedin time, the density current may be vented out of the reservoir.

The turbid open channel flow dives into the bottom atthe plunge point, which is the point of separation between theforward moving current and the induced reverse flow in the reser-voir. This point can be distinguished by the collection of floatingdebris on the reservoir surface. The head of the density current isthicker than the main body, as the head provides the potentialenergy necessary to overcome the inertia of the reservoir waterahead of the current. Resistance also exists at the interface, whichinduces the mixing of the density current and the surroundingwater.

The schematic diagram of density current and measureddata from Sanmenxia Reservoir are shown in Figure 5.11.

5.3.2.2 VENTING OF DENSITY CURRENT

Factors affect ing the venting of densi ty current are theincoming flow and sediment conditions, the topography of thereservoir, and outlet facilities (elevation, location, dischargecapacity, etc).

94 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.11 — Schematic diagram of density current.

Table 5.11Density differences of density current

Reservoir Density difference (kg m–3)

Guanting 20–100

Sanmenxia 10–50

Lake Mead 5–25

Sautet 0.5–1.0

Silting basin 0.5–100

Estuaries 0–2

Salt water intrusion Salt content difference 0–3%

Table 5.12Venting of density currents

Reservoir Dam height Capacity Annual Sediment load % of venting(m) (109 m3) runoff In Out

Iri Emda (Algeria) 75 0.16 0.21 5.21 1.31 251.52 0.65 437.47 3.64 49

Lake Mead (United States) 221 38.4 16.0 7.78 1.79 239.48 2.37 259.35 3.27 39

11.08 2.00 18

Nebeur (Tunisia) 65 0.30 0.18 Annual59–644.9 3.5

Fengjiashan (China) 77 0.40 0.48 0.46 0.11 231.18 0.77 65

Guanting (China) 45 2.27 1.40 7.86 2.70 3413.5 4.0 305.30 1.06 2020.5 4.56 226.34 1.58 251.63 0.29 18

Sanmenxia (China) 106 96.4* 43.2 1.70 0.30 181.47 0.31 21

* Before reconstruction

Twenty-seven sets of field data from Guanting Reservoirin 1956 to 1957 show that the discharge of density current is equalto more than one half of the incoming flow discharge, and thesediment load of the density current is about one quarter of theincoming sediment load; the rest deposits near the plunge pointand only the fine particles form the density current.

In Table 5.11 the density differences of density currentmeasured in some reservoirs are listed. In Table 5.12 field data ofventing of density currents are listed.

Figures 5.12 and 5.13 show the relationship between therelease efficiency of density current and the characteristics of thereservoir.

5.3.3 Erosion in reservoirs5.3.3.1 RETROGRESSIVE AND PROGRESSIVE EROSION

Although reservoirs are environments for sediment deposition,erosion can still take place when conditions are favourable. Inreservoirs, two types of erosion may occur, namely retrogressiveerosion and progressive erosion.

When the pool level drops by a certain amount, erosionmay first take place at the pivot point of the delta and then developupstream. This is retrogressive erosion.

Progressive erosion takes place when the sediment-carrying capacity is greater than the incoming sediment load.Erosion develops and its intensity decreases. This is a commonphenomenon caused by the imbalance of the incoming sedimentload and the sediment-carrying capacity. An example of retrogres-sive erosion is shown in Figure 5.14.

5.3.3.2 EROSION IN THE FLUCTUATING BACKWATER REGION

The fluctuating backwater region has dual characteristics: when itis submerged, it belongs to the reservoir; when the pool level

drops and this region is out of the effect of backwater, it belongsto the river. During the latter situation, erosion takes place in thisregion. Two types of erosion may occur in this region:(a) Erosion during drawdown: in dry seasons the pool level

gradually drops, and progressive erosion takes place on thepreviously deposited sediment bed;

(b) Erosion during reservoir filling: from the beginning of theflood season the river discharge gradually increases; duringthe filling process erosion may occur in the fluctuating back-water region, which may push the terminal of backwaterdeposits downward.

5.3.3.3 EMPIRICAL METHOD OF EROSION PREDICTION

The sediment carried by rivers is classified as suspended load andbed load. In some rivers most of the transported sediment belongsto suspended load, while in other rivers bed load accounts for amajor portion.

(1) Prediction of retrogressive erosion of suspendedload transportation. Figure 5.15 is the schematic diagram forcalculating the retrogressive erosion of suspended load.

During the period of ∆t, retrogressive erosion developsfrom point B to point A in pace with the pool level drawdown fromZo to Z1. The eroded volume ABC may be expressed as follows:

γ∆V = (Qso – Qsi)∆t (5.22)

where γ is the unit weight of the deposit in t m–3, ∆V is the erodedvolume in the period of t in m–3, Qso is the sediment load at theexit cross-section in t s–1, and Qsi is the sediment load at theentrance cross-section in t s–1.

If Qso can be determined, the value of ∆V may be calcu-lated. Some empirical formulae to determine Qso have been

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 95

Figure 5.12 — Release efficiency of density current with original riverbed slope.

Figure 5.13 — Release efficiency of density current with reservoirlength.

Figure 5.15 — Schematic diagram of retrogressive erosion ofsuspended load.

Figure 5.14 — Retrogressive erosion in Sanmenxia Reservoir.

Sanmenxia ReservoirGuanting ReservoirHeisonglin ReservoirLake Mead

Q (

m3

s–1 )

sedi

men

t dis

char

ge (t

s–1

)

derived on the basis of data from reservoirs in China. The mostcommonly used formula is as follows:

(5.23)

where Ψ is the parameter expressing the resistance of the river bedin the unit of s0.6t m–4.2, Q is the discharge in m3 s–1, J is theslope, and B is the channel width in m. It may be determined bythe method of hydraulic geometry.

The value of Ψ is determined by filed data from 10reservoirs and the Yellow, Weihe and Fenhe rivers where theerosion is progressive (Figure 5.16). In the diagram there are threelines: Ψ = 650, representing the river bed composed of newlydeposited fine sediment (D50 < 0.1 mm); Ψ = 300, representingthe medium situation (D50 > 0.1 mm); and Ψ = 180, representingthe river bed composed of cohesive sediment.

The range of parameters of the field data areQ = 0.1–5730 m3 s–1, J = (0.006–1.6)%, B = 10–1 000 m,Qso = 0.0006–777 t s–1.

(2) Prediction of retrogressive erosion of bed load trans-portation. The process and principle of the calculation of theretrogressive erosion of bed load transportation are the same asthose of suspended load transportation. The only difference lies inadopting the sediment transport capacity of the bed load instead ofthat of the suspended load.

Many bed load formulae are available, but they must beverified by the field data from the river where the calculation willbe carried out.

(3) Prediction of progressive erosion. The basic princi-ple for calculating progressive erosion is that the differencebetween the outgoing and incoming sediment loads of a riverreach is equal to the volume scoured from the river bed. This is the

same principle as that used to calculate retrogressive erosion.Consequently, the equation and section diagram may also be usedto predict progressive erosion.

5.4 EMPIRICAL ESTIMATION OF LONG-TERMDEPOSITION IN RESERVOIRS

5.4.1 Method of trap efficiencyThe ratio of the sediment deposited in a reservoir to the totalincoming sediment is called the trap efficiency of the reservoir.Trap efficiency is related to various parameters, such as the ratioof reservoir storage capacity, V, to the average annual runoff, W;the ratio of retention period to the average flow velocity in thereservoir; and the specific storage of the reservoir, i.e. the ratio ofthe reservoir storage to the river basin area above the reservoir.

The most commonly used method was developed byBrune (Brune, 1953). In Figure 5.17, Brune determined the rela-tionship between β and V/W based on large reservoirs in theUnited States. Data from reservoirs in China and the RussianFederation also follow the general trend of the Brune curve.

Other than the influence of V/W on trap efficiency, thesize of sediment particles, the operational mode of the reservoirand the type of outlets also influence trap efficiency, as shown inFigure 5.17. The average value of β may be determined by thefollowing expression:

(5.24)

Churchill presented a method to estimate the trapefficiency of a reservoir, using the sediment index of the reservoir,which is defined as the period of retention divided by meanvelocity (Figure 5.18). The sediment index may be expressed asV2/Q2L (s2 m–1), which is a dimensional index, where Q is theaverage daily mean discharge, and L is the length of the backwaterregion (Churchill, 1947).

The scattering of the points in the Churchill diagram isless than that in the Brune diagram. This may be explained by the

96 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.16 — The value of ΨΨ. Figure 5.18 — Trap efficiency (after Churchill, 1948).

Figure 5.17 — Trap efficiency (after Brune, 1953).

€

β =+

V

WV

W0 012 0 0102. .

QQ J

B50

16 12

0 6= Ψ

.

V/W

V2/(Q2L)θ = Q0.6J12/B0.6

Q10

(t s

–1)

fact that the sediment index includes more parameters than theBrune index. Data from reservoirs in China also confirm the valid-ity of the Churchill curve.

5.4.2 Method of rate of storage capacity lossThe rate of storage capacity loss may be expressed as:

(5.25)

If trap efficiency is determined by the Brune curve or theChurchill curve, then the value of α may be determined.

Where flow and sediment data are insufficient at theplanning stage of some small and medium-sized reservoirs, anempirical expression for determining the value of α is obtainedbased on 25 reservoirs mainly in North and Northwest China.

(5.26)

where G is the annual rate of erosion in the basin above a reservoirin t km–2a, F is the drainage area above the reservoir in m2, V isthe reservoir storage capacity in m3, and α is the rate of storagecapacity loss in %.

5.4.3 Process of depletion of reservoir storage capacity(lifespan of a reservoir)

The rate of siltation in a reservoir decreases with time as thestorage capacity is reduced, until a residual river channel remainsin the reservoir. The difference between the original total storagecapacity and the remaining storage is called the storage of silta-tion. It is important to estimate the process of siltation in order toestimate the benefit of a reservoir.

In the 1930s, the first expression for the estimation ofthe remaining storage capacity was presented as follows:

(5.27)

where Vt is the storage capacity at t years of the reservoir’s opera-tion in m3, Vo is the initial storage capacity in m3, and Ws is theannual sediment load in m3.

At present, there are many empirical expressions forestimating the process of depletion of storage capacity, as listed inTable 5.13.

In Table 5.13, Vi is the storage capacity at t years of the reservoir’soperation in m3, Vo is the initial storage capacity in m3, and Ws isthe annual sediment load in m3; Wo is the final volume of depositsin m3, Wt is the amount (volume) of deposition at t in m3, βo is theinitial trap efficiency, βt is the trap efficiency at t, Q is the annualincoming discharge in m3 s–1, S is the annual incoming sedimentconcentration in kg m–3, Wr is the residual channel volume whenthe equilibrium is established in m3, and γ's is the unit weight ofdeposit in kg m–3.

The expression presented by Tsinghua University isdescribed in detail as follows:

(5.28)

where βt is the trap efficiency at time t, βo is the initial trap effi-ciency, Ws is the amount of deposition at time t, Wo is the storageof siltation, and n is an index expressing the decreasing rate of trapefficiency.

After operation:

(5.29)

where n ≠ 1.Assuming Wt = ζWo, where ζ is the extent of siltation in

the reservoir, the following expression can be deduced:

(5.30)

where (5.31)

In practice, the value of n ranges between 0 and 1.The value of n in some reservoirs in China is listed in

Table 5.14.The less the sediment sluiced from the reservoir, the

smaller the value of n.

Because of the difficulty of determining the value of n,the original expression may be used to determine n. On a log-logpaper, the relationship between βt and (1 – Ws/Wo) is plotted. Theslope of the line represents the value of n.

5.5 NUMERICAL MODELLING OF RESERVOIRSEDIMENTATION

5.5.1 GeneralBased on the laws governing water flow and sediment transport,numerical models of reservoir sedimentation can be established

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 97

α =

−

0 0002 0 950 8

. ..

GV

F

α β βφ

= = = =∆ ∆W

V

W

W

W

V

W

Vs s

s

s s

V VW

Vt os

o= −

1

t

Table 5.13Expressions for estimating reservoir life span

Author Year Expression

Orlt 1930 Vt = Vo (1 – Ws/Vo)t

Shamov 1950 Vt = Wo (1 – βWs/Wo)t

Gangchalov 1960 Vt/Vo = 1 – (1 – W1/Wo)t

Shineer 1965

Tsinghua 1979University

tQS

W WV

V Ws

t ro

o t= +

γ'( ln )

(1) 1

(2) 1

n W Wn W t

W

n W WW

W

t os

o

n

t oo s

o

t

= = − −−

= = − −

−[ (( )

) ]

[ ( ) ]

1 11

1 1

1

1β

β

β βt os

o

nW

W= −

1

Tn

W

W

n

V

Vo s

o

o

o

=− −( )

−( )

=

−1 1

1

1ξα

α β

W Wn W t

Wt oo s

o

n= −

−( )

−1 1

11

1β _

Table 5.14Value of n

Reservoir n Note

Bajiazui 0.95 Sluicing sediment

Yanfuoxia 0.90 in between

Fenhe 0.75 in between

Gufengshan 0.75 in between

Hongshan 0.65 in between

Cetian 0.45 in between

Jioucheng 0 Storing sediment

and used to predict the future situation of reservoir sedimentation.The processes for establishing the numerical model include threesteps of approximate schematization and four steps of feedback.The first step of approximate schematization is to describe theengineering problem by physical processes; the second step is todescribe the physical processes by mathematical equations, andthe third one is to obtain the numerical solution of the mathemati-cal equations. Each feedback step is the process of verifying eachstep of approximate schematization.

Sediment transport and its induced channel deformationare the result of water flow motion. Simultaneously, the deformedchannel morphology has its effect on flow motion. Therefore, asediment numerical model includes two submodels of flow motionand sediment transport. These two submodels should be solvedsimultaneously, and their solutions are called coupled solutions.When channel deformation is not so intensive, to simplify thecomputation process, the two submodels can be solved step bystep, the first being that of flow motion and the second being thatof sediment transport. Such a solution is called an uncoupled solu-tion and is common practice nowadays.

The development of numerical models is seeing a movefrom one-dimensional to three-dimensional models. The naturalsituation is always a three-dimensional one. At present, three-dimensional numerical sediment models are still only on thehorizon, as the commonly used numerical models are either one-or two-dimensional. The selection of a suitable numerical modeldepends on the characteristics of the problem. If a one-dimensional model can simulate the problem, it is unnecessary touse a two-dimensional model, since the computer time of the latteris much longer than the former. In some special cases, a combinedmodel may be used. In some river reaches a one-dimensionalmodel is used, and in the remaining river reaches a two-dimen-sional model is used to meet engineering requirements.

At present, no analytical solution can be obtained forany sediment numerical model. Numerical approaches must beused to find the solution. There are a number of numericalapproaches, including the finite difference approach, which is themost common.

The numerical model must be calibrated and verified byseparate sets of field data. The accuracy of the result of verifica-tion must meet engineering requirements.

5.5.2 Basic equations (for unit width)5.5.2.1 CONTINUITY EQUATION

(1) Continuity equation of water:

(5.32)

(2) Continuity equation of sediment:

(5.33)

(3) Continuity equation of sediment-laden flow:

(5.34)

Equation 5.34 is a combination of Equations 5.32 and 5.33.Among these three equations, only two of them are independent.When z = 0, i.e. a fixed bed, then Equation 5.34 becomes thecontinuity equation of unsteady flow.

5.5.2.2 MOMENTUM EQUATION OF ONE-DIMENSIONAL

SEDIMENT-LADEN FLOW

(1) Forces include:(i) Pressure of sediment-water mixture

Where the specific weight of sediment-water mixture, γo is asfollows.

γo = γ + S (γs – γ) = γsS + (1 + S) γ (5.35)

where γo and γ are the specific weight of sediment and water,respectively.

(ii) Component of self-weight in x-direction(iii) Bed resistance

(5.36)

where τo is the resistance per unit area.(2) Change in momentum in unit time includes two parts:momentum change with time and difference of momentum goinginto and out of the unit section dx. The momentum equation ofone-dimensional sediment-laden flow is as follows:

(5.37)

where (5.38)

Compared with the momentum equation of clear flow,items 5 and 6 in Equation 5.37 are added. Item 5 is the water pres-sure induced by the longitudinal variation of sedimentconcentration, while item 6 is the change in momentum inducedby sediment deposition.

5.5.2.3 SUPPLEMENTARY EQUATION

There are three independent equations for computing river bedchanges, but there are four unknowns, v, h, s, and z. Therefore, onemore equation is needed. There are two methods to supplementone equation.

(1) Saturated sediment transport. Assuming thatsediment concentration is always equal to the sediment transportcapacity of flow, a formula of sediment transport capacity canbe adopted as the supplementary equation. This assumption isnearly true when the longitudinal variation of sediment transportcapacity is small, and the incoming sediment concentration ofthe river reach is not too different from the sediment transportcapacity.

A finite difference method is commonly used to solvethe equations. In most cases, unsteady flow is simplified as steadyflow.

(2) Non-saturated sediment transport. When the sedi-ment concentration is high and the longitudinal variation of thehydraulic parameters is significant, the assumption of saturatedsediment transport cannot be upheld. In such circumstances, non-saturated sediment transport must be considered in numericalmodels. By integrating the diffusion equation in two-dimensionalsteady flow along a vertical, one can obtain the basic equation oflongitudinal variation of mean sediment concentration in one-dimensional flow, as follows:

98 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

∂∂

− +∂∂

− + −∂∂

=x

vh St

h S pZ

t[ ( )] [ ( )] ( )1 1 1 0

∂∂

+∂∂

+∂∂

=x

vhSt

hS pZ

t( ) ( ) 0

∂∂

+∂∂

+∂∂

=x

vhh

t

Z

t( ) 0

Jhf

o

o=

τγ

1

2

1

g

v

t

v

g

v

x

h

x

Z

x

h S

x

v

h

p p

g

Z

tJ J

s

o

s

oo f

∂∂

+∂∂

+∂∂

+∂∂

+− ∂

∂−

+ − ∂∂

= −

γ γγ

γ γγ

[( )

]

JZ

xoo= −

∂∂

(5.39)

where qs is the unit sediment discharge, q is the unit discharge, Sg

is the bottom sediment concentration, and Sm is the mean sedimentconcentration of a cross-section.

After a series of operations, one finally obtains the basicequation of longitudinal variation of the mean sediment concentra-tion under a steady state:

(5.40)

where S* is the sediment transport capacity at the exit cross-section, So is the sediment concentration at the entrance, S*o is thesediment transport capacity at the entrance, α is a coefficient ofrecovery of sediment concentration, L is the length of the riverreach, and l is the horizontal distance for a particle settled within awater depth, ho.

From Equation 5.40, one can find that the sedimentconcentration at the exit cross-section is composed of three parts:item 1 — sediment concentration in saturation at the exit cross-section; item 2 — the attenuated value of residual sedimentconcentration at the entrance cross-section, (so – s*o), afterdistance L/l; item 3 — modified sediment concentration in satura-tion in the river reach.

In Table 5.15 field data from a desilting channel areused to check the necessity of the method of non-saturatedsediment transport. From the Table one can conclude that thevalues of items 2 and 3 in Equation 5.40 are too large to neglect,i.e. the consideration of non-saturated sediment transport is anecessity.

For non-uniform sediment, the total sediment is dividedinto n groups. For each group:

Si = Pi S (i = 1, 2, 3,…., n) (5.41)

S*i = Pi S* (i = 1, 2, 3,…., n) (5.42)

where Pi is the percentage of the ith group to the total (byweight). For each group, the basic equation of longitudinalvariation of mean sediment concentration is valid. Thecomputation procedure is quite lengthy. Those readers interestedin this subject are advised to read relevant literatures (e.g. Han,1990).

5.6 RESERVOIR SEDIMENTATION MANAGEMENT5.6.1 Universality of reservoir sedimentationIn Figure 5.19, reservoirs are classified according to φ and ψ, withsediment concentration as the third parameter. Here, φ and ψdenote the ratios of reservoir storage capacity to annual sedimentload and water runoff, respectively. In Figure 5.19, the points canbe classified into three groups. All the points fall close to one ofthe three lines representing different types of rivers. The firstgroup represents the reservoirs built on clear rivers with sedimentconcentrations lower than 1 kg m–3. The second group representsthe reservoirs built on rivers with medium concentrations from 1to 10 kg m–3. The third group represents the reservoirs built onheavily sediment-laden rivers with concentrations higher than10 kg m–3. For the first group, reservoir sedimentation is not aproblem, while for the third group it is very serious.

The features of deposition and experience of reservoirsedimentation management are more valuable a reference for

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 99

Table 5.15Verification of Equation 5.40 by field data (in kg m–3)

S S S S e S SL

eo o

L

lo

L

l= + − + − −−

* * * *( ) ( ) ( )α α

α1

11

Seconds Item Concentration Error

(1) (2) (3) Calculated Measured (%)

2 26.2 5.42 –8.22 23.4 30.8 –24.0

3 23.4 –1.64 2.24 24.0 27.8 –13.7

4 13.3 0.33 7.57 21.2 23.5 –9.8

5 9.77 5.64 3.09 18.5 19.2 –3.6

6 6.97 8.73 2.50 18.2 17.6 +3.4

7 7.46 9.56 –0.32 16.7 16.7 0

8 9.33 9.24 –1.77 16.8 14.8 +13.5

9 24.6 7.06 –14.8 16.9 14.3 +18.2

Figure 5.19 — Relationship between ΨΨ and φφof reservoirs.

dq

dxq

dS

dx

S

ySs m

y h

= =∂∂

−=

ε ω g

φ

Ψ

River of low sediment conc. (< 1 kg m–3)

River of medium sediment conc. (1-10 kg m–3)River of high sediment conc. (> 10 kg m–3)

°+

reservoirs in the same group, although the general law of reservoirsedimentation is the same.

5.6.2 Indicators of reservoir sedimentation problemsIn China, specifications for sediment design of hydropower andwater conservancy projects have been issued. In these specifica-tions, the states of distress caused by the sediment problems ofhydraulic projects are classified into two grades according to thedegree of seriousness with which sediment affects the safety andbenefits of the project, namely serious and non-serious effects.

When one of the following situations occurs, the state ofdistress is considered to be serious.(1) φ is less than 50 to 100.(2) Upstream extension of backwater deposits is so serious that

the safety of cities and industrial regions, etc. and the normaloperations of existing large or medium-sized hydraulicprojects are affected.

(3) A mouth bar may occur at the confluence of a tributary withthe main river, which may affect the functions of the reservoir.

(4) Deposition in the dam area may affect normal operation ofthe inlet or outlet structures.

(5) Sedimentation may impede navigation on the river.These specifications are mainly stipulated based on

practices in China during the past four decades.

5.6.3 Basic operating rulesOperating rules of reservoirs have a decisive influence on reser-voir sedimentation. Three basic types of operating rules have beenadopted in China, namely impoundment, impounding the clearand discharging the turbid water (I and D), and flood detention.The first two types are often adopted. In Table 5.16 some basiccharacteristics of reservoir operating rules are listed.

In Figure 5.19 various operating rules are also shown.The long-term capacity of a reservoir is the remaining

storage capacity when the equilibrium state in the reservoir isreached. The storage of a reservoir consists of two parts, namelythat over the flood plains and that of the main channel(Figure 5.20). The storage capacity over the flood plains will begradually lost by deposition of sediment carried by overflows offlood peaks and cannot be recovered. The loss of the storagecapacity over the flood plains is almost perpetual. A part of thestorage capacity in the main channel may be preserved throughrational use of the reservoir by lowering the pool level in the floodseason and storing during the rest of the year. This part of thestorage capacity is called the long-term capacity of the reservoir.

Under such an operational scheme, the pool level is keptlow to sluice the incoming sediment load during the flood seasons.For small or medium-sized reservoirs, drawdown flushing is oftennecessary to maintain the long-term capacity. During the rest ofthe year, the water carries much less sediment than during theflood seasons; storing water will not induce much deposition inthe reservoir.

One of the prerequisites for maintaining the long-termcapacity of a reservoir is to install sluicing outlets with suffi-cient capacity and proper bottom elevation in the reservoir. Withsuch facilities, sediment may be easily sluiced downstream anda useful storage capacity will be maintained on a permanentbasis.

5.6.4 Sediment design of hydrological projectsAt the feasibility study stage of large and medium-sized hydrolog-ical projects, sediment design should be carried out. Muchattention should be paid to basic data collection, the reservoir sedi-ment regulation mode should be carefully studied, and calculationapproaches of reservoir sedimentation should be properly selectedbased on the characteristics of river sediment and the project.States of distress caused by the sediment problems of hydrologicalprojects are classified into two categories according to the degree

100 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

No. Operating schemes Regulation of sediment Method of sediment sluicing Period of sediment sluicing

A1 Impoundment sediment None None or dredging Nonetotally trapped

A2 Impoundment sediment None Density current venting Beginning of flood seasonspartly trapped sluicing

B Impounding the clear and Yearly or seasonally Sluicing sediment during Flood seasonsdischarging the muddy water detention, density current

C Detention Sluicing Sediment during detention, Flood seasonsreservoir emptying

No. — Effect of sediment sluicing on downstream channels: A1-None; A2-No serious problems; B-Non-matching of flow and sediment may cause problems; C-same as B.

Table 5.16Reservoir operating rules

Figure 5.20 — Sketch of terminal capacity of reservoirs.

(a) Longitudinal profile

(b) Cross-section

of seriousness with which sediment problems affect the safety andbenefits of the projects, namely serious and non-serious effects.For projects in the serious category, sediment problems should bestudied specifically. If necessary, physical modelling should becarried out.

5.6.4.1 COLLECTION AND EVALUATION OF BASIC DATA

The basic conditions of a basin in which a hydrological projectwill be built need to be understood comprehensively. These condi-tions include physiographic and socio-economic conditions,climate, hydrologic and river characteristics, soil erosion, humanactivities and soil conservation, etc. The basic data include: topog-raphy charts, longitudinal profiles and cross-sections; watersurface profiles, bed materials, fluvial processes; and landslides,bank failures and debris flows. These data relate to the reservoirarea and the river reach below the hydrological project. Details onthe location and elevation of cities, towns, industrial areas, mines,and hydrological projects on the river reach affected by the designproject should be collected. Elements such as daily and monthlyflow discharges, suspended load discharges, bed load discharges,sediment particle composition, mineral composition and watertemperature are the basic hydrologic data required for designpurposes. Hydrologic data from other relevant hydrologic stationsin the same basin are also needed. The characteristics of otherprojects on the river, such as operational modes, should beanalysed to optimize the design for the project.

All collected data should be analysed and their rational-ity and reliability should be evaluated. For projects with serioussediment problems, measured sediment data are a must.

5.6.4.2 SEDIMENT INPUT

The distribution and characteristics of sediment source areas in theupstream basin of the design project need to be studied in detail.For large projects with serious sediment problems, the reconnais-sance of key sediment source areas should be carried out. Theeffect of existing upstream projects on the sediment input of thedesign project should be analysed.

For suspended load, the direct use of 20 years of consec-utive hydrologic data from a hydrologic station with a differenceof watersheds between the station and the project of less than threeper cent is necessary. When the difference of watersheds is largerthan three per cent and less than 20 per cent, the difference shouldbe calculated. The yearly and monthly variations of suspendedloads are the main items to be analysed.

For bed load, based on field measurements or empiricalmethods (formulae), the relationships between flow discharge andbed load discharge and between flow discharge and bed loaddiameter are analysed.

5.6.4.3 SEDIMENT DESIGN

For better reservoir management and to maximize resources, inaddition to hydraulic design, sediment design of a project shouldbe carried out, particularly for projects on sediment-laden rivers.With a rational sediment design, the project will maintain usefulstorage capacity for long-term usage.(1) Requirements

(i) For projects with serious sediment problems, the sedi-ment regulation modes of operation, reservoiroperations and sediment release facilities should bestudied comprehensively.

(ii) Predictions should be made relating to reservoir sedi-mentation (amount, location, elevation, spatialdistribution, and depletion process) and also releasedsediment discharge, concentration, and diameter.

(iii) For reservoirs with arms, the appearance of the rivermouth bar at the confluence should be studied.

(iv) For projects with a lengthy construction phase, sedi-ment problems during the construction stage,including the effects on diversion and project layout,etc., should be studied.

(v) The effect of upstream projects on the design projectand the effect of the design project on upstream anddownstream projects should be analysed.

(vi) For navigable rivers, the effect of scour and depositionin the fluctuating backwater region and the effect onnavigation of fluvial processes downstream of theproject should be studied.

(2) Sediment regulation modes of operation(i) A comparison should be made of various alternatives

according to river sediment characteristics, reservoircharacteristics (shape, objectives and regulationrequirements, etc.), and environmental requirements.

(ii) For storage reservoirs, the pool level should be kept ata certain level to sluice sediment during the whole, orpart, of the flood season; if the pool level is notcontrolled, sediment can be sluiced by venting thedensity current or reservoir emptying.

(iii) For low-head diversion projects, sediment regulationshould be carried out at several (a maximum of three)discharges, or all of the sluices can be opened to flushthe sediment.

(3) Calculation of reservoir sedimentation(i) Calculations can be carried out using numerical

models or empirical methods.(ii) Calibration and verification of numerical models or

verification of empirical methods must be carriedout.

(iii) The rationality of the calculated results should bechecked. For projects with serious sediment problems,several calculation methods may be adopted forcomparison.

(iv) As for the data series for calculation, long-term series,representative series, wet-normal-dry years, or a repre-sentative year may be adopted. The average annualsediment load and concentration of the adopted seriesshould approximate the long-term values.

(v) As regards the term of calculation, when the term ofquasi-equilibrium of deposition is longer than the termof the lifespan of the key structures of a project, thelatter should be adopted as the term of calculation.When the former is shorter than the latter, the formershould be adopted as the term of calculation.

(vi) When the trap efficiency is less than 10 per cent, it isconsidered that the quasi-equilibrium of depositionhas been reached.

5.6.4.4 PREVENTION OF SEDIMENT PROBLEMS

(1) The dam site, power house and tail channel, etc.should not be near a sediment-laden tributary (including abundantbed load) or an active debris flow valley, etc.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 101

(2) Where sediment deposition affects the normal opera-tions of a project, sediment prevention measures should beconsidered seriously and sediment release facilities should beconstructed in the project.

(3) For projects on navigable rivers, the approach chan-nels of a ship lock should be studied and the correspondingmeasures to mitigate sediment deposition should be adopted.

5.6.4.5 PREDICTION OF THE FLUVIAL PROCESSES BELOW A

PROJECT

This is mainly recommended for projects which significantlychange the natural flow and sediment regimes, as such fluvialprocesses may have serious implications below dams.

5.6.4.6 PLANNING FOR SEDIMENT MEASUREMENT

For large and medium-sized projects with serious sediment prob-lems, sediment measurement should be carried out from the verybeginning, or ideally before the impoundment of a reservoir.

5.6.5 Methods of reducing sediment input in reservoirsA range of measures can be adopted to reduce sediment supply inreservoirs.

5.6.5.1 SOIL CONSERVATION PRACTICE

The effectiveness of soil conservation in reducing sediment inputin a reservoir depends on the size of the watershed where thereservoir is built. For a large watershed with poor natural condi-tions, soil conservation can hardly be effective over a short periodof time. Nevertheless, if the watershed is not very large, the effectof soil conservation can be seen in a short time.

A good example of this is the Middle Yellow River basin.The hydrologic data of the Yellow River show an obvious

reduction in surface runoff and sediment load in the 1970s, and even

a remarkable reduction in sediment load in the 1980s, as listed inTable 5.17. Besides the climatic variations, human activity hasplayed an important role in such a reduction. The effect of humanactivity may be classified into two categories: water resourcesdevelopment and soil conservation.

The areas of soil conservation work above Sanmenxiaare listed in Table 5.18. The rapid development of soil conserva-tion work is obvious and shows a close association with thereduction in sediment loads.

Another example is Guanting Reservoir on the YongdingRiver, which controls a catchment of 43 000 km2. The meanannual river flow at the dam site is 1.4 billion m3 and the annualsediment load is 81 million tons. The reservoir storage is 2.27billion m3. The project was commissioned in 1955. Reservoir sedi-mentation is very serious, but it has been quite different atdifferent periods (see Table 5.19).

Although the average annual precipitation and precipita-tion in flood seasons in the 1950s, 1960s, and 1970s were almostthe same, the incoming runoff and sediment load in GuantingReservoir have declined significantly since 1960 under the influ-ence of human activities, as listed in Table 5.20.

The measures for reducing sediment load and theirrespective effects are listed in Table 5.21.

Another approach is to bypass the input of sediment.This method is mainly used for small or medium-sized

reservoirs where the topography is suitable for bypassing theincoming sediment. An example is shown in Figure 5.21.Unfortunately, this is not always successful as sediment can blockbypass channels and the topography may be unfavourable for sucha method.

From the example of Guanting Reservoir, it is obviousthat warping (colmatage) has a good impact in dealing withreservoir sedimentation. At the same time, it increases the fertility ofthe irrigated land.

The joint operation of reservoirs has also proven to be ofvalue at Guanting Reservoir (see section 5.8.5).

102 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 5.17Annual runoff and sediment load at Sanmenxia

Table 5.18Areas of soil conservation works above Sanmenxia (million ha)

Item 1940s 1950s 1960s 1970s 1980s

Annual runoff 47.8 52.6 59.3 51.3 55.8(billion m3)

Annual sediment 1.73 1.98 1.98 1.82 1.51load (billion t)

Year Terraces Reclaimed Afforestation Grassland Totalfarmland

1969 0.574 0.036 0.759 0.209 1.58

1979 1.84 0.086 1.35 0.317 3.60

1989 2.597 0.17 3.92 1.23 7.92

Table 5.19Sedimentation rate in Guanting Reservoir

Period Amount of Annual amount ofdeposition (million m3) deposition (million m3)

1956–1960 70

1961–1970 82 8.2

1971–1980 73 7.3

Table 5.20Runoff and sediment load in Guanting Reservoir

Period Precipitation (mm)Annual Annual sedimentrunoff load

Annual Flood (million m3) (million t)season

1951– 444 338 1 723 59.6919601961– 412 313 1 258 15.0819701971– 427 373 832 10.231980

Table 5.21Reduction of sediment load in Guanting Reservoir

Cause of reduction Annual reduction of sediment load(million t)

Upstream reservoirs 17

Irrigation and warping 19

Soil conservation 5

Total 41

5.6.6 Overview of remedial measures5.6.6.1 DRAWDOWN FLUSHING

When low-level outlets are opened, the pool level in a reservoirdrops. Consequently, the flow in the reservoir will be favourablefor scouring the previous deposits. Thus, the storage capacity ofthe reservoir can be enlarged. This method is mainly used forsmall and medium-sized reservoirs.

There are many examples of this method. ShuicaoziReservoir in China is one such example. Drawdown flushing wascarried out eight times during the period from 1964 to 1981. Thequantity of sediment flushed out each time was some 200 000 m3,corresponding to about one third of the annual incoming sedimentload (see section 5.8.4). Although this is beneficial in preventing aloss in reservoir capacity, there can be short-term impacts down-stream. Timing and the associated water discharges are importantconsiderations.

5.6.6.2 RESERVOIR EMPTYING

Reservoir emptying is the limit of drawdown flushing. It is veryefficient in eroding sediment out of a reservoir, but it can only beused in small reservoirs. Water consumption is the problem withthis measure.

Hengshan Reservoir in China is an example of reservoiremptying. In Hengshan Reservoir (V = 13.3 million m3), theemptying operation was not carried out annually. After the firsteight years of reservoir operation from 1966 to 1973, 3.2 millionm3 of deposits accumulated in the reservoir. The reservoir empty-ing operation took 37 days in 1974, and a storage capacity of0.8 million m3 was recovered. Reservoir emptying operationswere executed from 8 to 21 August 1976, 9 August to30 September 1979, and 28 May to 16 June 1982, and a storagecapacity of some 1 million m3 was recovered each time.

5.6.6.3 LATERAL EROSION

This technique is mainly used for recovering storage capacity onflood plains. The objective is to break flood plain deposits andflush them out by the combined actions of scouring and gravita-tional erosion caused by the large lateral gradient of the floodplains. In so doing, it is necessary to build a low dam at theupstream end of the reservoir for diverting water into diversioncanals along the perimeter of the reservoir, and the flow iscollected in trenches on the flood plains.

Guanshan Reservoir in China is an example of this tech-nique. A 2-metre high diversion dam was built at the upstream endof the reservoir. The diversion canal is 1 300 m long with a gradientof 0.001. The scouring discharge was 0.5 m3 s–1. Within twomonths, 0.4 million m3 of deposits were flushed out of the reservoir.

5.6.6.4 SIPHON DREDGING

Siphon dredging makes use of the water head difference betweenthe upstream and downstream levels of a dam as a power sourcefor the suction of deposits from the reservoir to the downstreamarea. It is an old method adopted for small reservoirs in somecountries. Since 1975 this method has been applied in some smallreservoirs in semi-arid areas in China, and the flushed mixture ofwater and sediment is diverted into farmland for warping and irri-gation. The diameters of the pipes used in China range from 0.3 to0.6 m, the discharges range from 0.2 to 1.2 m3 s–1, and themaximum sediment concentration of the flushed mixture rangesfrom 500 to 1 200 kg m–3.

5.6.6.5 DREDGING

Dredging is a measure to remove deposits in small and medium-sized reservoirs. The advantages of this method are: it is highlyefficient (less water consumption), the normal operations of theproject are maintained, it can be executed at any place, the evacu-ated fine material can be used on farmland, coarse material can beused for construction, and there is no limit for the recovery ofstorage capacity. The disadvantages are high costs (US$ 2–4 m–3

worldwide), difficulties in disposing of dredged deposits, andenvironmental problems.

5.6.6.6 DESIGN OF SEDIMENT SLUICING FACILITIES OF

RESERVOIRS

The location, elevation, size, and type of facilities are the designelements. Some empirical formulae have been derived for deter-mining the outlet capacity for flushing sediment and maintainingthe long-term capacity of a reservoir.(1) Shaanxi Institute of Hydrotechnical Research

Based on the sediment transport capacity in some reser-voirs, the adequate discharge capacity of a sluicing outlet, Qe, maybe determined by the following expression,

(5.43)

where Ws is the annual sediment load in tons, T is the duration ofsediment sluicing period in sec, J is the slope, determined as oneof the situations in Figure 5.22, and K is the coefficient (K = 3 inmost cases).(2) Tsinghua University

Based on data from existing reservoirs, TsinghuaUniversity proposed:

Qe = (30–50) Qfm0.6 (5.44)

where Qfm is the average discharge in the flood season.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 103

€

QW

KTJe

s= ( ).

/ .1 2

1 1 6

Figure 5.22 — Schematic diagram for determining the capacity ofoutlets.

Flood weir

Flood gatesMgeni River

To Pietermaritzburg

0 1 2 3 4 5 kg|__________________________

Scale

Figure 5.21 — Plan view of Mgeni Reservoir.

(a) Emptying (b) Controlled operation

5.7 FLUVIAL PROCESSES BELOW RESERVOIRS5.7.1 Fluvial processes below impounding reservoirs5.7.1.1 CHANGES IN FLOW REGIME

Dam construction leads to a change in the flow regime below thedam. As regards water flow, the main changes are the reduction inpeak discharges, an increase in the duration of medium flows, anincrease in low discharges, and a decrease in the seasonal andannual variation of discharges. As regards sediment transport, themain changes are the reduction of released sediment amount andthe loss of coarse sediment particles. However, the variations maydiffer significantly due to the difference between the reservoirstorage capacity and operational mode. Williams and Wolmananalysed 21 reservoirs in the prairie and semiarid western regionof the United States. The mean annual discharge of the riversranges between 1.5 and 930 m3 s–1. In Table 5.22, the changesafter the commissioning of dams are listed. The greatest change isthe reduction of peak discharge. The post-dam peak discharge isabout 45 per cent of the pre-dam value (Williams and Wolman,1984).

In Sanmenxia Reservoir, the incoming peak discharge of12 400 m3 s–1 in 1964 was reduced to 4 870 m3 s–1, or only about40 per cent of the original. The duration of medium flow,4 000 m3 s–1, increased from 59 to 73 days. Meanwhile, theseasonal and annual variation of discharge decreased.

5.7.1.2 DRASTIC REDUCTION IN SEDIMENT LOAD AND

CONCENTRATION

When most of the sediment is trapped in a reservoir, the releasedwater will be clear. Consequently, the sediment load and

concentration in the river reach below the reservoir will be muchlower than the pre-dam values. Table 5.23 lists the changes insediment concentration in several reservoirs. Below GuantingReservoir the sediment concentration of the river amounted to onlyone-tenth of the pre-dam value.

5.7.1.3 EROSION BELOW DAMS

Erosion takes place below the reservoir where the releasedwater is clear. The distance of erosion may be quite long anddepends on the released flow discharge. Erosion develops grad-ually downstream. Table 5.24 shows the development of erosionbelow Danjiangkou Reservoir.

In the Lower Yellow River erosion took place for800 km along the reach, and below the Aswan High Dam on theNile the length of eroded reaches extends for about 1 000 km.

The erosion thickness depends on many factors of theriver channel, and varies in different rivers. For example, in theYellow River below Sanmenxia Reservoir, the thickness oferosion after 4 years of clear water erosion was 1 m (the meandiameter of bed material ranged from 0.06 to 0.1 mm). On theNile, where the mean diameter of the bed material was0.15 mm, erosion thickness was 0.1 m after 3 years of erosion.

5.7.1.4 ARMOURING OF BED SEDIMENT

The selective process of water flow is the principal cause ofarmouring of bed sediment. In addition, the imbalancedexchange between suspended load, bed load, and bed material isalso responsible for the armouring of bed sediment.

104 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 5.22Ratios of post- and pre-dam discharge

Mean annual discharge Peak discharge 5% flood discharge 95% discharge

Range Mean Range Mean Range Mean Range Mean

0.46–1.48 0.91 0.15–0.91 0.45 0.31–2.1 0.88 0–2.67 1.01

Table 5.23Ratio of post-dam to pre-dam sediment concentration

(1) Guanting DamYear Jinmenzha1 Shifosi2

1956 0.21 0.241957 0.10 0.121958 0.83 0.101959 0.53 0.78

(2) Sanmenxia DamDischarge (m3 s–1) Huayuankou3 Gaocun4

1 000–2 000 0.36 0.423 000 0.18 0.24

1, 2, 3, and 4 are 160 km, 190 km, 280 km and 485 km below the dams, respectively.

Table 5.24Length of eroded reaches below Danjiangkou Reservoir

Year Length (km) Remarks

1960 Beginning of flood detention

1968 223 Commissioning of the dam

1972 465Figure 5.23 — Three types of bed armouring.

Fine

r th

an D

50by

wei

ght (

%)

Diameter (mm)

(a)

(c)

(b)

There are three types of armouring of bed sediment asshown in Figure 5.23: (1) Gravel bed covered with sand bed, suchas the Colorado River below the Hoover Dam (Figure 5.23a); (2)Gravel and sand bed, such as the Yongding River (Figure 5.23b);(3) Fine sand bed, such as the Colorado River below the ImperialDam (Figure 5.23c).

The armouring of bed material has three effects: (1)There is an increase in channel roughness, as at Yuma Station onthe Colorado River below the dams. When the diameter of the bedmaterial increased from 0.15 to 0.3 mm, the value of the Manningroughness coefficient, n, increased from 0.013 to 0.032; (2) Thereis a decrease in sediment-transport capacity; (3) There is a restric-tion on further degradation of the channel bed.

5.7.1.5 ADJUSTMENT OF LONGITUDINAL PROFILE

There are two different scenarios for the adjustment of the longi-tudinal profile after the release of clear water from impoundingreservoirs. If armouring of bed sediment is dominant, the sloperemains almost unchanged, such as the Colorado River below theParker Dam, or the Yongding River, or even becomes steeper,such as the Colorado River below the Hoover Dam (1.6–42.3 km)(see Figure 5.24). If armouring of bed material is not prominent,the slope may remain unchanged or become flatter; the slope ofwater surface at medium and low discharges of the HanjiangRiver below Danjiangkou Reservoir was 0.000 286 before damclosure in 1960, and in 1978 it was 0.000 268, almost the same asbefore.

5.7.1.6 ADJUSTMENT OF CROSS-SECTIONAL SHAPE

Erosion in river channels may manifest itself in two ways, namelydegradation of the channel bed and channel widening. In variousrivers the development of erosion is different, depending on thelocal conditions such as the basic characteristics of the river oroperational mode of the reservoir, etc.

The Yongding River below Guanting Reservoir is anexample of an increase in channel width. Table 5.25 shows thechange in channel width of the Yongding River, demonstrating thedrastic increase in channel width after the commissioning ofGuanting Reservoir.

Many rivers in the United Kingdom are examples ofanother pattern of changes in channel width. Petts (1979)analysed the variations of water depth and channel width of 14rivers in the United Kingdom. The water depth and channel widthof most of these rivers remained unchanged near the dams;further downstream in the meandering reach below the dams, thewater depth and channel width of two thirds of the riversremained unchanged, while the channel width of the rest of therivers decreased, and the water depth of two rivers decreased. Ingeneral, the cross-section pattern of these rivers remained almostunchanged, or became narrower and deeper. British rivers are notlong, their sediment particles are coarse, and river banks arecomposed of either coarse particles or silty clay with good vege-tation cover. After the commissioning of reservoirs, the reducedflood discharges may be incapable of eroding river banks, leadingto the above-mentioned variations of the cross-sections. The riverchannel cross-section below Danjiangkou Reservoir becamedeeper and narrower in a similar manner to that of the Britishrivers.

The variation of the channel cross-section below theSanmenxia Reservoir on the Yellow River during the period ofclear water release (1960–1964) was more complicated. Since thebed material of the Yellow River is fine and the medium and lowdischarges are comparatively large, channel degradation wasobvious in a 180 km reach; meanwhile, the collapse of floodplains and increases in channel width were also significant insome wide cross-sections. Since 1964, when the operational modechanged from impoundment to flood detention and sedimentrelease, sediment deposition has been taking place in the channel,leading to a serious collapse of flood plain banks and a wideningof the channel width.

For wandering rivers during the pre-dam period, a long-term balance between the loss and gain in flood plains prevails,and the channel width thus remains almost constant. During thepost-dam period, however, such a balance is upset by the changedflow and sediment regime. The collapse of flood plains increasesthe channel width and makes the cross-section wider.

The loss of flood plains takes place mainly at the begin-ning of the new stage.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 105

Figure 5.24 — Longitudinal profiles below dams.

Lugouqiao

Jinmenzha

Shifosi

Table 5.25Change in channel width of the Yongding River

Reach Length (km)Channel width (m)

Pre-dam Post-dam

1950 1956 1957 1958

Lugouqiao 30 790 1060 1210 1214

Jinmenzha 30 420 600 650 655

5.7.1.7 ADJUSTMENT OF CHANNEL PATTERN

Adjustment of the channel pattern below reservoirs is a long-termphenomenon. Until now, no field data have shown obviousevidence in this respect. However, laboratory tests show that thegeneral tendency of such an adjustment is a decrease in wanderingintensity and a gradual shift from wandering rivers to meanderingones.

In the Hanjiang River below Danjiangkou Reservoir, thewandering intensity in the braided-wandering reach has declined,with some mid-bars combining and others connecting to the bank.The river channel has become more regular than before. There hadbeen 26 river branches in a 240 km long reach just below the dam;at present 15 branches have disappeared. The ratio of sinuosity hasincreased from 1.25 to 1.50.

5.7.2 Fluvial processes below detention reservoirs5.7.2.1 CHANGES IN FLOW AND SEDIMENT REGIMES

Non-match of the flow regime with the sediment regime is theprominent phenomenon of the change in flow and sedimentregimes below detention reservoirs (see Figure 5.25). During the

rising leg of the flow discharge curve, the flood discharge isreduced and the released sediment load is significantly reduced;while during the falling leg of the discharge duration curve, a largeamount of sediment is released from the reservoir due to intensiveretrogressive erosion in the reservoir. Thus, the sediment peak lagssignificantly behind the flood peak.

5.7.2.2 AGGRAVATION OF DEPOSITION BELOW DAMS

The annual amount of deposition of the Lower Yellow Riverduring the detention period of Sanmenxia Reservoir was 438million tons, compared with 368 million tons under its naturalstate. The elevation difference between the channel bed and theflood plains dropped, as shown in Table 5.26, and therefore thebankfull discharges of the main channel were also reduced, asshown in Table 5.27.

5.8 CASE STUDIESSix projects have been selected for case studies to show variousexamples of reservoir sedimentation and related managementmeasures. The Liujiaxia Project is an example of how sedimentmay be a factor in the selection of a dam site. The SanmenxiaProject experienced several stages of reconstruction due to seriousreservoir sedimentation that had not been considered properly atthe original design stage. Through extensive studies, a new opera-tional rule of impounding clear and discharging turbid waters (Iand D) was developed. It is useful in maintaining the long-termstorage capacity of projects built on sediment-laden rivers. TheHeisonglin Project, though much smaller than the SanmenxiaProject, still faced similar sediment problems and solved theseproblems with almost the same measures. These two case studiesshow that I and D operational rules can be applied to hydrologicalprojects of different scales. The Shuicaozi Project was built on asmall river with a small amount of sediment load. However, sedi-ment problems were serious due to inadequate facilities to excludesediment from the reservoir. After several measures were adopted,including digging a new tunnel and dredging, the sediment prob-lems were solved satisfactorily. The Guanting Project is anexample showing the effectiveness of various measures to reducesediment input in the reservoir. The Tarbela Project on the IndusRiver is a key hydrological project in Pakistan. At the designstage, no sediment management measures had been adopted. Sincethe commissioning of the project in 1974, reservoir sedimentationhas emerged and become serious in recent years, as the Indus

106 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.25 — Flow and sediment regimes of Guanting reservoir.

Reach Natural Impoundment Detention

Huayuankou 6 300 9 000 3 500

Jiahetan 6 000 11 500 2 600

Gaocun 5 600 12 000 3 000

Luokou 8 800 5 000

Table 5.27Change in bankfull discharges (m3 s–1)

Reach Natural Impoundment Detention

Huayuankou 1.51 2.24 0.57

Jiahetan 0.97 2.34 0.95

Gaocun 1.64 2.44 1.02

Luokou 5.86 9.61 4.03

Table 5.26Elevation difference between channel bed and flood plains

Units in m

Table 5.28Characteristics of Liujiaxia and Sanmenxia Projects

Name of project Liujiaxia Sanmenxia

Reservoir capacity (109 m3) 5.74 35.4

Length of reservoir (km) 56

Dam height (m) 147 106

Pool level fluctuation (m) 41

Catchment area (109 km2) 181.8 688.4

Annual runoff (109 m3) 26.3 45.3

Annual sediment load (109t) 0.087 1.6

Average concentration (kg m–3) 3.31 35.6

D50 suspended load (mm) 0.025 0.038

D50 top-set deposit (mm) 0.02 0.02

Installed capacity (MW) 1 225 900

Q (

m3

s–1 )

Sedi

men

t (kg

m–3

)

8 9

River carries a large amount of sediment. How to deal with sedi-ment problems in the reservoir is still a pending issue for theauthorities. This case study shows how sediment managementshould be considered for reservoirs, except for those built on clearrivers.

5.8.1 Liujiaxia ProjectLiujiaxia Dam is the first large multipurpose hydrological projecton the Upper Yellow River for power generation, flood and ice jamcontrol, and irrigation. The first power came on line in 1969 andthe entire project was commissioned in 1974. Some pertinent dataare given in Table 5.28. The main dam is a concrete gravity dam.One discharging tunnel (Qmax = 2200 m3 s–1) and two sluicingtunnels (Qmax = 1 524 and 108 m3 s–1) are built in the dam.

Up until 1989, 1.41 billion m3 of sediment had beendeposited in the reservoir, accounting for 24.6 per cent of the originalstorage capacity. Of this deposit, some 70 per cent was in inactivestorage, accounting for about 45 per cent of the original inactivestorage, and only about 8 per cent of the original active storage.

In flood seasons, incoming sediment was deposited firstin the gorge near the end of the reservoir (Figure 5.26). When thepool level was drawn down during dry seasons, the deposits on thetop-set of the delta were eroded and transported, then deposited inthe inactive storage. The pivot point of the delta is still far awayfrom the dam. Thus, sedimentation in the main reservoir has notcaused any problems for the project so far.

Liujiaxia Reservoir has two small arms in the valleys ofthe Taohe River and the Daxia River. The storage capacity of thesetwo tributaries is only 2 per cent and 4 per cent of the total storagecapacity, respectively. The Taohe River joins the main stream at apoint 1.5 km above the dam and carries 28.6 million tons of sedi-ment per year, i.e. 31 per cent of the total sediment influx inLiujiaxia Reservoir. The substantial and rapid deposition wascaused by the large amount of incoming sediment load in the rela-tively small Taohe River, and has led to serious problems inLiujiaxia Reservoir.

The main problem is the formation of a mouth bar at theconfluence. By 1979, the inactive storage of the Taohe River wasfull and the mouth bar had risen to the minimum pool level. Theproximity of the mouth bar resulted in a rapid increase in theamount of sediment passing through the turbines. In June 1980,when more flow was required to meet an abrupt increase in powerdemand, the pool level in front of the dam suddenly dropped by alarge amplitude because the mouth bar impeded the flow of waterto the dam from the upstream part of the reservoir.

Abrasion of turbine blades and the lining of the outlettunnels for sediment sluicing had been very serious problems. Theannual amount of sediment passing through power unit 2 reachedits peak in 1978 and 1979 with 11.6 and 11.9 million tons, respec-tively, when the top of the mouth bar was the highest. Aftersediment sluicing in 1981,1984 and 1985, the amount wasreduced.

The abrasion of the turbine blades and the lining ofsluicing tunnels required a great amount of repair work. Forexample, power unit 2 was damaged to such an extent that it hadto undergo repair for 125 days. It was found that the maximumdepth of abrasion was 50 mm, and the abraded area was as muchas 28.9 m2. Welding rod consumption was as high as 3.5 tons.

If the dam site of the Liujiaxia Project had been selectedabove the confluence of the Yellow River and Taohe River, thesediment problems experienced by the Liujiaxia Project would nothave been so serious at the initial stage of the operation of theproject. The decrease in the benefit of the project would not havebeen large if remedial measures had been found to use the annualrunoff of the Taohe River (Qm = 178 m3 s–1).

At present, the major sediment problem of the LiujiaxiaProject is the existence of a mouth bar at the confluence of theTaohe River. To lower the top surface of the mouth bar and toreduce the amount of sediment passing through the turbines, draw-down flushing has been carried out four times. The effect ofsediment flushing was obviously positive and the top surface of themouth bar was lowered by 1.4 to 5.9 m. Sediment flushing alsorecovered a certain amount of the storage capacity in the TaoheRiver. The sediment flushing process was conducted at the end ofthe dry season, when the pool level was close to the minimum. Theeffect of sediment flushing depends on the schedule for the opera-tion of the tunnels. This was decided in view of previousexperience, although it could also be decided through a model test.

5.8.2 Sanmenxia ProjectThe Sanmenxia Project was the first large multi-purpose waterconservancy project on the Yellow River, where the catchmentarea accounts for 91.5 per cent of the total, and the runoff andsediment load account for 89 per cent and almost 100 per cent ofthe totals, respectively. The main characteristics of the project arelisted in Table 5.28.

The original planning of the Sanmenxia Project wasaffected to a large degree by the opinion that a “large reservoirstorage capacity has to be gained by large inundation”. In 1958,China decided to select 360 m as the normal pool level (NPL) in thedesign phase, but at the first stage of construction 350 and 325 mwere adopted as the NPL and dead pool level (DPL), respectively;the dam crest elevation was 353 m; the total storage capacity was35.4 billion m3, of which 14.7 billiion m3 was reserved for thesediment deposits; the installed capacity was 900MW. The mainobjectives of the reservoir were to reduce the 1 000-year flood from35 000 to 6 000 m3 s–1 and eliminate the flood threat in the LowerYellow River; to store all incoming sediment load and preventsediment deposits and bed levels from rising in the downstreamriver channel; to manage the water resources of the Yellow Riverand irrigate 1.48 million ha during the first stage and 5 million haduring the second stage; and to improve navigation in thedownstream reaches. Accoding to this planning, the reservoir wouldinundate 138 thousand ha of farmland, and 600 thousand peoplewould have to be resettled by the time the NPL was 350 m. The

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 107

Figure 5.26 — Plan of Liujiaxia Reservoir.

Yel

low

Riv

erD

axia

Riv

er

Taoh

e R

iver

Yanguoxia Dam

Liujiaxia Dam

reservoir lifespan was expected to be 25 to 30 years. By combiningthe reservoir with soil conservation works in the upstream reaches,its lifespan could increase to between 50 and 70 years. It wasestimated that the sediment load in 1967 would decline by 50 percent thanks to soil conservation works and reservoirs on thetributaries.

The general plan of the Sanmenxia Project and the reser-voir storage capacity curve are shown in Figure 5.27.

The Sanmenxia Project was commissioned in September1960. The operating rule in this period was to impound water andtrap incoming sediment load. The highest pool level was 332.58 m(9 February 1961). During this period, 1.74 billion tons of sedi-ment load entered the reservoir. However, only 7.1 per cent of thetotal sediment load was vented out of the reservoir by densitycurrent and 1.7 billion m3 of reservoir storage below 335 m wereoccupied by sediment deposits. Tongguan is at the confluence ofthe Yellow River and the Weihe River, which is the largest tribu-tary of the Yellow River. The Yellow River has a maximum widthof 18 km at the confluence zone but contracts downstream to alittle over 1 km at Tongguan. The pass at Tongguan thus serves asa local base level for the Weihe River and the Yellow Riverupstream. The bed elevation at Tongguan had risen by 4.5 m fromSeptember 1960 to March 1962. It induced a new problem,namely the upstream extension of backwater deposits, whichwould have very serious impacts on the Guanzhong Plain in theLower Weihe basin, a very important agricultural zone, and Xi’anCity, capital of Shaanxi Province. Such situations show that theplanning and design of the Sanmenxia Project in the 1950s were atfault in the following respects. First, the project’s targets were toohigh, such as the targets for power generation and navigation.Second, much attention was paid to retaining sediment in thereservoir to avoid aggradation in the Lower Yellow River, but theimpacts of reservoir sedimentation in the upstream area and reser-voir area were neglected. Third, the opinion that “reservoir storagecapacity has to be gained by inundation” made the reservoir scaletoo large, which was inconsistent with the national situation ofhigh population density and a shortage of farmland. Fourth, thebenefits of soil conservation were overestimated, since in 1967 theincoming sediment load had been expected to decrease by 50 percent. Actually, the goal has not been reached.

In March 1962, it had to be decided to change theoperating rule from impoundment to flood detention and sediment

discharge in order to reduce the rapid sedimentation in thereservoir.

In accordance with the operating rule of flood detentionand sediment discharging, just before the flood season the poollevel was drawn down to leave large reservoir storage for floodcontrol, and sediment was sluiced, with all sluicing gates fullyopened. As the capacity of the outlets was insufficient, two addi-tional tunnels with the bottom level of 290 m at the left bank andfour power penstocks were converted into sluiceways. Thedischarge capacity of the outlets was increased from 3 058 to6 102 m3 s–1. The reconstruction works were initiated in 1965 andgradually put into operation from June 1966 onwards. The trapefficiency fell to 20 per cent. As the annual sediment load of theYellow River is so large, sediment deposition in the reservoir wasstill too serious. In May 1969, it was decided that further recon-struction was needed, which included reopening eight diversionbottom outlets at an elevation of 280 m and lowering the intakesof five penstocks by 13 m, from an elevation of 300 m to 287 m.The flow discharge capacity at a pool level of 315 m increased to9 311 m3 s–1. The second stage of reconstruction started inDecember 1969 and was completed in 1973.

Based on the lessons learned from the periods ofimpoundment and flood detention, a new rational operating rule ofimpounding the clear and discharging the turbid water was devel-oped. During dry seasons the inflow with low sedimentconcentration is impounded in the reservoir for spring irrigationand power generation, and during flood seasons the reservoir poollevel is drawn down to sluice off most of the whole year’s sedi-ment load, so as to keep a balance of deposition and erosion in thereservoir in normal years and to reduce the aggradation underfavourable incoming flow and sediment conditions. Through theproper regulation of flow and sediment, reservoir sedimentation inSanmenxia Reservoir has been controlled. The reservoir storagecapacities below 330 and 335 m have recovered to 3.1–3.2 billionm3 and 5.9 billion m3, respectively. The bed elevation at Tongguanhas descended by 1.8 m. A narrow and deep channel and highflood plains have been established in the reservoir, so that thechannel storage capacity can be preserved in the long term. Thetrap efficiency has decreased to 0. The situation of the SanmenxiaProject at various stages is shown in Table 5.29.

5.8.3 Heisonglin ProjectThe Heisonglin Project is a small hydraulic project on a smallriver of Yeyu, China. The reservoir storage is 8.6 million m3,controlling a catchment area of 370 km2. The dam is 45 m highand a bottom outlet (2 × 1.5 m) with a discharge capacity of10 m3 s–1 is installed at the dead pool level. The mean annual

108 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.27 — Plan of Sanmenxia Reservoir.

Table 5.29Situation of Sanmenxia Project

Stage Time Operating Discharge Traprule capacity efficiency

(m3 s–1) (%)

1 September 1960 Impoundment 3 058 92.9to March 1962

2 March 1962 to Flood 6 102 20October 1973 detention

3 October 1973 I and D 9 311 0

Weihe River

Beiluo River

Yellow River

Fenhe River

runoff at the dam site is 14.2 million m3 (Qm = 0.45 m3 s–1) andthe mean annual sediment load is 0.70 million t. The mean annualsediment concentration is 49.3 kg m–3, while the mean sedimentconcentration in July and August is 113 kg m–3 and the maximumconcentration is 801 kg m–3. The suspended sediment is fine, withD50 of 0.025 mm. The D50 of the original bed material is 18 mm.The runoff in the flood season accounts for 45 per cent of thewhole year’s runoff, while the sediment load in the flood season is98 per cent. The reservoir is a gorge-type reservoir.

The project was commissioned in 1959 and the adoptedoperating rule was impoundment. During the first three years(May 1959 to June 1962) reservoir sedimentation was veryserious, with a cumulative amount of deposition of 1.62 millionm3, namely 18.8 per cent of the total storage capacity. If such anoperating rule had been continued, the reservoir would have beensilted up in 16 years. Therefore, the operating rule of the reservoirhad to be changed in 1962.

In dry years sediment concentration is lower thannormal, so impoundment of water prevails even in the floodseason, when sediment may be vented out of the reservoir bydensity currents. In wet years, discharging sediment prevails in theflood season.

In the course of a year, at the early stage of the floodseason (1 to 20 July) when the flood peaks are often not high andthe sediment concentration is also not too high, density currentventing is the main method of discharging sediment. In the middleof the flood season (21 July to 31 August), when floods frequentlytake place with high sediment concentration, the pool level shouldbe drawn down to the flood control level (FCL) to facilitatedischarging sediment. In September, when sediment concentrationis not high, impoundment may start; when a flood occurs, densitycurrent venting may be affected.

During the discharging of sediment, trap efficiency hasbeen as low as 10 per cent. Density current has formed easily inHeisonglin Reservoir, and its trap efficiency is also low, at about35 per cent.

Beginning in 1962, the overall trap efficiency ofHeisonglin Reservoir was 14.7 per cent. Consequently, the annualrate of deposition in the reservoir slowed down to 0.1 million m3,as compared with 0.54 million m3 in the first 3 years.

It should be emphasized that all the discharged sedimentfrom Heisonglin Reservoir was transported to an irrigation canaldownstream of the reservoir. The hyperconcentrated flow ofsediment contains organic manure and many nutrients, such asnitrogen. The irrigated farmland has become more fertile, resultingin an increased crop yield. Using the discharged hyperconcentratedflow from the reservoir for warping not only mitigates serioussedimentation in the reservoir, but also relieves deposition in thechannel downstream of the reservoir; it has three-fold benefits.

5.8.4 Shuicaozi ProjectThe Shuicaozi Project is located on the Yili River in YunnanProvince, China. It is the second stage of four hydropowerstations, and functions as a seasonal storage reservoir and a diver-sion work conveying water from the Yili River to the XiaojiangRiver for power generation.

The dam is 36.9 m high. The NPL is 2 100 m with acorresponding reservoir storage of 9.58 million m3, and the DPLis 2 096 m with a 5.93 million m3 storage capacity. The effectivestorage capacity is 3.65 million m3. The reservoir is 6 km long.The top elevation of the spillway is 2 089 m, and the elevation ofthe invert of the power station is 2 088 m. No bottom outlet wasinstalled in the dam. Sediment flushing was carried out throughthe drawdown of the pool level through the spillway.

The mean annual discharge at the dam site is16.3 m3 s–1, and the discharge for power generation is 2.9 m3 s–1.The inflow is regulated by an upstream reservoir (MaojiacunReservoir). The incoming sediment load in Shuicaozi Reservoir ismainly from the watershed between these two reservoirs. Theannual suspended load is 0.5 to 0.6 million tons, and the annualbed load is 20 to 30 thousand tons.

The project was commissioned in 1958. From 1958 toearly 1981, 8.17 million m3 of sediment was deposited in thereservoir, namely 85.3 per cent of the reservoir storage.

In order to recover a part of the storage capacity, draw-down flushing was carried out eight times during the period from1964 to 1981. Figure 5.28 shows the longitudinal profile beforeand after sediment flushing. A total of 1.22 million m3 of depositswere flushed out of the reservoir. Owing to the high elevation ofthe spillway, opportunities for drawing down the pool level werelimited. Flushing was divided into two stages. In the first stage,from 1964 to 1966, when the upstream reservoir was notimpounded, flushing was carried out in the flood season and theamount of flushing discharge was large. However, the top surfaceof the deposits at the dam was still low, and the volume of sedi-ment flushed out of the reservoir was small. In the second stage(which started in 1974), flushing was only carried out for two tothree days during the Spring Festival when the power demand waslower than normal. Since the top surface of the deposits was highat the dam, the volume of sediment flushed out of the reservoirwas large, although the flushing discharge was smaller than duringthe first stage. The quantity of sediment flushed out each time wassome 200 thousand m3, corresponding to about one third of theannual incoming sediment load. To increase the quantity of thesediment flushed out, a new tunnel for sediment flushing wasexcavated in 1988. The intake of the tunnel is 22 m below the topof the sediment deposits. The sluicing discharge is 50 to170 m3 s–1. The maximum velocity in the tunnel is 18.2 m s–1, andthe total length of the tunnel is 325 m. After the completion of themain part of the tunnel, more than 160 000 m3 of the reservoir’s

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 109

Figure 5.28 — Longitudinal profiles before and after sedimentflushing, Shuicaozi Reservoir.

1. Pre-flushing (1980)2. Post-flushing (1980)3. Pre-flushing (1965)4. Post-flushing (1965)

Ele

vatio

n (m

)

Distance (km)

storage capacity was recovered when the inlet to the tunnel wassuddenly opened by blasting of the rock plug. A funnel, 482 mlong, 18.5 m deep and 100 m wide, was scoured out in front of theintake. The sediment particles deposited near the dam were veryfine (D50 = 0.005 mm). The specific weight of the deposit was 1.1to 1.3 t m3. Deposition was mainly caused by density current.

5.8.5 Guanting ReservoirGuanting Reservoir is on the Yongding River in China, andcontrols a catchment area of 43 400 km2. The mean annualrunoff at the dam site is 1.4 × 109 m3 and the annual sedimentload is 81 million tons. The reservoir storage is 2.27 × 109 m3.The project was commissioned in 1955. The Yongding River isheavily sediment-laden with an average sediment concentrationof 34.6 kg m–3. Reservoir sedimentation was so serious that by1985, 612 million m3 of storage capacity had been silted up,accounting for 27 per cent of the original storage capacity.However, the siltation rates have been quite different in GuantingReservoir in different periods (Table 5.30). Although the averageannual precipitation and precipitation in the flood seasons of the1950s, 1960s and 1970s were almost the same, the incomingrunoff and sediment loads in Guanting Reservoir have declinedsignificantly since 1960 under the influence of human activities,as shown in Table 5.31.

It can be seen from Table 5.32 that sediment trapped inthe upstream reservoirs accounted for 41.5 per cent of the total

amount of reduction. Since 1958, 275 reservoirs with a totalstorage of 1.4 billion m3 have been constructed. Until 1983, 0.34billion m3 of sediment was deposited in 18 large and medium-sized reservoirs, of which the original total storage was1.39 billion m3. The average annual amount of deposition was17 million tons of sediment.

The largest reduction in sediment in Guanting Reservoirresulted from irrigation and warping, with an annual reduction insediment of 19 million tons. There are 267 thousand ha of irri-gated farmland upstream of Guanting Reservoir. Warping has beenapplied to half of the irrigated land.

From 1950 to 1980, 6 200 km2 of eroded area in theupper reaches of the Yongding River have been under control, i.e.one fourth of the total eroded area. It was estimated that theoverall reduction in sediment yield amounted to 10 million tons.However, in the meantime, the planting of astragalus membrana-ceous, a Chinese medicine herb, road construction, urbandevelopment and mining led to an increase in soil erosion by5 million tons. The net reduction by soil conservation measureswas therefore 5 million tons. From these data, it is evident that themeasures adopted to reduce sediment in Guanting Reservoir havebeen effective.

5.8.6 Tarbela Dam ProjectThe Tarbela Dam Project is on the Indus River in Pakistan. Thecatchment area of the Indus River is 969 000 km2, with anannual runoff of 175 billion m3 and an annual sediment load of470 million tons. Above the Tarbela Project the catchment areais 169 579 km2, with an annual sediment load of 287 tons.Although the catchment area above the Tarbela Project accountsfor only 17.5 per cent of the total catchment area of the Indus,the annual sediment load above the Tarbela Project accounts for66 per cent of the river’s total amount. The large amount ofsediment load carried by the Indus has been a great threat to theProject due to serious sedimentation in the reservoir. Since thedam’s first impounding in 1974, reservoir sedimentation at theTarbela Project has taken place rapidly. By 1990, 2.18 billionm3 of storage capacity had already been lost to deposition,accounting for 15.2 per cent of the original reservoir capacity. Adelta has rapidly developed, with its pivot point at 1 300 to1 310 ft elevation, which is close to the minimum pool level ofthe reservoir. The rate of advancement of the delta was from 0to 1 500 m per year, depending on the annual duration in whichthe pool level was kept below 1 320 ft. If the present scheme ofoperation continues, according to previous estimates the foresetslope of the delta will reach the tunnel intakes in the periodbetween 2005 and 2008. Consequently, the present operation ofthe Project will be impeded. Since the Tarbela Dam Projectplays an important role in the national economy (both for irriga-tion and power generation), its normal operation is vital forPakistan.

The problems induced by reservoir sedimentation inTarbela Reservoir above all include loss of storage capacity,abrasion of turbines and hydraulic structures, and the danger ofblocked tunnels. No measures for sediment mitigation have beenplanned. How to deal with the sediment problems in TarbelaReservoir is a question that is still under consideration. Thelesson is that for large hydraulic engineering projects, especiallythose built on sediment-laden rivers, sediment mitigationmeasures must be considered during the planning stage.

110 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 5.30Reduction of the rate of sedimentation in Guanting Reservoir

PeriodTotal amount of Annual rate of

deposition(106 m3) siltation (106 m3)

1956–1960 350 70

1961–1970 82 8.2

1971–1980 73 7.3

Table 5.31Reduction of runoff and sediment load in Guanting Reservoir

Precipitation (mm)Period Annual Annual sediment

Annual Flood runoff load (106t)season (106 m3)

1951–1960 444 338 1723 59.69

1961–1970 412 313 1258 15.08

1971–1980 427 373 832 10.23

Table 5.32Annual reduction of sediment load in Guanting Reservoir by

various measures

Trapping by IrrigationCauses of upstream and Soil Totalreduction reservoirs warping conservation

Annual reduc- 17 19 5 41tion of sedimentload (106 t)

Percentage of 41.5 46.5 12.0 100total reduction(%)

5.9 MEASUREMENT OF EROSION ANDDEPOSITION IN THE RESERVOIR

Sedimentation surveys in reservoirs and river reaches are used todetermine the total quantity of erosion and/or deposition, as wellas the pattern and distribution of deposits. Such surveys areusually made in order to modify the reservoir capacity curve andprovide data for studying the fluvial process upstream and down-stream from a dam in response to the variability of flow caused byvarious management measures in the river basin.

The range of surveys, frequency of measurements andproper timing of a survey are determined by the requirements ofthe research programme and according to the reservoir’s opera-tional requirements for flood control and the multi-purposeutilization of water resources. The range of reservoir sedimenta-tion surveys should meet the requirements for a revision of thereservoir capacity curve at normal high water levels and for anevaluation of the upstream extension of reservoir deposits.Repetitive surveys should be carried out whenever there is achange in capacity exceeding ±3 to 5 per cent. The survey shouldbe conducted before or after the flood season and under relativelystable flow conditions (Ministry of Water Resources, 1978).Similar requirements are adopted for conducting surveys in riverreaches. However, repetitive surveys of the range lines set up atreasonable densities are a cost effective and irreplaceable methodfor studying sedimentation problems in a river reach. Xiong, et al.(1983) made comparisons between the results of the amount ofsedimentation obtained by the range-line survey and that obtainedby the difference of sediment load observed at two terminal hydro-metric stations, considering the input and output in theintermediate areas. The study indicated that fairly good levels ofaccuracy may be achieved with range lines set up at a reasonabledensity. Also, the bias induced by the systematic error inherent inthe measurement of sediment discharge at hydrometric stationswould be too large if the amount of deposition or erosion was rela-tively small in comparison with the oncoming sediment load(Xiong, et al., 1983; Lin, 1982).

Progress in surveying and mapping methods and ininstrumentation has been rather pronounced in recent decades.Electronic distance meters such as microwaves, lasers and infraredlight devices are widely used. Aerial surveys together with under-water depth sounding are also commonly used. The GlobalPositioning System (GPS) and Geographic Information System(GIS) have caused a revolution in the field of surveying all overthe world. In this section, only the basic principles of conductingsedimentation surveys will be discussed.

5.9.1 MethodologyThree methods are most commonly used to measure erosion anddeposition in reservoirs and river reaches, namely the range-linemethod, the contour method (topographic survey), and thecomposite method, which is a combination of the range-line andcontour methods. Selecting a method depends mainly on the

topography of the studied reach and the accuracy desired. Prior tothe advent of electronic measuring and computerized data collec-tion and analysis systems, the range-line method was the preferredmethod of collecting field data because it involved lower costs andwas less time-consuming. The development of current collectionsystems has made the contour method the preferred method fordata collection and analysis.

Hydrographical surveys are recognized as either class 1,2 or 3, depending on the level of accuracy required. Class 1 is thehighest accuracy standard and generally pertains to surveys insupport of site planning in advance of design efforts, pre- andpost-dredging activities, and other uses. Class 2 is a medium accu-racy standard, and is generally used to determine channelconditions in headwater and tributary arms, and in cross-sectionsurveys for reservoir volume computations. Class 3 is the lowestaccuracy standard, and is used principally for reconnaissanceinvestigations. The recommended maximum allowable errors foreach survey class are given in Table 5.33 (Ferrari and Dorough,1996).

5.9.1.1 CONTOUR METHOD

A topographic survey covering the area of the whole studied reachor only a portion thereof is a precise method, and is employedwhen measuring the deposition or erosion in a reservoir or a riverreach, which is calculated from the difference in capacity at agiven elevation as measured from the topographic maps obtainedfrom two successive surveys. Surveys using the contour methodare employed as a control method for evaluating deposition in thelong term. The result provides a basis for the correction of thecapacities computed by the range method.

The scale of a topographic map for a reservoir or riverreach is determined by the desired accuracy of the computation oferosion and deposition. For a medium-size reservoir or a shortriver reach, a scale of 1:5 000 or 1:10 000 is preferred. For verylarge reservoirs or long reaches, a scale of 1:10 000 or 1:25 000should be used. If an accurate computation of the deposition isrequired, the scale should not be less precise than 1:25 000.

In general, prior to impounding water in a reservoiror conducting an experimental study of the fluvial processesin a river reach, topographic surveys are conducted to providebasic data for future studies. The topographic map is consid-ered to be a fundamental map, which is revised periodically inaccordance with later repetitive surveys. Repetitive surveyscover only an area over which a variation in land surfacetakes place. The highest contour drawn in the repetitivesurvey should, of course, coincide with the correspondingcontour on the original map, above which no change in land-scape takes place.

The elevation of the highest contour measured in thepreliminary survey should be 4 to 5 m above normal high waterlevel or, preferably, above the possible maximum level reached atdesign flood. The maximum probable range of bank erosionshould also be considered in deciding the range of the preliminarysurvey. Once the map scale is properly determined, the wholesurvey should be conducted according to the relevant specifica-tions and standards.

5.9.1.2 RANGE-LINE METHOD

Relatively speaking, the range method is advantageous becauseconducting the survey is simple and less time-consuming. If

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 111

Table 5.33Maximum allowable error in hydrographic surveying

Class 1 Class 2 Class 3

Horizontal positioning 3 m 6 m 100 m

Depth measurement ± 0.5 ft ± 1.0 ft ± 1.5 ft(15 cm) (30 cm) (45 cm)

ranges are arranged at reasonable intervals, the desired accuracycan be obtained within the tolerance limit for allowable error. Therange method is a conventional method in general use for mostreservoir studies.

A sedimentation survey for a reservoir should extend atleast to some distance or several ranges upstream of the end ofbackwater deposits. If the distance is large between the end of thebackwater deposits and the hydrometric station used as an inflowsediment measuring station, a number of ranges should be set upin such reaches. The river bed in this reach undergoes changes byself-adjustment of the alluvial channel. From measurementsperformed on these ranges, data may be obtained to verify thewater surface profile or to aid in the evaluation of sedimentbalance. For a river reach, ranges should be arranged to coverreasonably all the bends and transition regions, pools and riffles,and wide and narrow parts, etc.

Ranges should be positioned approximately perpendicu-lar to the major trend of the contour lines within which thereservoir is operated. At a confluence, ranges should be set up inlarge tributaries if the deposition in the tributary is estimated to beappreciable.

The number of ranges considered reasonable implies aminimum number of ranges established in a reservoir or riverreach which could reflect the essential pattern and distributionof sedimentation, both longitudinally and transversely, withoutbeing detrimental to the desired accuracy in the computation ofthe total sedimentation. As a general rule, it is recommendedthat the difference in the sedimentation computed by the range-line method and by the contour method should be kept within alimit of ± 5 per cent. Two methods may be used.

First method: On the preliminary topographic map witha scale of 1:10 000, ranges spaced at an equal distance, forexample 200 m, are drawn approximately perpendicular to thecontours below the elevation of normal high water. Capacity orvolume at the normal high water level is calculated by the range-line method and compared with the volume calculated from thecontour map. Computations are then made using fewer range linesso as to select one out of two range lines, and then one out of threerange lines, etc. The simplification or reduction of range linesshould proceed until the relative error for the computation ofcapacity or volume is still within the tolerance limit of ± 5 percent, using the volume computed by the contour method as areference.

Second method: Hakanson (1978) carried out studiesof the optimum arrangement of ranges in a lake survey.Adopting his idea, using the data obtained from four large reser-voirs in China, the optimum number of ranges may be computedfrom the following equation (Sanmenxia Reservoir ExperimentStation, 1980).

(5.45)

where Lr is the distance between range lines at optimum density, Arepresents the area enclosed by the highest contour line in km2, Lt

is the accumulative distance between ranges in km, and F = Lo/2(π A)1/2, where Lo is the length of the highest contour linemeasured in km. Based on studies of reservoir data, it was foundthat range line spacing according to the above equation will resultin surveys with a fair degree of accuracy. If the range intervals areproperly arranged, the accuracy of computing deposition by the

range-line method is within 5 per cent of that determined by thecontour method.

5.9.1.3 COMPOSITE METHOD

Contour or topographic and range-line methods may be combinedto gain a better understanding of the variations in ground surfacein a river reach or a reservoir. Bank failures usually take place at apoint not covered by pre-set range lines. The progress of deltaformation at the head of a reservoir may be studied by means of atopographic map. Thus, a local topographic survey may be indis-pensable to supplement the sedimentation survey. In fact, therewill be essentially no difference between the results obtained bythe two methods if the number of ranges is increased sufficientlyso that contour lines or a topographic map can be drawn from thedata obtained from the range-line survey. This is particularly truefor bed surveys conducted by underwater soundings. Aerialphotographs in combination with underwater soundings taken inthe portion still covered by water may be used advantageouslywhile the reservoir is at its lowest level.

5.9.2 Instrumentation for positioning and depth sounding5.9.2.1 DEPTH SOUNDING

Manual sounding poles, sounding weights and echo sounders arecommonly used for depth measurements. The appropriateselection of instruments or devices depends on the local depth,velocity, bed material composition and its degree of compaction. Abell-shaped sounding weight made of cast aluminium (weighing 2or 4 kg), or a sounding pole (aluminium sectional pole with eachsection about 1.5 m in length, fastened together with threadeddowels) may be used to take soundings where an echo sounder isnot available (Soil Conservation Service, 1973).

The type of echo sounder is selected mainly accordingto its ability to distinguish the bed surface. The results of depthmeasurements may differ with transducers of different power andfrequency response in the detection of the top of soft mud. Echosounders are usually specified by their relative accuracy. If adepth 50 times the deposit thickness is measured by an echosounder with a relative accuracy of 1 per cent or more, a largeand intolerable error will be included in the sounding results.Hence, a more precise instrument should be used. In general, anecho sounder equipped with transducers operated at a lowfrequency is preferred when measuring a river bed composed ofunconsolidated soft mud. A mini-echo sounder weighing onlyseveral kilograms has been developed in the Bureau of Hydrologyof the Yellow River Conservancy Commission and has beensuccessfully used in both reconnaissance surveys and routinework at hydrometric stations.

When an echo sounder is used for taking depth measure-ments in a reservoir or river reach, the instructions specified foreach instrument should be strictly obeyed. The transducer shouldbe properly installed on the bottom or the side of the measuringboat. Water temperature should be measured and the instrumentadjusted accordingly. The depth recorded by an echo soundershould be compared regularly with that measured by other reliablefixed-point methods during the operation. The calibration can becarried out by lowering an acoustic reflector, such as a flat metalplate, to a known depth below the transducer, and adjusting theinstrument to produce an equivalent depth reading. Deviations indepth recorded by the echo sounder can be used as a guide for anynecessary adjustment.

112 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

LA

L Fr

t

=1

3

5.9.2.2 POSITIONING OF SOUNDING POINTS

In a sedimentation survey carried out in small reservoirs or smallriver reaches, for reasons of economy, expediency and accuracy,the range-cable method of locating sounding points is mostcommonly used. Equipment used by the United States SoilConservation Service includes an aluminium reel holding about800 m of cable (galvanized aircraft cord with a diameter of2.4 mm or plastic water-ski tow cable with a diameter of 6.3 mm),equipped with a line meter (Soil Conservation Service, 1973).

In large reservoirs or broad river courses, sextants orintersection by transits from two or three points are still used as atraditional method of locating points. More advanced instrumentshave been adopted, such as positioning by microwave, laser orinfrared electronic distance measuring instruments or systems.The accuracy of a sedimentation survey, needless to say, relies onthe accurate positioning of measuring points, particularly in placeswhere the sediment deposition is not appreciable. Obviously, atpoints where no deposition or erosion takes place, the elevation ofthe bed surface should coincide with that measured in a previoussurvey. This is a good check of the accuracy and reliability of thesedimentation survey.

Aerial photographic techniques are most effective formaking base surveys before the reservoir is filled or at the start ofa research programme to study the fluvial processes in a riverreach. Capacity can be measured by photographic surveys, usingvertical air photography with permanent ground control points. Allsuch points should be coordinated and controlled in elevation tothe same degree of accuracy.

5.9.2.3 SURVEYING SYSTEM

To cope with the increasing demand for more complete and accurateinformation on hydrological and geomorphologic processes inreservoirs and river reaches, various types of surveying systemshave been developed and used. For instance, the Water Survey ofCanada had developed automated high-speed data collection andprocessing systems (Durette, 1977) by 1973. In the early 1990s theChangjiang Water Resources Commission (CWRC) developedsurveying systems, including software for communications betweenshore stations and surveying vessels for navigation and data storage,and processing methods for use with hardware such as electronictheodolite, laser or microwave distance measuring devices and PCs(Bureau of Hydrology 1990). Geomorphologic studies on riverreaches that undergo drastic changes during floods can beconducted more comprehensively and accurately at a low cost.

5.9.2.4 POSITIONING BY THE GLOBAL POSITIONING SYSTEM

The Global Positioning System (GPS) is an all weather radio-based satellite navigation system that enable users accurately todetermine three-dimensional positions (x,y,z) worldwide.Satellites are used as reference points for triangulating the positionof the receiver on earth. The position is calculated from thedistance measured using the time of transmission of the radiosignal. A minimum of four satellite observations is required tomathematically solve the four unknown receiver parameters (lati-tude, longitude, altitude and time). A single GPS receiver isusually not accurate enough for precise surveying and hydro-graphic positioning. Differential GPS (DGPS) is a collectionmethod to resolve the inherent errors of a single GPS receiver.More than two receivers are used in DGPS, one of which is set upat a known geographical benchmark. Differential GPS determines

the position of one receiver in reference to another and is a methodof increasing positioning accuracies by minimizing uncertainties.It is not concerned with the absolute position of each unit, but withthe relative difference between the positions of the two units thatare simultaneously receiving signals from the same satellites. In asedimentation survey of Cascade Reservoir, the use of DGPSmade possible positioning accuracies of 1 to 2 m, which is accept-able in a hydrographic survey (Ferrari and Dorough, 1998).

DGPS interfaced with underwater depth soundingsystems in reservoir sedimentation surveys has also been used inthe reservoir topographic survey of Xiaolangdi Reservoir, as wellas in a river range-line survey in the Lower Yellow River. Thismethod has become inceasingly popular all over the world inrecent years.

5.9.2.5 MEASURING SEDIMENT THICKNESS

In cases where accurate maps of the original reservoir basin arenot available, the thickness of sediment deposits must be measureddirectly to determine the original capacity and sediment volume. Ifthe water depth is not very deep, such as in small and medium-sized reservoirs, a spud or auger may be used. A sectional spud,made up of 0.9 m (3ft) sections which can be assembled up to alength of approximately 5.5 m (18 ft) with nickel-steel alloy dowelpins has been used by the United States Soil Conservation Service.The spuds are made of hardened case steel rods, 38 mm (1.5 in) indiameter, into which encircling triangular grooves are machined atintervals of 2.5 mm (0.1 in). The base of each groove is machinedto a depth of 3.2 mm (1/8 in) to form a cup in which sedimentdeposits can be caught and held. The layer of new deposits cangenerally be distinguished easily from the original bed material,and through these means the thickness of sediment deposits can bedetermined. In order to enhance accuracy, a combination of spudand sounding is preferable if there is a thin layer of deposits (SoilConservation Service, 1973).

5.9.3 Measurement of bed material compositionIf the intention is to measure the density and the particle sizedistribution of the deposits, undisturbed sediment samples shouldbe taken at representative locations in the reservoir or riverreaches. The grain size, composition and dry density (unit weight)of deposits are essential factors to be measured in a sedimentationsurvey. Disturbed and/or undisturbed samples are obtained byvarious means, and sent to the laboratory for further analysis. Avariety of equipment for taking samples was described by Vanoni,et al. (1975). Sampling apparatuses for bed material including thedeposits in reservoirs or river reaches have been described in stan-dards issued by ISO (1977c). If the size gradation of the bedmaterial as well as its unit weight are required, undisturbedsamples must be collected in the field. More often, only thesurface bed material is sampled for size analysis. In this case,samplers similar to those used in river conditions, such as theUS-B54 or US-BMH-80 samplers developed in the United States,or similar ones developed in other countries, may be used fortaking samples. For bed material composed mainly of coarsematerials, such as gravel and coarse sand on the flood plain or barsof a river, various random methods may also be used (Tang, 1992).

5.9.3.1 UNDISTURBED SAMPLING

Included in the apparatus used for taking undisturbed samples,ranging from simple to complicated equipment, are the ring-type,

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 113

axle-type, cylindrical revolving-type, gravity-core-type, pistontype, and vibration-type, etc. Each sampler has its particular rangeof application. A ring-type device is usually made of stainless steelpipe, 8 to 10 cm in height, with a sharpened knife-edge at one end.An undisturbed sample is taken with the ring on the exposedriverbed surface. The revolving cylindrical-type can be used inunconsolidated soft deposits (Bajiazui Reservoir ExperimentalStation, 1980). For shallow streams with fine bed material, theUS-BMH-53 sampler may be used. This sampler consists of astainless steel cutting cylinder, 5.1 cm in diameter and 20.3 cm inlength, with an internal retractable piston. For shallow streamscomposed of a slightly compacted river bed of fine material, thePhleger 840-A bottom corer may be used to take 3.5 cm coresamples (Durette, 1981). A gravity-core sampler can be used forsampling in deep water such as in a reservoir (Vanoni, et al.,1975). For detailed operating instructions, users of these devicesshould refer to the relevant manuals or specifications.

The pit method is suitable for an exposed river bed orflood plain. The procedure is to dig out a pit or hole of an appro-priate size. The volume of the pit is measured by weighing theamount of standard sand particles required to fill the pit and thepredetermined relationship between the volume and weight of thestandard sand. The unit weight of the deposit can then becomputed by weighing the sediment dug out of the pit. A cylin-drical ring with a knife-edge is used frequently for samplingdeposits composed mainly of fine particles. The volume of thecylindrical ring can be calculated by measuring its diameter andheight. After sealing the top and bottom of the sample, the sampletogether with the ring can be sent to the laboratory for the deter-mination of the unit weight as well as the moisture content(Vanoni, et al., 1975).

5.9.3.2 RADIOISOTOPE DENSITY PROBE

Measurement of the unit weight of sediment deposits may becarried out in situ with a radioisotope density probe, various typesof which are available. As with the nuclear gauge used in themeasurement of sediment concentration, the radioisotope probeshould be calibrated before its application in the field. The probecan be lowered from a raft by a cable or it may be fastened to theend of a drilling rod lowered along the outside pipe; by thesemeans, the unit weight of deposits in a lower layer can bemeasured directly in situ (Vanoni, et al., 1975).

5.9.3.3 SELECTION OF SAMPLING POINTS

Samples of bed material or deposits are usually taken along therange lines established for the sedimentation survey. The distancebetween sampling points is usually set at random and is preferablydetermined according to deposit thickness, although this may bedifficult to determine at the sampling time. The minimum numberof samples to be taken in a cross-section may be set at three formain channel widths of less than 500 m, and at five or more forwidths greater than 1 000 m. When the samples are taken on theflood plain, the number of sampling points required depends onthe deposit width over the flood plain and the variation in bedmaterial sizes. Ordinarily, the sampling points are evenly distrib-uted, or they can be distributed randomly.

As discussed in the previous section with reference tothe total sediment transport, the size and composition of the bedmaterial have an important influence on their transport and shouldnot be overlooked. The size and composition of an alluvial river

bed may change during floods, or it may change gradually when-ever the oncoming flow condition varies. The armouring effectdue to the coarsening of bed material during erosion is an impor-tant aspect that deserves thorough research. Sampling bed materialprovides valuable information on this subject and more samplesthan suggested above should be taken to achieve a better under-standing of the spatial distribution of bed material. The generallayout of sampling points can be arranged on a random basis ifthere are no other particular requirements.

For river beds composed mainly of gravel or even larger-sized particles, the sampling work should be carried out morecarefully than on sand beds, in order to obtain representativesamples. Grid or transect sampling procedures may be selected forsurface sampling in armour effect studies, as well as studies intothe initiation of motion and flow resistance. Samples may becollected by various samplers described elsewhere for subsurfaceexplorations that are related mainly to the study of bed materialtransport (ISO, 1981b).

5.9.4 Data processing5.9.4.1 COMPUTATION OF RESERVOIR CAPACITY OR AMOUNT OF

DEPOSITION OR EROSION IN RIVER REACHES

The frustum cone formula is a simple formula used generally forthis computation:

(5.46)

where V represents the volume or capacity occupied between twosections or two contour lines, A1 and A2 are the areas of sedimentdeposits or water at adjacent vertical sections or areas enclosed bycontour lines between which the volume is computed, V representsthe volume or capacity occupied between two cross-sections undera pre-assigned elevation or between two contour lines, A1 and A2are the areas of two adjacent cross-sections under the pre-assignedelevation or areas enclosed by contour lines between which thevolume is computed, and L is the distance between cross-sectionsor the interval between two contours. The difference of V for twosuccessive surveys in a reach gives the amount of deposition/erosion in a reach.

Let ∆A denote the difference of area under a certainelevation at the cross-section for two surveys. It is also the amountof deposition or erosion expressed in the area at the cross-section.If ∆A1 and ∆A2 do not differ by 40 per cent, the end area methodmay also be used:

(5.47)

In order to obtain consistent data from the preliminaryand successive surveys, the results obtained by the range-linemethod are correlated with those obtained by the topographicsurvey. Correction factors are found for every specific reach orportion of a reservoir and are applied to the results obtained by therange method in later surveys.

Two examples are shown in Figure 5.29 to demonstratethe characteristics of the reservoir capacity. The shaded area ingraph (a) represents the capacity between two elevations and thatin graph (b) represents the capacity between adjacent sections fora specific elevation, sometimes taken as the normal high waterlevel. Computations, of course, can be carried out using graphicalmethods (Vanoni, et al., 1975).

114 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

€

V A A A A L= + +( ) ⋅1

3 1 2 1 2

V A A L= +( ) ⋅1

2 1 2

In the Lower Yellow River, repetitive range surveys havebeen conducted for many years to monitor the sedimentationprocess, and experiment stations were established to study fluvialprocesses in some specific reaches. A software program(RGTOOLS) is now being worked on to incorporate the functionsof management of the database, examination of the reasonablenessof the surveying data, computation of the reservoir capacity oramount of sedimentation in river reaches, data processing and dataanalysis (Liang, 1999).

5.9.4.2 COMPUTATION OF CAPACITY FROM TOPOGRAPHIC

SURVEYS

There are two approaches in computing the reservoir capacityfrom topographic survey data: the point elevation-area method andthe conventional contour-area method. A computer program isused for efficient and economically feasible data evaluation. In thepoint elevation-area method, the surveyed area is large and has ahigh data density, as shown in Table 5.34.

With the development of electronic measuring andcomputerized collection and analysis systems, the contour methodof creating new reservoir topographic maps has become thepreferred method for reservoir sedimentation surveys. The UnitedStates Bureau of Reclamation uses the ARC/INFO package todevelop reservoir topography from the collected data, the aerialphotographic survey data. ARC/INFO is a software package forusing the GIS. Contours for the reservoir area at selected elevationintervals are computed from the compiled data using the TIN(triangular irregular network) modelling package withinARC/INFO. The Area-Capacity Computation Program is used to

generate elevation versus capacity and/or surface areas for thereservoir areas. The amount of deposition or erosion is the differ-ence of capacities under the specified elevation computed for twosurveys (Ferrari and Dorough, 1996).

5.9.4.3 UNIT WEIGHT OF SEDIMENT DEPOSITS

The unit weight of sediment deposits should be obtained to convertthe deposit volume into weight. It is also an important parameter inthe study of sediment transport. The unit weight of sediment is definedas the dry weight of sediment particles per unit volume of sedimentdeposit. Methods for determining unit weight in situ or in laboratorywere discussed in section 5.5. In general, undisturbed samples areobtained in the field and sent back to the laboratory for analysis. If itis difficult to obtain an undisturbed sample in the field, a disturbedsample may be taken instead and the unit weight may be estimated byempirical formulae from size analysis data.

The initial unit weight may be obtained by the followingempirical procedure: divide the sample into size groups and weigheach size group; mix each size group with water in separate cali-brated vessels and wait until the particles settle; the depositvolume may then be measured and the initial unit weight can becomputed. The result of an experiment conducted by Han, et al.,(1981) is shown in Figure 5.30.

The determination of unit weight through size gradationof deposits is suggested in literature. The initial unit weight in kgm–3 can be computed as follows:

W = WcPc + WmPm + WsPs (5.48)

where Wc, WM, Ws are the coefficients of unit weight for clay, siltand sand, respectively, in kg m–3 and Pc, Pm, Ps are the percent-ages of clay, silt and sand, respectively. For different operationmodes of the reservoir, the coefficients are given in Table 5.35.

In determining the dry density of sediment deposits aftercompaction, it is suggested that an additional value of unit weightshould be added to the initial value, as:

(5.49)

where T is in years and k is a constant based also on the type ofoperation and size gradation of sediment similar to the expressionfor the initial unit weight, as shown in Table 5.35.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 115

Table 5.34Number of point elevation data per km2 required in

topographic surveys

Quantity of point elevation data per km2

Detailed General Reconnaissancesurvey survey survey

Rough bottom 2 500–3 500 1 500–2 500 800–1 500

Relatively 1 500–2 500 800–1 500 400–800smooth bottom

Smooth bottom 800–1 500 400–800 100–400

Source: ISO, 1982b.

Figure 5.29 — Examples of reservoir characteristics and the computation of reservoir capacity.

W W kT

TT= +

−−0 0 4343

11. ( ln )

REFERENCESBajiazui Reservoir Experimental Station, 1980: Cylindrical

Revolving Sampler for Taking Undisturbed Samples of SoftMud.

Beaumont, P., 1978: Man’s impact on river systems: a world-widereview. Area, Volume 10.

Borland, W.M. and C.R. Miller, 1960: Distribution of sediment inlarge reservoirs. Transactions, ASCE, Volume 125.

Brune, G.M., 1953: Trap efficiency of reservoirs. Transactions,American Geophysical Union, Volume 34, Number 3.

Bureau of Hydrology, 1990: Development of River SurveyingSystems in the Changjiang River. Changjiang WaterResources Commission.

Bureau of Reclamation, 1987: Design of Small Dams. Thirdedition, Bureau of Reclamation, United States.

Churchill, M.A., 1947: Discussion of Analysis and Use ofReservoir Sedimentation Data. (ed.) Gottschalk, FederalInter-Agency Sedimentation Conference.

Dai, Dingzhong, 1994: River Sedimentation Problems .Chapter 12, Water resources development in China, (ed.)Qian, Zhengying, China Water and Power Press, Beijing,Central Board of Irrigation and Power, New Delhi.

Durette, Y.J., 1977: Hydac 100 — An automated system for hydro-graphic data acquisition and analysis. Technical Bulletin 105,Water Survey of Canada.

Durette, Y.J., 1981: Preliminary Sediment Survey EquipmentHandbook. Water Survey of Canada.

Ferrari, R. and W. Dorough, 1996: Chapter 3-4 — Measuringdeposited materials, and Chapter 3-5 — Determination ofvolume of deposits. International Conference on ReservoirSedimentation, Colorado State University, Fort Collins.

Ferrari, R. and W. Dorough, 1998: Cascade Reservoir 1995Sedimentation Survey. Sedimentation and River HydraulicsGroup, Water Resources Services, Technical Service Center,Denver.

Gottschalk, L.G., 1964: Reservoir sedimentation. Handbook ofApplied Hydrology, (ed.) V.T. Chow, McGraw-Hill.

Gu Wenshu, 1994: On the reduction of water and sediment yieldof the Yellow River in late years. International Journal ofSediment Research, Volume 9, Number 1.

Hakanson, L., 1978: Optimization of underwater topographicsurvey in lakes. Water Resources Research, Volume 14.

Han Qiwei, et al., 1981: Initial unit weight of reservoir deposits.Journal of Sediment Research, Volume 1.

Han Qiwei, 1990: A new mathematical model for reservoir sedi-mentation and fluvial process. International Journal ofSediment Research, Volume 5, Number 2.

Inland Waters Directorate, 1977b: SEDEX System OperationsManual. Water Survey of Canada.

Institute of Water Conservancy and Hydroelectric Power Researchand Beijing Municipal Bureau of Water Conservancy, 1986:Integrated Measures of Reducing Sedimentation in theGuanting Reservoir (in Chinese).

ISO, 1977: ISO Standard 4364. Liquid flow measurement in openchannels — bed material sampling.

ISO, 1981b: Technical Report on Methods of Sampling andAnalysis of Gravel Bed Material. ISOFFXC I 13/SC6N 152.

ISO, 1982b: Draft standard ISO/DIS 6421, Methods for measure-ment of sediment accumulation in reservoirs.

Jiao Enze, 1980: Shapes of reservoir deposit. Selected Papers ofYellow River Sediment Research, Number 4.

Lara, J.M. and E.L. Pemberton, 1965: Initial Unit Weight ofDeposited Sediments.

Liang Guoting, 1999: User’s Manual for RGTOOLS. Institute ofHydraulic Research, YRCC.

Lin Binwen, et al., 1982: Major causes of the deviations in evalua-tion of quantity of sedimentation by range-line method anddifference of sediment load method. Journal of SedimentResearch, Beijing.

Lin Bingnan, 1992: Watershed and sediment management inChina. Proceedings of the Fifth International Symposium onRiver Sedimentation, Karlsruhe.

Leeden, F. van der, et al., 1990: The Water Encyclopedia. SecondEdition, Lewis Publishers.

Long Yuqian and Li Songheng, 1995: Management of sediment inthe Sanmenxia Reservoir. Advances in Hydro-Science andEngineering, Volume II, Beijing.

Luo Minsun, 1977: Reservoir Delta and its Calculation (in Chinese).Miller, C.R., 1953: Determination of the Unit Weight of Sediment

for Use in Sediment Volume Computation. United StatesBureau of Reclamation.

Ministry of Water Resources, 1978: Tentative Standards forReservoir Sedimentation Survey. Beijing.

Petts, G.E., 1979: Complex response of river channel morphologysubsequent to reservoir construction. Progress in PhysicalGeography, Volume 3, Number 3, pp. 329-362.

116 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 5.30 — Variation of the unit weight with size(after Han, et al., 1981).

Dry

uni

t wei

ght (

t m–3

)

Sediment size (mm)

Table 5.35Unit weight as related to size gradation

Reservoir operation wc (kg m–3) kc wm km ws ks

Sediment submerged or nearly submerged 416 256 1 120 91 1 550 0

Normally moderate to considerably drawn-down 561 135 1 140 29 1 550 0

Reservoir normally empty 641 0 1 150 0 1 550 0

River bed sediment 961 1 170 1 550

Qian Ning, Zhang Ren and Zhou Zhide, 1987: Fluvial Processes.Kexue Press, Beijing (in Chinese).

Qian Zhengying, 1994: Water Resources Development in China.China Water and Power Press and Central Board of Irrigationand Power.

Sanmenxia Reservoir Experiment Station, 1980: OptimisticDensity of Ranges for Sedimentation Survey.

Shaanxi Institute of Hydrotechnical Research and TsinghuaUniversity, 1979: Reservoir Sedimentation. Water Resourcesand Electric Press, Beijing (in Chinese).

Shaanxi Provincial Bureau of Water Conservancy and SoilConservation, 1989: Techniques of Sediment Removal fromReservoirs. Water Conservancy and Electric Power Press,Beijing (in Chinese).

Shandong Provincial Office of Hydrology, 1980: Bed-loadTransport Estimated from Reservoir Sedimentation .Shandong.

Soil Conservation Service, 1973: National EngineeringHandbook. Section 3, Sediment, USDA, Washington, D.C.

Stevens, J.S., 1936: The silt problem. Transactions, ASCE,Volume 110.

Tang Yunnan, 1992: Study of Sampling Techniques of the BedMaterial on Gravel-bed Bars. Changjiang Water ResourcesCommission.

Task Group of Sanmenxia Project, 1994: Proceedings of theOperational Studies of Sanmenxia Project on the YellowRiver. Henan Renmin Press (in Chinese).

UNESCO, 1985: Methods of Computing Sedimentation in Lakesand Reservoirs, Paris.

Vanoni, V.A., et al., 1975: Sedimentation Engineering. ASCE,New York.

Williams, G.P. and M.G. Wolman, 1984: Downstream effects of damson alluvial rivers. Professional Paper 1286, USGS.

Working Group on Inventory of Reservoir Sedimentation inYellow River Basin, 1994: Report on Status of Reservoir

Sedimentation in Yellow River Basin. YRCC, Zhenzhou (inChinese).

Xia Maiding and Ren Zenghai, 1980: Methods of sluicing sedi-ment from Heisonglin Reservoir and its utilization.Proceedings of the International Symposium on RiverSedimentation, Beijing.

Xia Maiding, 1989: Lateral erosion — a storage recovery tech-nique of silted-up reservoirs. Proceedings of the FourthInternational Symposium on River Sedimentation, ChinaOcean Press.

Xia Zhenhuan, et al., 1980: The long-term capacity of a reservoir.Proceedings of the International Symposium on RiverSedimentation, Beijing.

Xiong Guishu, et al., 1983: Analysis of errors in the sedimentmeasurement in the Lower Yellow river. Proceedings of theSecond International Symposium on River Sedimentation,Nanjing.

Xu Mingquan, 1993: Strategy of reservoir sedimentation controlin China. International Journal of Sediment Research,Volume 8, Number 2.

Yellow River Conservancy Commission (YRCC), 1993: Survey onReservoirs in the Yellow River Basin (in Chinese).

Zhang Hengzhou, 1983: Sediment-controlling problems in Yunnanhydroelectric projects. Proceedings of the Second Interna-tional Symposium on River Sedimentation, Nanjing.

Zhang Zhenqiu and Du Guohan, 1984: The rational operation ofdrawdown flushing in the Shuicaozi Reservoir. Journal ofSediment Research, Number 4 (in Chinese).

Zhou Zhide and Wu Deyi, 1991: Sedimentation Management ofthe Tarbela Dam Project. International Research andTraining Centre on Erosion and Sedimentation.

Zhou Zhide and Yang Xiaoqing, 1995: Preservation of reservoirstorage capacity — experience of China. Proceedings of theInternational Reservoir Sedimentation Workshop, SanFrancisco.

CHAPTER 5 — RESERVIOIR SEDIMENTATION AND IMPACT ON RIVER PROCESSES 117

6.1 INTRODUCTION

6.1.1 Type of sediment loadSediment load may be classified as suspended load or bed loadaccording to the mode of movement in the river. Suspended load isthe sediment that moves in suspension in water under the influ-ence of turbulence. Bed load is the part of sediment load thatmoves in almost continuous contact with the streambed by salta-tion and traction, that is, by bouncing, sliding and rolling on ornear the streambed by the force of water.

According to its origin, or source of supply, the totalamount of sediment transported in rivers may be divided intotwo parts: wash load and bed material load. Wash load consistsof fine particles, which refers generally to sediment size finerthan 0.062 mm, and the amount depends mainly upon supplyfrom the source area. The discharge of bed material iscontrolled by the transport capacity of the stream, whichdepends upon bed composition and the relevant hydraulic para-meters. Wash load moves entirely in suspension, while the bedmaterial load may move either as temporarily suspended load oras bed load.

6.1.2 Network for measurement of sediment transportNetworks for stream gauging have been developed in many coun-tries to collect data relevant to the development and protection ofwater resources. In a sediment-laden river, sediment transport is animportant and significant item to be measured at a hydrometricstation within the framework of a stream gauging network.Observations are made of suspended and bed load discharge instreams with natural regimes as well as with regimes modified bymanagement activities. Stations that measure the sediment trans-port should function as components of the minimum stream flownetwork.

For studying sediment problems in a river system, sedi-mentation surveys in river reaches, reservoirs and/or in estuarineareas are also indispensable. Ranges or cross-sections spaced atappropriate intervals are usually set up to serve also as a part ofthe network for measuring sediment transport.

It is recommended in the Guide to HydrologicalPractices (WMO, 1994) that sediment discharge should bemeasured at 15 to 30 per cent of stations within the minimumnetwork of stream gauging stations. The minimum network stan-dard (expressed in area per station) is 1 000 to 2 500 km2 for flatregions, 300 to 1 000 km2 for mountainous regions, and 140 to300 km2 for small mountainous areas with very irregular precipi-tation. In arid regions or places where conditions are extremelydifficult, larger areas per station may be tolerated.

In general, the need to measure sediment discharge at ahydrometric station, or to conduct a sedimentation survey in ariver reach, is determined by the importance of the sedimentproblem in the development of water resources. It relates to a largeextent to the quantity of sediment transported in the river and thetemporal variation of this quantity. Measurements may be madeonly in the flood season at some of the stations. For some river

reaches, experimental or auxiliary stations may be set up to carryout detailed studies.

6.1.3 Classification of hydrometric stations for sedimentmeasurement

It is stipulated in the Chinese Standards for SedimentMeasurement in Rivers (Chinese Standards GB 50159-92,1992) that basic sediment measuring stations should beclassified into three categories, as follows (items of sedimentmeasurement and accuracy requirements in different class stationsare different):

Class I: Stations that play an important role in control-ling sediment yield from the drainage basin and are focal to thedesign and operation of major hydrological projects and to riverregulation or in the study of fluvial processes are classified asClass I stations in the stream gauging network. For Class Istations, suspended sediment discharge and sediment concentra-tion, size gradation of suspended sediment and bed materialshould be measured the whole year round. Bed load should also bemeasured at some of these stations by direct or indirect methods.

Class II: Stations at which the sediment yield is frommajor tributaries that are representative in the physio-geographicalregions in the drainage basin, or stations that are supplementary toa Class I station located on the main stem of the river, belong toClass II. The accuracy requirement for taking measurements islower than that required for Class I stations. Particle size gradationmust be measured at some of these stations. Sometimes, measure-ments may be conducted on a roving basis.

Class III: Stations at which the sediment yield is fromordinary or secondary tributaries are generally grouped into thiscategory. Stations that are representative of small watersheds witha drainage area less than 300 to 500 km2 in arid regions or 100 to200 km2 in wet regions also belong to this category. In Class IIIstations, simplified methods of taking measurements may be used,such that the sediment load in flood events is estimated withacceptable accuracy. Measurements are frequently taken on aroving basis.

This idea of classification of sediment measuringstations is useful in the planning and implementation of streamgauging networks.

6.1.4 Total loadThe purpose of sediment measurements at hydrometric stations orspecific locations in a river is to monitor the total sediment loadflowing through the section. Ideally, total load is the summation ofthe suspended load and the bed load, in view of the type of move-ment of the sediment. However, in practice, measurements cannotbe performed very well in zones very close to the river bed, wherethe sediment concentration is the greatest. Sometimes, there maybe an overlap in the portion of depth covered by the bed loadsampling apparatus, and part of the suspended sediment may beincluded in the sample collected by the bed load sampler.Furthermore, sediment covering a large range of areas in differenttypes of movement can have different types of behavour from a

CHAPTER 6

OPERATIONAL METHODS OF SEDIMENT MEASUREMENT

hydraulic point of view. The idea of total load should be kept inmind as a basis for taking sediment measurements.

6.1.5 Sedimentation surveysErosion in upland watersheds produces sediment in river systems.In the entire process of transportation from upland areas to the sea,the sediment may be deposited in some reaches, or scoured fromthe river bed at some other reaches. In managing the sedimentproblems in a river, the fluvial process, including the status ofsedimentation, must be well known. As far as the amount of depo-sition or erosion in a river reach is concerned, it is usually far lessthan the amount of sediment load transported through the riversystem. In some cases, the amount of deposition or erosion in ariver reach has an order of magnitude equivalent to the tolerancelimit of errors involved in the sediment measurement at hydromet-ric stations. Therefore, sedimentation surveys have to beconducted in the studied reach to provide more reliable and accu-rate data on the amount of sedimentation, rather than theestimation made with data obtained through hydrometric stationsand some other reconnaissance investigations. Measuring tech-niques are explained in Chapter 5.

The geomorphologic data of a river may be obtained byconducting a topographic survey, including a land survey andunderwater surveying, or by repetitive surveying on pre-deter-mined ranges. Besides the surveying data, bed material must besampled and its size distribution analysed. The dry density or unitweight should be determined with undisturbed samples that maybe collected occasionally. The data obtained through sedimenta-tion surveys in river reaches are irreplaceable for understandingthe fluvial process and the status of erosion or sedimentation ofthe river reach under study. They also provide basic data to studythe response of the river in its fluvial process to the modificationof incoming flow by human activities. A geomorphologic riverstudy provides a practical basis for the assessment, protection andenhancement of the physical environment of the river system. Apractical guide on the application of the geomorphologic approachto river management was provided by the Environment Agency ofthe United Kingdom (Universities of Nottingham, Newcastle andSouthampton, 1998).

6.1.6 Parameters to be collected for a complete sedimentdata set

For the collection of non-cohesive sediment data, guidelines wereissued by the American Society for Testing and Materials (ASTMD5387-1997) describing the parameters that should be measuredor collected to obtain a complete sediment and hydraulic data set.A complete data set should include the following parameters:(a) Sediment parameters: sediment discharge or sediment

concentration of suspended load; bed load; size distributionsof suspended load, bed load, bed material and their specificgravity;

(b) Hydraulic parameters: water discharge, velocity, width,depth and slope, gauge height;

(c) Other parameters: temperature;(d) Description of field conditions such as bed forms present at

time of data collection; methodology and instrumentation;site description.

If bed load is not measured, or the sediment load in theunsampled zone is to be evaluated, the data set can be used tocompute sediment transport using any prominently known and

verified transport formulae. With the data set, the total load maybe evaluated or estimated from the measured sediment load, forinstance by applying the Modified Einstein Procedure (Colby andHembree, 1955; Stevens, 1985).

6.2 MEASUREMENT OF SUSPENDED SEDIMENT6.2.1 Method of measurement6.2.1.1 MEASUREMENT OF SUSPENDED SEDIMENT DISCHARGE IN

A VERTICAL

Suspended sediment discharge over an entire cross-section isusually measured by dividing the cross-section into a number ofsections. Sediment discharge passing through each section isobtained by taking measurements along the vertical within theportion of the section it represents. It has been shown by field datathat the vertical distribution of sediment concentration for varioussize groups is quite different. Even for sizes finer than 0.062 mm agradient exists (Nordin, 1981). An example of the vertical distribu-tion of sediment concentration is shown in Figure 6.1.

The conventional methods used to measure sedimentconcentration in a vertical are sampling by point or depth integra-tion and/or in situ measurements. The measuring method is closelyrelated to the instrument used for taking the samples. Both time-integration samplers and instantaneous samplers are used for takingsamples. For a time-integration sampler, the nozzle of the samplerused for point or depth integration should be isokinetic, or, in otherwords, the velocity at the entrance of the nozzle should be equal orvery close to the ambient velocity. The same requirements are alsovalid for some in situ measuring apparatuses. Some apparatuses,such as nuclear gauges, ultra-sonic or vibration apparatuses, etc.,have been used for the in situ measurement of sediment concentra-tion. The measurement of sediment discharge at a point involvescollecting the accumulation of sediment in a specific period bymeans of apparatuses such as the Neyrpic sampler or the Delftbottle; these are integration samplers. By integration over a timeperiod, fluctuations of sediment concentration existing in naturalrivers may be minimized and temporal mean data can be obtained.(1) Sampling by point integration in a vertical

The selection of measuring points in a vertical has beenproposed by standards or manuals issued by various countries.The number of points can vary according to the depth of the riverand the size of sediment in suspension. In multi-point methods,it is common to sample at five points, i.e. at relative depths 0,0.2, 0.6, 0.8 and 1.0 (ratio of the depth of the sampler to the

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 119

Figure 6.1 — Vertical distribution of sediment concentration forseveral size fractions.

Sediment concentration of whole sampled <0.01d <0.025d <0.05d <0.1d = Sediment size (mm)

Concentration g.1–1

stream depth). In a frozen river, the bottom of the ice cover isused instead of the surface of the flow. Accuracy also depends onthe grain size of the suspended sediment and the shape of thedistribution curve.

In practice, in the interest of lessening the work involvedin taking and processing samples during a flood event, samplesmay be taken at fewer points, such as three points at relativedepths 0.2, 0.6 and 0.8, two points at relative depths 0.2 and 0.8,or one point at relative depth 0.5 or 0.6. A composite sample maybe obtained by direct mixing according to a proportion of thesamples determined through experiments in the field. In otherwords, the concentration at each point should be weighted againstthe proportion of discharge it represents. Such methods should beadopted only after their results are checked against measurementsobtained with multi-point or other more accurate methods.

The sediment discharge per unit width in each vertical isdetermined either by graphical integration of the product of veloc-ity and sediment concentration throughout the depth, or byEquation 6.1.

(6.1)

where qs is the sediment discharge per unit width in kg s–1 m–1, m isthe number of measuring points, Ci is the sediment concentration atthe measuring point as determined in a field laboratory or directlyby in situ instruments in g 1–1 or kg m–3, Vi is the velocity at themeasuring point in m s–1, d is the depth in m, ki is the fraction ofdepth each measurement represents, and n is the sum of theweighting factors at a vertical distance. In Equation 6.1, fractionsof depth ki are considered as a weighting factor to be applied to the

products of velocity and sediment concentration. Values of factor ki,as recommended in the Chinese Standards, are given in Table 6.1.

The average sediment concentration in a vertical can becomputed by dividing qs by q, the water discharge per unit width atthe vertical, which is obtained directly from discharge measurements.

It should be pointed out that the constant value of factorki is assigned to each measuring point purely by numerical inte-gration. In practice, no sample can be taken exactly at relativedepth 1.0. Sampling at the bottom of the river bed is usually takenwithin a varied relative depth ranging from 0.94 to 0.98 dependingon the structural design of the sampler, i.e. the lowest position ofthe sampler relative to the river bed. The gradient of concentrationfor coarse sediment is very large in the vicinity of the river bed.Hence, there is an error induced from the computation of sedimentdischarge by using the proposed factor ki. The error is a systematicerror in nature and should be minimal.(2) Sampling by depth integration in a vertical

Depth integration is usually performed with depth inte-grating samplers. Water and sediment mixture can then besampled continuously while the sampler is moving at a constant

120 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

qd

mk C Vs i

i

n

i i==∑

1

Table 6.1Value of factor ki

Measuring at relative depthNumber ofmeasuring points n 0 0.2 0.5 0.6 0.8 1.0in the vertical

5 10 1 3 3 2 13 3 1 1 12 2 1 11 1 1 or 1

Table 6.2Maximum transit rate ratios and depths for sampler bottle/nozzle configurations*

US sampler Nozzle size (mm) Nozzle color Container size (l) Maximum depth (m) Max.ratio Rt/Vm

DH-81 3.17 (1/8 in) White 0.4732 (1 pint) 4.57 (15 ft) 0.2

4.76 (3/16 in) White 0.4732 4.57 0.4

6.35 (1/4 in) White 0.4732 2.74 (9 ft) 0.4

7.93 (5/16 in) White 0.4732 1.83 (6 ft) 0.4

3.17 White 0.9464 (1 quart) 4.57 0.1

4.76 White 0.9464 4.57 0.2

6.35 White 0.9464 4.57 0.4

7.93 White 0.9464 3.05 (10 ft) 0.4

D-74 3.17 Green 0.4732 4.57 0.2

4.76 Green 0.4732 4.57 0.4

6.35 Green 0.4732 2.74 0.4

3.17 Green 0.9464 4.57 0.1

4.76 Green 0.9464 4.57 0.2

6.35 Green 0.9464 4.57 0.4

D-77 6.35 White 3 Liter 4.57 0.1

7.93 White 3 Liter 4.57 0.2

P-61 4.76 Blue 0.4732 54.86 (180 ft) 0.4

4.76 Blue 0.9464 36.58 (120 ft) 0.2

P-72 4.76 Blue 0.4732 21.95 (72 ft) 0.4

4.76 Blue 0.9464 15.54 (51 ft) 0.2

* Quoted from ASTM D6326-98. Here, only some versions are listed, for illustration. The standard United States samplers are designated by D for depth integration, P for point

integration, DH for hand-held depth integration, and also, by the year in which the version was developed.

transit rate along the vertical. If the ratio of intake velocity toambient velocity is equal to 1, the volume of samples at each pointwill be proportional to the local velocity. The sediment concentra-tion of the sample taken by the depth integration method is thedischarge-weighted average concentration in the vertical.

Sampling may be carried out by round trips of loweringand lifting or by just a single trip either from the surface to thebottom or from the bottom to the surface. Electromagnetic devicesmay be installed to open and close the intake. The transit rate oflowering and lifting the sampler should not exceed four-tenths ofthe mean velocity in the vertical and is also limited by the rate ofair compression in the sampling bottle. In order to obtain a repre-sentative sample, the container should not be filled entirely duringsampling.

In fact, the maximum transit rates are controlled by thecompression rate and the approach angle, and are functions of thesize of both the nozzle and the sampler container. It varies from0.1 to 0.4 Vm. For the United States, a series of isokineticsuspended samplers, maximum transit rate ratios and depths forsampler nozzle–container size configurations has been established.It appears in Table 6.2 [ASTM D6326-1998].

As illustrated by Edwards and Glysson (1999), a seriesof graphs used for determining the appropriate transit rate can beconstructed for various nozzle/container size combinations. As anexample, the graph as shown in Figure 6.2 is quoted. It was devel-oped for a nozzle size of 3/16 in (4.76 mm) and a container size of1 pint (0.473 l). For round trip depth integration, the transit rateused in raising the sampler need not be the same as the one used inlowering, but both rates must be kept constant.

6.2.1.2 MEASUREMENT OF SEDIMENT DISCHARGE IN A

CROSS-SECTION

(1) Selection of verticals based on the transverse distributionof concentration. The number of verticals required for sedimentdischarge measurements depends on the size distribution andconcentration distribution of the sediment, as well as on thedesired accuracy of data acquisition. Verticals should be spacedclosely in zones with large transverse variations in sedimentconcentration and in the main currents. In measuring sedimentdischarge, it is usual to measure the velocity simultaneously withthe sediment concentration. For new hydrometric stations, thenumber of sampling verticals is usually approximately half thosealong which velocities are measured. It is suggested in the Guideto Hydrological Practices (WMO, 1994) that for taking adischarge measurement, in general, the interval between any twoverticals should not be greater than one twentieth of the totalwidth, and that the discharge between any two sediment samplingverticals should not be more than 5 per cent of the total discharge.

(2) Multi-point and multi-vertical method. Thesemethods are used to determine as accurately as possible the sedi-ment concentration, size distribution and sediment discharge alonga vertical and across the entire section of a stream. They alsoprovide the basis for simplified measuring methods. Sampling bythese methods thus establishes the standard by which the adequacyof measurements made by other less detailed schemes or methodsis judged.

According to the requirement suggested in the ChineseStandard (Chinese Standard, 1992) quoted in Table 6.3, the accu-racy of conventional methods of sediment measurement should beassessed by conducting experiments at the station. If the error ofthe method currently in use exceeds the tolerable limits specifiedin the Standard, the method currently in use should be improved toreduce the error. The error limits specified in the table are some-what more restrictive than those used elsewhere.

For the middle or lower alluvial reaches of a river, flowsover flood plains often take place during floods. For a sediment-laden river, the distribution of water and sediment discharge in themain channel and the flood plain should be investigated. The sedi-ment distribution should be taken into account in the arrangementof the verticals. To shorten the duration of sampling, simplifiedmethods and a lesser number of verticals are usually used formeasurements over the flood plain.

(3) Selection of verticals based on equal dischargeincrement. In this method, verticals are arranged according to thedistribution of water discharge across the section. Each samplingvertical represents approximately an equal portion of discharge.The transit rate for each vertical may not be equal, but the samplevolume for each vertical should be kept approximately equal. Forround trip depth integration in a vertical, the transit rate during

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 121

Figure 6.2 — Example of transit rate determination using graphdeveloped for nozzle size 4.76 mm (3/16 in) and 1 pint samplecontainer (after Edwards and Glysson, 1999).

Table 6.3Allowable errors in sampling method along verticals in a cross-section

Relative standard error (%) Systematic error due to improper Systematic error due to insufficientStation class sampling methods in a vertical (%) numbers of verticals (%)

Sampling points Number of All sediment Bed material All sediment Bed materialin a vertical verticals

I 6.0 2.0 ±1.0 ±5.0 ±1.0 ±2.0

II 8.0 3.0 ±1.5 ±1.5

III 10.0 5.0 ±3.0 ±3.0

Transit rate divided by mean velocity

Dep

th (

feet

)

descending and ascending should be the same. The method isillustrated in Figure 6.3 and is known as the equal discharge incre-ment, or EDI, method. It is suggested in the Guide to HydrologicalPractices (WMO, 1994) that three to ten equal sections ofdischarge be selected. If the volumes of sediment-water mixturessampled at verticals are the same, a composite sample may beobtained by mixing all the samples to yield a cross-sectionalaverage sample from which the average concentration, as well assize gradation, can be determined by laboratory analysis. Thismethod is simple as regards sampling work and computation. Thedischarge distribution across the section must be estimated prior tothe sampling work. If the main current shifts its positionfrequently, or drastic scour or deposition takes place in the cross-section, sampling points representing equal portions of thedischarge should be promptly adjusted according to the variations.This may be difficult during floods.

(4) Selection of equally spaced verticals. The channelwidth at the water surface is divided into sections of equal widthcorresponding to the number of verticals required. The Guide toHydrological Practices (WMO, 1994) suggests that the wholewidth be divided into six to ten equal segments for taking depth-integrated samples. When the depth integration method isemployed, the transit rate of the sampler for all the verticalsshould be kept the same, that is, established at the deepest andfastest vertical in the cross-section. In round trip depth integration,the descending and ascending transit rates should also be kept thesame. The same nozzle is used at all verticals. The sample bottleshould not be allowed to fill completely. Ideally, the samplevolume will be directly proportional to the water discharge repre-sented by the vertical. The average concentration in thecross-section will be the concentration of the composite samplemade up by combining all samples at the cross-section. Thismethod, known as the equal width increment (EWI) method, isillustrated in Figure 6.3. It has an advantage over the EDI methodin that the distribution of flow in a measuring section is not neededbefore sediment samples are taken.

(5) Simplified Method. During a flood, adequatesampling using conventional methods may not be carried out dueto rapid changes in both discharge and sediment concentration.Hence, there is a need to develop a sampling method of greaterease and simplicity of operation to take samples to define thetemporal variation of concentration during the entire flood. Such asimplified method is called an index-sampling method. In theUnited States, it is sometimes referred to as the Box Sample.

Index samples should be taken at the same time that theconventional method is being used. The concentration of the indexsamples is correlated with the cross-sectional average concentra-tion obtained by a conventional method. If the relationship isstable, the ratio of cross-section concentration to index sampleconcentration is plotted against discharge or stage and used toconvert the index sample concentration to the cross-sectionalaverage value. Various methods for collecting index samples havebeen employed in rivers with different characteristics. Obviously,some of these are rather complicated and may even be consideredas conventional methods. It has been shown from actual data thatthe sediment distribution varies with flow conditions.

In small streams, structures already in existence or builtespecially can be utilized to take sediment measurements byinstalling sampling apparatuses or in situ instruments. Pumpingsamplers of various designs, radioisotope gauges, turbidity metersand depth-integration samplers have been used by many countriessuch as Indonesia, Italy, the United Kingdom and the UnitedStates, etc. (International Association of Hydrological Sciences(IAHS), 1981). These devices can be used to monitor the varia-tions in sediment concentration during flash floods. Samples takenby such devices are equivalent to index samples.

In some cases, the sediment carried by a current ismostly wash load and the distribution across the whole cross-section is fairly uniform. Samples taken at any point in a cross-section should be representative of the average value. However,for large alluvial rivers the situation is more complicated. Inreaches where erosion and deposition may take place on a largescale, an index sample taken at a fixed point in the river cannot beexpected to be representative, and the relationship of the concen-tration of the index sample to that of a cross-sectional averagesample will not be stable.

In summary, there are no definite and reliable rules forthe selection of measuring points for taking an index sample. Itserves as a supplement to the conventional method for measuringsuspended sediment discharge. By analysing data obtained byprecise and/or conventional methods, the sampling position maybe chosen, bearing in mind the desired accuracy. The followingare recommended:(a) When the variation in sediment along the transverse direc-

tion is relatively large, three to five verticals arranged in anequal discharge increment basis should be used for takingindex samples whenever possible. Depth integration ispreferred, but sampling at one to three points may be used;

(b) If the river bed in a wide river undergoes drastic changesduring floods, it would be impractical to determine thecentroid of equal portions of discharge accurately. Three tofive verticals covering roughly the deepest parts of thesection may be selected for taking samples. Depth integra-tion is preferred, but sampling at one or two points in eachvertical may be used. Samples should be combined for labo-ratory analysis;

122 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 6.3 — Sketch of methods for measuring sediment dischargeusing the depth integration method.

Equal discharge increment (EDI)

Samples taken atvertical throughcentroid of areas ofequal discharge

Equal transit rate forall verticals

Cross-sectional averageconcentration obtained bycomposite sample

Sample transit rateadjusted so that equalsample volumes aretaken at each vertical

Equal width increment (EWI)

(c) Three verticals at 1/6, 1/2 and 5/6 of the stream width formountain streams, or arranged by other appropriate divisionssuch as one in the main current, and two on both sides, etc.,are proposed by the Indian Standard (1966);

(d) In a flood event, when more verticals are precluded fromuse, one vertical may be used in order to shorten the durationof the measurement. One vertical located near the maincurrent is sometimes used to represent the cross-sectionalaverage conditions. It can be seen from the example of fielddata showing the transverse distribution curve of sedimentconcentration in Figure 6.4 that there are verticals located oneither side of the main current at which the ratio of localsediment concentration to the cross-sectional averageconcentration equals unity. The exact position varies withvelocity and sediment concentration. However, if it varieswithin a narrow range and is relatively stable, the verticalmay be used for taking the index sample with fair accuracy.When the transverse distribution of sediment is fairlyuniform, the sampling position may be fixed at a point deter-mined by analysing actual field data. During a flood event,one vertical located near one bank is allowed only when thepre-assigned position for sampling is inaccessible. Also,sampling on the water surface can only be allowed if othermethods cannot be used under practical conditions. Resultsshould be corrected by analysing more detailed actual obser-vational data.

6.2.1.3 SAMPLING FOR SIZE ANALYSIS

The purpose of sampling for size analysis is to provide informa-tion on temporal variations in grain size and to compute thesediment discharge of each size group. The distribution of sedi-ment size along a vertical and across a transverse section can alsobe used to assess the accuracy and reliability of the measurementof suspended sediment discharge.

A precise method for determining the size distributionover a cross-section may also be simplified so that more samplesmay be taken during a flood period. In general, samples used fordetermining concentration are used for determining size gradation.In selecting simplified methods, including methods for taking

index samples, the representatives of size distribution of thesample should be considered.

Along with velocity and channel shape, etc., sedimentsize is a major factor influencing the non-uniform distribution ofsediment concentration across a section. If coarse particles, suchas those greater than 0.062 mm, constitute only a small fraction ofthe total suspended sediment, the concentration obtained by asimplified method may be representative of the total suspendedsediment, but not for coarse particles. Vertical and transversedistribution of suspended sediment is affected by hydraulicelements such as water depth, slope, etc., as well as sediment char-acteristics such as grain size. The exponent z in the expression ofsediment distribution in a vertical based on diffusion theory maybe used as an index (ISO, 1977b; Vanoni, et al., 1975):

(6.2)

In Equation 6.2, ω is the average settling velocity for thesize group under study, κ is the Karman constant, and U* is thefriction velocity.

For sizes finer than 0.1 mm, settling velocity varies withthe square of particle diameter. Under the same hydraulic condi-tions, the value ω or z may differ one-hundred-fold for particles of0.1 and 0.01 mm in diameter. Different patterns of sediment distri-bution are found for these two size groups: for 0.01 mm sediment,the vertical and transverse distributions are rather uniform, whilefor 0.1 mm sediment, large gradients exist in a vertical and acrossthe stream. Errors, which may be involved in adopting simplifiedmethods, should not be overlooked when coarse sediment particlesare present in appreciable amounts.

When selecting a measuring method, a compromise hasto be made between simplification and desired accuracy. Ingeneral, the selection of measuring verticals or points for indexsampling has to take the characteristics of size distribution intoaccount if a better understanding of the sediment transport ofvarious size groups is desired. Errors which may be induced bythe simplified methods will be discussed in later sections.

Judging by the experiences gained from field observa-tions on sediment-laden rivers, the variation in particle size withtime may be less than the variation in sediment concentration.As far as sampling frequency is concerned, more samplingshould be carried out during floods in order to define clearly asediment hydrograph. If samples are expected to be representa-tive of both concentration and grain size, a composite sampletaken on the basis of equal portions of discharge by combiningmulti-point samples or depth integrated samples is recom-mended. Errors involved in simplified methods, such as anindex sample taken at one point in a vertical, would be toolarge, particularly if the sediment load contains an appreciableamount of coarse particles.

6.2.1.4 FREQUENCY AND TIMING OF SAMPLING

The desirable timing and frequency of sampling depends on therunoff characteristics of the basin. For many streams, an averageof 70 to 90 per cent of the annual sediment load is carried downthe river during the flood season. Suspended sediment should besampled more frequently during the flood period than during lowflow periods. During floods, hourly or even more frequentsampling may be required to define sediment concentration accu-rately. During the rest of the year sampling frequency can be

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 123

Figure 6.4 — Transverse distribution of sediment concentration.

€

zU

=∗

ωκ

reduced to daily or even weekly sampling. For watersheds with awide variety of soil and geological conditions and an unevendistribution of precipitation, sediment concentration in the streamdepends not only on the flood event in the year, but also on thesource of the runoff in the basin. Under such conditions, no defi-nite sediment measurement schedule can be assigned. Besides, thesampling of sediment concentration should be properly timed tocheck the temporal variation in sediment. In general, the accuracyneeded from the sediment data determines how often a streamshould be sampled. The greater the required accuracy and themore complicated the flow system, the more frequently it will benecessary to take measurements.

6.2.2 Computation of sediment dischargeWhen point samples of suspended sediment are taken for eachvertical, the sediment discharge per unit width is obtained byEquation 6.1. Sediment discharge of the entire cross-section canthen be computed by integration of the sediment discharge perunit width along the entire width of the stream. In practice, this iscarried out by summing the products of the sediment discharge perunit width and the section width each vertical represents.

If the sampling is conducted using the depth integrationmethod (either the EDI or EWI method), all samples are combinedinto a single representative discharge-weighted sample. The sedi-ment discharge in the entire cross-section is then computed as:

Qs = Cm Q (6.3)

where Qs is the sediment discharge of the entire cross-section inkg s–1, Q is the water discharge expressed in m3 s–1, and Cm repre-sents the cross-sectional average concentration expressed inkg m–3. If other units are used in expressing the parameters, acoefficient must be applied.

Some types of instruments, such as the Delft bottlesampler or the Neyrpic sampler, can only sample the accumulatedsediment passing into the sampler nozzle over a certain period oftime. Sediment discharge per unit area can then be computed bydividing the weight of sediment accumulated by the sampling timeand also by the area of the intake nozzle and the efficiency of thesampler (Jansen, et al., 1979).

The computation of average size gradation along a verti-cal or over the entire cross-section can be calculated by weighting

124 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 6.4Classification of suspended-sediment samplers

Classification Operation Basic Sample Description Depthfeature volume (L) limitation

May be opened or closed by springInstantaneous Point Sampling Horizontal 0.5, 1.0 or 2.0 dropping hammer or electro-magnetic None

switch; operated by rod or suspendedby cable

Pressure 0.47 US-D or US-DH series with nozzles 4.5 m round tripadjusted by in three different sizes;chamber 0.5 Bottle sampler with intake nozzle 4.5 m round trip

pointing to the flow and air exhaust;Depth integration

Pressure Plastic nozzle exchangeable; whileadjusted by 1.0–8.0 used in deep water the volume of Depends oncollapsible sample may be increased by using bag sizebag large plastic bags

Pressure 0.47 or US-P series with nozzles in three 25–40 m,Integration adjustable by 0.94 different sizes; max 55 m

opening orclosing value 1.0–2.5 JLC or AYX series with 4 mm nozzle

Pressure 1.0–3.0 Plastic nozzle exchangeable; Depends onadjusted by plastic bag bag size

Point integration collapsible 1.0–2.0 Nozzle exchangeable; rubber bag Depends onbag specially made bag size

Intake Practical Vacuum chamber used forvelocity may no limit adjusting pressure; may be Nonebe adjusted used near bed surface

Intake velocity adjusted byvarying pump speed; may be Noneused near bed surface

0.47 Single-stage sampler; used inflashy streams

Direct Water flows Delft bottle or Neyrpic-typeAccumulation accumulation out while for measuring suspended bedof sediment of sediment sediment material; discharge correction

retained factor should be applied

the amount of sediment in each size class in each sample accord-ing to the flow rate represented by the sample. The sediment sizesshould be divided into groups to meet data analysis requirements.In some countries, they are divided into three size groups, such assand (2.0 to 0.062 mm), silt (0.062 to 0.004 mm) and clay (finerthan 0.004 mm). If necessary, the number of size groups may beincreased. In India, suspended sediment coarser than 0.075 mm(Indian Standard 6339, 1971) is classified as coarse. In the ISOstandards (ISO, 1982a), the division line between coarse and finesediment is set at 0.06 mm.

6.2.3 Measuring devices and instrumentation6.2.3.1 SAMPLER FOR TAKING REPRESENTATIVE SAMPLES

Since 1947, a series of suspended sediment samplers, designed onthe basis of time integration and isokinetic nozzles, have beendeveloped through the Federal Interagency Sedimentation Project(FIASP) in the United States. This series of standard samplersincludes samplers with different types of suspension, i.e. usingrods or cable reels; different container sizes, i.e. 1 pint (0.473 l) or1 quart (0.946 l); and different construction materials, i.e.aluminium, bronze or plastic. A set of exchangeable nozzles withdifferent sizes varying from 0.3 to 0.8 cm is available for mostsamplers. In addition, epoxy-coated versions of all samplers areavailable for collecting trace metal samples (Edwards andGlysson, 1999).

Later on, various similar samplers were also developedin other countries, however, they are not listed in the Table. Basictypes of samplers are classified in Table 6.4. Although they maydiffer in structural design, type of suspension, sample volume, andnozzle size, etc., they may be classified in one of the categorieslisted in the Table 6.4.

Samplers are selected to meet data collection require-ments in consideration of suitable measuring methods. More thanone type of sampling device is sometimes found at key hydromet-ric stations, to meet various flow conditions. A comparison of theresults obtained with different samplers should be made if consis-tent data are to be obtained. Sometimes, it may be necessary tomake small modifications to the sampler to cope with local riverconditions, without sacrificing their basic properties.

Samplers designed on the basis of time integration havebeen widely adopted all over the world. Random errors due tofluctuations may be eliminated to a certain degree, improving thereliability of the results. During flash floods or the frozen season,when abundant debris or ice floes exist in the flow, which mayblock the intake nozzle of an integration-type sampler, instanta-neous samplers may be used instead. Instantaneous samplers arealso used when sediment concentration is very high, because theyare simple and easy to operate; however, errors due to fluctuationsin velocity and sediment concentration are inevitable and shouldbe compensated by repetitive sampling.

6.2.3.2 BASIC REQUIREMENTS FOR AN IDEAL SAMPLER

The basic requirements for an ideal sampler may be summarizedas follows:(1) The intake velocity of the nozzle for a time-integration

sampler should be equal or close to the ambient velocity. Toensure sampling accuracy, it is better to calibrate the intakevelocity of the nozzle. It has been proven by experiment thatthe error in the measurement of sediment concentration isless than 5 per cent if the ratio of the intake velocity to

ambient velocity is kept within 0.8 to 1.2 (USGS, 1976). It isspecified in China that the ratio should be 0.9 to 1.1 at aconfidence level of 75 per cent in flows with a velocity lessthan 5 m s–1 and a sediment concentration less than30 kg m–3. For flows with very high sediment concentra-tions, the ratio would fall below the above range, however,no appreciable differences in the observed sediment concen-trations have been found;

(2) The sampler should be able to collect samples close to thebed so that the unsampled zone can be kept as small aspossible. The distance from the centerline of the nozzle tothe bottom of the sampler should preferably be less than15 cm. This figure is 10 to 12 cm for American seriessamplers;

(3) Enough weight should be available for the sampler to main-tain its stability under water. Ease of operation andmaintenance is essential;

(4) The sampling volume should be sufficient to fulfil minimumrequirements for determining concentration as well as sizegradation. Repetitive sampling may be necessary to fulfil theminimum requirements for sample quantity.

In the design of a time-integration sampler, the intakevelocity is adjusted by pressure equalization in the samplercontainer. Limitations as to the depth within which the adjustmentis effective should be strictly observed. For instance, presentAmerican point-integration samplers can be operated to a depth of16 to 37 m, with a maximum of 55 m, while the United Statesdepth-integration sampler series can be used to a flow depth ofless than 4.5 m for round trip operation (Edwards and Glysson,1999).

6.2.3.3 SOME DEVELOPMENTS OF MECHANICAL DEVICES

Collapsible-bag samplers have been developed in the UnitedStates and China. One version of the American bag sampler, witha prefabricated plastic cap incorporated with an intake and air ventnozzle, is attached to a plastic bottle in which a lightweight plasticbag is inserted as a collapsible bag. With a newly developed sole-noid valve, it can be used either as a depth-integration orpoint-integration sampler (Stevens, et al., 1980; Szalona, 1982). Anew series of bag samplers that are more streamlined and have alower unsampled zone is now being developed in the UnitedStates. The Chinese version uses a specially made rubber bag asthe collapsible bag. By intercomparison, it was found that theyperform similarly and the concentration of samples taken by bothsamplers corresponded closely in flows with a velocity from 0.7 to3.0 m s–1 and concentrations from 4 to 90 kg m–3 [Long andNordin, 1989]. This type of sampler apparently has potential foruse in the field.

Automatic pumping devices have been used in smallrivers, canals and experimental basin outlets, etc. One of the char-acteristics of this type of sampler is its ability to collect samples atregular time intervals or in response to a rise or fall in stream flowat a definite point in the river. The entire variation in sedimentconcentration during a flash flood may be followed. Sufficientsamples can be obtained automatically to define the variations insediment concentration during a flood. It is particularly useful forstations located in remote areas. However, all automatic pumpingsystems are vulnerable to pipe blockages and may also requireefficient flushing systems. Different versions of the automaticpumping sampler developed from 1969 to 1982 have been tested

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 125

and evaluated by FIASP. It was found that almost all of the typeswere not isokinetic samplers, and improvements were needed toovercome shortcomings (Edwards and Glysson, 1999).

Portable pumping samplers may be used for takingpoint-integrated or depth-integrated samples at any point or verti-cal in a cross-section. A sampling nozzle may be mounted on thestreamlined sounding weight, together with velocity- or depth-measuring devices such as a propeller meter or an echo soundertransducer, etc. A device for measuring sediment and velocitydistributions in rivers and estuaries has been described byCrikmore (1981). A pumping sampler with an attached filteringdevice has also been developed and used in Pakistan.

6.2.3.4 SOME DEVELOPMENTS IN THE IN SITU MEASUREMENT OF

SEDIMENT CONCENTRATION

In situ monitoring of sediment concentration has been developedand applied in some countries with promising results. Themeasurement of sediment concentration by in situ nuclear gaugeshas been carried out in some rivers in Italy, Hungary, Poland andChina. In general, the following features are common for varioustypes of radioisotope gauges (Berke and Rakoczi, 1981; Lu Zhi,et al., 1981):(1) Range of measurement: different ranges are specified for

gauges of various designs. The lowest detectable concentra-tion within the tolerance of allowable error for hydrometricmeasurements is in general 0.5 g 1–1. The maximum concen-tration may well exceed 1 000 g 1–1;

(2) Accuracy and reliability are ensured by calibration at certainintervals of time or by comparison with traditional samplingmethods. The result of a field experiment indicates that thelower the concentration, the greater the relative error;

(3) The measured zone for portable nuclear gauges may extendto only 5 cm from the bed. In general, the unmeasured zoneextends 15 cm or more from the bed;

(4) Am241 or Cs137 is used as the source;(5) A continuous record of the temporal variation in concentra-

tion may be achieved by installing the sensor at a definitepoint in the cross-section. This is one of the advantages withwhich none of the existing apparatuses can compare;

(6) Sampling still has to be performed for size analysis.The development of the photoelectric turbidity meter is

based on the principle of attenuation of light transmitted throughsediment-laden water. From light scattering theory, the photo-density (the ratio of intensity of the transmitted light and incominglight, I/Io) depends not only on the concentration but also on theparticle size existing in the medium. It would be possible to estab-lish a relationship between the sediment concentration and aphoto-density reading only if the grain size were relativelyconstant. In operation, the instrument must be calibrated carefullyto establish such a relationship. Determination of sedimentconcentration on the basis of the photoelectric effect can only beadopted in rivers where variation in grain size is very small andthe concentration is fairly low. The upper limit of application is 1to 5 g l–1 (Brabben, 1981; Grobler, 1981).

There are two types of turbidity sensors based on lightscattering and absorptiometry (light attenuation). The former ismainly of value for the lower end of the turbidity range below0.5 g l–1, but can be relatively sensitive to variations in sedimentproperties. The absorptiometric systems tend to extend furtherup the turbidity range but are less sensitive at the lower concen-

tration end. Some works have used both systems in parallel(Leeks, 1999).

A vibration device was developed at the Institute ofHydraulic Research, YRCC. The apparatus has been installed atSanmenxia Hydropower Station for monitoring the sedimentconcentration passing through turbine runners (Ma and Zhao,1994). The Institute of Civil Engineering of the University ofFlorence, Italy, has developed an optical ultrasonic device tomeasure sediment concentration and mean particle size in thefield. By taking relative readings on two meters reflecting theultrasonic effect and the photoelectric effect, respectively, sedi-ment concentration and particle size can be interpolated by graphsobtained by calibration in the laboratory (Billi, et al., 1981).

A method has been developed based on the scattering ofultrasound (4.4 MHz) from suspended sediment particles. Bymeasuring the frequency as well as the intensity of the Dopplersignal within a sediment suspension, both the velocity and thesediment concentration can be measured simultaneously. It isreported that the instrument has been successfully applied foroffshore measurements (Jansen, 1978).

For low sediment concentrations such as those foundunder tidal conditions, a method is required which permits thesampling and handling of a large volume of water (for example,50 1) in order to obtain a reliable average value of the concentra-tion. Delft Hydraulics Laboratory has developed a pumpingsampler that is interfaced with a device for the separation of waterand sediment using a filter method. Sample volume is determinedby means of a calibrated vessel. Comparisons with the acousticDoppler method in the field gave satisfactory results (van Rijn,1980). As discussed in the previous section, an efficient flushingsystem is required to prevent pipe blockages.

The new developments in the measurement of sedimentconcentration cited in the above examples show promising results.Needless to say, these instruments are still in the process of beingdeveloped. More research work has to be done before they can beadopted for use in routine work.

6.2.3.5 INTERCOMPARISON OF MEASURING DEVICES

To ensure accurate and comparable results, observations withconventionally used sampling devices and/or in situ measuringinstruments should be compared for the standardization of sedi-ment samplers. The need for a better understanding of theprobable error involved in sediment measurement further empha-sizes the importance of the intercomparison of sedimentmeasuring devices. For time integration samplers, the hydraulicefficiency of the nozzle should be checked prior to its adoption forroutine work, both in the laboratory and in the field. Samplingefficiency may also be checked by comparison with a referencenozzle that has a sampling efficiency of 100 per cent.

It is recommended that comparisons be made by meansof parallel sampling with traditional samplers and new samplersbefore the latter are adopted. Attention must be paid to operationaltechniques to avoid any systematic errors. When data are collectedfor intercomparison, several samples should be collected andanalysed to minimize errors due to fluctuations. In situ measuringdevices have to be checked for deviations from the calibrationcurve determined previously in the laboratory. It is suggested thatthe results of parallel sampling (including measurements taken byin situ apparatuses) should not deviate by ±5 per cent at the 75 percent confidence level.

126 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

An intercomparison of four point-integration samplerswas made jointly by the Delft Hydraulics Laboratory(Netherlands) and the Cerni Institute (Yugoslavia) on the DanubeRiver near Belgrade in 1979. Velocity of the stream at thesampling point was approximately 1.0 m s–1. The sedimentconcentration was 0.1 to 0.2 kg m–3 with a mean diameter of0.2 mm (Dijkman and Milistic, 1982).

Through a WMO project, an intercomparison ofsuspended sediment samplers has also been carried out at Chutuohydrometric station on the Changjiang River in China, in whichseveral point integration samplers developed by different Chineseagencies were inter-compared and a USP61 type sampler was usedas a basis for comparison (Gao and Li, 1988). The sedimentconcentration at the site is in general several kilograms per cubicmeter. Later, similar work was carried out at Tongguan Station inthe Yellow River, where the sediment concentration is much higher.The results of the intercomparison are informative, not only withregard to the results from sediment transport values, but also for thecharacteristics and performance of the various samplers. It wasfound that at low sediment concentrations, the performances of theproperly designed point integration samplers are similar and themeasured sediment concentrations are comparable. For concentra-tions of more than 30 kg m–3, the ratio of the intake velocity toambient velocity is less than 1. It appears that further studies areneeded on the performance of an integration-type sampler inheavily sediment-laden flow (Gao and Li, 1988).

6.3 MEASUREMENT OF BED LOADBed load movement is an important type of sediment transport inrivers. The bed load, composed mainly of coarser particles, hasimportant effects on the fluvial process, even though its quantitymay be not as large as that of the suspended load. Bed load move-ment is quite uneven in both the transverse and longitudinaldirection and fluctuates considerably. In practice, it is more diffi-cult to measure the bed load discharge accurately than it is tomeasure suspended load. Research into the improvement ofsampling techniques is necessary.

6.3.1 Direct measurement of bed load dischargeThe direct method measures the bed load discharge by takingsamples directly from the stream with a properly designedsampler. Apparatuses or samplers used in the direct method maybe classified into the basket-type, pressure-difference-type, pan-type and pit-type categories. The weight of the sample taken bythese samplers in a specific time interval represents the bed loaddischarge over the width of the sampler.

The advantage of the direct method is that thesamplers are portable and are relatively easy to operate if properhoisting facilities are available. Temporal and spatial variationsmay be observed, and the sampling work may be laborious andtime-consuming. Sampling efficiency should be obtained bycalibration in laboratory flumes and also in the field when thebed load discharge can be determined by other reliable methods.The efficiency of a sampler is defined as the ratio of the quan-tity of sediment trapped in a bed load sampler to that beingactually transported as bed load in the space occupied by thesampler. Efficiency varies greatly from 10 to about 150 per centfor different types of samplers (ISO, 1981a; Hubbell, 1964;Xiang, 1980).

6.3.1.1 CHARACTERISTICS OF BED LOAD MOVEMENT

The factors affecting bed load transport are the hydraulicconditions in a river reach (velocity, depth and width, slope,size, shape and unit weight of bed composition, and morphol-ogy of bed forms, etc.) and the availability of sediment fromthe source area. Measured data appear to be rather random innature, with large fluctuations under relatively stable hydraulicand supply conditions. Figure 6.5 presents an example of thevariations in bed load discharge as measured in the field(CWRC, 1980).

Generally speaking, the bed load discharge increasesvery rapidly with increasing velocity. Consequently, the temporaldistribution of the bed load is characterized by its intensive trans-port during the flood season, particularly during several heavyfloods. For example, at Wutongqiao Station on the ChangjiangRiver in China, 60 per cent of the total bed load in 1965 wascarried down the river in just one day.

The spatial distribution of the bed load transport rateover a cross-section is also not uniform. Heavy transport maytake place over only fractions of the bed width, while the trans-port rate outside these strips may be very small or seem toapproach zero. Although bed load transport is strongly influ-enced by local currents and the availability of bed materials, it isquite common that the maximum velocity occurs within a stripother than where the bed load transport is the highest. Anexample of the transverse distribution of bed load transport forsand and gravel measured at Yichang Station on the ChangjiangRiver is shown in Figure 6.6.

The variation in bed load transport rates along the rivercourse is also pronounced. Measured data in the East Fork River inthe United States reveal that there is an orderly progression in bed

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 127

Figure 6.5 — Fluctuations in bed load discharge measured in the field(CWRC, 1980).

Figure 6.6 — Transverse distribution of bed load transport ratemeasured at Yichang Station, Changjiang River, China).

Width (m)

Tra

nspo

rt r

ate

for

grav

el (

kg s

–1)

Bed

load

dis

char

ge

(g s–1 m–1)

Time

load transport rate from pool to riffle, reflecting the phenomenon ofthe temporary storage of bed material (see Figure 6.7) (Emmett,et al., 1981, 1983).

6.3.1.2 FREQUENCY OF MEASUREMENTS

The frequency of measurements depends on the data requirementsfor the computation of the total amount of bed load discharge for aspecific flood period. The measurement of bed load discharge overan entire cross-section is laborious and time-consuming. In themeasurement of suspended sediment, simplified methods areusually adopted for routine work. However, fluctuations observedin bed load transport are far larger than those in suspended sedi-ment. Simplified methods may induce appreciable error andshould not generally be used.

In general, the measurement of bed load dischargeshould be planned to cover a large variation in water discharge.The frequency of measurements should be much higher duringfloods than in the low flow season. If bed load measurementcannot be carried out satisfactorily during the rising limb of alarge flood, the bed load discharge may be extrapolated from thedischarge-to-bed load transport relationship established under low

and medium flow conditions. On the Changjiang River, in China,the number of measurements taken in a year to monitor the entireprocess of bed load transport usually exceeds 100.

6.3.1.3 SELECTION OF SAMPLING VERTICALS

Sampling verticals are chosen to check the transverse variation ofbed load movement. According to the experience gained in theChangjiang River, sampling verticals should be in conformity withthe transverse distribution of the bed load transport, i.e. moreverticals, less than 15 m apart, are placed within the zone whereintensive bed load transport takes place, or any two adjacent verti-cals should cover no more than 15 per cent of the total bed loadtransport, and three to five repetitive samples are taken in eachvertical. Only a few verticals are placed in the weak bed loadzone. The portion of the bed where intensive bed load movementoccurs should be identified by trial, prior to selecting the samplingverticals (Huang, et al., 1983).

Experience gained in the East Fork River, in the UnitedStates, has shown that the collection of about 40 individual bedload transport rate measurements in a cross-section is, in mostcases, practical and economically feasible. Three differentmethods have been used. In the first method, called the singleequal-width increment (SEWI) method, samples are collected ateach traverse in a round trip at 20 equally spaced intervals in thecross-section. In the second method, called the multiple equal-width increment (MEWI) method, samples are collected to and froat four or more evenly spaced verticals, taking one sample at eachvertical in one traverse until a total of 40 samples are collected. Inthe third method, called the unequal width increment (UWI)method, samples are taken at unequal space width increments untila total of 40 samples are collected. It is clear that the SEWImethod is appropriate to define the transverse distribution in bedload transport rate, whereas the MEWI and UWI methods aremore effective to define the temporal variations at each vertical(Edwards and Glysson, 1999).

The duration of sampling, namely the time the sampleris left on the river bed to take a sample, is limited by the transportrate and the volume or capacity of the sampler. In general, thequantity of a sample should not exceed two thirds of the effectivevolume of the sampler. The experiment conducted in theChangjiang River shows that the duration is preferably 3 to 5 minfor gravel, and 0.5 to 3 min for sand bed load.

Owing to the extreme variability of the bed load move-ment at different sites, at present it would be difficult to set up

128 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 6.7 — Variations in bed elevations (East Fork River, UnitedStates).

Figure 6.8 — Variation of sampling errors in bed load samples.

Number of sampling verticals Number of sampling verticals

Days from 1 May 1980

Mea

n be

d el

evat

ion

(m)

Dis

char

ge (

m3

s–1 )

definite criteria in selecting the number of sampling verticals andthe number of required repetitions to be used at a particular site.Some compromise must be made to achieve a balance betweenthe representation of both the spatial and temporal variations.Experiments are encouraged at hydrometric stations to determinethe most appropriate sampling method to use for routine measure-ments. Probable relative error, which may be induced by aninsufficient number of verticals and repetitions, has been reportedby the Council for Mutual Economic Assistance (COMECON),as quoted by Operational Hydrology Report No. 16 (WMO,198l). Figure 6.8 is taken from that report.

6.3.2 Indirect method6.3.2.1 SEDIMENTATION PROCESS

If bed load constitutes the major part of deposits in a reservoir, themeasurement of the deposit volume by repetitive surveys shouldgive an average bed load rate. In the evaluation of bed load, finematerial transported into the reservoir mainly as suspended sedi-ment should be deducted from the total volume of deposits. Theunit weight of the deposits may be determined fairly accurately byfield measurements. Preferably, systematic suspended sedimentload data should be obtained at both inlet and outlet hydrometricstations. The amount of bed load is then the amount of depositedsediment, which is the difference between the amount of incomingand outgoing sediment load. This indirect method of bed loadmeasurement gives only an average rate of bed load discharge in aperiod between two successive surveys, rather than the instanta-neous rate. If the bed load discharge is not very large, a longperiod of time is necessary between repetitive surveys to obtain afair degree of accuracy (Shandong Provincial Office of Hydrology,1980).

6.3.2.2 DUNE TRACKING

The dune tracking method of measuring bed load dischargeinvolves measuring the rate of bed material movement in dune-shaped forms in the direction of flow. It is generally difficult tomeasure the bed load in an alluvial river that consists mainly offine sands by means of existing measuring methods. The dunetracking method has the advantage that only hydrographic survey-ing techniques are employed. With this method, a sounding systemshould be established which permits the recording of bottomprofiles along pre-fixed courses in a river reach. Bed load rate canbe estimated from the propagation of dunes, calculated by succes-sive surveys. The accuracy of the dune tracking methods relies onthe accurate determination of the bed elevation and positioning ofthe measuring points (Havinga, 1981).

Two methods are used in monitoring the movement ofdunes:(1) Moving boat technique: Longitudinal profiles are measured

repetitively by an echo sounder mounted on a boat. Thelength of the traversed reach should be long enough toinclude 20 to 25 well-defined dune forms. Usually, a straightreach is selected for this purpose. Accurate records of timeand the boat position should be maintained. In the active bedzone of the reach, five or more longitudinal profiles areusually measured during each survey;

(2) Echo sounding: Continuous soundings taken at a fixedpoint or several points in the flow cross-section monitorthe variation in depth and thus the movement of thedunes. The time for taking such measurements should be

sufficient for at least 20 to 25 dunes to pass the point ofmeasurement.

6.3.2.3 TRACER METHOD

The tracer method, as well as the dilution method, is based on thedetection of the sediment movement by tracers. This method isfeasible for measuring bed material discharge and sedimentdispersion. However, there are large variations in the techniquesused. Selecting the appropriate technique depends on the studypurpose and the river conditions in the measuring reach. Theprocedures and techniques involved are the selection and labellingof the sediment tracer particles, the method of introducing thetracer into the flow system, and the method of detection. Fielddata collection includes tracing the labelled particles, sampling thebed material and measuring hydraulic elements in the river reachunder investigation. The latter two are usually measured usingconventional methods.

Four labelling methods are available for use with thetracer method. The fluorescent tracer, radioactive tracer and stableisotope tracer can all be used in rivers where the bed material iscomposed of relatively coarse particles such as gravel and sand.However, only the radioactive tracer seems to be suitable for usein places where the bed material is composed mainly of fine sand,silt and clay. Fluorescent and stable isotope tracers have to bedetected in laboratories from samples taken in the field, but theradioactive tracer can be detected in situ with a portable instru-ment. With the fluorescent tracer method, the movement ofradioactive tracers of different sized sediments can be measuredby dyeing them various colours to represent particles in differentsize ranges. However, it is rather difficult to trace the movement ofradioactive tracer particles of different sizes. In contrast to aradioactive tracer, stable isotope tracers have no environmentalimpact since they do not involve radioactivity until the samplestaken from the field are neutron-activated in the laboratory.Magnetic methodologies have also been used. The magnetic prop-erties of sediment can be enhanced (by heating, inserting iron orusing electric coils). The particles are then traced using metaldetectors or specially designed detectors (Leeks, 1999). In allcases, the labeled particles should have the same hydraulic behav-iour after labelling as before and should resist leaching, abrasionand decay of their traceability.

6.3.2.4 INVESTIGATION OF THE LITHOLOGIC PROPERTIES OF

SEDIMENT

Bed load sediment is originally composed of rock fragmentsformed through weathering and wear during the transportcourse over long distances. Lithologic properties vary with thegeological conditions of individual watersheds. If, for instance,the bed load content of the tributary is known and the lithologiccomposition differs distinctively from that of the main stem, thelithologic composition of the bed load may be utilized as natu-rally labelled tracers in the estimation of bed load in the mainstem of the river.

In practice, the proposed method has been used to evalu-ate gravel bed load at Yichang in the Changjiang River. However,this method is rather laborious and time-consuming, since atremendous amount of field and laboratory work has to be carriedout if a fair degree of accuracy is expected. The method can stillprovide a feasible means of estimating bed load discharge whenother methods are impossible or too expensive.

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 129

6.3.3 Measuring devices6.3.3.1 TECHNICAL REQUIREMENTS FOR AN IDEAL BED LOAD

SAMPLER

The technical requirements for an ideal sampler may be listed asfollows:(1) The sampler should exert minimum disturbance on the flow,

especially in the vicinity of the sampler mouth;(2) The sampler should have a moderately high sampling effi-

ciency, for example, one exceeding 30 per cent, for differentsizes of bed load. The sampling efficiency should be cali-brated;

(3) The sampler should have a simple design and be robust. Aportable version should be sufficiently heavy and easy tooperate;

(4) The size of the entrance should be adequate to cope with themeasurement of suspended sediment and also be at least 1.5times the maximum size of the bed load;

(5) For pressure-difference samplers, the ratio of the intakevelocity to ambient velocity should be equal to or slightlyhigher than 1.

Techniques involved in the measurement of bed load arerather complicated. A commonly used bed load sampler may notfulfil all the above requirements, but good results may still beobtained if great care is taken in handling the sampler in theappropriate manner.

6.3.3.2 VARIOUS KINDS OF BED LOAD SAMPLERS

The bed load measuring devices or samplers currently in use maybe classified into four types: basket-type, pressure-difference-type,pan- or tray-type and slot- or pit-type. A variety of bed loadsamplers have been developed. Here, only some samplers arebriefly described.

(1) Basket-type sampler. A basket-type sampler isgenerally adopted for sampling coarse bed load material such asgravel and pebbles. Metal or nylon mesh is put on the side and topof a metal frame. Loosely woven iron rings or other elastic materi-als may be put at the bottom to deal with variations in bed surface.The average sampling efficiency of a basket-type sampler cali-brated in the laboratory is reportedly about 45 per cent, althoughthis may vary from 20 to 70 per cent (Hubbell, 1964). Experiencein China indicates that the sampling efficiency of this type ofsampler may still be much lower than this average value.

As an example, in order to take a direct measurement ofgravel bed load, several versions of basket type samplers weredeveloped and used in the Upper Changjiang River and some of itsmajor tributaries. The MB-2 sampler, weighing 700 kg, with anopening size of 50 (height) × 70 cm (width), was used in theMingjiang River and Qingyijiang River. It may be used in moun-tain streams with a velocity under 6 m s–1 and a depth of less than5.5 m, with bed load sizes of 5 to 500 mm. The Y80-2 sampler,weighing 200 kg, with an opening size of 30 × 30 cm, and itsformer versions have been used on the main stem of the UpperChangjiang River under flow conditions with a depth under 30 mand a velocity of less than 4 m s–1. The maximum size of the bedload to be sampled is 250 mm.

(2) Pressure-difference-type sampler. The main feature ofa pressure-difference bed load sampler is that the ratio of the intakevelocity to ambient velocity and the hydraulic efficiency does notdiffer much from 1. The pressure difference is obtained by enlargingthe flow section beyond the intake. Bed load sediment is collected

by the meshed bag at the rear or at the bottom of the flow section.The following is a general description of several samplers.(a) The Changjiang Y-78 bed load sampler. Several versions of

Y-78 samplers, i.e. type 78-1, weighing 50 kg (frameworknot included), and type 78-2, weighing 14 kg, are available.They may be used in streams with a velocity of less than2.5 m s–1 and a depth of less than 10 m. Type 78-1 has anopening of 10 × 10 cm and an effective capacity of 16 kg fortaking samples, while type 78-2 has an opening of 7 × 8 cm.The main feature of this sampler is the position of its centreof gravity, which is maintained in the front part of thesampler by heavy lead strips and by a buoy in the rear part. Aprotective plate in front of the sampler prevents unnecessarysettling and excessive scour around the entrance. Thehydraulic efficiency is close to 100 per cent. The samplingefficiency is about 60 per cent. The sampler is suitable forstreams with bed material that is predominantly sand sized(Zhou Dejia, et al., 1981);

(b) The BfG bed load sampler. This sampler was developed andis used by the Federal Institute of Hydrology in Germany.The intake nozzle is 8 (height) × 16 (width) cm. The collect-ing basket has a large capacity, allowing it to sample 6 kg ofbed load without affecting its efficiency. The inlet and thecollecting basket are connected by a flexible sleeve of rein-forced plastic of 15 cm length (Federal Institute ofHydrology, 1992);

(c) Helley-Smith (HS) bed load sampler. The intake section is7.62 × 7.62 cm. The area ratio of nozzle exit to entrance areaof the original version is 3.22, and the bed load is caught in anylon bag with a mesh opening of 0.25 mm. The hydraulicefficiency is 1.54. The overall sampling efficiency as cali-brated in the field is close to 100 per cent (Emmett, 1979). Alaboratory study with varying bed materials and a range oftransport rates carried out by Hubbell in 1985 indicates thatthe sampling efficiency varies with particle size, and that thetransport rate displays an approximate sampling efficiency of150 per cent for sand and small gravel, and close to 100 percent for coarse gravel (Edwards and Glysson, 1999).

In order to sample larger sizes of bed load, a modifiedversion with the intake opening enlarged to 15.2 × 15.2 cmhas also been developed. The hydraulic efficiency is alsowell over 100 per cent. However, its sampling efficiency isabout 100 per cent when used on medium to coarse sand andgravel beds with a bed material size of 0.5 to 16 mm. It canbe used in streams with a velocity of less than 3 m s–1. TheToutle River Sampler (TR2) is another modified version ofthe Helley-Smith sampler, but it can be used to take samplesthat are 2 to 150 mm in size. The intake section is 15.2 ×30.4 cm. During intercomparison work carried out in China,the sampler weight was increased to 230 kg by adding leadpieces to the sampler. The apparatus was used in flow with avelocity less of than 5 m s–1 (Xu, 1988);

After some modifications to the frame of the Helley-Smith sampler, using a nozzle of the same size and anexpansion ratio of 1.4, a new version designated US-BL-84was developed and adopted as a standard bed load samplerby the United States Geological Survey (USGS);

(d) Slot- or pit-type sampler or sampling method. Emmett ofthe USGS set up a bed load trapping system to collect bedload sediment. Concrete troughs or trenches 0.4 × 0.6 m are

130 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

constructed across the river to a width of approximately20 m. The slot is divided into eight sections fitted withgates. Along the bottom of the concrete trough a rubber belt0.3 m wide is threaded around drive and guidance pulleys,and then returns overhead. Sediment falling into the openslot is carried laterally to a sump in the riverbank. Aftercontinuous sieving and weighing, the sediment is returnedto the river downstream of the trap by a conveyor belt. Themeasurement of bed load using this type of installation isreliable and accurate. However, it is adaptable mainly forrelatively small rivers and particularly for experimentalstudies or the calibration of samplers (Emmett, 1979;Leopold and Emmett, 1997);

Bed load traps made of metal plates or other suitablematerial can be inserted into the riverbed to collect sedimentmoving as bed load. The length of the trap along the flowdirection may be 100 to 200 times the grain size. Instead ofsampling at regular intervals, this type of sampling method isused primarily to obtain the total amount of bed load in aflood period, since it is not easy to remove or replace thetraps during floods. Bed load traps can also be used to studythe bed load transport in small experimental basins. Acaisson mechanical trap was developed at the Bureau ofHydrology of Jiangxi Province, China. The top of the innercontainer may be adjusted to make it even with the riverbed.The height of deposition in the trap can be recorded and thesampled material can be extracted from the trap using asubmerged slurry pump. Traps such as vortex tubes havebeen used successfully in Nepal, China and other countriesto discard sediment moving in the vicinity of the bed ofcanals or streams. These can also be used as a bed loadmeasurement device.

6.3.3.3 NEW DEVELOPMENTS

(1) Intercomparison of bed load samplers. Bed load withmaterial of different sizes such as large gravel or fine sand has to besampled with different apparatuses such as a basket-type orpressure-difference-type sampler. To study the behaviour ofdifferent types of samplers, intercomparisons of bed load samplerswere carried out in the United States and China under a cooperativestudy programme from 1986 to 1988. The samplers used for thecomparison included the basket sampler (types MB2, Y80) and thepressure-difference sampler (types Y78-1, HS, TR2). The fieldworkfor the intercomparison was carried out in several rivers at sites withgravel bed or sandy bed load. Although the range of the size withwhich a specific sampler is applicable may be different, under thesame size range, the sampling result obtained by different samplersis still comparable. Relative sampling efficiency can be obtained,

which reflects the behaviour of the samplers under comparison (Gaoand Xu, 1989).(2) Development of a new bed load sampler for gravel andcoarse particles. Several important ideas were deduced throughintercomparison work. For sampling on gravel-bed rivers, theflexible bottom of a basket-type sampler may cope better with theriver bed, but the sampling efficiency may sometimes be too lowdue to its low hydraulic efficiency. The high hydraulic efficiencyof pressure-difference samplers such as the Helley-Smith samplerled to a high sampling efficiency, however, too much fine sedimentmay sometimes be sampled due to the suction effect. Also, ascouring effect may take place at the entrance due to the non-flexible bottom of the sampler. The different behaviour of thesetwo types of samplers was brought to light through theaforementioned intercomparison work. A new version of bed loadsampler to be used mainly for bed load of gravel and pebble sizewas therefore developed through a cooperative study by severalinstitutions in China. The new bed load sampler (Type AYT)provides a flexible bottom at the entrance and an expansion sectionto create a pressure difference. The sampler was designed,manufactured and calibrated through extensive studies byexperiments in flumes with scale models in various sizes. Thehydraulic efficiency is 1.02. The sampling efficiency is a functionof bed load discharge (η = 48.5 Qs

0.058, Qs in g s–1 m–1). Severalversions of this type of sampler are available, as shown in Table6.5 (Gao, et al., 1995).

6.3.4 Calibration of samplersA bed load sampler has to be calibrated for its sampling efficiency,with which the measured transport rate can be converted.

6.3.4.1 DIRECT FIELD CALIBRATION

Efficiency is determined by directly comparing the result ofmeasurement obtained by the sampler under study with the bedload measured directly by a more accurate and reliable method.The transverse slot with a conveyor belt installed on the East ForkRiver is a typical example of an accurate method for calibratingthe sampler and measuring the bed load (Leopold, 1997).

In practice, facilities similar to that installed at East ForkRiver are not available for calibrating various types of bed loadsamplers. It would appear that a carefully conducted intercompari-son of bed load samplers in the field would be a feasible way ofobtaining their relative efficiency. If the efficiency of the samplerserving as an index is known, then the efficiency of the sampler tobe compared may be determined.

If there is a highly turbulent section within the measur-ing reach, sediment that normally moves as bed load will besuspended, and can be sampled by a suspended-sediment sampler.

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 131

Table 6.5Basic versions of AYT sampler series used for coarse bed load

Dimension (mm) Effective Range of applicationSerial No. capacity Weight

Entrance Overall Particle size Depth Velocity

Width Height Length Max. height kg kg mm m m s–1

1 120 96 760 176 10 40 2–100 40 4.0

2 300 240 1 900 438 60 320 2–250 40 4.5

3 450 360 2 850 657 180 600 2–400 30 5.0

A good estimate of bed material discharge within the unmeasuredzone, including the bed load, may be obtained by measuring thedifference of the sediment discharge at both the turbulent and thenormal sections using standard suspended-sediment samplingtechniques. This principle has been used by the USGS to evaluatethe total sediment discharge in a turbulence flume built in anatural stream (Vanoni, et al., 1975).

6.3.4.2 LABORATORY CALIBRATION

Bed load samplers can be constructed to scale and tested in labo-ratory flumes. However, the sampling efficiency obtained byflume experiments in a laboratory with a model sampler is usuallylarger than the true efficiency. Large differences were observed inthe experiments carried out by CWRC in using model samplerswith a different scale ratio. It was found that the efficiency of asampler is not constant but varies with the flow parameters, trans-port rate, particle size and local bed conditions. For instance, theefficiency of a basket sampler with a flexible bottom for samplinggravel is very small at the moment when the gravel just starts tomove. The efficiency becomes greater as the transport rateincreases. For a pressure-difference sampler the efficiency changeswith the flow velocity.

The calibration of samplers on a reduced scale will leadto scale effects; it is therefore advisable to test the full-scaleinstrument. Hubbell of the USGS has reported recent refinementsin calibrating bed load samplers. Calibration curves, rather thanefficiency percentages, were derived by two independent methodsusing data collected with prototype versions of the Helley-Smithbed load sampler. The tests were conducted in a large calibrationflume capable of continuously measuring transport rates across itswidth. The flume was 2.7 m wide, 1.8 m deep and 83 m long, witha discharge as large as 8.5 m3 s–1. An adjustable width slotextended across the full width of the channel, dividing it intoseven lateral sections. The facility was designed to re-circulate bedload particles ranging in size from 2.75 mm at rates up to 12 to20 kg s–1. Apparently, with this type of facility the results obtainedby laboratory calibration should be much more reliable than theearlier laboratory calibrations using scale models (Hubbell, 1981;Druffel, et al., 1976).

As discussed in previous sections, different types of bedload samplers are designed for different bed conditions. Samplingefficiency is different for different types of samplers. At present, asampling efficiency of over 50 to 60 per cent should be consideredto be satisfactory for sand and gravel. In any case, the samplingefficiency of the bed load sampler should be determined by cali-bration in the field and also studied in the laboratory to correctlyinterpret the measured data.

6.3.5 Computation of bed load dischargeBed load discharge per unit width measured at each vertical maybe computed from the following equation:

qsb = 100 k Wb / (η b t) (6.4)

where qsb denotes the bed load discharge per unit width aftermodification according to the sampling efficiency η expressed in%, Wb is the weight of the sample collected in a period of time t(sampling duration), b represents the width of the sampler inlet,and k is a coefficient inserted for the conversion of units expressedfor various parameters.

The total bed load discharge over the entire cross-sectioncan then be computed by numerical integration along the streamwidth. This is done either by a graphical or analytical method. Inthe graphical method, the bed load discharge is plotted as the ordi-nate, and the horizontal distance along the entire width is plottedas the abscissa. In the graph, the distribution of mean velocities isalso plotted to make a visual inspection of the reasonableness ofthe measured results. The analytical method involves the computa-tion of bed load discharge by a trapezoidal formula, assuming alinear variation between two adjacent verticals.

The analytical method of computing the bed loaddischarge over a cross-section may be illustrated by Figure 6.6. Itis called the mid-section method and is expressed as (Edwards andGlysson, 1999):

QB = qb1b1/2 + Σ qbi [(bi + bi+1)/2] + qbi+1 + bi+1/2 (6.5)

The results obtained for each individual bed loadmeasurement can also be related to some hydraulic parametersuch as the discharge, or the stream power, during the period ofmeasurement. This relationship, together with the rating curve atthe same site, can provide a necessary tool in the further computa-tion of the total bed load. In flood events, it is difficult to take arepresentative bed load sample. In this case, the bed loaddischarge may be extrapolated through relationships between thebed load discharge and relevant hydraulic parameters which areestablished with measured data obtained in other periods.

6.4 MEASUREMENT OF TOTAL SEDIMENTDISCHARGE

6.4.1 Direct methodsThere are three types of direct methods for evaluating the totalsediment discharge, i.e. measurement of suspended and bed loaddischarge at a specific cross-section; measurement of sedimentaccumulation in reservoirs; and measurement by turbulence flume.

6.4.1.1 MEASUREMENT OF SUSPENDED SEDIMENT AND BED LOAD

DISCHARGE

The most direct and intuitive method is to take separate measure-ments of the suspended and bed load discharge simultaneously atthe cross-section. However, the total sediment discharge is not thesimple summation of the measured suspended sediment dischargeand bed load discharge. The reason for this is that there is anunmeasured zone when the suspended sediment is measured usingthe depth integration method. The lowest sampling point can onlybe set at 0.94 to 0.98 relative depth, and some suspended bedmaterial load in the vicinity of the bed may not be collected by thesampler. In sampling using the point method, some errors may beinduced by using the weighting factors in numerical integration.Sometimes, there may be an overlap in the portion of depthcovered by the bed load sampling apparatus, i.e. a part of thesuspended load may be included in the sample taken by the bedload sampler.

Under present technical conditions, the measurement ofbed load is both time-consuming and labour-intensive. In thelower reaches of an alluvial river, bed load usually constitutes onlya relatively small portion of the total load. Therefore, only a fewstations attempt to take both suspended load and bed loadmeasurements as routine work. Nevertheless, in evaluating thetotal sediment discharge, the transport in the so-called unmeasured

132 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

zone must be accounted for. Direct measurement of the bed load isencouraged and should be carried out whenever possible. For rivermanagement, it is necessary to have knowledge of the relativeresponse of bed load and suspended load since they representdifferent management problems. In such cases, the direct measure-ment of bed load is indispensable.

Installations and devices similar to those used by theUnited States Geological Survey on the East Fork River,supplemented by regular suspended sediment sampling over thesection, can satisfy the measurement requirements. However, theyrequire careful design and may be too expensive to operate on aroutine basis. Structures or weirs across a small river toconcentrate the sediment-laden flow have been constructed insome experimental basins in Italy. At the bottom of the weir, avortex tube is built to collect bed load materials. Devices such asautomatic-pumping samplers or other types of samplers may beused for suspended-load sampling (IAHS, 1981). Needless to say,such measuring installations can only be constructed on relativelysmall rivers. They would be impractical for normal sedimentmeasurement networks in large or medium-sized rivers. However,for some experimental reaches where the measurement of the totalload is significant, they provide an effective method worthadopting.

6.4.1.2 MEASUREMENT BY MEANS OF TURBULENCE FLUME

This method can be used at certain narrow constrictions insandbed streams with sections so turbulent that nearly all sedimentparticles moving through the reach are in suspension. The turbu-lence flume was so named in literature because artificialroughness elements were put on the floor of the flume to produceintense turbulence. In such a flume, measurement of the total sedi-ment transport may be conducted by taking only suspendedsamples. The turbulence flume set up at Dunning on the MiddleLoup River contained a series of baffle piers in a criss-crossarrangement on top of a concrete base. The base was placed at thesame elevation as the original river bed. The additional turbulencethus created was effective in putting all the sediment into a state ofsuspension. Suitable samplers could be used for depth integration(Vanoni, et al., 1975). Another example is at the outlet of a stillingbasin below a dam, where the flow is so turbulent that all the sedi-ment is in a state of suspension. It provides a place to take samplesrepresenting the total sediment load passing through the damoutlet structures.

The idea of a turbulence flume is practical. Its advantageis that a conventional method can be used without modification toobtain the transport rate of the total load. The total sedimentdischarge can be measured reliably and directly. However, thistype of construction may be not feasible for use on the main stemof large rivers.

6.4.1.3 MEASUREMENT BY SEDIMENT ACCUMULATION

The total deposition or the growth of deltas in a small reservoirover a certain period of time can be determined through repetitivesurveys. The volume of deposition, converted to weight anddivided by the duration, will give the average rate of accretion ofsediment discharge in the reservoir. The sediment passing throughthe reservoir can sometimes be measured accurately by takingonly suspended samples in fully developed turbulence sections atthe outlets. This amount can be added to the deposition to deter-mine the total sediment discharge.

Methods for conducting a reservoir survey are discussedin Chapter 4. A certain degree of accuracy can be achieved indetermining the total sediment discharge if the surveying work isdone strictly, according to the accepted standards.

6.4.2 Computation of total sediment load from measuredsuspended sediment discharge data at a hydrometricstation

Owing to the complexities of bed load movement, less labour-intensive techniques are still not very well developed formeasuring bed load discharge in rivers. In contrast, the measure-ment of suspended load, after long-term research anddevelopment, now yields acceptable results in most sediment-laden rivers. However, except in some experimental reaches orbasins, there are still no reliable means of measuring the total sedi-ment load in a river; neither can the suspended sediment dischargewhich exists in all verticals be accurately estimated by ordinarysampling procedures. Schroeder and Hembree (1956) pointed outthat in wide and shallow streams, the total quantity of bed loadand suspended load within the unmeasured zone may well amountto 20 to 60 per cent in some cases. It may exceed 100 per cent forcoarse particles. Chien and Wan (1998) pointed out that errorsexist for either the depth integration method or sampling bypoints, and described the methods of evaluating the correctioncoefficient for both sampling methods; these will be discussedlater in the section.

6.4.2.1 THEORETICAL BACKGROUND

The basic idea of computing the total sediment loadfrom the measured suspended sediment discharge data at a hydro-metric station may be illustrated by the following equation:

(6.6)

where QT is the total sediment discharge over the entire depth,including bed load; QM is the actual measured suspended sedimentdischarge, QSC is the computed theoretical sediment dischargeover the entire depth, and QSCM is the computed theoretical sedi-ment discharge for the measured zone.

For measurements in a vertical, the ratio of thecomputed sediment discharge over the entire depth to that in themeasured zone in a vertical may be evaluated by the Einstein totalload transport theory. If a case depth integration method is used, itmay be expressed as follows (Einstein, 1964):

(6.7)

where iTqT and iSqSM are the total sediment discharge over theentire depth and the suspended sediment discharge over themeasured zone, respectively, expressed in size fraction and perunit width, E is the ratio of the thickness of the unmeasured zoneto the flow depth, A is the ratio of thickness of the bed load layerto the flow depth (in the original Einstein formula, A stands for2D/d, where D is the grain diameter, and d is the depth of flow), zis the exponent in the sediment distribution formula, and equalsω/κU* (where ω is the settling velocity of the sediment grains, U*is the shear velocity, and κ is the universal coefficient), and theparameter P is computed by:

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 133

€

Q QQ

QT MSC

SCM=

i q

i q

E

A

A

E

PI I

PI IT T

S SM

z zA

E

=

−−

+ +( )+( )

−11 2

1 2

1

1

1

(6.8)

where ks is the dimension of roughness elements, χ is a functionof the ratio of ks and thickness of laminar sublayer as ks/δ, and I1and I2 are two definite integrals which are functions of A and z:

(6.9)

(6.10)

For sampling by points, the ratio of the computed sedi-ment discharge over the entire depth to that evaluated by dataobtained at points may be deduced as follows (let θ denote theratio):

(6.11)

The denominator in the expression is the formula used tocompute the sediment discharge in a vertical. In the expression, ki isthe weighting factor to be applied to each measuring point, and thetheoretical values of the point concentration are Cyi, and point veloc-ity uyi and total sediment discharge iT qST are given by the equationspresented below:

(6.12)

(6.13)

iTqST = ibqsb (1 + PI1 + I2) (6.14)

ibqsb = 11.6 (A • d • CA • U*') (6.15)

where CA is the sediment concentration in the size group in thebed layer (at a distance 2D from the bed in the original Einsteinformula), D is the mean grain size of the size group, z is the expo-nent in the sediment distribution formula, and equals ω/κU*',where ω is the mean settling velocity of the size group and κ is theuniversal coefficient, and U*' is the grain shear velocity;

After derivation, an equation for computing θ isobtained:

(6.16)

where xi is the relative depth. Once the weighting factor ki is deter-mined for the specific sampling method (by points), θ can beevaluated.

A graph quoted from Chien and Wan (1998) is shown inFigure 6.9. In the graph, the ratio is expressed by 1/θ for measure-ments taken at three points in a vertical with a weighing factor of1:2:1 and assuming P = 13. It can be seen that the ratio of theamount of unmeasured to measured sediment discharge would betoo large to be meaningful if the suspension index z exceeded 0.6to 0.8. In other words, for coarse sediment, direct measurement ofthe bed load and improvement of sampling methods are needed toobtain reliable data.

6.4.2.2 THE MODIFIED EINSTEIN PROCEDURE

The modified Einstein procedure (MEP), first proposed by Colbyand Hembree in late the 1950s, has been widely used in somerivers in the United States to estimate the total load. In practice,the proposed MEP method was formulated on the basis of theEinstein total load transport formula with some modifications, andit is applicable for computing the total sediment discharge over thewhole cross-section for streams where the bed material iscomposed mainly of sand and small gravel. It has been verified inmedium and small rivers where total sediment load data are avail-able. Schroeder and Hembree (1956) have applied this method inlarge rivers with sandy beds. Pemberton (1972) of the UnitedStates Bureau of Reclamation also proposed a modification to theEinstein formula for use in the planning and design of hydrologi-cal projects. Stevens (1985) worked out a computer program tofacilitate the computation. The procedure uses the data obtained inthe measurement of suspended sediment discharge, such asdischarge, width, average velocity or depth, water surface slope,average sediment concentration within the sampled zone, watertemperature, size gradation of measured suspended sediment andbed material to estimate the unmeasured suspended sediment andbed load discharge.

The MEP method was developed for rivers with bedmaterial predominantly of sand and small gravel on the basis ofthe depth integration sampling method. The study conducted byLin (Lin and Liang, 1997) indicated that the method could also beapplied to the Middle and Lower Yellow River with fine sand andcoarse silt bed materials and to stations in which the field datawere based on points-method sampling. They made some modifi-cations to the MEP computer program as proposed by Stevens tomake it suitable to the Yellow River. Li and Long applied thismodified program to compute the total load using data collected

134 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

u U ly

kyii

s=

∗5 75 30 2. .' og

χ

θ =− + +

−

+

−

=∑

4 6481 1

12 303

11 2

1

.( ) ( )

( . lg )

A

A

PI I

kx

xP x

z

z

ii

i

z

i

i

n

Figure 6.9 — Corrections applied to the measured data in a vertical bysampling at three points.

Pd

ks= ×

2 303 30 2. lg .

/ χ

IA

A A

y

ydy

z

zA

z

1

11

0 2161

1 1= ×

−−

−

∫.( )

IA

A A

y

yydy

z

zA

z

2

11

0 2161

1 1= ×

−−

−

∫.( )

ln

θ ι=

∑Τq

d k C u

ST

i yi yi

n

1

C Cd y

y

A

Ayi Di

i

z

=−

−

2 1

during the measurement of the suspended sediment discharge atseveral hydrometric stations in the Lower Yellow River, andapplied the coefficient to correct the daily suspended sedimentload which was derived from measured data after proper dataprocessing (Li and Long, 1994).

Deposition or erosion in river reaches or in the reservoirof the Middle and Lower Yellow River may be evaluated by thesediment balance equation (i.e. difference of sediment loadmeasured at two terminal stations taking account of the intermedi-ate in or out flows). If the total load was computed by theaforementioned method instead of the hydrometric stations’measured suspended sediment load in the balance equation, theresult would correspond much better to the amount of sedimenta-tion obtained directly through repetitive range surveys (Li andLong, 1994).

6.4.2.3 CORRECTION COEFFICIENT

To correct the measured sediment concentration in a vertical, theratio of iT·qT /iS·qSM expressed in section 6.4.2.1 may be consid-ered as a correction coefficient. For the depth integration method,the ratio may be computed by Equation 6.6, and similarly, for thesampling by point method, it may be computed by Equation 6.15.The measured suspended-sediment discharge of a given size frac-tion multiplied by θ will give the total sediment discharge per unitwidth of the size fraction at the vertical. Obviously, the summationof the total sediment discharge of all the size fractions will givethe total sediment discharge in the vertical. These can be summedfor all verticals at a section to give the total sediment discharge atthe measuring site of the stream.

In considering the correction of the measured sedimentconcentration accounting for the unmeasured zone in the depthintegration method, errors of discharge measurement should alsobe considered. For the point integration method, for instance, withfive points in a vertical, the lowest point for taking either velocitymeasurements or sampling for sediment concentration is usuallyaround a relative depth of 0.95 instead of the theoretical positionof relative depth of 1.0. On the one hand, the measured sedimentconcentration at this point will be lower than that at the riverbottom. On the other hand, the measured velocity will be greater.Hence, the deviation of the computed sediment discharge per unitarea from the true value depends on the relative magnitude of thevelocity and the concentration, or distribution, of the product ofUy and Sy.

6.4.2.4 RATIO OF BED LOAD DISCHARGE TO SUSPENDED-SEDIMENT DISCHARGE

Most published sediment data are limited to the suspended-sediment discharge. For a rough estimate, the ratio of the bed

load to suspended load may be used empirically for the estima-tion of the total sediment discharge. Following the same line ofapproach expressed in the preceding paragraphs, the ratio maybe written as:

(6.17)

where r is the ratio of bed load discharge to suspended-sedimentdischarge, and iTqT is the total sediment discharge per unit widthfor a certain size fraction.

The ratio varies with the diameter of transported sedi-ment and the boundary conditions of the flow, and may beestimated by Equation 6.16. According to an estimation based onfield data from some hydrometric stations on the Yellow River, theaverage value of r may vary from 0.14 to 0.88 per cent. Themaximum value of r of various stations varies from 0.8 to 4.2 percent. However, for rivers with relatively stable boundary andinflow conditions, the range of variation may not be so wide(Zhang and Long, 1998).

Maddock made a summary of the ratio of the bed load tosuspended load based on annual loads, as shown in Table 6.6. Theratio varies with different bed compositions and suspended sedi-ment concentrations (Vanoni, et al., 1975).

In an alluvial river, the suspended bed material load andthe bed load being transported may have the same correlation, asthey are related to the hydraulic conditions of the flow. The trans-port rate of wash load depends more on the available supply of thefine material contributed from the watershed. For this reason, theratio of the bed load to suspended load is valid only for bed mater-ial load.

6.4.3 CommentsThe following pertinent points are worth mentioning regarding theevaluation of the total sediment discharge. In the first place, thetotal load may be classified as bed material load and wash load.Wash load transport depends on the availability of the sedimentfrom the source area, and moves essentially as suspended load. Anaccurate estimation of wash load relies mainly on reliablemeasurement in the field, either by sampling or in situ measure-ment. An indirect method for estimating the total sedimentdischarge, as discussed in the previous section, would only give anevaluation of the bed material discharge and not of the wash load.Bed material discharge depends fundamentally on the transportcapacity of the flow, which may be evaluated by transport formu-lae for given hydraulic and morphological conditions. Transportformulae should be verified or modified if necessary usingobserved data. The amount of wash load can be estimated bydirect measurement.

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 135

€

ri q

i q PI Ib sb

T T= =

+ +1

1 1 2

Table 6.6Estimation of ratio of bed load to suspended load

Suspended sediment Bed composition Suspended load Ratio (r)concentration (ppm) composition

< 1 000 Sand Similar to bed 0.25–1.50Gravel, consolidated clay Small amount of sand 0.05–0.12

1 000–7 500 Sand Similar to bed 0.10–0.35Gravel, consolidated clay 25% sand or less 0.05–0.12

> 7 500 Sand Similar to bed 0.05–0.15Gravel, consolidated clay 25% sand or less 0.02–0.08

In small rivers, the overall ratio between bed load andsuspended sediment may be fairly constant over the years becauseit is controlled by the source properties. However, at instancesbetween floods it can be highly variable.

Evaluation of the total load based on measuredsuspended sediment data is a promising approach. However, thereliability of the computation of the total load relies upon theaccuracy of the proposed sediment transport formula used in eval-uation of the correction factors. The idea of applying a correctionfactor to the measured sediment discharge has been expressed. Inthis report, Einstein’s bed load function, as well as the total loadtransport formula, is used to illustrate the theoretical background,and is used after some modifications in the modified Einsteinprocedure (MEP). However, verification of the Einstein formula,since it was originally proposed with field data, indicates that thecomputed results do not match the field data very well, particu-larly in relatively low flows. On the basis of Einstein’s theory,Wang, et al., proposed a new transport formula (Wang, et al.,1995). The same line of approach was followed in the develop-ment of this new formula and some parameters originally used inthe Einstein formula were replaced by the results of newlyconducted experimental studies. The proposed formula has beenverified both by measured bed load and suspended load at hydro-metric stations in the Middle and Lower Yellow River, withsatisfactory results (Zhang and Long, 1998). The new transportformula will provide a sound theoretical basis for evaluating theratio QSC/QSCM expressed in Equation 6.6.

It should be noted that the sediment moving in the vicin-ity of the bed is composed of coarse bed material particles. Thecorrection coefficient is much greater for coarse sediment thanfine sediment. In an alluvial river, particularly the downstreamreaches, the bed load and the suspended bed material load beingtransported in the vicinity of the bed may constitute only a smallfraction of the total load. However, it is important in sedimentationstudies in determining the total transport load. It also plays animportant role in the fluvial process and reveals a proper relationbetween the transport rate and the hydraulics of the flow, which isa basic characteristic in the study of fluvial processes.

6.5 LABORATORY PROCEDURES6.5.1 Determination of sediment concentrationSuspended sediment samples obtained in the field must be treatedin the laboratory for the determination of sediment concentrationand particle size. Evaporation, filtration and displacement methodsare generally used in laboratories to determine the sedimentconcentration. The method is chosen on the basis of the quantityand the composition of sediment in the sample and the desiredaccuracy. In the Chinese Standards, a minimum weight of thesediment in the sample is required, in accordance with thesensitivity of the weighing apparatus, in order to make it possibleto use evaporation and filtration methods and specific gravity flasksof different capacities in the displacement method. In the UnitedStates, it was found that the filtration method might best be used onsamples containing sand concentrations of less than 10 000 mg/land clay concentrations of less than 200 mg/l. The evaporationmethod is applicable to samples ranging from 0.2 to 20 l in volume,from 5 to 500 000 mg/l in sediment concentration, and having lessthan 35 000 mg/l in dissolved-solid concentration. In addition, thewet sieving method is used if two concentration values are required:one for sand size particles and one for a combination of silt and clay

sized particles. The sample is separated by a sieve with 0.062 mmsquare apertures. The coarse fraction is treated by the evaporationmethod and the fine part, after splitting, may be weighed eitherthrough filtration or evaporation (ASTM Standard D3977-97).

In general, sediment concentration is determined by theweight of the dried sediment contained in the sample, divided bythe volume of the sediment-water mixture sample. An indirectmethod, for example, is to take a reading from a physical appara-tus, such as a turbidity meter, to obtain the sediment concentrationfrom a calibration curve that expresses the relationship betweenthe reading and the sediment concentration.

Sediment concentration is expressed in three differentways: CS represents the weight of dried sediment contained in aunit volume of sediment-water mixture commonly expressed inmg/1, g/l or kg m–3; CSG represents the weight of dried sedimentdivided by the weight of the sediment-water mixture and may beexpressed in percentage of weight (%) or in parts per million(ppm); and CSV represents the volume of sediment particlescontained in a unit volume of the sample, expressed in per cent(%) or as a ratio.

6.5.1.1 EVAPORATION METHOD

In the evaporation method, the wet sediment sample, after thesupernatant liquid is decanted from the vessel, is transferred to anevaporation dish and dried in an oven at a temperature slightlybelow the boiling point until the visible moisture is evaporated.The oven temperature is then raised to 105ºC for two hours. If thedissolved solids exceed 2 per cent of the sample weight, theirconcentration should be determined separately in the originalwater. The content of dissolved solids should be subtracted fromthe dried sediment weight in computing the sediment concentra-tion. The dry weight of the evaporation dish is usually preciselydetermined beforehand. In routine operations, it should bechecked to avoid any possible error.

6.5.1.2 FILTRATION METHOD

Filtration is used to determine concentration that is low. Thequality of the filter material influences the accuracy of thismethod to a great extent. Experiments should be carried out totest the filter material before it is finally selected. The firstexperiment is to determine the amount of sediment that may beleaking through the filter material. If the leak exceeds 2 per centof the total sampled sediment, better quality filter materialshould be used. The second experiment is to determine thecontent of soluble matter in the filter material. By comparing thedry weight of the filter material before and after immersion inwater, the weight loss can be determined and used to correct thedry weight of the sediment obtained by filtering. In the UnitedStates, it is considered that the filter pore size (filter ratings) andfilter diameter are critical in the filtration method. Filters withretention ratings of 1.5 micron and a filter diameter exceeding24 mm are commonly used in the sediment laboratory (Edwardsand Glysson, 1999).

In the United States, a crucible is used in conjunctionwith various types of filter material in the filtration method. Thecrucible is a small porcelain cup of about 25 ml in capacity with aperforated bottom. Glass fibre filter disks have proved satisfactoryfor the filtration of most types of sediments. Force filtering may beused, in which air pressure is applied to the water surface to speedup the filtering process. If much fine-grained material is contained

136 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

in the sample, a glass fibre filter disk may be used in conjunctionwith an asbestos mat. The crucible is adapted to an aspiratorsystem and vacuum filtration to speed up the filtration process(ASTM Standard D3977-97).

6.5.1.3 DISPLACEMENT METHOD

The displacement method involves determining the difference inweight between a sample of sediment-laden water and an equalvolume of clear water. This method can only be applied to sampleswith a relatively high sediment concentration. The dry weight ofsediment is computed by the following equation:

WS = k (WWS + WW) (6.18)

where:k = ρS/(ρS – ρ) (6.19)

where WS is the sediment weight to be determined in g, WWS is theweight of the specific gravity flask plus the weight of sedimentwater mixture in g, WW is the weight of the specific gravity flaskplus the clear water weight with volume and temperature equal tothat of the sediment-water mixture (during weighing the watertemperature should be constant), ρS is the density of sedimentparticles, and ρ is the density of water.

In routine work, the values of k and WW have been tabu-lated in forms for a given water temperature. The density of

sediment particles, ρS, should be checked occasionally. At normaltemperatures, k varies from 1.59 to 1.61 for a range of ρS from2.65 to 2.70. WW also varies with the temperature. In laboratories,for commonly used specific gravity flasks (usually 50, 100, 200 or250 ml in volume), it is calibrated once a year and its value caneasily be determined once the temperature is known. To ensureaccuracy, water temperature in the flask should be measured to0.1°C and WW should be weighed to 0.001 g.

In calibrating WW, the original water may be usedinstead of distilled water. If raw water is used in weighing WW,and the influence of dissolved solids on the concentration is negli-gible, no correction is needed. If the dissolved solids varysubstantially in a year, it would be better to calibrate the weight ofthe flask by using distilled water and to make necessary correc-tions for the dissolved solid content.

6.5.1.4 ACCURACY REQUIREMENT

The allowable error in the measurement of sediment concentrationis put forward in the Chinese Standards, as shown in Table 6.7.

Accuracy in determining sediment concentration reliesmainly on accuracy in weighing. For balances with differentsensitivity used for weighing, a minimum amount of sediment isrequired to ensure an acceptable accuracy such as specified in theChinese Standards. It is clear that either the balance should bechosen according to the sediment weight available to be sampled,or the quantity of samples should correspond to the available

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 137

Table 6.7Allowable error in determination of sediment concentration*

Method Random error (%) Systematic error (%)

Volume Sediment Loss during Dissolved Leakage through Absorption ofmeasurement weight decantation solids filter filter paper

Evaporation 0.5 1.0 1.0–2.0

Filtration 0.5 1.0 1.0–2.0 1.0–2.0 1.0–2.0

Displacement 0.5 2.0

* ranges of error are set for different class stations.

Table 6.8Size analysis methods commonly used in China and the United States

Range of application(diameter in mm) Concentration (g/l) Required sample weight (g)

Fine sediment--------Settling in clear water (two-layer system)Siltmeter 0.062–0.5; may be more if 0.3–5.0

longer tube is usedVisual accumulationtube 0.062–2.0 0.05–15.0

Fine sediment------- Settling in dispersed medium systemPipette 0.002–0.062 3.0–20.0 3.0–20.0 in 1 000 ml

0.002–0.062 2.0–5.0 1.0–5.0Photo-sedimentation 0.005–0.062; may be <1.0 <1.0

used for 0.005–0.1Hydrometer 0.005–0.062; may be 15.0–30.0 15.0–30.0 in 1 000 ml

used for 0.002–0.05

Coarse sedimentSieve 0.062–20.0 or more 100–200 if done independently

0.062–32.0 More than 20 for coarse particles;min. 0.05

Direct measurement Sufficient quantity

laboratory apparatus. Nevertheless, errors may be easily introducedin the measurement of sediment concentrations if sediment samplesare not properly treated. The procedures put forward in relevantstandards should be strictly observed.

6.5.2 Size analysis6.5.2.1 METHODS FOR SIZE ANALYSIS

There are many methods available for size analysis. The sizedistribution of a sediment sample may spread over a wide range.Two or even more methods may be necessary to analyse the wholesample. For instance, the sieve method may be used for small frac-tions of coarse particles while the visual accumulation (VA)method or its equivalent size-analyser method is used for particlesgreater than 0.062 mm, and the pipette or photo-sedimentationmethod is used for particles smaller than 0.062 mm. The methodscommonly used for size analysis in routine work in China and theUnited States are listed in Table 6.8.

Sediment size as obtained by different methods hasdifferent meanings. When the sediment particle is directlymeasured by a rule, its size is measured in three mutually perpen-dicular directions denoted by a, b and c, in which c and a are theshortest and longest axial lengths respectively. The mean diameteris the summation of a, b and c divided by three. The shape factorSF is given by the expression c (ab)–1/2. The nominal size isexpressed by the diameter of a sphere with the same volume as theparticle that is obtained by immersing the particle in water andmeasuring the volume of displaced water. Sediment size deter-mined by sieve analysis is called the sieve diameter. Sediment sizedetermined by methods based on the settling principle is definedas the diameter of the sphere that has the same settling velocityand the same density as the given particle. It is called the settlingdiameter, or sedimentation diameter.

There are overlaps in the range of application of thesieve method and methods based on the settling principle. Therelationship between sizes with different definitions has beenstudied and noted in relevant literatures or standards. Owing tothe different meanings in the definitions of particle size evalu-ated by the various methods, the size distribution curves will notcoincide with each other at the junction portion when twomethods based on different principles are employed in sizeanalysis. Empirical revisions or corrections are necessary at thejunction point. For fine sediment, methods based on the settlingprinciple are recommended, as no discontinuity in the gradationcurve is induced by the definition of size implied in the methodsused for size analysis.

The results of size analysis are usually expressed by asize gradation curve with an accumulated percentage finer as theordinate and a sediment diameter in the logarithm scale asabscissa. A log frequency curve may also be used. Nevertheless,characteristic figures can always be interpolated from the curve,such as D50, D35, D65, D90, P0.05 and P0.025, etc., where P repre-sents the percentage of the total sample that is finer than theindicated size. The mean diameter and mean settling velocity ofeach size fraction is usually expressed by its geometric meanvalue, i.e. √D1D2 or √ω1ω2.

In the ISO draft Standard and the Indian Standards, sizeanalysis for suspended load is performed by subdividing the totalsuspended load into three size groups: larger than 0.2 mm, 0.2 to0.075 mm and smaller than 0.075 mm, representing coarse,medium and fine particles. For bed load and bed material, thesediment sample is subdivided into two portions: smaller andlarger than 0.6 mm. Conventional methods are used for thedetailed analysis of each portion.

The treatment of suspended samples in three parts is akind of simplification of the method employed in the determina-tion of the percentages of each portion. Just as in thesimplification methods used in sampling suspended sediment, thesimplification for size analysis is worth studying. For instance, inan alluvial river with a bed composed mainly of coarse sand, siltand clay particles are the wash load. If the amount of sand (greaterthan 0.05 mm) could be roughly determined during floods byconsidering a simplified analysis of the index samples, a betterunderstanding of the role played by coarse particles in the fluvialprocess would be revealed.

The classification of sediment sizes in the size gradationof a sediment sample involves dividing the sizes into size groups.The demarcation is set at sizes so that the latter is twice as large asthe former, in ascending order. The nomenclature and division areshown in Table 6.9. In the Geological Department, the sedimentsize is usually represented by φ, which is defined as φ= – log2D.

(1) Sieve analysis. Sieve analysis is a traditional methodused for the mechanical analysis of sand and gravel. From a prac-tical point of view, the direct measurement of particles greaterthan 20 to 32 mm contained in a sample may be more convenientthan sieving. National standards for sieves, as well as operationalspecifications used for analysis, have been established in mostcountries. When sieve analysis is adopted for the size analysis offluvial sediment, two methods may be used. The wet-sievingmethod carries out the analysis by immersing the whole sample inwater while the sieving operation is performed, or a small water

138 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 6.9Nomenclature and division of sizes

Name Size range (mm) φphi-system Name Size range (mm) φphi-system

Gravel Very coarse 64–32 –6 Silt Coarse 0.062–0.031 +4Coarse 32–16 –5 Medium 0.031–0.016 +5Medium 16–8 –4 Fine 0.016–0.008 +6Fine 8–4 –3 Very fine 0.008–0.004 +7Very fine 4–2 –2 Clay Coarse 0.004–0.002 +8

Sand Very coarse 2–1 –1 Medium 0.002–0.001 +9Coarse 1-0.5– 0 Fine 0.001–0.0005 +10Medium 0.5–0.25 +1 Very fine 0.0005–0.00025 +11Fine 0.25–0.125 +2 Only the upper limit is expressed in the φ system.Very fine 0.125–0.062 +3

jet is used to rinse all the particles to speed up the process. In thedry-sieving method, sieving is performed in the usual way. Thesieves are shaken to speed up the process. To ensure accuracy insieve analysis, a comparison of the results obtained with sievesused in routine work should be compared against those obtainedwith standard sieves on a regular basis. Corrections should bemade if necessary.

At present, the lower limit of sizes within which sieveanalysis may be applied is 0.062 mm. It is recommended thatsettling diameter, rather than sieve diameter, be used in analysingsuspended sediment. Also, it is preferable to use methods based onthe settling principle, such as the VA-tube method or the sizeanalyser method for the analysis of sizes ranging from 0.062 to1.0 mm, which are commonly found in suspended sediment. Forbed materials, however, the major part of the sample will be in thesand range, and sieves are more convenient for this analysis. Thecharacteristics of bed material are usually expressed directly bysize, while for suspended sediment it is more common to givecharacteristics in terms of settling velocity or settling diameterrather than sieve size.

(2) Methods based on the settling principle. Accordingto the settling medium, methods based on the settling principlemay be classified into two groups: settling in clear water, or thetwo-layer system, and settling in sediment-laden water, or thedispersed system (Allen, 1977).(a) Two-layer System. This is also called the stratified system.

The settling tube is filled with clear water (distilled water)prior to analysis, and sediment is inserted into the tube fromthe top. Different-sized particles will separate automaticallyin the tube according to their own settling velocities. Theright-hand side graph in Figure 6.10 is a sketch illustratinghow this settling system works.

At time t, all particles of size D, which have fall velocityh/t1, have settled to position h. Sediment discharge per unitarea passing through the cross-section of the cylinder shouldbe C1ω1, where ω1 is the settling velocity of the particle of

size D1, and C1 is the sediment concentration at position h,corresponding to sediment size D1. During the period 0 to t1,the total sediment passing position h should be:

∫0

t1Cωdt (6.20)

This is the part of the sediment with size equal to orgreater than Di in the total sediment sample. The greaterpercentage in the total sediment sample could be expressedas:

(6.21)

where T is the time required for the settling of all the parti-cles in the sediment sample.

This is the basic principle of the visual accumulation(VA) tube and the size-analyser method. In practice, thesettled sediment weight is obtained directly in both methods.In the VA tube method, the height of accumulation at thebottom is recorded and converted into weights by the rela-tionships obtained from previous experiments. In thesize-analyser method, settled sediment from the tube is with-drawn at prescribed intervals and the weight can bedetermined directly. An apparatus developed by DelftUniversity of Technology in the Netherlands is called DUST,and has similar functions.

Settling in a clear water system is suitable for sizeanalysis of 0.062 to 1.0 mm sediment, i.e. medium and finesand. In practice, the settling velocity of a small group ofsediment particles is measured instead of the settling veloc-ity of a single particle. This does not fulfil the requirementset forth in the assumption on which the formula for settlingvelocity is based. It has been shown by experiments that tubesize and the quantity of the sample have an influence on theanalysis results. A method for correcting the results obtainedby the VA tube method was suggested in the report of theUnited States FIASP. Size analysis was performed by the VAtube method, and corrections could be made by comparisonof the results with the known size-gradation curve, if therewere any differences. A standard sample was prepared bysubdividing a sediment sample into groups by sieve analysis.One hundred particle grains were then picked out from eachgroup. Settling velocities were determined in the tube foreach individual particle, and the size distribution within eachgroup could be calculated. The composition of the sizedistribution curve for each group according to the weight ofeach group would give the size distribution curve of thecomposite sample, which was the known gradation of thestandard sample to be used for comparison (FIASP Report,1963). Based on a similar principle, a correction method forsize analysis by the size-analyser method has also worked inChina (Xiang and Li, 1994).

(b) Dispersed System. Size analysis methods that adopt thedispersed system and are commonly used in various coun-tries include the pipette, hydrometer, bottom withdrawal tubeand photo-sedimentation, etc. These methods are suitable forsizes of less than 0.062 mm, in the silt and clay range, or, inpractice, from 0.062 to 0.002 mm. With the pipette method,

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 139

Figure 6.10 — Sketch illustrating the settling process in two systemsfor determining size distribution of fluvial sediment.

€

PC

D

t

T> =∫∫1

1

0

0

dt

C dt

ω

ω

(a) Dispersed system (b) Stratified system

t = 0 t = t1 t = 0 t = t1

Dep

th (

cm)

water and sediment are mixed in a cylinder as shown in theleft-hand sketch of Figure 6.9. At time t, a small volume ofmixture v1 (in ml) is withdrawn at distance h below the watersurface. After treatment, the dry sediment weight w1 in thesmall sample can be obtained. The percentage P by weightof sediment finer than D1 can be computed by:

(6.22)

where V is the volume of the water-sediment mixture in thetest cylinder, and W is the total sample weight placed in thetest cylinder.

In the pipette method, a 1 000 ml graduated cylinder isusually used for the analysis. After sufficient dispersal of themixture in suspension, five or six samples (25 ml) are with-drawn from the centre of the cylinder intermittently atpositions 5, 10 or 20 cm from the water surface at predeter-mined times. A period of 10 seconds is allowed for eachwithdrawal by pipette. From experience, the optimumconcentration recommended for the suspension is 0.5 to 2.0per cent by weight. Evaporation or other appropriatemethods may be used to determine the dry sediment weightcontained in the pipette samples.

Photo-sedimentation is a method used extensively byvarious industries for determining size gradation. It is asimple, rapid method particularly suitable for size analysis offine sediment of silt size. It is based on the principle of thescattering of light transmitted through a sediment-ladenwater medium. From the light scattering theory, it can bededuced that the intensity of transmitted light is related tothe intensity of light before transmission, as follows:

I = Io exp [–k L C/D] (6.23)

where I and Io are the intensity of transmitted light and thatbefore transmission respectively, k is the extinction coeffi-cient, L is the distance between the light source and thedetector, C is the sediment concentration in g 1–1, and D isparticle size.

Photo-density (the ratio of I/Io) depends not only on theconcentration C but also on the particle size D existing in themedium. Figure 6.11 shows the relation of I/Io versus Cusing d as parameters, obtained from experiments withYellow River sediment. The extinction coefficient k doesvary with size, but approaches a constant when the particle

size exceeds 0.02 mm. Experimental data of the extinctioncoefficient k versus grain size fits quite well with thatdeduced from theory (Lu, et al., 1983). It would therefore bepossible to establish a relationship between the sedimentconcentration and a photo-density reading only if the grainsize was relatively constant. In operation, the instrumentmust be calibrated carefully to establish such a relationship.

Many comparisons have been made for the resultsobtained with the pipette and photo-sedimentation methodsby analysing the same sample. The average deviation for anumber of samples is less than 1.0 to 1.5 per cent, with amaximum deviation for a single sample of less than 4.5 percent. Repetitive analysis by the photo-sedimentation methodshows that the deviation from the average value of percent-age finer is less than 5 at the 80 per cent confidence level.

To ensure the reliability and consistency of the sizeanalysis, some standards have recommended that the adop-tion of new methods for size analysis should be based on theresults of comparisons with traditional methods. The allow-able error is specified (Chinese Standards, 1992).Comparisons may be made with the percentage finer for aspecific index size or another index size by which a sizegradation curve can be defined. It was found through theintercomparison of size analysis methods conducted inChina and the United States that the results obtained by thephoto-sedimentation method are comparable to thoseobtained by the traditional pipette method (Long, et al.,1989; Lu, 1995).

A semi-automatic pipette withdrawal apparatus has beendeveloped, as reported by FIASP. The auto-pipette is an appa-ratus that makes six scheduled withdrawals (for particle sizesof 2, 4, 8, 16, 31, 62 mm) automatically in the pipette sizeanalysis procedure. A fixed-elevation, 12-depth siphonsampling scheme is used instead of mechanically lowering thepipette to a predetermined depth for each withdrawal. Anoptical water level sensor stops the siphon when the correctvolume of sample is obtained. Flushing of the siphon lineprecedes each of the scheduled sub-samples (Beverage, 1982).

6.5.2.2 TREATMENT OF SAMPLES FOR SIZE ANALYSIS

Sediment samples should be treated in preparation for size analy-sis. The state of sediment particles moving in natural streams isquite complicated. Flocculation, coagulation and various physicalphenomena have been observed in natural rivers when the sedi-ment particles are transported, eroded or deposited throughout theriver course. It should be noted that these processes continue aftercollection. Dissolved salts, organic matter and flow turbulenceinfluence the physical state of sediment particles. This is whysamples should be treated prior to size analysis.

For suspended sediment analysed by settling methods,there are two schools of thought on the treatment of samples. Thefirst one is to treat the sample to achieve a standard state so thatthe analysis results obtained at different times can be compared toeach other. The other one is to keep the sample in a state as closeas possible to its natural state. Since the influence of water qualityand the physical state of the particles on the settling property offluvial sediment is still not well known, it is difficult to study thesettling property of sediment particles in different environments.This is true in particular for fine sediment such as fine silt and clayparticles, among which flocculation easily takes place. It has been

140 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 6.11 — Relation of I/Io versus C.

(

)

Pw

v

V

WD D< =1

1

1(%)

shown by experiments that flocculation occurs easily when thereare appreciable amounts of Ca++ and Mg++ ions present in theoriginal water, or when the organic matter absorbed or attached tosediment particles exceeds 1 per cent of the sediment weight. Theinfluence varies with sediment concentration in the river. If thesample has not been treated for organic matter and no dispersingagent has been applied to the suspension medium, no reasonableexplanations can be given for the results of size analysis in theoriginal water, due to the complicated relationships among thevariables. In some rivers, flocculation varies with the season,while in other rivers no change is noticeable. For this reason,analysers generally find it preferable to disperse the sedimentsample and use distilled water as a settling medium. In otherwords, size analysis is preferably carried out in a standard stateinstead of a natural state. In general, samples are treated fordissolved salts and organic matters, and dispersing agent is addedto the sample according to specifications or standards.

During field sampling for size analysis, appreciableamounts of high organic sediments or fragments are sometimespresent in the sample. The density of coal powder is quite differ-ent from that of ordinary sediment particles that are composedmainly of minerals. The separation of coal particles is necessaryto minimize the probable error induced by the difference indensities.

From the above discussion it may be concluded that,with the present state of knowledge, it is better to make size analy-sis by a standard method of sample treatment, keeping thesediment in a state of dispersion. Research, such as parallel analy-ses, should be conducted to study the potential flocculation andthe influence of organic matter on size distribution. The chemicalanalysis of water should occasionally be carried out with rawwater while collecting samples for size analysis.

6.5.2.3 MEASUREMENT OF PHYSICAL PROPERTIES

Density or specific gravity is an important physical property ofsediment particles, which may be measured with a specific-gravityflask. In general, the specific gravity of sediment particles variesfrom 2.60 to 2.70. For quartz sand particles a value of 2.65 isusually assumed. Sediment particles are composed of variouskinds of rock fragments, mineral fragments and clay minerals.Specific gravity determined by standard methods represents anaverage value of the composite sample. If an appreciable amountof coal powder is present, it should be separated from the sample.

The unit weight or dry density of the bed material is alsoan important parameter in the study of sediment transport. Themethod of sampling undisturbed samples for the determination ofunit weight is discussed in Chapter 4.

6.6 DATA PROCESSINGSediment data acquired by various means has to be processed in aunified manner. Daily, monthly and annual sediment load andvariations in size gradations are computed by appropriatemethods. The results are tabulated and published together with theobserved stream gauging data.

6.6.1 Data processing for suspended load6.6.1.1 COMPUTATION OF SEDIMENT DISCHARGE AND CROSS-

SECTIONAL AVERAGE SEDIMENT CONCENTRATION

Ideally, sufficient sediment discharge measurements should betaken routinely in cross-sections to define time and space varia-

tions. However, in practice, simplified methods such as indexsampling have to be used, particularly during floods. If theconcentration of the index sample is closely related to that of thecross-sectional average, and deviations from the regression lineare less than ±10 –15 per cent at a frequency of 75 per cent, therelationship may be considered relatively stable. The correlationcan be used to convert the index sample concentration to the cross-sectional average value. Sometimes the correlation may beestablished according to a variation of stages or to differentseasons of a year.

Another conversion method is to compute the propor-tional coefficient that is the ratio of measured cross-sectionalaverage concentration to the corresponding index sampleconcentration. The coefficient is plotted on a hydrograph andthe line representing the variation of coefficient with time maybe used for interpolation. The cross-sectional average concen-tration can then be obtained by multiplying the index sampleconcentration by the proportional coefficient interpolated fromthe graph.

Here, further comments on index sampling are calledfor. In section 6.1, the idea of taking index samples is consideredmerely as a simplified method to supplement a conventionalmethod. If three to five verticals, arranged on an equal dischargeincrement basis, are adopted as an index sampling method, theresult may be acceptable. However, if only a single vertical isused, the position for taking index samples should be carefullyselected so that the index sample can be better correlated to theaverage sediment concentration of the whole section. As can beseen from the transverse distribution, there should be a position, orone or two verticals in a cross-section, where the ratio of averageconcentration in the vertical to the cross-sectional average is equalto 1. If this position is relatively stable, it can be used for takingthe index samples. In some streams where the concentration is lowbut varies greatly in the transverse direction, or in some untrainedrivers, significant and time-dependent differences may exist evenin the higher concentration ranges. The advisability of adoptingthis kind of index sampling should be carefully examined bystudying actual data and should be determined in the light of expe-rience. At sites where optical or nuclear concentration gauges areused along with an automatic pumping device mounted at a fixedpoint of the cross-section, the data collected are equivalent to anindex sample concentration. The relationship with the cross-sectional average value should be examined to estimate theapplicability of these kinds of physical apparatuses.

6.6.1.2 COMPUTATION OF AVERAGE DAILY SEDIMENT DISCHARGE

AND CONCENTRATION

During the low-flow season or when the water discharge showslittle variation, only one sample is taken daily or even over severaldays. Average daily sediment concentration is usually obtained byinterpolation of an appropriate value from the sediment hydro-graph for the day. Sometimes, samples taken on successive daysare combined for treatment. The resulting concentration may beused as the average concentration for the period. If there is noappreciable change in discharge but the sediment concentrationshows variations, several samples may be taken in a day. Thearithmetic mean of the concentration may then be used as theaverage value.

If there are appreciable variations in discharge and sedi-ment concentration during a day, the errors resulting from the

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 141

computation of daily average sediment concentration by themethods discussed above will be intolerable. In such cases, theconcentrations should be weighted with the water discharge in thecomputation of daily average sediment discharge or concentration.The most common methods may be summarized as follows:

Assuming the temporal variation of sediment dischargeis linear, the mean sediment discharge in time period ti to ti+1 is:

(6.24)

where Q and ρ are the discharge and sediment concentrations,respectively. The daily sediment discharge is then:

(6.25)

In other words, the sediment discharge should beweighted by the time interval it represents to give the mean dailysediment discharge.

If it is assumed that the discharge Q and sedimentconcentration vary linearly, then the daily mean sedimentdischarge may be computed by:

(6.26)

The errors involved in this method may be smaller thanthose in the method of Equation 6.27 under conditions in whichboth discharge and sediment concentrations change drasticallyduring a day and the number of measurements is insufficient todelineate the changes. Nevertheless, the above methods areapproximate methods. To be exact, the average sediment dischargein a period should be computed by integration.

(6.27)

After integration and simplification:

(6.28)

The daily sediment discharge is computed as follows,dividing a day into n time periods:

(6.29)

In one computation of daily sediment discharge forseveral stations in a tributary of the Yellow River, it was found thatthe error induced by the approximate method using Equation 6.29ranges from -0.6 to 2.8 per cent, while for the method usingEquation 6.28, it ranges from +1.0 to 6.5 per cent.

6.6.1.3 SEDIMENT TRANSPORT CURVE

The relationship between water discharge and suspended sedimentdischarge, or sediment concentration, is sometimes called the sedi-ment transport curve, or the sediment rating curve. If a sufficientnumber of sediment discharge measurements is taken, the sedi-ment transport curve can be plotted and used for interpolation orextrapolation purposes. The transport curve can be drawn from

measurements at different stages of a flood. If the peak of the sedi-ment discharge lags behind the peak of discharge, a clockwiseloop is usually obtained, and vice versa. In cases where insuffi-cient observed data are available to define the loop, an averageline is drawn through the data as an approximation. However, theresult is less accurate or reliable than if the loop in the rating curvecan be drawn. Glysson described in detail the process of develop-ing sediment transport curves, including the choice of dependentand independent variables, procedures for developing a transportcurve, and the effects of seasonal variations, major sediment trans-port events and timing of peaks on the shape of transport curves.Curve fitting methods are also discussed for using the transportcurve to estimate the sediment load for periods when measuredsediment data are not available (Glysson, 1987).

The relationship of water discharge to sediment concen-tration may be drawn for different time intervals such asinstantaneous, daily, monthly, annual or flood period. The instan-taneous curve may reflect the effect of different factors on basictransport characteristics. However, it is not theoretically applicableto the direct computation of daily sediment discharge from dailywater discharge, except for days on which the rate of waterdischarge is approximately constant throughout the day. Daily orinstantaneous water-sediment discharge curves, adjusted forfactors that account for some of the scatter from an average curve,may be used to compute approximately the daily, monthly andannual sediment discharge (Mimikou, 1982).

6.6.1.4 DATA PROCESSING FOR SUSPENDED SEDIMENT SIZE

The percentage finer for a certain size is commonly used toexpress sediment size in computation. Usually, only a limitednumber of precise measurements for the distribution of sedimentconcentration and sediment size over an entire cross-section isavailable in a year. Therefore, the index sample used in themeasurement of sediment concentrations has also been used forsize analysis to define the variations in sediment size with time.Again, the relationship between the percentage finer for the unitsamples and for the cross-sectional average samples can be usedfor determining the average size distribution. It is recommendedthat deviations from the average correlation line should not exceed±3 to 5 per cent for 75 per cent of the points for coarse particlesand not exceed ±5 to 10 per cent for 75 per cent of the points forfine particles. The percentage finer for a certain size of the indexsample can be converted to a cross-sectional average value bymeans of this relationship.

Since the vertical and transverse distributions of sedi-ment concentration have different characteristics for differentsediment sizes and vary with the hydraulic elements of the flow, itis impossible to obtain a simple correlation between the sedimentconcentration for various size groups of the index sample and thatof the cross-sectional average sample. The method discussedabove is merely an approximation for practical purposes. Forrivers with a large amount of fine material, the errors induced maybe negligible. However, if coarse particles are predominant, theerrors should not be overlooked. The discharge of coarse particlesis usually underestimated.

Average daily, monthly and annual values of the percent-ages finer for a certain size of suspended sediment can becomputed by weighting individual values with the sedimentdischarge. If the sediment discharge is relatively stable, an arith-metic mean may be used without introducing appreciable error.

142 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Q Q Q tS i

n

i i i i= + +( )− +∑1

96 1

1

1( ) ρ ρ ∆

Qt t

Q dtSi i

t ti i=

−⋅ ⋅

+

−+∫1

1 0

1 ρ

Q Q Q Q QS i Si i i i i i i= + + ++ + + +1

3

1

61 1 1 1( ) ( )ρ ρ ρ ρ

Q Q Q t Q Q ts i i i i i

n

i i i i i

n

= +( )[ ] + +( )[ ]+ + + +∑ ∑1

72

1

1441 1

1

1 1

1

ρ ρ ρ ρ∆ ∆

Q Q Qsi i i i i= +( )+ +1

2 1 1ρ ρ

Q Q Q ts i i i i i

n

= +( )[ ]+ +∑1

48 1 1

1

ρ ρ ∆

Another approach in determining the mean size distribution ofsuspended sediment is to divide the sediment into size groups. Foreach size group, the procedures discussed in the previous sectionsshould be followed in the computation of average daily, monthlyand annual sediment concentrations, and that of sedimentdischarge.

6.6.2 Data processing for bed loadBed load is part of the bed material load. It varies with flowvelocity and other hydraulic properties. The measured bed loaddischarge should be plotted on the hydrograph of water stage,discharge and suspended sediment discharge to detect any incon-sistencies. Abrupt changes or deviations from an averagetendency should be checked for reliability in the measurement ofbed load.

If sufficient bed load measurements are made over asection, a hydrograph may be plotted to show bed load movementfor the duration of the hydrometric investigation. Daily bed loadtransport rates may be read directly from the hydrograph. Theaccuracy of each measurement relies, of course, upon the properselection of sampling verticals and the sampling techniques.

Although bed sediment moves at random underaverage conditions, a definite relationship exists between thebed load transport and hydraulic elements such as discharge orstream power, which can be used for computing the daily bedload. Empirical relationships between bed load discharge andhydraulic parameters established on the basis of measurementsin low and medium flows may be extrapolated to high flowconditions. After verification with the measured bed load, well-known formulae may be used in the calculation. With sizeanalysis data, relationships can also be established for differentsize groups and can be used for computing the bed loaddischarge in size groups.

6.6.3 Examination of processed data and data processingusing computers

The processed data, including the average daily, monthly andannual sediment concentration and sediment discharge, should becarefully examined for their reasonableness, and all calculationsshould be checked. For the data obtained at a single station, therelationship between the sediment concentration of an indexsample and a cross-sectional average concentration, and relation-ships between sediment discharge or sediment concentrationversus water discharge as shown by the data obtained over a year,should be compared with the relationships used in previous years.If there have been no changes in the operational methods, either inthe measurement of sediment discharge or in the collecting ofindex samples, the trends in the relationships should not vary.Points deviating from the trend should be checked for correctness,or possible reasons for the deviation should be explored.Hydrographs of discharge, water stage and sediment concentrationshould be drawn to detect any unreasonable bias. Inconsistencycan usually be judged by experience, and should be rectified ifnecessary.

Sediment and water balance data should also be used inthe examination of processed data. The monthly and annualsediment discharges at stations located on the same river should betabulated according to a sequence from upstream to downstream.Inflows from tributaries should be added to the sediment load atupstream stations and compared with the sediment load at

downstream stations. Amounts of sediment withdrawn from theriver, sediment inflow from intermediate regions and the amount ofdeposition or erosion should be estimated or measured in order todetect any bias. This process can be expedited by applying asediment-balance equation (WMO, 1994; Ministry of WaterResources, 1975b).

In the published yearly report, explanations should begiven concerning the major factors and procedures followed in thedata acquisition and processing stages to help users judge thequality of the data for their specific purposes. An explanation ofdata processing should include: (a) Operational methods forsampling suspended sediment, instrumentation, methodology,sampling frequency and problems to be solved, etc.; (b) Dataanalysis, checking for reasonableness and interpolation method, ifapplicable; (c) Assessment of accuracy and reliability of the data;and (d) Suggestions for future work and unsolved problems, etc.

The data-processing method discussed in the previoussections is the traditional method performed manually in manyhydrological offices. However, fundamental rules still have to beobserved if computers are adopted for data-processing purposessuch as the recording and transmission of observed data, process-ing of data according to a definite program and the storage,retrieval and publishing of the processed data.

Depending on the policy adopted by different countries,the analysed hydrological data may or may not be transmitted to acentralized office for further processing. This is particularly truefor sediment measurements such as samples taken in the field thathave to be sent to regional or district laboratories for size analysis,even though sediment concentration is usually determined in fieldlaboratories. Except in a few cases, it is unnecessary to transmitsediment data on a real-time basis. However, recent developmentsin automatic observation systems, as well as the widespread adop-tion of computerized systems for data processing, have created theneed for the efficient transmission of observed data after prelimi-nary processing.

Different transmission systems may be selected accord-ing to the speed at which data are required and the availability ofproper installations. Procedures for the transmission of hydrologi-cal data may include manual, semi-automatic and fully automaticmethods. Details of the transmission methods will not bediscussed in this report. Some general guidelines are discussed inthe Guide to Hydrological Practices (WMO, 1994). The vastquantities of observed and processed data being gathered call forcareful consideration regarding data storage. At present, most ofthese data are transferred to magnetic tape or discs for workingstorage.

After processing and tabulation in standardized formatsconvenient for various uses, sediment data are published. Theannual report is a common form of presentation for all fundamen-tal hydrometric data, including sediment data. Since sediment dataare always used in conjunction with water-flow data, they shouldbe published as a complete set rather than separately.

In general, the annual report is divided into volumesaccording to the river drainage basins. Every year, data obtained atvarious stations located in the same drainage basin receive prelim-inary processing at each station, and are then compared to rectifyany processing errors. If there are any unreasonable results, thereshould be a careful examination of the field and office work and, ifnecessary, additional field investigations should be conducted. Thenext procedure is a final examination of the data obtained at

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 143

various stations in the same drainage basin, within which waterand sediment balance should be achieved. The publication of thehydrological data is the final step in the annual data-processingexercise. Needless to say, the work should be published promptly,with errors kept to a minimum. In recent years, computer technol-ogy has been universally adopted for data processing, includingpublication. When processing data using computers, the normalfundamental procedures have to be followed if reliable data are tobe expected. Clear responsibility for the long-term stewardshipand long-term security of the raw and published data needs to beestablished.

The formats used for publication may be greatly influ-enced by whether or not computers are used. If the data have beencollected on machine-readable media, or if manually-collecteddata have already been transferred to machine-readable media,tabulation can be performed by computer line printers or by photocomposition much faster and more economically than bymanually-typed copy. Standardized data formats are usually usedin publications. Guidelines on data processing issued by relevantagencies are available for use. WMO/HOMS reference manualcomponents may also be of use.

6.7 ASSESSMENT OF ACCURACY ANDRELIABILITY IN MEASUREMENT OFSEDIMENT TRANSPORT

6.7.1 General descriptionMeasurement errors may be classified as systematic or randomerrors. Random errors, represented by the precision of measure-ment, are caused by many independent factors. As the number ofmeasurements is increased, the distribution of the deviations ofobserved data from the mean value tends to follow a normal distri-bution. Thus, if there are no systematic errors, a mean value can bedetermined which approaches the true value as the number ofobservations increases. However, if there are systematic errors, theproblem cannot be eliminated by merely increasing the number ofobservations. Hence, systematic errors will accumulate with anincrease in the number of observations. Systematic errors mayconstitute only a small fraction of the total amount of observedsediment discharge, yet intolerable errors can result if the measuredsediment load is used in the estimation of the total amount oferosion and deposition for certain reaches. Both random andsystematic errors should be controlled within allowable limits. Theelimination of systematic error in a measurement is a key problemwith regard to improving the reliability of sediment data.

6.7.2 Major factors influencing the reliability ofmeasurement of sediment transport

6.7.2.1 APPARATUS

The apparatus used in routine measurement should be chosencarefully and maintained to minimize probable errors. For time-integrating and depth-integrating suspended sediment samplers,the ratio of intake velocity to ambient velocity is an importantfactor that must be controlled. For a sediment size less than0.45 mm, the error would be less than ± 5 per cent if the veloc-ity ratio could be controlled within a range of 0.8 to 1.2(Edwards and Glysson, 1998). Errors may also be induced bymisuse of depth-integrating samplers. The transit rate of thesampler should be kept uniform and should be less than 0.4times the average velocity in the vertical; otherwise, samplesmay not be representative.

For an instantaneous trap-type sampler, natural fluctu-ations in sediment movement have a large influence on theobserved sediment concentration. The fluctuations vary with thecharacteristics of flow as well as with sediment concentration.Figure 6.12 gives examples obtained by means of radioisotopegauges at two hydrometric stations, one on the main stem of theYellow River and the other on a tributary. It is clearly shownthat fluctuations in sediment concentration appear less intensiveunder high concentration than under low concentration. Theconcentrations of the samples taken with horizontal trap-typeinstantaneous samplers at more than 10 stations on several largerivers in China were analysed for errors resulting from fluctua-tions in concentration. The study showed that the relativestandard error in measured concentration due to fluctuationscould reach ± 10 per cent.

6.7.2.2 CHARACTERISTICS OF MEASURING SECTIONS

The boundary and hydraulic conditions of the measuring sectionare closely related to the accuracy of the measurement. If themeasuring section is sited at a narrow constriction of the river andthe bed is composed mainly of gravel and pebbles, sufficientmixing will take place to suspend sand material due to the flowturbulence. The distribution of sand-size material should be fairlyuniform, both vertically and transversely. Under such conditions,samples taken by conventional or even simplified methods can beconsidered representative and accurate, in comparison withsamples taken at reaches in wide alluvial channels with a sandbed. At an ordinary cross-section in a river reach, however, thedistribution of the concentration of coarse sediment is not uniformeither in transverse or along a vertical. The gradient of sedimentconcentration for coarse particles in the vicinity of the riverbedincreases very rapidly. Errors involved in disregarding the sedi-ment load transported in the so-called unmeasured zone areinevitable. In the measurement of suspended sediment, the errorinvolved in sampling coarse particles is far greater than that forfine particles.

6.7.2.3 SAMPLING FREQUENCY

For rivers where the sediment source is from upland erosioncaused by storm rainfall, the major part of the annual water andsediment flow are concentrated in the flood season, and particu-larly in large floods. Continuous records of sedimentconcentration have been kept on the River Creedy in England, asreported by Walling, et al. (1981). These records have shown that80 per cent of the total yearly sediment load is transported in 3 percent of the time. In the Yellow River, on average, 68 per cent ofthe total sediment yield is transported in only two months of the

144 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 6.12 — Fluctuations of sediment concentration in the YellowRiver and its tributary.

Con

cent

ratio

n (g

1–3

)

Time (mins)

Beijiazuan Station

Huayuankou Station

year. In 1977, the sediment transported in three floods lasting only10 days amounted to at least 70 per cent of the annual load. Thetemporal variation in sediment discharge should be consideredwhen sampling frequency is selected.

6.7.2.4 IN SITU MEASUREMENTS

Radioisotope gauges and turbidity meters have been used in somecountries to measure sediment concentration in situ. The accuracyof in situ measurements obtained using nuclear gauges depends onthe characteristics of each apparatus. Other conditions beingequal, the precision of measurement is closely related to thecounting rate of the instrument in receiving radioactive signalsfrom the source. The higher the counting rate, the higher the preci-sion and the lower the smallest detectable concentration. Asregards the nuclear gauges currently in use, the lowest detectableconcentration is approximately 0.5 g/1, with an allowable relativeerror of 10 per cent. In the low concentration range, measurementerror increases with the decrease in sediment concentration. Thelowest concentration for which the use of a nuclear gauge ispermitted can then be determined by setting an allowable error forthe measurement of sediment concentration. To ensure the desiredaccuracy, attention must be paid to field calibration or to fieldchecks on the calibration curve by means of other reliablesampling methods. Changes in water quality and mineral compo-sition of the sediment may induce variations in the calibrationcurve. It is important to calibrate the instrument in the field by

comparison with the sediment concentrations obtained by tradi-tional methods.

6.7.2.5 MEASUREMENT OF CONCENTRATION AND SIZE ANALYSIS

IN THE LABORATORY

Errors involved in the treatment of sediment samples are one ofthe error sources when sediment concentration and size gradationare determined. The volume of the sample required to ensure acertain degree of accuracy in the determination of sedimentconcentration should be considered with reference to the sensitiv-ity of the balance available in the laboratory. The sample weightshould fulfil the minimum requirements of the size analysismethod. The minimum requirements are listed in Table 6.10.

The errors involved in the laboratory treatment ofsamples for sediment concentration and size analysis have beenanalysed in some countries. Systematic errors can easily result ifsome important procedures are not followed, for instance,correction for dissolved solids and calibration of the specificgravity flask, etc. The precision and bias for the concentration testmethod put forward in the ASTM Standard is as follows. Samplesfor collaborative testing were prepared by dispersing a speciallyprepared dry powder in approximately 350 ml of water. Mixtureswere shipped in sealed glass containers to the nine participatinglaboratories, where three Youden pairs at each of the threeconcentrations were tested. The results of the test for the threemethods specified in the Standard are shown in Table 6.11.

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 145

Table 6.11Precision and bias for sediment concentration test methods

Concentration Concentration Standard deviation of Standard deviation of Bias (%)added recovered test method (St) single operator (So)

Evaporation Filtration Evaporation Filtration Evaporation Filtration Evaporation Filtration

mg/l mg/l

10 9.4 8 2.5 2.6 2.3 2 –6 –20

100 91 5.3 5.1 –9

1 000 976 961 36.8 20.4 15.9 14.1 –2.4 –3.9

100 000 100 294 532 360 0.3

Source: ASTM Standard D3977-97.

Table 6.12Precision of photo-sedimentation method of size distribution

Particle size (mm)

0.005 0.01 0.025 0.05 0.05 0.10 0.25 0.5

Settling system Disperse system Clear water system

Deviation of cumulative percentfiner at 80% confidence level 1.8 2.3 3.2 4.8 0.6 3.0 3.0 2.5

VA tube Size analyser Pipette Hydrometer BW tube Photo-sedimentation

Suitable range Sand size Silt and clay size

Minimum weight 0.05–15.0 0.3–5.0 1.0–5.0 15.0–30.0 0.5–1.8 <0.5required 3.0–20.0

Table 6.10Minimum weight of sample required for size analysis based on settling principle

Source: Ministry of Water Conservancy, 1975a; Edwards and Glysson, 1998.

The precision of different size analysis methods has beenevaluated by many parallel experiments conducted by YRCC.Each sample is divided into more than 30 parts, and repetitivemeasurements using the same analysis method are made todetermine deviations from the mean value. Table 6.12 gives theresults of the experiments determining the precision of sedimentsize distribution using the photo-sedimentation method.

6.7.2.6 COMPUTATION METHOD AND DATA PROCESSING

The main purpose of processing sediment transport data is tocalculate the total amount of sediment transported in a month, ayear or in a flood period for a given river. Methods for dataprocessing may be grouped into extrapolation and interpolationmethods. By interpolation, the sediment concentration isdetermined from the actual measured values, and the totalsediment load is computed by integration of the product ofdischarge and sediment concentration with time. Byextrapolation, a sediment transport curve is established whichdefines the average relationship between instantaneous sedimentconcentration and water discharge. Other parameters, such asdifferent sources and seasonal variations, etc., may be used ifnecessary to improve the co-relationships. Sediment dischargedetermined by means of the discharge hydrograph of a givenperiod, together with the sediment transport curve, can only beused for a very rough estimate.

Walling, et al. (1981) studied the effect of various data-processing techniques and frequency of sampling on theaccuracy of calculated sediment yield by using continuousrecords of sediment concentration extending over seven years onthe River Creedy, in the United Kingdom. The ratio of sedimentyield estimated by taking concentrations at different samplingfrequencies to the actual measured sediment discharge obtainedby detailed computation is used as an index of precision. In thestudy, sediment discharge obtained by the interpolation of sedi-ment concentration and weighted by discharge provided a resultwith a relatively high accuracy. If the concentration is notweighted by discharge, the total sediment load is seriouslyunderestimated. In assessing the reliability of data-processingmethods, both accuracy and precision should be considered(Wallin, et al., 1981).

The average daily sediment concentration of the YellowRiver is usually computed by one of the following methods.Average daily concentration may be obtained by taking theaverage concentration value interpolated from the sedimentconcentration hydrograph, or it may be obtained by computingsediment discharge, integrating with time to obtain a daily amountof sediment and then dividing by the mean daily discharge. Acomparison of the sediment load during a flood event at Lintong,Weihe River, shows that the difference in the two methodsamounts to nearly 9 per cent of the total sediment load transportedin the flood. It is recommended that discharge-weighted sedimentconcentration be used rather than the average concentrationmethod in the computation of sediment discharged during floods.

6.7.3 Major factors influencing the reliability of bed loadmeasurement

The operational method for the measurement of bed loaddischarge differs considerably from that for suspended sedimentdue to the spatial and temporal variations in bed load movement.Experience on the East Fork River indicates that verticals

densely distributed across a river and measurements taken ondouble traverses are necessary to obtain an accurate and reliablebed load discharge. In routine measurements, such requirementsare not easily satisfied. The fact that the sampler efficiency is notstable and that bed load transport varies spatially and temporallymakes it very difficult, if not impossible, to obtain reliable bedload data.

In alluvial rivers, bed load material, including thedischarge of bed load and part of the suspended load, should beclosely related to the hydraulic and boundary conditions of theflow. Direct measurements taken under relatively stable conditionscan be used to establish or verify such relationships. An estimateof the yearly sediment yield can be made by extrapolation, usingmathematical or physical models in which the total sedimenttransport rate has been verified for stable flow conditions.

6.7.4 Analysis of systematic errorsThe systematic errors involved in sediment measurement are illus-trated here by two case studies. In the first case study, long-termdata on the amount of erosion and deposition obtained through asedimentation survey were accumulated for Sanmenxia Reservoirand the Lower Yellow River. The amount was compared with thatcomputed by the sediment balance equation. The elementsinvolved in the sediment balance equation included the differencein sediment load at two terminal hydrometric stations, the amountof bank erosion, inflow from the intermediate drainage basin, sedi-ment withdrawn together with water for irrigation, and the unitweight of deposits. It was found in this case study that the system-atic errors involved in the sediment measurement at the inflowhydrometric stations might be slightly greater than 2% and thatcoarse sediment constituted a major part of the deviations.According to the second case study, systematic errors induced bysediment measurement in a vertical may be estimated by themethods proposed in section 6.4. Einstein’s total load transportformula is used to estimate the probable error in sediment concen-tration for different size groups. According to the actual dataobtained at some stations on the Yellow River, the quantity Pusually varies from 10 to 16 and A varies from 10–5 to 10–3.Assuming an average value of 13 for P and 10–5 for A, the valueof the relative error may be computed. The factors P and A aredefined in section 6.4. The relative error is the ratio of the

146 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Figure 6.13 — Variation of relative error to the suspension index z.

difference between total and measured sediment discharge aspercentages of the total sediment discharge. Variation of thecomputed relative error versus z, the suspension index in thesediment distribution formula, is shown in Figure 6.13. It can beseen that as the value of z becomes greater than 0.47, the relativeerror is greater than 10 per cent for all the simplified methods forthe measurement of sediment discharge in a vertical. The greaterthe value of z, the larger the relative error will be.

6.7.5 Analysis of random errorsRandom errors may be eliminated by repetitive measurements, asdiscussed in previous sections. However, this is true only forcertain independent variables and not for quantities such as waterstage, discharge and sediment concentration, which are unsteadyin nature. Nevertheless, with statistics from long-term records it isstill possible to obtain an average value of the variable undercertain conditions. The accuracy of sediment measurement is ingeneral not very high. Random errors within a tolerance limit areeasier to deal with than systematic errors.

There are several sources of errors involved in themeasurement of cross-sectional average sediment concentration orsediment discharge. The first category relates to the measurementof width, depth and velocity and to the sampler’s performance andefficiency. The second concerns the fluctuation properties of thevelocity and sediment concentration. The third category belongs tothe errors involved in the laboratory analysis of samples. Thefourth category concerns errors related to the method of takingmeasurements, such as the number of points in a vertical or repre-sentatives of the index sample, etc.

When assessing the probable errors involved in themeasurement, experiments carried out on site are required tocompare the results obtained by a conventional method orinstrument with those obtained by a more precise method or astandard instrument. The random uncertainty for a measurementof cross-sectional average sediment concentration is composedof two sources of errors. The errors involved in average sedimentconcentration include: (i) errors inherent to the sampler, whichare the deviation of the results obtained with the apparatus usedin comparison with those obtained by a standard apparatus (A);(ii) errors involved in the laboratory analysis of the sample (L);(iii) fewer number of points in a vertical or method ofcomputation for the average concentration in a vertical (V); (iv)errors involved in the evaluation of the cross-sectional averagesediment concentration caused by an insufficient number ofverticals or method of computing the average sedimentconcentration in a cross-section (W); (v) errors caused by aninsufficient sampling duration, due to a temporal fluctuation ofsediment concentration (T).

For instance, in a vertical, if the average sedimentconcentration Csm obtained by taking measurements at more thanfive points in a vertical, for example, seven points, is used as a truevalue of the average concentration in the vertical, and the averagesediment concentration Cs is obtained by using fewer points, thenthe relative standard error is:

(6.30)

where Ei = (Csi/Csm) — 1, and Es = 1/n ∑ Ei.Similarly, the relative standard error in the measurement

of the cross-sectional average sediment concentration may beexpressed by similar equations. In the Chinese Standards, theuncertainty of a measurement is expressed by a percentage. Fornormal distribution, the random uncertainty X should be 2σ in itsvalue at a confidence level of 95 per cent. It is specified that therandom uncertainty and systematic error involved in the sedimentmeasurement should be limited, as shown in Table 6.14 (ChineseStandard GB 50159-92).

In Table 6.13, A denotes the uncertainty induced by theinstrument used in the measurements; it is obtained by intercom-parison with the standard calibrated instrument. L is theuncertainty induced in the treatment of sediment samples. V is theuncertainty of the mean sediment concentration in a vertical,which is induced by limited sampling points in the vertical(including that induced by the method of calculation of the meanconcentration in the vertical). W is the uncertainty of the averagesediment concentration in the cross-section, which is induced bythe number of verticals and also by the method of computing thecross-sectional average concentration.

The total random uncertainty and systematic error of ameasurement of cross-sectional average sediment concentrationshould be determined by a mixture of all the errors. i.e.:

XCT = [Xw2 + (1/(m+1)) (XA

2 + XL2 + XT

2 + XV2)]1/2 (6.31)

where X represents the uncertainty value, the subscripts representthe errors specified in the previous section, and m is the number ofverticals. The total random uncertainty of a measurement of sedi-ment discharge is computed by:

XQS = [XCT2 + XQ

2]1/2 (6.32)

where XQ represents the total random uncertainty of dischargemeasurement expressed as a percentage.

It is well known that systematic and random errors areinherent in sediment measurement. Systematic errors should be

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 147

Table 6.13Allowable error of suspended sediment measurement

Random error uncertainty X

Station (%) Systematic error (%)

A L V W A L V W

Grade I 10 4.2 12 4 ±1.0 –2.0 ±1.0 ±1.0

Grade II 16 4.2 16 6 ±1.5 –3.0 ±1.5 ±1.5

Grade III 20 4.2 20 10 ±3.0 -4.0 ±3.0 ±3.0

€

σ 2 21

1=

−

−( )∑

nE Ei s

minimized by improving measurement methods, or eliminated byapplying corrections to the measured data. Random errors shouldbe minimized by enhancing the precision of the measurement,including the operational methods used in the field, the treatmentof sediment samples in the laboratory and data-processingmethods.

Similarly, the error in sediment deposition measurementby survey should be studied. A 134-km long reach of the LowerYellow River was surveyed in the early 1960s by the rangemethod, with an average distance between ranges of about 1 to3 km. Xiong, et al., (1983) found that the relative standard error incomputing the amount of erosion or deposition with only half ofthe ranges amounted to 12 per cent.

Up until now, only limited research has been carried outto assess the accuracy and precision of sediment transportmeasurement. The measurement of sediment discharge, and of thebed load in particular, is relatively crude compared to the well-established methods for stream gauging. The systematic errorinvolved in the measurement of total sediment discharge hascreated indefinite factors in the evaluation of sediment depositionby the difference in the sediment load method. Deviations betweenthe range method and the difference in the sediment load methodare commonly found in river reaches or in reservoirs.Improvements in measurement methods are necessary to enhancethe accuracy and precision of the measurements.

6.8 SUMMARIES AND RECOMMENDATIONSA better understanding of sediment yield, sediment transport anderosion or deposition is of vital concern to all engineers engagedin the planning and development of water resources. The properselection of operational methods for sediment measurement reliesnot only on the basic knowledge of sediment movement in riversor in reservoirs, but also to a large extent on the accuracy requiredfor data acquisition. To summarize, the following recommenda-tions are listed for reference.

6.8.1 Fundamental conceptsThe data-acquisition programme for the study of sedimentationproblems in river basins is given in Table 6.14.

6.8.2 Implementation of measuring programmesOn sediment-laden rivers where sediment management in the riverbasin is a problem, a programme of sediment measurement shouldbe worked out to evaluate the amount and variation of sedimenttransport with existing and supplementary hydrometric networks.Sedimentation surveys should be carried out periodically inimportant river reaches and reservoirs for a better knowledge ofthe spatial variation of erosion or deposition.

For important river reaches or reservoirs, the inflowhydrometric stations should be able to measure the input frommore than 80 per cent of the drainage basin. Measurement of thetotal sediment discharge should be carried out at such stations. Forordinary reservoirs, a minimum of 60 per cent of the drainagebasin should be represented by inflow gauging stations at whichsediment measurement is taken. Inflows from tributaries contribut-ing more than 10 per cent of the total sediment inflow should alsobe measured.

6.8.3 Measuring siteChannel conditions, including the bed material composition andflow conditions in the main channel and over the flood plain, etc.,should be thoroughly investigated by reconnaissance. If it isnecessary to measure the total sediment discharge, a section ispreferred at which all sediment is well mixed in the flow by fullydeveloped turbulence. Such stations can be located at the outlet ofa dam, or at localities where artificial roughness can be set up. Atsuch stations, conventional suspended sediment measuring tech-niques may be employed to obtain the total load data. For smallrivers, measuring structures may be constructed in which vortextubes or trenches can be installed to collect the bed load. In riverswhere fine suspended sediment constitutes the major part of thetotal sediment load, an estimate of the total sediment discharge bytaking only suspended sediment measurements should providedata with a fair degree of accuracy. However, the probable bedload discharge can only be estimated by analytical methods. Theoperational method for suspended sediment measurements shouldbe chosen carefully.

148 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 6.14Programme of data acquisition according to the International Hydrological Programme (IHP)

Source: UNESCO, 1982.

Purpose of study Items of measurement

Surveying Sediment transport Relevant items

Annual sediment discharge Total sediment discharge orconcentration at hydrometric stations Water discharge, etc.

Erosion and deposition in Sedimentation survey by Total sediment discharge at inflow Size distribution and/or unitriver reach or reservoir; ranges in a river reach or and outflow gauging stations weight of depositsdepletion of reservoir reservoircapacity

Fluvial processes in river Repetitive survey over Bed material discharge at inflow Relevant hydraulic andreaches or in backwater entire reach or in localities stations sediment parameters such asreaches of a reservoir of interest: aerial photographs water surface slope, bed

if possible material composition, velocity,depth and width, watertemperature, size distributionof sediment

6.8.4 Measurement of suspended sediment dischargeIt is very important to select an appropriate operational method formeasuring suspended sediment discharge. For each measuringstation, field data obtained by multi-point methods should beanalysed to establish simplified methods that can be employedduring floods. The relationship between the sediment concentra-tion obtained from an index sample and the cross-sectionalaverage sediment concentration obtained by a multi-point methodshould be established for conversion purposes. An index sample isone taken at a pretermined vertical, or set of verticals, by depth orpoint integration methods.

6.8.5 Corrections for transport in the unmeasured zoneThe sediment discharge value as measured using conventionalmethods of suspended sediment measurement is usually inade-quate for coarse sediment in the sand-size category. The depthintegration method leaves an unmeasured zone in the vicinity ofthe bed due to the fact that the sampler nozzle is above the bedwhen the sampler rests on the bed. If the measurement involvessampling by points in a vertical, some errors will be induced sinceit is impossible to take samples right at the bed surface where theconcentration is the greatest. Corrections are necessary if the totalsediment discharge is to be obtained. Methods similar to the modi-fied Einstein procedure may be employed for correction purposes.

6.8.6 Frequency of measurementA common feature of rivers in which floods are produced mainlyby rainstorms is the non-uniformity of both water and sedimentflow. A sufficient number of measurements during floods isneeded to monitor the entire process. Experience plays an impor-tant role in finding a compromise between the proper timing ofmeasurements and the selection of adequate measurementmethods.

6.8.7 Sampling apparatus — suspended sedimentTime-integration samplers have been used extensively in recentdecades. Besides the well-adapted depth-integrating or point-integrating series, collapsible-bag samplers or portable pumpingsamplers can also be used advantageously. The in situ measurementof sediment concentration using newly developed instrumentsdesigned on the basis of physics such as nuclear gauges or ultra-sonic or vibration type apparatuses incorporated with computerdata processing units should be encouraged. However, the necessityof calibrating samplers or measuring devices in the laboratory andin the field prior to their adoption should be emphasized. New oruntested sampling methods should be evaluated by comparing theirdata with that obtained by conventional methods in flows with awide range of concentrations. One should be well aware of the factthat most suspended sediment samplers collect samples containingboth bed material and wash load. If morphological predictions haveto be made in which a transport formula is required, the wash loadshould be determined by size analysis of the sample and excluded,since it is the bed material that is of major importance in riverbehaviour. However, the wash load may have an influence on thetransport of bed material. As mentioned in the previous sections,some samplers such as the Delft bottle directly measure the sedi-ment discharge of bed material while others, such as thepump-filter sampler, measure the concentration of bed material insuspension. These samplers may be used advantageously to studytransport characteristics in rivers that carry a small amount of

sediment. The selection of an appropriate apparatus must be basedon the objectives and technical considerations of the measuringprogramme.

6.8.8 Sampling apparatus — bed sedimentAll bed load samplers should be properly calibrated to define theirsampling efficiency. An efficiency of more than 50 to 60 per centis considered to be satisfactory for use in the field, provided thatgreat attention is paid to the operation of the bed load sampler soas to overcome the uncertainties caused by the temporal andspatial distribution of bed load movement.

When studying the armouring effect on the transportcharacteristics of an alluvial river, sampling and analysis of thebed material are important. Samples of bed material from the sandand small gravel size categoires can be taken with conventionalsamplers currently in use. However, there are still some difficultiesinvolved in the sampling of coarse gravel.

6.8.9 Computation of total loadMethods for evaluating the total sediment discharge by a combina-tion of field measurement and analytical measures appear to bepromising, and should be studied further. The formulae used in theanalytical methods should be verified with actual measured data,when available. As regards coarse-grained sediment in the bedmaterial, the total annual sediment discharge may not be large, butit is significant in the study of stream behaviour.

6.8.10 Size analysisFluvial sediment samples should be analysed in the field or labo-ratory for size distribution. A rough estimate of suspendedsediment transport may be made for sediment in two to three sizegroups. Samples are separated using sieves and any one of themethods based on the settling principle. If the data are to be usedto study sediment transport characteristics, suspended load, bedload and bed material should be analysed for size distribution. Asize gradation curve should be prepared instead of only giving therelative amounts in just the two or three size groups.

6.8.11 Method of size analysisIn the size analysis of fluvial sediment, different methods havetheir own applications. In general, for sediment particles greaterthan 0.5 to 1 mm, sieving is preferable. For medium and fine sand,silt and clay, methods based on the settling principle are preferredbecause settling velocity is an important factor in the study ofsuspended sediment. A system of settling in clear water, such asthe visual accumulation tube method, is suitable for sediment sizesfrom 1.0 to 0.062 mm. For sediment finer than 0.062 mm, asystem of settling in a dispersed medium, such as the pipettemethod or the photo-sedimentation method, is preferable.

In current practice, it is necessary to consider thedissolved salts and organic matter in the samples to be analysed.All the sediment particles should be kept in a standard dispersedstate in still, distilled water for settling. The native water is used insize analysis only for comparison. The influence of water qualityon the settling characteristics still has to be determined.

6.8.12 Data processingIn the processing of sediment data, the stage-discharge relation-ship and the relationship of index sample sediment concentrationto the cross-sectional average concentration are of fundamental

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 149

importance. In a sediment-laden river where the bed is subject todrastic changes during floods, the development of stage-dischargerelationships is difficult. The relationship of index sedimentconcentration to average cross-sectional concentration may also bedifferent for different sediment sizes. The reliability and accuracyof sediment data rely not only on the measuring method, but alsoon the data-processing method. Therefore, in data-processingwork, the careful establishment of these two relationships accord-ing to the flow and sediment characteristics, using adequate actualmeasured data for the field, is essential. In data-processing work,checking the original data and some of the computed results fortheir reasonableness is an essential and important task that shouldbe taken seriously. Computer technology is already widely used inthe processing, publication and storage of data, which providesvery useful means for the study of sediment movement in riversand reservoirs.

6.8.13 Assessment of accuracy and reliabilityUnlike discharge measurement, there is still no established methodfor assessing the precision and accuracy of sediment measure-ment. As analysed in previous sections, systematic and randomerrors are inherent in sediment measurement. Systematic errorsshould be minimized by improving the measurement methods, oreliminated by applying corrections to the measured data. Randomerrors should be minimized by enhancing the precision ofmeasurement, including operational methods used in the field, thetreatment of sediment samples in the laboratory and data-process-ing methods. According to the purposes of the data acquisition,various degrees of accuracy should be maintained at differentstations engaged in the data acquisition programme. For instance,if it is necessary to estimate annual sediment yield in some smalltributary rivers, a simplified method of observation may beallowed. However, for an alluvial reach in the main tributary of asediment-laden river, if the measurement of sediment transport isrequired for studying the fluvial processes of the reach, a rela-tively high standard of accuracy is required, particularly for bedmaterial discharge. Data on the total sediment discharge, sizedistribution and relevant hydraulic parameters should be measuredand filed for further analysis.

6.8.14 Monitoring for sediment qualityThere is an increasing need for improved data collection for thestudy of sediment quality, as the latter is closely related to theenvironmental impact of a river. Sampling procedures similar tothose used in measuring sediment discharge may be adopted, butthe standardization of analysis and careful operation are essentialif reliable results are to be expected. Sediment is a pollutioncarrier and may be harmful to engineering works as a result ofsettling in reservoirs and silting of canals, etc. However, sedimentcan also be turned into a resource if it is well managed orcontrolled. The scope of sediment measurement programmesshould be broad enough to cover the quantity as well as the qualityof the sediment in order to obtain a better understanding of sedi-ment transport.

REFERENCESAmerican Society for Testing of Materials, 1997: ASTM

Standards. D5387-97, D3977-97.American Society for Testing of Materials, 1998: ASTM

Standards. D6326-1998.

Allen, T., 1977: Particle Size Measurement. second edition,Chapman and Hall, London.

Bajiazui Reservoir Experimental Station, 1980: CylindricalRevolving Sampler for Taking Undisturbed Samples of SoftMud. Bajiazui Reservoir Experimental Station, China.

Berke, B. and L. Rakoczi, 1981: Latest achievements in the devel-opment of nuclear suspended sediment gauges. Proceedingsof the Florence Symposium, IAHS.

Beverage, J.P., 1982: Theory and operation manual for the semi-automatic pipet withdrawal apparatus. Report Z, FederalInteragency Sediment Project, St. Anthony Falls HydraulicLaboratory, Minneapolis, Minn.

Billi, P., et al., 1981: Continuous measurement of suspended sedi-ment transport by an optical-ultrasonic device. Proceedingsof the Florence Symposium, IAHS.

Brabben, T.E., 1981: Use of turbidity monitors to assess sedimentyield in East Java, Indonesia. Proceedings of the FlorenceSymposium, IAHS.

Changjiang Water Resources Commission, 1980a: Compilation ofTechniques for Bed-load Measurements.

Chien Ning, 1952: The efficiency of depth integrating suspendedsediment sampling. Transactions, AGU, Volume 33.

Chien Ning and Wan Zhaohui, 1998: Mechanics of SedimentTransport. ASCE Press.

Chinese Standards, 1992: Standard for Measurement of SuspendedSediment in Rivers. GB 50159-92, Ministry of Construction.

Chinese Technical Standards, 1992: Technical Standard SL 42-92for Determination of Sediment Particle Size in OpenChannels. Ministry of Water Resources.

Colby, B.R. and C.H. Hembree, 1955: Computations of total sedi-ment discharge, Niobrara River. Water Supply Paper 1357,USGS.

Crikmore, M.J., 1981: Recent developments in pump samplers forthe measurement of sand transport. Proceedings of theFlorence Symposium, IAHS.

Dijkman, J.P.M. and V. Milistic, 1982: Investigations on SuspendedSediment Samplers Based on Measurements in the DanubeRiver, May 1979. Joint publication S410, DHL and JCI.

Druffel, L., et al., 1976: Laboratory hydraulic calibration of theHelley-Smith bedload sampler. Open File Report 76-752,USGS.

Durette, Y.J., 1977: Hydac 100-An automated system for hydro-graphic data acquisition and analysis. Technical Bulletin 105,Water Survey of Canada.

Durette, Y.J., 1981: Preliminary Sediment Survey EquipmentHandbook. Water Survey of Canada.

Edwards T.K. and G.D. Glysson, 1999: Field Methods forMeasurement of Fluvial Sediment. Book 3, Techniques ofWater Resources Investigations, USGS.

Einstein, H.A., 1964: Handbook of Applied Hydrology. (ed.) V.T.Chow, Section 17, Sedimentation, Part 11, RiverSedimentation, McGraw Hill, New York.

Emmett, W.W., 1979: Field calibration of the sediment trappingcharacteristics of the Helley-Smith bed-load sampler. OpenFile Report 79-411, USGS.

Emmett, W.W., et al., 1981: Measurement of bed-load in rivers.Proceedings of the Florence Symposium, IAHS.

Emmett, W.W., et al., 1983: Some characteristics of fluvialprocesses in rivers. Proceedings of the Second InternationalSymposium on River Sedimentation, Nanjing.

150 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Engel, R., 1981: The efficiency of basket type bed-load samplers.Proceedings of the Florence Symposium, IAHS.

Federal Interagency Committee on Water Resources: Reports 1-14.Subcommittee on Sedimentation 1940-1963, St. Anthony FallsHydraulic Laboratory, Minneapolis, Minn.

Federal Institute of Hydrology, 1992: Instruction for Bed Loadand Suspended Load Sampling. Koblenz.

Gao Huanjin, et al., 1995: Development of new type bed loadsampler and study of its sampling efficiency. TechnicalReport of the Joint Project, CWRC.

Gao Huanjin and Li Zhaonan, 1988: Intercomparison ofsuspended samplers. Series Report 8802, Working Group onSediment Measurement, Department of Hydrology, Ministryof Water Resources, China.

Gao Huanjin and Xu Shenguo, 1989: Intercomparison of bed loadsamplers. Series Report 8803, Working Group on SedimentMeasurement, Department of Hydrology, Ministry of WaterResources, China.

Glysson, G.D., 1987: Sediment transport curves. Open File Report87-218, USGS.

Grobler, D.C., 1981: Continuous measurement of suspended sedi-ment in rivers by means of a double beam turbidity meter.Proceedings of the Florence Symposium, IAHS.

Hakanson, L., 1978: Optimization of underwater topographicsurvey in lakes. Water Resources Research, Volume 14,Washington, D.C.

Han Qiwei, et al., 1981: Initial unit weight of reservoir deposits.Journal of Sediment Research, Volume 1, Beijing.

Havinga, H., 1981: Bed-load determination by dune tracking,ISO/TC113/SC6NI55 draft.

Huang Guanhua, et al., 1983: Study of bed-load measurementby direct sampling in the Yangtze River. Proceedings of theSecond International Symposium on River Sedimentation,Nanjing.

Hubbell, D.W., 1964: Apparatus and techniques for measuringbed-load. Water Supply Paper 1748, USGS.

Hubbell. D.W., 1981: Recent refinements in calibrating bed-loadsamplers. Water Forum, Volume 1, ASCE, New York.

Indian Standard 4890, 1966: Method for Measurement forSuspended Sediment in Open Channels.

Indian Standard 6339, 1971: Particle Size Distribution andSpecific Gravity of Sediment in Streams and Canals.

Inland Waters Directorate, 1977b: SEDEX System OperationsManual. Water Survey of Canada.

Inland Waters Directorate, 1979b: Automatic Suspended SedimentPump Sampler. Water Survey of Canada.

IAHS, 1981: Proceedings of the Florence Symposium on Erosionand Sediment Transport Measurement, IAHS.

International Organization for Standardization, 1977a: ISOStandard 3716: Requirements for Suspended SedimentSamplers.

International Organization for Standardization, 1977b: ISOStandard 4363: Method for Measurement of SuspendedSediment.

International Organization for Standardization, 1977c: ISOStandard 4364: Liquid Flow Measurement in OpenChannels — Bed Material Sampling.

International Organization for Standardization, 1981a: TechnicalReport on Methods for Measurement of Bed-load Discharge.ISO-CI13-SC6NI59.

International Organization for Standardization, 1981b: TechnicalReport on Methods of Sampling and Analysis of Gravel BedMaterial. ISO FFXC I 13/SC6N 152.

International Organization for Standardization, 1982a: DraftStandard ISO/DIS 4365: Determination of Concentration,Particle Size Distribution and Relative Density.

International Organization for Standardization, 1982b: DraftStandard ISO/DIS 6421: Methods for Measurement ofSediment Accumulation in Reservoirs.

Jansen, P., et al., 1979: Principles of River Engineering. Pitman,London.

Jansen, R.H.J., 1978: The in situ measurement of sediment trans-port by means of ultra-scattering. Publication 203, DelftHydraulics Laboratory.

Leeks, Wassand, 1999: Personal correspondence.Leopold, L.B. and W.W. Emmett, 1997: Bed load and river

hydraulics — Inferences from East Fork River, Wyoming.Professional Paper 1583, USGS.

Liang Guoting 1999: User’s Manual for RGTOOLS. Institute ofHydraulic Research, Yellow River Conservancy Commission(YRCC).

Li Songhen and Long Yuqian, 1994: Methods of modification ofthe observed sediment load and its application. Journal ofSediment Research, Volume 3, Beijing.

Li Zhaonan, 1980: Reduced Ranges for Reservoir SedimentationSurvey. YRCC.

Lin Binwen, 1981: Application of Einstein's bed-load function inthe computation of total sediment discharge in the YellowRiver. Interim Report, YRCC.

Lin Binwen, 1982: Major Causes of the Deviations in Evaluationof Quantity of Sedimentation by Range Method andDifference of Sediment Load Method.

Lin Binwen and Liang Guoting, 1997: Corrections of themeasured suspended sediment discharge in sand-bedstreams. Proceedings of the Scientific Exchange, Beijing.

Long Yuqian, et al., 1978: Photo-sedimentation size analysis forfluvial sediment. Journal of Peoples’ Yellow River, YellowRiver Press.

Long Yuqian and Lin Bingwen, et al., 1982: A preliminary analy-sis of the error in the measurement of suspended sediment.Journal of Sediment Research, Volume 4, Beijing.

Long Yuqian, W.W. Emmett and R.J. Janda, 1989: Comparison ofsome methods for particle size analysis of suspended sedi-ment samples. Proceedings of the Fourth InternationalSymposium on River Sedimentation, Beijing.

Long Yuqian and C.F. Nordin, 1989: Intercomparison of collapsi-ble bag suspended sediment samplers. Proceedings of theFourth International Symposium on River Sedimentation,Beijing.

Lu Yongsheng, et al., 1980: Application of photo-sedimentationtechnique in size analysis for fluvial sediment. Proceedingsof the First International Symposium on RiverSedimentation, Guanghua Press, Beijing.

Lu Yongsheng, et al., 1983: Automatic particle size analysis offluvial sediment by photo-electric techniques. Proceedingsof the Second International Symposium on RiverSedimentation, Water Resources Press, China.

Lu Yongsheng, 1995: Brief introduction on the intercomparisonof size analysis instruments. Journal of Hydrology ,Beijing.

CHAPTER 6 — OPERATIONAL METHODS OF SEDIMENT MEASUREMENT 151

Lu Zhi, et al., 1981: Development of nuclear gauge for use onYellow River. Proceedings of the Florence Symposium,IAHS, Washington, D.C.

Ma Jingsong and Zhao Yunzhi, 1994: Development and applica-tion of computer-aided measurement of sedimentconcentration. Proceedings of the First InternationalSymposium on Hydraulic Measurement, China ResearchInstitute of Water Conservancy and Hydropower, Beijing.

Mimikou, M., 1982: An investigation of suspended sedimentrating curves in western and northern Greece. Journal ofHydrological Sciences 27, IAHS.

Ministry of Water Resources, 1975a: Manuals on HydrometricMeasurement. Beijing.

Ministry of Water Resources, 1975b: Tentative Standards forHydrometric Measurement. Beijing.

Ministry of Water Resources, 1978: Tentative Standards forReservoir Sedimentation Survey. Beijing.

Niu Zhan, 1990: Method of computation of daily sedimentdischarge. Personal correspondence.

Niu Zhan, 1994: Assessment of the uncertainty of sediment measure-ment. Research Report, Bureau of Hydrology, YRCC.

Nordin, C.F. Jr., 1981: The sediment discharge of rivers — areview. Proceedings of the Florence Symposium, IAHS.

Pemberton, E.L., 1972: Einstein’s bed-load function applied tochannel design and degradation. Sedimentation Symposiumto Honor H.A. Einstein, (ed.) H.W. Shen, Fort Collins,Colorado.

Rakoczi, L., 1984: Guide to selection of appropriate technique andequipment for measurement of river sediment transport.WMO/HOMS Sequence for Acquisition and Processing ofRiver Sediment, Budapest.

Rijn, L.C. van, 1980: Measurement of sediment concentration intidal conditions. Publication 244 , Delft HydraulicsLaboratory.

Sanmenxia Reservoir Experiment Station, 1980: OptimisticDensity of Ranges for Sedimentation Survey.

Schroeder, K.B., and C.H. Hembree, 1956: Application of the modi-fied Einstein procedure for computation of total sediment load.Transactions, AGU, Volume 37, Washington, D.C.

Shandong Provincial Office of Hydrology, 1980: Bed-load TransportEstimated from Reservoir Sedimentation, Shandong.

Singhal, H.S.S., 1981: Sediment sampling in rivers and canals.Proceedings of the Florence Symposium, IAHS.

Skinner, J.V., et al., 1981: Test and Design of Automatic FluvialSuspended Sediment Samplers.

Soil Conservation Service, 1973: National EngineeringHandbook. Section 3, Sediment, Department of Agriculture,Washington, D.C.

Stelczer, K., et al., 1984: Technique for indirect estimation of bedsediment load. WMO/HOMS Sequence for Acquisition andProcessing of River Sediment, Budapest.

Stevens, H.H., et al., 1980: Collapsible-bag suspended sedimentsampler. Journal of the Hydraulics Division, ASCE (Hy4),April, New York.

Stevens, H.H. Jr., 1985: Computer program for the computation oftotal sediment discharge by the modified Einstein procedure.Water Resources Investigations Report 85-4047, USGS.

Stichling, W., 1969: Instruments and techniques in sediment survey-ing. Reprint Series 22, Inland Waters Directorate, Canada.

Szalona, J.J., 1982: Development of a bag type suspended sedi-ment sampler. Report Y, FIASP.

United Nationas Educational, Scientific and CulturalOrganization, 1982: Sedimentation problem in river basins.Technical Report, IHP project.

Universities of Nottingham, Newcastle and Southampton, 1998:River Geomorphology: A Practical Guide. National Centrefor Risk Analysis and Options Appraisal, EnvironmentAgency, United Kingdom.

US Geological Survey, 1976: Manuals on Techniques of WaterResources Investigations. Book 3, Chapter C2, Washington, D.C.

US Geological Survey, 1977: Manuals on Techniques of WaterResources Investigations. Book 5, Chapter C1, United States.

Vanoni, V.A., et al., 1975: Sedimentation Engineering. ASCE,New York.

Walling, D.E., et al., 1981: The reliability of suspended sedimentload data. Proceedings of the Florence Symposium, IAHS.

Wang Shijiang, et al., 1995: New equation for sediment transport.International Journal of Sediment Research, Volume 10,Number 3, Beijing.

World Meteorological Organization, 1981: Measurement of RiverSediments (L. Rakoczi). Operational Hydrology ReportNo. 16 (WMO-No. 561), Geneva.

World Meteorological Organization, 1994: Guide to HydrologicalPractices. Fifth Edition, WMO-No. 168, Geneva.

Xiang Zhian, 1980: Measurement of bed-load and data processing.Changjiang Water Resources Commission (CWRC), China.

Xiang Zhian and Li Keqin, 1994: Experimental Study of theMethod of Correction of the Size Gradation Analysed by SizeAnalyser Method. CWRC.

Xiong Guishu, et al., 1983: Analysis of errors in sedimentmeasurement in the Lower Yellow River. Proceedings of theSecond International Symposium on River Sedimentation,Water Resources Press.

Xu Shengguo, et al., 1990: Intercomparison of Gravel Bed LoadSamplers at Tizhiyan Station on the Qingyijiang River.Sichuan Provincial Bureau of Hydrology.

Yorke, T.H., 1976: Ten years of experience with automaticpumping sediment sampler. Proceedings of the Third FederalInteragency Sedimentation Conference, SedimentationCommittee of the Water Resources Council, United States.

Zhang Kuotail, 1981: Application of Computer Technology inSediment Data Processing. YRCC.

Zhang Yuanfeng and Long Yuqian, 1996: Improvement ofEinstein’s bed load function. Journal of Sediment Research,Volume 4.

Zhang Yuanfeng, Long Yuqian, 1998: Adaptability of sedimenttransport formula to the Yellow River. Proceedings of theInternational Symposium on River Sedimentation, HongKong.

Zhou Dejia, et al., 1981: The development of a sand bed-loadsampler for the Yangtze River. Proceedings of the FlorenceSymposium, IAHS.

Zhou Dejia, 1989: Methodology of bed load measurement. Seriesof Technology in Hydrometric Measurement, CWRC.

152 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

7.1 EFFECTS OF SEDIMENT AND HEAVY METALSON WATER QUALITY

7.1.1 Absorption of heavy metals in sediment particlesThe absorption of heavy metals in sediment particles depends notonly on sediment composition and properties, chemical propertiesand forms of the heavy metals, but also on the variety of environ-mental factors in the body of water. The factors affecting sedimentadsorption include pH, temperature, ionic capacity, adsorbentconcentration, oxido-reduction potential and particle size, etc.

(1) Effect of temperature. Temperature is one of theimportant factors relating to how sediment affects adsorption onmetal. For both adsorbent and adsorbate, the adsorptive temper-ature and the type of adsorbate determine adsorptive capacity.The study of adsorptive isobars shows that a quantitative rela-tionship exists between temperature and adsorptive capacity.Since physical adsorption and chemical adsorption are exother-mic reactions, adsorptive capacity generally drops when thetemperature rises. Because physical adsorption is a fast process,a balance is quickly found, and adsorptive capacity drops as thetemperature rises in the experiment. The speed of chemicaladsorption is low and rise in temperature speeds up the adsorp-tion process. It thus appears that adsorptive capacity increaseswhen the temperature rises.

(2) Effect of pH. The pH value is one of the mostimportant factors in the adsorption process of metal. The effectrelates to the solubility of metal, the surface adsorptive character-istics of sediment, and the sorption reaction of metal on thesurface of sediment.

In general, the adsorptive capacities of metal on sedi-ment particles, soil and suspended solids increase with theincrease of pH. Heavy metals appear to have the most adsorptivecapacity on sediment at its characteristic pH value. Table 7.1shows a study of the adsorption of heavy metal on particles in theJinsha River, in the upper reach of the Yangtze River, China.

(3) Effect of particle size. Heavy metals in water canbe adsorbed by sediment; its adsorptive capacity for the heavymetals is firstly determined by the particle size. According toFendler’s rule, the smaller the particle size, the greater theadsorptive capacity. Particle size greatly affects the distributionof heavy metal. Heavy metals exist on sediment particles finerthan 0.025 mm.(a) Transport and deposition. As noted previously, sediment can

be defined in terms of particle size and mineralogicalcomposition, both of which are inter-related. The chemicalcomposition of the sediment at its point of deposition is aproduct of the composition of the source material, size of the

source material, sorting during transport, and physicalconditions at the point of deposition. Transportation occurs ina similar fashion in both rivers and lakes, and is a directfunction of water movement. In rivers, water movement islinear, whereas in lakes water movement is mainly orbital oroscillatory due to the passage of wind-generated waves. Inlakes, wind stress also induces major water circulationpatterns involving low velocity currents, which influence thetransport directions of wave-perturbed sediment.

(b) Particle-size fractions. The size range (diameter µ) of trans-ported particles ranges upwards from the clay-sized material,conventionally defined as (<4 µ). This fraction consistsmostly of clay minerals such as montmorillonite and kaolin-ite etc., but may also include some other fine minerals andorganic debris. The silt fraction is medium-sized (4 µ–64 µ;and the sand (2 mm–64 µ) and gravel (>2 mm) make up thecoarser size fraction. These limits are only conventional andmay change slightly from one scale to another. There is amarked relationship between the particle size and its origin(rock minerals, rock fragments and pollutants, etc.).

(c) Grain-size influence. The specific surface area is a key parti-cle property which controls adsorption capacity. It isinversely proportional to particle size and decreases overthree orders of magnitude from clay-sized particles(10 m2 g–1) to sand grains (0.01 m2 g–1). Therefore, thefinest particles are generally the richest in trace elements.This effect is particularly evident when separate chemicalanalyses are made on different size fractions, as shown forCu and particulate matter in the Fly River Basin, Papua NewGuinea (Figure 7.1). When the total particulate matter isconsidered, the trace element content is usually directlyproportional to the amount of the finest fraction, as shown inthe Rhine River for the < 16 µm fraction.

CHAPTER 7

WATER QUALITY RELATED TO TRANSPORT OF SEDIMENT AND TOXIC MATERIAL

Table 7.1Characteristic pH value of heavy metal at maximum

adsorption (mg/g)

Element Zn Co Cu Ni Pb

pH 7.6 9.0 8.4 9.0 5.5

Max. adsorption 6.65 3.30 8.20 2.15 135.78 Figure 7.1 — Copper in various grain-size fractions in the Fly RiverBasin, Papua New Guinea.

0

200

400

600

800

1000

1200

<2 2–20 20–63 >63

Cop

per

(ug/

g)

Ok TediUpper FlyMiddle FlyLower FlyStrickland

Grain-size fraction (µm)

Cop

per

(µg/

g)

(4) Effect of sediment concentration on the adsorptionof heavy metal. The adsorption capacity of sediment is obviouslyaffected by the sediment concentration. There is a negative rela-tionship between them. The lower the concentration of sediment,the more marked the enrichment action of sediment, and thegreater the sediment adsorptive capacity. The total adsorptivecapacity of sediment to heavy metal increases quickly as the sedi-ment concentration increases.

7.1.2 Effects of sediment particles absorbing heavy metalson water quality

Sediment affects water environment and water quality consider-ably. The effect has dual characteristics. From one aspect,sediment is a chief pollutant which comes from a non-pointsource, causing physical, chemical and biological pollution on thewater body. It seriously affects the water quality and the aquaticecological environment.

From another aspect, on condition that the river has aspecific hydrochemistry, a high sand content and specific sedimentphysical and chemical characteristics, many kinds of pollutants,including heavy metals and toxic organic materials that enter thewater body from sewage, can be adsorbed by sediment. Theamount and intensity of its adsorption is closely related to thesediment content and size. Sediment adsorption is the process inwhich pollutants are divided between water and sand. The resultof sediment adsorbing pollutants is an improvement in waterquality once they are filtered out of the sediment. Therefore, suchadsorption reduces the concentration of pollutants in the water andimproves toxicity and the process of removing and transformingadsorbed pollutants in the water phase. This process is controlledby content, time-space distribution and the partition of sedimentparticle size in the water.

(1) Case study 1: Sediment and water quality of theKlagan River in the tropical rainforest of Sabah, Borneo Island. Thestudy of sediment and water quality was carried out on the KlaganRiver, a tributary of Labuk River in Sabah, north-east Borneo. Theriver courses through an uneven terrain largely composed of sand-stone, limestone, and basalt. The study was designed to gatherinformation on the water-quality and sediment characteristics of theabove-described riverine ecosystem. Water quality is affected bysediment and the nature of the rocks of the area.

A number of hydrological parameters of the KlaganRiver show wide variation. The data reveal a considerable degreeof erosion of the river banks. River bank erosion is mainly respon-sible for the increase of the concentration of suspended solids to alevel as high as 328 mg/l.

Phosphate content appears to be linked to the release ofthis chemical from sediment under certain conditions of temperature,anaerobic activity and pH. Desorption of phosphate from ferrichydroxide at high pH is a distinct possibility. Water content of sulfatevaries with the salinity. Obviously, it is high in the lower reaches.

Regarding the heavy metals in water, Cd, Mn, and Znare detectable, whereas Cr, Cu, and Pb are not. Co and Nioccurred in the first sampling at station KL7. Cd may representthe sediment-water exchangeable fraction.

Concentrations of heavy metals in the sediment followthe order: Mn > Ni > Cr > Zn > Cu > Co > Pb > Cd. Except Cd,Cu, and Zn, which are relatively constant, the remaining metalsdecrease in concentration from the upper to the lower reaches ofthe Klagan River. Cd occurs dominantly as a water-exchangeablefraction, and also appears to originate from carbonate compounds.Co, Cr, Mn, Ni, Pb, and Zn are mainly in the lithogenous fraction,and have low solubility. The non-lithogenous fraction accounts forless than 20 per cent of the detectable Cu, which is mainly linkedto the organic fraction. Pb is dominant in the lithogenous fraction.High Co content is attributed to ultrabasic rocks in the region. Mnis chiefly found in exchangeable and iron-manganeseoxide/hydroxide fractions. Relatively larger quantities of Ni in thesediment are derived from basaltic rocks.

(2) Case study 2: Background concentration of heavymetals in some rivers. A study of background concentration ofheavy metal was carried out for the Yellow River. The compar-isons of background of heavy metal in the middle reach of theYellow River and in other basins in China and other countries areshown in Tables 7.2 and 7.3.

(3) Discussion on absorption and flocculation.Flocculation, because it alters the hydrodynamic properties ofparticles in transport, significantly influences the fate and effect ofsediment and associated contaminants. It was found that thecomplex structure and composition of a floc would have a signifi-cant effect on its physical, chemical and biological behaviour. Animportant observation was the apparent structural dominance ofthe fibril extra cellular polymeric material within freshwater flocs.These fibrils are believed to be the dominant material for thedevelopment and stabilization of flocculated material. Eachgeneral component of a floc (organic and inorganic particles, pluswater and pores) is diverse and can possess a specific functionwithin a floc. The interactions between these constituents and theirfunctional processes can result in the modification of a floc’sbehaviour; how it is physically transported and settled, how itadsorbs and transforms contaminants and nutrients chemically,and biologically, how it develops a diverse microhabitat capable of

154 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 7.2Concentration of heavy metals in filtered water in some

rivers (µg kg–1)

Item Cu Pb Zn Ni Cr

Yellow River 3.14 5.25 117.9 1.36 14.3

Xiangjiang River 4.0 5.0 7.0 1.6

Rivers of the world 5.0 3.0 10.0 1.6

Surface fresh water 1.8 0.2 10.0 0.5

No. 2 Songhua River 2.6 2.5 6.9 9 3

Changbaishan 4.3 13.8 11.5 0.95Tianchi Lake

Table 7.3Concentration of heavy metals in bottom sediment in some

rivers (mg kg–1)

Item Cu Pb Zn Ni Cr

Yellow River 6.89 12.8 40.9 22.7 18.6

Lakes of the world 43 28 110 66 59

Xiangjiang River 13 22 59 32 37

No. 2 Songhua River 17.7 24 119 22 17.3

South lake in 38.0 13.8 69.6 25.8 8Changchun

Non-polluted sediment 45 34 118 62

modifying its structural, chemical and biological make-up. Theseinteractions and functions are summarized in Figure 7.2.

7.2 EFFECTS OF SEDIMENT AND TOXIC ORGANICMATERIAL ON WATER QUALITY

7.2.1 Absorption of toxic organic material on sedimentparticles(1) Resolvability of toxic organic material. The resolv-

ability of toxic organic material in water is closely related to thepartition coefficient of soil/sediment (Koc), the biological concen-tration coefficient (BCF), the partition coefficient of octanol-water(Kow) and the rate of degradation of carcinogenic action.Therefore, the solubility of the organic pollutant in water is animportant assessment parameter that forecasts its harmfulnesswith regard to the environment. The major environmental parame-ters of organic compounds are shown in Table 7.4.

(2) Absorption of toxic organic material in sedimentparticles. Most of the toxic organic compound, which is difficult todegrade and easily adsorbed in sediment and layers of biologicalfat, represents an accumulated and long-term toxic danger tobiology and the environment.

CHAPTER 7 — WATER QUALITY RELATED TO TRANSPORT OF SEDIMENT AND TOXIC MATERIAL 155

Figure 7.2 — Conceptual model of floc form and function.

InorganicBiota andBioorgani

Water

Active InactiveBound Free

Contaminationtransformationmicrobial growthexopolymersanaerobic/aerobicprocesses

Nutrient and metabolicand product transportelectrochemical anddiffusional gradientsFloc building

hydrodynamicchemical andbiologicalbehaviour

Colonization sitescation bridging conta-minant adsorption/desorption

Table 7.4The major environmental parameter of organic compound

Biota andBioorganic

Name of compound S Kow Koc KB Hc Pv BCF

Acrolein 2.1E5(20µ) 1.02 0.49 0.44 5.66E-5 220(20µ) 4.38

Acrylonitrile 7.9E4(25µ) 1.78 0.85 1.04 8.8E-5 100(23µ) 7.2

Benzene 1.78E3(25µ) 135 65 37 5.5E-3 95.2(25µ) 352.5

Benzidine 400(120µ) 21.9 10.5 10.1 3E-7 5E-4 68.7

Chlorobenzene 488(25µ) 690 330 164 3.58E-3 11.7(20µ) 1.5E3

1,2,4-Trichlorobenzene 30(25µ) 1.9E4 9.2E3 3.3E3 2.3E-3 0.29(25µ) 3.0E5

Hexachlorobenzene 6E-8(25µ) 2.6E6 1.2E6 2.9E5 6.8E-4 1.09E-5(20µ) 2.5E6

1,2-Dichloroethane 5.5E3(20µ) 63 30 19 4.26E-3 180(20µ) 177.7

1,1,1-Trichloroethane 720(25µ) 320 152 81 0.03 123(25µ) 765.8

Hexachloroethane 50(22µ) 4.2E4 2.0E4 6.75E3 2.49E-3 0.4(20µ) 6.1E4

1,1,2-Trichloroethane 4.5E3(20µ) 117 56 33 7.42E-4 19(20µ) 309.96

1,1,2,2-Tetrachloroethane 2.9E3(20µ) 245 118 91 3.8E-4 5(20µ) 6.0E2

Chloroethane 5.74E3(20µ) 30.9 14.9 9.8 0.148 1E3(20µ) 93.6

2-Chloronaphthalene 6.74(25µ) 1.0E4 4.8E3 1.8E3 5.4E-4 0.017(20µ) 1.7E4

1,2-Dichlorobenzene 100(20µ) 3.6E3 1.7E3 730 1.93E-3 1.0(20µ) 6.7E3

1,3-Dichlorobenzene 123(25µ) 3.6E3 1.7E3 730 3.61E-3 2.28(25µ) 6.7E3

1,4-Dichlorobenzene 79(25µ) 3.6E3 1.7E3 730 3.1E-3 1.18(25µ) 6.7E3

3,3’-Dichlorobenzidine 4.0(22µ) 3.236E3 1553 941 8E-7 1E-5(22µ) 6.1E3

1,1-Dichloroethylene 400(20µ) 135 65 53 0.190 591(25µ) 3.5E2

Trans-1,2-Dichloroethylene 600(20µ) 123 59 48 0.067 326(20µ) 3.2E2

1,2-Dichloropropane 2.7E-3 105 51 30 2.3E-3 42(20µ) 2.8E2

Trans-1,3-Dichloropropene 2.7E3(25µ) 100 48 40 1.33E-8 25(20µ) 2.7E2

2,4-Dinitrotoluene 270(22µ) 95 45 39 4.5E-6 5.1E-3(20µ) 2.6E2

2,6-Dinitrotoluene 180(20µ) 190 92 51 7.9E-6 0.018(20µ) 4.8E2

Fluor-anthene 0.26(25µ) 7.9E4 3.8E4 1.2E4 6.5E-6 5E-6(25µ) 1.1E5

1,2-Diphenylhydrazine 1.84E3 871 418 286 3.4E-9 2.6E-5(25µ) 1.9E3

Ethylbenzene 152(20µ) 2.2E3 1.1E3 470 6.6E-3 7(20µ) 4.3E3

4-Chlorophenylphenyl ether 3.3(25µ) 1.2E5 5.8E4 1.8E4 2.19E-4 2.7E-3 1.6E5

4-Bromophenylphenyl ether 4.8(25µ) 8.7E4 4.2E4 1.3E4 1.0E-4 1.5E-3(20µ) 1.2E5

Bis(2-Chloroethoxy)methane 8.1E4(25µ) 10.7 5.2 3.7 2.8E-7 <0.1(20µ) 36.1

Methylene chloride 2.0E4(20µ) 18.2 8.8 6.0 2.03E-3 362.4(20µ) 58.2Methyl chloride 6.45E3(20µ) 8.9 4.3 3.2 0.04 3.76E3(20µ) 30.6

Methyl bromide 900(20µ) 12.3 5.9 4.2 0.197 1.42E8(20µ) 40.9

Dichlorodifluoromethane 280(25µ) 120 58 33 2.98 4.87E3(25µ) 3.2E2

156 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 7.4 (cont’d)

Name of compound S Kow Koc KB Hc Pv BCF

Trichlorofluoromethane 1.1E3(20µ) 331 159 84 0.11 667.4(20µ) 7.9E2

Isophorone 1.2E4 180 87 48 5.75E-6 0.38(20µ) 4.6E2

Hexachlorobutadiene 2.0(20µ) 6.0E4 2.9E4 1.3E4 0.0256 0.15(20µ) 8.5E4

Hexachlorocyclopentadiene 1.8(25µ) 1.0E4 4.8E3 1.8E4 0.016 0.081(20µ) 1.6E4

Naphthalene 31.7(25µ) 1.95E3 940 420 4.6E4 0.087(25µ) 3.9E3

Nitrobenzene 1.9E3(20µ) 74 36 22 1.31E-5 0.15(20µ) 2.1E2

2-Nitrophenol 2.1E3(20µ) 56 27 17 7.56E-6 0.151(20µ) 1.6E2

4-Nitrophenol 1.6E4(25µ) 93 35 27 2.5E-5 2.2(46µ) 2.5E2

2,4-Dinitrophenol 290(25µ) 500 240 122 4E-5 5E-2(20µ) 1.1E3

Benzo(a)anthracene (20µ) 4.1E5 2.0E5 5.3E4 1E-6 2.2E-8(20µ) 4.7E5

Benzo(b)fluoroanthene 0.014(25µ) 1.15E6 5.5E5 1.4E5 1.22E-5 5E-7 1.2E6

Benzo(k)fluoroanthene 4.3E-3(25µ) 1.15E6 5.5E5 1.4E5 3.87E-5 5E-7 1.2E6

Benzo(g.h.i)perylene 2.6E-4(25µ) 3.2E6 1.6E6 3.5E5 1.44E-7 1.03E-10 3.0E6

Benzo(a)pyrene 2.8E-3(25µ) 1.15E6 5.5E6 1.4E5 4.9E-7 5.6E-9(25°C) 1.2E6

Chrysene 1.8E-3(25µ) 4.1E5 2.0E5 5.3E4 1.05E-6 6.3E-9(25µ) 4.8E5

Dibenzo(a,h)anthracene 5E-4(25µ) 6.9E6 3.3E6 6.9E5 7.3E-8 1E-10(20µ) 6.0E6

Indeno(1,2,3-cd)pyrene 5.3E-4(25µ) 3.2E6 1.6E6 3.5E5 6.95E-8 1E-10(20µ) 3.0E6

Fluorene 1.69(25µ) 1.5E4 7.3E3 3.8E3 6.4E-5 7.1E-4 2.4E4

Vinylchloride 2.7E3(25µ) 17.0 8.2 5.7 8.14E-2 2.66E3(25µ) 54.7

Trichloroethylene 1.1E3(20µ) 263 126 97 9.1E-3 57.9(20µ) 6.4E2

Tetrachloroethylene 200(20µ) 759 364 252 0.0154 14(20µ) 1.7E3

Toluene 534.8(25µ) 620 300 148 6.66E-3 28.7(20µ) 1.4E3

Phenanthrene 1.00(25µ) 2.8E4 1.4E4 4.7E3 2.26E-4 9.6E-4(25µ) 4.2E4

Pyrene 0.13(25µ) 8.0E4 3.8E4 1.2E4 5.1E-6 2.5E-6(20µ) 1.1E5

Dieldrin 0.195(25µ) 3.5E3 1.7E3 710 4.57E-10 1.78E-7(20µ) 6.6E3

Chlordane 0.056(25µ) 3E5 1.4E5 4E4 9.4E-5 1E-5(25µ) 3.6E5

Aldrin 0.180(25µ) 2E5 9.6E4 2.8E4 1.6E-5 6E-6(25µ) 2.5E5

Alpha-Endosulfan 0.53(25µ) 0.02 9.6E-3 0.012 1E-5 1E-5(25µ) 0.128

Beta-Endosulfan 0.28(25µ) 0.02 9.6E-3 0.012 1E-5 1.9E-5(25µ) 0.128

Endosulfan sulfate 0.22 0.05 0.024 0.029 2.6E-5 1E-5(25µ) 0.29

Endrin 0.25(25µ) 3.5E3 1.7E3 710 4E-7 2E-7(25µ) 6.6E3

Edrin aldehyde 50(25µ) 1.4E3 670 310 2E-9 2E-7(25µ) 2.9E3

Heptachlor 0.18(25µ) 2.6E4 1.2E4 4.4E3 4.0E-3 3E-4(25µ) 3.9E4

Heptachlor epoxide 450(25µ) 2.2E2 1.1E2 3.9E-4 3E-4 3E-12

Alpha-BHC 1.63(25µ) 7.8E3 3.8E3 1.5E3 6.0E-6 2.5E-5(20µ) 1.4E4

Beta-BHC 0.24(25µ) 7.8E3 3.8E8 1.5E3 4.5E-7 2.8E-7(20µ) 1.4E4

Delta-BHC 31.4(25µ) 1.4E4 6.6E3 3.5E3 2.07E-7 1.7E-5(20µ) 2.3E4

Gamma-BHC 7.8(25µ) 7.8E3 3.8E3 1.5E3 7.8E-6 1.6E-4(20µ) 1.4E4

PCB-1016 0.42(25µ) 3.8E5 1.8E5 5.0E4 3.3E-4 4E-4(25µ) 4.4E5

PCB-1221 40.0(25µ) 1.2E4 5.8E3 2.2E3 1.7E-4 6.7E-3(25µ) 1.99E4

PCB-1232 407(25µ) 1.6E3 771 351 1.13E-5 4.06E-3(25µ) 3.3E3

PCB-1242 0.23(25µ) 1.3E4 6.3E3 2.3E3 1.98E-3 1.3E-3(25µ) 2.1E4

PCB-1248 0.054(25µ) 5.75E5 2.77E5 7.29E4 3.6E-3 4.94E-4(25µ) 6.5E5

PCB-1254 0.031(25µ) 1.1E6 5.3E5 1.3E5 2.6E-3 7.71E-5(25µ) 1.2E6

PCB-1260 (25µ) 1.4E7 6.7E6 1.3E8 0.74 4.05E-5(25µ) 1.1E7

Toxaphene 0.50(25µ) 2E3 964 429 0.21 0.2-0.4(20µ) 3.9E3

Dimethyl phthalate 5.0E3(20µ) 3.63 17.4 16.0 2.15E-6 4.19E-3(20µ) 13.7

Diethyl phthalate 896(25µ) 295 142 107 1.2E-6 3.5E-3(25µ) 7.1E2

Di-n-butyl phthalate 13(25µ) 3.6E5 1.7E5 4.7E4 2.8E-7 1E-5(25µ) 4.2E5

Di-n-octyl phthalate 3.0(25µ) 7.4E9 3.6E9 3.9E8 1.7E-5 1.4E-4(25µ) 3.2E9

Bis(2-Ethylhexyl)phthalate 0.4(25µ) 4.1E9 2.0E9 2.3E8 3E-7 2E-7(20µ) 1.9E9

ButylBenzylphthalate 2.9 3.6E6 1.7E3 5.7E4 8.3E-6 6E-5 3.4E6

s: Resolvability in water (ppm) BCF: Biological concentration coefficientKoc: Partition coefficient of soil/sediment Hc: Constant of Henry (torr/mor)Kow: Partition coefficient of octanol-water Pv: Press of vapour (torr)KB: Partition coefficient of microbe-water (µg/g) (mg/L)

The comparison of partition and adsorption. Theadsorption mechanism of organic compounds in a water sedimentsystem is partition, and the adsorption mechanism of metals isadsorption. Their reactions present differences in the mechanisms.The differences between them relate to action, reaction thermal,type of sorption isothermal formula and sorption competitiveness(see Table 7.5).

The remaining concentration of organic compounds inriver sediment. The adsorption process is determined by thehydrophile or hydrophobe nature of the compound and the compo-sition of the adsorbent, of which the decisive factors are solubility,partition coefficient of octanol-water and the organic carboncontent of adsorbent.

The remaining concentration of organic compoundsdetermined by experiments in river sediment is shown inTable 7.6.

7.2.2 Effects of sediment particles absorbing toxic organicmaterial on water quality

As mentioned in section 7.1.2, the effects of sediment particles onwater quality are considerable and present a characteristic of adual nature. Toxic organic material can be adsorbed and kept bysediment for a long time, representing the sediment-waterexchange. An example of this is the following case study: In Italy,the use of pp’-DDT and lindane was forbidden in the 1970s, andthe application of lindane is currently restricted to agricultural useand the application of pp’-DDT to floriculture. The presence ofpp’-DDT metabolites indicates that the pesticide is no longer usedin the catchment basin, and that DDT contamination is due to thepast usage of this pesticide.

7.3 WATER QUALITY MODEL OF SEDIMENT ANDTOXIC ORGANIC MATERIAL AND HEAVYMETAL

Water quality models are designed to simulate the responses ofaquatic ecosystems under varying conditions. They have beenapplied to help explain and predict the effects of human activitieson water resources, such as lake eutrophication, dissolved oxygenconcentrations in rivers, the impacts of acid rain on natural waterbodies, and the fate, pathways, impacts and effects of toxicsubstances in freshwater systems. Mathematical models are veryuseful tools for water quality management because they allow:(1) The identification of important variables in a particular

aquatic system, and help interpretation of the system’sprocesses;

(2) Forecasting of the impacts of developments on water bodies;and

(3) Policy testing and analysis.The high degree of complexity, spatial and functional

heterogeneity, non-linearity, complex behavioural features (suchas adaptation and self-organization) and the considerable stochas-tic element of natural systems make model development a difficultand highly skilled task. Data requirements for model calibrationand for model use pose additional constraints on their widespreaduse. This complexity, and the limited knowledge of the processestaking place in rivers and lakes, requires that a high degree ofsimplification and a number of assumptions be built into anymodel. The model user must be aware of the model’s limitationsand its assumptions in order to draw appropriate conclusions. Atpresent, highly predictive models are not general and generalmodels are not highly predictive.

Model types: Mathematical models belong to one of twobasic classes, namely theoretical (or deterministic) and empirical.

Theoretical models: If the physical, chemical and/orbiological mechanisms underlying a process are well understood,a steady-state or dynamic model can be developed. Steady-statemodels cannot be used for predicting system responses over time,and they therefore have limited water management value. Time-variable models, on the other hand, can handle variable inputloads, and can be useful for establishing cause-effect relationships.

CHAPTER 7 — WATER QUALITY RELATED TO TRANSPORT OF SEDIMENT AND TOXIC MATERIAL 157

Table 7.5The characteristics of adsorption and partition

Adsorption Partition

High adsorption thermal Low adsorption thermal

Non-linear isotherm Linear isotherm

Competitive adsorption Non-competitive adsorption

Table 7.6Adsorption coefficient of PCBs and organic chloride pesticide

Compound Soil/sediment OC/OMµ%µ Kd Koc/Kom

2,2,4’-PCB Sandy soil 1.9(om) 460 24 000

2,5,2’-PCB Suspended sediment of river 4.1(oc) 10 000 250 000

Hexachlorinatedbiphenyls Sediment of Lake Michigan 2.9(oc) 9 000 310 000

Hexachlorinatedbiphenyls Suspended sediment of river 4.1(oc) 13 000 300 000

Aroclor 1254 Sediment of Lake Michigan 1.7(oc) 7 000 410 000

DDT Sediment of ocean 2.7(oc) 48 000 1 800 000

DDT Soil sample (om) 140 000

P,P’-DDE Suspended sediment of river 4.1(oc) 41 000 1 000 000

µ-BHC(Lindane) Sandy soil 1.9(om) 14 740

µ-BHC Creek sediment 2.8(om) 24 860

µ–Chlordane Suspended sediment of river 4.1(oc) 13 000 300 000

µ–Chlordane Suspended sediment of river 4.1(oc) 1 000 250 000

Endrin Sand 0.7(om) 58 8 300

Kepone Sediment of bay 1 700

oc: Organic carbon om: Organic material

When compared to empirical models, theoretical models aregenerally more complex. They require a longer period of observa-tion for calibration, and the number of variables and parameters tobe measured are greater. They also require a significant amount oftime for validation. Owing to their complexity, and because ourunderstanding of aquatic systems is usually incomplete, thesetypes of models are used less frequently than empirical models.

Empirical models: Empirical or statistically-basedmodels are generated from the data analysis of surveys at specificsites. The relationships thus identified are then described in one ormore mathematical equations. These models can be built relativelyquickly when compared with theoretical models, and they areeasier to use because they have fewer data requirements.Sometimes empirical models have to be generated from incom-plete or scattered information about the aquatic system. Forexample, the model may be supported by observations made overa limited range of conditions or a relatively short time period. Insuch cases, the model output should be interpreted with caution. Itis also important to remember that such models are not directlytransferable to other geographic areas or to different time scales.

Examples of water quality models: Hundreds of waterquality models have been developed. Some of them are specific toa given site or problem, while others are more general, such asmultimedia models. There is no single model that can be appliedto all situations. Some examples of models are described below.

Water Analysis Simulation Programme (WASP): Thistheoretical model is applicable to a wide variety of water qualityproblems, and can be adapted for site-specific uses. It is a time-variable model that can be applied to one, two or threedimensions. The input data consist of loads, boundary conditions,mass transfer rate, kinetic rates and concentrations of organiccompounds, trace elements and phytoplankton. The output listsvariable concentrations.

REFERENCESChapman, D., 1992: Water Quality Assessments. Chapman and

Hall, London.Evans, R.D., J.R. Wisniewski, and J. Wisniewski, 1997: The inter-

actions between sediments and water. Proceedings of theSeventh International Symposium , Baveno, Italy,22–25 September 1996, Kluwer Academic Publishers.

Jin Xiangcan, 1990: Pollution Chemistry of Organic Compounds.Qinghua University Publishe.

Jin Xiangcan, 1992: Pollution Chemistry of Sediment .Environmental Science Publisher.

Osterkamp, W.R., 1995: Effects of Scale on Interpretation andManagement of Sediment and Water Quality. IAHS.

Zhao Peilun, 1998: The Effect of Sediment on Water Quality of theYellow River and Control of Water Pollution in Major Rivers.Yellow River Hydropower Publisher.

158 MANUAL ON SEDIMENT MANAGEMENT AND MEASUREMENT

Table 7.7Main characteristics of the sampling stations and sediment samples

Sampling station Sediment samples

Stations and Sub- Depth of Type of Date of Water Organicsamples basin station (m) sample samples (%) (%) carbon

51 (0–10 cm) North 71 Core 1970–1992 65.4 9.96

51 (10–20 cm) North 71 Core 1948–1970 64.4 6.15

51 (38–48 cm) North 71 Core 1885–1970 58.5 4.15

52 North 33 Grab 1970–1992 59.6 4.87

53 North 19 Grab 1970–1992 75.1 11.65

54 North 30 Grab 1970–1992 67.0 9.84

43 North 89 Grab 1970–1992 39.0 7.44

56 North 58 Grab 1970–1992 42.5 4.53

45 (0–10 cm) North 143 Core 1970–1992 25.0 3.20

45 (10–20 cm) North 143 Core 1948–1970 31.0 4.11

45 (32–42 cm) North 143 Core 1900–1922 24.0 3.20

32A Central 90 Grab 1961–1992 42.2 5.83

13 (0–10 cm) South 47 Core 1950–1992 49.0 6.84

13 (10–20 cm) South 47 Core 1908–1950 41.0 4.78

13 (50–60 cm South 47 Core 1742–1784 28.0 5.80

1A South 11 Grab 1950–1992 74.6 4.97

WMO ReportNo. No.

332 No. 1 — Manual for estimation of probable maximum precipitation (Second edition)337 No. 2 — Automatic collection and transmission of hydrological observations*341 No. 3 — Benefit and cost analysis of hydrological forecasts. A state-of-the-art report356 No. 4 — Applications of hydrology to water resources management*419 No. 5 — Meteorological and hydrological data required in planning the development of water resources*425 No. 6 — Hydrological forecasting practices*429 No. 7 — Intercomparison of conceptual models used in operational hydrological forecasting433 No. 8 — Hydrological network design and information transfer461 No. 9 — Casebook of examples of organization and operation of Hydrological Services464 No. 10 — Statistical information on activities in operational hydrology*476 No. 11 — Hydrological application of atmospheric vapour-flux analyses*513 No. 12 — Applications of remote sensing to hydrology519 No. 13 — Manual on stream gauging — Volume 1: Field work — Volume 2: Computation of discharge559 No. 14 — Hydrological data transmission560 No. 15 — Selection of distribution types for extremes of precipitation561 No. 16 — Measurement of river sediments576 No. 17 — Case studies of national hydrological data banks (planning, development and organization)577 No. 18 — Flash flood forecasting580 No. 19 — Concepts and techniques in hydrological network design587 No. 20 — Long-range water-supply forecasting589 No. 21 — Methods of correction for systematic error in point precipitation measurement for operational use635 No. 22 — Casebook on operational assessment of areal evaporation646 No. 23 — Intercomparison of models of snowmelt runoff650 No. 24 — Level and discharge measurements under difficult conditions655 No. 25 — Tropical hydrology658 No. 26 — Methods of measurement and estimation of discharges at hydraulic structures680 No. 27 — Manual on water-quality monitoring683 No. 28 — Hydrological information referral service — INFOHYDRO Manual686 No. 29 — Manual on operational methods for the measurement of sediment transport704 No. 30 — Hydrological aspects of combined effects of storm surges and heavy rainfall on river flow705 No. 31 — Management of groundwater observation programmes717 No. 32 — Cost-benefit assessment techniques and user requirements for hydrological data718 No. 33 — Statistical distributions for flood frequency analysis740 No. 34 — Hydrological models for water-resources system design and operation749 No. 35 — Snow cover measurements and areal assessment of precipitation and soil moisture773 No. 36 — Remote sensing for hydrology — Progress and prospects754 No. 37 — Hydrological aspects of accidental pollution of water bodies779 No. 38 — Simulated real-time intercomparison of hydrological models804 No. 39 — Applications of remote sensing by satellite, radar and other methods to hydrology803 No. 40 — Land surface processes in large-scale hydrology806 No. 41 — An overview of selected techniques for analysing surface-water data networks813 No. 42 — Meteorological systems for hydrological purposes884 No. 43 — Current operational applications of remote sensing in hydrology885 No. 44 — Areal modelling in hydrology using remote sensing data and geographical information system886 No. 45 — Contaminants in rivers and streams — Prediction of travel time and longitudinal dispersion886 No. 46 — Precipitation estimation and forecasting

* Out of print

OPERATIONAL HYDROLOGY REPORTS