Nile River Sediment Modelling Challenges and Opportunities

7/29/2019 Nile River Sediment Modelling Challenges and Opportunities

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Nile River Sediment Modelling: Challenges and Opportunities

Abdalla A. Ahmed1

H. Sersawy2

V. Vanacker3

Usama. H. Ismail4

2- Mathematical Modeling Dep., Nile Research Institute, Elqanater El-Khiria,P.O. Box 13621, Egypt, [email protected]

3- Flemish Counterpart of STWMC, Belguim, [email protected]

4- Theme Researcher of DLFAC, UNESCO Chair in Water Resources, Sudan

[email protected]

Abstract

Alluvial river system is a dynamic one, which goes into complex responses where subject tochanges in flow hydraulics and sediment loads. Sediment modelling proved to be a usefultool in providing effective mitigation measures for problems related to sediment transport anddisposition. These models are also used to improve our understanding of river morphology

processes. In this paper the concept of sediment modelling is addressed. The majorcategories of sediment transport models are discussed and the limitations are pointed out.Moreover, the difficulties and constraints facing the sediment modelling in the Nile Basin are

discussed taking into consideration the four years experience with sediment transport andwatershed management within the FRIEND/ Nile project. Specifically, the problems andconstraints faced the implementation of Surface Water Modelling System (SMS) arehighlighted and thoroughly discussed.

Introduction

The Nile drainage basin represents the longest route of sediment transport on the world as itextends to 6671 km, Ahmed and El Daw, 2004. The production and transportation ofsediment in river basins are influenced by a complex set of geomorphic processes that vary in

time and space. There are various sources of sediment, such as upland rill erosion and interrillerosion, gully erosion, landslides, debris flows, channel bank erosion and wind erosion. Land

use practices such as logging and clearing, grazing, road construction, agriculture, andurbanization activities also affect sediment production and delivery. On small spatial andtemporal scales, the erosion rate is roughly equivalent to sediment yield. However, for larger

1 UNESCO Chair in Water Resources - Sudan2Nile Research Institute - Egypt

3Catholic University of Leuven Belgium

4UNESCO Chair in Water Resources - Sudan


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basins, like the Nile Basin, as the area and time scale s increase transport and depositionprocesses influence the sediment delivery, Fig (1).

The study of sediment transport of the Nile River is essential for the evaluation of the socio-

economic and environmental impacts of sediment processes in the river basin, not only tounderstand the soil loss characteristics in the upstream catchments area but also to assess the

socio-economic and environmental impacts of sediment transport and disposition in thedownstream region. The importance of such activities is especially important where thesocio-economic activities of a large number of people are directly linked to the Nile River.

A reliable and accurate sediment model is needed to assess sediment transport and dispositionso that management practices can be evaluated and downstream problems can be identified.

Numerical methods for simulating sediment transport processes are useful management toolsfor planning and assessing mitigation measures. Typically, these tools are applied for a

relatively short time span to provide necessary information for predicting impacts ofchanging hydraulic and sediment transport conditions.

The objective of this paper is to give an overview of present sediment transport models, and

to present the characteristics of the SMS software, which was chosen within theFRIEND/Nile project as an effective tool to model sediment transport and disposition in the

Nile River. Moreover, it reflects the main constraints and problems faced the implementationof SMS Model.

The Concept behind Sediment Modelling in River Channels

All models that deal with sediment transport in river channels share the basic notion ofcreating conceptions of physical reality that results in quantitative predictions. They are a set

of mass balance equations designed to quantitatively represent the processes that determinethe erosion and/or the transport of sediment. The equations of models are based on the

conservation of mass to properly account for all the inputs, transformations, outflows of thewater, solids, and contaminants in the surface water system. The study areas in the models aredivided into grids using either finite difference or finite elements methods for solving the

involved equations.

Most sediments models are uncoupled. First they solve the flow equations and use the resultsas input for sediment modeling. There exist currently two major techniques for solving flowequations. The first one is analytical and the other is numerical. Three numerical methods are

generally used in modeling (i) -finite difference (ii) - finite elementand (iii) - The methodof characteristics. Sediment modelling is complex, and suffers common limitations mainly

due to the estimation of channel roughness or friction slope as they are difficult to measure inthe field. The roughness can be approximated by using either Mannings (n) or Weisbachs

(f-coefficient). However, most of the existing formulas to calculate the channel roughness arederived for steady uniform flow.


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The Sediment Transport Modeling Framework

The framework that will be used to evaluate water flow and sediment transport requiresgenerally the following two types of model formulations: (i) hydrodynamic models and (ii)Sediment models. Table (1) gives some examples of hydraulic and hydrodynamic models,Summer and Walling, 2002.The hydrodynamic model is an important component of sediment transport models because itsimulates not only the stream velocity and stage height, but also the bottom shear stress that

in turn, controls the entrainment, transport and/or deposition of material.

Table (1) Examples of Hydraulic and Hydrodynamic Models

1D = One-dimensional 2D= Two- dimensional 3D= Three- dimensional

The sediment transport model represents the entrainment, transport and deposition ofsediment both as suspended load and bed load within a water column.

ModelType ofModel

Source Availability

HEC-2 1D USACE Hydrologic Engineering Center; HEC(1982) Public domain/ Intermediate

CE-QUAL-RIVI 1D USACE/ WES;ERL (1995) Public domain/ IntermediateFLDWAV2.0 1D NOAA; NWS (2001) Public domain/ Intermediate

RIVMOD 1D EPA CEAM; Hosseinipour & Martin (1993) Public domain/advanced

MIKE11 1D DHI Water & Environment Proprietary/ advanced

EFDC-ID 1D EPA Region IV; Tetra Tech (2001) Public domain/advancedMIKE21 2D DHI Water & Environment; Proprietary/ advanced

MODFLOW-HMS 2D Hydro Geologic; Huyakorn (2000) Proprietary/ Intermediate

TABS-2 2D USACE/ WES; Thomas &Mac Nally (1985) Public domain/advanced

HSCTM-2D 2D EPA CEAM; Hayter et al. (1999) Public domain/advanced

RMA-2 2D Resource Management Assoc. Proprietary/ advanced

FESWMS-2D 2D USGS; Froelich (1989) Public domain/advanced

BFHYDRO 3D Applied Science Assoc; Proprietary/ Intermediate

POM 3D NOAA Geophysical; Blumberg& Mellor (1987) Public domain/advanced

ECOM-3D 3D HydroQual; Blumberg & Mellor (1987) Public domain/advanced

CH3D-WES 3D USACE/WES; Johnson et al. (1991, 1993) Public domain/advanced

EFDC 3D EPA OS&T/Tetra Tech; Harmrick (1992, 1996) Public domain/advanced

FLOW-DELFT3D 3D Delft Hydraulics lab; Proprietary/ advanced

MIKE3 3D DHI Water & Environment Proprietary/ advancedRMA-10 3D Resource Management; King & DeGeorge (1996) Proprietary/ advanced

TRIM3D 3D USGS; Cheng et al. (1993) Proprietary/ advanced

GEMSS-HDM 3D J.E. Edinger & Associates; Edinger (2002a) Proprietary/ advanced


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Common Feature of Sediment Transport Models

In his famous paper, Show -shan, 1994, reviewed eleven different sediment models. Heconcluded that all models may have different sediment distribution assumptions, and that

models may produce significantly different results, even when run with the same set ofinputs. Most sediment models are uncoupled, in the sense that they solve the flow equation

first and consider the sediment transport later. Mathematically, they solve a boundary valueproblems and partial differential equations with finite difference, finite elements or themethod of characteristics. Table (2) summarizes the famous sediment transport models used

nowadays throughout the world as an effective tool for solving sediment problems.

Table (2) Examples of Sediment Transport Models

ModelType

ofModel

Source Availability

CTAP 1D HydroQual; EPA(1984) Public domain / Screening

SLSA 1D HydroQual; EPA(1984) Public domain / Screening

SMPTOX3 1D EPA CEAM; Limno-Tech (1993) Public domain / Screening

MICHRIV 1D EPA CEAM; EPA (1984)

Public domain/

Intermediate

HEC-6 1DUSACE/ Hydrologic Engineering Center;

HEC (1993)Public domain/Intermediate

RIVMOD 1D EPA CEAM; Hosseinpour and Martin(1993)Public domain/Intermediate

WASPS-

TOX

15

3D EPA CEAM; Ambrose et al. (1993)Public domain/

Intermediate

HSPF-RCH

RES

1D EPA OS& T; Bicknell et al. (1997-2001)Public domain/

Intermediate

SSFATE 3D Applied Science Assoc; w ww.appsci.com Proprietary/ Intermediate

SEDZL 2D Ziegler & Nesbit (1994-1995) Public domain/advanced

ECOMSED 3D HydroQual; Ziegler & Nesbitt (1994,1995) Public domain/advanced

EFDC 3D EPA OS & T; Tera Tech (1999a) Public domain/advanced

EFDC-ID 1D EPA Region IV; Tera Tech (2001) Public domain/advanced

CE-QAUL-

ICM3D USACE/WES Public domain/advanced

HSCTM-2D 2D EPA CEAM; Hayter et al. (1999) Public domain/advanced

CH3D-SED 3D USACE/WES; Spasojevic & Holly (1994) Public domain/advanced

WAQ-

DEL

FT 3D

3D Delft Hydraulics Lab;< www.wldelft.nl> Proprietary/ advanced

MIKE3-WQ 3DDelft Water & Env;

Proprietary/ advanced

RMA11 3DResource Management; King

&DeGeorge(1996)Proprietary/ advanced

TRIM2D 2D USGS; McDonald & Cheng (1997) Proprietary/ advanced

GEMSS-STM 3D J.E Edinger & Assoc; Edinger (2002a) Proprietary/ advanced

1D = One-dimensional 2D= Two- dimensional 3D= Three- dimensional

Generally, most models deal with steady flow conditions and with the following four mainhydraulic aspects: (i) the equation of motion for water (ii) the continuity equation for water

and sediment (iii) one or more sediment transport functions and (iv) a relationship for channelresistance. Thus, most models deal with unsteady flow conditions as steady one, and use

sediment transport functions which are not applicable to wash load or unsteady flow


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conditions. Only a few models in limited ways can model bank erosion, armoring effects onchannel geometry and morphology changes.

Application of Sediment Transport Models in the Nile Basin

i- Selection of a Sediment Transport Model for the Nile Basin

The selection of sediment transport function is a matter of professional judgment. At present,the sediment transport functions included in the stream sedimentation models for predicting

sediment transport capacity have mostly been developed from limited data for non-cohesivesediment transport in ideal steady and uniform flows. More importantly, the sedimenttransport functions were derived with the assumptions that there are close relations between

sediment transport capacity and one or more hydraulics variables, such as stream flow or/andvelocity. It has been proved that such correlation is often weak. Therefore, at present, few

sediment transport functions are applicable to wash load or unsteady flow conditions.

Sediment transport models are still in developing stage. These models are an approximation

of the reality, but are no true representation. Therefore at present times they cannot substitute

the professional experience. The interpretation of the model output by experts is highlyrequired for a successful application of those models. Computer modeling is a useful tool forscientists and engineers, but it can also be a source of mis-information for those who do notknow all the assumptions, capabilities, and limitations of the models.

Based on a comprehensive review of available sediment transport models, the Surface Water

Modelling System (SMS) was selected as a tool to model and analyze sediment transport inthe Nile River Basin. For this review, the available models were classified in terms of (i)

functionally; grouped into five major categories, watershed, stream, reservoir, estuarine, andcoastal sedimentation (ii) dimensionally; classified as one dimension, two dimensions andthree dimensional models. Table (3) illustrates the main components of SMS model in brief.


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Table (3) Gives Detailed Information of SMS Model ComponentsModel RMA2 SED-2D FESWMS

Type Flow Sediment Flow / Sediment

Dimensional

2D 2D 2D

Origin ofthe model

RMA2 developed by Norton,King &Orlob, 1973. Further

development carried byResource Management

Associates (RMA) and USA-ERDC culminating in thecurrent TABS-MD.

Developed by R.

Ariathurai (l974) andextended to include thevertical plane by

MacArthur & Krone,l977, then modernized

by WES ,1993 to create

SED2D.

FESWMS-2DH

developed for FHWA andUSGS, Wat. Res.Division, 1978.

Similar to TABS-MDCoded by David C.Froehlich.

Application

Flow patterns and levels in:-Rivers, reservoirs andestuaries

BridgesRiver junctions.

Hydropower plantsUser selectable turbulence,

roughness, temperature, etc.Model up to 5 different typesof flow control structures

SED2D can be applied toclay or sand bedsediments where flow

velocities can beconsidered 2D in the

horizontal plane. It isuseful for both

deposition and erosionstudies. Model treatsnoncohesive & cohesive.

Flow patterns and levelsin:

Bridges, River junctionsand contracting. Designof rip rap based on

computed bed shear.Easy modeling of weirs,culverts, drop inlets &

piers.Direct computation of

shear stresses. Variablewall roughness. Model

both super and subcriticalflow regimes.

Limitation

RMA2 operates underhydrostatic assumption i.e.

accelerations in verticaldirection are negligible.

2D in the horizontal plane. Itis not intended to be used fornear field problems where

vortices, vibrations, or verticalaccelerations are of primary

interest. It can not simulate thecritical flow.

Model considers a single,effective grain size

during each simulation.Hence, a separate model

run is required for eacheffective grain size. Model doesn't compute

water surface elevationsor velocities; data must

be provided from anexternal calculation ofthe flow field.

Free-Surface calculationmodel for sub-critical

flow problems

In the case study of the Nile Basin, the selection of the model was based on the following

criteria:- Usefulness of documentations.- User friendly (easy to interact with through Guide User Manual).- Reliability (verified / tested).- Technical support and after sale service.- Purpose of modeling to serve the specific aims of the study.- Degree and quality of required data input.- Flexibility.- Reasonable cost / and efficiency.


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Sondu R

Simiyu R

Awash R.

Blue Nile

AHD

Fi (2) Locations of the STWMC

ii- Modeling Approach in the Nile Basin

Sediments transport is a complex process in the Nile Basin.Soil erosion patterns in the basin are heterogeneous;

therefore, patterns of soil erosion and sediment transport aredifficult to model and predict particularly when data

availability becomes a second constraint. The fact that, theriver basin lying across ten countries has made the systemanalyses significantly a complex task.

Currently, research activities are undertaken in differentwatersheds of the Nile Basin countries. The Sediment andWatershed Management Component within the framework of

the FRIENF/Nile Project agreed on the catchments and/orrivers to be studied as case studied in each country as shown

in, Fig (2) and Fig (3), STWMC 2nd

Annual Year Report,2003.

1- Kenya Sondu River Basin..2- Tanzania Simiyu River Basin3- Ethiopia Awash River Basin.4- Sudan Blue Nile Basin.5- Egypt Aswan High Dam Reservoir.

Discussion

One of the most important factors affecting the sediment modelling results is dataavailability. A comprehensive set of data covering morphology and hydraulics is vital for anystudy area being modeled. Furthermore, in modeling stream sedimentation problems, thereare two types of mathematical limitations i) convergence and ii) solution procedures.

The application of SMS model in the Nile Basin faced several constraints and problems,

which are briefly described below.

i- Model Scale

Multiple scales of analysis (local, sub-regional, and regional scales) are needed for modellingsediment transport in the Nile Basin. For example at the local scale, erosion models can be

used to simulate the sediment yield at the catchment, including impacts of land managementpractices. At the sub-regional scale, flow and sediment are transported through a system ofchannels. At present there are several models to evaluate sediment transport and channelresponse using one, two and multi dimensional sediment transport models. Regional scaleanalysis consists of the entire river basin watershed and associated channels. At this scale,

simulations include sediment and water routing and channel response through the entire

sediment region.


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Analysis capabilities include channel stability and geomorphic response for all watershedchannels, distributaries, the main channel and receiving waters bodies (including reservoirs

and estuaries). Using these scales for sediment transport modeling in the Nile Basin givesflexibility in assessing the sediment process throughout the basin.

a- Sondu River Basin

$T

$T

$T

$T

$T

$T

#Y

#Y#Y

#Y

#Y

Addis ababa

Koka Reservoir

# Met Stations#Gauging stations

Awash Basin

b- Awash River Ethio ia

c- Simi u River-

d- Aswan Hi h Dam-

f- The Blue Nile catchment area

inside Ethiopia

Wad

Wad El Naw

e- The selected reach in

the Blue Nile- Sudan

Fig (3) Detailed locations of the case studies in the Nile River Basinand Awash River


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ii- Data Availability

A basic impediment to successful sediment modeling is the lack of adequate input data.Model calibration and verification require independent field data sets, preferably reflecting

different field conditions, for calibration and verification. The lack of proper sediment data inthe Nile Basin has been realized. The main reasons are (i) lack of trained manpower (ii)-lack

of laboratory facilities (iii)-lack of logistic supports and unavailability of the facilities formaintaining sophisticated laboratory equipment (iv)- lack of appropriate fund in the NileBasin. The quality of the data is always affected by the condition of the equipment and the

methodology of data collection. In this respect, the authors of this paper urge the Nile Basincountries to give more attention to the collection of sediment data of good quality. This isvital for the water resources management within the Nile Basin.

iii- Model Formulation

Similar to most of the sediment transport models, the SMS models incorporate certainsimplifying assumptions and approximations. Several difficulties are identified in model

formulation, particularly regarding the creation of the computational mesh.

- Finding an appropriate geo-referenced satellite image with a good resolution (for thebase map) is difficult to obtain.

- Mesh creation requires a lot of experience and time consuming.- The eddy viscosity parameter is highly sensitive and affects the model convergence.

iv- Model Calibration

Regardless of the formulation chosen for the sediment model, model calibration is a key stepin model application. Adequate data regarding the flow and channel characteristics are of

primary importance for a successful model calibration. The SMS model was tested andcalibrated using the data collected from Tuti area where the two main tributaries meet at

Khartoum City. The results obtained were fairly acceptable, for more details see STWMC 2ndAnnual Year Report, 2003.

v- Model Application

The application of SMS model on steep rivers flow conditions (e.g. in Ethiopian, and Kenyacases studies) require special professional experience and precaution regarding the conceptson which the hydrodynamic and sediment transport models used in SMS are based on. On the

other hand, large numbers of cross-sections with short intervals are required to providesuitable results when SMS models are implemented.

- Sudden falls in the longitudinal section (e.g. Awash River) complicate things and makethe application of the model more difficult.

- Model convergence is one of the difficulties facing the model application.- Intensive training on SMS application is required.- the SMS model developers failed to provide technical support in time.

vi- Predictive Capability

The experience of SMS model application in the Nile River and Awash River showeddifficulties in proofing to be a truly predictive tool, particularly when dealing with river

floods. The applications of SMS model in Egypt and Sudan case studies raised some


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396

397

398

399

400

0 5 10 15 20 25 30 35Distance (KM)

WaterLevel(M)

Distance (KM)

difficulties. For example, modeling of the sediment transport in Aswan High Dam Reservoirshowed that the predicted bed levels are higher than the measured ones in the whole inletzone as shown in Fig (4) for years 2001 and 2003.

Also, in the modeling of the sediment transport in the Blue Nile in Sudan, it is noted that boththe computed water level and bed level are higher than the measured ones for the entire

selected reach Fig (5) & Fig (6), STWMC 3rd

Year Annual Report, 2004.

Regarding the other case studies in Kenya, Tanzania and Ethiopia even more problems faced

the application of SMS model as a predictive tool.

Fig (5) Measured Water Surface Profile Medani Wad Elnaw Reach

3973983994004014 4 404405406

- -

-

WaterLevel(M)

Fig (6) Computed Water SurfaceProfile Wad Medani Wad Elnaw

Reach

Fi 4 Measured and Com uted lon itudinal Water Surface

Comparison of Calculated and Observed Water Surface

Elevations in AHDR (199)

Comparison Between Longitudinal Sections For (AHDR) at 2001 Comparison Between Longitudinal Sections For (AHDR) at 200 3


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Conclusions

Sediment models although are based on well-known flow equations, however, most of themselect sediment functions without any good justification for this selection. Therefore,

professional experience plays the major role in this.

The use of SMS to simulate sediment process in the Nile Basin River and Awash Riverwithin the FRIEND/ Nile Project has encountered some difficulties and problems. This could

be attributed to the different topography of the countries involved and the suitable data

availability.

Application of SMS model in the steep rivers as it can be seen from the Kenyan, Tanzanian

and Ethiopian case studies showed difficulties in obtaining straightforward results. It requireshigh professional experience on the nature of these rivers.

Modelling sediment is an effective tool in solving several water management problems;however, it may be a misleading one if not properly analyzed.

This paper without any hesitation recommends the Nile Basin countries should give moreattention to data collection in the field of water resources management, specially the sedimentdata for better and efficient water management.

Acknowledgments

This paper was prepared based on the research activities of the FRIEND/Nile Project which isfunded by the Flemish Government of Belgium through the Flanders-UNESCO Science TrustFund cooperation and executed by UNESCO Cairo Office. The authors would like to express

their great appreciation to the Flemish Government of Belgium, the Flemish experts anduniversities for their financial and technical support to the project. The authors are indebted

to UNESCO Cairo Office, the FRIEND/Nile Project management team, overall coordinator,thematic coordinators, themes researchers and the implementing institutes in the Nilecountries for the successful execution and smooth implementation of the project. Thanks are

also due to UNESCO Offices in Nairobi, Dar Es Salaam and Addis Ababa for their efforts tofacilitate the implementation of the FRIEND/Nile activities.

References

Show-Shan Fan, 1994, An Interagency Overview of Selected Stream SedimentationModels, Interagency Sedimentation Group, Washington, D. C, USA.

Summer . W & Walling D. E, 2002: Modeling Erosion, Sediment Transport and Sediment

yield, IHP VI, Technical Document in Hydrology No 60, UNESCO, Paris, France.

STWMC 2nd Annual Report, 2003, Sediment Transport and Watershed Management

Component, FRIEND/ Nile Program, Khartoum, Sudan.

STWMC 3rd

Annual Report, 2004, Sediment Transport and Watershed Management

Component, FRIEND/ Nile Program, Khartoum, Sudan.


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Ahmed A, A & Eldaw A. K, 2004, An Overview on Cooperation of Transboundary Water:Case of the Nile River Basin, 2nd Regional Arab Water Conference, Cairo, Egypt.

Nile River Sediment Modelling Challenges and Opportunities

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