University of ... 2.1 Physico-geographical conditions of the Niger delta 2.2 Climate of the Niger...

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  • University of Nigeria Research Publications

    TAMUNOENE, K.S Abam

    A ut

    ho r

    PG_PhD_88_6711

    Ti tle

    STABILITY OF RIVER BANKS IN THE NIGER DELTA

    Fa cu

    lty

    PHYSICAL SCIENCE

    D ep

    ar tm

    en t

    GEOLOGY

    D at

    e

    DECEMBER,1995

    Si gn

    at ur

    e

  • LI

    STABILITY OF RIVER BANKS IN THE NIGER DELTA

    TAMUNOENE K. S. ABAM

    ~ G / ~ h . ~ 1 8 8 / 6 7 1 1

    , DEPARTMENT OF GEOLOGY UNIVERSITY OF NIGERIA

    NSUKKA

    December, 1995

  • STABILITY OF RIVER BANKS IN THE NIGER DELTA

    TAMUNOENE K. S . ABAM PG/Ph.D/88/6711

    T'FESIS SUBMITTED TO THE DEPARTMENT OF GEOLOGY, FACULTY OF PHYSICAL SCIENCES IN FUWmLMENT OF THE REQUIREMENTS FOR THE AWARD OF IHX DEGREE OF DOCTOR OF RHILOSOPHY IN ENGINEERING GEOLOGY OF THE UNIVERSITY OF NIGERIA, NSUKKA

    December, 1995

  • CERTIFICATION

    Mr. Tcvnunoene K.S. Abam, a post graduate student in the Department of Geology, University of Nigeria, Nsukka has satisfactorily completed the requirements for the award of the degree of Doctor of Philosophy (Ph.D. Engineering Geology).

    The work embodied in this thesis is original and has not been submitted in part or full for any other diploma or degree of this or any other University.

    .-- I Prof. C.O. Okogbue (Supervisor) Department bf Geology University of Nigeria Nsukka \

    Prof. E.G. Akpokodje (External Examiner)

    ) -.A f -

    Prof. C.O. Okogbue Head of Department Department of Geology University of Nigeria Ns~~kka

  • ABSTRACT

    Processes of bank failure and erosion in the Niger Delta have been

    characterised, leading to the identification of common factors affecting

    the distribution of instability of river banks.

    The rates of bank recession depend on season, soil type, stratigraphy,

    bank height and inclination, The direction and rate of channel water

    level fluctuation, flow velocity and relative location along the river

    system have also been shown to have significant influence on bank

    recession rates.

    Bank failure events are episodic in nature and are concentrated at the

    early stages of lowering of channel water level and are mainly caused

    by the sensitivity of the banks to removal of passive resistance. High

    ground 'water level accentuates seepage erosion which reduces bank

    stability. Bank erosion occurs where ever the average channel flow

    velocity is greater that 0.5 m/sec. However, banks with height greater

    than 5 m and incfimtion steeper than 65" experience accelerated

    recession. Depending on the part of the bank exposed to erosion, soil

    removal can increase or decrease bank stability against rotational

    failure.

    Soil type and stratigraphy are identified as the major parameters that

    determine the mechanism of bank failure. Stratified banks with

    underlying sand strata easily developed overhangs which failed by

    cantilever or sliding mechanisms depending on the overhang height.

    Non cohesive banks were eroded piece-meal while cohesive banks

  • failed by mainly rotational mechanisms along the slip surface with a

    factor of safety equal or less than unity at the highest water level. To

    realistically predict the behaviour of river banks, the method of

    stability evaluation must recognise the dynamic nature of the

    controlling factors. The back analysis technique when combined with

    knowledge of operating processes can lead to an interpretation of bank

    development processes.

    Analytical and quantitative methods, including charts, were developed

    to facilitate the analysis of river bank stability+ The stability charts for

    rotational failure of partidy submerged banks considered

    l~ornogeneous soils and give conservative results. A model of river

    bank profiles based on shear strength was described. The model which

    can be applied to both homogeneous and stratified banks gives

    reasonably good fit when cornpcved to natural river banks.

  • ACKNOWLEDGEMENT

    I am indebted to Prof. C.O. Okogbue for supervising this work and for

    his numerous contibutions to this research. I am also grateful to Profs.

    John KniU, P.R. Vaughan, Drs. M.H, de-freitas, M. Rosenbaum, G.

    Evans and G. Wharton all of the University of London, U.K. for their

    active support and assistance at the initial stage of this research. I a m

    very grateful to the Institute of Flood, Erosion, Reclamation and

    Transportation (IFERT) of the Rivers State University of Science and

    Technology, for placing her data bank at my disposal.

    My special thanks go to the Association of C o m n w e a l t h Universities

    for their Academic Staff Scholarship and to the Rivers State University

    of Science and Technology, Port Hcucourt, for their financial support

    towards the fieldwork aspect of this research.

    I acknowledge with thanks the helpful advice of senior academics and colleagues including Profs. Mosto Onuoha, C.O. Ofoegbu, D.M.J.

    Fubara, C.S. Teme and Drs. Uma Kaln, L. Mamah and H. Ezeigbo. I a m

    sincerely grateful to Mr. N.I. Thomas for typing this work.

    Finally, I wish to express my sincere thanks to Engr. E.A.J. George and

    my family for inspiring me to complete this work and especially to my

    wife Mrs. Barbara Kingdom-Abarn for the inconveniences she had to

    endure in the course of this work.

    T.K.S. ABAM December 1995

  • TABLE OF CONTENTS

    Certification

    Abstract

    Acknowledgement

    List of Figures

    List of Tables

    List of Appendices

    CHAPTER ONE : Introduction

    ProbIem

    Objectives

    Scope of the Study

    Previous work

    Fluvial aspects of bank stability

    .The probSem of changing water level

    Bank morphoIogy

    Effective s&ss in partially saturated soils

    CHAPTER TWO: Characteristics of the Study Area

    2.1 Physico-geographical conditions of the Niger delta

    2.2 Climate of the Niger delta and its effects on bank morphology

    2.3 Distribution of soiI types in the Niger delta

    CHAPTER THREE: Review of SIope Stability Analysis

    3.1 Analysis of translational mechanism of failure

    3.2 Analysis of rotational mechanism of failwe

    ii

    iii

    v

    xi

  • The friction circle method

    The methods of slices

    Other methods of stability analysis for rotational failure mechanism

    Analysis of wedge mechanism of failure

    Toppling mechanism of failure

    Analysis of riverbank overhangs

    Shear nmechanism of failure

    Beam mechanism of fialure

    Tensional mechanism of fialure

    Probabilistic methods of analysis

    CHAPTER FOUR: Research Me thodology

    4.1 Introduction

    4.2 Field measurements

    4.3 Labor&ry tests

    4 4 Analysis of riverbanks \

    CHAPTER FIVE: Results

    Field measurements

    Water level

    Riverbank profiles

    Flow velocity

    Vane shear strength

    Results of pore pressure measurement using standpipe piezometer

    Laboratory test results

    Particle size distribution

    Moisture contents and consistency lilntts

  • 5.2,3 Bulk density 5.2.4 Shear strength

    5,2.5 Permeability

    CHAPTER SIX: Discussions 113

    Seasonal distribution of bank failure

    Areal distributoion of bank failure

    Analysis of factors affecting riverbank stability

    Influence of channeI water level

    Influence of reIative location in river system

    Influence of bank height

    Influence of shear strength

    Influence of unit weight

    Influence of ground water IeveI

    Influence of sim~dtaneous variation in bank height, water 'Ievel, unit weight and pore pressure 135 AnaIysis of processes of riverbank development and recession ' 140 Analysis of seepage erosion of alluvial river banks 140

    Analysis of bank faiIure and recessiona1 mechanism 148

    Determination of the roIe of fluvia1 erosion processes 179

    Remedial rn-easures for riverbank stabilisation in the Niger delta 181

    CHAPTER SEVEN: Development of stability charts and riverbank profile models 184

    7.1 Introduction 184

    7.2 RotationaI failure of partially submerged riverbank 186

    7.3 Riverbank profile model deveIopment 194

  • Model development

    Application of the model

    Comparison between model prediction and field observation of riverbank profile

    Design of stable riverbank profiIes

    CHAPTER EIGHT: Summaries, Conclusion and Recommendations

    Summaries

    ConcIusions

    Recommendations

    References

    Appendices

  • LIST OF FIGU'RES

    Map showing the study area in relationship to Nigeria and Africa I1

    Major rivers and creeks in the Niger DeIta 13

    Montly variations in rainfaII, evaporation and temperature in the Niger delta 14

    Distribution of annual rainfall in the Niger delta 16

    A typical evapotranspiration anc concurrent precipitation graph