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Hydraulic StructuresFourth Edition

P. Novak, A.I.B. Moffat and C. NalluriSchool of Civil Engineering and Geosciences,University of Newcastle upon Tyne, UK

and

R. NarayananFormerly Department of Civil and Structural Engineering, UMIST,University of Manchester, UK

Fourth edition published 2007 by Taylor & Francis2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN

Simultaneously published in the USA and Canadaby Taylor & Francis270 Madison Ave, New York, NY 10016

Taylor & Francis is an imprint of the Taylor & Francis Group, an informa business

© 1990, 1996, 2001, 2007 Pavel Novak, Iain Moffat, the estate of ChandraNalluri and Rangaswami Narayanan

The right of Pavel Novak, Iain Moffat, Chandra Nalluri and RangaswamiNarayanan to be identified as the Authors of this Work has been asserted bythem in accordance with the Copyright, Designs and Patents Act 1988

All rights reserved. No part of this book may be reprinted or reproduced orutilized in any form or by any electronic, mechanical, or other means, nowknown or hereafter invented, including photocopying and recording, or in anyinformation storage or retrieval system, without permission in writing from thepublishers.

The publisher makes no representation, express or implied, with regard to theaccuracy of the information contained in this book and cannot accept any legalresponsibility or liability for any efforts or omissions that may be made.

British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library

Library of Congress Cataloging in Publication DataHydraulic structures / P. Novak . . . [et al.]. — 4th ed.p. cm.Includes bibliographical references and index.ISBN-13: 978-0-415-38625-8 (alk. paper)ISBN-13: 978-0-415-38626-5 (pbk. : alk. paper)1. Hydraulic structures. I. Novák, Pavel.TC180.H95 2007627--dc22

ISBN10: 0-415-38625-X HardbackISBN10: 0-415-38626-8 PaperbackISBN10: 0-203-96463-2 e-book

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Contents

Preface xi

Preface to the third edition xiii

Preface to the second edition xv

Preface to the first edition xvii

Acknowledgements xix

List of tables xx

List of main symbols xxii

Part One Dam engineering 1

1 Elements of dam engineering 31.1 General 31.2 Introductory perspectives 41.3 Embankment dam types and characteristics 121.4 Concrete dam types and characteristics 161.5 Spillways, outlets and ancillary works 201.6 Site assessment and selection of type of dam 231.7 Loads on dams 35References 39

2 Embankment dam engineering 422.1 Introduction 422.2 Nature and classification of engineering soils 422.3 Engineering characteristics of soils 47

2.4 Principles of embankment dam design 602.5 Materials and construction 732.6 Seepage analysis 782.7 Stability and stress 822.8 Settlement and deformation 972.9 Rockfill embankments and rockfill 1002.10 Small embankment dams, farm dams and flood banks 1032.11 Tailing dams and storage lagoons 1072.12 Geosynthetics in embankment dams 1082.13 Upgrading and rehabilitation of embankment dams 109Worked examples 111References 116

3 Concrete dam engineering 1223.1 Loading: concepts and criteria 1223.2 Gravity dam analysis 1333.3 Buttress dam analysis 1553.4 Arch dam analysis 1573.5 Design features and construction 1643.6 Concrete for dams 1703.7 The roller-compacted concrete gravity dam 1743.8 Upgrading of masonry and concrete dams 180Worked examples 182References 188

4 Dam outlet works 1914.1 Introduction 1914.2 The design flood 1924.3 Flood routing 1954.4 Freeboard 1974.5 Sedimentation in reservoirs 2004.6 Cavitation 2044.7 Spillways 2064.8 Bottom outlets 231Worked examples 234References 239

5 Energy dissipation 2445.1 General 2445.2 Energy dissipation on spillways 2455.3 Stilling basins 2495.4 Plunge pools 2595.5 Energy dissipation at bottom outlets 261Worked examples 262References 264

vi CONTENTS

6 Gates and valves 2676.1 General 2676.2 Crest gates 2686.3 High-head gates and valves 2756.4 Tidal barrage and surge protection gates 2776.5 Hydrodynamic forces acting on gates 2796.6 Cavitation, aeration, vibration of gates 2836.7 Automation, control and reliability 284Worked example 285References 287

7 Dam safety: instrumentation and surveillance 2897.1 Introduction 2897.2 Instrumentation 2917.3 Surveillance 3047.4 Dam safety legislation 3067.5 Reservoir hazard and risk assessment 309References 315

Part Two Other hydraulic structures 319

8 River engineering 3218.1 Introduction 3218.2 Some basic principles of open-channel flow 3228.3 River morphology and régime 3278.4 River surveys 3318.5 Flow-measuring structures 3378.6 River flood routing 3388.7 River improvement 342Worked examples 353References 360

9 Diversion works 3649.1 Weirs and barrages; worked examples 3649.2 Intakes; worked examples 3929.3 Fish passes 410References 416

10 Cross-drainage and drop structures 41810.1 Aqueducts and canal inlets and outlets; worked examples 41810.2 Culverts, bridges and dips; worked examples 42810.3 Drop structures; worked example 448References 458

CONTENTS vii

11 Inland waterways 46111.1 Introduction 46111.2 Definitions, classification and some waterways 46311.3 Multipurpose utilization of waterways 46611.4 Transport on inland waterways 46911.5 Canalization and navigation canals 47111.6 Resistance of ships 47311.7 Wave action on banks 47511.8 Locks 47711.9 Thrift locks 48611.10 Lifts and inclined planes 48811.11 Lock approaches 49011.12 Inland ports 491Worked examples 493References 494

12 Hydroelectric power development 49612.1 Introduction 49612.2 Worldwide hydroelectric power development

in perspective 49712.3 Power supply and demand 49712.4 Some fundamental definitions 49812.5 Types of water power development 49912.6 Head classification of hydropower plants 50212.7 Streamflow data essential for the assessment of

water-power potential 50212.8 Hydraulic turbines and their selection 50512.9 Other components of hydropower plants 51712.10 Surge tanks 52512.11 Small hydraulic power plant development 52912.12 Other energy resources 530Worked examples 533References 546

13 Pumping stations 54813.1 Introduction 54813.2 Pumps and their classification 54813.3 Design of pumping mains 55413.4 Classification of pumping stations and intakes 55713.5 Sump design 55913.6 Screening devices 56213.7 Benching 56213.8 Surges 56213.9 General design considerations of pumping stations

and mains 566

viii CONTENTS

Worked examples 568References 574

14 Waves and offshore engineering 57514.1 Introduction 57514.2 Wave motion 57614.3 Range of validity of linear theory 58414.4 Waves approaching a shore 58614.5 Wave breaking 58814.6 Wave reflection 59114.7 Basin oscillations 59214.8 Wave diffraction 59314.9 Wave prediction 59414.10 Wave statistics 59914.11 Forces on cylindrical structures 60214.12 Vortex-induced oscillations 61214.13 Oscillations of cylinders in waves 617Worked examples 618References 624

15 Coastal engineering 62715.1 Introduction 62715.2 Coastal defence 62915.3 Wave forces on coastal structures 63615.4 Wave run-up 64115.5 Wave overtopping 64515.6 Rubble-mound breakwaters 64715.7 Sea outfalls 65315.8 Coastal management 662Worked examples 663References 670

16 Models in hydraulic engineering 67416.1 Hydraulic models 67416.2 Structural models 683Worked example 687References 688

Author index 691

Subject index 696

CONTENTS ix

Preface

The aim of the book, to provide a text for final year undergraduate andpostgraduate students, remains the same as in the previous editions; wealso trust that researchers, designers and operators of hydraulic structureswill continue to find the text of interest and a stimulating up-to-datereference source.

This new edition enabled us to update the text and referencesthroughout, and to introduce some important changes and additions react-ing to new developments in the field. We have also taken note of somecomments received on the previous edition; particular thanks for the con-structive comments and help provided by Professor J. Lewin in redraftingChapter 6 (Gates and valves).

The authorship of individual chapters remains the same as in previ-ous editions; (Dr Narayanan carried out the work on this edition duringhis stay in the Faculty of Civil Engineering, Universiti Teknologi Malaysia,Johor Bahru, Malaysia). However, as our colleague Dr C. Nalluri unfortu-nately died in December 2003 ‘his’ text was reviewed by Dr Narayanan(Chapter 13) and Professor Novak (Chapters 9, 10 and 12) who also againedited the whole text.

Readers of the previous (2001) edition may note the following majorchanges:

Chapter 1. Enhanced discussion of environmental issues including theWorld Commission on Dams report.

Chapter 2. New sections on partially saturated soils, small farm andamenity dams, tailing dams and lagoons and upgrading andrehabilitation of embankment dams; extended treatment ofupstream face protection/rock armouring.

Chapter 3. Extended discussion of roller-compacted concrete damsand a new section on upgrading of masonry and concretedams.

Chapter 4. Substantially enhanced discussion of flow over stepped spill-ways.

Chapter 5. Extended treatment of scour in plunge pools.Chapter 6. Enlarged treatment of hydrodynamic forces acting on low

and high-head gates and new sections dealing with cavita-tion, aeration and vibrations of gates and automation,control and reliability.

Chapter 7. Increased coverage of integrated risk analysis/managementand contingency/emergency planning in dam safety.

Chapter 9. Inclusion of barrages with raised sill.Chapter 12. New text on small hydraulic power development and tidal

and wave power.Chapter 14. More detailed treatment of wave breaking, wave statistics

and pipeline stability.Chapter 15. Enhanced treatment of beach profile and wave/structure

interaction and a new section on coastal modelling.Chapter 16. Enlarged discussion of mathematical, numerical and compu-

tational models in hydraulic engineering.

In order not to increase the size of the book unduly some less relevantmaterial has been omitted (particularly in Ch. 12).

P. Novak, A.I.B. Moffat and R. NarayananNewcastle upon Tyne, June 2006

xii PREFACE

Preface to the third

edition

The main aim of the book, i.e. to provide a text for final year undergradu-ate and for postgraduate students, remains the same as for the previoustwo editions; we also hope that researchers, designers and operators of themany types of structures covered in the book will continue to find the textof interest and a stimulating, up-to-date reference source.

It is now almost six years since the manuscript of the second editionwas completed and this new edition gave us the opportunity to correct thefew remaining errors and to update the text and references throughout. Atthe same time, as a reaction to some important developments in the field,certain parts of the text have been rewritten, enlarged or reorganized.Readers of the second edition may wish to note the following majorchanges:

Chapter 1. The environmental and social issues associated with majorreservoir projects are addressed in greater depth.

Chapter 2. New section on small embankments and flood banks andexpanded discussion of seismicity and seismic analysis.

Chapter 4. Enlarged text on design flood selection and reservoir floodstandards, aeration on spillways and in free flowing tunnels;extended treatment of stepped spillways.

Chapter 6. A new section on tidal barrage and surge protection gatesand enlarged text on forces acting on gates; a new workedexample.

Chapter 7. Enhanced text on reservoir hazard analysis and dam breakfloods.

Chapter 9. New paragraph on pressure distribution under piled founda-tion floors of weirs with a new worked example.

Chapter 14. This chapter – Coastal and offshore engineering in previousedition – has been divided into:Chapter 14 ‘Waves and offshore engineering’ and

Chapter 15 ‘Coastal engineering’Consequently the whole material has been reorganized. Thetreatment of forces on cylindrical bodies in waves and cur-rents has been significantly extended in Chapter 14. Chapter15 now includes an extended treatment of wave overtoppingand stability of breakwaters as well as a brief discussion ofcoastal management.

Chapter 16. (formerly ch. 15). Extended discussion of computational modelling of hydraulic structures.

P. Novak, A.I.B. Moffat, C. Nalluri and R. NarayananNewcastle upon Tyne, August 2000

xiv PREFACE TO THE THIRD EDITION

Preface to the second

edition

The main aim of the book, i.e. to provide a text for final year undergradu-ate and for postgraduate students, remains the same as for the first edition;equally we hope that researchers, designers and operators of the manytypes of hydraulic structures covered in the book will find the text of inter-est and a useful reference source.

We took the opportunity of a new edition to correct all (known)errors and to thoroughly update the text and references throughout. Atthe same time as a response to received comments and reviews as well as areaction to some new developments in the field, certain parts of the textwere rewritten or enlarged. Readers of the first edition may wish to notethe following major changes.

Chapter 1. Extended text on site assessment for dams.Chapter 2. Expanded treatment of geotechnical aspects, e.g. a new

paragraph (2.8.3) on performance indices for earthfill cores,and a new brief section (2.10) on geosynthetics.

Chapter 3. Extended coverage of RCC dams with a new paragraph(3.7.3) dealing with developments in RCC construction.

Chapter 4. Enlarged text dealing with design flood estimation, reservoirsedimentation, interference waves and aeration on spillwaysand a new paragraph (4.7.6) on stepped spillways.

Chapter 5. Enlarged section on scour below spillways.Chapter 6. A new paragraph (6.2.8) on overspill fusegates.Chapter 7. Enlarged text on reservoir downstream hazard assessment.Chapter 8. Enlarged text on multistage channels, geotextiles, Crump

weir computation and a new section (8.6) on river floodrouting.

Chapter 9. Extended text on fish passes and a new paragraph (9.1.6)on the effect of the operation of barrages on river waterquality.

Chapter 10. Enlarged text on canal inlets and scour at bridges and belowculvert outlets.

Chapter 13. A new short section (13.7) on benching.Chapter 14. Change of title (from Coastal engineering) to Coastal and

offshore engineering incorporating a substantial new section(14.7) on sea outfalls and the treatment of wave forces onpipelines in the shoaling region.

Chapter 15. Change of title (from Scale models in hydraulic engineering)to Models in hydraulic engineering to include in the generaldiscussion of hydraulic models (15.1.1) a typology of mathe-matical models; also included a short paragraph (15.2.4) onmodelling of seismic response.

The authors would like to thank the reviewers for their constructive com-ments and the publisher for providing the opportunity for this secondedition.

P. Novak, A.I.B. Moffat, C. Nalluri and R. NarayananNewcastle upon Tyne, December 1994

xvi PREFACE TO THE SECOND EDITION

Preface to the first

edition

This text is loosely based on a course on ‘Hydraulic Structures’ whichevolved over the years in the Department of Civil Engineering at the Uni-versity of Newcastle upon Tyne. The final-year undergraduate andDiploma/MSc postgraduate courses in hydraulic structures assume a goodfoundation in hydraulics, soil mechanics, and engineering materials, andare given in parallel with the more advanced treatment of these subjects,and of hydrology, in separate courses.

It soon became apparent that, although a number of good books maybe available on specific parts of the course, no text covered the requiredbreadth and depth of the subject, and thus the idea of a hydraulic structurestextbook based on the course lecture notes came about. The hydraulicstructures course has always been treated as the product of team-work.Although Professor Novak coordinated the course for many years, he andhis colleagues each covered those parts where they could make a personalinput based on their own professional experience. Mr Moffat, in particular,in his substantial part of the course, covered all geotechnical engineeringaspects. In the actual teaching some parts of the presented text may, ofcourse, have been omitted, while others, particularly case studies (includingthe discussion of their environmental, social, and economic impact), mayhave been enlarged, with the subject matter being continuously updated.

We are fully aware that a project of this kind creates the danger ofpresenting the subject matter in too broad and shallow a fashion; we hopethat we have avoided this trap and got it ‘about right’, with workedexamples supplementing the main text and extensive lists of referencesconcluding each chapter of the book.

This text is not meant to be a research monograph, nor a designmanual. The aim of the book is to provide a textbook for final-yearundergraduate and postgraduate students, although we hope thatresearchers, designers, and operators of the many types of hydraulic struc-tures will also find it of interest and a useful reference source.

The text is in two parts; Part One covers dam engineering, and PartTwo other hydraulic structures. Mr A.I.B. Moffat is the author of Chap-ters 1, 2, 3 and 7, and of section 15.2. Dr C. Nalluri wrote Chapters 9, 10,12 and 13, and sections 8.4 and 8.5. Dr R. Narayanan of UMIST wasinvited to lecture at Newcastle for two years, on coastal engineering, and isthe author of Chapter 14. The rest of the book was written by ProfessorP. Novak (Chapters 4, 5, 6 and 8, except for sections 8.4 and 8.5, Chapter11 and section 15.1), who also edited the whole text.

P. Novak, A.I.B. Moffat, C. Nalluri and R. NarayananNewcastle upon Tyne, 1989

xviii PREFACE TO THE FIRST EDITION

Acknowledgements

We are grateful to the following individuals and organizations who havekindly given permission for the reproduction of copyright material (figurenumbers in parentheses):

Thomas Telford Ltd (4.1, 4.2); US Bureau of Reclamation (4.3, 4.7,4.15, 4.16, 5.6, 5.7); Elsevier Science Publishers (4.5, 4.12, 4.13, 5.5, 5.8,5.10. 11.1, 11.2, 11.10, 11.11, 11.16, 11.17, 11.18, 12.17); British Hydro-mechanics Research Association (4.11, 13.6, 13.9); Institution of Waterand Environmental Management (4.18); ICOLD (4.19, 4.20); Figures 4.21,6.2, 6.3, 6.4 reproduced by permission of John Wiley & Sons Ltd, fromH.H. Thomas, The Engineering of Large Dams, © 1976; C.D. Smith (6.6,6.7); MMG Civil Engineering Systems Ltd (8.20); E. Mosonyi (9.12, 9.13,12.17); International Institute for Land Reclamation and Improvement,the Netherlands (10.14, 10.15); Morgan-Grampian Book Publishing (11.1,11.5); Delft Hydraulics (11.7); Macmillan (14.12); C.A.M. King (14.13);C. Sharpe (11.2); J. Lewin (6.1, 6.2).

Cover image courtesy of Ingetec S.A. Colombia (Dr A. Marulanda)

List of tables

1.1 Large dams: World Register statistics 41.2 Summary of numbers of British, US and Chinese dams 51.3 Highest dams 61.4 Largest-volume dams 61.5 Dams with largest-capacity reservoirs 61.6 Notional foundation stresses: dams 100m in height 321.7 Dam selection: type characteristics 342.1 Representative physical characteristics of soils 472.2 Descriptive consistency of clay soils 522.3 Illustrative engineering properties for selected soil types 572.4 Embankment dam defect mechanisms and preventive

measures 642.5 Characteristics of core soils 742.6 Indicative engineering properties for compacted earthfills 752.7 Guideline factors of safety: effective stress stability analysis 852.8 Seismic acceleration coefficients, �h, and earthquake

intensity levels 943.1 Seismic pressure factors, Ce 1313.2 Nominated load combinations 1323.3 Range of shearing resistance parameters 1373.4 Foundation rock shear strength characteristics 1373.5 Examples of shear strength degradation 1383.6 Recommended shear friction factors, FSF 1413.7 Comparative sliding stability factors: triangular gravity profile 1433.8 Permissible compressive stresses 1473.9 Illustrative values for coefficient, K0 1543.10 Characteristics of mass concrete for dams 1733.11 Characteristics of RCCs for dams 1774.1 Flood, wind and wave standards by dam category 1947.1 Selected major dam disasters 1959–1993 290

7.2 Primary monitoring parameters and their relationship topossible defects 294

7.3 Representative monitoring frequencies 3048.1 Types of weir 3399.1 Correction factors for submerged (non-modular) flows 3829.2 Values of � for parallel flow 39710.1 Types of flow in the barrel of a culvert 42910.2 Bridge loss coefficient, KB 43510.3 Values of K as a function of pier shape 43710.4 Values of KN and KA 43910.5 Permissible velocities to withstand erosion 44210.6 Range of values of C for free flow over the embankment 44310.7 Correction factor, f (non-modular flows) 44311.1 Freight on inland waterways: annual throughput of shipping 46912.1 Range of � values, specific speeds and heads 50712.2 Q–H–Ns data 50912.3 Runaway speeds and acceptable head variations 51012.4 Critical plant sigma values, �c 51113.1 Types of pumps and their applications 55013.2 Specific speeds for rotodynamic pumps 55113.3 Sludge flow head losses 56815.1 Factor r for various armour units 64415.2 Values of coefficients A and B for simple sea walls 64615.3 Values of KD in Hudson’s formula (SPM): no damage

and minor overtopping 64915.4 Layer coefficient K�D and porosity for various armour units 65015.5 Variation in damage number for failure conditions 65216.1 Scale factors 680

LIST OF TABLES xxi

Main symbols

a constant, gate opening, pressure wave celerity, wave amplitudeA cross-sectional areab breadth, channel width, constant, length of wave crestB water surface widthB_

porewater pressure coefficientc apparent cohesion, coefficient, constant, unit shearing strength,

wave celerityC Chezy coefficient, coefficient, concentrationCd coefficient of dischargeCD drag coefficientCv coefficient of consolidation, coefficient of velocityd depth, diameter, sediment grain sizeD diameter, displacement of vesselsE cut-off (core) efficiency, energy, Young’s moduluse energy loss, pipe wall thicknessf correction factor, frequency, function, Lacey’s silt factorF factor of safety, fetch, force, functionFD drag forceFr Froude numberFSL full supply levelg gravitational accelerationGWL ground water levelh� uplift pressure headh head, pump submergence, rise of water level above SWL, stageH total energy (head), head (on spillway etc.), wave (embankment)

heightHs seepage head, significant wave height, static liftHFL high flood leveli hydraulic gradientI inflow, influence factor, moment of inertia

k coefficient (of permeability), effective pipe roughness, wavenumber

K bulk modulus, channel conveyance, coefficientKc Keulegan–Carpenter numberl lengthL length, wavelengthm massmv coefficient of volume compressibilityM momentn Manning roughness coefficientN hydraulic exponent speed in rev/minNd number of increments of potential in flownetNf number of flow channels in flownetNs specific speedNWL normal water levelO outflowp number of poles, pressure intensitypv vapour pressureP force, power, wetted perimeterq specific dischargeQ dischargeQs discharge of sedimentr factor, radiusru pore pressure ratioR hydraulic radius, resistance, resultant, radiusRe Reynolds numberRs régime scour depthS maximum shearing resistance, slopeSc critical slopeSf friction slopeS0 bed slopeSh Strouhal numberSWL still water levelt thickness, timeT draught, time, wave periodu local velocity (x direction)uw porewater pressureU wind speedU* shear velocity� velocity (general), velocity (y direction)V mean cross-sectional velocity, storage, volumeVc critical velocityw moisture content, velocity (z direction)ws sediment fall velocityW régime width, weight

MAIN SYMBOLS xxiii

x distance, x coordinatey flow depth, y coordinatey� stilling basin depthy� depth of centroid of section Ayc critical depthym mean depth (�A/B)ys maximum scour (local) depth, turbine settingz depth, elevation relative to datum, z coordinate� angle, constant, energy (Coriolis) coefficient, (seismic)

coefficient, wave crest angle� angle, momentum (Boussinesq) coefficient, slope, angle specific (unit) weight (�pg)

boundary layer thickness, deflection settlement

� laminar sublayer thickness∆ relative density of sediment in water ((�s ��)/�)

strain� area reduction coefficient, efficiency� angle, velocity coefficient� Darcy–Weisbach friction factor, flownet scale transform factorµ dynamic viscosity of waterv kinematic viscosity of water, Poisson ratio� coefficient (head loss), parameter� density of water�s density of sediment particle� cavitation number, conveyance ratio, safety coefficient, stress,

surface tension�1,2,3 major, intermediate and minor principal stresses�� effective stress, safety coefficient� shear stress, time interval�c critical shear stress�0 boundary shear stress� angle of shearing resistance or internal friction, function,

sediment transport parameter, speed factor� flow parameter� angular velocity (radians s�1)

xxiv MAIN SYMBOLS

Part One

Dam engineering

Chap te r 1

Elements of dam

engineering

1.1 General

The construction of dams ranks with the earliest and most fundamental ofcivil engineering activities. All great civilizations have been identified withthe construction of storage reservoirs appropriate to their needs, in theearliest instances to satisfy irrigation demands arising through the devel-opment and expansion of organized agriculture. Operating within con-straints imposed by local circumstance, notably climate and terrain, theeconomic power of successive civilizations was related to proficiency inwater engineering. Prosperity, health and material progress becameincreasingly linked to the ability to store and direct water.

In an international context, the proper and timely utilization of waterresources remains one of the most vital contributions made to society bythe civil engineer. Dam construction represents a major investment inbasic infrastructure within all nations. The annual completion rate fordams of all sizes continues at a very high level in many countries, e.g.China, Turkey and India, and to a lesser degree in some more heavilyindustrialized nations including the United States.

Dams are individually unique structures. Irrespective of size andtype they demonstrate great complexity in their load response and in theirinteractive relationship with site hydrology and geology. In recognition ofthis, and reflecting the relatively indeterminate nature of many majordesign inputs, dam engineering is not a stylized and formal science. Aspractised, it is a highly specialist activity which draws upon many scientificdisciplines and balances them with a large element of engineering judge-ment; dam engineering is thus a uniquely challenging and stimulating fieldof endeavour.

1.2 Introductory perspectives

1.2.1 Structural philosophy and generic types of dams

The primary purpose of a dam may be defined as to provide for the saferetention and storage of water. As a corollary to this every dam must rep-resent a design solution specific to its site circumstances. The design there-fore also represents an optimum balance of local technical and economicconsiderations at the time of construction.

Reservoirs are readily classified in accordance with their primarypurpose, e.g. irrigation, water supply, hydroelectric power generation,river regulation, flood control, etc. Dams are of numerous types, and typeclassification is sometimes less clearly defined. An initial broad classifica-tion into two generic groups can be made in terms of the principal con-struction material employed.

1. Embankment dams are constructed of earthfill and/or rockfill. Upstreamand downstream face slopes are similar and of moderate angle, giving awide section and a high construction volume relative to height.

2. Concrete dams are constructed of mass concrete. Face slopes are dis-similar, generally steep downstream and near vertical upstream, anddams have relatively slender profiles dependent upon the type.

The second group can be considered to include also older dams of appro-priate structural type constructed in masonry. The principal types of damswithin the two generic groups are identified in Table 1.1. Essentialcharacteristics of each group and structural type are detailed further inSections 1.3 and 1.4.

Embankment dams are numerically dominant for technical and eco-nomic reasons, and account for an estimated 85–90% of all dams built.Older and simpler in structural concept than the early masonry dam, the

4 ELEMENTS OF DAM ENGINEERING

Table 1.1 Large dams: World Register statistics (ICOLD, 1998)

Group Type ICOLD code %

Embankment dams Earthfill TE 82.9Rockfill ER �Concrete dams(including masonry Gravity PG 11.3dams) Arch VA 4.4

Buttress CB 1.0Multiple arch MV 0.4

Total large dams 41413

embankment utilized locally available and untreated materials. As theembankment dam evolved it has proved to be increasingly adaptable to awide range of site circumstances. In contrast, concrete dams and theirmasonry predecessors are more demanding in relation to foundation con-ditions. Historically, they have also proved to be dependent upon relat-ively advanced and expensive construction skills and plant.

1.2.2 Statistical perspective

Statistics are not available to confirm the total number of dams in serviceworldwide. Accurate statistical data are confined to ‘large’ dams enteredunder national listings in the World Register of Dams, published by theInternational Commission on Large Dams.

ICOLD is a non-governmental but influential organization repre-sentative of some 80 major dam-building nations. It exists to promote theinterchange of ideas and experience in all areas of dam design, construc-tion, and operation, including related environmental issues. Large damsare defined by ICOLD as dams exceeding 15m in height or, in the case ofdams of 10–15m height, satisfying one of certain other criteria, e.g. astorage volume in excess of 1�106 m3 or a flood discharge capacity of over2000m3s�1 etc. The World Register of 1998 (ICOLD, 1998) reported41413 large dams completed or under construction. Of this total, whichexcluded separately registered industrial tailings dams, over 19000 wereclaimed by China and over 6000 by the US. These figures may be com-pared with a worldwide total of 5196 large dams recorded in 1950.

The 1998 edition of the World Register restricted the number ofentries for certain countries, notably China, in the interests of savingspace. This was achieved by listing only dams of 30m height and above, atotal of 25410 dams.

Few reliable estimates of national totals of dams of all sizes havebeen published. Estimated total numbers for the UK and for the US areavailable, however, following national surveys. They are presented along-side the corresponding national figures for large dams in Table 1.2. Fromthese statistics it may reasonably be inferred that the total number of damsin existence worldwide exceeds 300000.

INTRODUCTORY PERSPECTIVES 5

Table 1.2 Summary of numbers of British, US and Chinese dams (1998)

Large dams Estimated total dams Dams subject to national(national surveys) safety legislation

UK 0535 0�5500 2650USA 6375 �75000 N.K.China c. 19100 �90000 N.K.

Rapid growth in the number of large dams has been accompanied bya progressive increase in the size of the largest dams and reservoirs. Thephysical scale of the largest projects is demonstrated by the statistics ofheight, volume, and storage capacity given in Tables 1.3, 1.4 and 1.5respectively. Industrial tailings dams are excluded from Table 1.4.

In appreciating the progressive increase in the number of large damsand in the size of the largest, it must be recognized that the vast majorityof new dams continue to be relatively small structures. They lie most


Table 1.3 Highest dams

Dam Country Type Completed Height(m)

Rogun Tadjikistan TE–ER 1985 335Nurek Tadjikistan TE 1980 300Xiaowan China TE In progress 292Grand Dixence Switzerland PG 1962 285Inguri Georgia VA 1980 272Manuel M Torres Mexico TE–ER 1980 261

38 dams greater than 200m in height.

Table 1.4 Largest-volume dams

Dam Country Type Height Completed Fill volume(m) (�106 m3)

Tarbela Pakistan TE–ER 143 1976 105.9Fort Peck USA TE 076 1937 096.1Lower Usuma Nigeria TE 049 1990 093.0Tucurui Brazil TE–ER–PG 106 1984 085.2Ataturk Turkey TE–ER 184 1990 084.5Guri (Raul Leoni) Venezuela TE–ER–PG 162 1986 078.0

Tailings dams excluded.

Table 1.5 Dams with largest-capacity reservoirs

Dam Country Type Height Completed Reservoir(m) capacity

(�109 m3)

Kakhovskaya Ukraine TE–PG 037 1955 182.0Kariba Zimbabwe– VA 128 1959 180.6

ZambiaBratsk Russian Fedn. TE–PG 125 1964 169.3Aswan (High) Egypt TE–ER 111 1970 168.9Akosombo Ghana TE–ER 134 1965 153.0Daniel Johnson Canada VA 214 1968 141.8

commonly in the 5–10m height range. Earthfill embankments remaindominant, but rockfill is to some extent displacing earthfill for larger struc-tures as it offers several advantages.

It is also important to recognize that many major dams are nownecessarily built on less favourable and more difficult sites. For obviousreasons, the most attractive sites have generally been among the first to beexploited. A proportion of sites developed today would, in the past, havebeen rejected as uneconomic or even as quite unsuitable for a dam. Theability to build successfully on less desirable foundations is a reflection ofadvances in geotechnical understanding and of confidence in modernground-improvement processes.

1.2.3 Historical perspective

The history of dam building dates back to antiquity, and is bound up withthe earlier civilizations of the Middle East and the Far East. Countlesssmall dams, invariably simple embankment structures, were constructedfor irrigation purposes in, for example, China, Japan, India and Sri Lanka.Certain of these early dams remain in existence.

The dam built at Sadd-el-Kafara, Egypt, around 2600BC, is gener-ally accepted as the oldest known dam of real significance. Constructedwith an earthfill central zone flanked by rock shoulders and with rubblemasonry face protection, Sadd-el-Kafara was completed to a height of14m. The dam breached, probably in consequence of flood overtopping,after a relatively short period of service.

Numerous other significant dams were constructed in the MiddleEast by early civilizations, notably in modern Iraq, Iran and Saudi Arabia.The Marib embankment dam, completed in the Yemen around 750BC toservice a major irrigation project, was an example of particular note, asthis dam was raised to a final height of 20m. The first significant masonrydam, the 10m high Kesis Gölü (North) dam in Turkey, dates from thesame period.

The Romans made a significant later contribution in the Middle Eastand in countries bordering the Mediterranean. A number of Roman damsremain in service, and to the Romans probably falls the credit for firstadopting the arch principle in dam construction. The 12m high and 18mlong Baume arch dam, in France, was completed by the Romans in thesecond century AD.

In the Far East the construction of significant dams can be dated tothe period commencing c.380BC. Activity initially centred upon SriLanka, where a remarkable period of dam building commenced with the10m high Bassawak embankment and culminated in the Giritale andKantalai embankments (23m and 20m high respectively), completed in


AD610. The Japanese and Indian entry into major dam building com-menced c.AD750, and both nations made a notable contribution to theearly development of the embankment.

The period from AD1000 onwards saw a spread of dam-buildingactivity, with quite rapid growth in the height of dams and in the boldnessof their concept. Of particular note was the construction of a series ofmasonry gravity dams in Iran where the first true arch dam, i.e. a masonrydam too slender to be stable as a gravity structure, was also built. Thelatter dam, at Kebar, 26m high and of 55m crest length with a base thick-ness of 6m, was completed c.AD1300. The remarkable 31m high SultanMahmud dam in Afghanistan also dates from this time. This era also sawthe commencement of serious dam building activity in many parts ofEurope, e.g. the 6m high embankment at Alresford, in Britain (c.1195) orthe 10m high embankments at Mittlerer Pfauen, Germany (c.1298) and atDvor̆is̆tĕ, Czech Republic (c.1367) and many others.

The dam-builders of 16th-century Spain advanced masonry dam con-struction very considerably. The magnificent Tibi gravity dam, 42m inheight, was completed in 1594 and followed by a series of other outstand-ing masonry structures. The Elche masonry arch dam, 23m high and 120min length, was completed in 1640 and is also of particular merit. With therapid expansion of the Spanish Empire the expertise of the Spanish dam-builders was also exported to Central and South America. Representativeof their breadth of vision and their ability to plan and to mobilizeresources, the intensive metalliferous mining activity centred on Potosí(Bolivia) was, by the mid-17th century, served by a group of 32 reservoirs.

In the period from 1700 to 1800 the science of dam buildingadvanced relatively slowly. The dawn of the first Industrial Revolution andthe canal age gave considerable impetus to embankment dam constructionin Britain and in Western Europe in the period from about 1780. Designcontinued to be based on a combination of empirical rules and provenexperience. Despite the lack of rational design methods, dams steadilyincreased in size. As an example, the Entwistle embankment dam wascompleted in England in 1838 as the first of its type to exceed 30m inheight. In the 19th century British engineers advanced and developedembankment design and construction very successfully, notable projects inthe UK including the magnificent Longdendale series of five principaldams, completed between 1854 and 1877, and many similar large struc-tures constructed in India and elsewhere overseas.

Rational methods of analysis for masonry dams were developed andrefined in various countries, notably France, Britain and the US, fromabout 1865. The design of embankment dams continued to be very empiri-cal until much later. Advances in embankment construction were depend-ent upon the emergence of modern soil mechanics theory in the periodfrom 1930. Subsequent progress has been relatively rapid, and majoradvances have been made in consequence of improvements in understand-


ing of the behaviour of compacted earthfill and rockfill and with the intro-duction of modern high-capacity earthmoving plant. In the same period,partly in consequence of several major disasters, the vital importance ofthe interrelated disciplines of soil mechanics, rock mechanics and engin-eering geology to dam engineering was finally established.

Analytical techniques have also progressed rapidly in recent years,most specifically with the development of the elegant and extremelypowerful finite element analyses (FEA), now widely employed for themost advanced analysis of all types of dam. The application of sophisti-cated FEA techniques has, in turn, been dependent upon the ready avail-ability and power of the modern computer. However, limitations on theapplicability of FEA remain, and they arise essentially from the complexload response of all construction materials utilized in dams. These limita-tions will be referred to further in Chapters 2 and 3 (Sections 2.7.2 and3.2.8).

A comprehensive review of the history of dams lies beyond the scopeof this text. Reference should be made to the international and compre-hensive historical review of dams from earliest times published in Smith(1971) or to Schnitter (1994). The history prepared for the InternationalCommission on Irrigation and Drainage (Garbrecht, 1987) gives particu-larly detailed descriptions of the earliest dams in parts of the Middle Eastand of Central Europe; the text also includes a useful review of the devel-opment of dams in Britain. More detailed and comprehensive accounts ofearly British dams, and of 19th-century dams built by prominent engineersof the period, are published in Binnie (1987a) and Binnie (1981) respec-tively. The latter provides a valuable insight into the reasoning underlyingsome design features of many older embankment dams.

1.2.4 Environmental and related issues

The environmental, economic and other socio-political issues associatedwith reservoir development must in all instances be acknowledged at theoutset and fully addressed thereafter. This is especially important in thecase of the larger high-profile projects and all others, large or lesser, sitedin environmentally or politically sensitive locations.

Political and public consciousness with regard to environmentalissues, compounded by a heightened awareness of issues associated withclimate change and interest in promoting sustainable development, has ledto growing international debate over the benefit derived from major damprojects. This resulted in the setting-up of a 12-man ‘World Commissionon Dams’ (WCD; not to be confused with the International Commissionon Large Dams, ICOLD) under the auspices of the World Bank and theWorld Conservation Union in 1998. WCD was charged with reviewing


international experience in context with the emergent social and environ-mental controversies over large dam projects and reporting upon the roleof such projects in development strategies. Looking to the future, theCommission was also tasked with identifying best practice in addressingcritical policy and decision-making issues.

WCD reported in late 2000, stating that dams deliver significantdevelopment services in some 140 countries, with dam projects responsiblefor 19% of global electrical output, 12–15% of food production, and 12%of domestic and industrial water. It was also stated that dams provide forlarge-scale flood control and mitigation in at least 70 countries. The Com-mission examined alternatives for meeting water, energy and food needs,and identified a number of palliative organizational measures.

In terms of decision-making practice, the Commission’s guidelinesrecommend outcomes based on multi-criteria analysis of technical, social,environmental, economic and financial parameters. The recommendationsfor future decision-making also included:

• Five core values: equity; sustainability; efficiency; participatorydecision-making; accountability.

• A ‘rights and risk’ approach in negotiating development options.• Seven strategy priorities for water resource development:

Gain public acceptanceAssess optionsAddress existing damsSustain rivers and livelihoodsRecognize entitlements and share benefitsEnsure complianceShare rivers for peace, development and security

• Clear criteria for assessing compliance, with 26 guidelines for review-ing and approving projects at five key stages in the decision-makingprocess.

The WCD report has been criticized for not having given sufficient recog-nition to the positive dimension of major dam projects. The report has,however, made a significant contribution by stimulating considerabledebate. Issues associated with future decision-making for developmentand sustainability are further examined and discussed in Pritchard (2000),Morrison and Sims (2001), Workman (2001), Bridle (2006), Collier (2006)and UNEP (2006).

Environmental impact and associated socio-political considerationscan extend across a diverse spectrum of issues. The latter may range frompopulation displacement, with consequent economic impacts, to thepreservation of cultural or heritage sites; from the consequences of sedi-mentation and/or of changing flood regimes to altered patterns of disease.


The discussion of such an extensive and varied range of issues goes wellbeyond the scope of this textbook. Some general reference to selectedissues is, however, dispersed through the text, e.g. Section 4.5 on sedimen-tation, or Section 9.1.7 on the effects of river barrages on water quality.

The broader issues are examined and discussed within Golzé (1977),in ICOLD (1988, 1992, 1994) and in specialist texts. Hendry (1994) exam-ines legislative issues in the European context. The paper discusses therole of environmental assessment in terms of the appropriate EuropeanDirective (CEC, 1985), and discusses the provisions of the latter in rela-tion to relevant UK provisions, e.g. DoE (1989). General questions ofenvironmental evaluation, impact assessment and benefit appraisal areaddressed in Clifton (2000), Thomas, Kemm and McMullan (2000), andin Gosschalk and Rao (2000). The latter reference includes a concisesummary of the issues arising on three major high-profile dam projects, i.e.Aswan High (Lake Nasser, Egypt) completed in 1968, and projects cur-rently completing at Sardar Sarovar (Narmada River, India) and ThreeGorges (Yangtze River, China). The scale, and thus the overall impact, ofthe latter two multi-purpose projects is of particular note.

Sardar Sarovar, the principal component of the inter-state NarmadaRiver development, is intended to irrigate some 1.9 million ha of land inthe states of Gujarat and Rajasthan and provide 2450MW of hydro-electric generating capacity. The concrete gravity dam is intended to reacha height of 138m, and has a designed overflow capacity of 79�103 m3/s.Construction commenced in the late 1980s, but opposition in the courtscentred upon the displacement of an estimated 300000 people from thevery many village communities scheduled for inundation has delayed com-pletion of the dam beyond an interim height of 110m.

The Three Gorges project centres upon a 2331m long and 184m highconcrete gravity dam impounding the Yangtze River. Design dischargecapacity of the overflow system is 110�103 m3/s. The immediate benefitsassociated with Three Gorges on project completion in 2008/2009 will bethe availability of up to 22�109 m3 of storage capacity for flood control onthe notoriously difficult Yangtze and 18200MW of hydro-electric generat-ing capacity from 26 turbines (see also Section 12.2). Three Gorges is alsocentral to future development along some 600km length of the upperYangtze, the lock system which bypasses the dam (see also Sections 11.8.3and 11.10) providing direct access to the heart of China for ships of up to10000 tonnes. The project has engendered considerable controversyhowever, since creation of the reservoir is estimated to displace at least 1.3million people and submerge some 1300 known archaeological sites.Overall cost is officially stated as $14 billion, but it has been suggested thatthe true final figure will be considerably higher, with the most extremeestimates ranging up to $90–100 billion. An outline perspective on ThreeGorges which makes plain the enormous scale and societal/environmentalimpact of this regional development project is presented in Freer (2000).


1.2.5 Dams: focus points

Dams differ from all other major civil engineering structures in a numberof important regards:

• every dam, large or small, is quite unique; foundation geology,material characteristics, catchment flood hydrology etc. are each site-specific.

• dams are required to function at or close to their design loading forextended periods.

• dams do not have a structural lifespan; they may, however, have anotional life for accounting purposes, or a functional lifespan dic-tated by reservoir sedimentation.

• the overwhelming majority of dams are of earthfill, constructed froma range of natural soils; these are the least consistent of constructionmaterials.

• dam engineering draws together a range of disciplines, e.g. structuraland fluid mechanics, geology and geotechnics, flood hydrology andhydraulics, to a quite unique degree.

• the engineering of dams is critically dependent upon the applicationof informed engineering judgement.

In summary, dam engineering is a distinctive, broadly based and specialistdiscipline. The dam engineer is required to synthesize design solutionswhich, without compromise on safety, represent the optimal balancebetween technical, economic and environmental considerations.

1.3 Embankment dam types and characteristics

The embankment dam can be defined as a dam constructed from naturalmaterials excavated or obtained close by. The materials available are uti-lized to the best advantage in relation to their characteristics as an engi-neered bulk fill in defined zones within the dam section. The natural fillmaterials are placed and compacted without the addition of any bindingagent, using high-capacity mechanical plant. Embankment construction isconsequently now an almost continuous and highly mechanized process,weather and soil conditions permitting, and is thus plant intensive ratherthan labour intensive.

As indicated in Section 1.2.1, embankment dams can be classified inbroad terms as being earthfill or rockfill dams. The division between thetwo embankment variants is not absolute, many dams utilizing fill mater-ials of both types within appropriately designated internal zones. The con-ceptual relationship between earthfill and rockfill materials as employed in


embankment dams is illustrated in Fig.1.1. Secondary embankment damsand a small minority of larger embankments may employ a homogeneoussection, but in the majority of instances embankments employ an impervi-ous zone or core combined with supporting shoulders which may be ofrelatively pervious material. The purpose of the latter is entirely structural,providing stability to the impervious element and to the section as awhole.

Embankment dams can be of many types, depending upon how theyutilize the available materials. The initial classification into earthfill orrockfill embankments provides a convenient basis for considering the prin-cipal variants employed.

1. Earthfill embankments. An embankment may be categorized as anearthfill dam if compacted soils account for over 50% of the placedvolume of material. An earthfill dam is constructed primarily ofselected engineering soils compacted uniformly and intensively inrelatively thin layers and at a controlled moisture content. Outlinesections of some common variants of the earthfill embankment areillustrated in Fig.1.2.

2. Rockfill embankments. In the rockfill embankment the sectionincludes a discrete impervious element of compacted earthfill or aslender concrete or bituminous membrane. The designation ‘rockfillembankment’ is appropriate where over 50% of the fill materialmay be classified as rockfill, i.e. coarse-grained frictional material.

EMBANKMENT DAM TYPES AND CHARACTERISTICS 13

Fig.1.1 Earthfills and rockfills in dam construction

Modern practice is to specify a graded rockfill, heavily compacted inrelatively thin layers by heavy plant. The construction method istherefore essentially similar to that for the earthfill embankment.

The terms ‘zoned rockfill dam’ or ‘earthfill–rockfill dam’ are used todescribe rockfill embankments incorporating relatively wide imperviouszones of compacted earthfill. Rockfill embankments employing a thinupstream membrane of asphaltic concrete, reinforced concrete or othermanufactured material are referred to as ‘decked rockfill dams’.

Representative sections for rockfill embankments of different typesare illustrated in Fig.1.3. Comparison should be made between therepresentative profile geometries indicated on the sections of Figs1.2 and1.3. The saving in fill quantity arising from the use of rockfill for a dam ofgiven height is very considerable. It arises from the frictional nature ofrockfill, which gives relatively high shear strength, and from high perme-ability, resulting in the virtual elimination of porewater pressure problemsand permitting steeper slopes. Further savings arise from the reducedfoundation footprint and the reduction in length of outlet works etc.


(e) Wide rolled clay core: zoned withtransitions and drains: note base drainm � 2.5–3.5

(f) Earthfill/rockfill with central rolledclay core: zoned with transitions anddrainsm � 1.6–2.0

zone 2transition/drain

zone 1zone 3 zone 2

zone 2a

zone 4 transition/drain

zone 3

zone 2

(c) Slender central clay core:19th-century ‘Pennines’ type –obsolete post 1950m � 2.5–3.5

(d) Central concrete core:smaller dams – obsolescentm � 2.5–3.5

(a) Homogenous with toedrain:small secondary damsm � 2.0–2.5

(b) Modern homogeneous with internalchimney drainm � 2.5–3.5

Fig.1.2 Principal variants of earthfill and earthfill–rockfill embankmentdams (values of m are indicative only)

EMBANKMENT DAM TYPES AND CHARACTERISTICS 15

Fig.1.3 Principal variants of rockfill embankment dams (values of m areindicative only)

The variants of earthfill and rockfill embankments employed in prac-tice are too numerous to identify all individually. The more important arediscussed further in appropriate sections of Chapter 2.

The embankment dam possesses many outstanding merits whichcombine to ensure its continued dominance as a generic type. The moreimportant can be summarized as follows:

1. the suitability of the type to sites in wide valleys and relatively steep-sided gorges alike;

2. adaptability to a broad range of foundation conditions, ranging fromcompetent rock to soft and compressible or relatively pervious soilformations;

3. the use of natural materials, minimizing the need to import or trans-port large quantities of processed materials or cement to the site;

4. subject to satisfying essential design criteria, the embankment designis extremely flexible in its ability to accommodate different fill mater-ials, e.g. earthfills and/or rockfills, if suitably zoned internally;

5. the construction process is highly mechanized and is effectively con-tinuous;

6. largely in consequence of 5, the unit costs of earthfill and rockfillhave risen much more slowly in real terms than those for mass con-crete;

7. properly designed, the embankment can safely accommodate anappreciable degree of deformation and settlement without risk ofserious cracking and possible failure.

The relative disadvantages of the embankment dam are few. Themost important include an inherently greater susceptibility to damage ordestruction by overtopping, with a consequent need to ensure adequateflood relief and a separate spillway, and vulnerability to concealed leakageand internal erosion in dam or foundation. Examples of alternative typesof embankment dam are illustrated and described in Thomas (1976),Golzé (1977) and Fell, MacGregor and Stapledon (1992).

1.4 Concrete dam types and characteristics

Rubble masonry or random masonry was successfully employed for manyearly dams. In the latter half of the 19th century masonry was used forhigh dams constructed in accordance with the first rational design criteria.Cyclopean masonry (i.e. stones of up to c.10 t mass individually bedded ina dry mortar) was generally used, with a dressed masonry outer facing fordurability and appearance (Binnie, 1987b).

Mass concrete, initially without the formed transverse contractionjoints shown on Fig.1.4(a), began to displace masonry for the constructionof large non-embankment dams from about 1900 for economic reasonsand also for ease of construction for more complex dam profiles, e.g. thearch. Early mass concrete commonly employed large stone ‘displacers’ (cf.cyclopean masonry). From about 1950 mass concrete increasingly incorpo-rated bulk mineral additives, e.g. slags or pulverized fuel ash (PFA), inattempts to reduce thermal problems and cracking and to contain escalat-ing costs.

The principal variants of the modern concrete dam are defined below.

1. Gravity dams. A concrete gravity dam is entirely dependent upon itsown mass for stability. The gravity profile is essentially triangular,with the outline geometry indicated on Fig.1.4(a), to ensure stabilityand to avoid overstressing of the dam or its foundation. Some gravitydams are gently curved in plan for aesthetic or other reasons, andwithout placing any reliance upon arch action for stability. Where alimited degree of arch action is deliberately introduced in design,allowing a rather slimmer profile, the term arched or arch-gravitydam may be employed.

2. Buttress dams. In structural concept the buttress dam consists of acontinuous upstream face supported at regular intervals by down-stream buttresses. The solid head or massive buttress dam, as illus-trated by Figs1.4(b) and 1.4(c), is the most prominent modernvariant of the type, and may be considered for conceptual purposesas a lightened version of the gravity dam.

3. Arch dams. The arch dam has a considerable upstream curvature.Structurally it functions primarily as a horizontal arch, transmitting


the major portion of the water load to the abutments or valley sidesrather than to the floor of the valley. A relatively simple arch, i.e.with horizontal curvature only and a constant upstream radius, isshown in Fig.1.4(d). It is structurally more efficient than the gravityor buttress dam, greatly reducing the volume of concrete required. Aparticular derivative of the simple arch dam is the cupola or double-curvature arch dam (Fig.1.4(e)). The cupola dam introduces complexcurvatures in the vertical as well as the horizontal plane. It is themost sophisticated of concrete dams, being essentially a dome orshell structure, and is extremely economical in concrete. Abutmentstability is critical to the structural integrity and safety of both thecupola and the simple arch.

CONCRETE DAM TYPES AND CHARACTERISTICS 17

Fig.1.4 Principal variants of concrete dams (values of m and n indicativeonly; in (e) RH and RV generally vary over dam faces)

4. Other concrete dams. A number of less common variants of the majortypes of concrete dams illustrated in Fig.1.4 can also be identified.They include hollow gravity, decked buttress, flat slab (Ambursen)buttress, multiple arch, and multiple cupola dams, as illustrated inFig.1.5. The type names are self-explanatory, and the structuralparentage of each as a derivative of one or other of the principal typesis apparent from the figures. In view of this and the relative rarity ofthese variants they are not considered further in this text, but the com-parative vulnerability of the slender flat slab and similar types toseismic disturbance etc. may be noted.

The characteristics of concrete dams are outlined below with respectto the major types, i.e. gravity, massive buttress and arch or cupola dams.Certain characteristics are shared by all or most of these types; many are,however, specific to particular variants. Merits shared by most concretedams include the following.

1. Arch and cupola dams excepted, concrete dams are suitable to thesite topography of wide or narrow valleys alike, provided that a com-petent rock foundation is accessible at moderate depth (�5m).

2. Concrete dams are not sensitive to overtopping under extreme floodconditions (cf. the embankment dam).


Fig.1.5 Further variants of concrete dams

3. As a corollary to 2, all concrete dams can accommodate a crest spill-way, if necessary over their entire length, provided that steps aretaken to control downstream erosion and possible undermining ofthe dam. The cost of a separate spillway and channel are thereforeavoided.

4. Outlet pipework, valves and other ancillary works are readily andsafely housed in chambers or galleries within the dam.

5. The inherent ability to withstand seismic disturbance without cata-strophic collapse is generally high.

6. The cupola or double-curvature arch dam is an extremely strong andefficient structure, given a narrow valley with competent abutments.

Type-specific characteristics are largely determined through the dif-fering structural modus operandi associated with variants of the concretedam. In the case of gravity and buttress dams, for example, the dominantstructural response is in terms of vertical cantilever action. The reduceddownstream contact area of the buttress dam imposes significantly higherlocal foundation stresses than for the equivalent gravity structure. It istherefore a characteristic of the former to be more demanding in terms ofthe quality required of the underlying rock foundation.

The structural behaviour of the more sophisticated arch and cupolavariants of the concrete dam is predominantly arch action, with verticalcantilever action secondary. Such dams are totally dependent uponthe integrity of the rock abutments and their ability to withstand archthrust without excessive yielding. It is consequently characteristic ofarch and cupola dams that consideration of their suitability is confined toa minority of sites in relatively narrow steep-sided valleys or gorges,i.e. to sites with a width:height ratio at the dam crest level generally notexceeding 4–5.

A comparison of the general characteristics of concrete dams withthose of the embankment dam suggests the following inherent disadvant-ages for the former.

1. Concrete dams are relatively demanding with respect to foundationconditions, requiring sound and stable rock.

2. Concrete dams require processed natural materials of suitablequality and quantity for aggregate, and the importation to site andstorage of bulk cement and other materials.

3. Traditional mass concrete construction is relatively slow, beinglabour intensive and discontinuous, and requires certain skills, e.g.for formwork, concreting, etc.

4. Completed unit costs for mass concrete, i.e. cost per cubic metre, arevery much higher than for embankment fills, typically by an order ofmagnitude or more. This is seldom counterbalanced by the muchlower volumes of concrete required in a dam of given height.

CONCRETE DAM TYPES AND CHARACTERISTICS 19

A considered evaluation of the generalized characteristics in con-junction with Figs1.3 and 1.4 will suggest further conclusions as to the cor-responding advantages of embankment and concrete dams. However, thelimitations of generalizations on the merits of either type must be appreci-ated. An open mind must be maintained when considering possible damtypes in relation to a specific site, and evaluation must attach properweight to local circumstances. Economic comparisons apart, other non-engineering factors may be of importance: this is referred to further inSection 1.6.

The variants of the concrete dam illustrated and their merits arefurther compared with those for the embankment dam in Thomas (1976),Golzé (1977) and USBR (1987).

1.5 Spillways, outlets and ancillary works

Dams require certain ancillary structures and facilities to enable them todischarge their operational function safely and effectively. In particular,adequate provision must be made for the safe passage of extreme floodsand for the controlled draw-off and discharge of water in fulfilment of thepurpose of the reservoir. Spillways and outlet works are therefore essen-tial features. Other ancillary facilities are incorporated as necessary for thepurpose of the dam and appropriate to its type. Provision for permanentflood discharge and outlet works and for river diversion during construc-tion can prove to be technically difficult and therefore costly.

In this section, the more important structures and ancillary worksassociated with impounding dams are identified and briefly described. Assuch, it is intended as an introduction to subsequent chapters dealing withthe design of dams (Chapters 2 and 3), spillways and outlets (Chapter 4),energy dissipators (Chapter 5) and gates and valves (Chapter 6). Fora review of hydraulics of spillways and energy dissipators see alsoKhatsuria (2005)

1.5.1 Spillways

The purpose of the spillway is to pass flood water, and in particular thedesign flood, safely downstream when the reservoir is overflowing. It hastwo principal components: the controlling spillweir and the spillwaychannel, the purpose of the latter being to conduct flood flows safely down-stream of the dam. The latter may incorporate a stilling basin or otherenergy-dissipating devices. The spillway capacity must safely accommodatethe maximum design flood, the spillweir level dictating the maximum reten-tion level of the dam, i.e. the normal maximum water level (NWL).


Spillways are normally uncontrolled, i.e. they function automaticallyas the water level rises above NWL, but they may be controlled by gates.In some instances additional emergency spillway capacity is provided by afuse plug (see Section 4.7.7), i.e. an erodible subsidiary bank designed towash out if a predetermined extreme flood level is attained. Alternativeemergency provision can be made by reinforced concrete flap-gatesdesigned to tip over by hydrostatic pressure under extreme flood con-ditions or by the use of crest-mounted fusegates (see Section 6.2.8). Con-crete dams normally incorporate an overfall or crest spillway, butembankments generally require a separate side-channel or shaft spillwaystructure located adjacent to the dam.

1.5.2 Outlet works

Controlled outlet facilities are required to permit water to be drawn off asis operationally necessary. Provision must be made to accommodate therequired penstocks and pipework with their associated control gates orvalves. Such features are readily accommodated within a concrete dam, asnoted in Section 1.4. For embankment dams it is normal practice toprovide an external control structure or valve tower, which may be quiteseparate from the dam, controlling entry to an outlet tunnel or culvert.

A bottom discharge facility is provided in most dams to provide anadditional measure of drawdown control and, where reasonable, to allowemptying of the reservoir. The bottom outlet must be of as high a capacityas economically feasible and consistent with the reservoir managementplan. In most cases it is necessary to use special outlet valves (Section 6.3)and/or structures to avoid scouring and damage to the stream bed andbanks downstream of the dam.

1.5.3 River diversion

This provision is necessary to permit construction to proceed in dry con-ditions. An outlet tunnel or culvert may be temporarily adapted to thispurpose during construction, and subsequently employed as a dischargefacility for the completed dam. In the absence of such a tunnel of adequatecapacity alternative steps will be necessary, involving the construction oftemporary upstream and downstream cofferdams or, in the case of con-crete dams, by programming construction of one monolith or block toleave a temporary gap or formed tunnel through the structure.

The hydraulic aspects of river diversion are dealt with in detail inVischer and Hager (1998).

SPILLWAYS, OUTLETS AND ANCILLARY WORKS 21

1.5.4 Cut-offs

Seepage under and round the flank of a dam must be controlled. This isachieved by the construction of a cut-off below the structure, continued asnecessary on either flank. Modern embankment cut-offs are generallyformed from wide trenches backfilled with rolled clay, if impervious stratalie at moderate depths, and/or by drilling and grouting to form a cut-offscreen or barrier to greater depths. Grout screen cut-offs are also custom-arily formed in the rock foundation under a concrete dam.

1.5.5 Internal drainage

Seepage is always present within the body of any dam. Seepage flows andtheir resultant internal pressures must be directed and controlled. Internaldrainage systems for this purpose are therefore an essential and criticalfeature of all modern dams. In embankments drainage is effected by suit-ably located pervious zones leading to horizontal blanket drains or outletsat base level. In concrete dams vertical drains are formed inside theupstream face, and seepage pressure is relieved into an internal gallery oroutlet drain. In the case of arch dams, seepage pressures in the rock abut-ments are frequently relieved by systems of bored drains and/or drainageadits or tunnels.

1.5.6 Internal galleries and shafts

In addition to their function alongside drains in effecting local control ofseepage, galleries and shafts are provided as a means of allowing internalinspection, particularly in concrete dams. The galleries, shafts and anyassociated chambers to accommodate discharge valves or gates can also beused to accommodate instrumentation for structural monitoring and sur-veillance purposes (Chapter 7).

The ancillary structures and design features referred to are furtherdescribed in subsequent chapters. Additional illustrations of these andother ancillary works are also contained in Thomas (1976), USBR (1987),Fell, MacGregor and Stapledon (1992) and Kennard, Owens and Reader(1996).


1.6 Site assessment and selection of type of dam

1.6.1 General site appraisal

A satisfactory site for a reservoir must fulfil certain functional and tech-nical requirements. Functional suitability of a site is governed by thebalance between its natural physical characteristics and the purpose of thereservoir. Catchment hydrology, available head and storage volume etc.must be matched to operational parameters set by the nature and scale ofthe project served. Technical acceptability is dictated by the presence of asatisfactory site (or sites) for a dam, the availability of materials suitablefor dam construction, and by the integrity of the reservoir basin withrespect to leakage. The hydrological and geological or geotechnicalcharacteristics of catchment and site are the principal determinantsestablishing the technical suitability of a reservoir site. To these must beadded an assessment of the anticipated environmental consequences ofconstruction and operation of the dam, alluded to in Section 1.2.4. Theyare not considered further here.

The principal stages involved in site appraisal and leading to selec-tion of the optimum dam site and type of dam for a major project are asindicated schematically in Fig.1.6.

The considerable time which can elapse between initial strategicplanning, with identification of the project requirement, and commence-ment of construction on site will be noted. A significant proportion of thattime may be attributable to the ‘political’ decision-making processes andto arranging project funding.

In the reconnaissance phase, which may extend over a substantialperiod, the principal objective is to collect extensive topographical, geo-logical and hydrological survey data. Large-scale maps and any recordsalready available provide the starting point, but much more detailedsurveys will inevitably be required. Aerial reconnaissance, employingmodern sensors in addition to the traditional photogrammetric surveytechniques, has a particular rôle to play in the preparation of accurate andlarge-scale site plans (e.g. 1:5000 and larger). In the hands of an experi-enced engineering geologist as interpreter, aerial surveys also providevaluable information on geology, on possible dam sites and on the likelyavailability of construction materials. Hydrological catchment and riversurveys are directed to determining rainfall and run-off characteristics, andassessing historical evidence of floods etc.

The feasibility report prepared at the conclusion of the reconnais-sance phase assembles and interprets all available information, data andrecords, and makes initial recommendations with respect to the technicaland economic viability of the reservoir. Options with regard to the loca-tion, height and type of dam are set out, and comparisons drawn in terms

SITE ASSESSMENT AND SELECTION OF TYPE OF DAM 23

of estimated costs and construction programmes. Within the latter,account must be taken of the resource implications of each, i.e. financialoutlay, labour and plant requirements etc. On the strength of this report adecision can be made with respect to the further detailed investigationsrequired to confirm the suitability of the reservoir basin and preferreddam site (or sites).

Further investigation of the reservoir basin is principally directed toconfirming its integrity with respect to water retention. A thoroughgeological assessment is necessary for this purpose, particularly in karsticand similarly difficult formations and in areas with a history of miningactivity. The issue of less favourable sites for reservoirs and solution of the


Fig.1.6 Stages in dam site appraisal and project development: majorprojects

associated problems is addressed in ICOLD (1970) and in Fell, Mac-Gregor and Stapledon (1992). As specific examples, investigations andconclusions drawn for Cow Green reservoir (UK) are described byKennard and Knill (1969), and the initial leakage losses at May reservoir(Turkey) are discussed by Alpsü (1967).

Investigation of the reservoir margins to confirm the stability of poten-tially vulnerable areas, e.g. adjacent to the intended dam, is conducted asrequired. The availability of possible construction materials, e.g. suitablefills, sources of aggregates etc., is also assessed in considerable depth.

Hydrological studies are continued as necessary to confirm andextend the results of the initial investigations. In view of their very special-ist nature they are not considered further here; reference may be made toThomas (1976) and to Chapters 4 and 8 for details.

1.6.2 Dam site evaluation – general

The viability of the preferred dam site identified in a reservoir feasibilitystudy must be positively established. Extensive investigations are con-ducted to confirm that the site can be developed on the desired scale andat acceptable cost. The nature of the soil and rock formations present,critical to foundation integrity, must be proved by subsurface exploration.Emphasis is placed upon confirmation of site geology and geotechnicalcharacteristics, and on the evaluation of sources of construction materials(Sections 1.6.3–1.6.5).

Foundation competence is determined by stability, load-carryingcapacity, deformability, and effective impermeability. All are assessed inrelation to the type and size of dam proposed (Section 1.6.4).

In the case of a difficult site, the site evaluation programme can be pro-tracted and expensive. Expenditure may be of the order of 1% up to, excep-tionally, 2.5 or 3% of the anticipated cost of the dam. The scope of individualaspects of an investigation reflects circumstances unique to the site. Theinvestigation may also relate to a specific type of dam if site conditions aresuch that options are restricted, e.g. by depth of overburden (Section 1.6.6).

Only the general principles underlying dam site evaluation can bepresented here. A comprehensive review is provided in Thomas (1976),with outline summaries of example cases. An indication of the interactionwhich develops between site evaluation, local circumstance and type ofdam is given in Bridle, Vaughan and Jones (1985), Coats and Rocke(1983) and Collins and Humphreys (1974) for embankment dams, or Bassand Isherwood (1978) for a concrete dam and Kennard and Reader (1975)for a composite dam, part concrete and part embankment. Walters (1974)presents simplified but informative summaries of site geology in relation toan international selection of dams.


In parallel with these investigations, extensive and detailed surveysare required to establish the location and extent of potential sources ofconstruction materials in reasonable proximity to the site. The materials ofinterest may range from low-permeability cohesive soils and glacial tills forembankment cores through to sands and gravels suitable for shoulder fillor as concrete aggregates. Crushed rock may also be obtainable from exca-vations for underground works associated with the project.

Overall site viability is additionally subject to economic considera-tions, notably site preparation and construction material costs. It may alsobe influenced by seismicity, access development cost or other local con-straints, including environmental and socio-political considerations.

In summary, dam site investigations require careful planning and theinvestment of sufficient time and resources. Wherever possible, in situ andfield test techniques should be employed to supplement laboratory testingprogrammes. Proper and meticulous interpretation of geological and geo-technical data demands the closest cooperation between the engineeringgeologist, the geotechnical specialist and the dam engineer. Underinvestmentin reservoir site appraisal and in the investigation and assessment of the sitefor a dam can have grave consequences, both technical and economic.

1.6.3 Geological and geotechnical investigations

Geological and geotechnical investigation of a dam site selected fordetailed evaluation is directed to determination of geological structure,stratigraphy, faulting, foliation and jointing, and to establishing ground andgroundwater conditions adjacent to the dam site, including the abutments.

The general objectives of these and allied investigations are

(a) to determine engineering parameters which can reliably be used toevaluate stability of the dam foundation and, on compressible founda-tions, i.e. soils, to estimate probable settlement and deformation,

(b) the determination of seepage patterns and parameters enablingassessment of the probable seepage regime, including quantities andpressures, and

(c) to confirm the containment integrity of the reservoir basin and thestability of its margins.

The relative importance of (a), (b) or (c) is dependent upon the site andthe type of dam proposed. A fourth general objective is

(d) confirmation of the nature, suitability and availability of natural con-struction materials, including the determination of design parametersfor fill materials etc.


General features to be identified and defined in the course of thesite investigation include the interface between soil and rock, groundwaterconditions, unstable and caving ground, e.g. karstic formations etc., andall significant discontinuities, i.e. rock faults, shatter zones, fissuredor heavily fractured rock and the spacing and other characteristics of joint-ing and bedding surfaces etc. within the rock mass. Reference shouldbe made to Attewell and Farmer (1976) and/or to Bell (1993) for a com-prehensive perspective on engineering geology in relation to dam andreservoir sites.

Key features of this phase of the investigation include

(a) meticulous logging of all natural and excavated exposures and bore-hole records, etc.,

(b) careful correlation between all exposures, boreholes and other data,and

(c) excavation of additional trial pits, boreholes, shafts and exploratoryadits as considered necessary.

It is at this stage that more extensive geophysical and in situ testingprogrammes may also be conducted, with the primary intention of extend-ing and validating borehole and laboratory data. A further purpose of fieldtesting at this time is confirmation of the natural groundwater regime, e.g.through installation of piezometers, pumping tests, etc.

Extensive use is made of rotary drilling and coring techniques to estab-lish the rock structure at depth and to confirm its competence. Core recoveryis a crude but useful index of rock quality, e.g. in terms of rock quality desig-nation (RQD) (i.e. total recovered core in lengths of over 10cm as a percent-age of total borehole depth; RQD�70 is generally indicative of sound rock).In situ tests, e.g. for permeability, strength and deformability, are used toestimate rock mass characteristics in preference to small-scale laboratorysample testing wherever possible. All cores are systematically logged andshould ideally be retained indefinitely. Drilling, sampling and testing tech-niques are essentially those employed in conventional site investigation prac-tice. A comprehensive review of the latter is presented in Clayton, Simonsand Matthews (1995) and in the CIRIA site investigation manual (Weltmanand Head, 1983). More specialist techniques, e.g. for large-scale in situ tests,are illustrated in Thomas (1976) and in Fell, MacGregor and Stapledon(1992). The applicability of different equipment and exploratory methods inthe context of site investigation for dams are reviewed concisely in Wakelingand Manby (1989).

Evaluation of seismic risk for an important dam requires identificationof the regional geological structure, with particular attention being paid tofault complexes. Activity or inactivity within recent geological history willrequire to be established from study of historical records and field reconnais-sance. If historical records of apparent epicentres can be matched to key


geological structures it is possible to make a probabilistic assessment ofseismic risk in terms of specific intensities of seismic event. In the absence ofreliable historical information it will be necessary to monitor microseismicactivity as a basis for the probabilistic prediction of major seismic events.Either process is imprecise and will at best provide only an estimate of theorder of seismic risk. As a measure of reassurance over seismicity it has beensuggested that most well-engineered dams on a competent foundation canaccept a moderate seismic event, with peak accelerations in excess of 0.2g,without fatal damage. Dams constructed with or on low-density saturatedcohesionless soils, i.e. silts or sands, are, however, at some risk of failure inthe event of seismic disturbance due to porewater pressure buildup and liq-uefaction with consequent loss of stability.

Seismicity is discussed further in Sections 2.7 and 3.1, with a briefintroduction to the application of pseudo-static seismic analysis.

1.6.4 Foundation investigations

Foundation competence of the dam site must be assessed in terms of stability,load-carrying capacity, compressibility (soils) or deformability (rocks), andeffective mass permeability. The investigative techniques to be adopted willdepend upon the geomorphology and geology of the specific site.

(a) Dams on competent stiff clays and weathered rocks

Serious underseepage is unlikely to be a problem in extensive and uniformdeposits of competent clay. It is important, however, to identify and con-sider the influence of interbedded thin and more permeable horizonswhich may be present, e.g. silt lenses, fine laminations, etc. Considerablecare is required in the examination of recovered samples to detect all suchfeatures. The determination of appropriate shear strength parameters forevaluating foundation stability is of major importance.

For a foundation on rock positive identification of the weatheredrock profile may prove difficult. In situ determination of shear strengthparameters may also be necessary, using plate loading tests in trial pits oradits, or dilatometer or pressuremeter testing conducted within boreholes.The latter techniques are particularly suitable in softer rocks containingvery fine and closely spaced fissures.

(b) Dams on soft cohesive foundations

The presence of superficial soft and compressible clay deposits normallyensures that seepage is not a major consideration. The nature of such for-mations also ensures that investigations are, in principle, relativelystraightforward.


The soft consistency of the clays may necessitate the use of specialsampling techniques. In such situations continuous sampling or in situ conepenetrometer testing techniques offer advantages. Stability and settlementconsiderations will require the determination of drained shear strengthand consolidation parameters for the clay.

(c) Dams on pervious foundations

Seepage-associated problems are normally dominant where a dam is to befounded on a relatively pervious foundation. In a high proportion of suchinstances the soil conditions are very complex, with permeable and muchless permeable horizons present and closely interbedded.

(d) Dams on rock foundations

The nature of the investigation is dependent upon whether an embank-ment or a concrete dam is proposed. Where the decision is still open, theinvestigation must cover either option; both require a full understandingof the site geology.

C O N C R E T E D A M S

Concrete dam foundation stability requires careful assessment of the fre-quency, orientation and nature of the rock discontinuities, including thecharacteristics of infill material, e.g. clays etc. Foundation deformabilitywill be largely dependent upon rock load response characteristics and ondiscontinuity structure. Rotary coring is widely employed, but to assess therock structure reliably on the macroscale it is also advisable to expose andexamine it in trial excavations and, wherever justifiable, by drivingexploratory adits. The latter can be used subsequently for grouting or aspermanent drainage galleries. Abutment stability and deformability arevery important to all types of concrete dam in narrow steep-sided valleys,and most particularly if the design relies on some degree of arch action.Detailed investigations should, therefore, extend to the abutments, withparticular regard to the possibility of large-scale wedge or block instabilityor excessive deformation and yielding. Large-scale in situ loading tests toevaluate the strength and load-response characteristics of the rock, whilecostly, should be conducted in parallel with laboratory testing wheneverpracticable. In situ tests of this nature can be carried out in exploratory ordrainage adits, or at suitably prepared exposures, e.g

hydraulic structures: fourth edition...2.5 materials and construction 73 2.6 seepage analysis 78 2.7...

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