Table Of ContentRisk and uncertainty in dam
safety
CEA Technologies Dam Safety Interest Group
Desmond N. D. Hartford
Gregory B. Baecher
Published by Thomas Telford Publishing, Thomas Telford Ltd,
1 Heron Quay, London E14 4JD.
URL: http://www.thomastelford.com
Distributors for Thomas Telford books are
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First published 2004
Reprinted 2007
Also available from Thomas Telford Books
Interim guide to quantitative risk assessment for reservoirs. A. J. Brown and J. D. Gosden.
ISBN 07277 3267 6
Long term benefits and performance of dams. The British Dam Society. ISBN 07277 3268 4
Engineering judgement and risk. J. Parkin. ISBN 07277 2873 3
The philosophy of risk. J. Chicken and T. Posner. ISBN 07277 2666 8
This report was prepared by the CONTRACTOR and administered by CEA Technologies (CEATI) for the
ultimate benefit of CONSORTIUM MEMBERS (hereinafter called “SPONSORS”), who do not necessarily
agree with the opinions expressed herein.
Neither the SPONSORS, nor CEATI nor the CONTRACTOR, nor any other person acting on their behalf
makes any warranty, expressed or implied, or assumes any legal responsibility for the accuracy of any
information or for the completeness or usefulness of any apparatus, product or process disclosed, or accept
liability for the use, or damages resulting from the use, thereof. Neither do they represent that their use would
not infringe upon privately owned rights.
Furthermore, the SPONSORS, CEATI and the CONTRACTOR HEREBY DISCLAIM ANY AND ALL
WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANT-
ABILITY AND FITNESS FOR A PARTICULAR PURPOSE, WHETHER ARISING BY LAW, CUSTOM
OR CONDUCT, WITH RESPECT TO ANY OF THE INFORMATION CONTAINED IN THIS REPORT.
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damages because of use or any information contained in this report.
Any reference in this report to any specific commercial product, process or service by trade name, trade mark,
manufacturer or otherwise does not necessarily constitute or imply its endorsement or recommendation by the
CONTRACTOR, the SPONSORS or CEATI.
A catalogue record for this book is available from the British Library
ISBN 10: 0 7277 3270 6
ISBN 13: 978 0 7277 3270 5
© Authors and Thomas Telford Limited 2004
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Contents
Dedication vi
CEA Technologies Dam Safety Interest Group vii
Acknowledgements ix
The Sponsors’ Working Group x
Foreword xii
Preface xv
List of figures xix
List of tables xxv
Part I Guiding principles
1 Dam risk management 3
1.1 Dam risk management process 4
1.2 Potential benefits of integrated risk management 9
2 Guiding principles for risk analysis 11
2.1 Risk analysis methods 11
2.2 Risk analysis process 16
2.3 Iteration 19
2.4 Uncertainty and sensitivity analysis 20
2.5 Issues of good practice 20
3 Risk in decision-making 22
3.1 Significance of risk 22
3.2 Risk assessment 25
3.3 The value of risk information in decision-making 26
Part II Commentary on risk analysis
4 Reliability analysis 33
4.1 Failure modes, effects and criticality analysis 33
4.2 Event tree analysis 47
4.3 Fault tree analysis 67
Endnote 80
Contents
5 Consequence analysis 81
5.1 General considerations 82
5.2 General modelling considerations 84
5.3 Specific modelling considerations 90
5.4 Category considerations 94
5.5 Life safety analysis 97
6 Probability assignment 106
6.1 Philosophical preliminaries 106
6.2 Probability construct 114
6.3 Data 117
6.4 Phenomenological models 119
6.5 Experts and judgement 120
Endnote 123
Part III Theoretical considerations
7 Essential elements of probability theory 127
7.1 Uncertainty 127
7.2 Probability 134
7.3 The strength of evidence 140
7.4 Expert opinion 149
7.5 Appendix: mathematics of probability theory 166
Endnotes 175
8 Event tree analysis 176
8.1 Background 177
8.2 Concepts 184
8.3 Influence diagrams 191
8.4 Event trees 198
8.5 Branch probabilities 214
8.6 Calculations on event trees 228
Endnotes 233
9 Fault tree analysis 234
9.1 Mathematics of fault tree analysis 234
9.2 Fault tree evaluation 240
9.3 Quantitative evaluation 242
9.4 Component considerations 244
9.5 Operator actions 246
9.6 Probability 247
10 Consequence assessment 251
10.1 Importance of consequence analysis to risk 252
10.2 Modelling consequences 260
10.3 Measuring consequences 267
10.4 Public safety modelling and assessment 275
10.5 Economic modelling and assessment 282
10.6 Environmental modelling and assessment 288
Endnote 296
iv
Contents
11 Methods for probability assignment 297
11.1 Quantifying probabilities 298
11.2 What do assigned probabilities mean? 302
11.3 Reasoning with probabilities 306
11.4 Quantified probability assignments 318
11.5 Judgement and probability 348
Endnotes 356
References 357
Glossary 370
Index 377
v
Dedication
This book is dedicated to all those who lost their lives or suffered harm or loss from dam
failures in the hope that it will lead to safer dams.
Sketch by W. Nicholson of the failure of the Dale Dyke Dam, Sheffield, 1864.
Sheffield Libraries, Archives and Information retain rights to the use of this image.
The image was provided specifically for use in the book Risk and Uncertainty in
Dam Safety, with Thomas Telford named as the publisher.
vi
CEA Technologies Dam Safety Interest Group
The preparation of this book, Risk and Uncertainty in Dam Safety, has been sponsored
by the Dam Safety Interest Group (DSIG) under the auspices of CEA Technologies
Incorporated of Montreal, Canada (http://www.ceatech.ca). The DSIG is a group of
power utility owners interested in collaboration for the sharing of technical information
strategic to their business, and for the combining of resources (both staff and fiscal) to
applied research topics of mutual interest. Risk is one such topic.
At the present time there are thirteen DSIG members in North America, four in
Europe and one in Australia. Not all members fiscally support each initiative that is
proposed, but those members and their representatives that supported this book on risk
are shown here in Table 1.
Table1. Sponsors and their representatives
Sponsor Representative
BC Hydro & Power Authority Ray Stewart
Elforsk Urban Norstedt/Anders Isander
Great Lakes Power Harvey Walsh
Hydro Quebec Tai Mai Phat
Manitoba Hydro Gordon Schellenberg
Ontario Power Generation Tony Bennett
Scottish and Southern Energy Neil Sandilands
US Bureau of Reclamation Bruce Muller
US Army Corps of Engineers Charles Pearre
Gary Salmon, Technology Coordinator, CEA Technologies
The support came in the form of monetary contributions made on an annual basis by
the participating sponsors during the period 1997–2003, and from technical input and
discussion during the Spring and Fall Business Meetings held each year. In addition BC
Hydro and the US Bureau of Reclamation contributed considerable in-kind funding (i.e.
staff time not charged during this same six-year period) and, in doing so, helped
spearhead the initiative and kept it moving to a successful conclusion.
CEA Technologies Dam Safety Interest Group
Risk and Uncertainty in Dam Safetyis aimed at providing a contemporary description
of evolving techniques for risk-based dam safety management. The principal focus is
risk analysis, with the material presented representing the collective views of the
contributors, reviewers and sponsors. The book is intended to provide the sponsors and
the dam safety community with a basis for establishing their own Standards of Practice
for risk-based management of the safety of their dams, and concurrently provide a focus
for ongoing research and development into new and improved risk management
practices.
viii
Acknowledgements
BC Hydro served as Contractor for preparation of the book. Technical acknowledge-
ments (and there are a considerable number) are included in the Preface to the volume
as written by Desmond Hartford and Gregory Baecher, primary contributors to the
volume. Alan Imrie has been Project Manager for the period 1999 to date.
The Project Manager would like to thank Josef Lampa and Tonia Jurbin for their
careful editing of earlier drafts of the book and Donna Reedy for her patient and
competent approach to text processing and formatting. Final technical editing was
carried out by the sponsors’ Working Group (see Preface). The Project Manager would
also like to thank Gary Salmon who took on the challenging job of DSIG Technology
Coordinator in 1998 and used his persuasive powers to pave the way for the sponsors
and Contractor to move ahead in concert on the business side of the initiative. Finally,
it was a pleasure to work with Kaare Høeg who used his vision to write a thoughtful and
meaningful Foreword to the book, describing its usefulness to the dam safety
community. His articulate critique of an earlier draft helped move the Project Team to
the final product.
The sponsors’ Working Group
Desmond N. D. Hartford, BA, BAI, MA, PhD (Dublin), CEng FICE, CEng FIEI,
EurIng, PEng is Specialist Engineer – Dam Safety Risk Assessment at British
Columbia Hydropower Authority. Specialising in dam safety and catastrophic loss risk
management of large dams, hydroelectric facilities and water resource infrastructure, he
is primarily responsible for the development and implementation of risk management
solutions for BC Hydro’s portfolio of 43 dams. Dr Hartford also advises dam and water
resource agencies across Canada and internationally on dam safety and risk
management. Dr Hartford is one of five principal authors of the forthcoming
International Commission on Large Dams (ICOLD) bulletin on Risk Assessment for
Dams. He is active in the training and professional development of engineers in risk
management of dams and has presented workshops and seminars on dam risk
management to government agencies and professional dam safety organisations around
the world. Prior to joining BC Hydro, Dr Hartford held senior positions on international
infrastructure projects. He was Research Fellow in Civil Engineering at Trinity College
(1984–87) and subsequently Research Associate (1987–present).
Gregory B. Baecher, BSCE (Berkeley), MSc, PhD (MIT) is Professor of Civil and
Environmental Engineering at the University of Maryland at College Park (UMCP). Dr
Baecher is a consultant to government and industry on risk and reliability of civil
infrastructure, especially in water resources development, and has served on a wide
range of government and related committees. A member of the National Research
Council’s Water Science and Technology Board since 1998, Dr Baecher also served on
the NRC’s Board on Infrastructure and the Constructed Environment (1999–2002) and
the Consultative Council of the National Institute of Building Sciences (1998–2002).
Prior to joining UMCP he was President and co-founder of the information technology
company ConSolve Inc., providing software solutions for environmental engineering.
Earlier, he was Professor of Civil Engineering at MIT. He is recipient of the Thomas A.
Middlebrooks and State-of-the-Art Awards of the American Society of Civil Engineers,
and is Lilly Teaching Fellow and Freeman Foundation Fellow at the University of
Maryland. He is co-author, with John T. Christian, of Reliability and Statistics in
Geotechnical Engineering(Wiley, 2003).
Karl M. Dise, BSc (Cornell), MSc (Colorado), PE is a Civil Engineer and Technical
Specialist with the US Bureau of Reclamation in Denver, Colorado. He is a principal
investigator, consultant, advisor, and reviewer for extensive or complex geotechnical
engineering matters encountered in current reclamation work. Mr Dise is a member of
Reclamation’s Risk Analysis Cadre, responsible for writing reclamation guidelines on
