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Safety of Dams Flood and Earthquake Criteria Committee on Safety Criteria for Dams Water Science and Technology Board Commission on Engineering and Technical Systems National Research Council NATIONAL ACADEMY PRESS Washington, D. C. 1985

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NATIONAL ACADEMY PRESS 2101 Constitution Ave., NW Washington, DC 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors, according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Research Council was established by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and of advising the federal government. The Council operates in accord- ance with general policies determined by the Academy under the authority of its congressional charter of 1863, which establishes the Academy as a private, nonprofit, self-governing member- ship corporation. The Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in the conduct of their services to the government, the public, and the scientific and engineering communities. It is adminis- tered jointly by both Academies and the Institute of Medicine. The National Academy of Engineering and the Institute of Medicine were established in 1964 and 1970, respectively, under the charter of the National Academy of Sciences. This report represents work supported by Cooperative Agreement Number 4-FC-81-04270 between the Bureau of Reclamation (contracting agency for the Corps of Engineers and Bureau of Reclamation) and the National Academy of Sciences. Library of Congress Cataloging in Publication Data Main entry under title: Safety of dams. Bibliography: p. Includes index. 1. Dam safetyUnited States. 2. Floodcontrol- United States. 3. EarthquakesUnited States. I. National Research Council (U.S.~. Committee on Safety Criteria for Dams. TC556.S24 1984 363.3'497 85-2973 ISBN 0-309-03532-5 Copyright (I) 1985 by the National Academy of Sciences No part of this book may be reproduced by any mechanical, photographic, or electronic process, or in the form of a phonographic recording, nor may it be stored in a retrieval system, transmit- ted, or otherwise copied for public or private use, without written permission from the pub- lisher, except for the purpose of official use by the United States Government. Printed in the United States of America

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COMMITTEE ON SAFETY CRITERIA FOR DAMS GEORGE W. HOUSNER, Chairman, California Institute of Technology, Pasadena KEN ARI, University of Southern California, Los Angeles DONALD H. BABBITT, California Division of Safety Dams, Sacramento DENIS BINDER, Western New England College, Springfield, Massachusetts CATA~No B. Coo, Pacific Gas and Electric Company, San Francisco, California ALLEN T. CHWANG, The University of Iowa, Iowa City MERLIN D. COPEN, Consultant, Aurora, Colorado LLEWELLYN L. CROSS, Charles T. Main, Inc., Boston, Massachusetts CHAR[ES H. GARDNER, North Carolina Department of Natural Resources, Raleigh LESTER B. LAVE, Carnegie-Mellon University, Pittsburgh, Pennsylvania Douglas E. MAcLEAN, University of Maryland, College Park OTTO W. NUTTLI, St. Louis University, St. Louis, Missouri JOHN T. RIEDEL, Hydrometeorological Consultant, Hucl~eston, Virginia GURMUKH S. SARKARIA, International Engineering, Co., Inc. H. Bo~ToN SEED, University of California, Berkeley JERY R. S ~ STINGER, Cornell University, Ithaca, New York Ex-Officio Members WALTER R. LYNN, Cornell University, Ithaca, New York; Chairman, Water Science and Technology Board ROBERT L. SMITH, University of Kansas, Lawrence; Member, Commission on Engineering and Technical Systems; Member, Water Science and Technology Board -

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Technical Consultant HOMER B. WILLIS, Consulting Engineer, Bethesda, Maryland NRC Project Manager STEPHEN D. PARKER, Executive Director, Water Science and Technology Board NRC Project Secretary JEANNE AQuILINo Federal Agency Technical Representatives DoNALDDuNcAN, U.S. Army Corps of Engineers, Washington, D.C. DAVID PROSSER, U.S. Bureau of Reclamation, Washington, D.C. IV

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WATER SCIENCE AND TECHNOLOGY BOARD Members WALTER R. LYNN, Chairman, Cornell University, Ithaca, New York MARY P. ANDERSON, University of Wisconsin, Madison LEO R. BEARD (through 7/84), Espey, Huston & Associates, Inc., Austin, r ~ exas JOHN J. BOLAND, Johns Hopkins University, Baltimore, Maryland PAu~ BUSCH, Malcolm Pirnie Engineers, White Plains, New York JOHN Cairns, Virginia Polytechnic Institute and State University, Blacksburg PETER S. EAGLESON, Massachusetts Institute of Technology, Cambridge LEO EISEL, Wright Water Engineers, Denver, Colorado RICHARD S. ENGE~sREcHT, University of Illinois at Urbana-Champaign JEROME B. GILBERT, East Bay Municipal Utility District, Oakland, California YACOV Y. HAIMES (through 7/84), Case Western Reserve, Cleveland, Ohio HELEN INGRAM, University of Arizona, Tucson L. DouG~As JAMES (through 7/84), Utah State University, Logan ROBERT B. JANSEN (through 7/84), Consulting Civil Engineer, Spokane, Washington JOHN F. KENNEDY (through 7/84), The University of Iowa, Iowa City ORrE LoucKs, Butler University, Indianapolis, Indiana DAVID W. MILLER, Geraghty & Miller, Inc., Syosset, New York JEROME W. MILLIMAN, University of Florida, Gainesville STEPHEN E. REYNOLDS (through 7/84), State Engineer, Santa Fe, New Mexico DAN!FI. P. SHEER (through 7/84), Interstate Commission on the Potomac River Basin, Rockville, Maryland ROBERT L. SMITH, University of Kansas, Lawrence GARY WEATHERFORD, Center for Natural Resources Studies, Berkeley, California Staff STEPHEN D. PARK=, Executive Director SHEILA D. DAVID, Staff Officer CAROEE B. . CARSTATER, Staff Assistant JEANNE Ado, Administrative Secretary RENEE A. HAWKINS, Secretary v

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Preface Many thousands of dams have been constructed in the United States, and new dams continue to add to this total. The proper functioning of these dams under all conditions is an important matter of public safety and welfare. This report concerns the levels of safety to be provided at dams to withstand extreme floods and earthquakes. The occasional failure of a dam stimulates public concern, and in re- sponse, safety assessments are undertaken. Such assessments have recently been made by the U.S. Army Corps of Engineers and the Bureau of RecIama- tion, two federal agencies having major dam programs. However, many dams are also constructed and operated by other federal, state, and local government agencies, utilities, corporations, and individual owners. The study for this report was undertaken at the request of two departments of the federal government, but the report is for the most part relevant to all dams, both federal and nonfederal. On the average, about 10 significant clam failures have occurred some- where in the world in each decade, and many more damaging near-failures have occurred. Some of these events have resulted from incorrect decisions made during the design and construction process, whereas others have been the consequence of inadequate maintenance or operational mismanage- ment. Many have resulted from unanticipated large floods, and a few have resulted from intense earthquake shaking. The water retained in a large reservoir has enormous potential energy that can cause extensive loss of life and damage to property. In fact, few activities of man pose greater potential for destruction. Accordingly, engineers tend to take a very conservative V11

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viii Preface approach in designing dams; however, the more conservative the design, the greater the cost of safety. Also, relatively few dams will experience the ex- treme events for which they are designed, but the location and magnitude of these events cannot be predicted and, therefore, conservative designs gener- ally are provided at most dams to avoid catastrophic failures at a few. Earthquakes and floods pose a similar problem to designers of dams, in that both hazards have uncertainties associated with the occurrences of extreme events and decisions must be made as to the best way to handle these uncertainties. However, there are also significant differences in the prob- lems posed. For example, since most dams are built to retain runoff from a watershed, questions of extreme floods usually arise. On the other hand, some regions of the United States have a low seismic hazard, and ground shaking does not pose a serious threat to the safety of dams. The Committee on Safety Criteria for Dams was requested to report on the selection of appropriate flood and earthquake occurrences to be consid- ered in design of dams and safety evaluation of dams. This report represents a general consensus of the views and conclusions of the committee. Although the committee did not attempt to make the report a treatise on protecting dams from earthquakes and extreme floods, it did include background mate- rial to aid in understanding the bases for its findings and recommendations. The time and funding available to the committee precluded the undertaking of in-depth studies and research to develop detailed, new design criteria. Rather, the committee (1) reviewed current practices in the United States and abroad in regard to designing dams for extreme hydrologic and seismic events and (2) made recommendations for action and research aimed at improving safety evaluations of dams with respect to extreme events. The committee found that its deliberations led to questions of risk and responsi- bility; therefore, the report also addresses these matters. The members of this committee recognize that they have participated in an important and unusual activity, and they appreciate the responsibility this assignment has placed upon them. In most instances the formulation, or the review, of criteria for engineering work is accomplished by a group of similarly minded specialists in a narrow branch of technology. It would be difficult to find a parallel to the assignment and composition of this commit- tee. The charge to the group was a difficult one, and there are issues on which the committee did not reach complete agreement and the report recommen- dations represent a consensus of views. One particular issue of concern to the committee is the continued use of the probable maximum flood (PMF) as a principal basis for design of spillways for all new dams in high-hazard situa- tions. Some committee members felt such a design basis, in some cases, results in extravagant use of resources, but they also recognized that an adequate substitute design basis is not available at this time. Another con-

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Preface IX cern involves the lack of quantitative definitions for the dam hazard cIassifi- cations used by federal and state agencies along with a lack of uniformity in the spillway design floods assigned to each hazard and size category. The committee noted a considerable variety in these standards. Some members proposed that the committee attempt to formulate "hazard classification" standards that could be recommended; however, time did not permit such an effort, and the consensus was that such an activity should be pursued by other groups. The committee concluded, on the basis of information presented, that the Corps of Engineers, the Bureau of Reclamation, and some other federal agencies, as well as some state agencies and engineering firms, are generally using up-to-date methods of assessing flood and earthquake hazards. The committee felt that up-to-date methods could be further improved by re- search and by collection of relevant data. The importance to the nation of the problems of the safety of dams against extreme floods and earthquakes is widely recognized, and the committee foresees that the importance of these problems will increase as population density increases and water becomes a scarce resource. Thus, protection of dams against such events should receive the continued attention of federal ant] state governments, as well as the relevant engineering and science com- munities. The committee has been aided greatly in its work by many people and organizations. In the Acknowledgments that follow, some of the contribu- tions to this effort are briefly mentioned. For the committee, I express grati- tude for this help. For myself, I wish to thank all the committee members, members of the Water Science and Technology Board, members of the Na- tional Research Council anti federal agency staffs, the technical consultant, and others who have inspired and facilitates! the task at hand. GEORGE W. HOUSNER, Chairman Committee on Safety Criteria for Dams

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Acknowledgments Although the committee takes full responsibility for the material in this report, it wishes to acknowledge valuable contributions to the committee's work from many organizations and individuals, including the following: those organizations who responcled to the requests for information on dam safety criteria in use relating to extreme floods and earthquakes; staff mem- bers of a number of federal agencies, including the Bureau of Reclamation, U.S. Army Corps of Engineers, National Weather Service, U.S. Geological Survey, Tennessee Valley Authority, Soil Conservation Service, Forest Ser- vice, Federal Energy Regulatory Commission, Federal Emergency Manage- ment Agency, and Nuclear Regulatory Commission, who contributed to the committee's discussion and understanding of current practices and prob- lems; and the employers of the members of the committee who generously made those members available for the committee sessions. xi

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Contents Executive Summary 1 Introduction . . . 5 2 Extreme Floods and EarthquakesThe Nature of the Problem . . . . . . . . . . . . . . . . . 8 3 Summary of Present Practices on Dam Safety Standards . . . . . . . . . . . . . 4 History of Development of Present Practices 5 Design Flood Estimates: Methods and Critique 6 Design Earthquake Estimates: Methods and Critique . . . . . . . 7 Consideration of Risk in Dam Safety Evaluations 8 Risk and the Calculus of Legal Liability in Dam Failures 9 Proposed Hydrologic Criteria . 10 Proposed Earthquake Criteria. 11 Continuing Development of Hydrologic and Earthquake Engineering Technologies . X111 15 35 44 61 74 84 97 . 106 . 110

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xiv Appendixes A Design Criteria in Use for Dams Relative to Hazards of Extreme Floods . . B Design Criteria in Use for Dams Relative to Earthquake Hazards C Probable Maximum Precipitation (PMP) Estimates . D Concepts of Probability in Hydrology . E ~ Risk Analysis Approach to Dam Safety Evaluations F Glossary .......... G H Biographical Sketches of Committee Members Index References and Bibliography . Contents 15 75 2 . . . . . . . . 227 241 255 259 264 269

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List of Major Figures and Tables Figures 3-1 Earthquakes with maximum Modified Mercalli intensities of V or above in the United States and Puerto Rico through 1976, 28 3-2 Seismic risk map of the United States, 29 5-1 Comparison of generalize8PMP estimates for 24 hours and 200 square miles made in 1978 with those made in 1947,48 6-1 Fault map of California, 62 6-2 Idealized curve showing the approximate relation between the mag- nitucle of the earthquake and the length of the fault rupture, 64 6-3 Peak ground acceleration curves for stiff veils (Ms = 7.5), 66 6-4 Regression analysis of the peak accelerations recorcled cluring the Oc- tober 15, 1979, Imperial Valley earthquake, 67 6-5 Approximate relationships between maximum accelerations on rock and other local site conditions, 68 6-6 Design accelerogram and spectrum for Camanche Dam, 72 C-1 Regions covered by generalized PMP studies, 218 C-2 Ratios of estimated PMP for 10 square miles to estimated 100-year frequency rainfalls (both for 24-hour durations), eastern United States, 222 C-3 Ratios of estimated PMP for 10 square miles to estimated 100-year frequency rainfalls (both for 24-hour clurations), western United States, 223 x~

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xvi Figures (Continued) List ofMajor Tables and Figures Observed 24-hour, 10-square mile rainfall amounts expressed as per- cent of all-season PMP estimates, 224 E-1 Log normal plotting grid, 242 E-2 Sample decision matrix for risk analysis (expected damages Dij associ- ated with proposal i and inflow flood level j and also the corresponding expected loss of life), 245 Tables 3-1 Terms for Classifying Hazard Potentials, 16 3-2 Spillway Capacity Criteria Reported to Be in Current Use by Various Agencies, 18 3-3 Comparison of Indicated Spillway Capacity Criteria in Use or Pro- posed, 19 3-4 Summary of Practices Specifically Reported Relating to Evaluation of Safety of Dams Against Earthquake Hazards, 25 3-5 Modified Mercalli Intensity Ratings, 30 3-6 Richter Magnitudes and Square-Mile Areas as Affected by Peak Accel- erations, 31 5-1 PMP Estimates for Vicinity of Washington, D. C., 46 5-2 Boundaries for Hazard Classes, 55 6-1 Dams at Which Apparent Reservoir-Incluced Earthquakes Have Been Observed, 73 C-1 Generalized PMP Studies for United States, 216 D-1 Approximate Probabilities That No Flood Exceeds the 100-Year Flood, 230 D-2 Probability That Indicated Design Flood Will Be Exceeded During Specified Planning Periods, 231 D-3 Exceedance Probabilities for Different Values of T. 239 E-1 Design Options and Costs for Illustrative Example, 246 E-2 Matrices of Damages for Different Flows and Design Options, 248 E-3 Expected Damages for Different Design Options, Cases, and Return Periods, 250

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/ Al of Dams Flood ant! Earthquake Criteria Safety - -

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