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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 safety—United States. 2. Floodcontrol- United
States. 3. Earthquakes—United 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 Earthquakes—The 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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