The National Dam 

Safety Program 

Research Needs Workshop:

Hydrologic Issues for Dams
Preface 

One of the activities authorized by the Dam Safety and Security Act of 2002 is research 

to enhance the Nation’s ability to assure that adequate dam safety programs and practices 

are in place throughout the United States. The Act of 2002 states that the Director of the 

Federal Emergency Management Agency (FEMA), in cooperation with the National Dam 

Safety Review Board (Review Board), shall carry out a program of technical and archival 

research to develop and support: 

• improved techniques, historical experience, and equipment for rapid and effective 

dam construction, rehabilitation, and inspection; 

• devices for continued monitoring of the safety of dams; 

• development and maintenance of information resources systems needed to 

support managing the safety of dams; and 

• initiatives to guide the formulation of effective policy and advance improvements 

in dam safety engineering, security, and management. 

With the funding authorized by the Congress, the goal of the Review Board and the Dam 

Safety Research Work Group (Work Group) is to encourage research in those areas 

expected to make significant contributions to improving the safety and security of dams 

throughout the United States. The Work Group (formerly the Research Subcommittee of 

the Interagency Committee on Dam Safety) met initially in February 1998. To identify 

and prioritize research needs, the Subcommittee sponsored a workshop on Research 

Needs in Dam Safety in Washington D.C. in April 1999. Representatives of state and 

federal agencies, academia, and private industry attended the workshop. Seventeen broad 

area topics related to the research needs of the dam safety community were identified. 

To more fully develop the research needs identified, the Research Subcommittee 

subsequently sponsored a series of nine workshops. Each workshop addressed a broad 

research topic (listed below) identified in the initial workshop. Experts attending the 

workshops included international representatives as well as representatives of state, 

federal, and private organizations within the United States. 

• Impacts of Plants and Animals on Earthen Dams 

• Risk Assessment for Dams 

• Spillway Gates 

• Seepage through Embankment Dams 

• Embankment Dam Failure Analysis 

• Hydrologic Issues for Dams 

• Dam Spillways 

• Seismic Issues for Dams 

• Dam Outlet Works 

In April 2003, the Work Group developed a 5-year Strategic Plan that prioritizes research 

needs based on the results of the research workshops. The 5-year Strategic Plan ensures 

that priority will be given to those projects that demonstrate a high degree of 
collaboration and expertise, and the likelihood of producing products that will contribute 

to the safety of dams in the United States. As part of the Strategic Plan, the Work Group 

developed criteria for evaluating the research needs identified in the research workshops. 

Scoring criteria was broken down into three broad evaluation areas: value, technical 

scope, and product. The framework adopted by the Work Group involved the use of a 

“decision quadrant” to enable the National Dam Safety Program to move research along 

to produce easily developed, timely, and useful products in the near-term and to develop 

more difficult, but useful, research over a 5-year timeframe. The decision quadrant 

format also makes it possible to revisit research each year and to revise research priorities 

based on current needs and knowledge gained from ongoing research and other 

developments. 

Based on the research workshops, research topics have been proposed and pursued. 

Several topics have progressed to products of use to the dam safety community, such as 

technical manuals and guidelines. For future research, it is the goal of the Work Group to 

expand dam safety research to other institutions and professionals performing research in 

this field. 

The proceedings from the research workshops present a comprehensive and detailed 

discussion and analysis of the research topics addressed by the experts participating in the 

workshops. The participants at all of the research workshops are to be commended for 

their diligent and highly professional efforts on behalf of the National Dam Safety 

Program. 
Acknowledgments 

The National Dam Safety Program research needs workshop on Hydrologic Issues for 

Dams was held on November 14-15, 2001, in Davis, California. 

The Department of Homeland Security, Federal Emergency Management Agency, would 

like to acknowledge the contributions of the U.S. Army Corps of Engineers, Hydrologic 

Engineering Center, which was responsible for the development of the technical program, 

coordination of the workshop, and development of these workshop proceedings. A 

complete list of workshop facilitators, presenters, and participants is included in the 

proceedings. 
TABLE OF CONTENTS 

Page # 

FOREWORD................................................................................................................... v 

EXECUTIVE SUMMARY ............................................................................................ vi 

CORPS OF ENGINEERS HUNTINGTON DISTRICT HYDROLOGIC

RESEARCH NEEDS FOR DAM SAFETY ANALYSIS

 (Paper 1) 

 Jerry Webb 

 U.S. Army Corps of Engineers, Huntington District ................................ 1 

OMAHA DISTRICT’S CURRENT PRACTICES AND NEEDS FOR

DAM SAFETY ANALYSIS

 (Paper 2) 

 Jeffrey T. McClenathan, P.E. 

 U.S. Army Corps of Engineers, Omaha District....................................... 5 

H&H GUIDANCE FOR SAFETY OF DAMS

 (Paper 3) 

 Ming Tseng 

Hydro

 Headquarters, U.S. Army Corps of Engineers

logy and Hydraulics Branch

.......................................... 9 

H&H DAM SAFETY R&D NEEDS

(Paper 4) 

 Earl Eiker 

 Retired, Chief H&H Branch, 

 Headquarters, U.S. Army Corps of Engineers.............

FEMA WORKSHOP ON HYDROLOGIC

.............................11 

RESEARCH NEEDS FOR DAM 

SAFETY ANALYSIS FEDERAL ENERGY REGULATORY COMMISSION 

(Paper 5)

 Michael S. Davis, P.E. 

 Federal Energy Regulatory Commission, Office of

Division of Dam Safety and Inspections

HYDROLOGIC RESEARCH NEEDS FOR DAM SAFETY ANA

Energy Projects, 

, Chicago Regional Office .........15 

LYSIS

AT THE TENNESSEE VALLEY AUTHORITY

 (Paper 6) 

R. Cris Hughes, P.E., and Gregory W. Lowe, P.E. 

(Presented by Gregory Lowe)

 Tennessee Valley Authority.......................................................................21 

iii 
THE UTAH HYDROLOGICAL EXPERIENCE

(Paper 7) 

Matthew C. Lindon, P.E. 

 Utah Division of Water Rights, Dam Safety Hydrologist .........................27 

HYDROLOGIC PRACTICES IN THE STATE OF GEORGIA AND 

OTHER STATES EAST OF THE MISSISSIPPI RIVER

(Paper 8) 

Francis E. Fiegle II, P. E

 Program Manager, Georgia Safe Dams Program....................................33 

SOMERSET COUNTY DAM SAFETY PROGRAM

(Paper 9) 

Joseph J. Skupien, PE, PP 

 Principal Hydraulic Engineer Division of Engineering, 

Somerset County, New Jersey 

 (Presented by Arlen Feldman) 39

RESEARCH NEEDS IN DAM SAFETY ANALYSIS

, 

..................................................................

(Paper 10) 

 Samuel L. Hui

 Bechtel Corporation, San Francisco ........................................................45 

HYDROLOGIC ANALYSES RELATED TO DAM SAFETY

(Paper 11) 

Anand Prakash, Ph. D., P.E. 

 URS Corporation ......................................................................................49 

HYDROLOGY FOR DAM SAFETY – PRIVATE SECTOR

(Paper 12) 

 Cat Cecilio, P.E., P.H. 

 PG&E-retired, Consulting Engineer........................................................57 

ESTIMATION OF EXTREME HYDROLOGIC EVENTS IN AUSTRALIA: 

CURRENT PRACTICE AND RESEARCH NEEDS

(Paper 13) 

 Rory Nathan

 Sinclair Knight Merz, Australia................................................................73 

CURRENT AND FUTURE HYDROLOGIC RESEARCH NEEDS FOR DAM 

SAFETY ANALYSIS IN MATURE DAM SAFETY PROGRAMS

(Paper 14) 

 D.N.D. Hartford 

 BC Hydro ..................................................................................................85 

iv
NEW DEVELOPMENTS AND NEEDS IN SITE-SPECIFIC PROBABLE 

MAXIMUM PRECIPITATION (PMP) STUDIES

(Paper 15) 

Applied Weather Associates................................................................... 107 

YDROLOGIC RESEARCH NEEDS FOR DAM SAFETY ANALYSIS

Ed Tomlinson, PhD 

H

ineering Consultants, Inc ....................................................... 111 

(Paper 16) 

M.G. Schaefer Ph.D. P.E 

 MGS Eng

SIMPLIFIED DAM FAILURE ANALYSIS FOR INUNDATION MAPPING

USING HEC-1

d Consulting Engineers

(Paper 17) 

 J. J. DeVries

 David For and State of California ................... 119 

A FRAMEWORK FOR CHARACTERIZATION OF EXTREME FLOODS FOR

DAM SAFETY RISK ASSESSMENTS

wles, Robert Swain and Dean Ostenaa 

Bowles)

 

(Paper 18) 

David Bo

(Presented by David

David Bowles and the Bureau of Reclamation ...................................... 125 

HYDROLOGIC RESEARCH NEEDS FOR DAM SAFETY - EXPERIENCE 

LEARNED FROM ALAMO DAM RISK ASSESSMENT STUDY 

oe Evelyn

(Paper 19)

James Cheih and J

(Presented by James Cheih) 

 Los Angeles District, Corps of Engineers.............................................. 139 

PALEOFLOOD HYDROLOGY, DAM SAFETY AND FLOODPLAIN 

MANAGEMENT

(Paper 20) 

Robert D. Jarrett, John F. England Jr., Edward M. Tomlinson, Thomas M. 

US Geological Survey

Yanosky and Robert E. Swain 

(Presented by Robert Jarrett) 

 ........................................................................... 147 

v 
CURRENT HYDROLOGIC PRACTICES, PROBLEMS, and NEEDS WITHIN 

CALIFORNIA’S DAM SAFETY PROGRAM

California Department of Water Resources (DWR), Division of Safety of 

(Paper 21) 

Melissa Collord

Dams (DSOD) ....................................................................................... 167 

BUREAU OF RECLAMATION HYDROLOGIC RESEARCH

(Presented by Lou Schreiner) 

............................................ 169 

 (Paper 22) 

 Robert E. Swain, Louis C. Schreiner and Daniel R. Levish 

U.S. Bureau of Reclamation .....................

RESEARCH NEEDS SUMMARY ............................................................................ 175 

DISCUSSION .............................................................................................................. 181 

PARTICIPANTS ......................................................................................................... 188 

vi
FOREWORD 

) sponsored a workshop on

ydrol h Needs for Dam Safety. The workshop was held on 14-15 

ring Center (HEC). HEC was responsible for 

itation

MP) aximum Flood (PMF) and risk factors associated with dam safety and 

ase ex mples

articipants included representatives from Headquarters, Districts and the HEC. Non-

. Bure ation (USBR) 

ission (FERC)

. The States of California, Utah and Georgia dam safety programs 

. Univ lifornia at Davis and Utah State University 

. Sinc ir Kni dro, Canada

ed

e reasonableness

f the a dance probability (AEP) of a PMF and how the computed value of the 

MF co pares ses.

Dam Safety. 

The Federal Emergency Management Agency (FEMA

H ogic Researc

November 2001, at the Hydrologic Enginee

the technical program and workshop coordination. 

Information on current practices for computation of Probable Maximum Precip

(P , Probable M

c a were covered in the workshop. The workshop participants presented a 

variety of papers and information relating to research needs for dam safety. Corps 

p

Corps participants were from the following offices: 

1 au of Reclam

2. Federal Energy Regulatory Comm

3. Tennessee Valley Authority (TVA)

4

5. Pacific Gas and Electric (PG&E) 

6. US Geological Survey (USGS) 

7 ersity of Ca

8. MGS Engineering and Applied Weather Associates 

9 la ght Metz, Australia and BC Hy

10. Somerset County, New Jersey 

The workshop provided a forum for the exchange of ideas related to the risks involv

with operation of small to very large dams. There were discussions on th

o nnual excee

P m to data from paleoflood analy

The focus of the workshop was to prioritize recommendations for research needs related

to

vii 
HYDROLGIC RESEARCH NEEDS 

For 

DAM SAFETY

Needs for Dam Safety was held on 14-15, 

ovem er 200

orkshop provided a forum for the discussion of subjects important to the computation of 

Probable Maximum Flood (PMF), the uncertainty of parameters used to compute a 

develop the design flood and hydrologic risks involved with dams and dam operations. 

he main focus of the workshop was to generate a list of topics that are in need of 

he workshop proceedings are included in this document. 

PROBLEM STATEMENT 

Research problems can be divided into areas for fundamental research and for 

application. The fundamental research problems could involve investigating approaches 

for improving the state-of-the-art. This would include some possible new approaches for 

either estimating the design inflow flood or in using risk analysis instead of the currently 

accepted use of the probable maximum flood. Any new approach that is proposed needs 

to be commensurate with the funding and resources available for the dam safety 

evaluation. Large organizations owning dams in a high hazard category (where there is a 

significant population at risk downstream), may be able to afford a sophisticated 

engineering analysis. However, smaller dams, whose owners do not have the resources 

for sophisticated analyses, need methods that can be applied that provide an adequate 

analysis commensurate with the value of the dam and the consequences of failure. Some

possible avenues for research into developing simplified techniques that can provide 

useful answers given the most current thinking on hydrologic methods for dam safety 

analysis are discussed below. 

Inflow Design Flood Estimates 

The hydrologic problem typically addressed in dam safety analysis is the determination 

of the capacity of the spillway needed to prevent catastrophic failure of the dam due to 

overtopping. The PMF is generally accepted as the design inflow for evaluating the 

spillway when there is potential loss of life due to dam failure in high hazard situations. 

EXECUTIVE SUMMARY 

INTRODUCTION 

A workshop on Hydrologic Research

N b 1 at the Hydrologic Engineering Center (HEC) in Davis, California. The 

w

a

PMF, the continued use of the PMF as the “design flood”, using risk analysis procedures 

to

T

research that may help in generating solutions to problems related to dam safety. 

T

viii
The PMF represents an estimated up aximum runoff potential for a 

particular watershed. In some sense, the inherent assumption is that a dam with a 

most significant initial reevaluations 

f this standard is described in NRC (1985) . In this reevaluation, base safety analysis 

of the

ety analysis still need to be solved. 

hese problems are apparent when considering: 1) mechanisms in addition to 

ophic failures of the dam; 2) the recent research into 

n the past 10,000 years; and, 3) the

ility of extreme floods when performing a risk 

g, which have potential to cause failure of a dam, are 

on. Certainly, these

sas, Gila and Colorado River

isms as the duration of the 

ion events could result in severe erosion of a spillway, 

ith an insufficient intervening period to perform maintenance; and, consequently, cause 

the current application of the single

eak hydrograph of the PMF.

ach to addressing this design problem is to consider estimating the 

robable maximum precipitation over multi-month periods. Inflow hydrographs could be 

eveloped by simulating patterns of historic precipitation proportioned to have an equal 

volume to the probable maximum amount over the duration desired. 

Besides the volume issue, recent paleoflood evidence in the western United States 

indicates that the largest floods occurring in the past 10,000 years are significantly 

smaller than PMF estimates. This difference has caused some concern with regard to the 

magnitude of the PMF estimates. Possible reasons for this difference between the 

estimates might come from problems with models used to estimate peak flows from

paleo-stage indicators or the area reduction factors used to convert point estimates of the 

probable maximum precipitation (PMP) to a total storm depth. 

 

per bound on the m

spillway designed to pass this flood has zero risk of overtopping. 

The PMF design standard has been reexamined in the past and continues to be debated as

a design standard up to the present time. One of the

1 o

(an incremental deterministic evaluation) and risk analysis were explored as potential 

alternatives to the PMF criteria. 

Since this reevaluation, a considerable amount of research, which examines the use

PMF and explores the benefits of a risk approach, has been completed. Despite this 

research, significant hydrologic problems in dam saf

T

overtopping that might cause catastr

paleoflood estimates of the largest floods occurring i

need to estimate the exceedance probab

analysis. 

Mechanisms, other than overtoppin

erosion of the spillway or undermining of the spillway foundati

problems have been noticed in recent floods in the Kan

Basins. The potential for failure increases for these mechan

flooding increases. Extended durat

w

dam failure. As an extreme example, the great Upper Mississippi Basin flood of 1993 

included 5 major flood inflow events to Saylorville Reservoir on the Des Moines River 

over a period of 5 months. Examples like this illustrate that extended flow periods should 

be considered in the design process, as opposed to

p

A possible appro

p

d

1 National Research Council, 1985. Safety of Dams, Flood and Earthquake Criteria, Committee of Safety

Criteria for Dams, National Academy Press, Washington, D.C., 321p. 

ix 
Risk assessment, if applied, requires estim lood distributions for infrequent

quantiles where estimation method tistical hydrology are not valid.

Both the National Research Counc D (1986)

3

 have addressed the 

problem of estimating exceedance probabilities for large floods, without providing any 

guidance for extending estim e probability. Paleoflood

formation has more recently been used, at least in the western U. S., to extend the flood 

istribution to the 1/10,000 exceedance probability. Additional research is needed to 

ncorporate different sources of information (the gage record, 

ochastic precipitation and watershed models, and paleoflood information bounds) for 

btaini t.

g

nalysis. 

pplication techniques are needed that reflect the current thinking on estimating the 

the resources available to

erform the dam safety analysis. The need to develop simplified techniques is likely to 

e very important to owners of small dams who do not have the resources to 

meteorologic and hydrologic models. Opportunities for 

eveloping these simplifying techniques may reside in regional analyses. For example, 

watersh

r

resented suggestions for areas of research. 

ating f

ologies applied in sta

il (1988)

2 and IACW

ates beyond the 1/1000 exceedanc

in

d

address how best to i

st

o ng estimates of extreme flood exceedance probabilities needed for risk assessmen

These problems provide a focus for hydrologic research needs for dam safety analysis. 

The focus might be on developing long duration design inflow hydrographs, explainin

the reasons for differences between PMF and paleoflood estimates, and integrating 

different sources of information to obtain flood probability distributions needed for risk 

a

Application Research 

A

design inflow flood and can be applied commensurate with

p

b

formulate/apply sophisticated

d

ed characteristics might be related to existing estimates of the PMF. Obtaining

these regional relationships would require studies much like those done in developing the

U.S. Geological Survey regression equations for flow-frequency curves. In any case, the 

current research needs to develop methods that can be simply and economically applied 

given the appropriate dam safety analysis problem. 

 

PRESENTATIONS 

The majority of the workshop consisted of presentations by various agencies 

detailing what they felt are areas of research that could address the problems cited above. 

Federal Agencies, State and County Agencies, Private sector firms, and educational 

institutions made presentations. Presenters listed various problems encountered by thei

respective organizations and p

 

2

 National Research Council, 1988. “Estimating Probabilities of Extreme Floods, Methods and

Recommended Research”, ISBN 0-309-03791-3, National Academy Press, Washington, D.C. 141p. 

3

 Interagency Advisory Committee on Water Data (IACWD), 1986.“Feasibility of Assigning Probability to

e Probable Maximum Flood”, Office of Water Data Coordination, U.S. Department of the Interior, 

Geological Survey, Office of Water Data Coordination, Reston, VA. 

th

x
RESEARCH AREAS 

Research needs were broken into three areas. These were Risk based, Standards and

Meteorological needs. After a period of discussion, the workshop reassembled and each 

group boiled down the research needs presented in each paper and then presented their 

opinions. Some of the identified research needs are listed below. A more detailed list is 

located in the Discussion section of this document. 

Risk Analysis Group – Items relating to uncertainty factors that influence reservoir 

inflow values and the computation of the Annual Exceedance Probability (AEP) 

Improve technology transfer 

• Develop regional database 

d

of extreme floods. 

• Storms and flood database 

• Extension of flood frequency curves 

• Develop regional hydrology parameters 

Standards Group – Items relating to physical factors that influence the methodology for 

the computation of extreme floods, including the PMF. 

• 

• Loss rate function analysis 

Meteorology Group – Items relating to rainfall analysis from both the standards base

analysis and a risk-based analysis. 

• Precipitation analysis 

• Rainfall frequency analysis 

• Real time storm analysis 

xi 
Corps of Engineers Huntington District 

Hydrologic Research Needs for Dam Safety Analysis

untington District utilizes standard methodologies to develop Probable Maximum Flood 

District

altered ency, and nine (9) facilities remain that provide 53-79% PMF 

retentio H aluated using the "then-current" standards and 

spillwa d n several changes over the years in 

method e magnitude of the PMF, which 

ltered the extent of the deficiency and/or structural fixes required to correct the 

Operati cation to provide any

continu ty Assurance (DSA) requirements at Bluestone 

Dam w e roject exceeds the total construction cost of

the sev a f five. There is a significant need to develop 

consistent, supportable guidelines and methodologies for development of a design storm 

Proble ology.

Develo g od (PMF) for any project continues to rely upon the 

roper application of current knowledge and the development of individual experience in 

e field of hydrology. The final hydrograph for any PMF continues to be subjective and 

judgmental due to a large number of factors such as: 

Orographic Effects. 

The Huntington District includes a mountainous area that suffers from orographic lifting, 

whenever the wind has an onslope component. The rate of the lifting is determined by 

the magnitude of the onslope component and the degree of slope. The determination of 

the height of the air column to be lifted orographically presents a problem when dealing 

with Probable Maximum Rainfall (PMR). Studies suggest that a 3,000-ft mountain 

barrier in the path of a strong wind should certainly produces an upward wind component 

that would persist above the 6-km. level. This situation has resulted in the Huntington 

District requesting special studies by the NWS to develop site-specific PMR for these 

projects. The peak discharge has been impacted by 0-7% when compared to standard 

procedures. 

 

By 

Jerry W. Webb1

Current Practices. 

H

(PMF) for individual projects. Out of the 35 dams that have been constructed in the 

, nineteen (19) are considered adequate under current criteria, seven (7) were 

to correct the defici

n. istorically, the PMF was ev

y a equacy was assessed. There have bee

ology and standards, which have impacted th

a

hydrologic deficiencies. The assessment of hydrologic deficiencies is funded under the 

ons & Maintenance program and there is no specific allo

ity to the program. The Dam Safe

ill xceed $110 million. This single p

en ltered facilities by a factor o

to be used in Dam Safety analysis that is acceptable over a broad range of scenarios. 

ms with Existing Method

pin a Probable Maximum Flo

p

th

1

 Huntington District, U.S. Army Corps of Engineers 

1 Paper 1 - Webb 
Lack of Historical Data for Extreme Events. 

Historical stream flow data provides (when available) a means for calibrating the 

hydrology relating to the unit hydrograph development and stream routing procedures. 

The other variables (antecedent condition, initial and constant rainfall lose rate, etc.) 

cannot be readily determined for a Probable Maximum Flood event. Very few gaging 

stations provide data representative of extrem

of limited value in dam safety applications. 

Variation in Operational Scenarios. 

Huntington District develops daily forecasts for the purpose of making operational 

decisions. Most of our projects have not been operated for extreme events, and there is a 

perception that water control plans may be altered on a case-by-case basis during a major 

flood event. This consideration complicates the modeling process especially when 

dealing with the calibration issue. Current assessments are not consistent concerning 

operational assumptions during a PMF. 

 Antecedent Rainfall Conditions.

The rainfall breakdown for the design storm is not well documented relative to 

assumptions for antecedent rainfall that precedes the Probable Maximum Rainfall (PMR). 

The following National Weather Service criteria of incorporating the antecedent event 

into the total storm hydrograph is based on judgment (experience) and not supported by 

any particular study. The following diagram shows how the antecedent rainfall is 

currently used in the reservoir routing process. It provides a starting pool elevation prior 

to the main event. 

30% PMR + 3- Day Dry Period + 100% PMR 

39% PMR + 5-Day Dry Period + 100% PMR 

e events, which suggests that calibration is 

Paper 1 - Webb 2
Multiple Reservoir Systems. 

a system of reservoirs cannot be accurately modeled for Probable 

aximum Storm (PMS) spillway deficiency using the single storm centering approach of

his 

ects 

 safety studies. 

a portfolio of structures, including 

evaluations of risk and uncertainty of potential failure mechanisms and the consequences 

of such failures, provides a tool that can be used to justify incremental expenditures for

correction of some deficiencies while leaving others to a later date. The ongoing R & D 

s discussed above support the need for consistent guidelines that consider 

ffects, large drainage areas subject to frontal movement storms, multiple 

reservoir systems, lack of historic data for calibration, antecedent rainfall conditions, and 

variations in operational scenarios for extreme events. The cost to correct observed 

deficiencies is directly related to the magnitude of the design event. Provisions for more 

goal of any new R & D initiatives. Policy 

A drainage basin with

M

HMR51 and 52. Large storm events will drop significant amounts of rainfall over a 

larger area than the single storm exhibits. A single storm centering will not capture t

situation, when dealing with multiple reservoirs. The potential exists for multiple proj

to receive significant amounts of rainfall during rainfall event frontal movements. 

New Technologies. 

Geographical Information System Interface. 

The determination of basin hydrologic parameters (size, shape, slope, etc.) and 

hydrologic model development continue to improve with technology and ability to obtain

digital mapping. The future looks very promising from this standpoint and should 

continue to improve with onset of ground verified rainfall radar imaging. There is a 

definite need to incorporate the PMR distribution into the HEC-HMS modeling software.

The drainage area and latitude/longitude of the storm centering could be accurately 

assessed using currently available GIS utilities. Sensitivity to the design storm

assumptions could be easily applied to dam

Risk & Uncertainty Applications. 

The application of risk and uncertainty principles in the prioritization of dam safety 

requirements appears to have some merit. Developing

program in this arena will develop many tools that will assist dam safety officials 

nationwide in making decisions based on highest risk and/or highest consequences of 

failure. The concept can also be used to prioritize engineering studies necessary to assess

deficiencies.

R&D / Policy Needs. 

Address Current Known Problems & Inconsistencies. 

The problem

orographic e

objective evaluation procedures should be the

3 Paper 1 - Webb 
should be developed that will allow the issues, raised above, to be addressed in an 

fficient and economical manner. 

xtreme Event Volume/Duration/Frequency Criteria. 

dards.

s

mine 

he

 Special adjustm

Multiple Storm Tracks Storm Centerng 

e

E

There is significant need to develop a better understanding of the probability/frequency 

associated with the PMF. There is a perception that some agencies have started using 

different evaluation criteria for dam safety, which disregards the current design stan

Extreme event frequency projections are absolutely essential to any dam safety analysi

process. 

Special Problem/Example. 

The Huntington District has been involved in developing design storms to deter

spillway adequacy for several reservoirs in Nicaragua. HMR51 and HMR52 were 

employed to achieve this assigned task even though they were developed by NWS for t

mainland USA. Hurricane Mitch rainfall dropped extreme amounts of rainfall over a 

very large area. PMF storm amounts would not capture this condition when dealin

a large drainage area above an existing project. The outer isohyetal rainfall of the PMF 

did not approach the a

g with

ents were made to final 

carrying capability and distance from the 

r one of the study 

 

Apanas

Basin

I J

GABRIEL

MANCOTAL

ctual hurricane amounts.

HMR52 rainfall amounts based on air moisture 

sea. The diagram demo

areas. 

1⁄4"

1⁄4" 1⁄4"

1⁄4"1⁄4"1⁄4" 1⁄4" 1⁄4" 1⁄4"

1⁄4" 1⁄4"

»»"" 1"o" »o""

 ̧"#Y ̧" ̧ ̧""

#Y #Y#Y#Y #Y #Y #Y (X (X(X

GUATEMALA

nstrates how the rainfall pattern was placed ove

H G

TUMA

JINOTEGA

F E D APANAS C

30030 60 Kilometers

N

W E

S

#Y #Y

#Y #Y

(X (X (X

#

#

#

NICARAGUA

HONDURAS

Apanas

Basin

CHINANDEGA

LEON

M CHOLUTECA ictchtrack.shp

X( Tropical Depression

Y# Tropical Storm

 ̧" Category 1 Hurricane

1" Category 2 Hurricane

o" Category 3 Hurricane

»" Category 4 Hurricane

1⁄4" Category 5 Hurricane

Legend

N

W E

S

Paper 1 - Webb 4
Omaha District’s Current Practices and Needs for Dam Safety Analysis 

By 

Jeffrey T. McClenathan, P.E.1

occur and may be incorporated by assuming a full flood control 

tion occurring five days after the occurrence of a rainfall of onealf the Inflow Design Flood (IDF). For dam rehabilitation, Engineering Regulation ER 

1110-2 Safety Assurance Program was 

llowed. This document describes the determination of the Base Safety Condition to be 

hazard

ather

est a siteMS FACED

CURRENT PRACTICES 

Currently the Omaha District follows procedures determining Inflow Design 

Floods found in Engineering Regulation 1110-8-2(FR) dated 1 March 1991 and entitled:

Inflow Design Floods for Dams and Reservoirs. This document sets standards for four 

types of reservoirs and the data needed for their design. It also describes that an 

antecedent flood may

pool or the pool eleva

h

-1155 dated 12 September 1997 entitled: Dam

fo

used to determine the need for dam safety modifications allowing for even high

dams to be designed for less than a PMF if conditions warrant (that dam failure does not 

exceed damages and loss of life from an event without dam failure). 

 

Typically a study is initiated using a generalized Probable Maximum Precipitation

(PMP) based on the most recent applicable guidance received from the National We

Service (NWS). If questions arise concerning this study, the District could requ

specific PMP study from the NWS. 

PROBLE

ety

S and this was incorporated into the study. During the study 

re raised about the PMP analysis and some of the these issues are 

:

. The evidence showed the 

Problems concerning this process were encountered during a dam safety 

assurance study undertaken by the Omaha District in 1998 on Cherry Creek Dam in 

Denver, Colorado. Concerns were expressed during a Reconnaissance Study completed 

in 1993, that the PMP and Probable Maximum Flood (PMF) were too high for the Cherry

Creek drainage basin. A site-specific study was requested by the Omaha District from the

NWS and completed in 1995. Based on this study, the Omaha District began a dam saf

evaluation study in 1998. In addition, the antecedent flood had a tremendous impact on 

the PMF routing. Again the Omaha District requested a site-specific antecedent flood 

study from the NW

numerous questions we

summarized as follows

 

• Paleoflood evidence was not used in the study

largest paleoflood in the Cherry Creek basin was less than 100,000 cfs 

for the last 10,000 years. 

• Local topographic conditions (Palmer Divide) would block inflowing 

moisture preventing large intense rainfalls from occurring. 

 

1

 Omaha District, U.S. Army Corps of Engineers 

5 Paper 2 - McClenathan 
• Storms used in HMR 55A and in the site-specific study had rainfall 

values that questionable or that were not transposable to the Cherry 

Creek basin. The amount and aerial distribution of the storm was too 

large for the Cherry Creek basin. 

• Use of HMR 52 to orient the PMP was not appropriate. 

• Centering of the storm was not appropriate since most extreme rainfall is 

tied to topographic/orographic features. 

• The extreme frequency of a PMP/PMF event should not be used to

determine the hydrologic adequacy of dams. The frequency was of

quoted as being bet

ten

ween 1 in a million or 1 in a billion. 

es the 100-year rainfall: 

even times the 100-year 

Cherry Creek Background 

To adequately present the problems with the dam safety analysis a brief summary 

d

Table 1 

• As a rule of thumb, the PMP is two to three tim

the site-specific study for Cherry Creek was s

rainfall event. 

• The antecedent flood event study was not adequate. 

• The NWS site-specific PMP study had not been independently reviewed. 

• Dam safety risk analysis was not done 

of the project will be presented. Congress authorized Cherry Creek Dam in 1944 with

construction completed in 1950. The dam was located above downtown Denver an

diverted spillway flows to the neighboring Sand Creek basin to provide Denver 

“complete protection” from large flood events. The original spillway design flood was

based on the 1935 rainfall over the Republican River basin increased by 25-percent for a 

reliability factor. This data as well as subsequent studies are shown in Table 1. The 

original dam was also designed to include storage for irrigation that was never

incorporated. 

Cherry Creek Dam Spillway Design Flood and PMF History 

Event Rainfall in 

inches 

Runoff in 

inches 

Peak 

Discharge in 

cfs 

1944 Detailed Project Report HMR 13 11.5 8.1 180,700 

1970 Dam Safety Evaluation HMR 44 23.9 9.7 376,00 

1993 Reconnaissance Study HMR 55A 29.2 16.3 663,000 

1998 Dam Safety Study 1995 NWS SiteSpecific 24.7 12.8 524,000 

The 1995 PMP was compared to historic storms in Colorado as “reality” check on 

the site-specific PMP. Table 2 shows these values. In 1999 the NWS met in Denver with 

various interested parties to present and discuss their PMP and antecedent rainfall studies. 

At that time the NWS agreed to withdraw their antecedent study. Comments from the 

meeting were either addressed at the meeting or later by the NWS, however, 

disagreements on the use of the “best science” persisted after the meeting. At the end of 

Paper 2 - McClenathan 6
1998, legislation has been introduced preventing the Corps of Engineers from completing 

the dam safety study. The State of Colorado, lead by the Colorado Water Conservation 

Board, has lead a team of inte -specific PMP study and

ursuing various other methods including paleohydrology to perhaps be incorporated in 

e PMP study.

Table 2 

omp d to

rested parties reviewing the site

p

th

C arison of Average Rainfall Amounts for Historic Events in Colorado adjuste

a 386 square mile Basin (Cherry Creek)

Location (County) Date or Type of 

Storm 

Rainfall in 

inches 

6-Hour 

Rainfall in 

inc

12-Hour

hes

Pueblo, CO (Pueblo) June 4-5, 1921 7.0 10.3 

Cherry Creek (Elbert) May 30-31, 1935 9.9 (5-hour) - 

Hale, CO (Yuma) May 30-31, 1935 8.4 9.2 

Falcon, CO (El Paso) June 16-17, 1965 10.0 - 

Larkspur, CO (Douglas) June 16-17, 1965 6.8 - 

Pawnee Creek (Logan) July 29-30, 1997 6.7 - 

HMR 44-Cherry Creek PMP 12.5 14.3 

HMR 55A – Cherry Creek PMP 17.8 22.1 

S ecific for Cherry 

Creek 

ite-sp PMP 15.5 18.4

R&D EFFORTS 

As a member of the field review group for the Corps’ R&D effort to deve

procedures for performing risk analysis for dam safety, several research areas have be

pursued. The highlights include ways to perform portfolio risk analysis, a framework for

performing risk analysis, better ways to estimate the potential for loss of life, ways to 

extend discharge frequency curves beyond the 1000-year flood event level, and to 

develop probabilities for various types of failure mechanisms. 

A POINT OF CONCERN 

Often a frequency for the PMP/PMF is requested during dam safety 

investigations. Statistica

lop

en

l studies have indicated that in various parts of the country the 

stimated as between 1 in 1 million and 1 in 10 billion frequency of

occurrence.

gages with reco

As a m

Maximum Cred n a 1 in

10,000 frequency citing concerns over geologic changes during this time period. As 

to consider the impacts of climate change in the 

PMF has been e

These studies typically combine either rainfall or stream gage data from 

rds less than 200 years in length to estimate these extreme frequencies.

co parison, geologists are asked to perform the same estimate for the 

ible Earthquake. Often these estimates are limited to less tha

hydrologists and meteorologists, we need

7 Paper 2 - McClenathan 
last 20,000 y r last

ice age occurred ong

set of independe

If not, what wou e period from the last ice age? Even 

the period si e This will be an 

important po t

for dam safety.

As C r

PMP/PMF meth be

readily addressed by even the smallest dam owner. Can even federal agencies afford to 

perform site e agreement and 

judgment us i

about $170,0 . is since judgment will be needed 

extending the frequency curve. At least policy needs to address: 

• Concerns over site specific conditions such as topography impacting rainfall 

gment 

n to use risk analysis and the level of detail needed to incorporate it 

ea s in submitting the extreme frequencies for the PMP/PMF. Given the

between 10,000 and 15,000 years ago, do really have a sufficiently l

nt events from which to forecast frequencies in the 1 in a million range? 

ld be a suitable range? Perhaps th

nc the last ice age has had tremendous changes in climate.

in to consider for extending frequency curves to accomplished risk analysis 

POLICY NEEDS 

he ry Creek Dam demonstrates, policy for risk analysis and for the current 

odology need to consider the needs of specific areas and how they can

-sp cific PMP estimates when there seems to be much dis

ed n determining them? The Colorado PMP review was contracted for 

00 The same would be true for risk analys

in

and runoff 

• The need for independent review of work to overcome the potential 

disagreements from the use of judgment in the analysis 

• How to make consistent, reproducible studies given the amount of jud

that will be needed 

• How and when to include paleoflood evidence (when available) and the 

appropriate level of detail 

• Consistency on the use of antecedent storm conditions 

• Whe

Paper 2 - McClenathan 8
H&H Guidance for Safety of Dams 

by 

Ming Tseng1

1. Existing USACE Guidance

a. ER 1110-2-1155, Dam Safety Assurance Program, 12 Sept 1997. This 

ER lays out the USACE D y. Hydrologic Criteria is on page 3, 

letter dat

8-2(FR Floods nd Re

Mar 1991. This ER e s requirement ction of In

Design Floods at USACE projects. The ER sses the p

ed in the F m 3a below

-2-13, D rednes 96. This document 

ains general lementatio he USACE Da afety 

. 

ort, G uating Mo ations of Existing 

lated t ciencies, J 1980. This repo

uidelines for ming an increm alys

with poli t forth in items 1a and 1f. 

ort, Flood Emergency Plans, June 1980. This report contains 

detailed guidelines for preparation of EAP for USACE dams with an 

example. It encompasses the principles contained in the FEMA report in 

item 3c below. 

f. DAEN-CW/DAEN-EC Letter, Policy for Evaluating Modifications of

Existing Dams Related to Hydrologic Deficiencies, 8 April 1985. This 

is the original H&H of Hydrologic Deficiencies and

ams

1998.

c. IWR Report

Hydrologic R

3.

c. FEMA National Dam Safety Program Report, Emergency Action 

am Safet

paragraph 7, and reflects the policies originally established in the policy 

ed 8 April 1895 (item 1f below). 

b. ER 1110- ), Inflow Design

stablishe

for Dams a

s for sele

servoirs, 1

flow

encompa rinciples

contain EMA report (ite ).

c. EP 1110 am Safety Prepa s, 28 June 19

cont guidelines for imp n of t m S

Program

d. IWR Rep uidelines for eval dific

Dams Re o Hydrologic Defi une rt

contains g perfor ental hazard an is in

conformance cies se

e. HEC Rep

Policy for evaluation

is still in effect. 

2. Related USACE Guidance and Reports 

a. ER 1110-2-100, Periodic Inspection and Continuing Evaluation of 

Completed Civil Works Projects, 15 February 1995.

b. Proceedings of a 1998 Workshop, Modifications to Embankment D

to Accommodate Inflow Design Floods, February

96-R-13, Risk Analysis for Dam Safety Evaluation: 

isk, March 1996

Other Guidance (Non-Corps) 

a. FEMA National Dam Safety Program Report, Selecting and 

Accommodating Inflow Design Floods for Dams, October 1998. 

b. FEMA National Dam Safety Program Report, Hazard Potential 

Classification Systems for Dams, October 1998 

Planning for Dam Owners, October 1998 

 

1 Hydraulics and Hydrology Branch, Headquarters, U.S. Army Corps of Engineers 

9 Paper 3 - Tseng 
d. USBR Guidelines Publication, Dams and Public Safety, 1983 (Safety 

Evaluation of Existing Dams Program) 

Paper 3 - Tseng 10
H&H Dam Safety R&D Needs 

By 

rl E. Eiker, P.E.

1

General 

For the hydraulics (H&H) dam safety

related rese a

is closely align are

also pertinent

application of ions.

Traditional H

Flo

There i

evaluate the ef

single hypo

overtopping m

occurrence of

likely scenario

many of la

storms passing

Arkansas R. in

Upper Mississippi R. basin in 1993, and the Central Valley of California in 1995 and 

997. In all of these cases the individual storms were not unusually large and each 

res fall. Because of their persistence 

however, t to

exceed the cap of such conditions 

may not be lim

time but may a ution of the

individual these 

kinds of proble

Over th

of severe s in

1981, Lewisvi exas in 1990, Saylorville

Dam in Iow n

Kansas in 1993 In each of these cases erosion 

 

Ea

purpose of this discussion, hydrology and

arch and development (R&D) needs are divided into two areas. The first are

ed with traditional methods (deterministic approaches), although they

to risk analysis, while the second area is specifically related to the 

risk analysis (RA) methods to dam safety evaluat

&H

od Series and Flood Runoff Volume

s a very real and pressing need to develop systematic procedures to 

fect of flood series and flood runoff volumes on dam safety. The use of a 

thetical flood such as the PMF to evaluate the susceptibility of a dam to 

ay not be the appropriate approach in many instances. Further, the 

a series of floods with significant total runoff volume may be a much more

than the occurrence of a single rare flood. Over the last 10 to 15 years 

rgest floods that have occurred in the U.S. have been as a result of a series of 

over a given drainage basin. Examples of this type of flooding are: the 

Kansas and Arkansas in 1986, the Trinity River in Texas in1990, the 

1

ulted in only moderate to heavy amounts of rain

he cumulative effect of the runoff volume was substantial and threatened

acity of the reservoir system. In addition, the analysis

ited simply to an evaluation of total runoff volume over a given period of 

lso require the inclusion of the temporal and spatial distrib

storms. Guidelines are needed to allow practicing engineers to address

ms in a consistent manner. 

Spillway Erosion 

e last 20 years numerous USACE dams have been subjected to instances 

pillway erosion. Examples of the problem are: Grapevine Dam in Texas

lle Dam in Texas in 1990, Sam Rayburn Dam in T

a in 1984 and 1993, Painted Rock Dam in Arizona in1993, Milford Dam i

, and Tuttle Creek Dam in Kansas in1993.

rs 1 Retired Chief, Hydrology and Hydraulics Branch, Headquarters, Corps of Enginee

11 Paper 4 - Eiker 
was severe h

spillway with p ch instance costly remedial work 

as necessary to address the problem. Based on these experiences, there is a need to

continue development and testing of predictive techniques to identify conditions defining 

the onset and progression of erosion and a need for the identification of cost effective

fixes to resist erosion. The USSD (formally USCOLD) is now in the process of 

finalizing a technical bulletin on this subject that will provide a state of the art summary 

of current analysis procedures, but there is still much work to be done, particularly in the 

development of specific evaluation guidelines. 

H&H Dam Safety Risk Analysis 

The USACE is committed to pursuing a risk analysis (RA) approach to dam 

safety evaluation, but is also determined to accomplish this goal in a straightforward, 

deliberate manner. This commitment is demonstrated by the initiation, in 1999, of a fiveyear R&D program to address many of the unresolved issues related to RA for dam

safety. 

The use of RA, which is based on mathematical and statistical computations, 

implies a level of accuracy in the results and an understanding of the engineering 

uncertainty that does not, in my opinion, exist at this time. The risk analysis methods 

presently available require further R&D before they are ready for general application and 

use as decision tools. Prior to general application of RA by the practicing engineer in 

H&H dam safety evaluations, improved methods for estimation of the probability of 

extreme floods must be developed, uncertainty must be explicitly included in the 

computations, expected loss of life (LOL) estimates must be improved and decision 

criteria based on LOL and social and environmental consequences must be established. 

As noted above, there are four areas of major concern with the H&H portions of 

the RA methods that are currently being applied in dam safety evaluations. These areas 

of concern are: 1) the lack of proven statistical methods to estimate probabilities of 

extreme floods, 2) the large amount of uncertainty in the analysis that is not explicitly 

considered in the analytical approach, 3) the estimation of potential loss of life (LOL), 

particularly the heavy reliance on generalized data and flood warning systems, and 4) the 

lack of widely accepted decision criteria. 

A considerable amount of effort has been devoted to the problem of estimating 

extreme flood probability, which is the most critical component in the application of RA 

in H&H dam safety studies. Numerous methods have been used to develop 

discharge/frequency relationships for the range of possible reservoir inflows that could 

occur. Methods that have been employed to date have ranged from simple extrapolation 

of curves constructed based on finite periods of record, to the use of paleoflood 

hydrology, precipitation records, regional frequency analysis and stochastic hydrology to 

better estimate return periods that could serve as "anchor" points in extending the basic 

discharge/frequency curves. 

. T ere was a potential, if conditions of spillway flow persisted, to lose the 

ossible rapid loss of the reservoir. In ea

w

Paper 4 - Eiker 12
The RA methods currently being applied are for the most part based on event tree 

nalysis. This requires the assignment of probabilities of occurrence, often with little or 

no data, to each branch of e physical processes

that might occur. Thus, there is a heavy reliance on “expert elicitation.” Further, 

engineers are not generally comfor robabilities to physical events, so 

at the whole analysis becomes much more “art” than engineering. Yet, because RA 

end result made up of absolute numbers, the analysis takes on the appearance 

f providing far more understanding than is presently the case. 

is. In addition to

arning time LOL should be based on depth and velocity of flooding, how the flooding 

om flooded areas, adequacy of warning plans and other site specific 

ctors. USACE experiences with flood warning systems, as part of its flood damage 

reducti e been less than satisfactory. 

f lives

s

a

rology, stochastic hydrology and the use of 

eteorologic and historical flood data to extend the record length and at the same time 

identify new approaches. 

routing coefficients, dam breach formation, flood wave propagation, and operational 

uncertainty associated with hydraulic structure and hydraulic machinery performance. 

a

the event tree constructed to describe th

table with assigning p

th

leads to an

o

In estimating expected loss of life that would result from a dam failure, most of 

the estimates in current use are based on generalized studies with heavy emphasis on 

warning times. Loss of life estimates should be site specific, and to be complete,

uncertainty in the estimates should be explicitly included in the analys

w

will occur, egress fr

fa

on program, show that in general they hav

Finally, the decision criteria now being used are based on “best estimates” o

lost per year and the estimated probability of a given loading occurring. This approach 

has the effect of masking the true impacts of a dam failure. The long-term social and

environmental consequences that may result from a dam failure are not specifically

considered. The most common decision criteria now in use are 0.001 lives lost per year 

and a failure probability of 0.0001. In other words, if the estimated LOL per year is les

than 0.001 and the likelihood of failure is less than 0.0001, then there is not a compelling

reason to pursue remedial work, regardless of the social and environmental impacts. 

Furthermore, if the numbers indicate that no work is required, what is the chance that

non-technical decision maker will support remedial work, no matter what the long-term

social and environmental consequences might be? 

H&H R&D Needs for RA 

The most immediate R&D need in applying RA to H&H dam safety analysis is to

develop the means to more accurately estimate extreme flood probabilities. To 

accomplish this R&D must be expanded in areas that show promise. We need to 

continue and expand work on paleoflood hyd

m

Secondly, we must expand our efforts to better define H&H parameter 

uncertainties, including the explicit description of the error distributions associated with

"best estimates." Successful application of RA is dependent on accurately defining the 

uncertainties associated with, but not limited to, flood volumes, antecedent flood 

conditions, antecedent soil moisture conditions, unit hydrograph uncertainties, flood 

13 Paper 4 - Eiker 
Thirdly, we must develop better ways to estimate LOL. Estimates must be site 

specific and address not only warning time, but how flooding will occur (residual risk

considerations), rate of water surface rise, depth of flooding, flow velocities in flooded 

areas, availability of egress (initial and continuing conditions), and evacuation plan 

uncertainty.

 Finally, decision criteria must be established that are meaningful and 

understandable and take into account the catastrophic social and environmental impacts 

at are likely in the event of a dam failure. 

proving H&H Dam Safety Assurance using RA

duce

ty, determine the extent of problems and prioritize remedial work. 

nt

of

r

the full range of hydraulic loadings that is compatible with addressing 

th

Im

In summary, I believe that for the short term we should continue to use the 

incremental hazard analysis as an interim approach for addressing H&H dam safety 

problems. Over the long term, we need to develop an RA method that allows us to re

uncertain

Such an approach would entail the following steps: 

 

1. Complete the R&D necessary to develop statistically acceptable methods for

determining probabilities of extreme floods. 

2. Develop a method of analysis that explicitly addresses the uncertainty inhere

in the currently available risk RA procedures. 

3. Establish final risk-based decision criteria based on improved estimates

LOL and long-term social and environmental consequences. 

4. Develop an acceptable RA method that will allow dam safety evaluations ove

geotechnical and structural concerns. 

 

Paper 4 - Eiker 14
FE

y Regulatory

omm

MA Workshop on Hydrologic Research Needs for Dam Safety 

Analysis 

Federal Energy Regulatory Commission 

By 

Michael S. Davis, P.E.2

The purpose of this paper is to discuss the Federal Energ

C ission’s (Commission) current practice in evaluating the adequacy of the spillway

capacity of hydroelectric projects under it’s jurisdiction, and to suggest areas where 

future research would be a benefit to these projects. 

Current Practice 

 Regulations and Procedures 

3 Since Order 122 was issued in 1981 , the current practice of the Commission to 

ensure

s

ading or overtopping which may 

occur from a flood up to the probable maximum flood (PMF), or (ii) the capacity 

rvoir from rising to an elevation that would

endanger the project works. 

that the spillway capacity of all high and most significant hazard potential project 

is adequate has been governed by Sections 12.35(b) and 12.35(b)(1) of the Commission’

regulations, which are as follows: 

12.35(b) Evaluation of spillway adequacy. The adequacy of any spillway must be

evaluated by considering the hazard potential which would result from failure of 

the project works during flood flows. 

1235(b)(1) If structural failure would present a hazard to human life or cause 

significant property damage, the independent consultant must evaluate (i) the 

ability of the project works to withstand the lo

of spillways to prevent the rese

ams and 

e

n developed the Engineering Guidelines for the Evaluation of 

The Commission’s regulations require that for all high hazard potential d

most significant hazard potential dams

4

, an independent consultant must inspect th

project and evaluate its stability and spillway adequacy every five years. To give the

Commission staff and the independent consultant guidance and criteria for this 

evaluation, the Commissio

Hydropower Projects. 

2

 Lead Engineer, Federal Energy Regulatory Commission, Office of Energy Projects, Division of Dam

Safety and Inspections, Chicago Regional Office. 

3

 Order 122 established the current regulations for independent consultants inspecting and evaluating

projects on a five-year cycle. 

4

 Any significant hazard potential dam that is less than 32.8 feet high and impounds less than 2,000 acrefeet is not subject to this regulation.

15 Paper 5 - Davis 
d

f the

the independent consultant is required to perform a dambreak analysis in 

accordance with Chapter 2 of our engineering guidelines, or the licensee may choose to 

modify

flows between the existing spillway capacity 

nd the PMF show that the incremental increase in downstream flooding due to the 

significant property damage, then 

e spillway is adequate and no further studies are needed. However, this may need to be 

e-eval

r increase

e stability of the project structures to withstand the overtopping. Usually, an inflow 

atives for increasing the

acity or the stability of the structures, and the IDF can vary depending on the 

type of

demon am flooding due to failure of the 

dam fo

or coul

n demonstrate 

through

will be MF study 

is need DF.

Probable Maximum Flood Studies 

Chapter 8 of the Commissions Engineering Guidelines, entitled, “Determination 

of the Probable Maximum Flood”, was first developed in 1993, and was recently revised 

in September 2001. The purpose of these guidelines is to provide systematic procedures 

that will consistently produce a reasonable PMF hydrograph and appropriate reservoir 

flood levels for evaluation of project safety, given the limitations of basic hydrologic and 

meteorological data. Although the guidelines give procedures and make 

recommendations for parameters, alternate procedures and parameters outside the 

recommended ranges can be used provided they are justified for the basin under study 

and supported with adequate documentation. 

Runoff Model.

For most projects, the first step for the independent consultant is to develop the 

PMF for the project and to evaluate the spillway capacity and stability of the project 

during the PMF. If the project can safely pass the PMF, then the spillway is adequate an

no further studies are needed. If, however, the PMF exceeds the spillway capacity o

project, then

the dam to safely pass the PMF. 

If the dambreak analysis for all flood

a

failure is not a threat to downstream life or could cause

th

r uated during future inspections if there are any changes downstream. 

If there is a potential hazard to life or property downstream for these flood flows, 

then remedial measures are required to either increase the spillway capacity o

th

design flood (IDF) analysis is done considering various altern

spillway cap

fix. But once a fix is proposed for a flood less than the PMF, the license must 

strate that the incremental increase in downstre

r all flood flows between the IDF and the PMF is not a threat to downstream life 

d cause significant property damage. 

In a few cases, a PMF study is not needed if the consultant ca

a dambreak analysis that either the spillway capacity is adequate or that the IDF 

considerably less than any estimates of the PMF. But in most cases, a P

ed to establish the upper bound of the I

 This chapter recommends the use of the unit-hydrograph theory 

as the preferred runoff model. It also recommends that the Corps of Engineer’s HEC-1 or 

Paper 5 - Davis 16
HEC d

use and experience. 

Channel Ro

-HMS computer programs be used to model the runoff because of their widesprea

uting. Channel routing should be done using the Muskingum-Cunge

method, which is incorporated in . However, any acceptable

ynamic routing model such as t ice (NWS) DAMBRK 

omputer program can be used instead if the consultant chooses to refine the model. 

to these computer models

d he National Weather Serv

c

Probable Maximum Precipitation (PMP). The PMP should be developed from the

latest NWS Hydrometeorological Reports (HMR). However, a licensee may choose to 

develop a site-specific PMP study, although this is usually very costly. This may do

he basin under study (1) is not adequately covered b

ne if

y the HMRs, has unusual site

onditions that may not be address n benefit from a refinement of 

the HMR PMP values. To conduc tudy, the Commission requires 

at a Commission-approved Board of Consultants consisting of at least a hydrologist, a 

iew the study. As an example of an 

pproved study, the Commission accepted e 1993 EPRI PMP study for the States of 

iscon

es were reduced considerably, resulting in a significant 

ost savings to the licensee’s of these projects. Figure 8.8-1 of Chapter 8 shows the 

limitati

t

c ed by the HMRs, or ca

t a site-specific PMP s

th

meteorologist, and a hydrometeorologist rev

a th

W sin and Michigan, which resulted in PMP values as much as 15 percent lower 

than HMR No. 51. The 1993 EPRI study also refined the procedures in HMR 52 for 

developing the probable maximum storm from the PMP values. As a result, the PMF’s 

for many projects in these two Stat

c

ons of the latest HMRs and the 1993 EPRI study.

 Antecedent and Coincident Conditions. Rather than route an antecedent storm

through the model, this chapter recommends that the reservoir level be assumed at its 

annual at

the ons

conside

100-ye

using t

Loss Rates

maximum operating level and that saturated conditions exist in the entire basin

et of the PMP. Several alternatives for the starting reservoir level may be 

red which would required an analysis of the historical records or routing of the 

ar 24-hour antecedent storm 3 days prior to the start of the PMP. In most cases, 

he annual maximum operating level gives satisfactory results. 

 This subject received the most attention when Chapter 8 was recently 

the

of the hydrologic soil groups, or by a distributed loss rate 

method. The basin-averaged method is the traditional method that has been used for 

ge

ter 8 recommends that the minimum loss rates 

rom the National Resources Conservation Services (NRCS) 1955 Yearbook of 

A

revised. The preferred method is to use the uniform loss rate method. Consistent with 

the premise that saturated antecedent conditions exist in the basin prior to the start of

PMP, it is acceptable to assume the initial loss rate is set to zero. 

The losses in a basin can be developed either by the area-weighted basinaveraging of the loss rates

many years, and simply involves computing the average loss rate based on the percenta

of the four hydrologic spoil groups. Chap

f

griculture be used unless higher loss rates could be justified.

17 Paper 5 - Davis 
Instead of lumping the loss rates together in each basin, the distributed loss rate 

he

to

cs. A 

asin with a calibrated average loss rate for a specific storm may actually have a 

y

e

urrent criteria is to use the minimum value 

of the range for the least permeable layer, unless higher values can be justified 

he

am Break Studies 

uidelines and Criteria

method divides each basin into pseudo subbasins corresponding to each loss rate

class. The rainfall is then applied to each pseudo subbasin to determine the 

rainfall excess hyetographs. Then the rainfall excess hyetographs for all pseudo 

subbasins within a basin are summed to determine the rainfall excess hyetograph 

for that basin. These rainfall excess hyetographs are then input in HEC-1 with t

loss rates set to zero for that basin. This method also works for larger basins that

are subdivided into subbasins. 

The distributed loss rate method was developed in the 1990’s primarily to address 

an inconsistency with the basin-average method. Our procedures allows you

calibrate loss rates based on 3 to 5 historical events that meet certain criteria. 

However, loss rates calibrated using the basin-average method were found to be 

storm specific, particularly for basins with spatially diverse characteristi

b

significantly higher loss rate for the PMF since significantly more portions of the 

basin contribute to the runoff during the PMF than did during the specific storm. 

The second reason this method was developed was to incorporate the availabilit

of the digitalization of soil databases such as NRCS State Soil Geographic 

(STATSGO) database. The STATSGO database contains many properties of th

soil for several layers up to 60 inches deep. One such property is the hydraulic 

conductivity, which is the rate of flow through saturated soil, normally given as a

range of values. The Commission’s c

through calibration or additional investigation of the geological make-up of t

soils, the review of more detailed soils information such as county or local soils 

maps, or actual data obtained from any site investigations within the basin. 

D

G

l. The NWS 

the

e

ter analysis is not necessary. 

approximately 2.0 feet or less. However, the 2.0-foot increment is not an absolute 

The Commission’s criteria for conducting dam break studies is discussed in 

Chapter 2 of the Engineering Guidelines. The NWS DAMBRK and FLDWAV 

computer programs are the recommended unsteady flow model computer 

programs that can be used to route the flow downstream of the dam. However, 

the Commission has accepted studies using other programs as wel

SMPDBK program has been accepted in a few cases where the accuracy of

results is not as critical. In some cases, if a field reconnaissance shows that ther

are no structures downstream, a compu

In general, the consequences are considered to be acceptable when the 

incremental increase in flooding on downstream structures due to dam failure is 

Paper 5 - Davis 18
decision-making point. Sensitivity analyses and engineering judgment are 

required. For instance, inhabited trailers sitting on blocks can be moved with less 

an 2.0 feet of rise, and should be considered in this evaluation. 

or

ore monoliths up to one-half the length of the dam for gravity dams. Failure 

y

s a

th

The Commission’s guidelines for breach parameters is given in Table 1 of

Appendix A of Chapter 2. In general, the average breach width should be

between 2 and 4 times the height of the dam for earth or rock fill dams, and one

m

times range from 0.1 to 1.0 hours for earth or rock fill dams, and from 0.1 to 0.3 

hours for gravity dams. For arch dams, it’s appropriate to assume the entire dam

fails in 0.1 hours or less. 

Because of the uncertainty of breaches, the consultant should perform a sensitivit

analysis of these parameters. For projects with large reservoirs, conservative 

breach parameters should be adopted since the rate of draw down of the reservoir

during a breach is significantly slower than it is for projects with smaller 

reservoirs. In some cases, larger breach widths with longer failure times should

be considered, such as for a long 20-foot high earth embankment that impound

large storage reservoir. 

Common Modeling Problems

1. Failure to model the entire reservoir. If dynamic routing of the reservoir

stead of level pool routing is done, the consultant needs to make sure the crosse

.

ay be at the 

onservative end of the accepted range given in our criteria, a larger breach width 

. Improper use of the Manning’s n values. The NWS DAMBRK program 

ation.

e

. Improper spillway rating curve. In some cases, the reservoir was allowed to 

draw down during the beginning of the routing because the consultant did not 

in

sections extend upstream of the reservoir to the point where backwater effects no

longer exist. The shape of the cross-sections also needs to be examined to mak

sure all the storage between the cross-sections is accounted for. In some cases, 

the consultant extended the cross-sections only part way into the reservoir,

effectively negating the storage upstream that could be released through a breach

 

2. No sensitivity studies. Although the selected breach width m

c

may result in a substantially higher incremental rise downstream. If the 

incremental rise is highly sensitive to the breach width, then this needs to be 

considered when selecting the breach width. 

3

requires the user to provide the composite Manning’s n values at each elev

Therefore, for out-of-bank flood elevations the consultant needs to compute th

composite Manning’s n value based on the weighted wetted perimeter. In many 

cases, the consultant will select too high of a Manning’s value for the out-of-bank 

elevations. Although not a major factor, this can effect the results in some 

analyses. 

4

19 Paper 5 - Davis 
adjust the rating curve for when the gates are closed to maintain the normal pool

level. In other cases, the consultant adjusted the rating curve to correct this, but 

the simulation then appeared as though the licensee closed all the gates 

instantaneously when the reservoir receded below the normal maximum pool afte

the breach developed. 

5. The breach was assumed to initiate on the rising limb of the inflow hydrograph.

It’s imperative that a non-failure

r

case be run first so that the peak headwater 

elevation at the dam can be determined and used in the failure case. This 

n

t

xtreme temperatures, and the PMP. 

hanges in our guidelines point out the need for more research 

using the distributed loss rate method with STATSGO data, particularly since 

sitive to the selected loss rates. 

. Computer models. Research is needed on ways the current computer models 

users to 

becomes more complex when conducting a domino-type failure analysis of 

downstream dams. 

Research Needs

The following items are research needs that should be considered: 

Probable Maximum Flood Studies 

1. PMP. Many of the HMR’s cover very large areas that don’t take into account 

local terrain affects that may reduce the PMP for that area. Other refinements ca

be done that could reduce the probable maximum storm such as the EPRI 

Wisconsin/Michigan study. 

2. Snowmelt. The HMR’s in the western states have very approximate methods

for combining snowmelt with the PMP. More research is needed in this area as i

may be too conservative in some cases to combine 100-year snowpack with 

e

3. Loss Rates. The c

in

the PMF can be very sen

Dam Break Studies. 

1. Breach Parameters. More research is needed on the proper selection of breach

parameters, particularly for earth and rock fill dams. The FEMA/USDA 

Workshop on Issues, Resolutions, and Research Needs Related to Dam Failure 

Analysis, in Oklahoma City, OK, June 26-28, 2001, addressed this concern. 

2

for unsteady flow can be made easier to use and more flexible to allow

model more complex dams with multiple spillways. 

Paper 5 - Davis 20
Hydrologic Research Needs for Dam Safety Analysis 

At the Tennessee Valley Authority 

By 

R. Cris Hughes, P.E., and Gregory W. Lowe, P.E.1

Backgr

The Te

develop

“That for the purpose of . . . and to improve navigation in the Tennessee River and to 

control the destructive flood waters in the Tennessee River and Mississippi River Basins, 

there is

With

such le

ound

nnessee Valley Authority (TVA) was created in 1933 to provide for the unified 

ment of the Tennessee River Valley. The purpose of the Act is stated as follows:

hereby created . . . the ‘Tennessee Valley Authority’ “ - Preamble. 

respect to planning, Section 23 requires the President to recommend to Congress 

gislation as he deems proper “ . . . for the especial purpose of bringing abo

ity with said general purposes (1) the maximum amount of flood control, (2) the

um development of said Tennessee River for navigation purposes, (3) the 

um generation of electric p

ut . . . in

conform

maxim

maxim ower consistent with flood control and navigation; . . .” 

On the subject of operation of reservoirs, Section 9a states: “The (TVA) Board is 

hereby directed in the operation of any dam or reservoir . . . to regulate the stream flow 

primarily for the purposes of promoting navigation and controlling floods. So far as

istent with such purposes, the Board is authorized to provide and operate facilities

generation of electric energy . . . and the Board is further authorized, whenever an 

nity is afforded, to provide and operate facilities for the generation of electric 

in order to avoid the waste of water power

may 

be cons

for the

opportu

energy , . . .” 

TVA’s structural approach to minimizing flood risk was the construction of dams with 

flood control allocations to “keep the floods away from the people.” Today, TVA 

operate er

basin),

seven s ve been

realized

Mississ al benefits to the region including

navigation, hydropower generation, water supply, recreation, water quality, and land use 

for eco

TVA’s

reducti

floodin

degree

s an integrated reservoir system of 49 dams (1 project in the Cumberland Riv

in the 41,000-square mile Tennessee River drainage basin covering portions of 

tates. Since these dams were built, significant flood reduction benefits ha

along the Tennessee River and its tributaries, and along the lower Ohio and 

ippi Rivers. TVA dams also provide addition

nomic development. 

reservoir system has been effective in providing over $5B in flood damage 

on benefits. TVA also recognized that structural measures could not eliminate 

g, and that there were about 350 communities in the Tennessee Valley with some

of flood risk and damage potential. 

1

 Tennessee Valley Authority 

21 Paper 6 – Lowe, Hughes 
In resp

in 1953

plannin ith flood

problem

TVA p oiding development within the 100-year floodplain and 

encouraging to the extent possible the adoption of higher setback and elevation standards. 

In the l

Federal is today

the Nat A for

several years to develop flood information for many communities within the Tennessee 

Valley. During this period, TVA also demonstrated several different flood damage 

reduction measures at different communities including channel restoration/modifications, 

flood warning systems, acquisition and relocation, and flood proofing. 

Current Floodplain Management Activities 

Since 1994, TVA’s floodplain management efforts have focused on the lands and 

project on the floodplains along the rivers and

streams, which are affected by regulation from TVA dams. The objective of the program

is to mi

people

control s

reservo w the dams have seen substantial development over 

the last several years. Thus, flood risk is expected to continue to increase in the future 

from th

Dam S

TVA has maintained a dam safety program since its establishment in 1933. Following 

the fail

Federal

guideli of the interagency 

document, Federal Guidelines for Dam Safety (Reference 1), TVA formalized its dam 

safety program in 1982. At that time, TVA evaluated all of its dams for hydrologic and 

seismic d that 23 of its then 53 dams had some degree of deficiency 

and could be made even safer, consistent with these guidelines. Since that time, TVA has 

spent m

project

hazard

chose t

onse to this situation, TVA initiated a floodplain management assistance program

based on the concept of averting local flood damages by careful land use 

g. This approach of working with state and local governments to deal w

s was applied throughout the Tennessee River watershed. During this period, 

romoted the concept of av

ate 1960s, TVA utilized its floodplain management experience to assist the 

Emergency Management Agency (FEMA) with the development of what

ional Flood Insurance Program (NFIP). TVA served as a contractor to FEM

s that TVA holds in stewardship, and

nimize flood damages, ensure the safety of floodplain residents (by keeping the 

away from the water), preserve TVA’s reservoir operating flexibility for flood 

purposes, and ensure consistency with local floodplain regulations. TVA’

irs and the river reaches belo

is one factor alone.

afety Program Development

ure of Teton Dam, President Jimmy Carter issued a 1976 memorandum to all 

Agencies with responsibilities for dams to develop and implement formal 

nes for dam safety. After participating in the development

safety, and determine

ore than $75M modifying these dams, with work underway at the remaining two 

s to ensure its dams meet these guidelines. Because most of TVA’s dams are high 

structures with significant potential for loss of life and property damage, TVA 

o modify its dams to safely pass the Probable Maximum Flood (PMF). 

Paper 6 – Lowe, Hughes 22
TVA recogn robable

Maximum Precipitation to have studies

performed specific to the region rather than using generalized estimates for PMP. The 

National Weather S gories of extreme 

recipitation for the Tennessee Valley which included PMP and a standardized less 

ll referred to as the “TVA precipitation.” At this time, the NWS completed 

ydrometeorological Reports (HMR) 41 (Reference 2); 47 (Reference 3); and 56 

bove the City of Chattanooga (approximately 21,400 square miles) and for basins up to 

3,0

urrent Research Needs

rying

f Chattanooga, the major damage center in the Tennessee Valley. These reservoirs 

pro

o

or this flood storage (saying it is

o conservative) and have requested a delay of the drawdown of these reservoirs until 

down

h

d

s. This was the first comprehensive re-evaluation of 

servoir operating policy since the projects were built. However, the focus of this 

s,

, no alternatives were considered which 

ould change the winter flood storage allocations. However, the review did formalize 

the requirements for a minimum of one inch of flood storage space during the summer

months at 10 of the tributary reservoirs. 

ized the need to have an outside authority provide estimates of the P

(PMP). Further, TVA recognized the need

ervice (NWS) was funded by TVA to study two cate

p

extreme rainfa

H

(Reference 4). HMR 45, superseded by HMR 56 in 1986, was used in studies prior to 

HMR 56. These reports provided estimates of precipitation for large areas such as that 

a

00 square miles. These reports defined depth-area-duration characteristics and 

antecedent storm potentials. 

C

One of the most controversial aspects of the TVA reservoir system is the annual 

operating cycle for the tributary projects. There are 10 tributary projects, which have a 

summer-to-winter fluctuation of from 35 to as much as 90 feet. The seasonally va

allocation of flood storage was designed primarily to provide flood protection for the City

o

vide over 4 million acre-feet of flood storage space needed during the flood season

from mid-December through early April. However, the economic benefits attributable t

use of these reservoirs have changed over the years and now include enhanced lake front

property value, recreational boating, fishing, swimming, wildlife habitat, minimum flow 

and dissolved oxygen (DO) enhancements, and related functions. 

Stakeholders, for many years, have questioned the need f

to

later in the fall. These reservoirs are typically at their highest level by June 1 of each 

year depending on rainfall/runoff. During June and July, they are gradually drawn

to support downstream water quality and hydropower generation. After August 1 of eac

year, the reservoirs have an unrestricted drawdown to lower them to their January 1 floo

storage levels. 

In 1991, TVA completed the Tennessee River and Reservoir System Operation and 

Planning Review, an Environmental Impact Statement (EIS) that resulted in changes to 

its reservoir operating policie

re

review was on maintaining minimum flow below dams at critical times and location

increasing DO below 16 dams by aerating releases, and to delay unrestricted summer 

drawdown until August 1 on ten tributary reservoirs. While flood control was a 

consideration in review of these alternatives

w

23 Paper 6 – Lowe, Hughes 
TVA established a Regional Resource Stewardship Council in March 2000 under the 

Federal Advisory Committee Act (FACA). The purpose of the Council was to pro

advice to TVA on policies, priorities, and practices for managing its land and water 

resources and programs as part of its public responsibilities. The Council is made up o

20 representatives from across the Valley. They represent a range of interests in TVA’s 

stewardship activities, including representatives of the Governors of the seven TVA 

vide

f

ates, power distributors, industry, business, environment, recreation, consumers, and 

rform a 

omprehensive evaluation of reservoir operating policy in two years. This study will be 

olicy Act (NEPA) framework as an (EIS). 

n

on

d

e

l studies underway to address similar issues across the 

ountry (Upper Mississippi River Flow Frequency, Savannah River, and Lower Colorado 

iving this need including: (1) the Nation’s 

oodplains continue to be developed increasing the flood risk and damage potential; (2) 

t

hould

0-

supported

y the technical community, general public, and local, state, and federal land- and wateruse decision makers. 

st

educational and community leadership. 

This summer (2001) the Council recommended that TVA undertake a study of its 

reservoir operating policy to determine if changes could create greater overall value for 

TVA customers and stakeholders without reducing gains which had been realized in

water quality. The study will include evaluation of costs and benefits. The TVA Board 

responded to this recommendation in October with a commitment to pe

c

conducted within the National Environmental P

One of the major issues to be addressed will be the evaluation of potential change i

flood risk that could result from a change in reservoir operating policy. The evaluati

must ensure that the tools and analysis process must be capable of providing a clear 

understanding of how the flood risks could change. This should include impacts on floo

frequency throughout the full range of flood potential from the annual event through th

PMF, effect on local floodplain regulations as part of the NFIP, elevation and flow 

duration, and impact on dam safety. 

At this time there are severa

c

River Authority). Many factors are dr

fl

advances in weather forecast capabilities are viewed by the public as a reliable basis for 

reservoir operations well in advance of actual events; (3) studies raise questions abou

previously completed flood frequency analysis and whether these changes in turn s

result in changes to published information used for local floodplain regulations for 10

and 500-year flood boundaries and floodways; (4) the hydrologic period of records are 

increasing, coupled with more sophisticated computational methods and modeling 

capabilities; and (5) a growing interest on the part of the stakeholders that live along or 

use the water resource to change the allocation of benefits based on economics. 

Conclusion 

Research is needed to focus on flood risk assessment methodology that can be

b

Paper 6 – Lowe, Hughes 24
References:

1. Ad Hoc Interagency Committee on Dam Safety, Federal Coordinating Council for 

Science, Engineering and Technology, “Federal Guidelines for Dam Safety,” June 19

2. U.S. Weather Bureau, “Probable Maximum and TVA Precipitation Over the 

Tennessee River Basin Above Chattanooga,” Hydrometeorological Report No. 41, 1965

3. National Oceanic and Atmospheric Administration, “Meteorological Criteria for 

Extreme Floods for Four Basins in the Tennessee and Cumberland River Watershe

Hydrometeorological Report

79.

.

ds,”

No. 47, 1973.

e, “Probable Maximum and TVA Precipitation Estimates 

ith Area l Distribution for Tennessee River Drainages Less than 3000 Square Miles in 

4. National Weather Servic

w

Area,” Hydrometeorological Report No. 56, October 1986. 

25 Paper 6 – Lowe, Hughes 
The Utah Hydrological Experience 

By 

Matthew C. Lindon, P.E.1

The Utah Dam Safety Section is fairly deterministic in requiring the PMF to be 

o reduce 

MR – 49 Local storm PMP that is required for the PMF calculations has been revised to 

ate

design event, with its lower peaks, larger 

olume and longer runoff characteristics. 

ration difficult, if not

possible. Our typical calculation involves black box solutions such as synthetic unit 

ydrographs, curve numbers or constant infiltration rates. We need more physically 

ased runoff models tied to soil types, vegetation, spatial and temporal precipitation 

distributions, probably in a GIS type format with hydraulic, finite element, geographical 

routing. We need calibrated models that give accurate results as well as effective 

graphics. 

An example of long-term spillway outflows is the typical spring-time snow melt 

event in many of Utah’s high mountain dams. Drainage basins typically produce as 

much as 33 cfs per square mile in continual runoff, sometimes for weeks at a time. Rain 

on snow events can compound these flows and if the spillway is clogged with snow and 

ice, the dam can eventually overtop and fail, as was postulated for the failure of the Trial 

Lake dam in 1985 (although the failure was officially deemed an act of God). 

Another hydrologic modeling example was our experience with the Quail Creek 

Dike failure. After completing all the modeling classes given by HEC and the NWS we 

experienced a large dam failure and subsequent flood. When we tried to simulate the 

breach and the flood with our models we had a difficult time recreating the reality that 

had been observed. Breach size, timing, piping initiation and breach migration were 

difficult to reproduce. Routing required huge Manning numbers to simulate roughness 

and we had difficulty accurately portraying the observed hydraulic jumps, attenuation and 

timing of the flood wave. If we had such a hard time modeling a flood we experienced 

first hand, how could we have faith in our models of hypothetical floods? 

Again, we need to have physically based models that can be calibrated and tied to 

reality. We need improved methods that take advantage of new technology such as 2d 

and 3d models, hydraulic routing, GIS, and NEXRAD radar, to match the apparent 

veracity of the amazing graphics and animations of the input and output. We need to 

 

routed by all High Hazard dams. Incremental damage assessments can be used t

the design flood, especially for debris basins and smaller flood control structures. The 

H

accommodate a state specific areal reduction developed by Dr Don Jensen at Utah St

University. In the event that the 6-hour local storm PMP is less than the general storm, 

72-hour PMP, the general storm becomes the

v

Problems that we have encountered in design storm calculations include the lack 

of rain and flow gages that make statistical analysis and calib

im

h

b

1

 Utah DNR, Dam Safety Hydrologist

27 Paper 7 – Lindon 
honestly express our confidence limits, sensi vity and fuzziness of our answers. We

need to get out of our boxes to observe and m mic the complex ways that nature works. 

ti

i

Paper 7 – Lindon 28
The Utah Hydrological Experience 

By 

Matthew C. Lindon, P.E.1

Small reservoir, large dams, 

ocal and General Storm - 6 - 72 hour 

 20 or 200 square miles 

 Point precip possible?? 

 

 Design Inflow Flood Calculation 

Deterministic calculation of PMP/PMF for High Haz 

 

Some Incremental Damage Assessment 

 

Debris basins, Flood Control 

100 year storm minimum design storm

 Some Risk Assessment 

 

Prioritize resource allocation, 

 Bang for the Buck 

L

Curve Number and Constant Infiltration rates 

 Little gaging and calibration 

 HMR 49 and State Specific PMP 

 Black Box Spatial and Temporal Distribution 

 State specific PMP 

 

Morgan Storm of August 1958 

 

Dr Jensen, USU review 

 

7 inches in 1-hour hub cap study 

 Base storm for HMR-49 - western USA 

 Dr Jarrett review 

 No Paleo evidence of the storm locally or regionally 

1

 Utah DNR, Dam Safety Hydrologist

29 Paper 7 – Lindon 
 Revised Areal Reduction 

 Revised Temporal reduction from storm data 

 Reduces Local 6 hour storm LT 15 sq miles 

 Defaults to General Storm - 72 hr HMR-49 

 Smaller peaks, more volume, longer duration 

 Site/State Specific General Storm study done 

 Canned PMP programs for consistent calculations 

 PMF Improvements 

 Close only counts in Horseshoes and Hand Grenades 

 Calibration, Correlate, Verify

 Little rain gage info and correlated flow gage 

 Less history or storm data base 

 Curve number and Infiltration rates 

 Cook Book temporal and spatial distribution 

 Black Box Solution Methods 

 Better Models Better Modelers 

 Physically based - no black box 

 Spatial and temporal data - GIS coverage 

 Hydraulic routing - combine RAS and HMS 

 Input quality matches output 

 Apparent Veracity - Know what model does 

 Computers lie and liars use computers 

 Trial Lake Spillway 

 

Negligent Overtopping or Act of God 

 Long, Cold winter 

 North facing spillway 

 Clogged with snow and ice 

Paper 7 – Lindon 30
 Late Spring 

 

 Rain on snow event 

Model overtopp flow

 Organic foundation contact blamed 

Design for long

uail reek

God ha e's sure of. 

MBRK, BREACH...

ry to tion

REA

HEC I

EC II - constant flow - no hydrogra h routing

 DAMBRK 

ing, bridges, debris, eddies 

 Timing of flood 

ime steps 

Adjust reality

 

liars use computers 

Physic

 Calibrate, Correlate, Verify - tie to reality 

ing with no spillway/outlet

duration snow melt flows 

General Storm long duration flows 

Q C Experience 

ve mercy on the man who doubts what h

 Competed HEC I, II, DA

T recreate the Breach and inunda

B CH

Initiation piping channel size

 Trapezoidal migration of Breach 

 Timing and size of breach 

- backwater and attenuation

H p

 Manning Numbers bulk

 Hydraulic jumps

T

to fit the model 

 Computers lie and

ally based models - no black boxes 

31 Paper 7 – Lindon 
Educate Modelers to what the model does 

 Garbage in Groovy out - Apparent Veracity 

 Express confidence, sensitivity, fuzziness 

 Use new technology and methods 

 2d, 3d, hydraulic routing 

v, slope...

 Combine HECRAS and HECHMS 

 Get out of the box and see how nature works 

 GIS coverages for soils, vegetation, ele

Paper 7 – Lindon 32
FEMA Workshop on Hydrologic Research 

Needs for Dam Safety Analysis 

gia and

other states east of the Mississippi River

y

Francis E. Fiegle II, P. E.

1

State of Georgia 

The Georgia Safe Dams Act regulates high hazard dams that are either 25 feet tall or 

ore more than 100 acre-feet of water at maximum pool. These regulated dams are 

ey have adequate spillway capacity and wave action freeboard 

t maximum design pool to pass the design storm as defined by the Georgia Safe Dams

Act (A is prescribed in the 

ct based on the height of the dam and/or its maximum storage capacity. 

1) mall dams - Those dams with a storage capacity not exceeding 500 acre-feet and 

MP spillway design capacity.

2) s - Those dams with a storage capacity exceeding 500 acre-feet but 

not exceeding 1000 acre-feet or a height exceeding 25 feet but not exceeding 35 

feet - 3 capacity. 

3) Large d rage capacity exceeding 1000 acre-feet but 

ot exceeding 50,000 acre-feet or a height exceeding 35 feet but not exceeding 

esign capacity. 

4) acity exceeding 50,000 acre-feet 

r a height exceeding 100 feet - 100 percent PMP spillway design capacity. 

Typically, the Georgia Safe Dams Program develops Visual Inspection Reports 

which are a str orts that were done in 

the late 1970's or state dam safety programs across the United States. 

In that report, our Program evaluates the spillway capacity of existing dams with respect 

to compliance with the Act and Rules. Except in rare instances, the hydrology evaluation 

ses the NRCS/SCS Curve Number, Lag Time methodology. Of note, because the 

illway capacity requirements are set forth in the Act, and the design storm is restricted 

quires the use of Antecedent Moisture Condition III 

n of spillway capacity. This dramatically affects the Curve Numbers 

used. W storm. The spillway rating curves are 

done using standard hydraulic practice accounting for weir, orifice, and full pipe flow for 

princip emergency spillways, etc. This 

Hydrologic Practices in the State of Geor

B

st

evaluated to determine if th

a

ct) and Rules for Dam Safety (Rules). The spillway capacity

A

S

a height not exceeding 25 feet - 25 percent P

Medium dam

3 percent PMP spillway design

ams - Those dams with a sto

n

100 feet - 50 percent PMP spillway d

Very large dams - Those dams with a storage cap

o

eamed lined version of the old USACOE Phase I rep

and early 1980's f

u

sp

to the 6-hour PMP, the Program re

for the evaluatio

e use HMR 51/52 to develop the design

al spillway pipes/open channel flow in earth

1

 Program Manager - Georgia Safe Dams Program 

33 Paper 8 - Fiegle 
information is imputed into the HECI model with a Type II distribution to evaluate the 

hydraulic adeq

On very large drainage basins, a unit hydrograph is developed based on existing 

stream rea with similar watershed 

haracteristics.

or more detailed information, please see the attached section on Hydrology and 

Hydraulics, pages 28 to 33 of the Georgia Safe Dams Program's Engineering Guidelines. 

The Guidelines gineers who are

involved with

Other

Because there is a need for the state dam safety regulators to provide significant 

input on practices, research needs and development needs in the field of hydrology and 

hydraulics modeling, I polled 26 states east of the Mississippi River including: Maine, 

New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, New York, 

Pennsylvania, Ohio, Michigan, Indiana, Illinois, Wisconsin, Kentucky, West Virginia, 

Tennessee, Virginia, North Carolina, South Carolina, Florida, Maryland, New Jersey, 

Mississippi, Louisiana, Arkansas, and Puerto Rico. Delaware and Alabama were not 

polled because they had no dam safety laws or programs. I received 19 state responses. 

The following questions were asked in the survey and where appropriate, the 

number of responses are tallied. In some cases, a state regulatory agency may use/accept 

more than one methodology. 

1. What Hydrologic Model(s) are used/accepted by your state? Do you accept 

more than one methodology? 

a. NRCS/SCS Curve Number/Lag Equations - 16 

b. HECI - 17 

c. HECHMS - 10 

d. USGS/State Regression Equations - 9 

e. Unit Hydrographs - 6 

f. Synder Unit Hydrograph - 5 

g. SITES - 6 

h. TR20- 5 

i. TR55 - 3 

j. HydroCAD 5.11 - 3 

k. Others - 1 

2. What are the difficulties/problems you encounter with these models? 

uacy of the dam.

data on the particular stream or one in the a

c

F

were developed in conjunction with consulting en

the design of dams in Georgia. 

states east of the Mississippi River 

Paper 8 – Fiegle 34
a. HEC1 

• DOS based program - glitches sometimes when loaded on a Windows 

• tion with the

use o

• Time distribution of rainfall losses. 

• Unit hydrograph for eastern shore needs to be modified. 

specific data (mentioned 5 times.) 

f

. dant

n

4. 

ummaries 

Operating System. 

PM card record is outdated for PMF determination in conjunc

f HMR51/52.

b. Lack of site

c. HECHMS

• Does not have the ability to run multiple storm events 

• Bizarre results sometimes. 

d. Engineers who do not know how to use the model appropriately. Identified as an 

issue by six states. 

e. Hydro CAD 5.11/SCS unit hydrograph is limited a 24 hour duration storm, 

surcharge flows are not allowed. 

O note, two states reported no difficulties.

3 Are extended flow periods (days) vs. single events an issue? Is it depen

o the size of the drainage basin or the location of the dam? 

Yes - One 

No - Ten 

Maybe - (extremely large drainage basin) - Seven 

Please rank the following issues in priority: 

a. low cost/rapid assessment of inflow flood (47 points) 

b. extensive detailed investigative assessment of the inflow flood (54 

points) 

c. risk assessment vs. PMF (34 points) 

d. determinate inflow design flood (55 points) 

e. all equally important (20 points) 

*To evaluate the responses, I assigned a point value of 5 to 1 for the priority ranking with 

5 being the highest priority and 1 being the lowest priority. 

5. Any suggestions for short and long terms hydrologic needs and 

development?

Short term hydrologic needs 

A. HECHMS needs the following: 

• level pool routing/results & s

35 Paper 8 - Fiegle 
• overtopping subroutine 

• breaching subroutine 

• multiplan feature 

nk HECRAS to HECHMS

blish

.

and other Federal agencies.

on

United States are regulated by the states. 

ydrologic s storms that will produce 

reasonably athering and interactive 

modeling. The results should be reproducible from universally accepted data sources. 

I be owing:

evelopment. Make it compatible with other 

d link it to HECRAS, ARCVIEW, and NWS DAMBREAK. 

ooperation between Federal Agencies to develop hydrologic 

dels that states can use.

comparison of various hydrologic methodologies that produce 

inflow floods including sensitivity evaluations. Make the comparison 

meaningful by region/sub region. Publish results. 

• printout of inputs 

• other routing subroutines (Muskingum-Cunge) 

• li

• add Arcview 

• develop multiple stage outlet modeling 

B. Comparison of USGS Regression inflow curves vs. other methodology. Pu

results for different parts of the country

C. Cooperation between NWS and HEC

Long-term hydrology needs 

A. Update NRCS/SCS Curve Number methodology. 

B. What is the appropriate time period of the design storm event? PMP is variable

across the country and changes within a region. 

C. Define what ancedent moisture condition is appropriate for what type of design

storm.

D. Couple NEXRAD or IFLOWS for river basins for real time hydrologic 

forecasting (Virginia). 

E. Further development of the Green-Ampt loss rate function nationwide. 

F. Additional continuous stream gaging streams. 

C clusions and Recommendations:

The majority of the dams in the

H re earch should focus on creating models for design

accurate results that do not involve extensive data g

lieve the short terms needs include the foll

• Complete H

n

ECHMS model d

software a

• Improve c

data and software mo

• Do detailed

Paper 8 – Fiegle 36
Lon te

models/modeli

• Curve Numbers and lag time routines. 

ll events.

t loss rate function nationwide. If 

as as usable as the NRCS/SCS Curve Number/soils mapping 

be doable with

accuracy in hydrologic modeling is important, so is a certain 

mea xist in

every d ublic safety.

g rm hydrologic needs include further development of certain hydrologic

ng methods. 

Update NRCS/SCS

• Better regionalization of PMP rainfa

• Define and update Antecedent Moisture Conditions. 

• Further development of the Green-Amp

this w

methodology, modeling long terms storm event would

reasonable results. 

While I believe that

sure of conservatism to account for the undefined and unknown variables that e

rainage basin when dealing with p

37 Paper 8 - Fiegle 
Hydrologic Research Needs for Dam Safety Analyses 

by 

Somerset County Dam Safety Program 

The follow eeds of the dam safety 

program of s 305 square miles in 

the central r

population den

experienced sig t

abatement expected in the foreseeable future. 

Somers

10 square miles. Eight of these 

ams serve as regional stormwater detention facilities that help offset the adverse 

ydrologic impacts of land developments within their watersheds. The remaining dam is 

former water intake dam located on property acquired from a private water purveyor 

nd converted into a County park. Since their original design, engineering activities at 

e dams include the determination and periodic reassessment of their hazard 

lassifications and Spillway Design Floods (SDFs) as well as any subsequent revisions to 

eir Emergency Action Plans (EAPs). These activities are performed in accordance with 

the Dam Safety Standards (N.J.A.C. 7:20) of the Dam Safety Section of the N.J. 

Department of Environmental Protection (NJDEP). 

In addition to the dam safety activities described above, the County Planning Board also 

regulates that stormwater management aspects of all new land developments in the 

County that front on a County road and/or drain to a County bridge or culvert. These 

regulations typically result in the design and construction of one or more onsite 

stormwater detention basins that can range up to approximately 15 feet in height and up 

to 100 acres in drainage area. While the design of such basins do not normally require a 

dam break and downstream inundation analysis, the design does include the 

determination of emergency spillway size and top of dam elevation. This is done using 

County-based emergency spillway criteria at all dams not subject to the NJDEP Dam 

Safety Standards noted above. An unofficial survey indicates that there are presently 

more than 500 such basins in County, most close to residential, commercial, and/or 

industrial structures. 

 

Joseph J. Skupien, PE, PP1

ing describes both the current scope and research n

Somerset County, New Jersey. The County encompasse

po tion of the state and has a population of approximately 300,000, yielding a 

sity of approximately 1000 people per square mile. The County has

nificant land development pressure since the 1970’s with no significan

et County’s dam safety program includes the analysis, operation, inspection, and 

maintenance of nine Significant Hazard dams ranging in height from 13 to 40 feet with 

drainage areas ranging from approximately 50 acres to

d

h

a

a

th

c

th

1

 Principal Hydraulic Engineer, Division of Engineering, Somerset County, New Jersey

39 Paper 9- Skupien 
Current State and County Practices 

As noted above, all County-owned dams are gulated by the Dam Safety Standards of

the NJDEP’s Dam Safety Section. Similar to the FERC’s Engineering Guidelines for the 

re

Evaluation of Hydropower Projects, these state standards classify dams by Hazard Class 

based upon the threat posed by a dam failure to downstream structures and roadways. A 

summary of the various NJDEP Hazard Classes is presented in Table 1 below. 

Table 1 

Summary of Dam Hazard Classes and Spillway Requirements 

NJDEP Dam Safety Standards

Hazard Class Downstream Failure Threat Spillway Design Storm

I – High Loss of Life or Extensive Property Damage PMP 

II- Significant Significant Property Damage One-Half PMP 

III – Low Minor Property Damage 100-Year NRCS Type III 

IV – Small 

Dams

Height < 15 Feet, Storage < 15 Acre-Feet, 

Drainage Area < 150 Acres* 

100-Year + 50 Percent 

NRCS Type III 

* Class IV Dam criteria only applicable if Class I or II threats do not exist. 

According to discussions with l, PMP rainfall amounts are 

typically obtained from HMR ojects, spatial distribution of 

the rainfall is not necessary and the rainfalls are temporally distributed in accordance 

with a variety of storm distributions, including the NRCS Type III distribution and the 

U.S. Army Corps of Engineers PMP distribution as contained in HEC-1 Flood 

Hydrograph Package. One foot of freeboard is required above the routed SDS water 

surface. The NRCS Runoff Equation and Dimensionless Unit Hydrograph are typically 

used to compute losses and convert the rainfall to runoff. Antecedent Moisture 

Conditions are typically assumed to be either average (AMC II) or high (AMC III). 

Sources of dam failure parameters vary, with those published in the FERC Engineering 

Guidelines or derived from the Froelich equations utilized most frequently. 

Downstream discharges and water surface profiles are typically computed using steady

flow assumptions with HEC-1 used for discharge and failure computations and either 

HEC-2 Water Surface Profiles or HEC-RAS River Analysis System used for profile 

computations. Unsteady flow computations using either DAMBRK or FLDWAV are 

used on approximately 10 percent of the projects, which typically involve large dams and 

drainage areas. Downstream failure impacts are typically assessed based upon the 

difference in water surface with and without a dam failure, with a difference greater than 

two feet considered excessive. A minority of projects utilizes the Downstream Hazard 

NJDEP Dam Safety personne

No. 51. For the majority of pr

Classification Guidelines published by the Bureau of Reclamation. 

As noted above, the Somerset County dam safety criteria, which were originally 

established in 1975, apply to all structures subject to County Planning Board approval 

unless superceded by the NJDEP regulations described above. Hazard classification for 

Paper 9 - Skupien 40
all dams is a nce of

downstream conditions warrants a higher classification. Safe spillway capacity and 

freeboard requirements are bas pillway (ESH) and

reeboard (FBH) Hydrographs through the facility based upon 24-hour rainfalls of 10 

and 17 inches, respe e equations used 

y the NRCS to develop the Service’s 6-hour ESH and FBH rainfalls. It should also be 

summary of technical and operational problems and related hydrologic research needs 

of

e

creased

nfortunately, these models and

atabases have exceeded, in many instances, the knowledge and expertise of the modelers 

ety

only

tion and/or

. A technical problem that has grown more acute in recent years involves the presence 

of concrete corewalls within older earthen dams that have come under closer scrutiny by 

dam safety personnel. Similar problems also arise with older dams that have been 

constructed with masonry or concrete walls along their upstream face. These walls were 

commonly utilized to prevent seepage from the upstream impoundment and, while they 

theoretically have little structural strength, experience has shown that some have been 

able to remain intact even after the loss of large portions of the downstream fill material. 

dequate under 

ssumed to be Class B as defined by the NRCS unless a reconnaissa

ed upon routing Emergency S

F

ctively. These rainfalls were derived from the sam

b

noted that the County’s 17 inch FBH rainfall is equivalent to one-half of the 24-hour 

PMP for the County. 

Problems and Research Needs 

A

of both the NJDEP and Somerset County dam safety programs is presented below. The 

NJDEP problems and needs were identified during interviews with representatives of the

NJDEP’s Dam Safety Section. Resolution of these problems and needs, either through 

research and/or policy decisions, will help improve the effectiveness and efficiency

both programs. 

1. Perhaps the most significant operational problem facing the State and County dam

safety programs is the lack of hydrologic and hydraulic expertise in many of th

engineers performing dam failure, inundation, and hazard classification studies and/or 

preparing emergency spillway designs. In the last 20 years, both the sophistication and 

availability of computer-based models to perform such studies and designs has in

significantly, along with the data to base them on. U

d

and designers of small dams. This “ability gap” leads to inaccurate studies and designs 

and prevents many dam safety programs from realizing their full potential, thereby 

negating the efforts of researchers and regulators to create advanced, effective dam saf

programs. Under such conditions, further technical advances can be expected to

widen this gap. 

Research Request: Investigate the feasibility of establishing minimum educa

experience requirements for engineers performing dam safety studies and designs for 

small dams. Alternately, develop a basic set of small dam hydrology and hydraulic 

courses that can be presented by state and/or local dam safety officials to engineers 

within their jurisdictions. 

2

Such dams, due to their age, typically have principal spillways that are ina

41 Paper 9- Skupien 
present standards and, . However,

odeling this failure can be difficult, particularly in the selection of failure width and 

e

Request: Develop typical failure parameters and/or policies for dams with 

corewalls an state dam

safety programs. 

putation of Con

typ nded graphs for fr cies up

. H ience sugges

sho d when comp hs from la

su F, d v

st utat w

ven h can lead to n and underd

e

e 

to address this 

ners

as such, are prone to failure due to overtopping

m

time due to the presence of the concrete wall within or upstream of the earthen fill. Whil

conservatively large width and short time parameters can be selected, their use may result 

in overestimates of hazard class and Spillway Design Storm. This, in turn, may require

the construction of excessive remedial measures that the owners, who are predominantly 

private individuals or organizations, may find cost-prohibitive. 

Research

d upstream face walls that can be utilized or adopted by local and

3. Standard com methods for drainage area Time centration (TC) are

ically inte for use in estimating flood peaks and hydro equen

to the 100-Year storm owever, both theory and exper t that shorter TCs 

uld be use uting peaks and hydrograp rger, less frequent events

ch as the PM el

andard comp

ts, whic

ue primarily to the greater flow depths and

ion methods may be yielding lower peak flo

 inaccurate hazard classificatio

ocities. As such, the 

estimates for such 

e esigned remedial

measures. 

Research Request: Investigate accuracy of standard TC computation methods under 

extreme storm conditions. If necessary, develop alternative methods or policies for 

estimating drainage area TCs for such extreme events. 

4. Since the majority of dams subject to local or state dam safety regulations have 

drainage areas less than 10 square miles, questions arise over the most appropriate 

temporal rainfall distribution to utilize in a hazard classification and/or Spillway Design

Storm analysis. These questions include concerns whether the NRCS design storm

distributions are appropriate for extreme rainfall amounts and whether other popular 

storm distributions such as the Corps of Engineers’ Standard Project Storm is appropriat

for small drainage areas. 

Research Request: Investigate suitability of popular design storm distributions for 

extreme rainfall amounts falling over small drainage areas. Develop appropriate 

distribution for such rainfalls and areas through research and/or policy. 

5. As noted above, the majority of water surface profile computations downstream of a 

dam failure are based upon steady rather than unsteady flow assumptions. In most 

instances, however, the selection of steady over unsteady flow is not based upon 

theoretical grounds but rather on the difficulty in performing the unsteady flow

computations with the DAMBRK and FLDWAV computer programs (see 1 above). Th

recent inclusion of unsteady flow in HEC-RAS offers an opportunity

problem. However, limited training has been available to date to small dam desig

and analysts and local and state dam safety personnel. 

Paper 9 - Skupien 42
Research Request: Develop guidelines and limitations for use of steady flow models i

dam safety studies. Also develop effective HEC-RAS unsteady flow and FLDWAV

training courses for small dam analysts and designers and local and state dam safety 

personnel. 

6. The most significant and, at times, prohibitive cost of an accurate hazard classification

or Spillway Design Storm study for a small dam is incurred during the development o

the hydraulic model of

n

f

the downstream waterway. Costs associated with field surveys, 

pographic mapping, and model data development often exceed the remaining study 

osts.

esearch Request: Develop inexpensive techniques to develop waterway and 

d 

umption of failure starting at the maximum water surface should be 

ade. However, instances have arisen where greater discharges are computed when the 

ilure

ter 

ects.

Acknowledgements 

State

to

c

R

floodplain topography and cross sections in a digital format that can readily be input into 

available hydraulic computer models. 

7. According to the FERC’s Engineering Guidelines, dam failures should not be assume

to begin on the rising limb of the inflow hydrograph. Instead, the presumably more 

conservative ass

m

failure is assumed to occur somewhat before the maximum water surface is achieved.

This is particularly true at certain small dams with very short (10 to 15 minute) fa

times. In these instances, the rapid enlargement of the dam breach begins while the wa

surface is still rising, leading to ultimately greater heads and resultant peak outflows due

to the failure. While these greater peak outflows can be identified through sensitivity 

analyses of failure starting times, such analyses can increase costs for small dam proj

Research Request: Determine the sensitivity of starting water surface elevation to peak

outflows from small dam failures and establish parameters to identify the most 

conservative starting water surface assumptions. 

The author gratefully acknowledges the assistance of Mr. John Ritchey and Mr. Joseph

Ruggeri of the NJDEP’s Dam Safety Section in preparing the discussions of current

of New Jersey practices, problems, and research needs. 

 

43 Paper 9- Skupien 
Research Needs in Dam Safety Analysis 

Samuel L. Hui1

ed

o areas: 

swers for

ess-level improvements to the dam safety analysis 

off

n

ng

Basin Runoff Models and Parameters 

he inflow

llenge, particularly for an un-gauged

basin. While large organizations can devote a great deal of effort in seeking out 

regional data that could be useful in the development of the appropriate basin runoff 

model, the smaller firms, wh ely, cannot afford to do so. The

easiest means, without an exhaustive research, is to use the dimensionless unit 

g Guidelines: Chapter 8 –

Guidelines for Probable Maximum Flood (PMF) Determination allows the use of the 

NRCS dimensionless unit hydrograph for un-gauged watersheds provided that: 

• there are no regional data available 

• drainage area at the dam site is 100 square miles or less and 

each of the sub-basins, if the basin is sub-divided, does not exceed 20 square 

miles in area

 

Introduction

As pointed out by Dave Goldman in his write-up of May 2, 2001 on problem definitions

for this workshop, the research needs for hydrology in dam safety analysis can be divid

into tw

1. Applied research to develop simplified techniques that can provide useful an

dam safety analysis 

2. Fundamental research to bring proc

Coming from the industrial sector, I would like to discuss two items that belong to the 

applied research category: simplifying the processes required in developing basin run

models and parameters, and providing guidance related the use of confidence limits i

flood frequency analyses. On the fundamental research side, I would like to have the 

participants to consider the urgent need to improve the analytical techniques in simulati

dam break mud-wave propagation resulting from the failure of tailings dams. 

Applied Research Needs 

The selection of a proper basin runoff model for use in the development of t

design flood (IDF) hydrograph is always a cha

ich bid the job competitiv

hydrograph suggested by the Natural Resources Conservation Service (NRCS) as 

currently provided in HEC-HMS. In fact, the soon-to-be-released draft of the Federal 

Energy Regulatory Commission (FERC) Engineerin

1

 Bechtel Corporation, San Francisco 

45 Paper 10- Hui 
We know that the NRCS dimensionless unit hydrograph is applicable for “average” 

basin conditions. Its applicability to steep terrain without proper adjustments is 

problematic and could lead to under-estim tion of the flood peak discharge. For 

dams in a high hazard category, a drainage area of 100 square miles is a sizable area. 

The failure of such dams could be devast the community and inhabitants 

downstream. 

For the practitioners who use the NRCS dimensionless unit hydrograph “liberally” for

flood analyses, they need to have easy access to data on regional basin runoff 

parameters. I do know many, if not all, of the districts of the U.S. Army Corps of 

Engineers (USACE) have unit hydrographs for use in flood studies for watersheds in 

their respective districts; likewise, the U.S. Bureau of Reclamation (USBR), 

Tennessee Valley Authority (TVA) and some of the districts of the U.S. Geological 

Survey (USGS) as well as the NRCS. FERC also has regional basin runoff data from 

dam owners submitted through Part 12 of the Federal Dam Safety Program and so do 

many states. Major utilities, such as Pacific Gas & Electric (PG&E) Company, 

Southern California Edison Company, Southern Services, New York Power 

Authority, etc. have their own regional basin runoff model and parameters. The 

compilation and publication of these data in one place

a

ating to

 will go a long way in 

improving dam safety analyses in this country. 

Many of you may remember a publication by the USACE back in the 1950(?), which 

provides the Snyder coeffi the river basins in this

country. I envision that similar efforts will be required. But, this effort will be more 

comprehensive since it would involve many federal and state government agencies, 

major utilities and deal with a host of different basin runoff models and parameters. 

Flood Frequency Analysis 

As risk assessment (RA) is gaining more support from the engineering community as 

the preferred approach for dam design, a frequently asked question is how we can 

accurately predict the exceedance probability of large floods, such as the 1,000-year 

flood event. From my own perspective, before we address the problems related to 

exceedance probability of large floods, we need to first address a fundamental issue 

related to frequency analysis: the uncertainty intrinsic to the frequency analysis. 

We all recognize that a specific record of flood peak discharges at a site provides for 

a limited random sample of the underlying population of the flood peak discharges at 

that location. The statistics developed from this limited sample reflect only those of 

the sample and are only approximations of those of the population. There is a good 

chance that the developed flood peak discharges would either over- or under-estimate 

the “true” flood statistics of the population. Confidence bands are, therefore, 

constructed to bracket, with some degrees of certainty, the “true” population statistics. 

The flood peak discharge commonly referred to is the expected probability value. 

cients Ct and Cp for many of

Paper 10 - Hui 46
Looking at the f Frequency

Analysis (HEC-FFA) published by the USACE Hydrologic Engineering Center, for a 

500-year flood, the 5% upper bou ence limits are higher than the 

respective expected probability values by 32% to 104%. One would expect that the 

spread between the upper bounds of the confidence limits and their respective 

obability values would grow even larger for more infrequent floods. The 

logical question asked is how we should incorporate these uncertainties in the 

lletin No. 17 B - Guidelines for Determining Flood Flow Frequency states 

“Application of confidence limits in reaching water resources planning decision 

practitioners who want to incorporate the confidence limits in 

within many of the government agencies. They should be made available more 

Fundamental Research Needs 

s for Mud-Wave Propagation in Rivers 

neglected is that regarding tailings dams

because they are usually located in remote areas with scarce population or in 

y

nt of

ot

agation of the mud-waves resulting from the 

failure of tailings d

f we are to use risk assessment and/or the “incremental hazard 

evaluation” to determine inflow design floods for the design of dams, including 

tailings dams, we need to develop analytical approaches or computer codes that 

would allow the simulation of mud-wave propagation in a river channel. I do not 

know of any such methods or computer codes. The Dam Break Model marketed 

our test cases given in the User’s Manual of Flood

nds of the confid

expected pr

development of an inflow design flood and in the ensuing risk assessment. No 

guidance statement is given in this HEC-FFA User’s Manual. The only statement 

found in Bu

depends upon the needs of the users.” This is by no means adequate as guidance for 

the less-sophisticated

the design or risk assessment. I am sure that there are guidance documents available 

readily to the hydrologic engineering community. A standardized approach in 

incorporating the confidence limits in frequency analysis will certainly help the 

practitioners in the industrial sector, who conduct dam safety analyses. 

Dam Break Analysi

One of the dam safety issues often

developing countries, which may have different regulatory requirements. The 

way tailings dams are constructed (using highly erodible sand fractions recovered

from the tailings); their heights, large storage capacities and locations (in man

seismic active areas of the world) put them directly in the high hazard category. If

my memory serves me correctly, it was the failure of a tailings dam in Buffalo 

Creek back in 1968 (?) that triggered the implementation of the dam safety

program in the United States. However, historically, we seldom treat tailings 

dams with the urgency and vigorous regulatory requirements demanded of water 

impoundment dams. I am not aware of any requirements for the developme

Emergency Action Plan (EAP) for river reaches downstream of tailings dams, like

those required for water impoundment dams. I surmise that, maybe, we are n

quite sure how to determine the prop

ams. 

Specifically, i

47 Paper 10- Hui 
by BOSS International has an option for considering mud-wave propagation usin

Bingham Plastic Flow Theory, but it does not appear to work satisfactorily. S

we are building tailings dams higher and higher, we must have the necessary 

analytical tools that enable us to address the safety issues related to this type of

dam, particularly how they could affect the downstream community and 

inhabitants

g

ince

when they fail. (Bechtel has recently completed, in high Chilean 

Andes, a 120-m high tailings starter dam, which will be built-out to an ultimate 

as well

Clo

I ha r, the 

app y in dam safety analysis. They are:

• 

• 

• 

I hope that we can give these topics serious considerations and include them in the list of 

rese nalysis to be formulated by this Workshop. 

height of 175 m high in 8 years, and retain more than 60 million cubic meters of

tailings consisting of fine soil particles and slime.)

The development of such computer codes is, by no means, an easy undertaking. 

It will involve hydrologic and hydraulic engineers, geotechnical engineers

as mining process engineers. But it is a task we must do in order to improve our 

state-of-the-art knowledge in dam safety analysis. 

sing Remarks

ve stated, from my own perspective as a practitioner from the industrial secto

lied and fundamental research needs for hydrolog

the compilation and publication of regional basin runoff models and parameters 

the provision of guidance in incorporating confidence limits in flood frequency

analysis 

the development of computer codes for the simulation of dam break mud-wave 

propagation in river channel resulting from breaching of tailings dams

arch needs for dam safety a

Paper 10 - Hui 48
Hydrologic Analyses Related to Dam Safety 

By 

Anand Prakash, Ph. D., P. E.1

Abstract 

drologic design of dams involves consideration of risks and costs associated wit

ure due to hydrologic factors. This paper presents a procedure to characterize da

the basis of hydrologic risk associated with potential failure. Methods are presented

mate economic risks associated with different design-basis floods lower than the 

bable maximum flood (PMF). Practical difficulties in estimating appropriate 

ameters to be used for the determination of the PMF and to assess the appropriateness 

he estimated PMF are described. A fuzzy set approach is presented, which includes 

sideration of both tangible and intangible risk factors in determining risk-based design

is flood for a dam. Research needs related to risk analysis for hydrologic safety o

s are identified. 

Introduction

Hy h dam

fail ms

on to

esti

pro

par

of t

con

bas f

dam

n most cases, hydrologic safety of dams is assessed in terms of their capability to safely 

pass the probable maximum flood (PMF) or a fraction thereof depending on the size and 

haz F is defined as a hypothetical flood that can be 

expected from the most severe combination of critical meteorologic and hydrologic 

conditi m the probable 

ma mum precipitation (PMP) using an event-based rainfall-runoff model. The PMP is, 

theoret y

possibl me

of the y

From t

three c

(i) 

uccessive storm events less severe than the PMP must be 

the hydrologic analysis. 

(ii) ted

I

ard potential of the dam. The PM

ons that are reasonably possible in a region. It is derived fro

xi

ically, the greatest depth of precipitation for a given duration that is physicall

e over a given size storm area at a particular geographic location at a certain ti

ear.

Hydrologic Categorization of Dams 

he standpoint of hydrologic safety and risk analysis, dams may be divided into 

ategories (ASCE, 1988). 

Category 1 includes dams where potential loss of life, economic loss, and social 

and environmental damages resulting from failure are unacceptable. For these 

dams, PMF is the design-basis flood (DBF) and a detailed risk analysis, other 

than hydrologic analysis to estimate the PMF, may not be necessary. Of course,

the potential for overtopping and severe scour or erosion due to a long-term PMF

event or more than one s

evaluated as part of 

Category 2 includes dams where the DBF is PMF unless it can be demonstra

that a smaller DBF results in total costs (including failure consequences) lower 

rporation, One Continental Towers, 1701 Golf Rd., Suite 1000 

eadows, Illinois, 60008 

1 URS Co

 Rolling M

49 Paper 11- Prakash 
than the PMF design. These dams require detailed risk analysis (USACE, 1988; 

Prakash 1992). 

Category 3 includes dams where failure is not expected to result in loss of life 

and significant economic, social, and environmental damages. For such dams, 

regulatory guidelines prescribed by the respective state and federal agenc

be adopted. However, the owner/operator must be made aware of the risk of

failure during the life of the project. This can be done using the risk probability

equation: 

(iii) 

ies may 

R = 1 – (1- P)n

 (1) 

in whic

occur d

severe d include those where dam

failure is likely due to overtopping or erosion and scour on the downstream toe. 

Risk-Based Economic Analysis

vent. The intangible factors may include potential community disruption, population at 

e

from ration of the dam. The economic factors can be evaluated using hydrologic 

crit gory 2 dams

ay be conducted using one of the following approaches: 

ted with

servoir outflows of different probability densities or frequencies are weighted by the 

respective frequencies up to the exceedance probability of the DBF. Economic 

consequences of dam failure are weighted by the frequencies of floods between the DBF 

and PMF. The sum of these two gives the expected annual damage or costs associated 

with the DBF, which is less than PMF. This approach camouflages the enormity and 

catastrophic nature of damages resulting from extreme flood events, including dam

failure, by weighting them with the respective low exceedance probabilities. However, 

the consequences of such a single event may be so severe that the owners and other 

impacted parties may not be able to recover from the catastrophe. This most relevant and 

significant event for them is de-emphasized in this approach. 

(ii) Indemnification Cost Approach- This approach provides for the establishment of an 

escrow account sufficient to compensate for monetary damages resulting from dam

failure during the life of the dam, which is designed for a DBF less than the PMF. 

Variations of this approach include considerations such as the owner opting or not opting 

to indemnify himself against the loss of his investments and escalation in economic 

consequences of dam failure with time (ASCE, 1988; Prakash, 1992; Prakash, 1990). 

h R = probability that one or more events of exceedance probability, P, would 

uring n years of project life. The exceedance probability, P, refers to events more

than the DBF. Such severe hydrologic events shoul

Determination of the DBF for Category 2 dams must include economic considerations 

and intangible factors associated with the risk of failure during the PMF or equivalent 

e

risk, political and institutional consequences of potential dam failure, loss of capital 

invested in dam construction, and loss of benefits which the community would deriv

safe ope

and economic analyses. The intangible factors have to be evaluated using subjective

eria. Hydrologic and economic analyses to determine the DBF for Cate

m

(i) Expected Value Approach (NRC, 1988)- In this approach damages associa

re

Paper 11 - Prakash 50
(iii) Modified Expected Cost Approach- Th proach considers the probabilities of 

specific economic damages o n the absolute 

probabilities in evaluating the expected cost of dam failure. 

he aforementioned economic and intangible factors may be included in a rectangular 

phi

and

e

Therefore, it may be desirable to include indemnification or 

surance costs against potential failure consequences in risk-based designs and analyses. 

Other difficulties associated with risk s include uncertainties and 

bjectivities in the estimation of the PMF peak and volume, probability of PMF and 

., Creager, Crippen, Matthai, Myer/Jarvis equations, and previous estimates 

of PMF peaks for other facilities in the region. A second set of alternative methods 

includes estimates based o ows and those estimated 

y tree-ring and paleoflood analyses. Usually, the range of values estimated by various 

Prakash, 1994 ). A common difficulty with the estimates 

ased on statistical, tree-ring, or paleoflood analyses is that these estimates are often 

modelin

Possible

• The

por

• The orm event of shorter duration. 

he

dis

• The historic flood may reflect the response of a storm cell moving upgradient through 

reflect drier antecedent moisture conditions. 

is ap

ccurring within the life of the dam rather tha

T

matrix including subjective relative scores of all evaluation factors for each alternative 

DBF as its elements and a column vector including weights for different evaluation 

factors. The product of this rectangular matrix and column vector gives the weighted 

score or rank of each alternative DBF. The decision making process may involve del

and fuzzy-set approaches to assign due scores and weights to all relevant tangible

intangible factors. 

Hydrologic designs based on industry or agency standards provide an implicit protection 

against liability for failure consequences. Such liability protection may not be availabl

in risk-based designs.

in

analysis for dam

su

other extreme flood events, and rates development and sizes of potential breaches used

for risk analysis. 

Reasonableness of PMF Estimates 

Generally, the reasonableness of the estimated PMF is assessed by comparing the PMF 

peak with peak flows estimated by other alternative methods. Alternative methods to 

estimate peak flows approaching the PMF peak include those based on enveloping 

equations, e.g

n statistical analysis of reported peak fl

b

enveloping equations and previously estimated PMF peaks is fairly wide so that only a

ballpark assessment is possible (

b

found to be significantly lower than those obtained by event-based rainfall-runoff 

g, particularly for large watersheds comprised of several sub-watersheds. 

reasons for this under-estimation include the following: 

historic flood may reflect the response of a severe storm event covering only a 

tion of the watershed.

historic flood may reflect the response of a st

• T historic flood may reflect the response of a severe storm of a different temporal 

tribution of precipitation.

the watershed. 

• The historic flood may

51 Paper 11- Prakash 
• The

relatively large. The reported standard errors associated with the USGS regional 

eg

ind

ran

In some

high peak flows resulting from catastrophic events such as dam-break or ice-jam break, 

c. In these cases, treatment of outliers becomes a problem. Peak flows caused by such 

may not be relevant to statistical analysis of natural floods, which must

e caused by events such as high precipitation or rain with snowmelt. Usually, storm 

e

ays

roduce peak flows equal to the T-year peak flows estimated by statistical, tree-ring, or 

paleoflood analyses. 

ets

Duration of PMP 

hed of

rs

in

d streams in Maryland 

oncluded that a 24-hour storm duration may be appropriate for drainage areas in the 

imes 

cting

on

band of uncertainty on the reported flood peaks used in these analyses may be 

r ression equations based on statistical analysis of reported peak flows provide an

ication of the magnitude of potential uncertainties. The reported standard errors

ge from 15% to 135%, most values being above 25% (USGS, 1993). 

 exceptional cases, statistical data for peak flows may include extraordinarily 

et

abnormal events

b

runoff hydrographs are developed assuming uniform distribution of precipitation on th

respective sub-watersheds or contributing drainage areas (CDAs). In most, cases, the 

depth, duration, areal coverage, and temporal distribution of the storms associated with

historic peak flows are not known. The result is that T-year storm events do not alw

p

Parameters for PMF Estimation

At present, there are no consistent methods to estimate the parameters used to develop the

PMF hydrograph. There are numerous methods to develop sequences of incremental

precipitation for PMP and to estimate basin lag times and loss rates. Use of different s

of methods and assumptions often results in widely different estimates of PMF peak and 

runoff volume (Prakash, 1986). Future applied research should focus on developing 

guidelines such that consistent estimates are obtained by different hydrologists. 

The determination of storm duration appropriate for a given watershed is mostly 

subjective. For instance, design storm duration used for the Pacoima Dam waters

28.2 sq miles and Lake Wholford watershed of 8 sq miles in California was 72 hours; 48 

hours for a drainage area of 807 sq miles for Lyman Dam watershed in Arizona; 48 hou

for a drainage area of 206.6 sq miles for Jiguey Dam watershed in the Dominican 

Republic; 24 hours for a drainage area of 184 sq miles for Harriman Dam watershed

Massachusetts; and 6 hours for a drainage area of 54 sq miles for Sutherland Dam

watershed in California. 

An analysis of the annual maximum discharge records of selecte

c

range of 2 to 50 sq miles ( Levy and McCuen, 1999). Similar studies need to be 

conducted for watersheds of different sizes in other regions. At the preset time, the t

of concentrations of different sub-watersheds along with travel times through intera

channels are used for a preliminary estimate of the design storm duration. But, there are 

wide variations in the estimation of the times of concentration and travel times based

different empirical equations (Prakash, 1996). 

Paper 11 - Prakash 52
Parameter Estimation Based on Joint Probabilities 

One approach to assess the reasonableness of the events or data used to develop the PMF 

ydrograph using an event-based rainfall-runoff model and route it through the reservoir 

tion,

) P(C) P(D) (2) 

tion;

0-5 to 

he expected probability of about 10-6 to10-8 for the PMF. If all the

ctors are appropriately considered, the probability of the estimated PMF may be 

unreasonably low. The ab ed if probabilities of

ther events, such as times of concentration and travel times are considered. Evidently, 

and

ts

I

inflow flood events experienced by the Saylorville Reservoir on the Des Moines River 

o se

g

sugg ore successive flood events and long-duration floods 

s

appears logical to select these successive or long-duration flood events in such a way that 

t

h

includes estimation of the joint probability of combined events. The estimated joint 

probability is compared with the heuristic probability of the PMF. Relevant events may

include antecedent moisture condition, loss rate, PMP depth and its temporal distribu

and wind wave activity (NRC, 1985; Prakash, 1983). For the sake of simplicity, it is 

assumed that these events are independent of one another. Thus, 

P(PMF) = P(A) P(B

in which P(PMF) = combined or joint probability of PMF; P(A) = probability of the 

occurrence of selected antecedent moisture condition; P(B) = probability of the 

occurrence of selected loss rate; P(C) = probability of PMP and its temporal distribu

and P(D) = probability of selected wind wave activity. The probability of PMP itself,

P(C), is reported to vary form 10-12

 to 10-19 in the eastern United States and from 1

10-14

 in the western United States (ASCE, 1988). With probabilities of less than 1.0 

assigned to the other three events, the combined probability of the PMF may approach 

much lower than t

fa

ove probability may be further modifi

o

additional research is needed in the probability aspects of PMF determination. 

PMF Peak and Volume 

The statistical, tree-ring, and paleoflood analyses and estimates based on enveloping

regression equations provide estimates of peak flows. To evaluate the hydrologic safety 

of dams, estimation of the PMF hydrograph is extremely important. Methods and 

guidelines are needed to assess the reasonableness of PMF hydrograph as also its peak.

Some times, a hydrograph with a slightly lower peak but much larger runoff volume may 

be more critical. In such cases, the aforementioned tests of the reasonableness of PMF

peak may not be adequate. 

Long-Duration or Successive Flood Even

t is reported that the Upper Mississippi River Basin flood of 1993 included five major 

ver a period of five months. Such successive or long-duration flood events may cau

reater scour and erosion downstream of the spillway than a single flood event. This 

ests that the effect of two or m

hould also be evaluated, particularly on potential downstream scour and erosion. It 

heir probability of occurrence is commensurate with that of the PMF. 

53 Paper 11- Prakash 
If tw

cons

floo

even robability

at three 50-year flood events would occur in a period of three months, would be about 

f

etermined. Available information includes PMP durations of only 

p to 4 days or so.

Conclusion and Research Needs

ve. 

g all

levant factors with due weightage assigned to each based on input from the interested 

or impacted parties. 

esigns

• Identification of methods to identify categories of dams where risk-based 

y

rs,

o or three independent flood events within a period of three months or so are to be

idered, then their probability of occurrence should be about 10-6 or so. Assuming the

d events in a region to be a Poisson process, the probability that three 100-year flood

ts would occur in a period of three months, would be about 3 x 10-9; the p

th

2.1x 10-8; and the probability that three 25-year flood events would occur in a period o

three months, would be about 1.67x 10-7. This suggests that three 25-year floods in a 3-

month period may have a probability equivalent to that of the PMF. One or three 25-year

floods may or may not cause scour and erosion comparable to a single PMF event. Also,

additional research is needed to estimate scour and erosion for different types of rock, 

concrete, vegetation, and soil protection due to long-term exposure to high velocities or 

exposure to successively occurring high velocities several times during a given period.

Determination of a long-duration PMF requires estimation of the long-duration PMP. 

Methods to determine one-month or 5-month duration storm events equivalent to the 

PMP have yet to be d

u

Determination of a reasonably safe DBF for a dam should be based on economic and 

several intangible factors associated with the consequences of dam failure. Economic

consequences may be estimated using analytical approaches outlined in this paper. 

Evaluation of the relative significance of all tangible and intangible factors is subjecti

A combination of the delphi and fuzzy-set approaches may be useful in considerin

re

The methods and assumptions used for hydrologic analyses related to risk-based d

of dams are inherently subjective. As a result, different hydrologists may obtain widely 

different results from risk analysis for one and the same dam. Research efforts must 

focus on reducing this inherent subjectivity so that results of risk analyses performed by 

different hydrologists are as close to one another as practicable. This requires 

development of guidelines and methods for each specific computational element of risk

analysis. Research needs relevant to this goal are indicated below: 

hydrologic design is acceptable and preferable to that based on industry or agenc

standards related to size and hazard classification. It must be recognized that 

designs based on standards imply a certain degree of protection to designe

operators, and owners against liability for failure consequences. 

• Identification of consistent methods and assumptions related to PMF 

determination including magnitude, duration, and sequencing of PMP and 

Paper 11 - Prakash 54
estimation of lag times and loss rate parameters for different types and sizes of 

watershed

• Development of consistent methods for estimation of the probability of PMF and 

.g.,

ity combined with a 

severe storm event; successive storm events which may cause erosional damages 

than PMF, etc. 

• Development of methods to estimate long-duration storm events equivalent to 

tes of

l

rministic and Probabilistic Perspectives of the Probable 

Maximum Flood, Proceedings of the Hydraulics Division Conference, ASCE, 

Cambridge, Massa

Prakash, A., 1986, Current State of Hydrologic Modeling and Its Limitations, 

,

g

oint

s.

similar extreme flood events. 

• Identification of methods to estimate the reasonableness of the estimated PMF 

peak and volume so that consistent estimates are obtained by different 

hydrologists. 

• Development of consistent methods to estimate combined events equivalent to the 

PMF in probability of exceedance and magnitudes of peak flow and volume , e

severe rainfall combined with snowmelt; wind wave activ

similar to or more severe the

PMP and PMF, which may generate larger volumes of reservoir inflows. 

• Identification of methods to select plausible modes of dam failure and ra

development and sizes of breaches for different field conditions. 

• Identification of methods to incorporate both economic (i.e., tangible) and 

intangible consequences of dam operation and potential dam failure in risk

analysis and evaluation of dam safety. 

• Development of methods to include indemnification or insurance costs to cover

dam failure consequences in risk analysis. 

• Development of methods to arrive at consensus on how to account for potentia

loss of life associated with dam operation or failure in risk-based hydrologic 

design of dams. 

References

♦ American Society of Civil Engineers (ASCE), 1988, Evaluation Procedures for 

Hydrologic Safety of Dams, New York, NY. 

♦ Levy, B. and McCuen, R., 1999, Assessment of Storm Duration for Hydrologic

Design, Journal of Hydrologic Engineering, ASCE, Vol. 4, No. 3, July 1999. 

♦ National Research Council (NRC), 1985, Safety of Dams, Flood and Earthquake 

Criteria, National Academy Press, Washington, D.C. 

♦ Prakash, A., 1983, Dete

chuttes, August 1983.

♦

Proceedings of the International Symposium on Flood Frequency and Risk Analysis

14-17 May 1986, Louisiana State University, Baton Rouge, LA, D. Reidel Publishin

Company, Dordrecht, Holland. 

♦ Prakash, A., 1990, Estimation of Expected Damages, Indemnification Costs, and J

Probabilities of Dam Failures, Proceedings of International Symposium, ASCE, New

York, N.Y, July 30-August 3, 1990. 

♦ Prakash, A., 1992, Design-Basis Flood for Rehabilitation of Existing Dams, Journal 

of Hydraulic Engineering, ASCE, Vol. 118, No. 2, February 1992.

55 Paper 11- Prakash 
♦ Prakash, A., 1994, Probable Maximum Flood Estimation for New and Existing Dams, 

Proceedings of the National Conference on Hydraulic Engineering, ASCE, Buffalo, 

N.Y., August 1-5, 1994. 

♦ U. S. Geological Survey (USGS), 1993, Nationwide Summary of U.S. Geological 

Survey Regional Regression Equations for Estimating Magnitude and Frequency of 

Floods for Ungaged Sites, Water-Resources Investigations Report 94-4002. 

Paper 11 - Prakash 56
Hydrology for Dam Safety – Private Sector 

By 

Althou s 

throughout the world, the concept of hydrology for dam safety for owners of existing 

dam h

of Cali

1929, i

guarant

Baldwi ff-stream dams to be under the 

juri c

inspect

Pacific at that time for

the g

Divisio

Inspect were evaluated and investigated.

Hyd l

At t

Energy ires

the inspection and review of safety of non-federal dams by an Independent Consultant. 

Order 315 was later superseded by FERC Order 122 which expanded the safety 

evaluation of dams under FERC jurisd

ial at

a da

Cal ure 

s 

nes

The pothetical floods requires the determination 

ds to

inte

hyd

infiltration, base flow, channel and reservoir routing. Routing involves the investigation 

safe

Cat Cecilio, P.E., P.H.1

INTRODUCTION 

gh dams have been designed and built by private engineering consultant

s ave never really been addressed until perhaps in the late 1960s. Although the state 

fornia has an existing dam safety program implemented by the Legislature in 

ts function was more of issuance of certificate of operations to dam owners to 

ee the safe operations of the reservoirs. However, in 1966 after the failure of 

n Hills Dam, the law was revised to include o

sdi tion of the Division of Safety of Dams. At the same time a program of dam

ion and review of design criteria was implemented. 

Gas and Electric Company (PG&E) which owned some 200 dams

lar est private dam owner in the country began discussing with the California 

n of Safety of Dams in 1967 to implement the new dam safety program. 

ion programs and criteria for dam safety

ro ogic criteria were one of them.

tha same time also, the Federal Power Commission (now known as the Federal 

Regulatory Commission) Order No. 315 was passed in 1969. The Order requ

FPC

iction

.

This paper will give an overview of existing methods in developing the flood potent

m, especially the methods employed in PG&E from 1968 to comply with the 

requirements for dam safety by the Federal Energy Regulatory Commission and the

ifornia Division of Safety of Dams. Advantages and disadvantages of each proced

together with their inherent problems or limitations will be mentioned. Research need

for some items are discussed throughout the paper. A comparison and a set of guideli

to determine reasonable values are also described. 

HYDROLOGIC CONSIDERATIONS 

design and evaluation of dams against hy

of the hydrologic response (infiltration, base, flow, routing and runoff) of watershe

nse rainfall or snowmelt, verification based on historical storm and runoff data (flood 

rograph analysis). It also requires the determination of the hypothetical storms, 

of the adequacy of existing structures above and below the dam under investigation to 

ly pass the hypothetical floods.

 &E-Retired, Consulting Engineer 1 PG

57 Paper 12 - Cecilio 
Rainfall-Runoff Relations 

h drainage basin requires a separate study to determine the rainfall-runoff relation of

basin. There is often gre

Eac

that at variation of rainfall-runoff relations between neighboring 

read

reg

Hydrologic Response 

The hydrologic response of the watershed to precipitation is usually determined and 

verified from historical flood records. In 1969, PG&E embarked into a major project of 

“reconstituting” historical floods to determine the hydrologic response to extreme 

rainfall. Reconstitution of historical large floods substantiate the use of the runoff model 

to estimate extreme floods up to the probable maximum flood (PMF). PG&E obtained 

through the California Division of Safety of Dams (DSOD) a newly developed computer 

program by Leo Beard of the Hydrologic Engineering Center called “Unit Hydrograph 

and Loss Rate Optimization.” The first few studies were limited to small drainage basins 

(35 sq mi) and the results were shared with DSOD. Later when the program was 

incorporated into the new HEC-1, further studies included unimpaired flows from larger 

drainage areas. Figures 1 to 4 show four examples of reconstituted flood from major 

historical floods using the computer program, HEC-1, “Flood Hydrograph Package.” (1) 

Between 300 to 500 floods from 1938 to 1986 in California Sierra Nevada slopes were 

reconstituted with HEC-1. 

Development of Design Flood 

The development of a design flood from the selected design storm involves several tools 

of meteorology, hydrology and hydrologic engineering. Since the workshop is composed 

of experts in hydrology, the detailed process of development will not be discussed 

anymore. 

HYPOTHETICAL STORMS 

It is the general practice to obtain design flood hydrographs from design storms. Two 

general types of design storms are used: (a) the frequency-based storm, and the (b) 

probable maximum precipitation, PMP. 

Frequency-Based Storm 

This design storm is defined as the depth of rainfall with certain return interval in years 

ranging from 2 to 100 years. The depths are obtained from generalized studies prepared 

by the National Service such as those in references 2 and 3. 

To comply with California DSOD criteria, PG&E uses the frequency-based design storm

derived in accordance with the method introduced by James D. Goodridge in 1969 and 

later published as Bulletin No. 195. (3) The method estimates the frequency-based 

storms for a specific precipitation station up to the 1,000-year event using the Pearson 

Type III probability distribution. The bulletin even provides estimates of statistical PMP 

for each rain station based on the concept introduced by David Hershfield of the former 

U.S. Weather Bureau. In some cases, when the hazards downstream are high, DSOD 

extrapolates storms beyond the 1,000-year event. 

drainage areas. This variation may be caused by physical characteristics which are not

ily apparent, such as snow accumulation in high elevations bands in mountainous 

ions similar to the Sierra Nevada in California. 

Paper 12 - Cecilio 58
Probable Maximum Precipitation 

All of PG&E dams have been evaluated against the probable maximum precipitation 

derived in accordance with the U drometeorological Report 

(HMR) No. 36. (4) Recently, a few dams have been evaluated against the new National 

Weather Service (NWS), Hydrome o. 58 and No. 59. (5,6). The

E

ar

y through September) appear 

create problems for reservoirs with small drainage areas. The estimates appear to 

f a PMP.

500

ort duration rainfall that it produces. The 

understorm has a duration of six hours.

loss

the size or subdivision of the basin, the 

sence or absence of snow, antecedent or subsequent storms, and channel routing 

area which is tributary to the river 

cation in question. This requires separate storm centerings (areas) for each location 

where floods are to be co

ine 

. S. Weather Bureau, Hy

teorological Report N

PMPs from these publications have durations up to 72 hours. 

In HMR 36, estimates of the PMP are prepared for the months of October, November, 

December, January-February, March and April. For the small drainage areas of PG&

reservoirs, the October PMP controls. For larger drainage areas, the November PMP 

provides the controlling flood. 

In HMR 58 & 59, estimates of the PMP are prepared for the 12 months of the calend

year. Recent estimates for the summer and fall months (Ma

to

provide higher rainfall and thus would require revised operating rule curves for these 

reservoirs. This is where, the writer feel that additional research needs to be done.

An additional PMP estimate is done using what is called the thunderstorm type o

This type of storm is very critical for all reservoirs with small drainage areas (up to

sq mi) because of the high intensity, sh

th

FACTORS THAT AFFECT FGLOOD ESTIMATES 

There are several factors which affect the determination of hypothetical floods up to the 

PMF. The major ones are the areal and temporal distribution of the design storm, the

rates or infiltration rates, the unit hydrograph,

pre

methods. 

Areal Distribution 

The storm area used for the point-to-area adjustment is critical. This adjustment 

compensates for the fact that storm covering larger areas will have lower average 

intensities. Such a relationship between storm area and average intensity can be 

determined for different frequency storms in a region, using historical precipitation 

statistics. Thus, the storm area used should be that

lo

mputed. 

For areas where an elliptical pattern of design storm is given, such as in HMR 52, the 

contribution for each sub-basin in a stream system is easily estimated because they are 

already areally reduced. However, in orographic areas such as in HMR 36 or the new

HMR 57 & 58, there are no standard techniques to distribute the PMP among sub-basins 

in large drainage areas. PG&E uses the isohyets of a major storm in the area to determ

the areal distribution of the PMP. 

59 Paper 12 - Cecilio 
Temporal Distribution 

To compute the flood hydrograph for a given design storm, it is necessary to specify the 

time sequence of the precipitation. These increments should be arranged in a sequence 

that will result in a reasonably critical flood hydrograph. Ideally, the design storm

sequence should be modeled after historically observed storms if such storms show tha

major storm rainfalls have a predominant pattern. T

t

herefore, the hydrologist is left to 

dopt his own distribution pattern. This is where significant differences in answers of 

peak flows will occur. 

uce

e

d is the “initial and uniform” technique which has a single 

il moisture (and interception) deficit which must be satisfied before runoff can occur. 

Once this initial deficit is sat nfall intensities which 

ce)

ber technique was developed for determining total runoff volume from total 

infall volume; but, the technique has been widely used for incremental rainfall 

infiltration as well. It has rec its simplicity and because 

he

The variable loss rate function i ngineers (L = KPE

) and is 

a

HMR 36 (4) shows five acceptable time sequence of storms. These time sequences are 

also adopted for HMR 57 & 58. If all of these sequences are tried, one would prod

the most severe flood hydrograph, but not necessarily the most reasonable. Engineers 

would have a tendency to adopt the most conservative; meaning, one with the highest 

peak flow. However, this conservative value might not be the most reasonable because it

could be the result of compounding too many low probability events. Figure 5 shows th

time sequences recommended in HMR 36, 57 and 58. 

Loss Rates or Infiltration Rates 

The many methods that have been proposed to account for rainfall losses are also the 

causes of the variations of estimates of the probable maximum floods. These methods 

range from the simple empirical to complex conceptual models of the surface and soil 

system. The simplest metho

so

isfied, runoff will occur for all rai

exceed a given “uniform” infiltration rate. 

The U. S. Soil conservation Service’s (now National Resources Conservation Servi

Curve Num

ra

eived much attention because of

the curve number (infiltration rate) can be obtained from readily measurable geographic

characteristics (soil type and land use). The “curve number” computations neglect t

effects of rainfall intensity. 

ntroduced by the Corps of E

part of the computer program HEC-1 is used by DSOD and PG&E in all their studies. 

Unit Hydrograph 

The most common method of transforming rainfall excess to runoff is the unit 

hydrograph technique. This technique is the biggest cause of differences in PMF 

determination because there are several methods of deriving a unit hydrograph for a

basin. When there is no measured streamflow hydrograph of a major flood at the study

basin from which a unit hydrograph can be derived, the hydrologist resorts to synthetic 

unit hydrograph. 

Paper 12 - Cecilio 60
The most common synthet t hydrograph method (7), 

less

r

. As a matter of fact, there

re instances when the triangular method will estimate a flood twice that derived by the 

velop unit hydrographs for the small basins of their 

servoirs. With the aid of the computer program HEC-1, the Clark’s unit hydrograph 

est

ile. 

iver

estern slopes of California where PG&E have 

servoirs has a plot of this unit hydrograph. The author determined that the variability of 

runoff within eac hydrographs.

r

all watersheds in California where these were performed also. 

it is

s

he presence of snow on the ground will determine whether there should be larger runoff 

e

” from the North Pacific Division of the U. S. Army 

orps of Engineers has been a very useful reference. However, the publication was 

issued in June 30, 1956. It appears that it needs some kind of an update based on current 

information on snow hydrology in the western region. 

ic methods are the SCS triangular uni

Clark’s method (8), Snyder’s method (9), and the Bureau of Reclamation’s dimension

graph-lag method (10). All of these unit hydrograph techniques are defended by thei

practitioners as acceptable. However, if the unit hydrograph is not calibrated or verified 

against a major flood event, the synthetic method will overestimate (or even 

underestimate in some cases) the PMF peak flow considerably

a

calibrated Clark'’ method. 

As mentioned earlier, PG&E began the studies of some 300-500 major floods in 

California. This was done to de

re

coefficients of TC and R were calibrated and related to the basin characteristics of 

LLca/S1⁄2 where: L is the length of the longest watercourse from the point of interest to the

watershed divide in miles; Lca is the length of the watercourse from the point of inter

to the intersection of the perpendicular from the centroid of the basin to the stream

alignment in miles and S is the overall slope of the longest watercourse in foot per m

The ratio of R/(TC + R) was assumed as the average for a particular rivershed or 

watershed so that a specific R can be obtained for a sub-basin using this relationship in 

the basin under study. Figure 6 shows a typical plot of Clark’s TC for the Kings R

Basin only. Each river watershed in the w

re

h river watershed justified a separate plot of derived unit

The river watershed where the author has plotted calibrated unit hydrographs similar to 

Figure 5 are: Pit and McCloud Rivers, North Fork Feather River, West Branch of the 

Feather River, South Fork American River, Yuba River, Mokelumne River, Stanislaus

River, San Joaquin River above Millerton Lake and the Kings River. There are othe

isolated sm

Snow

In California where snowfall is part of the meteorological process of precipitation,

important that an antecedent snowpack and coincident snow be considered in the

estimation of design floods. Generally, the National Weather Service reports such a

HMR 36, HMR 58 and 59 provide also the coincident wind and temperatures for 

snowmelt process. 

T

or less. In areas of high elevation the presence of snow could reduce the runoff becaus

as the elevation band increases to higher elevation, the coincident temperatures become

colder by the lapse rate. Therefore, there would be more rainfall that would be changed

into snow rather than runoff. 

The publication “Snow Hydrology

C

61 Paper 12 - Cecilio 
For current practices in PMF det ergy Regulatory Commission 

ts

ore of 

s

is

Channel Routing 

Another major technique th stimate of hypothetical 

s of

f

ervoirs against hypothetical floods, there were 

o hydrologic criteria for dams under regulatory jurisdiction. At that time the PMF 

 potential for loss of life and economic damage to a design 

ood. Although the approach is generally adopted throughout the country, the specific 

of

stimate of the PMF by different engineers or federal agencies may vary significantly, as 

discussed earlier. 

rmining Design Basis Flooding at Power Reactor Sites.” (11) However, 

ecause it carried the name of the nuclear industry, it was never available to the ordinary 

dam safety program. In addition, it was probably the belief that the criteria suggested in 

ermination, the Federal En

recommends the use of an antecedent 100-yr snowpack. The effect of the 100-yr 

snowpack depth is minimal since the melt is dictated by the available melting coefficien

and parameters under the PMP event. The requirement for a 100-yr snowpack is m

a criteria rather than a physical characteristics in the meteorological process. The snow 

can melt only as much as the temperatures or wind coincident with the PMP can melt.

However, a deeper snowpack should affect the loss rate function in the basin during the

PMP event. 

Antecedent or Subsequent Storms 

The application of detailed analysis requiring routing of antecedent or subsequent storm

are applicable only to large watersheds because the critical storms for these watersheds

are usually due to long duration storms. PG&E reservoirs have small watersheds so it

generally assumed that the reservoirs are full prior to the occurrence of the PMP or other 

design storms. 

at causes a major difference in the e

floods is the method chosen for channel routing. HEC-1 has currently several method

channel routing. Each of these methods is acceptable but would give varying degrees o

results to the routed floods. If one performs channel routing with an unsteady flow 

model, the answers would also be different. 

HYDROLOGIC CRITERIA 

When PG&E started evaluating all their res

n

concept was an exclusive specialty of the Corps of Engineers. Additionally, all PG&E 

reservoirs have small drainage areas and the imposition of the PMF at the start appeared

too severe. The California Division of Safety of Dams came up with a minimum 1,000-

yr storm event as the minimum design criteria for all dams in California. 

Current criteria for hydrologic safety design of dams usually relate dam height, storage

volume, and the downstream

fl

criteria vary significantly. In addition, although the PMF is usually adopted for design

dams where the potential economic and social consequences of failure are large, the 

e

The ANSI/ANS 2.8 Standards 

The first attempt to provide a uniform approach in developing a probable maximum flood 

was made by the American Nuclear Society Working Group ANS 2.8. The American 

National Standard Institute (ANSI) published the work as the “American National 

Standard for Dete

b

Paper 12 - Cecilio 62
the standard would be very conservative in order to be compatible with other nuclear 

standards. 

The National Research Council Report 

The report prepared by the Committee on Safety Criteria for Dams under the

the National Research Council (12) provided new insights into the present thinking in the 

selection process of inflow design floods. The “blue book” as it became to be known ha

made an impact on the curr

auspices of

s

ent thinking of choosing a spillway design flood. However, 

e book contained only the criteria for the so-called “high-hazard” dams owned by the 

to

orking

gic Safety of

ams,” in 1988. (15) A quantitative risk assessment was the basis for the proposed 

procedures. The design selection process sed upon the three categories of dams

l Energy Regulatory Commission (FERC) issued their own guidelines 

determining the Inflow Design Flood and the Probable Maximum Flood. These 

CONCLUSION 

ven with all these activities, the dam safety community still needs more research in 

th

Corps of Engineers and Bureau of Reclamation. There were no criteria for small

medium hazard dams. 

The ICODS Report 

An Ad Hoc interagency committee on dam safety of the Federal Coordinating Council 

for Science, Engineering, and Technology, prepared “Federal Guidelines for Dam

Safety,” (13) which were published in June 1979. To provide general guidelines on

procedures for selecting and accommodating inflow design floods (IDF) for use by 

Federal Agencies, the Interagency Committee on Dam Safety (ICODS) formed a w

group on inflow design floods. The result of their work is published in a manual called,

“Federal Guidelines for Selecting and Accommodating Inflow Design Floods for Dams.” 

(14) 

ASCE Guidelines 

In October 1984 the Surface Water Hydrology Committee of the Hydraulics Division of

the American Society of Civil Engineers formed the Task Committee on Spillway Design 

Flood Selection in response to the growing national concern for dam safety and the 

hydrologic procedures used in evaluation. After three years of work and peer reviews,

the committee released the publication “Evaluation Procedures for Hydrolo

D

was ba

which depend on the failure consequences and the level of effort required to select a 

design flood. 

FERC Guidelines 

In 1993, the Federa

in

guidelines are contained in their “Engineering Guidelines for the Evaluation of 

Hydropower Projects.” (16) All dams under the jurisdiction of FERC apply the 

guidelines in evaluating the hydrologic safety of dams. 

E

developing better approach and selection of the appropriate spillway design flood of a 

dam. The author recommends the following research activities for funding: 

• Update “Snow Hydrology” previously published by the Northwest Division of the

U. S. Army Corps of Engineers. 

63 Paper 12 - Cecilio 
• Update Sacramento District 1962 studies, “Generalized Snowmelt Runoff 

Frequencies.” 

• Continue to upgrade the hydrologic models used for dam safety evaluation such 

as the Corps of Engineers HEC-1, HEC-2, HEC-HMS and HEC-RAS. The HEC1 model needs to be retained because it contains the snowmelt computations 

required for hypothetical floods in the Western Regions such as California, 

Oregon and Washington. 

• Develop a uniform method of areal distribution of the PMP especially applicable 

in orographic areas. 

• Technical review o 7 and 58) reports for

el is used for Inflow Design Flood selection process. Danny Fread has 

retired from service and the program development of his 

world-renowned DAMBRK ionally, this model is DOS-based 

f the recently published PMP (HMR 5

reasonableness. 

• Develop a temperature sequence for PMP estimates with snow so that it reflects 

the diurnal variation of temperatures. 

• Develop a “Windows-based” dambreak model. This is very important because 

the mod

continuation of the

is unknown. Addit

only. 

Paper 12 - Cecilio 64
65 Paper 12 - Cecilio 
Paper 12 - Cecilio 66
67 Paper 12 - Cecilio 
Paper 12 - Cecilio 68
69 Paper 12 - Cecilio 
Paper 12 - Cecilio 70
REFERENCES 

1. U. S. Army Corps of Engineers, “Flood Hydrograph Package,” HEC-1 Computer 

Program, Hydrologic Engineering Center, Davis, California. 

2. National Weather Service, “Precipitation Frequency Atlas of Western United States,” 

NOAA Atlas 2, Washington, D.C., 1974. 

3. Department of Water Resources, “Rainfall Analysis for Drainage Design,” Bulletin 

No. 195, State of California, The Resources Agency, October 1976. 

4. U. S. Weather Bureau, “Interim Report – Probable Maximum Precipitation in 

California,” Hydrometeorological Report No. 36, Washington, D.C., October 1961 

with revisions of October 1969. 

5. U. S. National Weather Service, “Probable Maximum Precipitation for California – 

Calculation Procedures,” Hydrometeorological Report No. 58, Silver Spring, 

Maryland, October 1998. 

6. U. S. National Weather Service, “Probable Maximum Precipitation for California,” 

Hydrometeorological Report No. 59, Silver Spring, Maryland, February 1999. 

7. Soil Conservation Service, “Hydrology,” SCS National Engineering Handbook, 

Section 4, Washington, D.C., August 1972. 

8. Clark, C. O., “Storage and the Unit Hydrograph,” ASCE Transaction, Vol. 110, 1945, 

pp. 1419-1488. 

9. Snyder, F. F., “Synthetic Unit-Graphs,” Transactions, American Geophysical Union, 

Vol. 19, Part 1, August 1938, pp. 447-454. 

10. U. S. Bureau of Reclamation, “Design of Small Dams,” 2nd Edition, Denver, 

Colorado, 1973. 

11. American Nuclear Society, “American National Standard for Determining Design 

Basis Flooding at Power Reactor Sites, ANSI/ANS 2.8, 1976. 

12. National Research Council, ‘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 Academy Press, 

Washington, D.C., 1985. 

13. Federal Coordinating Council for Science, Engineering and Technology, “Federal 

Guidelines for Dam Safety,” Ad Hoc Interagency Committee on dam Safety, 1979. 

14. Federal Emergency Management Agency, “Federal Guidelines for Selecting and 

Accommodating Inflow Design Floods for Dams,” Prepared by the Working Group 

on Inflow Design Floods, Subcommittee 1, Interagency Committee on Dam Safety, 

1986. 

15. American Society of Civil Engineers, “Evaluation Procedures for Hydrologic Safety 

of Dams,” A report prepared by the Task Committee on Spillway Design Flood 

Selection of the Committee on Surface Water Hydrology of the Hydraulics Division 

of the American Society of Civil Engineers, New York, New York, 1988. 

16. Federal Energy Regulatory Commission, “Engineering Guidelines for the Evaluation 

of Hydropower Projects,” Office of Hydropower Licensing, Washington, D.C.

71 Paper 12 - Cecilio 
Estimation of Extreme H ts in Australia: Current

Rec

f

Eng ty for

ian

d

to its

s a

risk the

larg

.

ben

use

Des

assu

The objective of this paper is to summarise the nature of the methods used to characterise 

risk for evaluation of dam safety, and to highlight those areas of research that are 

considered to be of most importance in the short term. 

ydrologic Even

Practice and Research Needs 

Rory Nathan, Sinclair Knight Merz, Australia

Introduction 

ently two important documents relating to the assessment of flood-induced failure of

dams have been released in Australia. Firstly, Book VI of Australian Rainfall and Runof

(Nathan and Weinmann, 1999) has been revised and updated by the Institution of 

ineers, and secondly the guidelines on the selection of acceptable flood capaci

dams was released by ANCOLD (2000). 

Book VI provides guidance on the derivation of Large to Extreme Floods for Austral

catchments. The focus of the Book VI guidelines is on the estimation of floods with 

annual exceedance probabilities (AEP) rarer than 1 in 100. As such, they are relevant to 

the assessment of dam safety. In effect, Book VI describes the basis of the hydrologic 

techniques required to estimate the range of floods relevant to the selection of acceptable 

flood capacity for dams. The relevant factors and issues to be considered in the 

assessment of dam safety are covered in the ANCOLD (2000) guidelines. 

One of the salient changes common to both of the aforementioned guidelines is the 

explicit consideration of risk. In the past, the assessment of hydrologic safety was base

on deterministic standards in which the safety of the dam was assessed by reference

ability to pass a flood of a given magnitude. The new guidelines have moved toward

-based approach, in which attention is focused on establishing the probability of

est flood that can be safely passed by the spillway.

This change in focus from a standards-based approach to a risk-based one has 

necessitated a revision to the conceptual framework underlying flood estimation practice

In the past we focused on estimating a “probable maximum” type of event to use as a 

chmark to compare with the flood capacity of the spillway. The common benchmark

d was the “Probable Maximum Flood” (PMF), which was defined as the limiting 

value of flood that could be reasonably expected to occur (Pilgrim and Rowbottom,

1987). Book VI has introduced the concept of the Probable Maximum Precipitation 

ign Flood (PMP DF), which is the flood derived from the PMP under “AEP-neutral” 

mptions, that is, under assumptions that aim to ensure that the AEP of the flood is the

same as the rainfall that caused it. The PMP DF represents the upper limit of the flood 

frequency curve and it is estimated to have the same AEP as the PMP. 

73 Paper 13 - Nathan 
Overview of Flood Estimation Procedures 

The procedures can be loosely grouped into three classes of flood magnitude over the 

range of AEPs under consideration. The type of available information, degree of 

uncertainty, and hence nature of procedure that can be used in the analysis varies with 

flood magnitude. The notional classes are summarised in Figure 1. This figure broadly 

divides the floods and rainfalls of interest into Large, Rare, and Extreme ranges, though it 

should be stressed that the adopted classes represent a continuum of increasing 

uncertainty and not discrete intervals. 

Design

timate

Pragmatic

U antifiable, but notionally

very large

ible

limit of extrapolation

Extrapolation

Moderate to large

Beyond

to the credible

limit of extrapolation

Design

rainfall

or flood

Annual Exceedance Probability (1 in Y)

50 100 2000 104 105 106 

Large Rare Extreme Event class

Nature of procedures

Nature of uncertainty

Range of AEP

Credible limit 

extrapolation

AEP of PMP varies between

1 in 104 and 1 in 107

Interpolation

Moderate

From 1 in 50

to 1 in 100

Upper and lower

limits of uncertainty

Figure 1: Design characteristics of notional design event classes.

Large Floods 

Large floods are intended to represent those events for which direct observations relevant 

to the site of interest are available. The most common sources of information for this 

range of floods are the systematic records of rainfalls or streamflow (available either at 

the site of interest or else transposed from similar catchments), though they include 

historic information for notable events that occurred prior to the beginning of continuous 

gauged records. Accessible records in general only extend back to the past 100 years, and 

thus notionally the AEPs corresponding to this category are limited to events more 

frequent than 1 in 100. 

Large flood estimates are derived by either at-site/regional flood frequency analyses of 

annual flood maxima, or else by rainfall-based flood event models. The current 

guidelines (Pilgrim and Doran, 1987) recommend that the annual flood maxima be fitted 

by the method of moments to the log-Pearson III distribution, though this is currently 

of

es

nqu

1 in 100 Beyond the cred

Paper 13 - Nathan 74
unde

Generalised Extreme Valu

The analyses are ba e of

irect observations, and thus are essentially interpolative in nature. A large body of 

xperience and a great variety of procedures are available to derive flood estimates within 

this range, and the associated degree the estimates can be readily 

uantified.

erence to the latter concept, it is worth noting 

at the term: 

nd,

that can

used which

re based on the regional pooling of data, and the quality of the extrapolation depends on 

ost optimistic bounds. At 

resent in Australia using regional pooling techniques the credible limit of extrapolation 

d

r review and is likely to be replaced by the use of LH-Moments fitted to the 

e distribution (Wang, 1997). 

sed on deriving design flood estimates that lie within the rang

d

e

of uncertainty in

q

Rare Floods 

Rare floods represent the range of events between the largest direct observations and the

“credible limit of extrapolation”. With ref

th

• “credible” is used to represent the limit of extrapolation that can be estimated without

the use of other confirming information from an essentially independent source; a

• “extrapolation” is used to denote estimates that are made outside the range of 

observations that are available at a single site. 

The credible limit of extrapolation is dependent upon the nature of available data

be obtained at and/or transposed to the site of interest. Procedures are often

a

the strength of the assumptions made. 

The notional credible limits of extrapolation for a range of data types in Australia are 

shown in Table 1. This table indicates the lower AEPs corresponding to both typical and

the most optimistic situations, though in most cases the credible AEP limits are likely to

be considerably closer to the typical estimates than the m

p

(for rainfalls) is between 1 in 5000 and 1 in 10 000 AEP. 

Table 1: Limit of credible extrapolation for different types of data in Australia (modifie

after USBR, 1999). 

Type of data used for frequency analysis Credible limit of extrapolation

(AEP) 

Typical Most optimistic

At-site gauged flood data 1 in 50 1 in 200 

At-site gauged rainfall data 1 in 100 1 in 200 

At-site/regional gauged flood data 1 in 200 1 in 500 

At-site gauged and paleoflood data 1 in 5 000 1 in 10 000 

Regional rainfall data 1 in 2 000 1 in 10 000 

75 Paper 13 - Nathan 
Extreme Floods 

Extreme floods, the third class, represent the range of floods where even a high level o

expertise cannot reduce the level of uncertainty substantially, that is the region wh

borders on the “unknowable”. Estimates of such events lie beyond the credible limit of

extrapolation, but are hopefully based on our broadest understanding of the physical 

limits of hydrometeorological processes. The procedures employ a consensus approach 

that provides consistent and reasonable values for pragmatic design. The proced

relating to this range of estimates are in

f

ich

ures

herently prescriptive, as without empirical 

vidence or scientific justification there can be no rational basis for departing from the 

onsensus approach.

Floods over this range can only be derived using rainfall-based flood models. The 

rainfalls used to input into the models are derived by interpolation between the credible 

limit of extrapolation and estimates of Probable Maximum Precipitation. 

Rainfall-based Flood Event Models 

In Australia, the most common method used to estimate a flood hydrograph is by runoff 

routing models. With this approach, a hydrograph is calculated by some form of routing 

of rainfall excess through a representation of the storage within the catchment. Runoffrouting models provide an alternative to unit hydrographs. They are not restricted to the 

assumption of linear behaviour, and in most applications non-linear behaviour is 

assumed. Most runoff routing models contain spatially-distributed conceptual storages to 

represent the catchment storage afforded by overland flow paths. Some models lump this 

overland storage together with the attenuation arising from channel storage, and some

models simulate overland and channel storages separately. Probably the most common 

g and Partners, 

997).

rocedures are largely focused on the adoption of an AEP-neutral 

pproach, in which it is assumed that the estimated design flood characteristic (e.g. peak 

puts

s

joint probability methods are commonly used (e.g. Laurenson, 

974) to take into account the likelihood that the reservoir may not be full prior to the 

he

e

c

models used are RORB (Laurenson and Mein, 1997) and RAFTS (Willin

1

The Book VI p

a

flood) has the same AEP as its causative design rainfall. In order to satisfy this 

assumption it is necessary to incorporate representative design values of all inputs and

parameters in such a way that they are AEP-neutral. In practice this commonly requires

that a designer selects a single representative value from a wide range of all design in

(such as losses, temporal and spatial patterns, and model parameters), though joint 

probability approaches are currently being explored to better achieve AEP-neutrality (e.g. 

Weinmann et al., 1998). With respect to achieving AEP-neutrality for outflow flood

from reservoirs, explicit

1

onset of the flood. 

Where suitable rainfall and runoff data are available, the flood event models are 

calibrated using observed floods on the catchment of interest and, where appropriate, t

Paper 13 - Nathan 76
parameter values are adjusted to help reconcile differences between design values

from flood frequency analysis and rainfall-based method

derived

s. In other cases, design values

r the model parameters are estimated from calibration on adjacent gauged catchments, 

alls used as input to the flood event models are derived from a variety of 

urces (see Figure 2). For Large design rainfalls the depths are obtained from mapped 

r durations ranging between 15 minutes and 72 hours. These rainfalls were 

erived by the subjective smoothing of rainfall quantiles derived from at-site frequency 

n the effective record length of the combined data sets. 

Est

t ralised

met gical Organisation (World

eteorological Organisation, 1986). These methods use data from a large region and 

d

ates based on conceptually different methods and 

ifferent data sets; the interpolation procedures are not supported by scientific reasoning. 

fo

regional relationships, or other relevant information. 

Design Rainfalls Inputs 

Design rainf

so

information fo

d

analyses of annual rainfall maxima. Rainfalls up to the credible limit are derived using a 

regional pooling procedure (Nandakumar et al, 1997) which takes into account the effect

of correlation o

imates of Probable Maximum Precipitation (PMP) are provided by a federal 

me eorology agency (the Bureau of Meteorology). These estimates are based gene

hods, as recommended by the World Meteorolo

M

make adjustments for moisture availability and topographic effects on extreme rainfall 

depths. Estimates of design rainfall depths between the credible limit of extrapolation an

the PMP are based on interpolation procedures. These procedures are necessarily 

pragmatic as they attempts to link estim

d

Large Rare Extreme

Credible limit of

extrapolation

AEP of PMP varies between

1 in 104 and 1 in 107

ceedance Probability (

 2000 104 6

Frequency Regional pooling

thods that

space for

me

Pragmatic i

procedures bet

credible limit an

PMP estimates

based on

Des methods ign

generalised

rainfall

depth

Annual Ex

50 100 

1 in Y)

 10 5 10

analysis of me

annual

maxima

trade

ti

nterpolation

ween the

d the PMP

Figure 2: Summary of procedures used to derive design rainfall depths.

77 Paper 13 - Nathan 
Temporal patterns used to distribute the rainfall depths in time are based on methods that 

ere derived explicitly with the objective of AEP-neutrality (Pilgrim et al., 1969; Nathan, 

off)

oss – Proportional Loss (IL-PL) models. The initial 

ss is that which occurs prior to the commencement of surface runoff. The continuing 

,

s

al loss

rate (equivalent to 1-C, where C is the runoff coefficient) can be understood to be the 

unsaturated proportion of the catchment which produces very little runoff. 

ne important distinction made in the application of loss rates is whether or not design 

ith

ll

w

1992). Importantly, “pre-burst” patterns are provided that allow intense bursts of design 

rainfalls to be converted to storm rainfalls (see next section). Procedures are also 

provided on how to spatially distribute the rainfalls across the catchment. 

Estimation of Rainfall Excess 

A loss model is needed to partition the design rainfall input into rainfall excess (run

and loss. The most common loss models used are the simple Initial Loss – Continuing

Loss (IL-CL) model, and the Initial L

lo

loss is the average rate of loss, and the proportional loss the average fraction of loss

throughout the remainder of the storm. The continuing loss rate is often conceptualised a

an average rate of potential infiltration over the catchment, while the proportion

O

bursts of rainfall are used or complete storms; a schematic diagram illustrating the 

difference between these two concepts is shown in Figure 3. Design bursts are generally 

preceded by some lower intensity rainfall (pre-burst rainfall), they do not represent 

complete storms. Accordingly the larger the rainfall the larger the expected cumulative 

depth of pre-burst rainfall. This expected variation means that if losses are used w

design rainfall bursts then they are decreased in some fashion with increasing rainfall 

depths. On the other hand, if losses are used with design rainfall storms then the available

empirical evidence (Hill et al, 1997) suggests that they may be held constant over the fu

range of rainfall depths considered. 

Design Storm

Time

Pre-burst rainfall Design burst

Rainfall / stre

a

mflo

w

Storm Initial Loss (ILs)

Burst Initial

Loss (ILB)

Start of

hydrograph

rise

Pre-storm

rainfall

Figure 3: Schematic diagram illustrating concepts used in design loss formulation 

(adapted from Hill et al, 1997). 

Paper 13 - Nathan 78
Estimation of the AEP of the PMP

Estimation of the AEP of the PMP (and hence the PMP DF) is needed to define the upper

nd of the frequency curve (Figures 1 and 2). While assigning an AEP to the PMP is 

er limiting” concept of the PMP, it is recognised that 

MP are estimates only, and their accuracy is crucially 

rable uncertainty

e

inconsistent with the “upp

operational estimates of P

dependent on the validity of both the method and the data used to derive them. Thus 

operational estimates of PMP may conceivably be exceeded. 

The method proposed to assign an AEP to the PMP is based on the review by Laurenson 

and Kuczera (1999) of a range of procedures developed both in Australia and overseas. 

The AEP of PMP estimates are considered to vary solely as a function of catchment area 

(Figure 4), and are consistent with the recommendations of Kennedy and Hart (1984), 

Pearse and Laurenson (1997) and Nathan et al. (1999). There is conside

surrounding these recommendations as they are for events beyond the realm of 

experience and are based on methods whose conceptual foundations are unclear. It is not 

surprising that the notional 75% confidence and upper and lower limits are larger than is 

desirable, though they are regarded as a realistic assessment of the true uncertainty. A

probability mass function is provided to allow the incorporation of uncertainty into risk

analysis. Although the probabilities are subjective, they do reflect the considerable 

uncertainty in the AEP estimates. 

1.E-02

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

10 100 1000 10000 100000

Area (km2)

AEP of PMP

 Recommended

Notional 75% confidence limits

Notional upper and low er limits

Figure 4 Recommended values of AEP of PMP. 

79 Paper 13 - Nathan 
Preliminary Estimates of Rainfall and Flood Frequency Curves 

It is recognised that there are many design situations in which preliminary estimates of

design floods are required. Accordingly the guidelines allow for the derivation of

preliminary estimates using “quick” procedures based on regional information. Full flood

requency curves can be derived using the probabilistic rational method to estimate 

and regional estimates of the Probable Maximum Flood 

athan et al, 1994) to define the upper end of the curve. Regional prediction equations 

ess

he AEP of the PMP defines the location of the upper tail of the rainfall (and hence 

g

rch

ts to estimate the conditional likelihood that if a 

MP does occur in the region, that it occurs over the catchment of interest (Agho and 

Kuczera, 2000). 

Techniques required to minimise bias in the AEP transformation between rainfall and 

floods. 

If rainfalls are used in event-based models to estimate flood peaks, then in a risk-based 

context special attention needs to be given to ensuring that the AEP of the incident 

rainfall is preserved in the consequent flood, e.g. if an estimate of the 1:5000 AEP 

rainfalls is available from regional pooling, then what considerations are required to 

ensure that the AEP of the resulting flood peak is also 1:5000. 

There are a number of ways of ensuring that this is achieved for frequent floods, but as 

the AEP of the flood decreases it becomes increasingly difficult to validate. The factors 

that influence this are many, though possibly the major factors are non-linearity of flood 

response in overland flowpaths and channels, rainfall losses, temporal patterns, and initial 

s, and 

in the near future. Physically-based models provide a 

f

frequent floods (Pilgrim, 1987)

(N

have also been derived to provide quick estimates of PMP depths (Nathan et al, 1999). 

Research Priorities 

There are two major areas of research that are considered to be a high priority to addr

in the short to medium term: 

Annual Exceedance Probability of the Probable Maximum Precipitation 

T

flood) frequency distributions. The location of the upper tail influences estimates of risk

beyond the credible limit of extrapolation, and this is a region of importance when 

assessing the risk of overtopping failure for many existing dams. 

The problem of estimating the AEP of the PMP has been considered a problem for a lon

time – indeed it is viewed by some as being unsolvable. However, there is some resea

being undertaken currently in Australia at two different institutions that shows 

considerable promise: (i) use of joint probability approaches to estimate the likelihood of

a PMP occurring anywhere in a homogeneous meteorological region, and (ii) the use of 

stochastic storm transposition concep

P

snowpack. Some of these issues can be solved, some are tractable research problem

some are unlikely to be resolved

Paper 13 - Nathan 80
vehicle to explore ways of using physical reasoning to condition extrapolations of flood 

niques provide another.

risking.

e

am

References

Australian National Committee on Large Dams (ANCOLD; 2000) Guidelines on 

Selection of Acceptable Flood Capacity for Dams. ANCOLD, March 2000. 

Hill, P.I., Mein, R.G., Weinmann, P.E. (1997): Development and testing of new design 

losses for south-eastern Australia. Proc. 24th Hydrology and Water Resources 

Symposium, Auckland New Zealand, 71-76. 

Kennedy, M.R and Hart, T.L. (1984): The estimation of probable maximum precipitation 

in Australia. Civ. Engg Trans, Inst Engrs Aust, Vol. CE26, pp. 29-36. 

Agho, K. and Kuczera, G. (2000): Estimation of extreme rainfall exceedance 

probabilities: non-dimensional stochastic storm transposition. Proceedings of the 3rd

International Hydrology and Water Resources Symposium, Perth, November 2000, pp 

60-65. 

Laurenson, E.M. (1974). Modelling of stochastic-deterministic hydrologic systems. 

Water Resources Research, Vol. 10, No. 5, 955-961. 

Laurenson, E.M. and Kuczera, G. A. (1999): Annual exceedance probability of probable 

maximum precipitation, Aus. J. Water Resour., 3(2), 167-173. 

Laurenson, E.M. and Mein, R.G. (1997): RORB - Version 4, Runoff Routing Program, 

Us sity.

Nandakumar, N., P. E. Weinmann, R.G. Mein, and R.J. Nathan (1997): Estimation of 

extreme rainfalls for Victoria using the CRC-FORGE method (for rainfall durations 

response, and paleo-hydrological tech

Conclusions 

Australian guidelines have recently been revised to incorporate a shift towards a

based framework for characterising the likelihood of dam failure due to overtopp

They provide both a framework and design inputs for estimation of floods down to 

annual exceedance probabilities as rare as 1 in 104

 to 1 in 107. 

At present in Australia the credible limit of extrapolation of rainfalls is limited to 

exceedance probabilities of 1 in 5 000 and 1 in 10 000. Estimates of risk beyond this limit 

are subject to considerable uncertainty and are based on a mixture of engineering 

pragmatism and physical reasoning. While there is some justification for estimates of th

magnitude of PMP depths (and to a lesser extent the resulting floods), the guidelines do 

provide a rational and consistent framework for characterising risks relevant to d

safety. 

The most urgent needs for research are considered to be in the refinement of the annual

exceedance probability of extreme rainfall depths, and in the further investigation of 

factors that influence the bias in AEP-transformation between rainfall and floods. 

er Manual, Dept. Civil Engineering, Monash Univer

81 Paper 13 - Nathan 
24 to 72 hours). CRC Research Report 97/4, Cooperative Research Centre for 

Catchment Hydrology, Melbourne. 

e

probable maximum flood in South East Australia. Int. Hydrology and Water 

ium: Water Down Under, November, Adelaide, I.E. Aust. Natl.

4, 229-234.

MP Events in Southeast Australia, Aus. J. Water Resour.

3(1), 143-154. 

for estimating 

Probable Maximum Precipitation in the tropical and south-east regions of Australia. 

Runoff: A Guide to Flood Estimation. Institution of Engineers, Australia. 

H. (ed.)

fall

ilgrim, D.H. and Rowbottom, I.A. (1987): Chapter 10 – Flood frequency analysis, In 

ilgrim, D.H. and Rowbottom, I.A. (1987): Chapter 13 - Estimation of large and extreme 

ood

9.

e events. Water Research 

Research 133(12): 2841-2848. 

National Committee on Water Engineering (NCWE, 1999): Australian Rainfall and 

Runoff: A Guide to Flood Estimation. Institution of Engineers, Australia. 

Nathan, R.J. (1992): The derivation of design temporal patterns for use with generalised 

estimates of Probable Maximum Precipitation. Civil Engng. Trans., I.E. Aust. Vol 

CE34(2), 139-150. 

Nathan, R.J., Weinmann, P.E., and Gato, S (1994): A quick method for estimation of th

Resources Sympos

Conf. Publ. No. /9

Nathan, R.J., Weinmann, P.E., and Minty, L. (1999): Estimation of the Annual 

Exceedance Probability of P

Nathan, R.J., Crowe, P., Hill, P.I. and Green, J.H. (1999): A quick method

Proc. Water 99 Joint Congress, Brisbane, July 1999, Institution of Engineers, 

Australia, 703-708. 

Nathan, R.J., Weinmann, P.E., (1999): Book VI “Estimation of Large to Extreme 

Floods”. In National Committee on Water Engineering (ed): Australian Rainfall and 

Pearse, M.A. and Laurenson, E.M. (1997): Real probabilities for real floods. Proc., 

ANCOLD 1997 Conference on Dams. ANCOLD. 

Pilgrim, D.H. (1987): Chapter 5 - Estimation of peak discharges. In Pilgrim, D.

Australian Rainfall and Runoff: A Guide to Flood Estimation, Institution of 

Engineers, Australia. 

Pilgrim, D.H. and Cordery, I., and French, R. (1969): Temporal patterns of design rain

for Sydney, Civil Engng. Trans., I.E. Aust. Vol CE11(1), 9-14. 

P

Pilgrim, D.H. (ed.) Australian Rainfall and Runoff: A Guide to Flood Estimation, 

Institution of Engineers, Australia. 

P

floods, In Pilgrim, D.H. (ed.) Australian Rainfall and Runoff: A Guide to Flood 

Estimation, Institution of Engineers, Australia. 

U.S. Bureau of Reclamation (USBR, 1999): A framework for developing extreme fl

inputs for dam safety risk assessments, Part I - Framework, USBR, February 199

Wang, Q. (1997). LH Moments for statistical analysis of extrem

Weinmann, P.E., Rahman, A., Hoang, T., Laurenson, E.M., and Nathan, R.J. (1998): A 

new modelling framework for design flood estimation. ’HydraStorm’98 Conference

Proceedings, Adelaide, Australia, Institution of Engineers, Australia, pp 393-398. 

Willing and Partners Pty Ltd (1997): XP-RAFTS runoff training and flow system

(Version 5.1). 

Paper 13 - Nathan 82
World Meteorological Organisation (1986): Manual for estimation of probable maximum

precipitation, Second Edition, Operational Hydrology Report No. 1, WMO - No. 

332, Geneva. 

83 Paper 13 - Nathan 
CURRENT AND FUTURE HYDROLOGIC RESEARCH NEEDS FOR 

DAM SAFETY ANALYSIS IN MATURE DAM SAFETY PROGRAMS

By 

D.N.D. Hartford1

Considerations for Dam Owners 

Dam owners have a duty of care to protect the public, property and the environment from

harm that might result from their operations. In this regard, the dam owner can be 

considered to be the creator of the risk and increasingly there are requirements in law for 

the dam owner to identify the hazards, assess the risks, prepare a safety case 

demonstrating how the risks will be prevented or otherwise controlled, and set out a 

safety management system showing how the safety case will be implemented and 

maintained. 

Risk assessment, which provides a structured, systematic examination of the likelihood of 

harmful events and the associated consequences should the events occur, is the essential 

anticipatory element that underpins the safety case. Therefore, in the preparation of the 

safety case and in demonstrating due diligence in risk management and control the 

quality of the risk assessment is of paramount importance to the owner. Therefore, 

research into the quality of the risk assessment process is of obvious interest to dam 

owners. 

A second very important consideration pertains to the context within which a dam safety 

decision is to be made. The modern view from the perspective of a risk regulator (HSE, 

2001) is that ”the proper use of risk assessment would require inter alia that: 

• more often than not, the results of a risk assessment should be expressed as a value

judgement rather than a number; 

• the risk problem should be properly framed 

• the nature and limitations of the risk assessment are understood; and 

• the results of the risk assessment are used to inform rather than to dictate decisions 

and are only one of the many factors taken into account in reaching a decision.”

Researching what all of this means and its implications are a second interest to dam 

owners. 

The dam owners risk profile, characterized in large part by the consequences of dam

failures for the portfolio, provides a third important consideration which must be 

accounted for in the risk assessment and decision-making processes. Societal 

catastrophic loss risk management is different to the other risks managed by dam owners. 

The rules for decision-making for catastrophic loss risks are also different, as are the 

representations of the risk. The decision and risk assessment processes used by an owner 

whose losses are limited or insurable can be expected to be different to those owners 

managing catastrophic loss risks. 

 

1

 BC Hydro

85 Paper 14 - Hartford 
Fourth, the degree of maturity of the owner’s dam safety program will influence the 

approach to risk assessment and decision-ma ing. Here the term “degree of maturity” is 

used in the sense of a long track record of demonstrably effective risk identification and 

risk control as opposed to simply being long established and/or having no track record of 

incidents yet which in itself does not demonstrate effectiveness in risk management. 

It is clear that the actions that dam owners take with regard to dam safety management 

are governed by a number of complex, interacting and often opposing factors. The 

prevailing legal, regulatory and societal expectations, business considerations and the 

owner’s values and principles govern dam ow r’s decision-making processes. Having 

determined these boundary conditions, the dam owner must establish a decision process 

that meets the needs of these boundary conditions and also ensures the viability of the 

operation (be it business or some level of government). The effectiveness, in the sense 

of assuring that dams are safe enough and legal defensibility of the decision process, 

where the term legal defensibility is used in its broadest sense, that lead to the 

conclusions concerning the safety of dams are primary considerations for dam owners. 

Context of this Paper 

The above suggests that owner’s dam safety search needs are multifaceted and driven 

by the owner’s decision proces track record (including

economic efficiency - a debate for another forum) of established dam safety practice, it 

seems reasonable to hold the view that most dam owner’s research needs relate to dealing 

with problems that are beyond conventional engineering wisdom (traditional dam safety 

practice). Some of the research needs that owners must address are of an engineering 

nature, others are not, making it necessary to understand where the balance changes from

one of engineering dominance of the decision process to one of engineering support for 

the decision process. 

Since the owner is responsible for both ensuring the safety of the dam and the decision 

process (to varying degrees depending on the regulatory environment) that determines the 

level of safety of the dam, one can infer that dam safety research needs are fundamentally 

determined by owner’s needs. The regulator is the other principal stake holder and while 

the regulator’s research needs may be the same as those of the owner in some respects,

they can be expected to be different in others. Thus owners and regulators dam safety 

research needs govern the form, nature and future direction of research initiatives. 

The need for research 

In to-day’s challenging business environment research funding is scarce and 

owners require demonstrable evidence that research is necessary prior to 

considering funding research work. Research can only be justified if it is 

necessary or beneficial to improve existing practices or create new practices 

k

ne

re

s. Given the generally good

Paper 14 – Hartford 86
because existing procedures are inadequate in some way. Therefore, prior to 

embarking on a research initiative, owners need to identify (or have identified for 

them) inadequacies (from the perspectives of improved economic efficiency or 

legal defensibility and due d ting state of knowledge and 

procedures that require rectification.

The view that “m dam failure are poorly

al

ents. 

lood

ge Reduction studies (NRC, 2000) provides further evidence of a need to 

ds should be determined through analysis of the present state of affairs in a 

ncertainty and the underlying bases for 

mining owner’s research needs. 

Dec

rt, 2001). This framework reflects established dam safety practice that has 

rved dam owners and the public well thus far and permits the extension of this practice 

fety 

and

concern place towards the bottom right. 

iligence) in the exis

ethodologies for estimating the chance of

developed and, at the present time, do not provide a defensible basis for the 

conclusive sign off on the safety status of a dam” (McDonald et al. 2000) provides 

a basis for concluding that there might be good reasons for owners to question 

current capability in dam safety risk assessment. The view is supported by recent 

data collected by BC Hydro, which although carried out to investigate potenti

dam safety deficiencies, also provided a basis for validating past risk assessm

The National Research Council’s report Risk Analysis and Uncertainty in F

Dama

improve risk assessment capability. 

Focus of the paper 

This paper, written from the perspective of a commercially focused owner of 

dams ranging in size and hazard potential from the very small (by any standards) 

to amongst the largest in the world, reflects the view that dam owner’s research 

nee

way that reveals the gaps in current capability, thereby revealing dam owner’s 

research needs. 

The need to understand the uncertainties in all important aspects of the inputs to 

the dam safety decision processes is central to the paper. The manner in which 

decisions are made under conditions of u

these decisions provides direction for exa

ision Context

A conceptual form of a dam owner’s decision context is illustrated in Figure 1 (Hartford

and Stewa

se

to incorporate the emerging realities of explicit consideration of risk issues in dam sa

decision-making. 

Figure 1 is conceptual in nature and the decision context in any particular case is broadly 

determined by utilizing the horizontal and vertical axes. Decisions that can be made in

terms of building codes or standards established by Government would place in the upper

left hand section whereas those decisions where no precedents or standards exist

which involve significant societal

87 Paper 14 - Hartford 
Category “A” can be considered applicable to “routine” risk management situations for 

s to 

e

nce

ut where it is necessary to make significant trade-offs between benefits and risks. The 

he

high

ose

d

s

on

,

ade in terms of codes and standards and accepted good practice. The need for 

pplication of risk assessment increases as the decision context moves through categories 

B and C ght hand corner. It is in this lower region that the research 

needs of dam owners lie. 

Decisio

dams where the application of codes and standards is appropriate. Category B relate

those situations that are less “cut and dried” but where there is general guidance

concerning how to implement risk controls, with certain aspects of the risk being uniqu

to the situation at hand. Category “C” can be related to the more complex safety

decisions, where the public interest and/or the environment are of paramount importa

b

risk control measures can be expected to be novel and potentially extremely costly for t

risk reduction benefits gained. Societal interest and involvement are potentially

requiring a participatory role for the public. Decisions in this context also include th

where the complete spectrum of long-term implications are highly uncertain an

potentially devastating. An example is retrofitting dams to meet new safety requirement

that have not achieved the status of authoritative good practice, where the retrofit soluti

to meet a new standard is unique, where retrofit technology does not exist or is unproved

or where serious risk trading obfuscates the decision. 

Significance to

des

&

dards

Good

Means of Calibration Decision Context Type

Nothing new or unusual

Well understood risks

Established practice A

Decision-making Process

Co

stan

practice

Engine ring judgement

Verification

Peer Review

Benchmarking

Internal Stakeholder

Consultation

No major stakeholder implications

B

C

Lifecycle implications

Judgements of risk trade-offs/transfers

Increased precautions to account for

any uncertainty or deviation from

standard or best practice

Significant economic implications

Very novel or challenging

Stakeholder considerations embodied

in profession practice

Further increase in precautions to account for S

uncertainty or deviations from standard practice ocietal values

Region of formal analyticdeliberative risk assessment

involving

Owner's values

Codes & standards

e

Figure 1. Proposed Decision Context Framework for Dam Safety Decision-Making

(Hartford and Stewart (2001) based on UKOAA as reported by Brinded (2000)) 

Figure 1 provides general guidance for both the establishment and management of a dam 

safety program as well as the research needs of dam owners with mature dam safety 

programs. From the perspective of a commercially focused dam owner, there is little 

value in applying quantitative risk assessment to situations where the decision is to be 

m

a

towards the bottom ri

n Process

Paper 14 – Hartford 88
The de dam 

owner’

modern

relevan

1. Use the decision-making process and criteria 

adopted are such that the actions taken are inherently precautionary, commensurate 

wit

unc

2. Ens

goo

3. If t

con ut

to d

4. Imp l 

me

cision process is the second feature of a dam safety program that influences

s research needs. A decision process suitable for decision-making in terms of the

view of risk assessment is illustrated in Figure 2. The aspects of the process

t hydrologic research needs are summarized as follows: 

the Precautionary principle where

h the level or risk to individuals, the societal concerns and the degree of

ertainty.

ure that at a minimum the Inflow Design Flood (IDF) implements authoritative 

d practice irrespective of situation-specific risk estimates. 

here is no reliable base of good practice for ensuring that risks are adequately

trolled, a process of risk analysis, evaluation and assessment should be carried o

ecide the extent of the risk control measures. 

lement risk controls until the residual risk is reduced to the extent that additiona

asures are likely grossly disproportionate to the risk reductions achieved. 

Step 5:

Determine if IFF condition is

clearly unacceptable

NO

Step 1:

Assess dam against

"Authoritative good practice" for

Assess the IFF condition with emphasis on a

maximally exposed individual (MEI)

YES

VEMENT AND COMMUN CI

AFETY CASE

MENT

OUTLINE OF PROCEDURE FOR SELECTING FLOOD PERFORMANCE GOALS FOR EXISTING DAMS

(a similar procedure can be followed for earthquakes)

YES:

Step 8:

Step 2:

Determine means and

resources required to meet

"Authoritative good practice"

for new dams

new dams

(Cconfirmed as being

"ALARP")

NO: Dam does not meet "Authoritative good practice"

RISK-BASED S

DEVELOP

Step 3:

Determine characteristics of the Imminent Failure

Flood (IFF) for the dam

(defined as exceeding one or more design basis

criterion - not the load at which the dam is just on the

point of failure) YES

Dam can

be "readily" upgraded

to meet "good practice" for

new dams

A

Step 4:

TION

Dam meets

"good

practice"

for new dams

Step 6:

Determine and implement "interim"

risk control measures as soon as

possible NO:

May

be tolerable

REGULATORY INVOL

Step 7:

Develop Safety Case for appropriate performance goal

within ALARP framework

Schedule and prioritize upgrade

within ALARP framework

DAM IS SAFE ENOUGH (ALARP)

Submit Risk-Based Safety Case for regulatory and public

approval

Implement agreed risk management measures on a priority

basis

Agreement reached

89 Paper 14 - Hartford 
Figure 2. Dam Safety Decision Process 

Point 2 above suggests that there is little or no opportunity not to adhere strictly to 

established practice in selecting the Inflow Design Flood for a dam, with the PMF being 

the standard for all High Consequence dams. However, this is not the case for at least 

two reasons: 

• What constitutes ‘authoritative good practice’ for selecting the IDF for a dam remains

undefined (see Hartford and Stewart. ibid.); and, much more importantly, 

• Accepted dam safety practice, including selection of the PMF as the IDF, has not 

been submitted to the rigorous tests of risk assessment to determine if they actually 

provide effective risk control in terms of the As Low As Reasonably Practicable

(ALARP) principle. 

This is not an argument against selection of the PMF as the IDF for High Consequence 

dams. Rather, if PMF protection can be achieved, if the associated risks are sufficiently 

w and if it passes the ALARP test, then there is little basis to argue against it. Thus, 

rather than assuming that PMF protection is adequate, the proposed approach requires 

that selection of the PMF be justified in terms of a broader, more comprehensive set of 

societal values. Conversely, if less than PMF protection results in a very low level of risk 

and if the incremental cost of achieving PMF protection is grossly disproportionate to the 

risk reduction benefits gained, then we propose that it is reasonable to expect owners to 

prepare safety cases aimed at demonstrating that less than PMF protection is appropriate, 

that society or individuals are not exposed to excessive risks, and that it is worth taking

the risk to accrue the associated benefits. 

When viewed in this way the decision process is clearly both an enhancement of 

traditional practice by subjecting traditional practice to the ALARP test, and an advance 

over traditional practice which provides a basis for decision-making in situations where 

there are clear difficulties in applying traditional practice. The process has two important 

features that sets it aside from some contemporary views of risk assessment: 

. By embodying traditional practice this risk assessment approach to decision-making 

e

mechanical way by estimating the risk and comparing the estimate with a criterion 

ng

c

es in the early 1990’s (e.g. ANCOLD, BC Hydro) as decisions 

oncerning the tolerability of risk cannot be made in such a simple way. 

lo

1

is not presented as an alternative to traditional practice. 

2. The decision process is not a technocratic one whereby the decision is made in som

(e.g. expected value or F-N curve). 

Feature 1 means that the view that “the immediate objective of many of those advocati

risk assessment in current practice is to provide defensible design solutions as economi

optima that are likely to be of lower cost than those that result from a traditional

engineering standards approach to design” (ANCOLD, 1994) does not apply. Feature 2 

means that when applying risk assessment, it is necessary to go significantly beyond the 

view presented in guidelin

c

Paper 14 – Hartford 90
Decision Basis

The third feature of the decision process that drives dam owner’s research needs is the 

decision basis which includes the parameters required to make the decision. The risk to

risk

wis

important public concern and for the purpose of simplicity at the moment, it can be 

spe nce, it can be assumed 

sim

ove f the dam. Further, a more realistic 

and

Other parameters include the various measures of societal risk, including the risk to the 

population downstream of the dam, property damage, environmental degradation, and 

broader socio-economic activity impacts. 

To be meaningful for decision-making, it is important that the measure of risk to the 

individual include a full characterization of the uncertainty in the estimate of risk. The 

measure of uncertainty can be presented as a mean value with an associated probability 

distribution, or as a single value obtained by integrating over the probability distribution. 

This measure of uncertainty in the risk estimate (Figure 3) is something that is usually not 

found in contemporary applications of risk assessment, although some might argue that it 

is done in some cases where Monte Carlo simulation is used. The need to fully 

characterize the uncertainty on the risk estimate and the associated need to be able to 

demonstrate that the estimate of risk and associated uncertainties have been made in a 

scientifically valid and legally defensible way provide a basis for determining the 

research needs of dam owners. 

Figure 3 Full Characterization of the Risk 

The risk estimate and associated uncertainty can then be incorporated in a decision 

framework such as that of the Health and Safety Executive approach to judging the 

tolerability of risk (HSE, ibid.) is of the form illustrated in Figure 4.

the maximally exposed individual is one such parameter. People who are exposed to 

s are becoming increasingly interested in the nature and degree of the risk and often 

h to be in the position to make choices concerning their exposure. To address this 

assumed that the maximally exposed individual (MEI) has no means of escape and 

nds his/her entire life immediately downstream of the dam. He

that the risk to the MEI is equal to the probability of dam failure. Again this 

plification is conceptually sound for the purposes of this proposal because it 

restimates the risk to any individual downstream o

characterization of the risk to individuals downstream of dams is now available (Assaf 

Hartford 2001), and this more advanced approach can be used if required. 

Mean Value

Associated Uncertainty

91 Paper 14 - Hartford 
Figure 4. Tolerabi

Mean

Va ul e

BROADLY ACCEPTABLE

region: residual risk

insignificant

risk tow a r d s the broadly

acceptable region.

ty

UNACCEPTABLE

region : risks only

justified under

extr a ordinary

circumstances

TOLERABLE region: risk

control measures must be

introduced to drive the residual

Associated

U

ncerta ni

lity of Risk (HSE, 2001) 

qua

F

ed in terms of 

e time tested approach of open challenge, and have not been overtaken by modern 

cientific advances (cannot be improved upon at the moment) little in the way of research 

can be justified. 

A Basis for Determining Research Needs in Dam Safety Analysis 

It is possible to determine the research needs by examining how well the inputs to and 

outputs from the decision process meet requirements for quality, scientific validity, 

verifiability and legal defensibility. The quality and validity of the risk estimates as 

illustrated in Figure 3 and the reliability and legal defensibility of the decisions based on 

these estimates as utilized in the decision process of Figures 2 and 4 are dependent on the

lity and robustness of the procedures used to generate the risk information illustrated

in igure 3.

The determination of the quality and reliability of the risk estimates can be determined 

through analysis of the underlying procedures and the process used to integrate the 

outputs of the individual procedures. This entails decomposing the risk measure, 

including associated uncertainties into its fundamental parts and examining the 

robustness of each of the constituent procedures and the robustness of the integration 

process. Clearly, if each constituent procedure and the method of integration are robust 

and meet the accepted norms of broad professional acceptability establish

th

s

Paper 14 – Hartford 92
However, should it be revealed that the capability of characterizing the risk in a 

or

at the

the 

ng the risks

ting a

truction 

f a number of sub-models for the different sub-systems and elements. Linking all of the 

in Figure 5 (Hartford et al. 2001). 

scientifically valid and legally defensible way, is somewhat questionable, or that recent

scientific advances permit a marked improvement in the quality and economic efficiency

of the dam safety assurance process while not reducing safety standards, then a basis f

justifying research projects exists. 

While risk analysis may be based on past experience, with an implicit assumption th

past is a good predictor of the future, the relatively small number of dam failures in

historic record together with the lack of homogeneity across the population of dam

failures mean that statistical methods cannot be relied on as a basis for assessi

posed by individual dams. Therefore, the risk analysis process involves construc

description of how events may develop from a given initial state. This description is 

termed a model of the behavior of the different elements of the system under the various

conditions that can exist. Usually this modelling endeavor will require the cons

o

sub-models together constitutes part of the overall analysis process. 

One possible “process” model is illustrated

Figure 5. Process model for analyzing risk 

Annual Exceedance Frequency (AE

UNACCEPTABLE

REGION

TOLERABLE

REGION

10-3

10-4

10-5

10-6

Characteristics of PMF

F)

QIFF

10-2

10-1

Characteristics of IFF

Pseudo Pf 0

1

Q

QPMF

93 Paper 14 - Hartford 
Figure 5 which illustrates a model to make a conservative estimate of the risk to the 

individual, (it does not present all of the sub-models that are required to fully 

characterize all of the consequences of dam failure and associated uncertainties), 

provides insight into the form of the outputs of the sub models and provides a guide as to 

what attributes of these sub-models require verification. 

The question as to how reliably existing procedures generate the outputs of the submodels needs to be answered as does the question as to how reliably existing procedures

integrate all of the sub-model outputs to generate the model outputs. These questions are

important to owners who are responsible for risk control and to regulators who need to 

be satisfied that the owner’s risk control measures are reliable and that the risk 

assessment process used is genuinely informing the decision process. These questions 

represent two topic of research of particular interest to owners who, for legal due 

diligence reasons need to have substantial indicia of reliability of all aspects of the 

processes that generate the risk information. 

Fundamentally this requires demonstrating the scientific validity of the procedures where 

the term science refers to knowledge ascertained by observation and experiment, 

critically tested, systemized and brought under general principles. In this regard, 

researching the scientific validity of risk analysis and risk assessment procedures is 

of paramount importance to owners concerned about legal defensibility. 

In its decomposed form prise models that 

ermit construction of binatorial logic that 

cludes full characterization of all uncertainties in the models and the input parameters

igure 6)

, the overall model can be considered to com

p the risk measure by means of a process of com

in

(F

Figure 6. Conceptual form of the logic the fully decomposed form of the model

Investigations into the scientific validity of the overall risk assessment process

necessarily requires investigation into the scientific validity and completeness of the

Consequences

States of Nature

Logic T ree

Event Tree

System Events

Consequence T ree

Paper 14 – Hartford 94
supporting sub-processes. Therefore there is a need to demonstrate, through 

nce of 

Figure 7 illustrates the form of the fully characterized hazard in terms of the 

Figure 7. Fully characterization of the hydrologic hazard

Figure 7 provides an indication of the research needs for the hazard as in principle 

it is desirable to be able to reliably generate scientifically valid characterizations 

of the magnitude and probability of the hazard over the full range of physically 

possible realizations of the hazard. If the hazard cannot characterized in

this way then there may be a basis for owners supporting research aimed at

completely (to the extend that is possible) characterizing the hazard. 

Full Characterization of the Dam Response 

The uncertaint loading provides a

second area of research interest to dam owners (Figure 8). From a safety 

management perspective and with reference to Figure 2, the magnitude of the 

research, the extent to which the all aspects of the hazard and the performa

the dam under the influence of the hazard are fully characterized.

Full Characterization of the Hazard 

magnitude of the hazard and the associated uncertainties. 

10-2

10-1

Characteristics of

IFF

10-3

10-4

10-5

10-6

QIFF QPMF

Characteristics of

PMF

Q

Annual Exceedance Frequency

be fully

ies in the response of the dam to the applied

95 Paper 14 - Hartford 
“gap” between the existing design and the desirable situation is of paramount

 fruitful endeavor than research

into the dam response. 

it

rs that they mask the manner in which the probability of failure under static 

loading conditions might be represented in a risk analysis. Figure 8 appears to 

provide one “key” to estimating the probability of failure by piping, the second 

“key” being the logic and mathematics of the construction of epistemic 

probability distributions. In this regard, we can now propose that the fragility 

curves for the hydrologic hazard should be constructed over all loading conditions

from the annual (normal) through to the very extreme as illustrated in Figure 9. 

importance. Figure 8 also provides a perspective on the usefulness of researching 

the uncertainties in the response of the dam under the influence of the hazard. It 

appears from Figure 8, that the uncertainties in the dam response only represent a 

small part of the total uncertainty in the risk estimate. If this is the case, then

research into the nature of the hazard is a more Probability of Failure Pf 0

1

EXISTING DESIGN "DESIRABLE" STANDARDSBASED DESIGN "Hypothetical" fragility

curves

QIFF uncertainty in the actual

value of outflow Q that

causes the design basis

(e.g. overtopping of core)

to be exceeded.

Probability

Distribution

around QIFF

QPMF

Probability

Distribution

around QPMF

Knowledge (epistemic)

Q

Figure 8. Characterization of Uncertainties in dam response 

However, the fragility curves in Figure 8 may be somewhat deceptive as

appea

Paper 14 – Hartford 96
Probability of

Failure Pf 1

0

Suggested form of the

fragility curve under

"normal" loading

conditions

bability of

failure by

Pro

Figure 9. Suggested representation of the probability of failure under 

normal hydrologic loads 

This said, the debate around this matter is one for the geotechnical engineers as

the uncertainties in the hydrologic hazard under normal loading conditions are the 

least problematic from a characterization perspective. 

 

Full characterization of the consequences of dam failure 

There is significant scope for research into physically-based modelling of the 

consequences of dam failure. Two dimensional dam breach modelling, advances

in artificial intelligence (AI), digital terrain mapping, GIS systems and virtual 

reality modelling provide powerful tools that are helpful in this endeavor. 

undamentally it is desirable to that subjective ‘engineering judgments’ that 

usually co alyses be

replaced by with a physically-based procedure that provides a transparent basis 

s

fic

alistic

eavor of this

ature is to ensure that it is possible for peers to review and if necessary

produce all calculations. This requires that the basis for the models and the 

ecified and the ingredient data made

vailable (Cooke, 1991).

piping

F

nstitute substantial proportions of dam breach consequence an

for making inferences concerning the range of possible outcomes. A key

objective, consistent with the advice of the National Academies (NRC, 2000), i

that “the likelihoods of consequences (risks) should be estimated using scienti

reasoning from data”. Since loss of life data from dam failures is scarce, it is

necessary to generate synthetic data from simulations that are based on re

models of the situations that might evolve. Good practice in an end

n

re

inherent calculation procedures be fully sp

a

Outputs from a physically based model are of the form illustrated in Figure 10. 

97 Paper 14 - Hartford 
FE SCALED TO PAR DISTANT

LOCATION

LOSS OF LIFE SCALED TO PAR

PROBABILITY

SUMMER

SPRING

WINTER FALL

PAR (Spring/

Summer

PAR (Fall)

PAR (Winter) LOSS OF LI

PROBABILITY

NEAR DAM

LOCATION ENTIRE

INUNDATED AREA

Figure 10. Probability distributions of loss of life from dam failure generated by 

physically-based models 

The need for research in this area stem from three sources. First, from an 

emergency planning perspective these new modelling techniques can be used to 

simulate the various dam breach emergency scenarios that possibly unfold. 

Second, from the perspective of determining societal risk, statistical methods of 

estimating the loss of life from dam failures cannot be used to develop realistic 

probability distributions describing the full range of possible loss of life from dam 

failures because the historic record for each dam represents just one of a very 

large number (0 to the Population at Risk) of possible outcomes. Third, by 

accounting for the spatial and temporal variation of everyone in the vicinity of the

dam breach and not just those in the inundated area, these models permit 

simulation of the risk to individuals making it possible to characterize the risk to 

individual members of population groups

addition to physically-based modelling of life safety considerations, the 

“De

In recent years there has been some but not sufficient discussion on this potentially

divisive matter, but unfortunately it has not received sufficient attention. There is a very

significant possibility that this ongoing debate concerning ‘rigorous’ and ‘simplified’ 

approaches could degenerate into a serious dispute between the two ‘schools of thought’. 

This would be unfortunate and every effort should be made to prevent it from happening 

while at the same time fostering the difficult debate that is necessary to resolve any 

differences of opinion that might exist. 

 

However, regardless of which view one holds, there is really no basis to argue against the 

view that both rigorous and simplified approaches to estimating risk should be validated, 

i.e. demonstrated as being a reasonable and realistic simplification of reality, before being 

relied upon in risk assessment. 

 

In

technology now exists to rapidly and efficiently determine the property damage 

on a structure-by-structure basis. The computational capability now exists to 

maintain up-to-date inventories of everyone and everything that might be 

impacted by a dam failure. 

tailed” (Rigorous) vs. “Simplified” Approaches to Risk Analysis 

Paper 14 – Hartford 98
Since any rigorous analysis is an inevitable simplification of reality, the reasonableness 

of the approximations and simplifications introduced to permit a less rigorous analysis of 

the same problem should be demonstrated. Therefore, any simplified approach to risk 

analysis must be based on a more complex analysis process. In cases where the 

‘simplified’ approach is not based on a more complex approach, then the problem that is 

the subject of the analysis is a simple problem and the ‘simplified’ approach is ‘de-facto’ 

the complex approach. Here it becomes necessary for practitioners to be explicit about 

what they mean by ‘simplified’ or ‘practical’ approaches as well as the validity of any 

conclusions that might be drawn from the analysis. 

The development of any simplified approach entails the following steps: 

1. Develop a solid understanding of the physical processes involved. 

2. Develop a robust, theoretically sound and scientifically verifiable theory or model of 

the physical process recognizing that it is a simplification of reality. 

3. Introdu ise

the theory or modelling process

Therefo

‘rigoro

have ev ssible to arrive at ‘stage 3’

above without first completing stages 1 and 2, unless of course stages 1, 2 and 3 are one 

and the same. In practice, there may be grounds for debate if the simplified approaches 

are not actual problem in hand. 

However, such a debate is not one of ‘rigorous’ vs. ‘simplified’ rather it is one of values 

which

to whic

Obviou ches

to risk analysis by advocating scientific correctness, validation and verification. If the so 

called “

scientif

such “p

manage

three st

analysi

“what i

approac

One de

of ratio

the fac

princip subjective probability although traditional scientific 

method does not explicitly accommodate the use of “subjective probability” as scientific 

data. I ld

only relate to the extent to which uncertainty is described and the degree of defensibility 

ce additional valid simplifications and approximations that do not comprom

re, in principle, there are no grounds for divisive debates between advocates of 

us’ and ‘simplified’ approaches. In fact, the proponents of simplified approaches 

en more work to do initially as it is not generally po

demonstrated as being reasonable simplifications of the

reflects the extent to which there are differences of opinion concerning the extent 

h scientific correctness, validation and verification is necessary. 

sly this position might be misinterpreted as opposition to “practical” approa

practical or pragmatic approach” involves not conforming to the principles of

ic inference and probabilistic reasoning, then it is difficult to demonstrate that 

ractical” approaches would lead to legally defensible decisions and safety 

ment actions. In this regard, a simplified approach developed in terms of the 

eps outlined above remains faithful to these fundamental principles of risk 

s. Therefore, if the terms “practical” and “pragmatic” are used in the sense of 

s realistic” as opposed to “what is cheap and easy to perform”, then “practical” 

hes are in fact the ‘simplified’ approaches described above. 

sirable attribute of scientific defensibility of risk analysis is it entails the building 

nal consensus, an approach that has evolved over the centuries to compensate for

t that, “scientific proof” is notoriously fickle (Singh, 1997). These scientific 

les also find application in

n principle, the differences between “simplified” and “rigorous” analyses shou

99 Paper 14 - Hartford 
required. How far one goes in characterizing uncertainty, and ensuring defensiblity is, in 

part, a value judgement, either the owner’s values and/or the values of society and in part 

a professional matter where engineers are duty bound to comply to their codes of 

professional practice. 

Risk analysis problems are very often complex and difficult. Their simplification, if done 

without thorough understanding can lead to false results and not approximate results. 

Importantly, the analysts might not even realize this. There is considerable danger that 

ana n

and unacceptable trivialization. 

The sc

scientif

analysi

knowle

conditi

assessm

uncerta

seems

inquiry s, the analysis of risk involves the 

application of scientific analysis principles to reveal the state of knowledge of the

perform

In its d 96)

noted t g a risk

decision is not a matter for a recipe. Every step involves judgement, and the right 

choices are situation dependent. Still it is possible to identify objectives that also serve 

s criteria for success. These objectives and criteria are: Getting the science right;

Ge t,

nd Developing an accurate, balanced and informative synthesis.

nches of mathematics, logic and theories of the 

cs being the language of science. While the inputs may 

lysts are often ill equipped to distinguish between adequate and correct simplificatio

A Comment on the Science of Risk Assessment 

ience of risk assessment should not be confused with scientific research. The 

ic approach to risk assessment involves the application of scientific principles of

s in the gathering of data and in the synthesis of all facets of the existing

dge of the dam including its expected performance under all existing and future 

ons. It is through the application of scientific analysis principles that risk 

ent derives its systematic features. Since risk pertains to uncertainty and since

inty pertains to the state of knowledge and since knowledge pertains to science, it 

reasonable to view the analysis of risk as fundamentally a matter of scientific 

. For engineered systems such as dam

ance characteristics of the dam system. 

eliberations on “Understanding Risk” the National Research Council (NRC, 19

hat: “Structuring an effective analytic-deliberative process for informin

a

tting the right science; Getting the right participation; Getting the participation righ

a

This then raises two issues for dam owners, the research need to get the right science for the risk

problem in hand and the need to get the science right. 

The scientific approach to risk assessment does not exclude empirical data, personal 

experience of the phenomena involved and sound judgement. Rather, they are essential 

elements of the scientific approach, but they are not in themselves sufficient to construct 

the probability distributions. Certain bra

physical processes involved are also required to construct coherent, properly conditioned

probability distributions which are determined through the process of scientific inference.

The probability distributions are logical constructs of the state of knowledge expressed in 

mathematical form, mathemati

Paper 14 – Hartford 100
well, and usually do include subjective elements, it has been demonstrated (Howson an

Urbach) that the process of constructing scientifically valid probability distribution is

entirely objective. According to Howson and Urbach, “there is nothing subjective in t

Bayesian theory as a theory of inference: its canons of inductive reasoning a

impartial and objective.” The Bayesian approach to scientific reasoning provides the

fundamental rules for inferring probability distributions from elementary knowledge (e.g.

data). They note that “the probabilities might be personal, but the constraints impose

on them by the condition of consistency are certainly not.” 

d

he

re quite

d

The im ortance of demonstrating the scientific validity or the results of a risk analysis

used as a basis

for dam afety decisions, then the legal defensibility of these decisions will rest in part on 

le to reflect on the

cas al judges have a

mu theories - even if the expert witness is not a scientist 

(Ridenour, 1999). 

In other words, the Supreme Court cracked down on the use of junk science and crackpot 

t

akes no

zed'

ult for

ific'

id,

ific expert witnesses

should also apply to expert witnesses claiming only technical or other specialized 

p

should not be underestimated. If the results of a quantitative analysis are

s

the scientific validity of the analysis method. In this regard, it is valuab

admissibility of expert testimony in US Federal Courts, because in March 1999, in the 

e Kumho Tire Co. v. Carmichael, the Supreme Court held that tri

responsibility under Federal Rule of Evidence 702 (3) to ensure that all expert testimony 

st be based on reliable scientific

The implications of this case are very significant. Previously in 1993, in the Daubert 

case, the Supreme Court found that that, “in examining the admissibility of scientific

testimony, trial judges may exclude expert testimony if the judge decides that an expert 

scientific witness' theory has not been or cannot be tested; if it has not been peerreviewed or published; if the error rate of the theory or technique is unknown and if lowquality standards, or no standards, were in use during the testing or operation of the 

theory or technique in question. 

theories by expert (often paid) witnesses in federal courtrooms, even when the exper

witness claims only technical or other specialized, rather than purely scientific,

knowledge.”

According to Ridenour, “in Kumho Tire, Justice Breyer noted that Rule 702 "m

relevant distinction between 'scientific' knowledge and 'technical' or 'other speciali

knowledge" in expert testimony. Furthermore, said Breyer, it would prove diffic

judges to administer rules that "depended upon a distinction between 'scient

knowledge and 'technical' or 'other specialized' knowledge.” As a result, he sa

Daubert's rules allowing judges to judge the reliability of scient

knowledge.” 

Clearly, there are grounds for dam owners (including those outside the United States) to 

determine the implications of the Daubert and Kumho Tire rulings in relation to the legal 

defensibility of dam safety decisions based on the results of risk analyses. It is worth 

noting that the debate about the process for generating probabilities is about as old as the 

discipline itself. The reason for the development of the probability calculus was to 

101 Paper 14 - Hartford 
transform the early seventeenth century laws of probability from the intuition and 

experience of gamblers to a set of mathematical rules that more accurately describe the 

laws of chance. Even though Blaise Pascal was capable of developing an answer to the 

gambling problem posed to him by the professional Parisian gambler Antoine Gombaud, 

e Chevalier de Méré, he collaborated with the reclusive Pierre de Fermat to speed up 

ne does not normally associate Fermat’s name with the probability calculus but, 

ccording to Singh (p. 43), “together, Fermat and Pascal founded the essential rules that 

govern all game ect playing

nd betting strategies. Furthermore, these laws of probability have found applications in 

tributions through some internal 

ubjective) cognitive process.

quivalent propositions

equivalent.” (See Cooke, Chapter 3 for an explanation as to why probabilistic 

he process of correctly handling uncertainty is notoriously difficult and there are 

irectly.

th

the process of obtaining a rigorous solution to the problem. In doing so, they were led to 

the more subtle and sophisticated questions related to probability. Much was understood 

about the nature of these problems in the 17th Century, however their resolution provided 

immense challenges for philosophers until the end of the 20th Century. 

O

a

s of chance and that can be used by gamblers to define perf

a

a whole series of situations, ranging from speculating on the stock market to estimating 

the probability of a nuclear accident.”

However, it is vitally important that the mathematics of probabilistic analysis be correct 

because, as described by Singh, “probability problems are sometimes controversial 

because the mathematical answer, the true answer, is often contrary to what intuition 

might suggest. The failure of intuition is perhaps surprising because ‘survival of the 

fittest’ ought to provide a strong evolutionary pressure in favour of a brain naturally 

capable of analysing questions of probability. A talent for analysing probability should 

be part of our genetic make-up, and yet our intuition misleads us.” In short, the human 

brain is not equipped to construct probability dis

(s

The issue of using correct logic in assigning probabilities can be outlined as follows: 

According to Cooke (ibid.), “Arguments that are valid when the premises are known with 

certainty are not ‘almost valid’ when the premises are ‘almost certain’. Premises that 

are equivalent when known with certainty are not ‘almost equivalent’ when the premises 

are ‘almost certain’. Rather, discontinuities arise and just the slightest bit of uncertainty 

can change a valid argument into an invalid one or can make e

in

reasoning is much more difficult than deterministic reasoning.) 

T

numerous examples of relatively simple probability problems in everyday life where 

elementary errors in logic are made even by very knowledgeable people. It is also well 

known that uncertainty also plays tricks with one’s intuition with the result that intuition 

can’t be relied on in constructing probability distributions. The only way to guard against 

making these errors and avoiding the pitfalls inherent to reasoning under uncertainty is to 

ensure that the procedures used are mathematically sound. Legal defensibility follows

d

Paper 14 – Hartford 102
Against this background, there is no reason, apart perhaps for reasons of cost, that the 

scientific validity of dam safety risk analyses cannot be revealed by explicitly describing 

the logic used in the process of constructing the probability distributions. There is 

nothing subjective about logic. 

 

Conclusions 

On the basis of the above, it can be concluded that dam owners research needs can be 

ole process” with proper consideration of the links between subrocesses and the procedures used to integrate the outputs of the various sub-models. 

ith

 the 

r

hereby

should be fully characterized over the full range of loads as,

epending on the shape of the dam response fragility curve, the loading conditions substantially 

sment should be conducted, with the view to ensuring that the practice 

f risk estimation is robust and scientifically valid. 

s own right. 

s

ure

t they

present their view of the current capability as there is no incentive for owners to invest in 

broadly grouped into two categories: 

1. Research into the physical nature of the phenomena involved, the full characterization 

of all associated uncertainties and the development of new/better theoretical “Type

A” predictive models (Lambe, 1973) to assist in the endeavor of risk characterization. 

2. Research into the quality and the reliability the analytical processes and procedures 

used to inform dam safety decision-making, and, 

Concerning point 1, hydrologic research initiatives should be carried out within the 

context of the “wh

p

The suggested method of characterizing the flood hazard is fully compatible w

contemporary procedures for probabilistically characterizing earthquake hazards. In

light of Figure 9, it may now be possible, at least in principle to estimate the risks fo

static failure modes, floods and earthquakes in the same fundamental way t

permitting a unified way of estimating the total risk posed by a dam. 

Uncertainty in hydrologic hazards 

d

less than the PMF could be a significant contributor to the total risk. 

Research into the manner in which all forms of probability distributions that enter 

the risk asses

o

The matter of the extent of the detail in a risk analysis requires considerable 

research to ensure that appropriate guidance is available concerning the level of

rigor required in the analysis process for defensible decisions. However, provided 

the “less detailed” method is a valid simplification of the “fully detailed” method, 

then “less detailed methods can be used with some known confidence. The “science 

of simplification” of complex analysis problems is itself complex and worthy of 

research in it

Concerning point 2, there is presently no basis for owners to hold the view that method

are available to fully and reliably quantify the risk for all hydrologic failure modes. In

fact suggestions that methods are available to estimate the risks for dams for all fail

modes have the potential to seriously hinder the funding of much needed research.

Consequently proponents of risk assessment must be very careful in the way tha

103 Paper 14 - Hartford 
research if in fact sound methods are available to estimate the risks for dams for all 

failure modes. On the matter of quality and reliability of the risk assessments, owne

need to know how well calibrated the process and the people using it actually are

Reliance on the “ipse dixit” of the proponent is simply inadequate, rather substantial

indicia of reliability and well designed quality procedures for verification an

validation purposes will have to be developed to assure credibility and legal

defensibility of the risk assessment and decision-making processes. Against this

background there is good reason for research into how best to implement the

scientific approach to estimating probability dist

rs

.

d

or dam safety 

ssessments.

o

y

ff on the safety status of a dam in terms of the 

odern view of risk assessment as outlined by HSE (HSE, ibid.). Of course McDonald 

 sign 

f a

ce? 

the

t

ations

Safety Executive. [2001] Reducing Risks, Protecting People. HMSO. 

ributions f

a

Returning to McDonald et al’s observation (McDonald et al, ibid.) “methodologies for

estimating the chance of dam failure are poorly developed and, at the present time, d

not provide a defensible basis for the conclusive sign off on the safety status of a dam” it

is necessary to debate whether the analysis process presented in this paper will ultimatel

provide the basis for conclusive sign-o

m

et al’s observation creates another line of research that has not been discussed in

this paper, specifically, if “methodologies are not adequate for the conclusive

off on the safety status of a dam”, then precisely what can the results o

contemporary risk analysis be used for and with what degree of confiden

Perhaps this question alone is worthy of particular and urgent informed debate as

answer is not at all obvious and yet an answer must be found before risk assessmen

techniques can be used with any confidence in dam safety decision-making. 

References 

Assaf, H. and Hartford, D.N.D. [2001] “Physically-Based Modelling of Life Safety 

Considerations in Water Resource Decision-making”, Proceedings of the 1st ASCE

World Water and Environmental Resources Congress, Orlando, ASCE. 

Australian National Committee on Large Dams. [1994] “Guidelines on Risk 

Assessment.” 

BC Hydro[1993] “Interim Guidelines for Consequence-Based Dam Safety Evalu

and Improvements”. 

Cooke, R.M. [1991] “Experts in Uncertainty” Cambridge University Press. 

Hartford, D.N.D., and Stewart, R.A. [2001] “Handling Uncertainty in Dam Safety

Decision-making” Proc. of the Annual Meeting of the US Society on Dams, Denver. 

Health and

Paper 14 – Hartford 104
Howson, Colin., and Peter. Urbach. 1989. Scientific Reasoning; the Bayesian Approach. 

Open Court. 

Lambe, T.W. [1973] “Predictions in Soil Engineering” Rankine Lecture. Geotechnique 

, No. 2.

McDonald, L.A., B. Cooper and C-F me lessons from use of risk 

ssessment to aid Dam Safety Evaluation. Proc. 20 ICOLD Congress Q.76-R.18, Vol. 

dem

Red

23

Wan. [2000]. So

th a

1, pp. 279 - 302 

National Research Council. [1996] “Understanding Risk – making decisions in a 

ocratic society”. National Academies Press. 

National Research Council. [2000] “Risk Analysis and Uncertainty in Flood Damage 

uction Studies”.

Ridenour, A. [1999] “Supreme Court and Congress Act Against Junk Science”. 

National Policy Analysis # 237. National Center for Public Policy Research. 

Singh, S. 1997. Fermat’s Enigma. Viking 

105 Paper 14 - Hartford 
New Developments and Needs in Site-Specific 

Probable Maximum Precipitation (PMP) Studies 

Ed Tomlinson, PhD 

urrent Procedures

retical values that represent the 

aximum rainfall possible at particular geographic locations during a certain times of 

theoretically determine PMP values is not available. The 

rocedures that have been developed rely on analyzing the largest observed storms and 

asis 

m location. 

on and Analyses 

This procedure is based on having a reliable inventory of all historic extreme 

storms that have occurred over the region that is “geographically and climatologically” 

similar to the location being evaluated. Historically the Corps of Engineers, Bureau of 

Reclamation and the Weather Bureau (currently National Weather Service(NWS)) have 

conducted extreme storm searches and storm studies for the largest storm identified. 

These are detailed analyses that provided extensive information on the storm 

meteorological characteristics and the resulting rainfall amounts, timing and spatial 

patterns. During the past several decades, these storm searches and storm studies have

not been completed and site-specific PMP studies are required to perform storm searches 

and storm analyses to update the historic extreme storm rainfall data base for the 

geographic region around the drainage basin being studied. These storm searches and 

storm analyses are provided by the government agency or consultant performing the sitespecific PMP study. The results provide a partial update to the historic storm analyses 

data base but are not systematically added to the publicly available storm studies archives 

at the NWS and the Bureau of Reclamation. For most geographic regions, extreme storm

rainfall events are not systematically identified and the resulting rainfall characteristics 

are not evaluated. 

By 

Applied Weather Associates 

C

Probable maximum (PMP) values are theo

m

year assuming no change in climate. Historically it has been recognized that the 

technology needed to

p

applying standard procedures to compute extreme rainfall values that provide the b

for estimating PMP values for the geographic region surround the historic stor

Extreme Rainfall Storm Identificati

107 Paper 15 - Tomlinson 
Procedures and Analysis Technologies used in PMP Studies 

The initial task in performing PMP studies is the identification of extreme storm

events. During the first half of the last centu , there appears to have been considerable

effort expended on the identification of extreme storm events as soon as possible after 

they occurred. For the largest of these storm s were dispatched to the storm

location to collect additional rainfall information to aid in storm analyses. 

During more recent years, identification and study of extreme rainfall storm events has 

not accomplished as extensively. Where there is particular interest, storm studies have

been completed, e.g. Tropical Storm Alberto Georgia 1994 and Fort Collins, Colorado

1997. 

Once extreme rainfall producing storms have been identified, the rainfall amounts 

and patterns need to be evaluated. Isoheytal analyses of the rainfall amounts are 

developed along with mass curves for rainfall at locations with the heaviest rainfall. 

Depth-area-duration analyses (D-A-Ds) are completed to quantify the amount of rainfall 

that fell over various areas sizes for various durations. These analyses become the basis 

for describing the rainfall that actually resulted from the historic storm as well as 

estimating the maximum rainfall that the storm could have produced. 

Standard procedures have been developed to increase observed extreme rainfall 

amounts to estimate the maximum rainfall potential for each historic extreme rainfall 

storm. This procedure identifies dewpoint te peratures that are associated with the air 

mass that provided the atmosp at was converted to rain on

the ground. A climatology of maximum dewpoint temperatures is used to determine the 

maximum dewpoint temperature that could have potentially have been associated with

the storm and could have potentially contributed to an increase in the rainfall. The 

current climatology for maximum dewpoint temperatures for the US was published in 

1965 and provides maximum observed dewpoint temperatures. It was developed from a 

limited data base using procedures that are not well documented. The most recent 

Hydrometeorological Reports (HMR 57 and HMR 59) as well as recent site-specific PMP 

studies have developed updated climatologies to provide values for maximum dewpoint 

temperatures. These updates provide regional climatologies and use various methods to 

compute the maximum dewpoint climatologies used in each study. 

Historically 12-hour persisting dewpoint temperature values are used in the PMP 

procedure. This value provides the maximum dewpoint temperature that has persisted for 

12 hours or in other words, the minimum dewpoint temperature that occurred during the 

12 hour period. At least one regional PMP study has use average dewpoint temperatures 

averaged over time period consistent with the duration of the rainfall in an effort to more 

effectively represent the atmospheric moisture available to the storm for conversion to 

rainfall. 

The procedure for identifying the location for where the dewpoint temperatures 

are selected is based on an analysis of the winds fields associated with the storm

environment. For older storms (pre-1950), surface wind observations have been the 

primary data source for determining the inflow wind direction and magnitude. More 

recent storms have occurred when upper air wind analyses derived from weather balloon 

observations have been available and those wind fields have been used together with the 

ry

s, team

in

m

heric moisture to the storm th

Paper 15 - Tomlinson 108
surface winds to en the storm

moisture sourc to determine 

e appropriate winds to use and to determine a dewpoint one a location has been 

identified. Sea surface temperatures have been use as a substitute for dewpoint 

temperatures. 

In an effort to increase the torms for a particular location, 

storms which have occurred o the same climate region area 

considered. Under slightly different atmospheric conditions, those storms could have 

occurred over the location being studied while still maintained their basic characteristics. 

in the PMP analysis after being modified for differences in 

tmospheric moisture availability between the actual storm occurrence location and the 

location

uring the first half of the last century but identification and analyses of extreme rainfall 

tury. The data base should

be updated and catalog extreme rainfall events completed. Fortunately, extensive rainfall 

data fro

be

le with

Hydro

s

d made available to projects involved with site-specific PMP and 

hydrologic analyses. Evaluation of the exclusive use of 12 hour persisting dewpoint 

temperatures instead of average dewpoint temperatures for various durations needs to be 

evaluated. Maximum observed dewpoint temperature have been used and are still used to 

construct maximum dewpoint temperature climatologies. With over 50 years of 

dewpoint temperature observations digitally available for a large number of weather 

stations, return frequency climatologies should be developed to provide return frequency 

values for use in the PMP procedure. 

determine the storm winds. Particular difficulties arise wh

e is over ocean regions. Historically it has been very difficult

th

data base of extreme s

ver similar topography within

These storms are used

a

being studied.

 

Extreme Storm Analysis Needs 

 

Systemic identification of extreme rainfall events has not been a priority during 

the past several decades. Storms studies for a large number of storms were completed 

d

events was not continued throughout the latter half of the cen

m weather stations exist and can be efficiently used to search for the severe 

storms. 

A program should be established where extreme rainfall events are identified in 

real time. Procedures should be in place to supplement standard observation with 

additional information immediately after the storm occurrence. This information can

extremely valuable for detailed storm analyses and quickly becomes unavailab

time. 

Once a storm has been identified and supplemental information collected, the 

storm analysis should be completed using standard procedures and should provide 

standard products. Software has been developed, such has been used in the Maine

site-specific study, to efficiently blend hourly with daily rainfall observation, produce 

hourly isoheytal analyses and produce depth-area-duration tables. The storm analysis can

been completely in a timely manner and be made available to both the meteorology and

hydrology communities for use in evaluating the impact of the storm and potential 

implication on existing design criteria such as PMP values. 

For PMP applications, an update to the US maximum dewpoint climatology need

to be developed an

109 Paper 15 - Tomlinson 
urrently transposition limits for storms (determining the geographic region 

where a potentially occur)

are subjectively determined. More objective techniques have been developed using GIS 

to comp

ture and

e

s procedure.

nd

es 

C

n extreme storm could have occurred or a similar storm could

are the topography of the historic site location with locations where a sitespecific PMP study is being conducted. Use of these techniques in a standard procedure

can provide increased reliability in transpositioning storms. 

Recently the National Center for Environmental Prediction (NCEP) has 

developed a data base with modeled winds. Wind fields, along with tempera

other parameter fields, are available on a regular grid for various levels in the atmospher

four time a day or every six hours. This data base provides modeled data fields back to

about 1950. Procedures should be developed to incorporate these data into the PMP 

analysi

Meteorological models development during the past decade has provides a 

significant increase in the reliability of these models to simulate storm behavior a

rainfall amounts. The use of these models to provide maximum potential rainfall valu

should be investigated. Successful use of computer models will aid significantly in the 

understanding of extreme storms and in providing estimates of the maximum rainfall 

possible from these storms. 

Paper 15 - Tomlinson 110
HYDROLOGIC RESEARCH NEEDS FOR DAM SAFETY 

ANALYSIS 

by 

M.G. Schaefer Ph.D. P.E. 

nts

ory authority or meeting the project-specific needs of the client. 

low Design Floods (IDFs) are computed using standard rainfall-runoff watershed 

models via a variety of deterministic and probabilistic approaches. The watershed models 

may be event models with lu EC-HMS31, or may be 

els with distributed inputs such as WATFLOOD11,12. 

ntage of the Probable Maximum Precipitation (PMP). Hydrometeorological 

inp

icy or

of

than extreme values. 

Acc

um 

Flood 

eterministic flood computation model and treat the 

input parameters as variables instead of fixed values. Monte Carlo sampling 

MGS Engineering Consultants, Inc. 

November 14, 2001 

CURRENT PRACTICES IN ORGANIZATION 

As a consulting firm, our hydrologic practices are based on meeting either the requireme

of the governing regulat

Inf

mped inputs such as HEC-130 or H

continuous mod

Conventional Deterministic Method 

A conventional deterministic method is used in those cases where the regulatory 

authority utilizes a standards-based approach. This approach is typically used for 

computing Probable Maximum Floods (PMFs) or floods produced by a fixed 

perce

uts and watershed model parameters such as, antecedent watershed conditions,

minimal infiltration rates, snowpack magnitude, etc. are primarily based on pol

guidelines set by the governing regulatory authority. The magnitude of the design 

precipitation and the storm temporal pattern are likewise set by policy or rule. 

Mixed Deterministic-Probabilistic Method 

A mixed deterministic-probabilistic method is used in those cases where the 

regulatory authority utilizes frequency-based standards. Hydrometeorological inputs 

and some watershed model parameters are selected based on probabilistic analyses

historical data. The goal is to produce a flood with the same Annual Exceedance 

Probability (AEP) as the design storm magnitude specified by the governing agency.

This is what is commonly called an “AEP Neutral” approach and utilizes 

hydrometeorological inputs that are nearer typical values

ordingly, mean or median values of the hydrometeorological inputs are 

commonly used rather than conservative or worst-case values. 

Stochastic Modeling of Extreme Floods 

Stochastic modeling is utilized when information is needed on the magnitude-frequency

and seasonal characteristics of flood peak discharge, runoff volume and maxim

reservoir level. To address these needs we developed the Stochastic Event-based

Model (SEFM29) and also employ project-specific variations of that model. The basic

concept of SEFM is to employ a d

111 Paper 16 - Schaefer 
pro

at

contains

meters. The simulated floods 

d by

c

flood

e, flood runoff volume, and maximum reservoir level. Frequency 

in

off

ve been spent in the past, and will be spent in the future, for 

pgrading of dams due to perceived spillway inadequacies. The decision to upgrade is often 

ased largely on the findings of rainfall-runoff modeling for extreme floods. Currently, 

hydrologists use simplified conceptual and empirical models in conjunction with numerous 

policies, procedures, guidelines, and standards-of-practice in modeling extreme floods. This 

heavy reliance on institutionalized policies and procedures provides consistency of

application, but does not necessarily provide accurate estimates of extreme floods. 

If accuracy is defined as nearness to the truth, there are several factors that currently limit the 

ability of the engineering community to consistently achieve accurate results. First, in most 

applications, there is no benchmark for measuring accuracy. By the very nature of the 

problem, we are estimating extreme floods, events that are beyond, oftentimes well beyond, 

the historical data. Without a benchmark for accuracy, it is often difficult to make a 

compelling case that a given model, method, or procedure is superior to another. While 

models can be calibrated to past floods, there is almost always a need to extrapolate to obtain 

solutions for the extreme conditions of interest. 

Second, while there may be a large set of models to choose amongst for modeling the 

hydrologic processes, we are typically woefully short of obtaining the field data necessary 

for properly applying any of those models on a watershed basis. This leads to the use of 

simplified physically-based models and conceptual models of the hydrologic processes and 

excessive lumping of inputs. Some of these models may not perform well beyond the limits 

for which they were originally developed or calibrated. 

Given the life-safety and economic considerations involved in the decision to upgrade 

spillways, it is important that the rainfall-runoff modeling be conducted in a manner that 

truly emulates watershed flood response for extreme floods. Consideration of the discussion 

above indicates that greater research emphasis should be placed on analyses of extreme 

historical floods. The goal would be to improve the knowledge base and identify how 

cedures are used to allow the hydrometeorological input parameters to vary in

accordance with that observed in nature while preserving the natural dependencies th

exist between many of the climatic and hydrologic parameters. 

Multi-thousand computer simulations are conducted where each simulation

a set of input parameters that were selected based on the historical record and 

collectively preserves the dependencies between para

constitute elements of an annual maxima flood series that can be analyze

standard flood-frequency methods. The resultant flood magnitude-frequency 

estimates reflect the likelihood of occurrence of the various combinations of 

hydrometeorological factors that affect flood magnitude. The use of the stochasti

approach allows the development of separate magnitude-frequency curves for

peak discharg

information about maximum reservoir level is particularly important for use

hydrologic risk assessments because it accounts for flood peak discharge, run

volume, hydrograph shape, initial reservoir level, and reservoir operations. 

PROBLEMS FACED 

Enormous sums of money ha

u

b

Paper 16 - Schaefer 112
modeli nly

available for calibration. In addition, it is suggested that applied research be conducted in 

modeling that can have the greatest affect on the magnitude of 

extreme floods and for which questions and knowledge gaps exist. 

Example of a Specific

Use of the linear unit-hydrogra ian overland flow concepts remain 

mainstays of current practice. These methods/concepts originated over 70-years ago 

and application of these approaches has changed little since their origin. Since that 

time, it has been licated in terms of 

noff

the

y contributors in

forested watersheds and are common during long-duration general storms containing 

low to moderate rainfall intensities in the western US. 

ere

ing

theory is appropriate and when significant non-linearities occur. These are subject 

areas that warrant

NE

We at

wou

roach.

ns of the model, 

on watersheds for five large dams in the western North America18,19,20,26

. It has a 

promising future in e-frequency

ng of extreme floods differs from modeling of floods of a magnitude commo

those aspects of rainfall-runoff 

Problem

ph theory and Horton

learned that the runoff response can be very comp

the runoff pathways to a watercourse. While the runoff response is typically classified 

as surface runoff, interflow, and groundwater flow, the important distinction is the 

elapsed time in arriving at the receiving watercourse. It is now recognized that ru

during a flood includes elements of overland flow, saturated overland flow, 

throughflow, and the source area for these contributions varies during and following

storm event. In particular, interflow and throughflow1,10 are primar

Nearly all of the project-specific unit-hydrographs and synthetic unit-hydrographs

have been developed based on Hortonian overland flow concepts. Accordingly, th

are valid questions about the utility of a one size fits all unit-hydrograph for a 

watershed when the runoff process includes various runoff mechanisms with differ

response times. There is particular uncertainty when large extrapolation is required

during modeling of truly extraordinary storms approaching PMP and for estimation of

extraordinary floods approaching the magnitude of the PMF. Likewise, few studies

have been conducted to identify the conditions for which linear unit-hydrograph

additional research. 

W TECHNOLOGIES BEING EMPLOYED 

are currently using the following computer models and methods of data analysis th

ld be classified as new or recent technology. 

Stochastic Modeling of Extreme Floods 

The stochastic modeling of extreme floods is a relatively new hydrologic app

In the past 3-years, we have employed the SEFM model, or variatio

that it offers the ability to compute magnitud

relationships for a number of flood-related characteristics and can also provide 

seasonal characterizations. However, the model is data intensive and requires 

numerous probabilistic analyses of historical data to determine the frequencies of

occurrence for the various hydrometeorological inputs. The methods of analysis are 

continuing to evolve as more is learned about the probabilistic and seasonal behavior 

of the various hydrometeorological inputs. 

113 Paper 16 - Schaefer 
Use of Atmospheric Models in Rainfall-Runoff Modeling

An

Mic ed

dist d

for ture

gag

con d spatial

dist

pro

We atial

and fornia

21. 

hese patterns will be used in a resampling procedure for the stochastic modeling of floods 

for Folsom Dam on the Amer is currently under development that 

used

ample 

tic

Spatial Mapping of Regional Precipitation-Frequency Information 

olution

d 72-hour

ram. 

esearch about the hydrologic processes has been conducted for well over 70-years. 

ll of

plying advanced models in

ituations where the necessary input data are either, unavailable, inadequate, or overly 

take

o

ras.

atmospheric model (Danard6,7

) was used in stochastic modeling of extreme floods for

a Dam on the Upper Columbia River26 in British Columbia. The model provid

ributed daily precipitation and temperature information for a historical 96-year perio

the Upper Columbia watershed that was calibrated to precipitation and tempera

e measurements. This information was used to provide antecedent watershed 

ditions for WATFLOOD11,12 a distributed hydrologic model. The temporal an

ributions of extreme storms in the historical period were also used in a resampling 

cedure for stochastic modeling of extreme floods. 

Resampling Spatial and Temporal Patterns from Historical Storms 

 are presently working with the Army Corps of Engineers in the analysis of the sp

temporal patterns of historical storms on the American River in central Cali

T

ican River. A procedure

would allow combinations of spatial and temporal patterns from separate storms to be

in rainfall-runoff modeling. This would significantly increase the dataset of storms and 

include combinations both more severe and less severe than observed in the original s

of storms. The resampling approach replaces the conventional practice of using a synthe

design storm and can be used with either the conventional deterministic, mixed 

deterministic-probabilistic, or stochastic modeling approaches. 

 

Regional precipitation-frequency analysis9

 has been conducted at numerous locations 

around the globe since the late-1980’s. Recent improvements to the spatial mapping of 

precipitation pioneered by Daly4,5 and Oregon Climate Service15 now allow high-res

mapping of precipitation-frequency information. The spatial mapping of 100-year and 

1000-year precipitation has recently been completed27 for the 6-hour, 24-hour, an

durations for southwest British Columbia for use by BChydro in their dam safety prog

RESEARCH AND DEVELOPMENT NEEDS

R

Through past research, a sound scientific foundation now exists for describing nearly a

the hydrologic and hydraulic processes that are important for estimation of extreme floods.

However, there continues to be a large chasm between the tools developed by the research

community and the tools in common use by hydrologic engineers. 

This chasm is primarily due to the practical problem of ap

s

expensive to obtain on a watershed basis. Because of these constraints, it is difficult to

advantage of the capabilities of the advanced models. As a result, hydrologists continue t

use simplified conceptual and empirical models in conjunction with numerous policies, 

procedures, guidelines, and standards-of-practice. In many cases, existing policies and

procedures were developed decades ago based on information and experiences of past e

Paper 16 - Schaefer 114
These same policies and procedures commonly receive reinforcement through requireme

or guidelines set forth by regulatory agencies. 

 

The continued use of these institutionalized policies and procedures

nts 

has caused many 

ngineers to become comfortable with fixed values, or ranges of values for 

hyd shed model parameters. This heavy reliance on 

inst

con

ther

sup

The nue to

be

bas avoidable

reli

app

scie

e

rometeorological inputs and water

itutionalized policies, procedures, and guidelines has resulted in a false sense of 

fidence about the accuracy of our computations. It has also fostered the belief that 

e is a greater understanding of extreme storms and extreme floods than can be 

ported by the actual knowledge base.

ability to incorporate new hydrologic tools from the research community will conti

a problem due to the difficulties in obtaining the necessary input data on a watershed 

is. Therefore, watershed models will continue to slowly evolve with some un

ance on policies, procedures and guidelines. While hydrology is considered a science, 

lication of that science to practical problems on a watershed basis is as much art as 

nce. Given the current situation, application-based research is the primary need in the

dam safety area rather than pure or basic research. 

As d

ma

Co low.

a practical matter, those topics/issues that have the greatest potential impact on floo

gnitude, public/project safety, or rehabilitation cost should receive the higher priority. 

nsidering all the background above, several specific research needs are listed be

Short-Term R&D Needs 

Create a resource center for research findings and case-studies pertaining to 

historical extreme floods – It would be very useful to have a resource center for 

research findings and case studies pertaining to historical extreme floods. Th

could start as a simple electronic bibliography of research articles and case studi

for truly extreme floods. It could evol

is

es

ve into a University being a repository for 

information on extreme floods. It is likely that much information already exists in 

various research documents that could help address many of the questions and 

ds.

Needs

Non-linearity of the hydraulic response of watersheds

uncertainties about modeling of extreme floo

Long-Term R&D

 - Conduct research and 

sical conditions for which the time-lag 

ar cases.

examine case studies to identify the phy

and peaking characteristics of the hydraulic response of a watershed can be 

considered linear. Likewise, identify the physical conditions for which the 

hydraulic response of a watershed should be considered non-linear. Develop 

recommendations and procedures for hydrologic modeling of the non-line

Current policies on this issue vary greatly amongst agencies. 

Surface runoff versus interflow runoff for unit hydrographs - Most unithydrographs in the western US were developed at a time when runoff was

considered to be Hortonian surface runoff. More recent experience indicates that 

interflow runoff can be a major component of the runoff volume in western US 

115 Paper 16 - Schaefer 
mountain watersheds in response to low to moderate intensities in long-duration 

general storms

1,10. Misapplication of hydrologic models can result in large errors 

for situ icant 

be

Updating and Improve Depth-Area-Duration Storm Analyses for Mountainous 

ations where both surface runoff and interflow are both signif

contributors to the runoff volume. Depending on the method of analysis and the 

choice of the unit-hydrograph, situations can arise to produce either significant 

under-estimation or over-estimation of flood peak discharge. Past studies for 

development of synthetic unit-hydrographs should be revisited to determine if 

interflow was a major component. Where applicable, unit-hydrographs should

revised and policy changes implemented to better model the situation where both 

surface runoff and interflow are present. 

Areas

Update the current method for conducting depth-area-duration analyses for storms

in mountainous areas. Review the current procedures for conducting storm 

analyses for opportunities to incorporate recent technological improvements such 

as NEXRAD radar and satellite imagery. Conduct research to improve procedures 

for interpolation of precipitation in mountainous terrain. Conduct research to 

improve procedures for transposition of storm-related characteristics such as 

spatial patterns from one site to another in mountainous terrain. 

LICY NEEDS

Review of Hydrologic/Flood Safety Criteria

PO

- A review of hydrologic criteria for

sign of new dams and safety evaluation of existing dams is needed for those State 

Dam entage

roviding the desired level of protection. 

de

Safety Programs that employ flood safety criteria expressed as some perc

of Probable Maximum Precipitation (PMP) or Probable Maximum Flood (PMF). The

level of safety afforded by these types of criteria varies dramatically across the United

States (Schaefer17). This review may be conducted by examination of the frequency 

of occurrence of extreme storms in the state (Riedel and Schreiner16) and comparison 

with the results of regional precipitation-frequency analyses (Hosking and Wallis9,23

). 

This information will allow an assessment if the current flood safety criteria is 

p

References 

1. Barker BL and Johnson DL, Implications of Interflow Modeling on Spillway Design

Computations, Dam Safety ‘95, 1995 ASDSO Annual Conference Proceedings, 

Lexington KY, August 1995. 

2. Barker BL, Schaefer MG, Mumford JA, Swain RE, A Monte Carlo Approach to

Determine the Variability of PMF Estimates (Bumping Lake Dam), prepared fo

USBR Flood Hydrology Group, MGS Engineering Consultants, Final Report, July 

1997. 

3. Bell FC, The Areal Reduction Factors in Rainfall-Frequency Estimation

r

, Natural 

Environmental Research Council, (NERC), Report 35, Institute of Hydrology, 

Wallingford UK, 1976. 

Paper 16 - Schaefer 116
4. Daly C, PRISM, Parameter-Elevation Regression on Independent Slopes Model, 

Oregon State University, Oregon Climate Service, Corvallis Oregon, 1994. 

5. Daly C, Neilson RP, and Phillips DL, PRISM, A Statistical-Topographic Model for

Mapping Climatological Precipitation over Mountainous Terrain, Journal of App

Meteorology, Vol 33, pp140-158, 1994. 

6. Danard M, and Gailbraith J, Boundary Layer Models for Analyzing Precipitati

lied

on,

Maximum and Minimum Temperature and Snow Water Equivalent, Atmospheric

Dynamics Corporation for Hydroelectric Engineering Division BChydro, De

1994. 

cember 

7. Danard M, and Gailbraith J, Numerical Simulation of Daily Climatic Record for the 

r

Upper Columbia River Watershed 1899-1994, on CD-ROM files, Volume A, 

Atmospheric Dynamics Corporation for BChydro, February 1995A, revised March

1995. 

8. Hosking JRM, L-Moments: Analysis and Estimation of Distributions using Linea

Combinations of Order Statistics, Journal Royal Statistical Society, Ser B, 52, pp105-

124, 1990. 

9. Hosking JRM, and Wallis JR, Regional Frequency Analysis - An Approach Based on 

L-Moments, Cambridge Press, 1997. 

10. James DL, and Burges SJ, Hydrologic Modeling of Small Watersheds, ASAE 

Monograph No. 5, Chapter 11, 1982. 

11. Kouwen NJ, Soulis ED, Seglenieks F, Graham A, The Use of Distributed Rainfall

Data and Distributed Hydrologic Models for Estimation of Peak Flows for the

Columbia River Basin, for Hydroelectric Engineering Division, BChydro, Water

Research Institute, University of Waterloo, March 1997. 

Kouwen NJ, The WATFLOOD Hydrologic Model, WATFLOOD Users Manual, 

loo

12.

WATFLOOD SPL8, Flood Forecasting System, University of Waterloo, March 199

Miller JF, Frederick RH, and Tracey RJ, NOAA Atlas 2, Precipitation Frequenc

7.

13. y

Atlas of the Western United States, US Dept. of Commerce, NOAA, National 

Weather Service, 1973. 

14. National Research Council (NRC), Estimating Probabilities of Extreme Floods, 

Methods and Recommended Research, National Academy Press, Washington D

1988. 

Oregon Climate Service, Mean Annual Precipitation Maps for Western United

PRISM Model, Corvallis Oregon, 1997. 

Riedel JT, and Schreiner LC, Comparison of Generalized Estimates of Probable 

Maximum Precipitation and Greatest Observed Rainfalls, NOAA

C,

15. States, 

16.

Technical Report

WS 25, US Dept. of Commerce, NOAA, National Weather Service, 1980. 

17.

N

Schaefer MG, PMP and Other Extreme Storms, Concepts and Probabilities

Safety ‘95, 1995 ASDSO Annual Conference Proceedings, Lexington KY, Octobe

1995. 

, Dam 

r

117 Paper 16 - Schaefer 
18. Schaefer MG, Stochastic Modeling of Extreme Floods for A.R. Bowman Dam, M

Engineering Consultants Inc, November 1997. 

Schaefer MG and Barker BL, Assessment of Risk Reduction Measures at A.R. 

GS

19.

Bowman Dam using a Stochastic Model of Extreme Floods, MGS Engineering 

Consultants Inc, October 1998. 

20. Schaefer MG and Barker BL, Stochastic Modeling of Extreme Floods for Keechelus

Dam, MGS Engineering Consultants, September 1999. 

21. Schaefer MG and Barker BL, Precipitation Magnitude-Frequency Characteristics for

the American River Watershed, MGS Engineering Consultants, January 2000.

22. Schaefer MG, Characteristics of Extreme Precipitation Events in Washington State

Washin

, 

gton State Department of Ecology, Water Resources Program, 89-51, October 

23. gton

1989. 

Schaefer MG, Regional Analyses of Precipitation Annual Maxima in Washin

State, Water Resources Research, Vol. 26, No. 1, pp. 119-132, January 1990. 

24. Schaefer MG and Hosking JRM, Regional Analyses of Precipitation Annual Maxima

in the Pacific Northwest, American Geophysical Union Spring Conference, Bos

MA, May 1992 

ton

25. Schaefer MG, Magnitude Frequency Characteristics of Precipitation Annual Maxima

in Southern British Columbia, MGS Engineering Consultants, Inc., December 1997. 

26. Schaefer MG and Barker BL, Stochastic Modeling of Extreme Floods for Mica Dam

MGS Engineering Consultants, Inc., prepared for BC Hydro Power Supply and

Engineering, October 1999. 

27. Schaefer MG and Taylor GH, Spatial Mapping of 100-Year and 1000-Year 

Precipitation for 6-Hour, 24-Hour, and 72-Hour Durations for Southwestern British

Columbia, MGS Engineering Consultants, Inc. and Oregon Climate Service, prepa

for BC Hydro Power Supply and Engineering, March 2001. 

28. Stedinger JR, Vogel RM, and Foufoula-Georgiou E, Frequency Analysis of

red

Extreme

Events, Chapter 18, Handbook of Hydrolo

, 

gy, McGraw Hill, 1992. 

29. SEFM, Stochastic Event Flood Model (SEFM), MGS Engineering Consultants Inc, 

prepared for US Bureau of Reclamation Flood Hydrology Group, 1997-2001.

30. US Army Corps of Engineers; HEC-1 Hydrograph Package; Hydrologic Engineering 

Center, Davis, California, 1990.

31. US Army Corps of Engineers; HEC-HMS Hydrologic Modeling System; Hydrologic 

32. g

Engineering Center, Davis, California, 2000. 

US Department of Interior, Bureau of Reclamation, A Framework for Characterizin

Extreme Floods for Dam Safety Risk Assessment, prepared by Utah State University 

and USBR Flood Hydrology Group, Denver CO, November 1999. 

Paper 16 - Schaefer 118
Draft 

Simp EC-1

This pa

provide

combination with digital elevation models (DEM) to analyze the dambreak flood and 

pro

The Ca or of

the Sta

the state. The flood mapping was done using a 2-D hydrodynamic model (Mike 21) using 

exis

inu

not to produce detailed dam-failure flood maps, but to reveal those dams for which 

detailed tudies are required. This was a screening study and not a study to 

pro e

The modeling tools were chosen on the basis of convenience of use and the ability to link 

the a

modified version of NWS DAMBRK and the Danish Hydraulic Institute Mike 21 model 

for the dambreak flood analyses. However, nearly all of the maps were based on the 2-D 

Mik

Mike 2 re

or prov

develo mulate the dam failures. 

HE

files. A

The

mai

Resour

dam, crest elevation and length, reservoir storage capacity, and reservoir area. 

To

time, an

published values of crest elevation, crest length, dam height, and reservoir storage 

cap

Bac

The Ca m failure inundation mapping and 

em

making

agencie m.

The

establis

 

lified Dam Failure Analysis for Inundation Mapping Using H

By

J. J. DeVries1

Introduction

per describes a simplified approach to dam failure analysis using HEC-1 to 

input hydrographs for the hydrodynamic model Mike 21. Mike 21 was used in 

duce inundation maps. 

lifornia Office of Emergency Services (CA OES) was directed by the Govern

te of California to develop dam failure inundation maps for all non-federal dams in

ting topographic information available from the USGS. Nearly 500 dam failure 

ndation maps were produced in a four-month period. The purpose of this study was 

mapping s

vid the final detailed inundation maps for individual dams. 

generation of flood maps to the inundation process. CA OES selected two models:

e 21 modeling. 

1 requires an input hydrograph since the program does not simulate a dam failu

ide reservoir routing to compute the outflow hydrograph through the breach as it 

ps with time. HEC-1 has these features so it was used to si

C-1 also has an option for writing the calculated hydrograph data to user-formatted

utility program was used to reformat the HEC-1 output as Mike 21 input.

database for dam and reservoir characteristics for the State of California is 

ntained by the Division of Dam Safety of the California Department of Water 

ces for dams under their jurisdiction. The information includes the height of the

simulate a dam failure HEC-1 requires breach characteristics, including total failure 

d an elevation-capacity curve for the reservoir. These were developed from the 

acity.

kground

lifornia Office of Emergency Services da

ergency procedure program applies to dams meeting specific size requirements 

them subject to the jurisdiction of the State of California. Dams owned by 

s of the United States government are not under the jurisdiction of this progra

legislative intent of the original CA OES seismic safety of dams legislation was to

h emergency procedures for the evacuation and control of populated areas below 

1

 David Ford Consulting Engineers and State of California 

119 Paper 17 - DeVries
dam

Dam ow view and approval in accordance 

wit

wat

upon ap y

in the m t emergency procedures for the evacuation and 

con

HE

t

possible to define specific characteristics for each dam am and reservoir. Dams were 

clas

the par

Assum oncerning dam failure: 

• 

• Failure is for full height of dam 

Tab

Param Earth/rockfill Dams Concrete Dams 

s which could be used to save lives and reduce injury in the event of a dam failure. 

ners submit inundation maps to CA OES for re

h guidance issued by CA OES. Inundation maps represent the best estimate of where

er would flow if a dam failed completely and suddenly with a full reservoir. Based

proved inundation maps, or the delineated areas, cities and counties with territor

apped areas are required to adop

trol of populated areas below the dams. 

C-1 dam failure analysis

Breach analysis—Because a large number of dams had to be analyzed, it was no

sified as either an earth or rockfill dam or a concrete dam. The following table shows 

ameters for the two types of dams. 

ptions c

• “Sunny-day” failure 

Reservoir is full 

• Failure is rapid

 Earth dam: 15 minutes for full failure

 Concrete dam: 9 minutes for full failure 

• The maximum expected breach width occurs 

le 1. Dam breach parameters:

eter 

Side slope of breach 1.0 0.2 

ntil breach reaches its

um size (hr)

0.25 0.15

width of breach (ft) 5 * H 1 *

Time u

maxim

Bottom H

Discharge through turbines(cfs) 10, 100, 1000 Value kept as small as 

possible. 

Elevation of water surface in Weir Crest Elevation Weir Cre

res

st Elevation

ervoir at failure

Inflow at upstream end of Small dams : 10 cfs Inflow to reservoir =

reservoir Medium dams: 100 cfs

Large dams : 1000 cfs

discharge through

turbines

Paper 17 – DeVries 120
Figure 1. Breach geometry for earthen dams

5 Hd

1:1 side slopes

Final breach area 

d

Dam Width, W

H

Figure 2: Dam failure hydrograph generated by HEC-1 

121 Paper 17 - DeVries
Hydrodynamic model 

MIKE 21 BACKGROUND 

The version of Mike 21 used in this study simulates water level variations and flows in 

response to a variety of forcing functions in water bodies. The water levels and flows are

resolved on a rectangular grid covering the area of interest when provided with the

athymetry, bed resistance coefficients, wind field, hydrographic boundary conditions, 

time-dependent non-linear equations of continuity and 

conserv

rs used for all individual inundation models 

r boundary resistance. Higher values were used in 

canyon areas than in the fl alue was used for the entire model. No 

effort was made to vary M d use. Each 30-meter grid cell was 

represented by a single ele a single roughness coefficient. The dam

failure tial steady flow and a constant base

flow ad bility problems occurred without 

adding these to the hydrog

able 2. Parameters used in Mike 21.

Parameter Value used 

b

etc. The system solves the full

ation of momentum. The solution is obtained using an implicit ADI finite 

difference scheme of second-order accuracy. 

Mike 21 parameters: 

The only changes to the modeling paramete

were to the Manning’s n values used fo

oodplains. A single v

anning’s n by lan

vation value and

hydrograph generated by HEC-1 had an ini

ded at the end of the hydrograph. Model sta

raph.

T

Eddy Viscosity 5 m2/s 

Wind Conditions No wind 

Dry Depth 0.04 m 

Flood Depth 0.10 m 

Manning’s n - Canyons 0.06 to 0.08 

Manning’s n - Floodplain 0.

Results 

The assumptions used produc

failure; the

servatively large flows the simulated dam

considered conservativel low; and many of the

etry were not represented the 30-m

04 to 0.05

ed con from

Manning’s n-values were y

detailed features of the surface geom in DEMs.

was concluded that the inundation maps developed using these data typically overredict expected inundation areas that could occur as a result of a dam failure. 

primary purpose of this study was to identify dams for which flooding due to dam

ilure could cause major risk to people and property. It is the dam owner’s responsibility 

submit inundation maps to CA OES for review and approval in accordance with 

uidance issued by CA OES. Based on the results of this study various dam owners were 

directed to have dam failure inundation studies made. 

It

p

A

fa

to

g

Paper 17 – DeVries 122
Several maps developed in this study were compared with inundation maps prepared by 

engineering firms for dam owners. None of the previous comparison studies used twodimensional flood models. In general the tools used for dambreak flood analysis have 

been the one-dimensional unsteady-flow DAMBRK model or steady flow onedimensional models (HEC-RAS or HEC-2). For the latter, dambreak flood hydrographs 

were generated by HEC-1 or the National Weather Service model BREACH. 

Maps developed in this study showed good agreement with maps produced in previous 

studies, especially in steeper areas and areas with well-defined channelizing features. The 

results differed in flatter floodplain areas, especially where flow could split around small 

hills and other topographic forms. 

More accurate definition of potential inundation areas could be obtained using twodimensional hydrodynamic models. However, accurate topographic information is

required. The widely available 30-m DEMs do not provide very accurate information on 

topography, and more detailed topographic data should be obtained. 

sions

This study was conducted to establish emergency procedures for the evacuation and 

control of populated areas below dams that could be used to save lives and reduce injury 

in case of a dam failure. Inundation maps represent the best estimate of where water 

would flow if a dam failed completely and suddenly with a full reservoir. Based upon 

approved inundation maps, or the delineated areas, cities and counties with territory in 

the mapped areas are required to adopt emergency procedures for the evacuation and 

control of populated areas below the dams. 

More detailed investigations of individual dams would produce more accurate definition 

of potential inundation areas, and these are expected to be done in the future. The results 

from this study were judged to be appropriate in the light of the purpose of the study, and 

on the basis of comparisons of inundation maps submitted by dam owners appeared to 

provide approximately the same level of accuracy as obtained by commonly used 

inundation mapping methods. 

Conclu

123 Paper 17 - DeVries
A FRAMEWORK FOR CHARACTERIZATION OF EXTREME 

FLOODS FOR DAM SAFETY RISK ASSESSMENTS

Robert E. Swain5, David Bowles6, and Dean Ostenaa7

Abstract

Risk-based decisions require different types of information than standards-based

decisions. Traditional sources of information used for estimating probabilities of extreme

loods include gaged streamflow records, indirect discharge measurements, and 

n 100

es the

 1 in 10,000, or less, 

mphasis is placed on developing probabilistic estimates using regional 

ydrometeorological data and paleofl n. The uncertainties associated with

escriptions of flood flow exceedanc are likely to be substantial and an 

,

od

he

al

the

sk

6). An

ty Risk Assessment is the development of probabilistic 

xtreme flood estimates. This shifts the focus for dam safety flood evaluation from 

uting a single “maximum” event (i.e. the probable maximum flood, PMF) to 

consideration of the entire range of plausible inflow flood events, and ultimately to the 

magnitude-frequency relationship of maximum reservoir stages. 

 

f

precipitation records. Generally these data sources have records that are less tha

years in length. This framework for flood characterization for risk assessments us

length of the data record and other characteristics of the data to determine the credible

extrapolation limits used in the flood frequency analysis. Because risk assessments

require estimation of floods with annual exceedance probabilities of

e

h ood informatio

d e probabilities

important attribute to convey into the risk assessment. 

No single approach is capable of providing estimates of extreme floods over the

full range of annual exceedance probabilities required for risk assessment. Therefore

results from a number of approaches need to be combined to yield a composite flo

characterization; this means several methods and sources of data are needed. T

application of several independent methods applicable to the same range of annu

exceedance probabilities will increase the credibility and resulting confidence in

results.

Introduction

The U.S. Bureau of Reclamation is now making extensive use of quantitative ri

assessment in support of dam safety decision making (Von Thun and Smart, 199

important input to Dam Safe

e

ro

5

Technical Specialist - Flood Hydrology, Bureau of Reclamation, P.O. Box 25007, Denver CO ; 80225-

0007; E-mail-rswain@do.usbr.gov 6

Professor of Civil and Environmental Engineering, Utah Water Research Laboratory, Utah State 

University, Logan UT 84322-8200; E-mail-dbowl@pub.uwrl.usu.edu 7

Lead Geologist, Geophysics, Paleohydrology and Seismotectonics Group, D-8330, Bureau of

Reclamation, P.O. Box 25007, Denver CO 80225-0007; E-mail-dostenaa@do.usbr.gov 

 125 Paper 18 – Swain, Bowles, Ostenaa
Paper 18 – Swain, Bowles, Ostenaa 126

For floods, the risk assessment process involves selecting a spillway evaluation flood (SEF) 

based on the probability of dam failure and the severity of the incremental consequences of dam 

failure. Past practice also examined consequence , but without formal consideration of probability 

of failure; if consequences were judged to be large, the SEF was chosen as the PMF. 

Reclamation has identified the need for a review of its present procedures for developing 

probabilistic extreme flood estimates and their associated uncertainties for use in dam safety risk 

assessment. Where practical, Reclamation would ke to develop improved procedures. The overall 

objective is to develop a practical, robust, con istent, and credible framework for developing 

probabilistic extreme flood estimates for Dam Sa ty Risk Assessment. The desired outcome is a 

robust framework in which components can be improved in the future as the state-of-the-art 

develops. 

The framework was developed by inviting a group of approximately 20 professionals with 

extensive experience in the theoretical and practical aspects of physical, paleo-, and statistical flood 

hydrology and hydrometeorology to participate in a one-week workshop held at Utah State 

University in June 1997. Participants from North America, Australia, and the United Kingdom 

reviewed current Reclamation practice, and evaluated various advances in developing probabilistic 

extreme flood estimates for their potential role in the needed framework. A smaller group met in 

Denver to develop the details of the framework. This paper summarizes the findings of these 

groups. 

Risk Assessment Stages

Present Reclamation risk assessment practice uses a staged approach for conducting risk assessments 

(USBR, 1997a). Project schedule and budget constraints are considered in determining the type of 

flood assessment prepared at each stage. While each risk assessment is unique, the following stages 

are generally used in Reclamation risk assessments: 

a) Screening Level Risk Assessment: An evaluation of risk that includes definition of load 

probabilities and consequences for all load classes (flood, earthquake, and static). Structure 

failure probabilities and associated uncertainties are also considered in a global sense, but 

detailed event trees are not usually prepared. An emphasis at this stage is to maximize the 

use of available information, without conducting new analyses or collecting additional data. 

The intent is to identify areas where risks are potentially high and to determine the need for 

further evaluations and data collection. Results of these evaluations are used to determine 

Reclamation’s risk profile and to “screen” out dam safety issues where additional funding 

and effort appears to have little potential for reducing dam safety risks. 

b) Scoping Level Risk Assessment: A more detailed evaluation of risks is performed for the 

dam safety issues identified in a screening level risk assessment. This level of risk 

assessment typically involves more detailed treatment of event trees, load probabilities, 

structural response, and consequences. The intent is to invest sufficient effort so that the risk 

assessment team understands the major contributors to risk to enable formulation of risk 

reduction strategies and to determine the need for additional analyses and investigations. 

s

li

s

fe
ecision Level Risk Assessment: At this level, more detailed evaluation of 

d to provide decision makers with the information necessary to reac

 stenaa

c) D risks is 

performe h a dam safety 

decision for a structure. The decision may be related to continuing project operations, 

correcting dam tion alternatives, or 

determining the need for interim actions to reduce risk while long term plans are developed. 

The intent is to provide decision makers with sufficient pertinent risk information such that 

the risk reduction objective can be considered along with other Reclamation 

objectives. At this level of risk assessment, detailed loading information, structural response 

analy e of 

risk 

risk 

asses the 

extrap d 

hazar ave 

recor ded 

to ab precipitation and streamflow data can 

create pooled s of 

flood

Strea

eme 

flood s for risk assessment. Streamflow data are used in flood hazard assessment as input 

for frequency studies or as the basis for developing flood hydrographs. The usual source of these 

data is the streamflow records collected a the U.S. Geological Survey. However, 

similar data are collected and archived by many other Federal and State government agencies and 

some non-go shed 

gagin lude 

estim ost 

stream that 

far. ars. 

Com

Climate Data

Precipitation and weather data used in hydrologic models can include rainfall, snowfall, snow 

water equivalent, temperature, solar radiation, and wind speed and direction from individual weather 

statio s. Data 

types s vary greatly in record length and quality throughout the United 

States n, and wind) 

 127 Paper 18 – Swain, Bowles, O

 safety deficiencies, selecting among risk reduc

 effectively 

ses, and consequence evaluations are developed for all significant issues. This typ

risk assessment focuses on reducing uncertainties in the risk estimates and evaluating 

reduction actions. 

Data Sources

The proposed framework for developing probabilistic extreme flood estimates for 

sment uses the length of record and other characteristics of the data to determine

olation limits for flood frequency analysis. Traditional sources of information used for floo

d analyses include streamflow and precipitation records. Generally, these data sources h

ds that are less than 100 years in length, although in some cases these records can be exten

out 150 years using historical information. Regional

 data sets from short periods of observation, and paleoflood data can extend record

s to periods of up to several thousand years. 

mflow Data 

Many different types of streamflow information are used in developing probabilistic extr

 estimate

nd maintained by

vernment organizations. Streamflow records consist of data collected at establi

g stations and indirect measurements of streamflow at other sites. Streamflow data can inc

ates of peak discharge, as well as average or mean discharge for various time periods. M

flow measurements on U.S. streams began after 1900 with only a few records dating back 

Most often, streamflow records at a single site range in length from about 20 to 60 ye

pleteness of the data set may vary from station to station. 

ns, as well as remote sensing information and radar information for broader region

 available from various source

. Some of these types of data (i.e., snowfall, snow water equivalent, solar radiatio
are lim

eling calculations. However, as with any type of historical data, the accuracy and validity

of the o

istorical data, paleoflood data do not involve direct human observation of the flood events. 

nstead, the paleoflood investigator studies geomorphic and stratigraphic records (various indicators) 

as the evidence of past floods and streamflow derived from historical, 

rcheological, dendrochronologic, or other sources. The advantage of paleoflood data is that it is 

h can put exceptional annual peak discharge 

stimates in context and assist in reconciliation of conflicting historical records. 

stages

dischar

compon

which

dischar

for ext

treated

Extrapolation Limits for Different Data Types

from th

analysi

extensi

differe data

ources should be selected to meet project requirements. The greatest gains to be made in providing 

ited to record lengths of less than about 30 years; basic rainfall and temperature data are 

available for some stations for up to 150 years, but in most cases are limited to less than 100 years. 

Historical Data

Historical data can provide a means for extending the length of record for many types of

data, in particular for observations of the most extreme events. These data are most commonly used

to extend streamflow records of peak discharge prior to organized stream gaging. Historical

observations can provide information for other types of data such as weather patterns and the 

frequency of extreme storm events, or changes in land use or vegetation that may be significant to 

runoff mod

bservations must be carefully assessed and compared to the other types of data used in the

analysis. 

Paleoflood Data

Paleoflood hydrology is the study of past or ancient flood events which occurred prior to the 

time of human observation or direct measurement by modern hydrological procedures (Baker, 1987). 

Unlike h

I

of past floods, as well

a

often possible to develop records that are 10 to 100 times longer than conventional or historical

records from other data sources in the western United States. In addition, the paleoflood record is a 

long-term measure of the tendency of a river to produce large floods. In many cases, paleoflood 

studies can provide a long-term perspective, whic

e

Paleoflood data generally include records of the largest floods, or commonly the limits on the 

of the largest floods over long time periods. This information can be converted to peak 

ges using a hydraulic flow model. Generally, paleoflood data consist of two independent 

ents. One component is a peak discharge estimate; the second is a time period or age over 

the peak discharge estimate applies. Paleoflood studies can provide estimates of peak 

ge for specific floods in the past, or they can provide exceedance and non-exceedance bounds 

ended time periods. Each of these differing types of paleoflood data must be appropriately 

in flood frequency analyses.

The primary basis for a limit on credible extrapolation of extreme flood estimates derives

e characteristics of the data and the record length used in the analysis. The data used in the 

s provide the only basis for verification of the analysis or modeling results, and as such, 

ons beyond the data cannot be verified. Different risk assessments require flood estimates for

nt ranges of annual exceedance probability (AEP), and therefore analysis procedures and

Paper 18 – Swain, Bowles, Ostenaa 128

s
 129 Paper 18 – Swain, Bowles, Ostenaa

ibl

sources

regiona

credibl

1 in 10

low AE

lists the

typical 

subsequ

however, opinions differed amongst workshop participants. In general, the optimal limits are based 

n the best combination(s) of data envisioned in the western U.S. in the foreseeable future. Typical 

limits are based on the combination(s) of be commonly available and analyzed for 

ost sites. 

se types of records to have meaning in estimates of 

xtreme floods for dam safety risk assessment. This climatic constraint indicates that an optimal 

ion from paleoflood data, when combined with at-site gaged data, for a single 

tream should be about 1 in 10,000 AEP. For regional precipitation data, a similar limit is imposed 

becaus

any paleoflood records do not extend 

 10,000 years, and extensive regional paleoflood data sets do not currently exist. Using a record 

t 4,000 years, a typical limit of credible extrapolation might be an AEP of 1 in 15,000 

ased on regional streamflow and regional paleoflood data. 

cred e estimates of extreme floods can be achieved by combining regional data from multiple 

. Thus, analysis approaches that pool data and information from regional precipitation, 

l streamflow, and regional paleoflood sources should provide the highest assurance of 

e characterization of low AEP floods. 

For many Reclamation dam safety risk assessments, flood estimates are needed for AEPs of 

,000 and ranging down to 1 in 100,000, or even lower. Developing credible estimates at these 

Ps generally require combining data from multiple sources and a regional approach. Table 1 

 different types of data which can be used as a basis for flood frequency estimates, and the 

and optimal limits of credible extrapolation for AEP, based on workshop discussions or 

ent communications. The limits presented in the table represent a general group consensus; 

o

data which would 

m

Many factors can affect the equivalent independent record length for the optimal case. For 

example, gaged streamflow records in the western United States only rarely exceed 100 years in 

length, and extrapolation beyond twice the length of record, or to about 1 in 200 AEP, is generally 

not recommended (IACWD, 1982). Likewise, for regional streamflow data the optimal limit of 

credible extrapolation is established at 1 in 1,000 AEP by considering the number of stations in the 

region, lengths of record, and degree of independence of these data (Hosking and Wallis, 1997). For 

paleoflood data, only in the Holocene epoch, or the past 10,000 years, is climate judged to be 

sufficiently like that of the present climate, for the

e

limit for extrapolat

s

e of the difficulty in collecting sufficient station-years of clearly independent precipitation 

records in the orographically complex regions of the western United States. Combined data sets of 

regional gaged and regional paleoflood data can be extended to smaller AEPs, perhaps to about 1 in 

40,000, in regions with abundant paleoflood data. Analysis approaches that combine all types of 

data are judged to be capable of providing credible estimates to an AEP limit of about 1 in 100,000 

under optimal conditions. 

In many situations, credible extrapolation limits may be less than optimal. Typical limits 

would need to reflect the practical constraints on the equivalent independent record length that apply 

for a particular location. For example, many at-site streamflow record lengths are shorter than 100 

years. If in a typical situation the record length is only 50 years, then the limit of credible 

extrapolation might be an AEP of about 1 in 100. Similarly, m

to

length of abou

b

The information presented in Table 1 is intended as a guide; each situation is different and 

should be assessed individually. The limits of extrapolation should be determined by evaluating the 

length of record, number of stations in a hydrologically homogeneous region, degree of correlation 

between stations, and other data characteristics which may affect the accuracy of the data. 
ractical limits on the range of the floods to which AEPs can be assigned. There does not appear to 

to justify computation of AEPs less than 1 in 100,000. In general, the scientific 

mit to which the flood frequency relationship can be credibly extended, based upon any 

charact

Hydrometeorological Data Types and Extrapolation Limits for Flood Frequency 

Analysis

Ideally, one would like to construct the flood frequency distribution for all floods that could 

conceivably occur. However, the limits of data and flood experience for any site or region place

p

be sufficient data

li

eristics of the data and the record length, will fall short of the probable maximum flood 

(PMF) for a site. PMF estimates provide a useful reference to past practice and can be compared 

with extreme floods characterized for risk assessment. However, the workshop participants 

concluded that there is limited scientific basis for assigning an AEP to the PMF. For precipitation 

data, similar limitations apply to extrapolations that approach values described by probable

maximum precipitation. 

Table 1. 

Limit of Credible Extrapolation 

for Annual Exceedance 

Type of Data Used for Flood Frequency Analysis Probability

Typical Optimal 

At-site streamflow data 1 in 100 1 in 200 

Regional streamflow data 1 in 750 1 in 1,000 

At-site streamflow and at-site paleoflood data 1 in 4,000 1 in 10,000 

Regional precipitation data 1 in 2,000 1 in 10,000 

Regional streamflow and regional paleoflood data 1 in 15,000 1 in 40,000 

Combinations of regional data sets and extrapolation 1 in 40,000 1 in 100,000 

Methods of Analysis

At Site Flood Frequency Analysis

Frequency an long record of flood

lows, or perhaps rainfall for a basin, then a frequency distribution for a site could be determined 

with go

alysis is an information problem: if one had a sufficiently

f

od precision, so long as change over time due to anthropogenic or natural processes did not

alter the distribution of floods. In most situations available data are insufficient to precisely define 

the annual exceedance probability of large floods. This forces hydrologists to use practical 

knowledge of the physical processes involved, and efficient and robust statistical techniques, to 

develop their estimates (Stedinger et al., 1993). 

Paper 18 – Swain, Bowles, Ostenaa 130
 131 Paper 18 – Swain, Bowles, Ostenaa

d quantiles and expectations can be calculated with the 

"fitted"

ther method that may be used to estimate the parameters of a distribution for at-site 

frequen

s. As a result L-moment estimators of the dimensionless coefficients of 

variatio

 The National Research Council (1988) 

has pro

frequency 

analysis: (1) average parameter approach; (2) index flood approach; and (3) specific frequency 

Fitting a distribution to data sets allows both a compact and smoothed representation of the 

frequency distribution revealed by the available data, and a systematic procedure for extrapolation to 

frequencies beyond the range of the data set. Given a family of distributions, one can estimate the 

parameters of that distribution so that require

 model. Appropriate choices for distribution functions can be based upon examination of the 

data using probability plots, the physical origins of the data, previous experience, or prescriptive 

guidelines. 

Several general approaches are available for estimating the parameters of a distribution. A 

simple approach is the method of moments, which uses the available sample to compute estimators of 

the distribution’s parameters. The Federal guidelines published in Bulletin 17B (IACWD, 1982) 

recommend fitting a Pearson type 3 distribution to the common base 10 logarithms of the peak 

discharges. It uses at-site data to estimate the sample mean and variance of the logarithms of the 

flood flows, and a combination of at-site and regional information to estimate skewness. 

Ano

cy analysis is the Expected Moments Algorithm (EMA). EMA (Cohn et al., 1997) is a 

moments-based estimation procedure and is identical to the existing Bulletin 17B (IAWCD, 1982) 

approach when no high or low outliers are present. The EMA method was developed to utilize 

historical and paleoflood information in a censored data framework. This approach explicitly 

acknowledges the number of known and unknown values above and below a threshold, similar to a 

maximum-likelihood approach. Three types of at-site flood information are used: systematic stream 

gage records; information about the magnitudes of historical floods; and knowledge of the number of 

years in the historical period when no large flood occurred. 

Still another method, which has strong statistical motivation, is the method of maximum 

likelihood. Maximum likelihood estimators (MLEs) have very good statistical properties in large 

samples, and experience has shown that they generally do well with records available in hydrology. 

In many cases MLEs cannot be reduced to simple formulas, so estimates must be calculated using 

numerical methods (Stedinger et al., 1988; O’Connell, 1997). 

L-moments are another way to summarize the statistical properties of hydrologic data. 

Sample estimators of L-moments are linear combinations (and hence the name L-moments) of the 

ranked observations, and thus do not involve squaring or cubing the observed values as do the 

product-moment estimator

n and skewness are almost unbiased and have very nearly a normal distribution (Hosking and 

Wallis, 1997). 

Regional Flood Frequency Analysis

In hydrology, sufficient information is seldom available at a site to adequately determine the 

frequency of rare events using frequency analysis. This is certainly the case for the extremely rare 

events which are of interest in dam safety risk assessment. 

posed several general strategies, including substituting space for time for estimating extreme 

floods. One substitutes space for time by using hydrologic information at different locations in a 

region to compensate for short records at a single site. 

Three approaches (Cudworth, 1989) have been considered for regional flood 
approac

ta from several sites without adjustment to obtain a large composite record 

support frequency analyses. The concept underlying the index flood method is that the 

parameter which reflects the size, rainfall and runoff characteristics of each watershed. Generally 

e mean is employed as the index flood (Hosking and Wallis, 1997). 

Average Shape Parameter. As at-site records increa e

two parameters, with at-site data to be used with a regional sh o

perfo edinger an ngths

of ev estim te of the third shape parameter, are 

generally more accurate than are 3-parameter estimators (Lu and Stedinger, 1992; Stedinger and Lu, 

1995). However, whether or not it is better to also regionalize the coefficient of variation depends 

upon the heterogeneity of the regions and the coefficients of variability of the flows. In regions with 

high coefficients of variation (and high coefficients of skewness) index flood methods are more 

a

sis can be use t uati s the 

v ding means, sta ation , nd

n uantiles) as a function of physio racteri her

parameters. Stedinger and Tasker (1985, 1986a, 1986b) developed a specialized Generalized Least 

S s the region hydro cs.

A nt para e wh ve

s better des e relat en

ydrologic data and information for hydrologic network analysis and design. 

esign Event-Based Precipitation-Runoff Modeling

Precipitation-runoff modeling is typically used as an event-based method for determining 

extrem

ified locations, maximum peak discharges, and total 

h. With the average parameter approach, some parameters are assigned average values

based upon regional analyses, such as the log-space skew or standard deviation. Other parameters 

are estimated using at-site data, or regression on physiographic basin characteristics, perhaps the real 

or log-space mean. The index flood method is a special case of the average parameter approach. 

The specific frequency approach employs regression relationships between drainage basin 

characteristics and particular quantiles of a flood frequency distribution. 

Index Flood Method. The index flood procedure is a simple regionalization technique with 

a long history in hydrology and flood frequency analysis (Dalrymple, 1960). It uses data sets from

several sites in an effort to construct more reliable flood-quantile estimators. A similar

regionalization approach in precipitation frequency analysis is the station-year method, which 

combines precipitation da

to

distributions of floods at different sites in a "region" are the same except for a scale or index-flood 

th

se in length, procedures that estimat

ape parameter, have been shown t

rm better that index flood methods in many cases (St d Lu, 1995). For record le

en 100 years, 2-parameter estimators with a good a

ttractive.

Regional Regression. Regional analy

lues of various hydrologic statistics (inclu

d o derive eq

ndard devi

on to predict

a s quantiles, a

ormalized regional flood q graphic cha stics and ot

quares (GLS) regression methodology to addres alization of logic statisti

dvantages of the GLS procedure include more efficie

hort records, an unbiased model-error estimator, and a

m ter estimates

cription of th

en some sites ha

ionship betwe

h

D

e floods. A single set of hydrometeorological parameters and watershed characteristics are 

used to simulate a design flood event. The major inputs to a design event-based precipitation-runoff 

model are: (1) climate data (rainfall, snowfall, and other variables needed to predict snowmelt); (2)

losses (infiltration/interception); (3) physical watershed characteristics for runoff and routing

simulations (drainage areas, watershed and channel slopes, lag times, antecedent moisture, etc.); (4) 

precipitation-runoff transformation function; and (5) runoff conveyance/routing mechanisms. Model 

output includes runoff hydrographs at user-spec

Paper 18 – Swain, Bowles, Ostenaa 132
 133 Paper 18 – Swain, Bowles, Ostenaa

runoff 

res are used to allow the input variables to vary in accordance with their 

observe

orological variables for several sites 

concur

Combining Methods and Data Types

 of providing the needed characterization of extreme floods 

ver the full range of annual exceedance probabilities that may be required for risk assessment. In 

particu

nsidered for characterizing extreme floods to 

support

volumes. Examples of this type of model include HEC-1 (USACE, 1990) and RORB 

(Laurenson and Mein, 1995). 

Stochastic Event-Based Precipitation-Runoff Modeling

In the stochastic approach, hydrologic model inputs are treated as random variables. Monte 

Carlo sampling procedu

d distributions, including the observed dependencies among some climatic and hydrologic 

parameters. The use of the stochastic approach with regional precipitation information allows the 

estimation of flood magnitude-frequency curves for flood peak discharge, flood runoff volume, and 

reservoir level. An example of this type of model is discussed by Barker et al. (1997). 

Atmospheric Storm Modeling and Continuous Precipitation-Runoff Modeling

This method combines the work of atmospheric modelers and regional precipitation analysis 

to derive a precipitation magnitude-frequency curve (Chin et al., 1997). The atmospheric model is 

used to generate storms over the watershed, and the findings from the regional analysis are used to 

estimate the annual exceedance probability of point and areal precipitation generated by the model. 

Using distributed precipitation-runoff modeling, snowpack and other antecedent conditions can be 

combined to estimate a simulated flood frequency curve using a Monte Carlo approach. 

Data Generation and Continuous Simulation Modeling

The data generation and continuous simulation modeling approach is based on Monte Carlo 

generation of long and detailed sequences of hydrometeorological variables, including precipitation, 

air temperature, and wind speed and direction. In order to represent spatial differences across the 

watershed adequately, it is necessary to generate hydromete

rently. Hydrological models of watershed behavior and hydraulic models of confluences, 

wave effects and reservoir outlets are used to simulate the reservoir water level continuously. An 

estimated magnitude-frequency relationship of maximum reservoir stages is input to the risk 

assessment (Calver and Lamb, 1996). 

No single approach is capable

o

lar, characterization of floods with AEPs less than 1 in 10,000 can be expected to require that 

results from a number of approaches, based on multiple data sources, need to be combined to yield a 

composite flood frequency description. The application of several independent methods and types of 

data applicable to the same range of annual exceedance probabilities will increase the credibility and 

resulting confidence in the results. 

Table 2 lists various methodologies that were co

 dam safety risk assessment. A flood frequency analysis must be combined with each of 

these methodologies to assign annual exceedance probabilities to the floods. 
d scientific reasoning for

weight

The framework developed for Reclamation does not propose a specific methodology for 

rigorously combining information from these differing data sources and methodologies in an overall 

statistical framework. In some cases the information may be combined statistically, and in other 

cases one set of results may be used as a bound on the frequency distribution obtained by analysis of 

other data. Clearly, this process will require a measure of judgement. Regardless of the approach 

taken for combining results, it should incorporate sound physical an

ing or combining results. 

All floods characterized for the risk assessment process should display the uncertainties 

resulting from the analysis. As the risk assessment moves from the screening and scoping levels to 

the decision level, uncertainty should be reduced and better quantified so that appropriate 

information is included in the dam safety decision-making process. 

Table 2. Applicability of Hydrologic Methods of Analysis to Various Risk Assessment 

Levels

 Risk Assessment Level 

Method of Analysis Screening Scoping Decision 

Flood frequency analysis Yes Yes Yes 

Design event-based precipitation-runoff modeling No Yes Yes 

Stochastic event-based precipitation-runoff modeling No Yes Yes 

Distributed simulation modeling No No Yes 

Atmospheric modeling and distributed precipitation-runoff modeling No No Yes

Evaluation of Uncertainty

Uncertainty can be evaluated by applying Monte Carlo analysis to the overall risk assessment

calculations. For example, consider the estimation of threat to life consequences and probability of 

failure associated with an existing dam and various risk reduction alternatives. One is concerned

ith uncertainty due to such risk assessment inputs as flood frequency distribution parameters, 

time, and estimated loss of life. Then in each 

eration of Monte Carlo analysis, one could generate likely values of each of these inputs and 

evaluat

w

system response estimates, population at risk, warning

it

e the threat to life and probability of failure. The expected annual life loss and the annual 

exceedance probability of failure, which are both used as Reclamation Public Protection Guidelines 

(USBR, 1997b), could be computed for each iteration. By generating many replicates, one obtains 

samples that describe the possible values of these risk measures (performance metrics). 

Averaging over the replicates provides “expected” values of the quantities reflecting both the 

modeled probability distributions of the phenomena (risk assessment inputs) that are considered to 

be random variables, and the uncertainty in the parameters describing those distributions. The 

sample standard deviations describe the variability of the performance metrics. Replicates can be 

Paper 18 – Swain, Bowles, Ostenaa 134
 135 Paper 18 – Swain, Bowles, Ostenaa

tainty in the risk assessment inputs. 

ency Quantiles

bly different. 

hus, while calibration of a model provides valuable information on the flood response of a drainage 

ds of much larger 

agnitudes (Pilgrim and Cordery, 1993). 

s associated 

ith the greatest uncertainty can be varied within appropriate limits to ensure agreement with the 

dertaken for a range of AEPs to ensure a 

onsistent variation of parameters with flood magnitude or AEP. 

te, where the decrease in rigor is offset by the computational 

onvenience and the transparency of the adopted functional relationships. For the least important 

parameters it may be appr ) value instead of the full 

istribution. However, the relationship between rainfall and runoff is non-linear, and adoption of a 

single 

loped for characterizing extreme floods for the purposes of dam 

safety 

EP limits may exist in 

used to estimate frequency distributions which can be used for describing and evaluating the 

decision implications of uncer

Calibration to Flood Frequ

The ability of a flood event model to reproduce historic events certainly gives some 

confidence to the validity of subsequent estimates. However, even in a well gaged watershed the 

annual exceedance probabilities of the calibration floods are likely to range between 1 in 5 to 1 in 

20, and only occasionally up to 1 in 100. While it would be expected that floods of these 

magnitudes will activate some floodplain storage, the non-linear nature of drainage basin flood 

response is such that the routing characteristics of larger events may be considera

T

basin, caution is needed when using the calibrated model to estimate floo

m

Calibration to flood frequency quantiles using design rainfall inputs can provide important 

information on flood response characteristics for extreme design events (Nathan and Bowles, 1997; 

Nathan, 1992). With this approach, design rainfall information is prepared for a specified AEP, and 

then used with a given set of model parameters and input assumptions to derive a design hydrograph. 

The peak (or volume) of the design hydrograph can then be compared to the corresponding quantile 

obtained from a combined at-site/regional flood frequency analysis. The model input

w

selected flood quantile. Model calibration should be un

c

For risk-based studies based on a “design storm concept”, it is necessary to adopt an AEPneutral approach, where the objective is to derive a flood with an AEP equivalent to its concomitant 

precipitation (Nathan and Bowles, 1997). The factors that influence the transfer between 

precipitation and runoff can be characterized by probability distributions, and ideally the design 

hydrograph should be determined by considering the joint probabilities of all the input factors. 

Monte-Carlo methods are ideally suited to the AEP-neutral objective, as they accommodate the 

observed variability of the inputs while still preserving the interdependencies between parameters. 

Simpler approaches may be appropria

c

opriate to adopt a single representative (mean

d

representative value for the major inputs will introduce bias into the transformation. 

Accordingly, for more important inputs it is necessary to adopt a joint probability approach. The 

nature of the method can be tailored to suit the relative importance of the parameter concerned. 

Conclusions

A framework has been deve

risk assessment. By incorporating regional information on precipitation, floods, and 

paleofloods with good at-site records, it is possible to provide scientifically credible flood estimates 

to annual exceedance probabilities as low as 1 in 100,000, although higher A
many c

hould be combined to yield a composite characterization. The

applica

on of extreme floods. Flood 

haracterization should include a "best estimate" of the annual exceedance probability of floods of 

agn

be honestly represented and considered throughout the risk assessment process. 

Acknowledgments

The U.S. Bureau of Reclamation’s Dam Safety Office sponsored the Workshops and other

activities which have resulted in the proposed framework for characterization of extreme floods for

d ssment. Some twenty professionals fro

Europe participated in this effort and each contributed in som ay to the ulting fra ork.

These individuals were: David Achterberg, Victor Baker, David Bowles, David Cattanach, Sanjay 

C Chuck Hennig, D n J n, Lesley Julian, Jong-Seok Lee, 

D rd, Rory Nathan, Dan O’Connell, Dean Ostenaa, Duncan Reed, Mel

Schaefer, Lou Schreiner, Jery Stedinger, Robert Swain, Jim as, and Ed Tomlinson. This paper 

was also presented at the Eighteenth U.S. Committee on Large Dams Annual Meeting and Lecture

eld in Buffalo, New York and is included in the conference proceedings. 

References

ases. In general, the scientific limit to which the flood frequency relationship can be

extended based upon available data will fall short of the PMF for a site. PMF estimates provide a 

useful reference to past practice and can be compared with floods characterized for risk assessment; 

however, there is limited scientific basis for assigning an annual exceedance probability to the PMF. 

No single approach is capable of providing the needed characterization of extreme floods 

over the full range of AEPs required for risk assessment. Therefore, the results from several 

methods and sources of data s

tion of several independent methods applicable to the same range of AEPs will increase the 

credibility and resulting confidence of the results. 

Uncertainties associated with descriptions of flood flow exceedance probabilities are likely to 

be substantial and an important attribute for the characterizati

c

different m itudes and a description of the uncertainty in such results. Such uncertainties need to

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e w res mew

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intervals and a comparison, Jour. of Hydrol., 138(1⁄2), p. 247-268. 

Nathan, R.J., 1992. The derivation of design temporal patterns for use with generalized estimates of probable maximum 

precipitation, Civil Engineering Transactions, I.E. Aust., CE34(2), p. 139-150. 

Nathan R.J. and D.S. Bowles, 1997. A probability-neutral approach to the estimation of design snowmelt floods, 

Conference proceedings, 24th Hydrology and Water Resources Symposium, Auckland, New Zealand. 

National Research Council, 1988. Estimating Probabilities of Extreme Floods, Methods and Recommended Research,

Report by the Committee on Techniques

Press, Washington D.C. 

O’Connell, D.R.H., 1997. FLFRQ3, Three-Parameter Maximum Likelihood Flood-Frequency Estimation with Optional 

Probability Regions using Parameter Grid Integration: Users Guide (Beta Edition). 

Pilgrim, D.H., and I. Cordery, 1993. "Flood Runoff", Chapter 9 in Maidment, D.R. (ed.) Handbook of Hydrology, 

McGraw-Hill, New York, p. 9.1-9.42. 

Stedinger, J.R., and G.D. Tasker, 1985. Regional hydrologic analysis, 1. Ordinary, weighted and generalized least 

squares compared, Water Resour. Res., 21(9), p. 1421-32. 

Stedinger, J.R., and G.D. Tasker, 1986a. Correction to 'Regional hydrologic analysis, 1. Ordinary, weighted and 

generalized least squares compared,' Water Resour. Res., 22(5), p. 844. 

Stedinger, J.R., and G.D. Tasker, 1986b. Regional hydrologic analysis, 2. Model error estimates, estimation of sigma, 

and log-Pearson Type 3 distributions, Water Resour. Res., 22(10), p. 1

r, J.R., R. Surani, and R. Therivel, 1988. Max Users Guide: A Program for Flood Frequency Analysis using 

Systematic-Record, Historical, Botanical, Physical Paleohydrologic and Regional Hydrologic Information 

Using Maximum Likelihood Techniques, Department of Environmental Engineering, Cornell University. 

Stedinger, J.R., R.M.Vogel, and E. Foufoula-Georgiou, 1993. Frequency Analysis of Extreme Events, Chapter 18, 

Handbook of Hydrology, D.Maidment (ed.), McGraw-Hill, Inc., New York. 

Stedinger, J.R. , and L. Lu, 1995. Appraisal of Regional and Index Flood Quantile Estimators, Stochastic Hydrology and 

Hydraulics, 9(1), p. 49-75. 

U.S. Army Corps of Engineers, 1990, HEC-1 Flood Hydrograph Package, User’s Manual, Hydrologic Engineering 

Center, Davis, CA, 283 pp. 

U.S. Bureau of Reclamation, 1997a. Risk Assessment Methods for Dam Safety Decision Making, Draft, U.S. Department 

of Interior, Denver, CO. 

U.S. Bureau of Reclamation, 1997b. Guidelines for Achieving Public Protection in Dam Safety Decision Making, U.S. 

Department of Interior, Denver, CO, 19 pp. 

Von Thun, J.L. and J.D. Smart, 1996. Risk assessments support dam safety decisions, USCOLD Newsletter, Issue No. 

110, Nov 1996, U.S. Committee on Large Dams.
 139 Paper 19 – Chieh and Evelyn 

James Chieh1 and Joseph B. Evelyn2

Introdu

Hydrol tandard basis to a 

of this 

Assessm of its reservoir projects. Some specific outcomes of the demonstration 

odes 

outcom

reservo

and dow information (floodplain depths, flood wave velocities, and travel 

other ri otential loss of life downstream. 

Study, ch needs identified for dam safety. 

Alamo 

Alamo Dam was completed in 1968 as a multipurpose project for flood control, water conservation 

and supply, and recreation. Alamo Dam is located on the Bill Williams River, 39 miles upstream 

from its confluence with the Colorado River in Havasu Lake near Parker, Arizona. The dam is on 

the border of Yuma and Mohave Counties, Arizona, about 2.5 miles downstream from Alamo 

Crossing. The drainage area above Alamo Dam is 4770 square miles, and is generally mountainous, 

 

HYDROLOGIC RESEARCH NEEDS FOR DAM SAFETY 

Experience Learned from Alamo Dam Risk Assessment Study 

By 

ction 

ogic dam safety evaluation is moving from a deterministic design s

probabilistic (risk based) approach. Because of this trend, the U.S. Army Corps of Engineers is in 

the process of developing risk assessment guidance for its Dam Safety Assurance Program. As part 

effort the Los Angeles District participated in a demonstration project, the Alamo Dam Risk 

ent Study, using one

risk assessment study were the following: 

. An understanding of potential failure m

 . An evaluation of the risk posed by the existing dam against various risk-based 

criteria. 

 . An assessment of risk reduction and the cost effectiveness of risk reduction expected 

for various structural and non-structural measures. 

A hydrologic analysis was conducted to support the study to achieve the above expected project 

es. To perform the analysis from the hydrologic aspect required the determination of 

ir inflow volume-frequency relationships, reservoir elevation-frequency relationships, 

reservoir elevation-duration relations for flood loading conditions, and the “Threshhold Flood”. The 

hydrology results were then used for the hydraulic analysis to develop dam break flood hydrographs, 

nstream overflow area

times) for consequence analysis. The hydrology results were also used for economic analysis and 

sk assessment, such as p

This paper briefly describes the hydrologic analysis performed on the Alamo Dam Risk Assessment 

and summarizes the hydrologic resear

Dam 

1

 Senior Engineer, Los Angeles District, Corps of Engineers 

2

 Chief, Hydrology and Hydraulics Branch, Los Angeles District, Corps of Engineers 
in west central Arizona. The drainage area is bounded on the north by the Cottonwood Cliffs; on the 

east by the Juniper and Santa Maria Mountains; on the south by the Date Creek and Harcuvar 

Mountains; and on the west by the Hualpai Mountains. Figure 1 is a picture of Alamo Dam; some 

physical characteristics and hydrologic information regarding the dam and spillway is presented 

below the figure. 

Hydrologic Analysis for Alamo Dam Risk Assessment Study 

Brief descriptions of the hydrologic analysis conducted for the Alamo Dam Risk Assessment 

including volume frequency analysis, balanced hydrographs, design flood routings, filling frequency 

analysis, and elevation duration analysis are presented in this section. 

 (a) Volume Frequency Analysis 

An analysis was performed on the Alamo reservoir inflow record to establish 1-, 2-, 3-, 5-, 10-, 20-, 

and 30-day volume frequency curves. The inflow records were from October 1928 to December 

1993, a total of 65 years records. The volume freq

recurrence interval of 10-6 using judgment, comparison with the volume duration characteristics of

the SPF and PMF, and the shapes of the filling fre es relative to each other. The PMF 

exceedance frequency was assumed to be 10-6 based on engineering judgment. Likewise, median 

plotting positions and a logarithmic scale were applied for graphical frequency analysis. For the risk 

assessment flood loading analysis it was necessary o extrapolate the volume frequency curves 

beyond 10-6 to plot the results of Co alling for SPF plus PMF as the

inflow design flood. Figure 2 presents the inflow volume frequency curves for the Alamo reservoir. 

(b) Balanced Hydrographs 

Balanced hydrographs (50-; 100-; 200-; 500-; 1000-; 5000-; 10,000-; 50,000-; 100,000-; and 

1,000,000- year) were constructed for reservoir inflow based on the volume frequency relationships 

described above. The historical flood hydrograph from the February- March 1978 event, and the 

PMF hydrograph were used to determine an appropriate pattern hydrograph. The reasonableness of 

balanced hydrographs was verified by visual inspection of graphical plots of hydrographs, and 

comparison of 50-year and 100-year balanced hydrograph routing results with the statistical estimate 

of 2 percent and 1 percent exceedance water surface elevations. 

(c) Design Flood Routings 

To be consistent with standard practice in the evaluation of design flood routing of Standard Project 

Flood (SPF) and Probable Maximum Flood (PMF), the starting reservoir water surface elevation for 

routings was assumed to be the target objective elevation (1125 feet) at the current reservoir 

operation plan, and the maximum reservoir release is 7,000 cfs. Routings assumed the 7,000 cfs 

release was continued throughout spill events. 

uency relationships were extrapolated to a 

quency curv

t

rps spillway design practice c

Paper 19 – Chieh and Evelyn 140
 141 Paper 19 – Chieh and Evelyn 

In order to ad on 

alternatives to ha elevations, 

flood routings were performed through Alamo reservoir using the net storage capacity curve for the 

following cases. 

 

 Case 1. Starting en SPF. 

 Case 2. Starting water surface elev ion of 1125 feet, then PMF. 

 Case 3. Starting water surface elev ion of 1125 feet, then SPF, followed 23 days 

later by PMF. This routing results in a starting WSE corresponding to about 50 

percent of flood control pool filled at start of PMF. 

 up to and including SPF plus PMF were determined for 

e existing dam configuration and all flood risk reduction alternatives. 

r 

Alamo Dam was performed using the adopted reservoir operation plan to determine maximum 

maximum valu le for probability. Since the 

ighest reservoir pool elevation was well below the 

g 

00 

l. The filling frequency curve was 

xtended to the maximum water surface elevation attained during each routing (i.e., the filling 

outings of the balanced hydrographs were made for each risk reduction alternative dealing with 

ficiency. Results from the routing of the balanced hydrographs were plotted along 

ith the simulation results from the historical records (the starting portion of the filling frequency 

dress issues concerning the capability of the existing project and risk reducti

ndle design floods (SPF and PMF) at different starting water surface

 water surface elevation of 1125 feet, th

at

at

Case 4. Starting water surface elevation of 1125 feet, then SPF, followed 

immediately by PMF 

 

 (d) Filling Frequency Analysis 

Alamo Dam filling frequency relationships

th

 

An HEC5 reservoir operation simulation of the historical daily inflow record (1929 to 1998) fo

annual water surface elevation. The HEC-FFA computer program was used to plot the annual 

es using median plotting positions and log cycle sca

h

spillway crest for any risk reduction alternatives, the results of this analysis were used as the startin

portion of the filling frequency curve estimated for all alternatives. 

 

The balanced hydrographs were routed through the reservoir for existing conditions with 

overtopping of the existing embankment (no failure). Reservoir routings were made using the 

current reservoir operation plan modified to limit the maximum release to 5000 cfs (instead of 70

cfs) until spillway crest is reached to account for the likelihood of release reductions required by 

lower Colorado River system reservoir operation for flood contro

e

frequency curve was not truncated due to probable dam failure at this point of the analysis). 

R

hydrologic de

w

curve) to generate filling frequency relationships for each risk reduction alternative. In graphically 

drawing the filling frequency curve, the influence of the spillway discharge relation on the slope of

the filling frequency curve was considered in each case. Results of maximum surcharge water 

surface elevation and maximum outflow from the dam were tabulated and plotted on Excel 

spreadsheets. Figure 3 shows the Excel spreadsheet for the existing condition. 
oard

n Duration Analysis

rvoir pool elevation versus total 

uration relationship was computed using the available 70 years of simulated daily WSE values. 

es 

ately 

s presented in the above section, the risk based approach to dam safety analysis required: 

y relationships

b. Flood hydrograph shape 

analysis

c. Procedures applicable nationwide 

ilable hydrologic/meteorologic/geologic data 

e. Confidence limits/reliability of estimates. 

large areas due to need to assess multiple dam

failures in same watershed 

Trial and error reservoir routings were made to determine the Threshold Flood as about 83 percent of 

PMF (largest flood event safely handled by dam and spillway without failure using Corps freeb

criteria). The starting water surface elevation for the Threshold Flood determination was 1125 feet.

 (e) Elevatio

For the existing condition dam (with net storage curve) the rese

d

This total duration curve helped in determining the probability of geotechnically related failur

(seepage, piping, seismic, etc.) because the relationship of number of days at any given elevation is 

given. 

Hydrologic Research Needs for Dam Safety 

During 1982 through 1997, the Corps of Engineers Los Angeles District conducted several Alamo 

Dam safety studies based a deterministic approach. The district accomplished a lengthy and costly

Corps reporting and evaluation process based on spillway design flood standard that was ultim

unsuccessful in upgrading spillway capacity due to legitimate cost vs. benefit concerns. 

Our experience with the Alamo Dam Demonstration Risk Assessment (1998-1999) was: 

a. Holistic approach covering all dam safety aspects 

b. Hydrologic risk put into perspective of total project risk 

c. Risk reduction benefit vs. cost of alternatives displayed.

A

a. N-year volume frequenc

c. Entire range of frequencies from 2-year to PMF and beyond 

d. Elevation-duration frequency relationship. 

During the demonstration dam safety study, the characteristics of hydrologic procedures needed 

were identified as: 

a. Quick & dirty (cheap) methods for reconnaissance and “portfolio” studies 

b. More comprehensive (expensive) methods for in-depth

d. Procedures based on ava

Through these dam safety analyses some hydrologic research needs were identified: 

b. Method for assigning a reasonable estimate to the probability of PMF 

c. Procedure for filling the large gap between observed hydrologic data and PMF estimates 

d. Hydrologic methods consistent over

Paper 19 – Chieh and Evelyn 142
 143 Paper 19 – Chieh and Evelyn 

items were also identified: 

Case studies of observed extreme flood events as compared with PMF and discharge and 

vol

. Rec

c. Co PMF & 

discharge-frequency estimates; generalize findings if possible. 

ts. 

o 

e. A practical approach to uncertainty analysis to enable better (more meaningful) 

evaluations than sensitivity analysis. 

Specific research 

a. 

ume frequency estimates 

b onstitution of rainfall-runoff relationships for extreme events 

a. Antecedent precipitation and soil moisture 

b. Unit hydrograph characteristics (efficiency of runoff or time of concentration

findings) 

mpilation of paleo-hydrologic information nationwide and integration with 

Conclusion 

We need hydrologic tools that enable advancing from strictly deterministic (design flood) 

evaluations to performing risk based (probabilistic) dam safety assessmen

References 

1. Alamo Dam Risk Assessment Study, Los Angeles District Corps of Engineers, July 1999. 

2. Alamo Dam, Supplemental Reconnaissance Report on Inadequate Spillway Capacity at Alam

Dam on the Bill Williams River, Los Angeles District Corps of Engineers, July 1993. 

3. Colorado River Basin, Hoover Dam, Review of Flood Control Regulation, Corps of Engineers, 

Los Angeles District and Bureau of Reclamation, Lower Colorado Region, July 1982. 
Figure 1 Alamo Dam

Spillway

height = 283 feet

Reserv

Reserv

Existin

PMF in

PMF o

hreshold Flood = 83% of PMF

Alamo Dam and

Drainage area = 4,770 square miles 

Dam embankment

oir storage at spillway crest = 1 million acre-feet 

oir storage at TOD = 1.3 million acre-feet 

g spillway capacity = 41,500 cfs 

flow = 820,000 cfs and 1.39 million acre-feet 

vertops existing dam by 20 feet

T

PMF Peak Outflow

With dam failure = 2.9 million cfs 

Without dam failure = 362,000 cfs 

Paper 19 – Chieh and Evelyn 144
 145 Paper 19 – Chieh and Evelyn
Paper 19 – Chieh and Evelyn 146
Paleoflood Hydrology, Dam Safety, and Floodplain Management 

Robert D. Jarrett, U.S. Geological Survey, Denver, Colorado 

John F. England, Jr., U.S. Bureau of Reclamation, Denver, Colorado 

Edward M. Tomlinson, Applied Weather Associates, Monument, Colorado 

Thomas M. Yanosky, U.S. Geological Survey, Reston, Virginia 

Robert E. Swain, U.S. Bureau of Reclamation, Denver, Colorado

Abstract 

In the past decade, there has been a growing interest by dam-safety officials to 

incorporate a risk-based analysis for design-flood hydrology. Extreme or rare floods, 

with annual exceedance probabilities (AEPs) in the range of about 10-2 to 10-7 chance of 

occurrence per year, are of continuing interest to the hydrologic and engineering 

communities for purposes of planning, design, and maintenance of structures such as 

dams [National Research Council, 1988]. Flood-frequency analysis is synonymous with 

flood-risk assessment. Flood-frequency relations also are difficult to estimate when using 

short gage record lengths typical of streamflow-gaging stations in the United States. 

Reliable flood-frequency estimates is needed as input to risk assessments for determining 

appropriate levels of public safety, prioritizing projects, and allocating limited resources 

in a wide range of water-resources investigations such as flood-plain management, flood 

forecasting, and related environmental studies. The NRC stresses that as much 

information as possible about floods needs to be used for evaluation of the risk and 

consequences of any decision. Paleoflood hydrology provides useful information to 

assist dam-safety officials and flood-plain managers in their assessments of the risk of 

large floods. Documenting maximum paleofloods combined with regional analyses of 

contemporary extreme rainfall and flood data help provide reliable flood estimates with 

very small AEPs. This paper provides an overview of contributions in paleoflood 

hydrology by the U.S. Geological Survey research project “Paleohydrology and Climate 

Change,” recent research findings, an example applied of an paleoflood study, and 

suggestions for future directions in hydrologic research needs for improving hydrologic 

estimates for dam safety, and flood-plain management. 

Introduction

Worldwide, floods are among the most destructive events related to meteorological 

processes. In the United States, the average annual death toll of 125 is accompanied by 

about $2.4 billion in damages from floods (Federal Emergency Management Agency, 

FEMA, 1997a). In 2001, natural disasters caused at least 25,000 deaths and total 

economic losses were $36 billion worldwide (Environmental News Network, ENN, 

2001). Storms and floods dominated these statistics, contributing more than two thirds to 

the 700 major disasters and causing 91 percent of all insured natural disaster losses. The 

worst weather-related disaster in 2001 was tropical storm Allison that produced record

flooding in parts of Texas, which caused losses of about $6 billion, making it “the most

expensive tropical storm in history” (ENN, 2001). Although large expenditures of 

147 Paper 20 – Jarrett et al 
Paper 20 – Jarrett et al 148

resources are made to mitigate flood losses, damages from floods continue to rise at an 

alarming rate, and thus, better approaches are needed to reverse this trend. 

Effective planning, design, and maintenance of flood-risk management projects require 

accurate estimates of flood risk (National Research Council, NRC, 1999). Poor 

understanding of flood frequency contributes to unnecessary loss of life and increased 

flood damages in some cases, and conversely, leads to costly overdesign of hydraulic 

structures located on floodplains and questionable hydrology in flood-plain management 

for other situations (Jarrett and Tomlinson, 2000). In an evaluation of extreme floods and 

the use of the probable maximum flood (PMF) methodology to estimate design floods, 

the Interagency Advisory Committee on Water Data (IACWD, 1986) raised a major 

concern about PMF’s being either dangerously small or wastefully large, and they 

emphasized the importance of accurately estimating the risk of extreme flooding. Thus, 

the objective of a flood study should be to generate as much information as practicable 

about the flood potential at a site (NRC, 1988), which should be the basis for evaluation 

of the risk and consequences of any decision. 

For about the past 50 years, the design criteria for construction of structures such as dams 

has included an estimate of the Probable Maximum Flood (PMF) (Cudworth, 1989). The 

PMF is an estimate of the maximum flood potential for a given drainage basin and is 

derived from an analysis of the Probable Maximum Precipitation (PMP) (Cudworth, 

1989). The NRC (1988, 1994) recognized the: 1.) limited hydroclimatic data available to 

estimate PMP/PMF values for mountain basins less than about 1,050 km2

; 2.) subjectivity 

and variation of PMP estimates among experienced meteorologists; 3.) critical need for 

regional analyses of extreme precipitation and flooding; 4.) the need to better use historic 

and paleoflood data; and 5.) potential use of probability-based methods for providing an 

alternative to the PMP/PMF approach. Greater emphasis is needed to better understand 

processes that generate floods to improve methods for estimating flood risk, mitigating 

flood impacts, and designing infrastructure in floodplains. 

Other countries have made major investments in updating their procedures for 

determining flood flow frequency. For example, the United Kingdom updated their 

procedures in 1999 in the Flood Estimation Handbook (Reed et al., 1999). This 

handbook replaced the 1975 Flood Studies Report by the Natural Environment Research 

Council. Australia has also updated their flood frequency approach in Australian 

Rainfall and Runoff – a guide to flood estimation (Nathan and Weinmann, 1998). 

Although there is substantial research on improving flood-frequency methods in the 

United States, progress in integrating this research into standard hydrologic engineering 

practice generally has been slow. One exception is Reclamation where significant 

resources have been committed for risk-assessments for dam safety in about the past ten 

years. Reclamation has been using quantitative risk assessment for dam safety decision 

making for several years (U.S. Bureau of Reclamation, 1999; Levish, 2001). 

There also is substantial interest and need for improving methods to estimate flood 

frequency in floodplain management, flood inundation mapping, water resources 
man

Swain and Jarrett, 2000; Vecchia, 2001; Jones et al., 2001; Levish, 2001). Floodplain 

), and there is

g (NRC, 1999;

other

streams) require 

plexity in floodazard assessments. 

EMA, which administers the National Flood Insurance Program, and other federal 

tly

e and regional water

sources agencies, U.S. Geological Survey (USGS) Paleohydrology and Climate 

project scientists and engineers are conducting research on the 

ydraulics, hydrometeorology, and paleoflood hydrology to help assess the frequency 

hydrology related to dam safety. The U.S. Army Corps of Engineers (COE) also is 

agement, and many related environmental disciplines (e.g., NRC, 1988, 1999; 

management is an issue of national concern (Vogel et al., 2001

increasing research to address effects of climatic variability on floodin

Lins and Slack, 1999; Jarrett and Tomlinson, 2000). Climatic variability and

nonstationarity issues (e.g., land-use changes and regulated flow

more robust flood-frequency analysis and add another level of com

h

F

agencies rely on the procedures for estimating flood flow frequency presented in

Bulletin 17B Guidelines for Determining Flood Flow Frequency, Interagency 

Committee on Water Data (IACWD, 1982), to map flood-plain hazards throughout 

the United States. However, Bulletin 17B guidelines were developed in the late1960s and last revised in 1982 (IACWD, 1982). Many of the effective flood maps 

were created in the late 1970’s and early 1980’s, and suffer from a number of well 

recognized deficiencies such as out-of-date or inaccurate flood-frequency estimates, 

changes in basin characteristics, and have additional flood data since the frequency 

studies were originally done (Jarrett and Tomlinson, 2000; Jones et al., 2001). Fortyfive percent of effective maps are over 10 years old and many communities do not 

have effective flood mapping. The National Flood Insurance Reform Act of 1994 

mandates that the Nation’s flood maps be revised in the near term and subsequen

reviewed every 5 years. FEMA estimates that updates to existing flood insurance 

studies and creating new maps for communities without mapping will cost between 

$800 and $1,000 million. FEMA’s Map Modernization Plan also emphasizes that 

more cost-effective methods to produce flood maps are essential (FEMA, 1997a, 

1997b, 1998). 

USGS Paleohydrology and Climate Change Project 

To help support efforts of the Bureau of Reclamation, Corps of Engineers, Federal 

Emergency Management Agency, and other federal, stat

re

Change (PCC)

h

of extreme or rare floods, with AEPs in the range of about 10-2 to 10-4

 or smaller. A 

primary focus of this interdisciplinary research is to develop cost-effective paleoflood 

techniques that can be used to complement meteorologic, hydrologic, and engineering 

methods to improve estimation of the magnitude, frequency, and risk of floods. This 

objective is achieved through basic and applied research that includes improving 

methods for estimating flood discharge, hydrometeorologic processes of floods, 

flood-frequency analysis, better use of historic and paleoflood data at gaged sites, 

ungaged sites, and in regional settings, and quantifying the effects of natural and 

anthropogenic climatic variability on flooding. For about the past decade, the USGS 

has worked closely with Reclamation on technical consultation for paleoflood 

149 Paper 20 – Jarrett et al 
implementing a risk-assessment method to evaluate potential safety problems for its 

more than 550 dams to aid decision-makers in prioritizing investment decisions

oster, 1999). Two PCC projects in progress focus on incorporating paleoflood data 

ude

n floods at gaged and ungaged sites

aker et al., 1988). Floods leave distinctive sedimentary deposits (Webb et al., 

-conveyance methods 

ebb and Jarrett, 2001). 

th

n

e used in risk assessments and in the assessment of "nonstationarity" flood 

(F

into flood-frequency analyses for two COE projects. The COE requested that the 

flood hydrology for the American River in California be reanalyzed to better incl

historical and paleoflood data to better extrapolate frequency relations for AEPs less 

than 0.01. In a complementary PCC study for the COE, paleoflood data are being 

incorporated in a regional flood-frequency study to be used to revise the hydrology 

for a revised FEMA flood-insurance study for Fountain Creek basin, Colorado. 

A Role for Paleoflood Hydrology in Water-Resources Investigations 

Paleoflood hydrology is the science of reconstructing the magnitude and frequency of 

large floods using geological evidence and a variety of interdisciplinary techniques 

(House et al., 2001). Although most studies involve prehistoric floods, the 

methodology is applicable to historic or moder

(B

2001, House et al., 2001; Jarrett and England, 2001), botanical evidence (Yanosky 

and Jarrett, 2001), erosional features on channel margins (O’Connor et al., 1993), and 

modifications of geomorphic surfaces by floodwaters (Levish, 2001) in channels and 

on floodplains. These features, termed paleostage indicators (PSIs), can be used to 

infer the maximum stage of past floods (Figure 1). In paleoflood studies, the most 

commonly used PSIs are slackwater deposits (SWDs) of silt and sand rapidly

deposited from suspension in sediment-laden waters where velocities are minimal 

during the time that inundation occurs. Other types of PSIs used in paleoflood studies 

include flood bars (FBs) of sand, gravel, cobble, and boulder deposits. Estimating 

paleoflood discharge using PSIs (which can be viewed as old high-water marks, 

HWMs) is similar to estimating peak discharge using recent HWMs with stepbackwater analysis, the slope-area, critical-depth, and slope

(W

There has been a misunderstanding that paleoflood techniques are only used for 

estimating very old and extreme floods. Paleoflood studies to obtain data for 

contemporary floods (recent to about 500 years) also are used to complement short 

gage records and can be used to estimate magnitude-frequency relations at sites wi

minimal or no gage data. Paleoflood studies provide important information that ca

b

Paper 20 – Jarrett et al 150
Figure 1. Diagramatic section across a stream channel showing a peak flood stage 

and various paleoflood features used as paleostage indicators (PSIs) such as 

slackwater deposits, gravel, cobble and boulder bars, trees scars, and erosional scars 

(Source: Jarrett and England, 2001). 

at various temporal and spatial scales (NRC, 1999; Jarrett and Tomlinson, 2000; 

Vecchia, 2001). Non-stationarity typically includes effects of natural and

anthropogenic climatic variability, but also effects of land-use changes such as 

urbanization, detention storage, and wildfire. Paleoflood data are particularly usefu

in providing upper limits of the largest floods that have occurred in long time spans 

(Enzel et al., 1993; Jarrett and Tomlinson, 2000). The extreme upper tail of a flood

frequency distribution is most important for design of high-risk structures (Vecchi

2001). Paleoflo

l

a,

od data can provide unique insight for estimating the upper tails of 

equency distributions.

ecause of the important role of paleoflood hydrology is increasingly being used in a 

t

a task

in source 

SIs.

fr

B

range of water-resources investigations, the American Geophysical Union (House e

al., 2001) published a book Ancient Floods, Modern Hazards, Principles and 

Applications of Paleoflood Hydrology that provides a state-of-the-art review of 

paleoflood hydrology. In 1999, the American Society of Civil Engineers began

committee on paleoflood hydrology as it relates to dam safety and risk-based 

assessments as well as better use of historical data and paleoflood data in waterresources investigations. The task committee on Paleoflood Hydrology also is 

preparing a monograph entitled Use of Paleoflood and Historical Data in Water 

Resources Applications (Swain and Jarrett, 2000), which will emphasize using 

paleoflood techniques and applications by practicing hydrologists, engineers, and

scientists in related fields. 

Recent Paleoflood Research Results 

This section summarizes two recent research studies of the PCC project. A ma

of uncertainty in paleoflood reconstructions is maximum flood stage inferred from P

151 Paper 20 – Jarrett et al 
Typically, the elevation of the top of the PSI is used as the minimum elevation of the

flood that deposited the sediments (Figure 1), but little evidence has been provide

support this assumption. Jarrett and England (2001) documented the first systematic 

assessment for one aspect of paleoflood data: the relation between paleostage indi

(PSIs) and the peak stage of floods responsible for their emplacement. Recent floodtransported sediments (fresh PSIs) deposited as flood bars (FBs) and/or slack-water 

deposits (SWDs) from recent large floods can be directly related to flood high-wate

marks (HWMs) at the same location. A flood-chasing approach was developed to v

many extreme flood sites as possible after a flood, and obtained PSI-HWM data

d to

cators 

r

isit as

from

cent large floods at 192 sites. This systematic documentation and analysis of recently 

emplace study 

the physical evidence of floods in the United States. Data were obtained from 13 states, 

3),

nd

in the

f the

r

ients

es

 higher 

vary

h

ion

ts nearest

od.

re

d flood-deposited sediments (new PSIs) provided a unique opportunity to

including: Alaska (4), Arizona (1), California (23), Colorado (126), Hawaii (1), Idaho (

Montana (2), Nebraska (1), New Mexico (6), Oregon (16), Utah (2), Wyoming (5), a

Washington (4). 

Data from recent large floods (median recurrence interval of 75 years), primarily

western and west-central United States, were used for a comprehensive evaluation o

relation between flood-transported sediments deposited as flood bars and/or slack-wate

deposits and flood high-water marks (HWMs). Surveys of flood-deposited sediments,

HWMs, and channel geometry were made for 192 sites, primarily streams with grad

larger than 0.002 m/m and depths less than about 4.5 m. Analysis of the data indicat

that the elevation at the top of the flood sediments (PSIs) is on average 15 mm

than the HWMs (Figure 2) with a range of –914 mm to +2,347 mm; results slightly

with type of deposit and location. No statistically significant relation was identified wit

channel gradient, particle size, discharge, recurrence interval, expansion or contract

ratios, width, or depth variables. The main result from this study is that use of the

elevation at the top of flood-deposited sediments (new PSIs), preferably deposi

to channel margins, provide a reliable and accurate (±5 percent) indication of the 

maximum height of the flo

Paper 20 – Jarrett et al 152
ed

urrence interval for recent floods (Source: Jarrett 

nd England, 2001).

rmed by an 

olorado

ver

nit

cars

rges ranged from about 85 m /s at the upstream study reach to 

500 m /s at the downstream end of the reach. The study was conducted along an 8-km 

veral

las fir,

ed to the

 to +150 

0 cm

tudy

he mean

40 cm

ng the lower gradient Skeena River in British Columbia 

igure 3) in a comprehensive study by Gottesfeld (1996). Gottesfeld studied scar height 

r produced by a snowmelt flood in 1990 having a 

recurrence interval of about 9 years. The height of scars ranged from +9 cm to –80 cm

Figure 2. Graph of difference between the elevation of new PSIs and HWMs stratifi

by deposit type with approximate rec

Approximate recurrence interval, years

-1000

-500

0

500

1000

1500

2000

2500

3000

1 10 100 1000 10000 100000

PSI-HWM, mm

1 overbank SWD

2 main channel SWD

3 main channel FB

4 overbank FB

5 delta

6 fan apex

a

In a similar study, Yanosky and Jarrett (2001) studied the height of scars fo

extraordinary flood in 1996 along Buffalo Creek, a high gradient stream in the C

Rocky Mountains southwest of Denver, Colorado. A rainstorm on July 12, 1996, of o

130 mm of rain in about one hour over the rugged mountains that had been severely 

burned by wildfire on May 18, 1996. The wildfire exacerbated flooding producing u

discharges (peak discharge divided by drainage area) of up to 60 m3

/s/km2; adjacent

unburned basins had little to no runoff. Along the reach of Buffalo Creek that tree s

were studied, peak discha 3

3

reach with bed gradients ranging from 0.01 to 0.04 m/m, thus extending the range of 

hydraulic conditions for documented tree scar data available for lower gradient rivers

(Gottesfeld, 1996). The streambed contains small gravel and boulders up to se

meters in diameter. Riparian trees include ponderosa pine, lodgepole pine, Doug

and cottonwood. Scar data were collected from 102 riparian trees along the reach of 

Buffalo Creek. The elevation of each HWM along the streambank was extend

top of the highest scar on each tree. The heights of all scars ranged from –60 cm

cm relative to 1996 HWMs (mean +21 cm); 64 scars (63 percent) were within 2

(Figure 3). Scar heights relative to HWMs were slightly greater along the upper s

reach, possibly owing to increased flow depths, velocity, and flow turbulence. T

height of scars relative to measured crest elevation along Buffalo Creek was about

greater than determined alo

(F

of 48 trees along a 170-m reach of rive

relative to the flood crest (mean was –20 cm). It is possible that extraordinary floods on 

low-gradient streams might possess sufficient energy to produce a tree-scar distribution 

153 Paper 20 – Jarrett et al 
similar to that of the high-gradient Buffalo Creek. Although additional tree-scar data a

needed from a wider range of hydraulic and botanical conditions, the few studies 

conducted to date (see Yanosky and Jarrett, 2001) suggest that maximum scar heights

provide acceptable estimates of paleoflood stage along both low- and high-gradient 

streams. 

re

ate 

mining the peak discharge of floods, particularly 

xtraordinarily large floods in higher gradient (~0.01 m/m or larger) rivers, typically has 

roduced overestimated discharges by about 60 percent (Jarrett, 1987). Thus, it is 

ssential that validation of methods and results be used in investigations where possible. 

arrett and England (2001) obtained forty-six peak discharges (Qsite) using the criticalepth (35 sites) and slope-conveyance (11 sites) methods for comparison with peak 

ischarges obtained from gage records (Qgage) at or near streamflow-gaging stations by 

thers (Figure 4). Of these Qgage floods, 22 peak discharges were determined using 

urrent-meter measurements, and 12 were estimated by using modest rating-curve 

xtensions. Peak discharges also were derived using flow through a flume or over a 

iversion dam (6 sites), which are believed to be more reliable than the critical-depth and 

lope-conveyance methods. In addition, Qgage peak discharges were estimated using the 

lope-area method at six sites for comparison with the critical-depth discharge estimates. 

he range in the difference of the peak discharge using the critical-depth and slopeonveyance methods is –45 to +43 percent with an average difference of +1 percent. 

hey found 42 of the 46 estimates were within ±25 percent of the gage peak discharge 

easurements (Figure 4). These results have larger uncertainty in critical-depth and 

Figure 3. Maximum heights of scars on riparian trees relative to peak flood crest on 

Buffalo Creek, Colorado (N=102 trees), and on the Skeena River, British, Columbia, 

(N=48 trees) (Gottesfeld, 1996) (Source: Yanosky and Jarrett, 2001). 

A second research summary is provided concerning indirect methods used to estim

peak discharge. Indirectly deter

e

p

e

J

d

d

o

c

e

d

s

s

T

c

T

m

Paper 20 – Jarrett et al 154
slope-conveyance method estimates compared with Qgage peak discharges of ±12 percent 

for streams in northwestern Colorado noted by Jarrett and Tomlinson (2000). The floods 

documented in northwestern Colorado rivers were from relatively small magnitude, 

snowmelt peak discharges (AEPs of ~0.04). These results for peak-discharge estimation 

are encouraging because of their ease of use when resources and time preclude use of 1-D 

(step-backwater modelling) and/or 2-D hydraulic models commonly used in flood and 

paleoflood studies. The validation with gage data indicates the critical-depth and slopeconveyance methods, when applied in hydraulically good channel reaches, provide 

reasonable flood estimates. 

igure 4. Relation of peak discharge between streamflow-gaging station values and peak 

aleoflood techniques have inherent assumptions and limitations that produce uncertain 

ere 

in

1

10

100

1000

10000

1 10 100 1000 10000

Peak discharge at gage, m

3

/s

Peak dischar

ge at site, m3/s

Critical Depth

Slope Conveyance

Linear Fit

log(Qsite) = 0.03 + 0.986 log(Qgage)

Upper and Lower 95%

Prediction Intervals

F

discharge computed with the critical-depth and slope-conveyance methods (Source: 

Jarrett and England, 2001). 

P

flood estimates. Although paleoflood estimates also involve uncertainties, the estimates

are based on interpretations of physical data preserved in channels and on floodplains 

spanning thousands of years. Paleoflood uncertainties primarily are related to possible 

post-flood changes in channel geometry and flood heights interpreted from PSIs. Wh

possible, paleoflood estimates are obtained in bedrock controlled channels that minimize

changes in channel geometry. The HWM-PSI relations developed from recent floods

the western United States (Jarrett and England, 2001; Yanosky and Jarrett, 2001; House 

et al., 2001) help to improve the reliability of paleodischarge estimates. Validation of the

critical-depth and slope-conveyance methods to estimate peak discharge is encouraging

155 Paper 20 – Jarrett et al 
and provides a cost-effective framework for additional validation (Jarrett and Englan

2001). 

A Recent Applied Paleoflood Study: Elkhead Reservoir, Colorado 

This section provides a summary of an applied paleoflood study for Elkhead 

Reservoir on Elkhead Creek near Craig in the Colorado Rocky Mountains (Jarrett and 

Tomlinson, 2000). The cost-effective approach, which can be used in many other 

hydrometeorologic settings, was applied to Elkhead Creek basin (531 km2

) in 

northwestern Colorado; the regional study area was 10,900 km2

. A regional, 

interdisciplinary paleoflood approach provides a more thorough assessment of 

flooding and with site-specific PMP/PMF studies provide dam safety official with 

new information to assess extreme flood potential. The study was conducted for the 

Colorado River Water Conservation District to complement a site-specific PMP by 

Tomlinson and Solak (1997) and a PMF study by Ayres Associates Inc. in Fort 

Collins, Colorado, for Elkhead Reservoir. Elkhead Reservoir was being recertified 

for hydrology safety by the Colorado State Engineer. PMP estimates are considered

of lesser r

d,

eliability along the Continental Divide, which includes the upper Yampa 

iver basin. Therefore, a site-specific PMP study was conducted to address issues 

ised by the NRC (1988, 1994) pertaining to the hydrometeorology for the basin and 

e surrounding geographical and climatologically similar region. Inherent in a sitepecific PMP study are analyses of extreme storms that have occurred in the region 

ince the generalized hydrometeorology report was published. Site-specific 

ydrometeorologic studies are being conducted because dam-safety officials 

ecognize the difficult problems inherent in PMP estimates in the Rocky Mountains. 

tilization of an interdisciplinary regional paleoflood study provides additional 

upporting information for understanding the magnitude of the largest contemporary 

oods and paleofloods with estimates of the PMF potential for a particular basin. 

terdisciplinary components included documenting maximum paleofloods, and 

nalyses of contemporary extreme rainfall and flood data in a basin and in a broader 

gional setting. Site-specific PMP studies were conducted to better understand 

xtreme rainfall processes by analyzing the rainstorms with similar hydroclimatic 

onditions (Tomlinson and Solak, 1997). The approach provides scientific 

formation to help determine the delicate balance between cost of infrastructure and 

ublic safety.

aleoflood data using bouldery flood deposits and non-inundation surfaces for 88 streams 

-

l) were used to

determine the paleoflood record length for paleoflood deposits and non-inundation 

sical

R

ra

th

s

s

h

r

U

s

fl

In

a

re

e

c

in

p

P

were used to document maximum flood discharges that have occurred during the 

Holocene. A variety of relative dating techniques (degree of soil development, surface

rock weathering, surface morphology, lichenometry, and boulder buria

surfaces. Peak discharge for a paleoflood deposit was obtained primarily using the 

critical-depth and slope-conveyance methods. Maximum paleofloods provide phy

evidence of an upper bound on maximum peak discharge for any combination of rainfallPaper 20 – Jarrett et al 156
or snowmelt-runoff in northwestern Colorado in at least the last 5,000 to 10,000 yea

(since deglaciation). 

No evidence of substantial flooding was found in the study area. The maximum 

paleoflood of 135 m3

/s for Elkhead Creek is about 13 percent of the site-specific PMF o

1,020 m3

/s. Envelope curves encompassing maximum flood at 218 sites (Figure 5) a

rainfall at 181 sites were developed for northwestern Colorado to help define maxim

contemporary and Holocene flooding in Elkhead Creek and in a regional frequency 

context. Paleoflood estimates incorporate the effects of climatic changes on hydrology 

during the period of the paleoflood record. Certainly, moderate climate changes (or other

changes such as wildfire effects on flooding or vegetation changes) have occurred during 

the Holocene; however, these effects are reflected in the maximum flood preserved a

site. Paleoflood data where the maximum age during which the flood occurred is at le

5,000 years are denoted with large, solid triangles and as small, solid triangles for less 

than 5,000 years (Figure 5). The envelope curve of maximum flooding incorporating

paleoflood data (Figure 5) is about 20 to 25 percent larger than contemporary maximum 

flooding in about the past 100 years since streamflow monitoring began. This modest 

increase likely is due to the large spatial extent of the data and relatively low-magnitu

flooding in northwestern Colorado. Variability in climate and basin conditions during the

Holocene does not appear to have had a large impact of flood magnitude, and that the 

assumption of stationarity may be valid for the upper end of the flood-frequency curves

in the study area. Thus, the envelope curve probably reflects an upper bound of flooding

during the Holocene in northwestern Colorado, which includes the effects of climate

change and other factors (wildf

rs

f

nd

um 

t a

ast

the

de

ire and vegetation changes) during the Holocene. 

157 Paper 20 – Jarrett et al 
10000

drainage

ett

for

l

g in

to

6-hour

e

reams 

. However,

needs to be

f

to

ons for

.1

1

10

100

1000

paleoflood <5000 yrs

gage

ungaged site

Peak Discharge, m /s

NW Colo.

NW Colo.

(contemporary)

Elkhead Reservoir

3

.1 1 10 100 1000 10000 100000

.01

paleoflood >5000 yrs

Drainage area, km

E. Colorado

<2,300 m (Jarrett)

2

Figure 5. Relation between contemporary and paleoflood peak discharge and

area with envelope curves for northwestern Colorado. The envelope curve of maximum

flooding for eastern Colorado is shown for comparison (Source: modified from Jarr

and Tomlinson, 2000). 

Maximum observed 24-hr rainfall is about 150 mm in about the past 100 years

northwestern Colorado (see Jarrett and Tomlinson, 2000), which provides additiona

support for the lack of flood and paleoflood evidence. Maximum rainfall and floodin

northwestern Colorado is substantial less than in eastern Colorado, which is subject

some of the most extreme rainfall flooding in the United States where maximum

observed rainfall has approached 610 mm. Large floods, if as hypothesized by 

transposition of such large rainstorms into northwestern Colorado and PMF magnitud

floods, would have left recognizable paleoflood evidence in at least one of the 88 st

studied, but no substantial out-of-bank flooding was identified. 

A critical assumption for calculation of synthetic rainfall-runoff modelling approach,

including PMP estimates, is geographic transposition of storm events from 

geographically and climatologically similar locations to watershed of interest

the NRC (1994) cautions that storm transposition and moisture maximization

for a slightly different location in the same hydroclimatic region. Regional analyses o

rainfall, streamflow, and paleoflood data in the present study provide information

evaluate the assumptions about large rainstorms in northwestern Colorado. The 

assumption that large rainstorms or rain-on-snow produce large floods in the Rocky

Mountains (FEMA, 1976; Hansen et al., 1977; Hansen, et al., 1988) has implicati

dam safety and floodplain management. Although a number of streamflow-gaging 

Paper 20 – Jarrett et al 158
stations in the northwestern Colorado had over 75 years of record, but no large ra

floods, these long-term gage data were assumed not to be representative of extreme flood 

potential from rainfall by FEMA (1976). Thus, the flood hydrology for som

based on transposing distant, large rainstorms from Arizona, New Mexico, and 

southwestern C

infall

e studies was 

olorado into northwestern Colorado and using rainfall-runoff modelling to 

djust the upper end of the gage flood-frequency relation (FEMA, 1976). 

),

e 

ons

-

m this study up to about the 20-year flood. The Ayres relation 

harply increases above the 20-year flood due to rainfall-runoff modelling, falls outside 

flood,

a

Flood-frequency relations using the Expected Moments Algorithm (England, 1998

which better incorporates paleoflood data, were developed to assess the risk of extreme 

floods. Flood-frequency analyses were made for eight streamflow-gaging stations. Th

flood-frequency relations for Elkhead Creek basin are shown in Figure 6. The relati

were developed at a gage site upstream from Elkhead Reservoir and using regional 

regression relations to estimate magnitude frequency at the reservoir. The floodfrequency relation for Elkhead Creek at Elkhead Reservoir developed by Ayres 

Associates, Inc. (written commun., 1996) (Figure 6) essentially is the same as the flood

frequency relations fro

s

the confidence limits of the regional flood-frequency relations above the 50-year

exceeds the maximum paleoflood for the basin at a recurrence interval of about 150 

years, and exceeds the envelope curve value of 250 m3

/s, which is not reasonable 

hydrometeorologically. Similar results were noted for other streams in northwestern 

Colorado. The difference for larger recurrence intervals primarily results from

transposition of distant rainstorms over basins in northwestern Colorado and then using 

rainfall-runoff modelling to estimate the upper end of flood-frequency relation as well as 

the PMF. The gage and paleoflood data based on the evaluation of physical evidence in 

channels provide information that can be used to refine assumptions used to estimate 

extreme flooding using storm transposition and rainfall-runoff modelling to at least a 

recurrence interval of 5,000 years. The paleoflood data provide no support for sharp 

upward slope increase of the frequency curve. The lack of substantial rainstorms and 

flood evidence in northwestern Colorado probably is explained by high mountain 

barriers, which substantially reduce the available atmospheric moisture from the Pacific

Ocean or Gulf of Mexico.

159 Paper 20 – Jarrett et al 
1 10 100 1000 10000 1

10

100

1000

10000

Recurrence Interval, years

Peak discharge, m /s

PMF

Ayres

Kircher et al.

 (at dam)

EMA (P=5,000 yrs;

station skew)

Max paleofloods

 Reservoir

 Gage

 x Peak Discharge

 95% confidence limits

EMA (P=5,000 yrs; 

 ave. skew=-0.03)

envelope curve for 

Elkhead Creek (fig. 5) 3

Figure 6. Flood-frequency relations for Elkhead Creek at Elkhead Reservoir using 

;

f

rve value (Figure 5) for maximum flooding in the Holocene is 

lso shown. The flood-frequency relation and probably maximum flood for Elkhead 

wn

he PMP 

, downslope wind flows 

nder PMP storm conditions and high altitude moisture depletion. 

vertopped

$184

.,

regional regression equations with paleoflood data (shown as a rectangle) at the reservoir

near Elkhead gage (0924500) with paleoflood data (shown as a rectangle) using at-site 

and average skew with 95 percent confidence limits. P denotes the paleoflood length o

record. The envelope cu

a

Reservoir (Ayres Associates, Inc., Fort Collins, Colorado, unpublished data) are sho

for comparison (Source: Jarrett and Tomlinson, 2000). 

The site-specific PMP study conducted for the Elkhead Creek drainage basin west of the

Continental Divide in northwestern Colorado revisited various issues related to t

under the explicit conditions which exist at Elkhead Reservoir and other reservoirs in 

northern Colorado (Tomlinson and Solak, 1997). These issues included a physical 

accounting of the effect of topography on storm transpositioning

u

The combined results of the site-specific PMP/PMF study and the regional 

interdisciplinary paleoflood study showed that Elkhead Dam would not be o

from the site-specific PMP. These results were accepted by the Colorado State Engineer 

for dam-safety certification with no modifications to the existing structure.

Changnon and McKee (1986) estimated the cost for modifying just the 162 high-risk

dams in Colorado to the PMP standards (Hansen et al., 1988) was approximately

million. This modification cost appears low as the estimated modification cost for 

proposed modifications of the Cherry Creek dam are as high as $250 million for Cherry 

Creek dam near Denver, Colorado (U.S. Army Corps of Engineers, written commun

Paper 20 – Jarrett et al 160
1997). There are over 10,000 dams in the Rocky Mountain region that may need to be 

modified for current PMP criteria during dam safety recertification. Thus, given the large

differences in maximum paleoflood and PMF values in the Rocky Mountains, it seems 

prudent to conduct additional hydrometeorologic and paleoflood research to help

the uncertainty in estimates of maximum flood potential. This regional interdisciplinary 

paleoflood approach, which is cost-effective, can be used in other hydrom

reduce

eteorologic 

ettings to improve flood-frequency relations and provide information for a risk-based 

a

have

to

ray

au

dy 

he

es”

technologies in applied flood frequency analyses. 

Initially, Bulletin 17B guidelines (log-Pearson 3, LP3), expected moments algorithm

(EMA), maximum likelihood estimators (MLE), and L-moments (LMOM), and other 

flood frequency estimation techniques, including regional approaches will be 

evaluated. A compilation and comparison of flood frequency estimates using EMA, 

MLE, and other new methods is then planned. A comprehensive model validation 

with field data is the critical component to the success of the new guidelines. Input 

on future directions and needs in flood frequency will be obtained from other 

agencies, universities, and consulting engineers and scientists. Five representative 

hydrologic regions in the United States will be used to evaluate proposed new floodfrequency methods and guidelines. Recommendations will be proposed for changing 

the flood frequency analysis guidelines for consideration by the IACWD and other 

water management agencies. 

s

approach for hydrologic aspects of dam safety. 

Considerations for Future Research 

The IACWD (1982, p. ii) indicated that the Nation’s flood frequency guidelines are

continuing effort and that much additional study is needed to achieve correct and

consistent guidelines. Many alternative approaches for flood-frequency analysis

been developed since 1982. Updated, consistent, reliable, and uniform

guidelines/approaches are needed to determine flood flow frequencies in order to 

have an effective flood damage abatement and related environmental assessment 

programs within Reclamation, the COE, the USGS, and other water agencies. 

Increasing flood losses, increasing emphasis on risk assessments for dam safety, 

improved floodplain management, and numerous related needs emphasize the need

improve methods to estimate the magnitude-frequency relations of flooding. Credible

flood estimates, particularly with very small AEPs, are needed as input to risk 

assessments for determining appropriate levels of public safety, prioritizing projects, 

and allocating limited resources. 

The need to develop more efficient and robust flood-frequency estimates for a wide ar

of water resources applications as quickly as feasible are widely recognized. The Bure

of Reclamation and U.S. Geological Survey recently proposed a $5 million, 5-year stu

to update and improve the Nation’s guidelines of flood frequency and related issues. T

goals of the proposed study “Improvements to Flood Frequency Analysis Techniqu

are to develop new guidelines for flood frequency analysis and to demonstrate the 

viability of using emerging

161 Paper 20 – Jarrett et al 
An integrated science approach will be used to involve participation of Reclamation 

and USGS engineers and scientists as well as full IACWD participation to help meet 

the needs of the water-resources community. Probabilistic hazard-estimation requires 

extrapolation well beyond the limits of gage and historical data. Thus, extrapolation 

requires improved understanding of physical processes and extending the 

observational record with paleoflood data. Though not a direct component of this 

proposal, a strong interagency commitment is needed to enhance detailed storm and 

flood documentation, long-term basic and applied research, applied flood-hazard 

assessment, and improved model interfaces for model users. The following tasks will 

be addressed; tasks are prioritized by need but may change depending on needs of 

funding agencies.

Extrapolation techniques for probabilistic risk assessment. Extrapolation of floodfrequency relations for return periods exceeding the 50-year flood (AEPs <0.02), 

which are most important to flood-plain managers and dam-safety officials. This 

analysis will focus on floods for AEPs ranging from 0.02 to 0.0001 (50- to 10,000-

year range), though reliability of the entire frequency curve will be emphasized. 

Determining the amount and types of data needed to credibly extend flood frequency 

relations to small AEPs are a primary focus of this element. Various regional 

approaches for flood-frequency analysis for probabilistic risk assessments will be 

evaluated. 

Improve methods to evaluate the effects of land-use changes on flood hydrology. 

These changes include hydrologic effects of wildfire, urbanization, and changes in 

vegetation patterns on flood magnitude and frequency (forms of non-stationarity 

issues). Separate, regional flood data sets for recent flooding, which document the 

effect of wildfire and urbanization, derived using paleohydrologic techniques, are 

available and proposed for study. The flood data and associated detailed rainfall data 

ill be evaluated with rainfall-runoff modelling and geospatial (GIS-based) statistical 

ethods. Generally, few data are available to properly assess non-stationarity issues. 

w

m

Thus, flood-hazard assessments can conducted using precipitation modelling and 

hydraulic modelling practices (Leavesley et al., 1996; Vecchia, 2001; Smith et al., 

2000). These analytical tools will be used to develop and evaluate viable methods to 

quantify the effects of land-use changes on flood hydrology. 

Flood frequency for regulated-flow streams. Develop a procedure for estimating 

rs of the

fields.

flood-flow frequency downstream from flood regulating structures. Membe

IACWD Subcommittee on Hydrology have begun collecting approaches used by 

various agencies. However, the information should be evaluated and assembled in a 

fashion usable to practicing engineers, hydrologists, and scientists in related

Climatic variability, non-stationarity issues. Evaluate the effects of natural and 

anthropogenic climate change on flood frequency. Paleoflood, historical, and gage 

data would be examined to evaluate non-stationarity of the flood record over varyin

time scales and the impact on the resulting flood-frequency relations. Regional 

paleoflood data available for five diverse hydrologic regions in the west-central 

g

Paper 20 – Jarrett et al 162
United States will be used to assess the effects of climate change on flood magnitude. 

[Currently there are no regional paleoflood data for the eastern United States, 

however, if they become available, they will incorporate them into the analyses.] 

These data then will be used to test and evaluate various assumptions of hydrologic 

modeling scenarios of climate change effects on flood magnitude and frequency. 

Products include several reports and supporting journal articles summarizing unified, 

state-of-the-art techniques for flood-frequency estimation that have general 

pplication for water-resources investigations. Recommendations will be proposed 

sis guidelines for consideration by the IACWD 

nd other water management agencies. Benefits of the proposed study will include 

94 mandates 

d

es

0

at

of research needs, including optimizing a balance of 

flood chasing” and detailed modeling approaches of paleoflood investigations. Our

er

bilities

r types of

SIs, if they exist, and to better understand geomorphic effectiveness of floods. More 

ood

age

-frequency

ata,

-plain

plex

, discharge

by the

have

tera

for changing the flood frequency analy

a

improve guidelines for flood hazard assessments, flood inundation mapping and as 

input to risk assessments for determining appropriate levels of public safety, 

prioritizing projects, and allocating limited resources. A major benefit of the 

proposed study supports the National Flood Insurance Reform Act of 19

that the Nation’s flood maps be revised in the near term and subsequently reviewe

every 5 years. The FEMA estimates that updates to existing flood insurance studi

and creating new maps for communities without mapping will cost between $80

million and $1,000 million. FEMA’s Map Modernization Plan also emphasizes th

more cost-effective methods to produce flood maps are essential. 

Concluding Remarks 

We hope to stimulate discussion

“

results indicate further research is needed that better accounts for physical processes of

flooding, for sediment transport and deposition during flood conditions, and the need to

improve methods for estimating peak discharge of floods and paleoflood. There is much

work to be done to improve our understanding of the distribution and location of

sediment deposits and erosion thresholds of surfaces from extreme floods. A bett

understanding of the physics of large flood response and better predictive capa

than those documented here is needed. Research is needed to identify othe

P

quantification (e.g., using 1-D or 2-D hydraulic modelling to calculate paleofl

discharges, using absolute-age dating of flood deposits and validation of relativedating methods, particularly developing guidelines for more robust flood

parameter estimation procedures, regional flood-frequency analysis with paleoflood d

etc.) are essential for dam safety risk assessments, flood-inundation mapping, flood

management, and related environmental investigations. While use of com

procedures provide slightly more precise quantitative description of the data

and frequency estimates of extreme floods in a basin may be readily estimated

cost-effective approach paleoflood techniques described above, the ability to

multiple approaches that fits the needs and resources specific to a specific wa

resources application are needed. 

163 Paper 20 – Jarrett et al 
References

Baker, V.R., Kochel, R.C., and Patton, P.C., (eds.), 1988, Flood Geomorphology, New

York, John Wiley, 503 p. 

Changnon, David, and McKee, T.B., 1986, Economic impacts and analysi

of extreme precipitation estimates for Eastern Colorado, Cooperative Institute f

Research in the Atmosphere, Colorado State University, CIRA Paper No. 5, 75 

pp. 

Cudworth, A.G., Jr., 1989, Flood hydrology manual, U.S. Bureau of Reclamation, A 

Water Resources Technical Publication, Lakewood, Colorado, 243 p. 

England, J.F. Jr., 1

s methods 

or

999, Draft User's manual for program EMA, at-site flood 

frequency analysis with historical/paleohydrologic data. Flood Hydrology Group, 

01,

f Steamboat Springs, Routt Co., Colorado, revised 1983, 40 p. 

____, 1997a, Multihazard identification and Risk Assessment, A cornerstone of the 

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evidence for a natural upper bound to flood magnitudes in the Colorado River 

Basin: Water Resources Research, v. 29, no. 7, p. 2287-2297. 

Federal Emergency Management Agency (FEMA), 1976, Flood Insurance Study, 

Town o

_

National Mitigation Strategy, Subpart C Hydrologic Hazards, Washington., D.C

133-147. 

_____, 1997b, Modernizing FEMA’s flood hazard mapping program - A progress

Report, Washington, D.C., November 1997. 

_____, 1998, Modernizing FEMA’s flood hazard mapping program - A progress 

report, December 1998, URL: http://www.fema.gov/library/mapmod.pdf

Foster, Jerry, 1999, Improving the state of the art in risk analysis, Hydro Review, 

38(6), p. 12-14. 

Gottesfeld, A.S., 1996, British Columbia flood scars: maximum flood-indicators, 

Geomorphology, 14, p. 319-325. 

Hansen, E.M., Schwarz, F.K., and Riedel, J.T., 1977, Probable maximum precipitation

estimates, Colorado River and Great Basin Drainages, Hydrometeorological Report 

No. 49, NWS, NOAA, U.S. Dept. of Commerce, Silver Spring, MD, 161 p. 

robable

and

, U.S.

Floods, Modern Hazards, Principles and Applications of Paleoflood Hydrology: 

,

Hansen, E.M., Fenn, D.D., Schreiner, L.C., Stodt, R.W., and Miller, J.F., 1988, P

maximum precipitation estimates-United States between the Continental Divide

the 103rd meridian, Hydrometeorological Report 55A, NWS, NOAA

Department of Commerce, Silver Spring, MD, 242 p. 

House, P.K., Webb, R.H., Baker, V.R., and Levish, D.R. (editors), 2001, Ancient 

American Geophysical Union, Water Science and Application 5, Washington

D.C., with accompanying CD-ROM, 385 pp. 

Interagency Advisory Committee on Water Data (IACWD), 1982, Guidelines for 

determining flood-flow frequency (2nd ed., revised), Reston, Virginia, U.S. 

Geological Survey Office of Water Data Coordination, 28 p, 14 appendices. 

Paper 20 – Jarrett et al 164
_____, 1986, Feasibility of assigning a probability to the probable maximum flood, 

Reston, VA., U.S. Geological Survey Office of Water Data Coordination, 7

Jarrett, R.D., 1987, Peak discharge errors in slope-area computation in mountain 

streams: Journal of Hydrology, vol. 96, (1-4), p. 53-67. 

Jarrett, R.D., and Tomlinson, E.M., 2000, Regional interdisciplinary paleoflood 

9 pp.

pproach to assess extreme flood potential: Water Resources Research, 36, (10), 

for

using

phical

rmation for flood 

hazard assessment, in House, P.K., Webb, R.H., Baker, V.R., and Levish, D.R. 

ient Floods, Modern Hazards, Principles and Applications of 

Paleoflood Hydrology: American Geophysical Union, Water Science and 

wain, R. E., and Jarrett, R.D., 2000, Monograph for using paleoflood data in water 

resources application, in Hotchkiss, R.H., and Glade, Michael, editors, Building 

a

p. 2957-2984. 

Jarrett, R.D., and England, J.F., Jr., 2001, Reliability of paleostage indicators

paleoflood studies, in House, P.K., Webb, R.H., Baker, V.R., and Levish, D.R. 

(editors), 2001, Ancient Floods, Modern Hazards, Principles and Applications of 

Paleoflood Hydrology: American Geophysical Union, Water Science and 

Application 5, Washington, D.C., with accompanying CD-ROM, p. 91-109. 

Jones, J.L., Haluska, and Kresch, D.L., 2001, Updating flood maps efficiently

existing hydraulic models, very-high-accuracy elevation data, and a geogra

information system - A pilot study on the Nisqually River, Washington: U.S. 

Geological Survey Water Resources Investigations Report 01-4051, 25 p. 

Leavesley, G.H., Restrepo, P.J., Markstrom, S.L., Dixon, M., and Stannard, L.G., 

1996, The modular modelling system (MMS) - user’s manual: U.S. Geological

Survey Open-File Report 96-151, 142 p. 

Levish, D.R., 2001, Paleohydrologic bounds, non-exceedance info

(editors), 2001, Anc

Application 5, Washington, D.C., with accompanying CD-ROM, p. 175-190. 

Lins, H.F. and Slack, J.R., 1999, Streamflow trends in the United States: Geophysical 

Research Letters 26 (2), p. 227-230. 

Nathan, R., and Weinmann, P.E., 1998, Australian Rainfall and Runoff, A guide to 

Flood Estimation, Volume 1, Book VI Estimation of Large and Extreme Floods: 

The Institute of Engineers, Australia, various pages. 

National Research Council, 1988, Estimating probabilities of extreme floods, 

methods and recommended research: National Academy Press, Washington, D.C., 

141 p. 

National Research Council, 1994, Estimating bounds on extreme precipitation events: 

National Academy Press, Washington, D.C., 25 p. 

_____, 1999, Improving American River flood frequency analyses, Committee on 

American River Flood Frequencies, National Academy Press, Washington, D.C., 

120 p. 

Reed, D., Faulkner, D., Robson, A., Houghton-Carr, H., and Bayliss, A., 1999, Flood 

Estimation Handbook, Procedures for Flood Frequency Estimation: Institute of 

Hydrology, Crowmarsh Gifford, Wallingford, Oxfordshire, UK, 5 volumes, 

various pages. 

Smith, J.A., Baeck, M.L., Morrison, J.E., and Sturdevant-Rees, Paula, 2000, 

Catastrophic rainfall and flooding in Texas: Journal of Hydrometeorology, 1, p. 5-

25. 

S

165 Paper 20 – Jarrett et al 
Partnerships, Proceeding of the 2000 Joint Conference on Water Resources 

Engineering and Water Resources Planning and Management, July 30-August 2, 

on

U.S. Bureau of Reclamation (USBR) (1999), A framework for characterizing extreme 

ssments 

Colorado, November 16-17, 1999,

port 01-324, 77 p.

Vog .C., 2001, Frequency of

.

46,

M

2000, Minneapolis, MN, American Society of Civil Engineers, Reston, VA, p. 1-

7. 

Tomlinson, E.M., and Solak, M.E., 1997, Site-specific probable maximum precipitati

(PMP) study of Elkhead drainage basin, TRC North American Weather 

Consultants, Salt Lake City, Utah, NAWC Report WM 95-6. 

floods for dam safety risk assessment. Prepared by Utah State University and 

Bureau of Reclamation, November 1999, 67 p. 

Vecchia, A.V., (compiler), 2001, A unified approach to probabilistic risk asse

for earthquakes, floods, landslides, and volcanoes, Proceedings of a 

Multidisciplinary Workshop held in Golden,

Sponsored by the U.S. Geological Survey Urban Hazards Initiative: U.S. 

Geological Survey Open-File Re

el, R.M., Antigoni Zafirakou-Koulouris, Matalas, N

record-breaking floods in the United States Water Resources Research, 37 (6), p

1723-1731. 

Additional Information: Robert D. Jarrett, U.S. Geological Survey, P.O. Box 250

S 412, Denver, Colo., 80225-0046: rjarrett@usgs.gov, ph: 303-236-6447

Paper 20 – Jarrett et al 166
Current Hydrologic Practices, Problems, and Needs 

Within California’s Dam Safety Program 

By 

Melissa Collord

Introduction 

nt of 

Wa

thro f an acre to

tho capable

esti y

eva

981 to determine the hydrologic adequacy 

1. 

2. 

3. 

5. 

7. 

8. 

sho gs.

Un

unitgraph parameters are obtained from a generalized study, conducted in 1971, of 

cha rnia

exc Sierra

Nevada Divide. The regression equations relate the drainage basin characteristics of 

t ) and 

Cla he

study also presents guidelines for estimating loss rate parameters. 

There are over 1,200 dams within the jurisdiction of the California Departme

ter Resources (DWR), Division of Safety of Dams (DSOD). The dams are located

ughout the state and have drainage basins that vary in size from portions o

usands of square miles. DSOD requires that all dams within its jurisdiction be

of adequately passing a selected design flood. A method was developed by DSOD to

mate flood hydrographs for ungaged or poorly gaged watersheds for use in spillwa

luation.

Methodology

The procedure devised by DSOD in 1

of any spillway in California consists of eight parts: 

Assessment of the potential downstream hazard 

Determination of appropriate storm return period 

Development of precipitation 

4. Development of synthetic unit hydrograph parameters 

Development of loss rate parameters 

6. Computation of the flood hydrograph 

Routing of the flood hydrograph through the reservoir 

Evaluation of the spillway adequacy 

The following discussion will focus on parts 4 thru 6, their basic concepts and 

rtcomin

it Hydrograph

DSOD utilizes Clark’s method to develop a synthetic unit hydrograph. Clark’s

observed rainfall and runoff events, which related these parameters to drainage basin 

racteristics by regression analysis. The study is applicable to the State of Califo

ept the area south of the Tehachapi Mountain Divide and the area east of the

stream length, area, elevation, and ground cover to the time of concentration ( c

rk’s storage coefficient (R) for development of a basin-specific unit hydrograph. T

167 Paper 21 – Collord 
Flo

(Hydrologic Engineering Center, 1981). The program obtains the flood hydrograph by 

surf tio on

wit d not be less than 70

sho

r

stor obtain the

des

Pro

o

developing a basin-specific synthetic unit hydrograph. However, with the advent of 

ster and more sophisticated computers and programs, such as HEC-HMS, calibration of 

b

f h observed

rainfall and flow data. Unfortunately, there still exists the problem of a lack of basic 

hydrologic data in most small drainage basins. This makes it impossible to calibrate a 

basin’s unitgraph and loss rate or to check the reasonableness of a synthetic unit 

hydrograph. Therefore, we highly recommend that more research be conducted into 

developing unit hydrographs and determining loss rates for small ungaged basins. 

Modeling large ungaged drainage basins that range in size from 100 to 1000 

square miles present an even more difficult task. In addition to determining the unit 

hydrograph parameters for each subbasin, channel routing or lag time must now be 

included in the analysis. The methods that are available in determining routing 

parameters (DSOD typically uses the Muskingum routing method) are imprecise and our 

level of confidence in accurately defining the routing conditions is low. 

Utilization of HEC-HMS as our primary tool to perform our hydrology and 

reservoir routing calculations will occur in a very gradual process as more staff members 

are exposed or are given training to operate the program. Several staff members have 

already received some training on HEC-HMS, but are unable to use the program since the 

exponential loss rate function is no longer available and in some cases snowmelt 

calculation is needed. DSOD can certainly modify loss rate assumptions to be 

compatible with HEC-HMS, but we do encourage HEC to incorporate snowmelt 

calculations into HMS as soon as practical. 

od Hydrograph

The flood hydrograph is developed using the computer program HEC-1 

convolution of the effective rainfall increments with Clark’s unitgraph. Losses due to 

ace reten n and infiltration are estimated by the exponential loss rate functi

hin HEC-1. The general criteria is that the percent runoff shoul

when the mean annual precipitation (MAP) at the basin is greater than 25 inches and 

uld not be less than 60 when the MAP is 25 inches or less.

If applicable, allowances for snowmelt, base flow in the basin, runoff from prio

ms, import of water, etc., are added to the storm runoff hydrograph to

ign flood hydrograph for the watershed.

blems Faced 

The regression equations provide a rational, consistent and simplistic method t

fa

asin models has become easier and a more widely used approach. The optimization 

eature in HEC-HMS estimates unit hydrograph and loss rate parameters wit

Paper 21 – Collord 168
Bureau of Reclamation Hydrologic Research 

Presented at the FEMA Workshop on H Safety 

Robert E. Swain, Louis C. Schreiner, and Daniel R. Levish 

troduction

k

t

events, and ultimately to the magnitude-frequency relationship of maximum 

servoir stages. 

ation has identified the need for a review of its present procedures for 

eveloping probabilistic extreme flood estimates and their associated uncertainties for use 

dam

paper describe ongoing hydrologic

search activities for use in dam safety analysis. 

put in flood frequency analyses, probabilistic 

dels and other hydrology studies aimed at 

k nalyses. The physical 

lood events requires collection and review of a 

ariety of flood hydrology data including peak discharge and mean daily stream flow 

records

treme rainfall data, and many other sources of information. A searchable 

atabase that includes organized and connected lists of available data, visual selection 

s to the data would allow faster and more thorough assessment. 

a,

,

ydrology studies. The eventual goal of this project is to provide spatial and temporal 

data for use in probabilistic flood hazard studies in the 17 western states. 

ydrologic Research Needs for Dam

By 

In

The U.S. Bureau of Reclamation is now making extensive use of quantitative ris

assessment in support of dam safety decision-making (Von Thun and Smart, 1996). An 

important input to Dam Safety Risk Assessment is the development of probabilistic 

extreme flood estimates. The focus has shifted from routing a single “maximum” even

(i.e. the probable maximum flood, PMF) to consideration of the entire range of plausible

inflow flood

re

Reclam

d

in safety risk assessment. Where practical, Reclamation would like to develop 

improved procedures. The following sections of this

re

Flood Hydrology Database 

A variety of hydrologic data are used as in

hydrographs, stochastic rainfall-runoff mo

ma ing flood hazard probability statements for use in risk a

understanding and modeling of extreme f

v

, precipitation (rainfall and snowfall) and temperature data, soil data, paleoflood

information, ex

d

capability, and link

The purpose of this research is to continue development of a hydrology database that will 

include a variety of hydrology data such as peak discharge estimates, paleoflood dat

precipitation and temperature data, as well as potential sources of infiltration 

characteristics and other geologic properties of drainage basins. These data would be 

used as input into flood frequency analyses, probabilistic hydrograph development, and

prediction of basin response in stochastic modeling of extreme flooding. This project

focuses on the development of a flood hydrology database that identifies, summarizes

and links hydrologic data that is needed for developing flood frequency analyses and 

probabilistic hydrographs, as input for stochastic rainfall-runoff models, and other 

h

169 Paper 22 – Swain et al 
A variety of data including information on extreme peak discharge have been gathered 

for the Sierra Nevada region of northern California. Paleoflood data have been gathered

throughout the western U.S., as well as in a database at the University of Arizona, 

tree.ltrr.arizona.edu/~katie/paleofld.html. Currently, Reclamation has a paleoflood 

database in Microsoft Access and hydrology database in Arcview exist as separate 

databases. These databases need to be integrated into one database in order to efficiently

store and access information for flood-related studies. In addition, computer code and

user interface have been developed for the hydrology database

a

that will allow the user to

ccess records by graphically selecting an area. By the end of 2002, a preliminary userRevision and Update of Precipitation-Frequency Studies for the United States and Its 

Possessions 

date

,

or

c.), 

for

t

recipitation-frequency information that can be incorporated in risk assessments that are 

ies

S),

-kind-services) dependent on their interest/needs to provide support for the 

tates,

ion, the

next couple of years, Reclamation expects 

to continue with data set development for the Upper Midwest, but concentrate its 

effort on development of Depth-Area (DA) relationships used to adjust point 

precipitation-frequency values to representative areal average 

precipitation-frequency estimates.

a

friendly database is scheduled for completion. This data information system will require

updating and continual maintenance. 

Since the mid 90's, meteorologists in both Reclamation’s Flood Hydrology Group and 

River Systems & Meteorology Group have been addressing the need to revise and up

precipitation-frequency estimates for the United States. The demand for this work is 

obvious in that precipitation-frequency atlases presently used by Reclamation are 

woefully outdated, with the far majority of the previous studies dating back 30 to 40 

years ago, and lack extensions to important meteorological parameters (duration, area

return period, etc.). This information is used in establishing hydrologic design criteria f

the safety evaluation of water control structures (dams, canals, levees, culverts, et

design of other types of construction (roads, bridges, flood warning systems, etc.), and

establishing project operational criteria. Results of this work will provide consisten

p

used in current/future flood hydrology studies. 

The project is a cooperative effort among several federal, state, and local agenc

involved in water resource management. The National Weather Service (NW

Hydrometeorological Design Studies Center (HDSC) is the lead agency for 

accomplishing the work with participation from other agencies (financial, 

in

particular region under investigation. Because of the large amounts of data to 

process and the need to test new meteorological and statistical analysis techniques, 

the United States and its Possessions were broken into nine separate zones. 

Presently, four of these zones are under development in varying degrees. These 

zones include: Semiarid Southwest, the Ohio River Basin and Surrounding S

Hawaii, and Puerto Rico and the Virgin Islands. Work on a fifth study reg

Upper Midwest, has been started by Reclamation to assemble maximum daily

precipitation (prior to 1949) data. For the

Paper 22 – Swain et al 170
The Semiarid Southwest, Ohio River Basin and Surrounding States, and Puerto Rico and 

the

completion in 2003. Current updates/progr

available at: htt

Virgin Islands zones are scheduled for completion in 2002. Hawaii is scheduled for

ess reports concerning all work underway is 

p://www.nws.noaa.gov/oh/hdsc. The precipitation-frequency work as

ompleted will be published as NOAA Atlas 14, Volume ( # ). It is expected that the 

ntire Atlas will be completed by 2006. 

Probabilistic Flood Hydrographs 

the

s are needed for situations where: the reservoir inflow peak discharge is 

ed 

Basic streamflow hydrograph methods (e.g., Chow et al., 1988; Bras, 1990) are used to 

bilistic hydrographs. These methods include peak and oneay mean discharge identification, selection of hydrograph shape and duration, base flow 

e past

e

rest

re used as a sample to represent potential extreme flood shapes and volumes. The largest 

ability of

s

s,

c

e

Flood runoff hydrographs integrate the drainage basin and channel response to 

precipitation and snowmelt, given some initial, variable state of moisture throughout

watershed. Probabilistic flood hydrographs are developed to assess the adequacy of the 

spillway and reservoir flood/surcharge space to temporarily store a portion of the flood 

volume, and to attenuate or pass the hydrograph peak without overtopping the dam. 

Flood hydrograph

greater than the maximum spillway capacity; the reservoir has a large, carry-over storage; 

and/or the reservoir has dedicated flood control space. The focus of this research is to 

develop a simplified approach for estimating probabilistic hydrographs that can be us

for appraisal or feasibility level studies, and to develop a simplified method of 

extrapolating flood frequency curves. 

estimate properties for proba

d

identification and separation, and direct runoff volume estimation. Peak discharge and 

mean-daily streamflow records are used because this source is the best information on

flood magnitudes that are likely to occur in the future, based on what occurred in th

(Pilgrim and Cordery, 1993). 

The key idea is calibration or scaling of hydrographs to match a particular peak discharg

for a given probability. The approach relies completely upon the specification of a peak 

flow frequency curve that describes the probabilities of interest. Peak discharge 

estimates, n-day maximum mean flows, and observed hydrographs at the site of inte

a

peak and volume hydrographs are utilized as a basis to scale. 

There are five major assumptions for developing the hydrographs: (1) the prob

peak discharge is sufficient to represent a probability of the composite hydrograph; (2)

unit hydrograph (e.g., linearity) assumptions apply to the basin; (3) direct runoff volume

can be estimated from daily flow hydrographs; (4) peak discharge - maximum mean nday flow relationships can be extrapolated; and (5) the recorded streamflow observation

historical information, and paleoflood data provide an adequate sample to base 

extrapolations to extreme floods. 

The anticipated completion date for this project is early 2002. 

171 Paper 22 – Swain et al 
Improved Flood Frequency Extrapolations and Runoff Modeling 

The purpose of this research project is to develop improved methods to extrapolate flood

ood

on. CASC2D is a 2-dimensional, distributed 

infall-runoff model that has successfully reproduced the 1997 Fort Collins flood. The 

m

It is anticipated that model selection and extrapolation functions can be derived from the 

atershed topography, hydraulic routing characteristics, and precipitation characteristics 

d 

00 mi

2

). This research project attempts to 

ddress many of these concerns. 

s

ch can

r than

d

n

rate.

ated by snowmelt 

the study was based on rainfall distributions. The 

frequency curves and develop extreme flood hydrographs. The major approach to fl

frequency extrapolation will be based on a combination of rainfall extrapolation and

derivation from physically based runoff mechanisms. Rainfall-runoff models will be 

used to derive the peak discharge frequency distribution from input basin characteristics 

and precipitation, and be used as the basis for frequency curve extrapolation. The 

CASC2D rainfall-runoff model will be evaluated and tested for application at 

Reclamation sites, and compared with a stochastic event runoff model (SEFM) developed

by Dr. Melvin Schaefer for Reclamati

ra

ain precipitation and stochastic components used in SEFM will be added to CASC2D.

w

at Reclamation dams. Input rainfall will be derived from frequency analysis or from

stochastic storm generation. Flood frequency and hydrograph uncertainty bounds will be 

approximated by simulation. Models will be compared on a large (>500 mi

2

) basin 

where paleoflood data are available. 

Progress has been made in developing hydrograph-scaling techniques for appraisal and

feasibility studies that require low effort and expense. These techniques have been 

applied to several projects such as Pineview/Deer Creek, Red Willow, North Platte, an

Folsom Dams. Internal and external reviewers have pointed out several shortcomings of 

that work including, assumptions of linear runoff and extrapolation, use of observed 

hydrographs, failure to separate rainfall and snowmelt, and the challenges of using the 

techniques at larger basins (greater than about 5

a

This research can be applied to Dam Safety projects where flood peaks and hydrograph

are needed with return periods that exceed 1,000 years. The extrapolation resear

be applied to sites where loss of life is large, as floods with return periods greate

10,000 years are sometimes needed. The research will be span three fiscal years an

conclude in 2004. 

Rainfall-Runoff Modeling Using National Weather Service 1,000-year Retur

Period Precipitation Estimates at Causey Dam, Utah 

Recently, the Flood Hydrology Group completed a study where the frequency estimates 

were extrapolated to a return period of 200,000 years. The method used a two-point

extrapolation to 200,000 years using the mean of the gage data and the mean of the 

paleoflood range. This was the first attempt to extrapolate frequency data beyond a 

10,000-year return period and several assumptions were made that may not be accu

For example, the stream gage data in the Wasatch Range are domin

events, yet the distribution selected in

Paper 22 – Swain et al 172
intention of this study is to provide another independent data point and to verify the 

off

1,000-year thunderstorm flood event for Causey 

am. This peak discharge estimate will be compared with the frequency analysis 

developed for Causey Dam. st case because there is 

xisting at site paleoflood data to use in comparison, the drainage area is relatively small 

data

ent. 

lood hazards will be brought in to Reclamation on 

n individual basis. It is intended that each person present a Technical Update Lecture 

eferences

Handbook of Hydrology,

Maidment, D.R. (ed.), McGraw-Hill, New York, Ch. 9, pp. 9.1-9.42. 

conclusions reached in the previous analysis. 

This investigation will use the draft precipitation values developed by the National 

Weather Service (NOAA Atlas 14, Vol 1, DRAFT) to produce a 1,000-year assumed 

thunderstorm event at Causey Dam, Utah. These values will be input to a rainfall-run

model (HEC-1 or FHAR) to develop the

D

 Causey Dam was selected as the te

e

(137 mi

2

), and it has a significant amount of streamflow and other comparative

developed. The estimated completion date would be June 2001. 

Probabilistic Flood Hazard Workshop

The introduction of risk analysis for dam safety signaled a significant change in the way 

the Dam Safety Office and the Technical Service Center conduct flood hazard 

assessments. The purpose of this project is to compile, review, and evaluate current 

state-of-the-knowledge on probabilistic techniques used in flood hazard assessm

External experts in various aspects of f

a

(1-2 hours). Members of the Flood Hydrology Group will subsequently meet with them

to discuss their research in detail and potential technology transfer to Reclamation. 

 

About 12 experts participated in the workshop last year. These experts have helped the

Flood Hydrology Group map out future methods, improve current methods, and plan a 

program for probabilistic flood hazard analysis to meet Dam Safety Office needs. 

R

Bras, R.L. (1990) Hydrology, An Introduction to Hydrologic Science. AddisonWesley, Reading, MA, 643 p. 

Chow, V.T., Maidment, D.R. and Mays, L.W. (1988) Applied Hydrology. 

McGraw-Hill, New York, 572 p. 

Pilgrim, D.H. and Cordery, I. (1993) Flood Runoff. In

Von Thun, J.L. and J.D. Smart (1996) Risk assessments support dam safety 

decisions, USCOLD Newsletter, Issue No. 110, Nov 1996, U.S. Committee on 

Large Dams. 

173 Paper 22 – Swain et al 
RESEARCH NEEDS SUMMARY 

Jerry Webb, Huntington District, Corps of Engineers 

o Addressing lack of historic data for calibration 

o Antecedent rainfall conditions 

ional scenarios for extreme events 

o Concerns over site specific conditions such as topography impacting 

o How to make consistent, reproducible studies given the amount of 

judgment that will be needed 

o How and when to include paleoflood evidence (when available) and 

use of antecedent storm conditions 

porate

Jam

cedure for filling the large gap between observed hydrologic data 

and PMF estimates 

o Hydrologic methods consistent over large areas due to need to assess 

multiple dam failures in same watershed 

o A practical approach to uncertainty analysis to enable better (more 

meaningful) evaluations than sensitivity analysis.

• Specific Research Items 

• Antecedent rainfall 

• Multiple Reservoir Systems 

• Probable Maximum Rainfall distribution included in HMS 

• AEP of PMF 

• Guidelines that incorporate 

o Orographic effects 

o Large drainage areas subject to frontal movement storms

o Multiple reservoir systems 

o Variations in operat

Jeff McClenathan – Omaha District, Corps of Engineers 

• R&D Efforts 

o Portfolio risk analysis 

o Loss of life 

o Extend frequency curves beyond .001 event 

o Develop probabilities of various types of failure mechanisms

o AEP of PMF 

• Policy Needs 

rainfall and runoff 

o The need for independent review of work to overcome the potential 

disagreements from the use of judgment in the analysis 

the appropriate level of detail 

o Consistency on the

o When to use risk analysis and the level of detail needed to incor

it 

es Chieh, Los Angeles District, Corps of Engineers 

• General Research Needs

o AEP of PMF 

o Pro

175 Research Needs Summary
o Case studies of observed extreme flood events as compared with PMF 

and discharge and volume frequency estimates 

o Antecedent precipitation and soil moisture 

o Unit hydrograph characteristics (efficiency of runoff or time of 

concentration findings) 

o Compilation of paleo-hydrologic information nationwide and 

integration with PMF & discharge-frequency estimates; generalize 

findings if possible. 

Earl Eiker – Headquarters, Corps of Engineers - Retired 

• Traditional H&H Research Needs

o Flood Series and Flood Runoff Volume

o Spillway Erosin 

• Risk Analysis Research Needs 

o Paleoflood hydrology 

o Stochastic hydrology 

o Use of meteorological and historical flood data to extend record 

lengths 

o H&H Parameters (antecedent conditions, unit hydrograph 

uncertainties, routing coefficients, dam breach formation, wave 

propagation, operational uncertainties) 

o Loss of Life estimates 

Lou Schreiner, Bureau of Reclamation 

• Current Research Projects 

o Flood hydrology database 

 Discharge estimates 

 Paleoflood data 

 Precipitation and temperature data 

 Soil characteristics 

o Precipitation-Frequency studies for the United States 

 Update precipitation-frequency atlases 

 Zone based analysis 

o Flood Frequency Extrapolations 

 Extrapolate flood-frequency curves and develop extreme 

hydrographs. 

Michael Davis – Federal Energy Regulatory Commission 

• Probable Maximum Flood Studies 

o Orographic impacts on the PMP 

o Additional research on combination of snowmelt contribution factors 

such as snowpack, temperature wind. 

o Distributed loss rate method and STATSGO data 

• Dam Break Studies 

o Breach parameters for rock and earth fill dams

Research Needs Summary 176
o Uns s and multiple 

spillways 

Gre L lley Authority

ing from

floodplain regulations relating to NFIP, elevation 

n

1. 

Matthew C

• Des

l precipitation

S)

Melissa Co a Division of Dam Safety

Joe Skupie

• Lac

• Failure rameters/policies for dams with corewalls or upstream face walls 

tion methods 

timate time of concentration for extreme events 

• Sm

ution

• Hyd

is

levation for 

teady flow computer models for complex dam

g owe – Tennessee Va

• Risk Analysis 

o Development of methodology to analyze risks result

operational changes

 Able to show impacts on full range of potential events 

 Impacts on local

and flow duratio

 Impacts on Dam Safety

. Lindon – Utah DNR 

ign Storm Analysis

o Develop physically based models 

 Soil types

 Vegetation

 Spatial and tempora

 Routing (RAS, HM

 Calibrated models 

o Grow experienced modelers 

llord – Californi

• Ungaged watersheds

o Unit hydrograph development 

o Loss Rate determination

o Channel routing 

o Flood hydrograph computation methods 

o Snowmelt impacts 

n – Somerset County, New Jersey 

k of experienced modelers 

o Develop minimum education requirements 

o Small dam hydrology/hydraulic courses 

pa

• Unit Hydrographs 

o Standardize Time of Concentration computa

o Methodology to es

all Dams

o Design storm distrib

raulic Model Development 

o Develop guidelines for steady flow analys

o Develop inexpensive techniques to develop digital topography 

o Identification of applicable starting water surface e

hydraulic models

177 Research Needs Summary
Ed Fiegle

• Short T

EAK 

o p

• Long Term Needs 

P rainfall events 

onditions

he Green-Ampt loss rate function for nationwide 

Sam Hui –

• App

o eters 

o 

• Fundamental Research Needs 

d wave propagation in rivers

Anand Prakas

• Catego

o cations

• Probab lopment 

P

tes vs size of basin

o nsistent

resu rologists

o ain on snow

• Dev ure risks

• Insu

• Los

Cat e d

• Update “Snow Hydrology” a Northwest Division publication 

– Georgia Safe Dams

erm Needs 

o Software Development 

 Complete HECHMS and make compatible with HECRAS, 

ARCVIEW, DAMBR

o Develop software models for use at the state level 

Perform sensitivity analysis of various hydrologic methods. Develo

results by region. 

o Update Curve Numbers and lag time routines 

o Better regionalization of PM

o Define and update Antecedent Moisture C

o Development of t

application 

Bechtel Corporation 

lied Research Needs

Basin runoff model param

 Unit Hydrograph development 

Flood Frequency Analysis 

 Inclusion of uncertainty factors 

 Confidence limits 

o Dam Break analysis for mu

o Tailings Dams

h – URS Corporation

rize Dams

Risk based vs Standards related classifi

le Maximum Flood deve

o Consistent methods 

 Magnitude, duration, sequence of the PM

 Lag times and loss ra

o AEP of PMF 

Methods to standardize reasonableness of PMP to generate co

lts among hyd

R

o Long duration storm events equivalent to PMF

• Dam failure rates and breach sizes 

elop tangible and intangible dam fail

rance costs of dam failures 

s of Life estimates 

C cilio – PG&E, retire

Research Needs Summary 178
• Upd

District publication 

• Continue update existing HEC hydrologic and hydraulic models. 

ribution of the PMP especially 

ographic areas.

• Tec

• Develop a temperature sequence for PMP estimates with snow so that it 

reflects

• Window

Rory Nathan tralia

AEP of PMP 

ize bias in the AEP transformation between 

• AE

Des Hartford

• Physica

• Charac

• Mo tion

• Analytical process and procedures used for dam safety decision making

Ed Tomlinson – Applied Weather Associates 

• Upd all data base

• Com fall events

• Upd dewpoint climatology

• Sto

• Incorporate modeled data fields into PMP procedure 

• Com l

Me c ineering

• Sho

ectronic bibliography

o Unit Hydrographs 

ate “Generalized Snowmelt Runoff Frequencies”, a 1962 Sacramento

• Develop uniform method of areal dist

applicable in or

h review HMR 57 and HMR 58

the diurnal variation of temperatures 

s based DAMBREAK

– Sinclair Knight Merz, Aus

• 

• Techniques required to minim

rainfall and floods.

P of extreme rainfall depths

– BC Hydro, Canada

l nature of phenomena involved

terization of uncertainties

dels to help with risk characteriza

ate the historic extreme storm rainf

plete catalog of extreme rain

ate the US maximum

o 12 hour persisting dewpoint temperatures

rm transpositioning 

o Wind 

o Temperature 

o Other parameters 

puter models for maximum potential rainfal

l S haefer – MGS Eng

rt Term

o Resource center for research findings and case studies 

 El

 University maintained 

• Long Term

o Hydraulic Response of watersheds 

 Linear 

 Non-Linear 

 Develop Policy

179 Research Needs Summary
 Surface Runoff 

 Interflow

-Area-Duration storm analysis 

 

• Pol

truction

dam evaluation 

David Bow

• Ris

o urve extrapolation

o AEP of extreme events 

Bob Jarrett – US Geological Survey 

• Phy

o 

o 

o olds 

• Pal

ling

o Absolute Age Dating of Deposits 

ge Dating Methods 

nes for Flood-Frequency Parameter Estimation 

leoflood Data

o Mountainous area Depth

 NEXRAD 

Satellite Imagery 

icy

o Hydrologic and Flood Safety Criteria 

 New dam cons

 Existing

les – Utah State University 

k Assessment 

o Probabilistic extreme flood estimates 

Frequency c

sical Processes of flooding

Sediment Transport 

o Deposition during Floods 

Sediment Deposition 

Surface Erosion Thresh

• Peak Discharge Estimates 

eoflood Peak Discharge Estimates 

o Hydraulic Mode

o Validation of Relative A

 Guideli

 Regional Flood-Frequency Analysis with Pa

Research Needs Summary 180
DISCUSSION 

1. Gen l sion, the

worksh p d their

discussion

Ris n

era – After the presentation of papers, and some general group discus

op articipants divided into 3 smaller discussion groups. These groups, an

subjects, were:

k A alysis Group – Items relating to uncertainty factors that influence reservo

ow values and the computation of the Annual

ir 

infl Exceedance Probability (AEP)

• Storms and flood database 

nal hydrology parameters. 

Standa

of extreme floods. 

• Extension of flood frequency curves 

• Develop regio

rds Group – Items relating to physical factors that influence the methodology 

s, including the PMF.

• Improve Technology Transfer 

Me r

for the computation of extreme flood

• Develop Regional Databases 

• Loss Rate Function Analysis 

teo ology Group – Items relating to rainfall analysis from both the standards based 

• Precipitation Analysis 

Each gr p presented by each

organization th e most pressing needs for 

dam sa ter a period of discussion in the small groups, 

the wor h their opinions on 

research needs. The group related research items are presented below, in their order of

group-defined

2. Rese c p participants reassembled 

nd each group boiled down the research needs presented in each paper and presented 

thei earch needs generated by each group are 

presented below in their order of group-defined priority. 

Risk Analy

analysis and a risk-based analysis. 

• Rainfall frequency analysis 

• Real time storm analysis

ou was challenged to boil down the research needs

en generate, and prioritize, what they felt are th

fety research within their group. Af

ks op participants reassembled and each group presented

importance. 

ar h Needs – After a period of discussion, the worksho

a

r opinions on research needs. The res

sis Group

• Develop a historical database of storms and floods that can be used for 

• Dev ncies to smaller AEP’s. 

• Develop regional hydrology parameters. 

• Develop an approach to consider storm series. 

• Continu odels. 

analysis. 

elop an approach to extend flood freque

e to develop stochastic rainfall/runoff m

181 Discussion 
• Continue to develop Paleo methods and expand the use of Paleo analysis into 

the Eastern United States. 

• Dev

• Improv rameters. 

Standards Group 

elop site-specific Loss of Life (LOL) estimates. 

e Dam break pa

• Imp

o Mo g

o GIS – NEXRad – Metorologic Data 

• Develop Regional Database 

isture

o 

o 

o 

o 

o 

o 

o 

o 

o 

o 

o Non

o Init

eteorology Group

rove Technology Transfer

del development and trainin

o Storms

o Antecedent mo

Unit hydrographs 

Storm durations and patterns 

Instrumentation 

• Loss Rate Function Analysis 

o Curve Number 

Green-Ampt

STATSGO 

Initial and constant 

Variable 

• Watershed Modeling 

o Calibration, verification, accuracy 

Basin runoff parameters 

Model adjustment 

Instrumentation 

-linear basin response

ial reservoir levels 

M

• Precipitation Analysis 

• Antecedent storm analysis 

• Real time storm analysis

• Extend rainfall frequency analysis to return periods > 1000 yr 

• Analysis of the last 10 years storm data 

• Analysis of older storms

• Application manual for orographic areas 

• Standardize storm development 

3. Ranking of Research Needs - After the workshop, all participants were sent a ballot 

and asked to rank the above research subjects in terms of importance and difficulty. Of 

the 26 workshop participants, 16 replied with their rankings. The subjects were to be 

rated 1 to 8 in terms of importance with one being the most important. No subject could 

have the same rating. Each subject was given a difficulty rating of 1 to 10 with 1 being 

the easiest. More than one subject could receive the same difficulty rating. Also, each 

Discussion 182
participant was asked to pick what t e the 10 most important subjects 

overall. These were again to be ranked 1 to 10 with 1 being the most important. Each of 

Once the rankings were received, they were tabulated based on their ranked importance 

and t

very di

not qui as easier to study. The weighted rankings are presented in the 

graphs belo

Risk Analy

hey considered to b

the 10 was also given a difficulty rating that ranged from 1 to 10 with 10 being the 

hardest. Only 15 of the 16 responses were usable for the overall ranking. 

then weighted by their difficulty. For example, a subject that was very important bu

fficult to study may actually end up with a lower ranking than a subject that was 

te as important but w

w.

sis

T ers of the Risk Analysis group rated the ‘d

o

he memb evelopment of a historical database

f s

cur

rated as bject to a ranking of

3rd. 

Standards

torms and floods’ as most important, as shown in Figure 1. Extension of ‘frequency 

ves to rare AEP’ was ranked as the 2nd most important subject. However, it also was 

one of the more difficult subjects. Weighting pushed this su

he members of the Standards group ranked ‘technology transfer’ as the most important 

sub

databas nked the highest from the overall workgroup. 

Weigh b al database development’ ends up being the highest 

ranked subject. After weighting, ‘technology transfer’ ends up as the fourth ranked 

subject. These results are shown in Figure 2. 

T

ject. However, as shown in the Standards graph below, the ‘development of regional 

e of storm and flood information’ ra

ted y difficulty, the’ region

Meteorology

Figure 3 shows that the members of the Meteorology Group ranked ‘rainfall frequency 

analysis to return periods greater than the .001 event’ as the most important subject. 

However, when weighted by difficulty, this subject dropped to 5th. ‘Precipitation 

analysis’ was the 2nd rated in terms of importance and ends up the top rated when 

weighted by difficulty. 

Overall Top Ten

The overall top ten rankings are shown on Figure 4. For the top ten ranking, ‘historical

database of storms and floods’ was ranked as the most important subject. After we

by difficulty, it remained as the top rated subject. Of the 1

ighting

5 usable ratings, 10 participants

cluded this subject within their top ten. 

Conclusion

in

It is important to remember that these rankings are the opinions of 16 people. It is 

possibl t said, the

intent o h anizations which have vested interest 

in the f d ld provide a good cross-section

of opin n for itemizing research 

subjects. 

e that 16 other people could generate a different set of rankings. With tha

f t e workshop was to invite people from org

iel of dam safety. It was felt that these people wou

io s on dam safety issues and therefore, a good basis

183 Discussion 
An overview of the results from the three groups shows a common thread among 

all the g u nion of each group is that

there is n ase of information on both floods and 

storms that can be shared among all. The Risk Analysis and Standards groups both listed 

stor the most important research item. The Meteorology group listed 

precipi o t ranked subject.

To ppears that the next step would be to

come u of rainfall and flow-frequency curves to extreme 

frequencies along with developing an antecedent soil moisture database.

ro ps that there is a need for basic data. The general opi

a eed to generate an organized datab

m and flood data as

tati n analysis as the second highes

further interpret the results, it generally a

p with methods for extrapolation

Discussion 184
Figure 2. Standards Graph 

Number Name Weighted Value

1 Historical Database of Storms and Floods 54.3

2 Extend Frequency Curves 28.6

Develop Regional Hydrology Parameters 35.8

Storm Series Analysis 26.7

Figure 1. Risk Analysis Graph 

5 Development of Stochastis Rainfall/Runoff Models 21.5

6 Develop Paleo models and extend to Eastern US 14.8

7 Site-Specific Loss of Life estimates 12.6

8 Dam Break Parameters 20.0

3

4

SUBJECT

0

1 2 3 4 5 6 7 8

Subject Number Importance

RISK ANALYSIS

100

20

40

60

Relative Comparison

80

Difficulty

Weighted

Number Name Weighted Value

1 Improve Model Development and Training 40.4

2 Improve Technology Transfer of GIS, NEXRad and Meteorological Data 28.0

3 Regional Database of Storm Amounts, Durations and Patterns 45.2

4 Regional database of Unit Hydrographs 31.9

5 Regional Database of Antecedant Moisture 23.8

SUBJECT

0

1 2 345 6 7 8

Subject Number Importance

Difficulty

Weighted

6 Loss Rate Function Analysis 20.6

7 Basin Model Parameters for Watershed Development 25.3

8 Watershed Model Calibration 23.6

STANDARDS

20

40

Relative Comparison80

100

60

185 Discussion 
Figure 4. Overall Top Ten Graph 

Figure 3. Meteorology Graph 

Number Name Weighted Value

1 Precipitation Analysis 43.9

2 Antecedent Storm Analysis 29.5

3 Real Time Storm Analysis 19.6

4 Rainfall Frequency Analysis to Return Periods > 1000 yr 26.0

5 Analysis of last 10 Yrs Data 31.6

6 Analysis of Older Storms 30.1

7 Manual for Orographic Areas 25.1

8 Standardize Storm Development 23.2

SUBJECT

METEOROLOGY

0

20

123 4 5 678

Subject Number

Relative Comparison 60

80

100

40

Importance

Difficulty

Weighted

Weighted Group

Number Name Value

1 Historical Database of Storms and Floods 35.8 Risk

2 Extend Frequencies 20.6 Risk

3 Develop Regional Hydrology Parameters 12.4 Risk

4 Develop Paleo models and extend to Eastern US 11.4 Risk

5 Dam Break Parameters 13.6 Risk

6 Precipitation Analysis 12.4 Met

7 Rainfall Frequency Analysis to Return Periods > 1000 yr 21.1 Met

8 Improve Model Development and Training 19.8 Standards

9 Improve Technology Transfer of GIS, NEXRad and Meteorological Data 11.5 Standards

10 Regional Database of Storm Amounts, Durations and Patterns 24.8 Standards

SUBJECT

TOP TEN 

0

20

40

60

80

100

1 2 345 6 7 8 9 10

Subject

Relative Comparison

Importance

Difficulty

Weighted

Discussion 186
WORKSHOP 

ON 

HYDROLGIC RESEARCH NEEDS 

For 

DAM SAFETY 

WORKSHOP PARTICIPANTS 

November 14-15, 2001 

DAVID BOWLES 

Utah Water Research Laboratory 

Utah State University 

8200 University Blvd. 

Logan, Utah 84322-8200 

University Fax: 435-797-4010/3663 

Home Office: 435-753-6004 

david.bowles@usu.edu

CAT CECILIO 

PG&E, Retired 

2009 Carignan Way 

San Jose, CA 95135-1248

408-528-9909 

cat@cecilio-consulting.com

JAMES CHIEH 

Corps of Engineers, Los Angeles District 

911 Wilshire Blvd 

Los Angeles, CA 90017 

213-452-3571 

shih.h.chieh@spl01.usace.army.mil

ROBERT COLLINS 

District Hydrologist 

Water Management Section 

Corps of Engineers, Sacramento District 

1325 J Street 

Sacramento, CA 95814 

916-557-7132 

robert.f.collins@usace.army.mil

MELISSA COLLORD 

State of California 

Division of Safety of Dams

2200 X Street, Suite 200 

Sacramento, CA 95818 

916-323-5495 

melissac@water.ca.gov

MICHAEL DAVIS 

Federal Energy Regulatory Commission 

230 S. Dearborn Street, Suite 3130 

Chicago, Il 60604 

312-353-3787 

michael.davis@ferc.fed.us

JOE DeVRIES 

David Ford Consulting Engineers 

2015 J Street Suite 200 

Sacramento, CA 95818 

916-447-8779 

jjdevries@ford-consulting.com

EARL EIKER 

Corps of Engineers, Retired 

2651 Legends Way 

Ellicott City, MD 21042 

410-465-2120 

eeeiker1@aol.com

Discussion 188
ARLEN FELDMAN 

Chief, Hydrology and Hydraulics 

Technology Division 

Hydrologic Engineering Center 

609 Second Street 

Davis, CA 95616 

530-756-1104 

arlen.d.feldman@usace.army.mil

ED FIEGLE 

Georgia Safe Dams Program

4244 International Parkway Ste 110 

Atlanta, GA 30354 

404-362-2678 

ed_fiegle@mail.dnr.state.ga.us

DES HARTFORD 

BC Hydro 

6911 Southpoint Drive 

Burnaby, BC, Canada 

V3N 4XB 

604-528-2423 

des.hartford@bchydro.com

SAM HUI 

Bechtel Corporation 

328 Trestle Glen Court 

Walnut Creek, CA 94598 

415-768-7734 

shui@bechtel.com

BOB JARRETT 

US Geological Survey 

Box 25046, MS 412 

Denver, CO 80225 

303-236-6447 

rjarrett@usgs.gov

MATT LINDON 

Utah Dept of Natural Resources 

1594 W North Temple Street Suite 220 

PO Box 146300 

Salt Lake City, Utah 84114-6300 

801-538-7372 

mattlindon@utah.gov

GREG LOWE 

Tennessee Valley Authority 

400 West Summit Hill Dr., WT10B 

Knoxville, TN 37902 

865-632-6857 

gwlowe@tva.gov

JEFF McCLENATHAN 

Corps of Engineers, Omaha District 

215 N 17th Street 

Omaha, NE 68102 

402-221-4578 

jeffrey.t.mcclenathan@nwo02.usace.arm

y.mil

RORY NATHAN 

Sinclair Knight Merz 

PO Box 2500 

Malvern VIC AUS 3144 

+61 3 9248 3322 

rnathan@skm.com.au

RAKASH 

URS Corporation 

One Continental Towers

1701 Golf Road, Suite 1000 

Rolling Meadows, IL 60008 

847-228-0707 x234 

anand_prakash@urscorp.com

ANAND P

189 Workshop Participants 

MEL SCHAEFER 

MGS Engineering Consultants, Inc. 

7326 Boston Harbor Road NE 

Olympia, WA 98506 

360-570-3450 

mgschaefer@thurstom.com

LOU SCHREINER 

Bureau of Reclamation 

Lou Schreiner, D-8530 

PO Box 25007 

Denver, CO 80225-0007 

303-445-2546 

lschreiner@do.usbr.gov
JOE SKUPIEN 

Principal Hydraulic Engineer 

Somerset County, New

PO Box 3000 

Somerville, NJ 08876 

Phone 

skupien@co.somerset.nj.us

Jersey

ROBERT SWAIN 

Bureau of Reclamation 

0

7

Robert Swain, D-853

PO Box 25007 

Denver, CO 80225-000

303-445-2547 

rswain@do.usbr.gov

ED TOMLINSON 

Applied Weather Associates

O Box 680

32

P

Monument, CO 801

719-488-9117 

awa@divide.net

MING TSENG

eers

41 G Street NW

314-1000

sace.army.mil

Headquarters, Corps of Engin

4

Washington DC 20

202-761-7177 

ming.tseng@hq02.u

rict

ater Resources Engineering Branch 

01-2070 

JERRY WEBB

Corps of Engineers, Huntington Dist

W

502 Eighth Street 

Huntington, WV 257

304-529-5606 

jerry.w.webb@lrh01.usace.army.mil

GENE ZEIZEL 

Engineering Sciences Division

gency

00 C Street S.W.

ashington, D.C. 20472

02-646-2802

ene.zeizel@fema.gov

Federal Emergency Management

A

5

W

2

g

Workshop Participants 190