Regulatory Guide 1.59

Design Basis Floods for Nuclear Power Plants

ML13350A359
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Issue date:	08/31/1973
From:	US Atomic Energy Commission (AEC)
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August

1973 at.

August 1973 U.S. ATOMIC ENERGY COMMISSION

REGULATORY GUIDE

DIRE"W"TORATE OF REGULATORY STANDARDS

REGULATORY GUIDE 1.59 DESIGN BASIS FLOODS FOR NUCLEAR POWER PLANTS

A. INTRODUCTION

General Design Criterion 2. "-Design Bases for Protection Against Natural Phenomentia." of Appendix A

to 10 CFR Part 50. **General Design Criteria for Nuclear Power Plants." requires. in part. that structures. systems.

and components important to safety be designed to withstand the effects of natural phenomena such as floods, tsunami. and seiches without loss of capability to perform their safety functions. Criterion 2 also requires that the design bases for these structures, systems. and components reflect: (I) appropriate consideration of the most severe of tihe natural phenomena that have been historically reported for the site and surrounding region.

with sufficient margin for the limited accuracy and quantity of the historical data and the period of time ill which the data have been accumulated. (2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural plhenonlena.

and (3) the importance of the safety functions to be performed.

Paragraph 100.10 (c) of 10 CFR Part 100,"Reactor Site Criteria," requires that physical characteristics of the site, including seismology. meteorology, geology.

and hydrology, be taken into account in determining the acceptability of a site for a nuclear power reactor.

Appendix A. "Seismic arid Geologic Siting Criteria for Nuclear Power Plants." was published in the Federal Register on November 25, 1971 (36 FR 22601) as a proposed amendment to

10 CFR Part

100. The proposed appendix would specify investigations required for a detailed study of seismically induced floods and water waves. Proposed Appendix A to 10 CFR Part 100

would also require that (lie determination of design bases for seismically induced floods and water waves be based on the results of the required geologic and seismic investigations and that these design bases be taken into account in the design of tile nuclear power plant.

TlThis guide describes a1n acceplahl'

ntl lhod (it determinirng fOr siles aloi*g strealis tit riveis ilie design basis floods that nuclear power plants maust lie designed to withstand without loss of saltety-related functions. It further discusses tlie phenomlena producing colmpar*able design basis floods for coastal. estuary; and Gieat Lakes sites. It does not discuss the design requirements for flood protection. The Advisory Committee on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.

B. DISCUSSION

Nuclear poower plants must be designed itf prevent the loss of safety-relat ed functions resulltig front the most severe flood conditions thai call reasonably be predicted to occur at a site as a result of sevele hydrometenrological conditions, seismic activity.

or both.

The Corps of Engineers for many years has studied conditions arid circumstances relating to floods and flood control. As a result of these studies, it has developed a definition for a probable niaxinmui

'lood (PM F)'

and attendant analytical techniques for estimating with an acceptable degree oft conservattsm flood levels on streatis or rivers resulting fromi hydromLeteorological conditions.

For estimating seismtiically induced flood levels. an acceptable degree of

'Corps ot tEngincecr Pribahltc Ma',intsni ItIodt definlililn appears in many publication, of thait :g00ncy sch 1is IEngineering Circular EC-I 110-2-27, Change I.

'T"ngincering

snd Design -Policies and Procedures Perlaining 10 t)eerminaition of Spillway Capalities and Frecboard Allowances fir t)jn<,. dated

19 Feb. 1968. Ttie probahble niamimuni fhlood is atso direclly analogous to ftte Corps (if 1'ngineers "Spillway Design Itlod" as used for darns whose failures would result in a significant toss of lire and property.

USAEC REGULATORY GUIDES

Copies of published guides may be obtained by request indicating the divietoat desired to the US. Atomic Energy Commrstiori, Washington. D.C. 20545, Regulatory Guides e issued to describe and make available to the public Attention: Director of Regulatory Standards. Comments and stuggetions fot methods aeceptsble to the AEC Regulatory staff of implementing specific parts of Irtroovements In these guides are encouraged and should be sent to the Secrets'y the Commission's regulations. to delineate techniques used by the stafl in of the Commission, U.S. Atomic Energy Commission. Washington, D.C. 20545.

evaluating ecilfic problems or posttulatd accidents, or to provide guidane to Attention: Chief, Public ProctedingtStlff.

eaplicants. RegAnftory Guides are not substitutes for regulationt and compliance with thern is not required. Methods and solutions different from those set out in The guides are issued In the following ten broad divisions:

the guides will be acceptable if they provide a basis for the findings requisite to the itauence or continuance of a permit or license by the Commitsion.

2. Research and Test Reactors

6. Tranportation

3. Fuels ard Materials racilitien

8. Occupational Health Published guides will be revised periodically, as appropriate, to accommodate

4. Environmentall and Siting

9. Antitrust Review comments end to reflect new information or experlence.

5. Materialt and Plant Protection

1

0. General

conservatism for evaluating the effects of lte initiating event is provided by the proposed Appendix A to 10 CFR Part 100.

The *onditions resulting I'rom the worst site-related flood precHble at the nuclear power plant (e.g.. PMF,

seismically induced flood, seiche. surge. severe local precipitation)

with attendant wind-generatcd wave activily constitute the design basis flood conditions that safety-related structures.

systems. and components identified in Regulatory Guide 1.292 must he designed ito withstand and remain functional.

For sites along streams or rivers, a hypothetical probable maximum iflood of the severity defined by the Corps of Engineers generally provides the design basis flood. Ior sites alone lakes or seashores, a flood Condition of cotinparahle severity could be produced by the most severe combination of hydrometeorological parameters reasonably possible, such as may be protduced by a probable maxinmum hurricane" . or by a probable matximum seiche. On estuaries. a probable inaxinitun rivet c

lood. a probable maximum surge. a probable tuaximnuni seiche. or a reasonable combination of less severe phenomenologically caused flooding events should all he considered in arriving at design basis flood conditions comparable in frequency of occurrence with a probable ;naximum flood on streams and rivers.

Ini addition to floods produced by severe Ih y d rometeorological conditions.

Ihe most severe seismically induced floods reasonably possible should be considered for each site. Along streams. rivers, and estuaries, seisinically induced floods may be produced by dam failures or landslides. Along lakeshores, coastlines, and estuaries.

seismically induced or tst, namit-ype flooding should be considered.

Consideration of seismically induced floods should include the same range of seismic events as is postulated

2 Regulatory Guide 1L29 (Safety Guide 29), "Seismic Design Classification," identifies waler.cooled nuclear power plant structures. system,. and components that should be designed to withstand the effects of the Safe Shutdown Earthquake and remain funetionalt These structures. systems. and components are those necessary to assure (I) the integrity of the reactor coolant pressure boundary, (2) the capability to shut down the reactor and maintain it in a ,.afe shutdown condition, or (3) the capability to prevent or mitigate the consequences of accidents which could result in potential offsite exposures comparable to the guideline exposures, of I1t CFR Part tI0O. These same structure%, systems, and components should also be designed to withstand conditions resulting from the design basis flood and remain functional.

If is expected that safety-related structures, systemns. and components of other types of nuclear power plants will be identified in future Regulatory guides. In the interim. Regulatory Guide 1.29 should be used as guidance when identifying rafety-related structures, systems, and components of other types of nuclear power plants.

'See Corps of Engineers Coastal Engineering Research Center "Technical Report No. 4, Shore Protection, Planning and Design." third edition. 1966.

for the design of the nuclear plant. For instance, the analysis of floods caused by darn failures, landslides, or tsunami requires consideration of seismic events of the severity of the Safe Shutdown Earthquake occurring at the location that would produce the worst such flood at the nuclear power plant site. In the case of seismically induced floods along rivers, lakes, and estuaries which may be produced by events less severe than a Safe Shutdown Earthquake, consideration should be given to the coincident occurrence of floods due to severe hydrometeorological conditions, but only where the effects on the plant are worse, and the probability of such combined events may be greater, than the effects on the plant of an individual occurrence of the most severe event of either type. For example. a seismically induced flood produced by an earthquake of approximately one-hal f the Safe Shutdown severity coincident with a runoff-type flood produced by tihe worst regional storm of record may be considered to have approximately the same severity as an earthquake of Safe Shutdown severity coincident with about a

25-year flood. For the specific case of seismically induced floods due it) dam failures, an evaluat ion should be made of flood wave! which may be caused by domino-type darn failures triggered by a seismically induced failure of a critically located dam and of flood waves which may be caused by multiple darn failur':s in a region where dams may be located close enough together that a single seismic event can cause multiple failutes.

Each of the severe flood types discussed above should represent the upper limit of all phenomenologically caused flood potential combi- nations considered reasonably possible, and analytical techniques are available and should generally be used for their prediction for individual sites. Those techniques applicable to PMF

and seismically induced flood estimates on streams and rivers are presented in Appendix A to this guide. Similar apperdices for coastal, estuary. and Great Lakes sites, reflecting comparable levels of risk. will be issued as they become available.

Analyses of only the most severe flood conditions may not indicate potential threats to safety-related systems that might result from combinations of flood conditions thought to be less severe. Therefore.

reasonable combinations of less-severe flood conditions should also be considered to the extent needed for a consistent level of conservatism. Such combinations should be evaluated in cases where the probability of their existing at the same time and having significant consequences is at least comparable to that associated with the most severe hydrometeorological or seismically induced flood. For example, a failure of relatively high levees adjacent to a plant could occur during floods less severe than the worst site-related flood, but would produce conditions more severe than would result during a greater flood (where a levee failure elsewhere would produce less severe conditions a[ the plant site).

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Wind-generated wave activity may produce severe flood-induced static and dynamic conditions either independent of or coincident with severe hydromelcorological or scisnmic flood-producing mechanisms. For example, along a lake. reservoir. river, or seashore, reasonably severe wave action should he considered coincident with the probable maximum water level conditions.4 The coincidence of wave activily with probable maximum water level conditions should take into account the fact that sufficient time can elapse between the occurrence of the assumed meteorological mechanism and the maximum water level to allow subsequent meteorological activity to produce substantial wind-generated waves coincident with the high water level produced by the initial event. In addition, the most severe wave activity at the site that can be generated by distant hydrometeorological activity should be considered. For instance, coastal locations may be subjected to severe wave action caused by a distant storm that, although not as severe as a local storm (e.g., a probable maximum hurricane), may produce more severe wave action because of a very long wave-generating fetch. The most severe wave activity at tile site that may be generated by conditions at a distance from the site should be considered in such cases. In addition, assurance should be provided that safety systems necessary for cold shutdown and maintenance thereof are designed to withstand the static and dynamic effects resulting from frequent flood levels coincident with the waves that would be produced by the maximum gradient wind for the site (based on a study of historical regional meteorology).

C. REGULATORY POSITION

I. The conditions resulting from the worst site-related flood probable at a nuclear power plant (e.g., PNIF.

seismically induced flood, hurricane. seiche, surge. heavy local precipitation) with attendant wind-generated wave activity constitute the design basis flood conditions that safety-related structures. systems, and compor.Ents identified in Regulatory Guide 1.292 must be designed to withstand and remain functional.

a.

On streams and rivers, the Corps of Engineers definition of a probable maximum flood (PMF) with attendant analytical techniques (summarized in Appendix A of this guide) provides an acceptable level of conservatism for estimating flood levels caused by severe hydrometeorological conditions.

4 Probable Maximum Water Level Is deflined by the Corps of Engineers as "the maximum still water level (i.e.. exclusive of local coincident wave runup) which can be produced by the most severe combination or hydrometeorological and/or seismic parameters reasonably possible for a particular location. Such phenomena are hurricanes, moving squall lines, other cyclonic meteorological events. tsunami, etc., which, when combined with the physical response of a body of water and severe ambient hydrological conditions, would produce a still water level that has virtually no risk of being exceeded."

(Sec Appendix A to this guide)

b.

Along lakeshores. coastlines, and estuaries.

eslimales of flood levels resulting frorn severe surges.

seiches. and wave action caused by hydronteteorological activity should he based on criteria cOl uparahle in conservatism to those used for probable maximum Ihoods. Criteria and analytical techniques providing this level of conservatism for the analysis of these events will he summai'zed in subsequent appendices to ilbis guide.

c.

Flood Aronditions Ihat could be caused by earthquakes of the severity used in thie design of the nuclear facility should also be considered in establishing the design hasis flood. A simplified analytical technique for evaluating the hydrologic effects of seismically induced dam failures disctrssed herein is presented in Appendix A of this guide. Techniques for evaluating the effects of tsunami will be presented in future appendices.

d.

In addition to the analyses of the most severe floods I hat may be induced by either hydrometeorological or seismic mechanisms. reasonable combinations of less-severe flood conditions should also be considered to the extent needed for a consistent level of conservatism, Such combinations should be evaluated in cases where the probability of their existing at the same time and having significant consequenceL is at least comparable to that associated with the most severe hydrometeorological or seismically induced flood.

e.

To the water levels associated with the worst site-related flood possible (as determined from paragraphs a.. b.. c.. or d. above) should be added the effects of coincident wind-generated wave activity to generally define the upper limit of flood potential. An acceptable analytical basis for wind-generated wave activity coincident with probable maximum water levels is the assumption of a 40-mph overland wind from the most critical wind-wave-producing direction, unless historical windstorm data can be used to substantiate that such an event (i.e., wind direction and/or speed) is more extreme than has occurred regionally. However. if the mechanism producing the maximum water level.

such as a hurricane, would itself produce higher waves, then these higher waves should be used as the design basis.

2.

As an alternative to designing

"hardened"

protection- for all safety-related structures. systems. and components as specified in regulatory position I . above, it is permissible to not provide hardened protection for some of these features if:

a.

Sufficient warning time is shown to be available to shut the plant down and implement adequate emergency procedures"

b.

All safety-related structures.

systems. and components identified in Regulatory Guide 1.29) are I tardened portection means structural provisions incorporated in the plant design that will protect %afcty-related structures, systems, and components from the static and dynamic effects of floods. Examples of the types of flood protection to be provided for nuclear power plants will le the subject of a separate regulatory guide.

1.59-3

designed to withstand the flood conditions resulting from a severe slorm such as tie worst regional storm of record"' with attendant wind-generated wave activity Ihl1 mw. lie produced by the worst winds of record and reiain functional:

c.

In addition to the analyses required by paragraph

2.b.

above, reasonable combinations of For sites along streams and rivers thik event is characterized by the Corps of. Engineer! definition of a Standard Projcct Flood. Such floods have been found to produce tlow rates generally 40 wo fill percenrtl tihte P.SIF. For sites along seahorc, this event m)*" le ch;taracterized b% the Corp, oi t" :ineinctrs defiNition of j Standard Projecl Ilurricane. For other 'ijC

a comparable level olf risk should le assumed.

less-severe flood conditions are also considered to the extent needed for the consistent level of conservatism:

and d.

In addition it) paragraph 2.b. above, at least those structutres, systems, and components necessary for coldl shutdown and maintenance thereof are designed with "hardened" protective fealtures to withstand tlie entire range of flo0d conditions up to and including the worst site-related flood probable (e.g., PM F. seismically induced flood. hutricane, surge, seiclhe, heavy local iercipitalion)

with coincident wind-generated wave act ion a s discussed in regulatory positiotn I. above and remain funictiolnal.

i

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a

0

APPENDIX A

TABLE OF CONTENTS

A.I

A.2 A.3 A.4 A.5 A.6 A.?

AS8 A.9 A.10

A.1 I

Introduction

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

Probable Maxinmum Flood (PMF) ..........

Hydrologic Characieristics ................

Hlood Hydrograph Analyses

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

Precipitation Losses and Base Flow .........

Runoff M odel .........................

Probable Maximum Precipitation Estimates

..

Channel and Reservoir Routing ............

PNI F llydrograph Estimates ...............

Seismically Induced Floods ..............

Water Level Detei minations

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

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

.5(1.5

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

I .q

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

.5' .6 I..,. I...................

1.59*.7

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

1.59-7

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

59 -8

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

1.5 -

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

1.59-1 I

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

1.5 .i 1 2

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

1.59 -12

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

1.59-)13 A.1 2 Coincident Wind-Wave Activity .................................

1.59-13 References .......................................

........

1.59-15 PROBAELE MAXIMUM AND SEISMICALLY INDUCED FLOODS

ON STREAMS AND RIVERS

A.1 INTRODUCTION

This appendix has been prepared to provide guidance for flood analyses required in support of applications for licenses for nuclear power plants to be located on streams and rivers. Because of the depth and diversity of presently available techniques. this appendix summarizes acceptable methods for estimating probable maximum precipitation, for developing rainfall-runoff models, for analyzing seismically induced dam failures.

and for estimating the resulting water levels.

The probable maximum flood may be thought of as one generated by precipitation, and a seismically induced flood as one caused by dam failure. For.many sites, however, these two types do not constitute the worst potential flood danger to the safety of the nuclear power plant.

Analyses of other flood types (e.g.,

tsunami, seiches, surges) will be discussed in subsequent appendices.

The probable maximum flood (PMF) on streams and rivers is compared with the upper limit of flood potential that may be caused by other phenomena to develop a basis for the design of safety-related structures and systems required to initiate and maintain safe shu.tdown of a nuclear pow'er plant. This appendix.

outlines the nature and scope of detailed hydrologic engineering activities involved in determining estimates for the PMF and for seismically induced floods resulting from dam failures, and describes the situations for which less extensive analyses are acceptable.

Estimation of a probable maximum flood (PMF)

requires the determination of the hydrologic response (losses, base flow, routing, and runoff model) of watersheds to intense rainfall, verification based on historical storm and runoff data (fhood hydrograph analysis).

the most severe precipitation reasonably possible (probable maximurn precipitation-.lPI

riinimum losses. tnaximum base flow. channel and reservoir routing, the adequacy of existing and propetsed river control structures to safely pass a PMF. water level determinations, and the superposition of potential wind-generated wave activity. Seismically induced Ihoods such as may be produced by dam failures or landslides.

may be analytically evaluated using many PMF

estimating components (e.g.. routing techniques. water level determinations) after conservative assumptions of flood wave initiation (such as dam failures) have been made. Each potential flood component requires an in-depth analysis. and the basic data and results should be evaluated to assure that the PMF estimate is conservative.

In addition. the flood potential from seismically induced causes must be compared with the PMF to provideappropriate flood design bases. but the seismically induced flood potential may be evaluated by simplified methods when conservatively determined results provide acceptable design bases.

Three exceptions to use of the above-descrihed analyses are considered acceptable as follows:

a.

No flood analysis is required for nuclear power plant sites where it is obvious that a PMF or sismically induced flooding has no bearing. Examples of such sites are coastal locations (where it is obvious that surges.

wave action, or tsunami would produce controlling water levels and flood conditions) and hilltop or "dry"

sites.

b.

Where PNIF or seismically induced flood estimates of a quality comparable to that indicated herein exist for locations near the site of the nuclear power planw, they may be extrapolated directly to the site, if such extrapolations do not introduce potential

1.59-5

errors of more than about a foot in PMF water level estimates.

c.

It is recognized that an in-depth PNF estimate may not le warranted because of the inherent capability of lihe design of some nuclear power plants to function sofely with little or no special provisions or because the time and costs of making such an estinate ate not coninmensurate with the cost of providing protection. In such cases, other nieans of estimating design basis flnois are acceptable if it can he demonstrated that the technique utiliied or the estimate itself' is conservative.

Similarly. conservative estimates of seisinically induced flood potenti:al may provide adequate denmonstration of nuclear power plant safety.

A.2. PROBABLE MAXIMUM FLOOD (PMF)

Probable maxir'inn Ilood sttid:,-

should be coiripatible with the specific definitions and criteria summnnarized as follows:

a.

The Corp; of Engineers defines the PMF as "the hyp.,thetical I1(x)d characteristics (peak discharge.

Volmnc. arid hydroge?

ih shape) that are considered to he the most severe reasonrabl\\

possible at a particular location.

haised on relatIively comprehensive hvdr ometeoro logic:' I analysis o f critical rt niill-producing precip tation (and snowmell.

if pertinent)

and hydroltgic factors favorable for ima*inuirm fltiod ruinoff." Detailed PM F determinations are usuially prepared by estimating the areal distribution of *'prohbahe maximurn" precipitation (PNIP) over flie subject drainage basin in critical periods of time. and computing the residual runoff hydrograph likely to result with critical coincident conditions of ground wetness and related factors. PMF estimates are usually based un the observed and deduced characteristics of hi St ori:al flood-producing storms anid associated hy d ro log ic factors modified on the basis of hydronietecorological analyses to represent the most severe runoff conditions considered to be "reasonably possible" in the particular drainage basin under study. In addition to determining the PMF for adjacent large rivers and strearims. a local PMF should be estimated for each local drainae coUrSe that can influence safety-related facilities, including lie roofs of safety-related buildings.

to assure that local intense precipitation cannot constitule a threat to tile safety of tlie nuclear power plant.

b.

Probable maxinium precipitation is defined by tile Corps of Engineers and the National Oceanic and Atnmospheric Administrat ion (NOAA) as "thie t liheret ically greatest depth of precipitation for a given duration that is nieleorologically possible over the applicable drainage area that would produce flood flows of which there is virtually no risk of being exceeded. These estimates usually involve detailed analyses of historical flood-producing storms in the general region of the drainage basin under study. arid certain nmodificalions and extrapolations of historical data and reflect more severe rainfall-runoff relations than actually recorded.

insofar as these are deemed reasonably possible of occurrence on the basis of hydrometeorological reasoning." The PMP should represent the depth, time, and space distribution of precipitation that approaches tile upper limit of what the atmosphere and regional topography can i Iroduce.

The critical PMP

meteorological conditions are based on an analysis of air-mass properties (e.g., effective precipitable water, depth of inflow layer, temperatures, winds), synoptic situations prevailing during recorded storms in tile region, topographical features, season of occurrence, and location oh the respective areas involved. The values thus derived are designated as the PMP, since they are deterinited wit thin I lie limitations of current meteorological theory and available data and are based on the most effective combinalion of critical factors con Iollinrg.

A.3 HYDROLOGIC CHARACTERISTICS

Hydrologic characteristics of the watershed and sireani channels relative to the plant site should be duierniniied fromt the Iollowing:

a.

A topographic map of the drainage basin showing watershed boundaries for the entire basin and principal tributaries and other subbasins that are pertinent. The mnap should include ;

location of principal stream gaging stations and other hydrologically related record collection stations (e.g., streamflow, precipitation) and the locations of existing and proposed reseroirs.

b.

The drainage areas in each of the pertinent watersheds or subbasins above gaging stations, reservoirs, any river control structures, and any unusual terrain features that could affect flood runoff. All major reservoirs and channel improvements that will have a major influence on streamfnow during flood periods should be considered. In addition, the age of existing structures and information concerning proposed projects affecting runoff characteristics or streamflow is needed to adjust streamflow records to "pre-project(s)"

and

"with project(s)" conditions as follows:

(1) The term "pre-project(s) conditions" refers to all characteristics of watershed features and developments that affect runoff characteristics. Existing conditions are assumed to exist in the fiture if projects are to be operated in a similar manner during the life of the proposed nuclear power plant and watershed runoff characteristics are not expected to change due to development.

(2) The term "with project(s)" refers to the future effects of projects being analyzed, assuming they will exist in the future and operate as specified. If existing projects were not operational during historical floods and may be expected to be effective during the lifetime of the nuce.r, power plant. their effects on historical floods should be determined as part of the analyses out lined in Sections A.5. A.6. and A.8.

c.

Surface and subsurface characteristics that affecl runoff and streamiflow to a major degree, (e.g..

1.59-6

large swamp areas, noncontributing drainage areas, groundwater flow, and other watershed features of an unusual nature to the extent needed to explain unusual characteristics of streamflow).

d.

Topographic features of the watershed and hi-!orical flood profiles or high water marks. particularly in the vicinity of the nuclear power plant.

e.

Stream channel distances hetween river control structures, major tributaries, and the plant site.

f.

Data on major storms and resulting floods of record in the drainage basin. Primary at tcntion should be given to those events having a major bearing on hydrologic computations. It is usually necessary to analyze a few major floods of record in order to develop such things as unit hydrograph relations, infiltration indices, base flow relationships, information on flood routing relationships, and flood profiles. lxcept in unusual cases, climatological data available from the Department of Commerce. The U.S. Army Corps of Engineers.

National Oceanic and Atmospheric Administration and other public sources are adequate to meet the data requirements for storm precipitation histories. The data should include:

(I) Hydrographs of major historical floods for pertinent locations in the basin, where available, from the U.S. Geological Survey or other sources.

(2) St o rmi precipitation records, depth-area-duration data, and any available isohyetal maps for the most severe local historical storms or floods that will be used to estimate basin hydrological characteristics.

A.4 FLOOD HYDROGRAPH ANALYSES

Flood hydrograph analyses and related computations should be used to derive and verify the fundamental hydrologic factors of precipitation losses (see Section A.5) and the runoff model (see Section A.6). The analyses of observed flood hydrographs' of streamflow and related storm precipitation (Ref. I) use basic data and information referred to in Section A.3 above. The sizes and topographic freatures of the subbasin drainage areas upstream of the location of interest should be used to estimate runoff response for each individual hydrologically similar subbasin utilized in the total basin runoff model.

Subbasin runof'

response characteristics are estimated from historical storm precipitation and streamflow records where suchi are available, and by synthetic means where no streamflow records are available. The analysis of flood hydrographs (Ref. 2) should include the following:

a.

Estimates of the intensity, depth, and areal distribution of precipitation causing the runoff for each historical storm (and rate of snowmelt. where this is significant). Time distributions of storm precipitation are generally based on recording rainfall gage

s. Total

'Strcamflow hydrographs (of major floods) are available in publications by the US. Geological Survey. National Weather Service, State agencies, and other public Sources.

precipitation measurements are usua~ly distributed, in time, using precipitation recorders. Areal distributions of precipitation. for each time increment, are generally based on a weighting procedure in which tihe incremental precipitation over a particular drainage area is computed as tile sum of tihe corresponding incremental precipitation for each precipitation gage where cacch value is separately weighted by the percL1ntage of the drainage area considered to be represented by the rain gage.

b.

The determination of base flow as the time distribution( of the difference between gross runoff arnd net runoff.

c. Computation of distributed (in time)

differences between precipitation and net direct runoff.

the difference being considered herein as initial and inflitrafion losses.

d.

The determination of the combined effect of drainage area. channel characteristics, and reservoirs on the runoff regimen, herein referred to as the "'runoff model." (Channel and reservoir effects are discussed separately in Section A.8.)

A.5 PRECIPITATION LOSSES AND BASE FLOW

Determination of the absorption capability of the basin should consider antecedent and initial conditions and infiltration during each storm considered.

Antecedent precipitation conditions affect precipitation losses and base flow. These assumptions should be verified by studies in the region or by detailed storm-runoff studies.

Tile fundamental hydrologic factors should be derived by analyzing observed hydrographs of streamflow and related stormis. A

thorough study is essential to determine basin characteristics and meteorological influences affecting runoff from a specific basin. Additional discussion and procedures for analyses are contained in various publications such as Reference

2. The following discussion briefly describes the considerations to be taken into account in determining the minimum losses applicable to the PMF:

a.

Experience indicates the capacity of a given soil and its cover to absorb rainfall applied continuously at an excessive rate may rapidly decrease until a fairly definite minimum rate of infiltration is rcached. usually within a period of a few hours. Infiltration relationships are defined as direct precipitation losses such that the accumulated difference between incremental precipitation and incremental infiltration equals the volume of net direct runoff. The infiltration loss relationships may include initial conditions directly, or may require separate determinations of initial losses. The order of decrease in infiltration capacity and the minimum rate attained are primarily dependent upon the vegetative or other cover, the size of soil pores within the zone of aeration, and the conditions alfecting the rate of removal f" capillary water from the zone of aeration.

The infiltration theory, with certain approximations, offers a practical means of estimating

1.59.7

the volume of surface runoll fronm intense rainlfall.

However. in applying tile method to natural drainage basins, tile following factors must be considered:

(I) Since the infiltration capacity of a given soil at the beginning of a storm is related to antecedent field moisture and the physical condition ofthe soil. the infiltration capacity for the same soil may vary appreciably from storm to storm.

(.2) The infiltration capacity of' a soil is normally highest at the beginning of rainfall, and since rainfall frequently begins at relatively moderate rates, a substantial period of time may elapse before the rainfall intensity exceeds the prevailing infiltralion capacily. It is gnerally accepted that a fairly definite quantity of waler loss is required to satisfv initial soil moislture deficiencies before nnoff will occur, the amount of initial loss depending upon antecedent conditions.

(3)

Rainfall does not normally cover the entire drainage basin during all periods of* precipitation with intensities exceeding infillration capacities. Futhermore.

soils and infiltration capacities vary throughout a

drainage basin. Therefore, a rational application of any loss.rate technique must consider varying rainfall intensities in various portions of the basin in order to de te rmine tile area covered by effective runolf-producing rainfall.

b.

Initial loss is defined as thie maximnum amount of precipitation that can occur without producing runoff. Initial loss values may range from a minimum value of a few tenths of an inch during relatively wet seasons to several inches during dry summer and fall months. Tile initial loss conditions conducive to major floods usually range from about 0.2 to 0.5 inch and are relatively small in comparison with the flood runoff volume. Consequently. in estimating loss rates from data for major floods, allowances for initial losses may be estimated approximately without introducing important errors in the results.

c.

Base flow is defined herein as that portion of a flood hydrograph which represents antecedent runoff condition and that portion of the storm precipitation which infiltrates the ground surface and moves either laterally toward stream channels, or which percolates into the ground, becomes groundwater, and is discharged into stream channels (sometimes referred to as bank flow). The storm precipitation, reduced by surface losses, is then resolved into the two runoff components:

direct runoff and base flow. Many techniques exist for estimating thie base flow component. It is generally assumed that base flow conditions which could exist during a PMF are conservatively high. the rationale being that a storm producing relatively high runoff could meteorologically occur over most watersheds about a week earlier than that capable of producing a PMF. One assumption sometimes made for relatively large basins is that a flood about half as severe as a PMF can occur three to five days earlier. Another method for evaluating base flow relates historical floods to their corresponding base flow. The base flow analyies of historical floods.

there" fore, may he readily utilized in PMF

determinations.

A.6 RUNOFF MODEL

The hydrologic response characteristics of the watershed to precipitation (such as unit hydrographs)

should be determined and verified from historical floods or by conservative synthetic procedures. The model should include consideration of nonlinear runoff response due to high rainfall intensities or unexplainable factors. In conjunction with data and analyses discussed above, a runoff model should be developed, where data are available, by analytically "reconstituting" historical floods to substantiate its use for estimating a PMF. The raiitfall-runofft lime-areal distribution of historical floods should be used to verify that tile "reconstituted"

hydrographs correspond reasonably well with flood hydrographs actually recorded at selected gaging stations kRef. 2). In most cases. reconstil ut ion studies should he made with respect to two or more floods and possibly at two or more key locations, particularly where possible errors in the determinations could have a serious impact on decisions required in the use of* the runoff model for the PMF. In some cases, the lack of sufficient time and areal precipitation definition, or unexplained causes.

have not allowed development of' reliable predictive runoff models, and a conservative PMF model should be assured by other means such as conservatively developed synthetic unit hydrographs. Basin runoff' models for a PMF determination should provide a conservative estimate of the runoff that could be expected during the life of the nuclear power plant. The basic analyses used in deriving thie runoff model are not rigorous, but may be conservatively undertaken by considering the rate of runoff from a unit rainfall (and snowmelt. if pertincnt)

of some unit duration and specific time-ae.ral distribution (called a unit hydrograph). The applicability of a unit hydrograph. or other technique, for use in computing the runoff from an e..'uiiated probable maximum rainfall over a basin may be partially verified by reproducing observed major flood hydrographs. An estimated unit hydrograph is first applied to estimated historical rainfall-excess values to obtain a hypothetical runoff hydrograph for comparison with the observed runoff hydrograph (exclusive of base flow-net ninoff),

and the loss rate, the unit hydrograph. or both. are subsequently adjusted to provide accurate verification. A

study of the runoff response of a large number of basins for several historical floods in which a variety of valley storage characteristics, basin configurations, topographical features, and meteorological conditions are represented provides the basis for estimating the relative effects of predominating influenm-i for use in PMF analyses. In detailed hydrological studies, each of the following procedures may be used to advantage:

a.

Analysis of rainfall-runoff records for major storms;

b.

Computation of synthetic runoff response models by (I) direct analogy with basins of similar characteristics and/or (2) indirect analogy with a large number of other basins through the application of empirical relationships. In basins for which historical streamflow and/or storm data are unavailable, synthetic i .59.9

4 techniques are the only known means for estimating hydrologic response characteristics. However, care must be taken ito assure that a synthetic model conse.rvatively reflects tile runoff response expected froin precipitation as severe as thie estimated PMP.

Detailed flood hydrograph analysis techniques and studies fkor specific basins are available from many agencies. Published studies such as those by tile Corps of Engineers, Bureau of Reclamation.

and Soil Conservation Service may be utilized directly where it can be demonstrated that they are of a level of' quality comparable with that indicated herein. In particular, the Corps of Engineers have developed analysis techniques (Rfs. 2, 3) and have accomplished a large number of studies in connection with their water resources development activities.

Computerized runoff models (Ref. 3) offer an extremely efficient tool for estimating PMF runoff rates and for evaluating tihe sensitivity of PMF estimates to possible variations in parameters. Such techniques have been used successfully in making detailed flood estimates.

Snowmelt may be a substantial runoff component for both historical floods and the PMF. In cases where it is necessary to provide for snowmelt in the runoff model, additional hydrometeorological parameters must

.

be incorporated. The primary parameters are the depth of assumed existing snowpack. the areal distribution of assumed existing snowpack ( and in basins with distinct changes in elevation, the areal distribution of snowpack with respect to elevation), the snowpack temperature and density distributions, the moisture content of the snowpack. the type of soil or rock surface and cover of the snowpack, the type of soil or rock surface and cover in different portions of the basin, and the time and elevation distribution of air temperatures and heat input during the storm and subsequent runoff period.

Techniques that have been developed to reconstitute historical snowmelt floods may be used in both historical flood hydrograph analysis and PMF (Ref. 4)

determinations.

A.7 PROBABLE MAXIMUM

PRECIPITATION ESTIMATES

Probable maximum precipitation (PMP) estimates are the time and areal precipitation distributions compatible with the definition of Section A.2 and are based on detailed comprehensive meteorological analyses of severe storms of record.

The analysis uses precipitation data and synoptic situations of major storms of record in a region surrounding the basin under study in order to determine characteristic combinations of meteorological conditions that result in various

.

rainfall patterns and depth-area-duration relations. On the basis of an analysis of airmass properties and synoptic situations prevailing during the record storms, estimates are made of tile amount of increase in rainfall quantities that would have resulted if condilions during the actual storm had been as critical as those considered probable of occurrence in tile region. Consideralion is given to the modifications in meteorological conditions that would have been required IOr each of" the record storms to have occurred over the drainage haisin under study. considering topographical features and locations of the respective areas involved.

The physical linimiations in meteorological mechanisms

• or the maximum depth. time. and space distribution of precipitation over a basin are I )

humidity (precipitable water) in tile air flow over the watershed. (2) the rate at which wind may carty lhie humid air into tile basin. :ind (3) tile fraction of tile inflowing atmospheric water vapor that can be precipitated.

Each of these limitations is handled differently to estimate tile probable miaximum precipitation over a basin, and is modified further for regions where topography causes marked orographic control (designated as the orographic model) as opposed to the general model (with little topographic effect}) 0

precipitation.

Further details on the models and acceptable procedures ate contained in References 5 and 6.

a.

The PNIP in regions of limited t opographic influence (mostly convergence precipitation) may he estimated by maximizing observed intense storm patterns in thie site region for various durations.

intensities, and depth-area relations and transposing them to basins of interest. The increase in rainfall quantities that might have resulte! from maximizing meteorological conditions during the rtcord storm and tile adjustments necessary to transpose the respective storms to the basin under study should be taken into account. The maximum storm should represent tli.. most critical rainfall depth-area-duration relation for the particular drainage area during various seasons o" ithe year (Refs. 7. 8. 9, 10). In practice. the parameters considered are (I) the representative storm dewpoint adjusted to inflow moisture producing the maximum dewpoint (precipitable water), (2) seasonal variations in parameters. (3)

the temperature contrast. (4)

thie geographical relocation, and

(5)

thie depth-area distribution. Examples of these analyses are explained and utilized in a number of published reports (Refs. 7.8.

9. 10).

This procedure, supported with an appropriate analysis. is usually satisfactory where a sufficient number of historical intense storms have been maximized and transported to the basin and where at least one of them contains a convergent wind

"mechanism" very near the maximum that nature can be expected to produce in the region (which is generally the case in the United States east of the Rocky Mountains).

A general principle for PMP estimates is: The numher and seperily of JnaximiyathiV

steps must balance ihe adequacy of the storm sample, additional inaximizatioun

1.59-9

• ..

.

steps are required in regions of more limiteid storm sanmples.

b.

PMI1 determinations in regions of orograplhit influences generally are for hlie high mountain regions that lie in the path of Ithe prevailing moist wind.

Additional maximization steps front paragraph A.77.a.

above are required in the use of the orographic model (Refs. 5, 6). The orographic moxlel is developed for the orographic component of precipitation where severe precipitation is expected it) be caused largely by tire lifting imparted to fie ait by' mounwains. This orographic influence gives a basis for a wind model with maximized inflo

w. Assuming laminar

%low of air over any particular mountain cross section. one can calctlate Ihe liife" of the air. the levels at which raindrops and snowflakes are formed. and their drift with the air before they strike lhe ground. Such mnodels are verified by reproducing the precipitation'in observed storms and are then used for estimating PIMP by introducing maximum values of mtoisture and wind as inllow at thie foot of thie mountains. Maximum moisture is evaluated just as in nonorogiaphic regions. In mnotntainous regions, where storms cannot readily be transposed (paragraph A.7.a.

above)

because of !heir intimate relation to the immnediate tuderlying topography. historical stornits are resolved into their convective and orographic compnecnts and maximnized as follows: (I)

mraximuim moisture is assunied. (2) maxinmum winds are assumed.

and finally (3)

maximum values of tIle orographic consponent and convective component (convective as in nonorographic areas'l of precipitation are considered to occur simultanretously. Some of the published reports that ill ustr:ute the combination of orographic and convective components. including seasonal variation, are References II. 12, and 13.

In somne large watersheds. major floods ate often the result of melting snowpack or of snownilt combined with rain.

Acco:dingly.

the probable maxinmum precipitation (rainfall)

and maximunt associated runoff-producing snowpacks are both estimated on a seasonal and elevation basis. The probable maximum seasonal snowpack water equivalent should be determined by study of accumulations on local watersheds from historical records of the region.

Several methods of estimating the upper limit of ultimnate snowpack and rueling are summarized in References 4 and 5. The methods have been applied in the Columbia River basin, the Yukon basin in Alaska.

the tipper Missouri River basin, and the upper Mississippi in Minnesota and are described in a number of reports of the Corps of Engineers. In many internmediate-latitude basins, the greatest flood will likely result from a combination of critical snowpack (water equivalent) and PMP. Thie seasonal variation in both optimum snow depth (i.e.,

the greatest water equivalent inl the snowpack) and the associated PMP combination should be meteorologically compatible. Temperature and winds associated with PMP are two important snowmelt factors amenable to generalization for snowinell computations (Ref. 14). The meteorological (e.g., wind, temperature, dewpoints) sequences prior to, during, and after the postulated PMP-producing storm should be compatible with the sequential occurrence of the PMIP, The user should place the PNIP over the basin and adjust the sequence of olher parameters to give the most critical runof flor t(ie season considered.

The meteorological parameters for snowniel comIpu tations associated with PNIP are discussed in more detail in References II 12, and 14.

Other items that need to be considered in determining basin melh are optimntum depth. areal extent.

and type of snowpack. and other snowmuell factors (see Section A.8). all of which must he compatible with the most critical arrangement of the PMP and associated nueiiorological paramneters.

Critical piobable maxiniuni storm estimates for very large drainage areas are determined as above, but may differ somewhat in flood-producing storm rainfall from those encountered in preparing similar estimates for small basins. As a general rule. the critical PMP in a small basin results primarily from extremely intense small-area storms; whereas in large basins the PMP usually results from a series of less intense, large-area storms. In very large river basins (about 100,000 square miles or larger)

si.:h as the Ohio and Mississippi River basins, it may be necessary to develop hypothetical PMP storm sequences (one storm period followed by another) and storm tracks with an appropriate limte interval between storms.

The type of meteorological analyses required and typical examples thereof are contained in References 9, 15, and

1 6.

The position of probable maximum rainfall centers.

identified by "isolyetal patterns" (lines of constant rainfall depth), may have a very great effect on the regimen of runoff from a given volume of rainfall excess.

particularly in large drainage basins in which a wide range of basin hydrologic runoff characteristics exist.

Several trials may be necessary to determine the critical position of the hypothetical PMP storm pattern (Refs. 8.

17) or the selected record storm pattern (Refs. 9, 16) to determine the critical isohyetal pattern that produces the inaxiumtm rate of runoff at thie designated site. This may be accomplished by superimposing an outline of the drainage basin (above the site) on the total-storm PMP isohyetal contour map in such a manner as to place the largest rainfall quantities in a position that would result in the maximum flood runoff (see Section A.8 on probable maximuni flood runoff). Thi isohyetal pattern should be reasonably consistent with the assumptions regarding the meteorological causes of the storm. A -

considerable range in assumptions regarding rainfall patterns (Ref. 11) and intensity variations can be made in developing PMP storm criteria for relatively small basins, without being inconsistent with meteorological

1.59-10

L

,1 0.

0

causes. Drainage basins less than a tew thousand square miles in area (particularly if only one unit hydrograph is available) may be expressed as average depth over tile drainage area. However. in deoerntining the BilP pattern for large drainage basins (with varing basin hydrologic characteristics, including reservoir etfects).

runoff estimates are required for different storm pattern locations and orientations to ohtain the final PMF.

Where historical rainfall patterns are not used for PMP,

two other methods are generally employed as follows:

a.

Average depth over the entire basin is based onl the maximized areal distribution of Ihe PMP.

h. A hypothetical isohyclal pattern is assumed.

Studies of areal rainfall distribution from intense storms indicate elliptical patterns may be assumed as representative of such events. Examples are the typical patterns presented in References 8. 14. 17. and 18.

To compute a flood hydrograph from the probable maximum storm, it is necessary to specify the time sequence of precipitalion in a feasible and critical meteorological time sequence.

Two meteorological factors must be considered in devising the time sequences: ( I ) the time sequence in observed storms and

(2) the manner of deriving the PMP estimates. The first imposes little limitations: the lhetographs (rainfall time sequences) for observed storms are quite varied. There is some tendency for the two or three time increments with thie highest rainfall in a storm to bunch together. as sonie time is rcouired for the influence of a severe precipitation-producing weather situation to pass a given region. The second consideration uses meteorological parameters developed from PMP estimates.

An example of 6-hour increments for obtaining a critical 24-hour PMP sequence would be that the most severe 6-hour increments should be adjacent to each other in time (Ref. 17). In this arrangement the second highest increment should bc adjacent to the highest. the third highest should be immediately before or after this

12-hour sequence. and the fourth highest should be before or after the 18-hour sequence. This procedure may also be used in the distribution of the lesser second

(24-48 hours) and third (48-72 hours) 24-hour periods.

These arrangements are permissible because separate bursts of precipitation could have occurred within each

24-hour period (Reference

7). The three 24-hour precipitation periods are interchangeable.

Other arrangements that fulfill the sequential requirements would be equally reasonable. The hyclograph. or precipitation time sequence. selected should be the most severe reasonably possible that would produce critical runoff at the project location based on tihe general appraisal of the hydrometeorologic conditions in the project basin. Examples of PMP time sequences fulfilling the sequential requirements are illustrated in References I1, 12. and 17. For small areas. maximized local records should be considered to assure that the PMP time sequence selected is severe.

The Corps of Engineers arnd the Hydrometeorological Branch of NOAA

(under a

cooperative arrane tientI

since

19)39))

have made cor n prchlenrsive inet corological studies of extremno flood-producing storms ( Ref. I ) and have developed a ntuimbe r o(f estimates of

"probahle maximunm precipilation." The PMP estimates arc presented in various unpublished mnemoranda and published reports.

The series of' published reports is listed on the lyv sheet of referenced Hydronietcorological Reports such as Reference I8. The published memoranda reports mtay he obtained from thi e Corps of i Engineers or HyJrometeorological Branch. NOAA. These reports and memoranda present pgneral techniques: included among the reports are several that contain "generalized"

estimates of PM I' for different river basins.

The generalized studies (Refs. 7. 12) usually assure reliable and consistent estimates for various locatlions in the region for which they have been developed inasniuch as they 'are based on coordinated studies of all available data.

supplemented by thorough meteorological analyses. In sonic cases. however, additional detailed analyses are needed for specific river basins (Refs. 7. 8)

to take into account unusually large areas. storm series, topography, or orientation of drainage basins not fully reflected in the generalized estimates. In many river basins available studies may be utilized to obtain the PMP without the in-depth analysis herein or in tihe referenced reports.

A.8 CHANNEL AND RESERVOIR ROUTING

Channel and reservoir routing of floods is generally an integral part of the runoff model for subdivided basins, and care should be taken to assure not only that the characteristics determined represent historical conditions (which may be verified by reconstituting historical floods) but ;dso that they would conservatively represent conditions to be expected during a PMF.

Channel and reservoir routing methods of many types have been developed to model the progressive downstream translation of flood waves. Tihe same theoretical relationships hold for both channel and reservoir routing. However, in the case of flood wave translation through reservoirs, simplified procedures have been developed that are generally not used for channel routing because of the inability of such simplified methods to model frictional effects. The simplified channel routing procedures that have been developed have been found useful in modeling historical floods, but particular care must be exercised in using such models for severe hypothetical floods such as the PMF because the coefficients developed from analysis of historical floods may not conservatively rellect flood wave translation for more severe events.

Most of tihe older procedures were basically attempts to model unsteady-flow phenomena using simplifying approximations. The evolutiorn of computer

1.59-1 I

use has allowed development ,,ofI analysis techniques that permit direct solution tit' basic 'Instead% flow equations mlilizinig ntimerical analysis teclinitques adaptable to the digital comptuter (Ref.

I19). In addition. most of' the older techniques have been adapted for computer use (Ref. 3).

In all rout ing techniques. care must be ,:xercised in assurinig hat1 ijmiramet ers selectLed Jor model verification are based on several hislorical floods (whenever possible)

and that their applicationl Ith1 PMF will restilt in conserv

a. liVe est mates

1 l'h\\

ata Cles. water levels.

velocities, and ilIpacM

torceI . Theoretical discussions of1 the many methods availahle for such analyses are contained in Refelences 2. 19). 20.- I . mnd 22.

A.9 PMF HYDROGRAPH ESTIMATES

PM F net runolf hydrograph estimates are made bh sequentially applying critically located and distributed PM P estinmt tes using the runoff timodel. conservatively low%, estimates of prcipitalioti losses, and conservatively hilh estimates (1' base Ilow z'nd antecedent reservoir levels.

lit PlMF determinationis it is cenerall v assumed that short-lerin reservoir flood control storage would be depleted by possible antecedent floods. An exception would be whet it cat be demonstrated that tile occurrence oif a measonably seveie flood I say aboolu;

one-h:alf ofl a P1I\\)

less than a week (usually a tinitnrtni oit' 3 to- 5 days prior :ii a lIFM

c:nli be evacialetl frotil the reservoir helfre tile artival otf a PMVF. However, it is unusual to use all antecedent storage level less than one-halftile flood control storage available'

Time applicatiomn (i P\\MP in bhasins whose hydrologic features vat fron llcation to location requires the detenriiimatit, that thie estimated PM F hydrograph represents the most critical centering of the PIMP storm with respect to the site. ('are must be taken in basins witlhi substantial headwater flood control storage to assure that maoire highly concentrated PMP over a smaller area dowistireant of' the reservoirs would not produce a greater PNIF tIan a total basin storm that is partially controlled. In siich cases more than oCe P['NIP

runoff analysis mayl he required. Usually. only a few trials oft a total basin l.NI' are required to determine the most critical centering.

The antecedent snowpack and its contribution to the PNIF are included when it is determined that snowrnell coilrihntions to thie flood Would produce a PNIF

(see Section A.7). However.

these typcs of hypothetical floods are generally the controlling events only in the far west and northern United States.

Runoff hydrogruphs should be prepared at key hydrologic hlcations (e.g.. strcanigages and dams) as well as at the site of mnclear facilities. For all reservoirs itnv olvedt.

in flvw.

out hllow, and pool elevat ion hydrographs should be prepared.

Many existing and proposed dams and oilier river control structures may niot be capaible of safely passing floods as severe as a PMF. Tile capability of river control structures to safely pass a PMF and local coincident wind.generated wave activity must be determined as part of' the PM F atnalysis. Where it is poissible that such structures imay nitot safely survive Iloods as severe as a PM F.

tile

\\vtwrst such conidition withi resipect to downstream nuclear lpower plants is assuimied (hut should be suhtsltanlialed hr analysis ohl lpsl eamn PNIF poi':litiall to be their failuore during a PMF.

and the PM F

detertminatiion should include the resuiltant effects. This analysis: also requires that tihe consequncces otf lupsreamii dam failures on downtstreanm damis ( domtino effects) he considered.

A.10 SEISMICALLY INDUCED FLOODS

S.isinically induced bloods on streams and rivers may be caused hr landslides or dain failures. Where river Coitrol structures are widely spaced, their arbitrarily as.suilied indiciduwil total.l instantaneous failure and resul tinig downsttreailmi flotodl wave atltenuation (routing)

mliar be showII to coTIns6lcite lbi) threat to nuclear facilities. Where the relative size. location, and proximity of' dams !o ptentiial seismic generators indicate a threat to nuclear power plants. tite capability of suIch structures (cither singly or in combination) Ito resist severe earthquakes (critically located) shimald he considered. Ili river basins where the flood a unoff season may constitute a significant portion of' the year (such as the Mississippi. Columbia. or Ohio River basins). f'ull flood control reservoirs willi ai 25-year flood is assunied coincident with the Safe Shutdown t..artliquake. Also.

cotnsideration should he given to the occurrence of' a flood of approximately one-half the severity of a PM F

with frill flood control reservoirs coincident wi\\h the maximumi earthquake determined on the basis of'

historic seismicity ito mainlain a consistent level of analysis I'or Other combinations of such events. As with failures dime to inadequiate flood control capacity, domino and essentially simultaneous multiple f'ailures may also require consideration.

If the arbitrarily assumed total failure of the most critically located (from a hydrolh.:,ic standpoint ) struct ures indicates flood risks at the nuclear power plant site more severe than a PMF, a progessively more detailed analysis of the seismic capability of the dam is warranted. Without benefit of detailed geologic and seisunic investigations. the flood potential at the nuclear power plant site is next generally evaluated assuming the most probable mechanistic-type failure of' the quest ioned struci tires. IfI tile results of each step of the above analysis cannot be safely acconmnodated at the nuclear power plant site in an acceptable manner, the seismic potential at tile site of each questioned structure is then evaluated in detail, the structural capability is evaluated in the same depth as for

-

I

1.59. 12

°

nuclear power plant sites, and the resulting seismically induced flood is routed to the site of the nuclear power plant. This last detailed analysis is not generally required since intermediate investigalions usually provide sufficient conscrvalive inflormiation to allow determinalion of an adequate design basis flood.

A.11 WATER LEVEL DETERMINATIONS

All the preceding discussion has been concerned primarily with determinations of flow rates. The Ilow rate or discharge must be converted to water level elevation for use in design.

This may involve determination of' elevation-discharge relations Ifor natural stream valleys or reservoir conditions. The reservoir elevation estimates involv,:

the spillway discharge capacity and peak reservoir level likely to be attaiiied during the PMF as governed by the inflow hydrograph.

the reservoir level at the beginning of the 'M[:. and the reservoir regulation plan with respect to total releases while the reservoir is rising to peak stage. Most river water level deterininations involve the assumption of steady, or nonvarying, flow for which standard methods are used to estimate flood levels. Where little floodplain geometry definition exists, a

technique called

"slope-area" may be employed wherein the assumptions are made that the water surface is parallel to the average bed slope, any available floodplain geometry information is typical of the river reach under study, and no upstream or downstream hydraulic controls affect the river reach fronting the site under study. Where such computations can be shown to indicate conservatively high flood levels, they may be used. However, the usual method of estimating water surface profiles for flood conditions that may be characterized as involving essentially steady flow is a technique called the Itstandard-step method." This technique utilizes thle i- .grated differential equation of steady fluid motion commonly referred to as the Bernoulli equation (References 22. 23, 24, and 25) where, depending on whether supercritical or subcritical Rlow is tinder study, water levels in the direction of flow computation are determined by the trial and error balance of upstream and downstream energy, respectively. Frictional and other types of head losses arc usually estimated in detail with the use of characteristic loss equations whose coefficients have been estimated from computational reconstitution of historical floods, and from detailed floodplain geometry informatio

n. Application of the

"standard-step method" has been developed into very sophisticated computerized models such as the one described in Reference 23. Theoretical discussions of the techniques involved are presented in References 22, 24, and 25.

Unsteady-flow models may also be used to estimate water levels. Since steady flow may be consider,:d a class

.

of unsteady flow, such models may also be used for the steady-flow water level estimaLion, Compnterized unsteady-flow models require generally the same floodplain georrit tv definition as steady-fiowv models.

and thelrefore hit li use may allowv more accurate water surface level t"'caini;ws whiiere ýid'*'-t~i'w approxinmatlions are inlle.

()n.e such iilwloidV-Iw coriputier 1t1odel is dicused ill Re*('tih. e 11).

All ieas.omahly i,'cnr:ile wvacr h'ct, c*{irnwilii'u nlrdels reqmuire

11;1,lpl:1

&lfiminitiori l :11c.ts that cat1 inatetialklv affect w* ticl levels. I.ood wa%(

t .l;:iriom .

and c:litihratlini lv by rnr:henirl~ical iecii.,-iwii of hislorical l*d)ts (tit mte

,hcclioit of- c.1iblat:ioi cocttficiellts based (it l the cil 'itsa,;li'c liallnIerl of information derived torll SAilr 'lildies

-I' oilier iv,.r reaches). Particular c:are s hould he cxercis-d it, asstiie that corntrolling tlfomd lc.el est iniates tic tilwvayvs conservatively high.

A.12 COINCIDENT WIND-WAVE ACTIVITY

The superposition tlt \\n'd-wave :activitv on I'MF tir seismically induced wael!

level dcte rnin ltions is required to assure that. in 11le event Cilt hr coildit ito did occur, ambient nieteorological activityv would Inot cause a loss of safe ty-related tun t iotn due to wav, act ion.

The selection of' wind spejeds andtI critical wind directions assu.med coincident with mnxiiniini I'MI: or seismically i.'duced water levels should provide :t,,n; i rincc of virtually no risk to safety-reialed equipmientr icces.arnV

to plant shutdowvn. The ('orps of'

ngineecrs .uqiests (Refs. 26. 27) that average rmaximum %%-itnd siced% of'

approximately 40 to (10 inph have occurred in miajor windstorms in most regions of the United States. For application to the safety analysis of nuclear facilities, the worst regional winds of record should le :ssnmned coincident with the PMF. However. the postuhlted winds should be meteorologically compatible with the conditions that induced tire PMF

or with tlie flood conditions assunred coincident with seismically induced dam failures) such as the season of tfie year. the ntite required for the PMP storon to 11r0%'e our of the area and be replaced by meteorological conditions that could produce the postulated winds, ard the restrictions on wind speed and direction produced by topography. As an alternative to a detailed study of hitorical regional winds, a sustained 40-inph overland wind speed t'romr any. critical direction is an acceptable positulation.

Wind-generated set up (or wind tide) atd wave action (runup and impact torces) may be estimated using the techniques described in References 26 and 28. Tire method for estimating wave action is based on stutistical analyses of a wave spectrum. For nuclear power planrts.

protection against the maximuin wave, defincd in Refernce 28 as tire average of tire upper one percent ofl"

the waves in the anticipated wave spectrumI

, should bIe assumed.

Where depths of water ill tronit r0'

safety-related structures are sufficient (Cusually about seven-tenths the wave height), the wave-induiced forces will be equal to the hydrostatic forces estimated frort

1.59-13

the maxilunm rurup level. Where the waves can be

-tripped' and caused to break both before reaching and on safeiy.related structures, dynamic Irces may. be estimated from Reference 28. Where waves may induce surging in intake structure sumps. pressures on walls and the underside of' exposed floors should be considered, particularly where such sumps are not vented and air Colmpression call greatly increase dynamic forces.

.

In addition, assurance should be provided that safety systems ncessary for cold shutdown and maintenance thereof are designed to withstand the static and dynamic effects resulting from frequent flood levels coincident with the waves that would be produced by the nmaximumn gradient wind for the site (based on a study of historical regional meteorology).

1.59.14 I

V

6

4 REFERENCES

I. Precipitation station data and unpublished records of Federal, State, municipal, and other agencies may be obtained from the U.S. Weather Bureau (now called National Weather Service).

In addition, studies of some large storms are available in the

"Storm Rainfall in the Un it ed States.

Depth.Area-Duration Data." summaries published by Corps of Engineers, U.S. Army.

2. Corps of Engineers publications, such as EM

1110-2-1405 dated 31 August 1959 and entitled,

"Engineering and Design-Flood Hydrograph Analyses and Computations."

provide excellent criteria for the necessary flood hydrograph analyses.

(Copies are for sale by Superintendent of Documents.

U.S.

Government Printing Office, Washington, D.C. 20402.) Isohyetal patterns and related precipitation data are in the files of the Chief of Engineering, Corps of Engineers.

3. Two computerized models arc "Flood Hydrograph Package. HEC-I Generalized Computer Program,"

available from the Corps of Engineers Hydrologic Engineering Center, Sacramento, California, dated October

1970

and

"Hydrocomp Simulation Programming-HSP," Hydrocomp Intl.. Stanford, Calif.

4. One technique for the analysis of snowmelt is contained in Corps of Engineers EM 1100-2.406,

"Engineering and Design-Runoff From Snowmelt,"

January 5, 1960. Included in this reference is also an explanation of the derivation of probable maximum and standard project snowmelt floods.

5. "Technical Note No. 98-Estimation of Maximum Floods,"

WMO-No.

233.TP.126, World Meteorological Organization, United Nations, 1969 and "Manual for Depth-Area-Duration Analysis of Storm Precipitation," WMO-No. 237.TP.129, World Meteorological Organization, United Nations, 1969.

6. "Meteorological Estimation of Extreme Precipitation for Spillway Design Floods", Tech.

Memo WBTM HYDRO-5. U.S. Weather Bureau (now NOAA) Office of Hydrology. 1967.

7. "Seasonal Variation of the Probable Maximum Precipitation East of the 105th Meridian for Areas from 10 to 1,000 Square Miles and Durations of 6,

12, 24, and 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />," Hydromneteorological Report No. 33, U.S. Weather Bureau (now NOAA), 1956.

8. "Probable Maximum Precipitation. Susquehanna River Drainage Above Harrisburg, Pa.,

"Hydrometeorological Report No. 40. U.S. Weather Bureau (now NOAA), 1965.

9. "Meteorology of Flood Producing Storms in the Ohio River Basin," Hydronieteorological Report No. 38. U.S. Weather Bureau (now NOAA). 196L.

10. "Probable Maximum and TVA Precipitation Over the Tennessee River Basin Above Chltllanooea."

Hydrometeorological Report No. 43, U.S. Weather Bureau (now NOAA), 1965.

11. "Interim Report- -Probable Maximum Precipitation in California." Hydrometeorological Report No. 36.

U.S. Weather Bureau (now NOAA). 1961.

12. "Probable Maximuni Precipitation, Northwest States," Hydrometeorological Report No. 43. U.S.

Weather Bureau (now NOAA), 1966.

13. "Probable Maximum Precipitation in the Hawaiian Islands," Hydrometeorological Report No. 39. U.S.

Weather Bureau (now NOAA). 19)63.

14. "Meteorological Conditions for the Probable Maximum Flood on the Yukon River Above Rampart, Alaska," Hydronieteorological Report No.

42, U.S. Weather Bureau (now NOAA), 1966.

15. "Meteorology of Flood-Producing Storms in the Mississippi River Basin."

Hydrometeorological Report No. 34, U.S. Weather Bureau (now NOAA).

1965.

16. "Meteorology of Hypothetical Flood Sequences in the Mississippi River Basin," Hydrometeorological Report No. 35, U.S. Weather Bureau (now NOAA),

1959.

17. "Engineering and Design-Standard Project Flood Determinations,"

Corps of Engineers EM

1110.2-1411, March 1965, originally published as Civil Engineer Bulletin No. 52-8.26 March 1952.

18. "Probable Maximum Precipitation Over South Platte River, Colorado.

and Minnesota River.

Minnesota," Hydrometeorological Report No. 44.

U.S. Weather Bureau (now NOAA).

1961).

19. "Unsteady Flow Simulation in Rivers and Reservoirs," by J. M. Garrison. J. P. Granju and J.

T. Price.

pp

1559-1576, Vol. 95. No. IIYS,

(September

1969), Journal of the Ilyt'draulics Division. ASCE. (paper 6771).

20. "Handbook of Applied Hydrology." edited by Ven Te Chou, McGraw.Hill.

9)64. Chapter 25.

21. "Routing of Floods Through River Channels." EM

H 10-2-1408. U.S. Army Corps of Engineers. I

March 1960.

1.59-15

.2. "'l~nLiti .'riig 1 yvdiauilics". e.'dited hy Hlu tier Rouse.

John WViley & Sons. l1tc. 19Q50.

.. 1 eW

c Sil face Plroilies.

HI.I-2 Genraliued Co nipmiaUt Program.'

available from( tie Corps of

1:-ni neers Hydrologic Engineering Center.

Sacrameilnito. C:ail.

_'4. "()pen Chalnel Ilydratlic'" by Ven Te Choli;

-j

"lack%:%tlctr (Cirv es in River (Channels."

EM

I I 1

40-).I4. U.S.

Ariny Corps of Elpgineeis.

Dc),. a',:.

cr "7. I*)g!

2o. "Compiitation of Freeboard Allowances ,fr Waves in Reservoirs."

I-ngineca Technic;al Leiter lTL

I1 10-2-). U.S. Army Corps of lingineer

s. I Augist

1960.*

27. "Policies a nd Proceedures PerIaining to D)etermination of Spillway ('apaci ties anid Frecehoard Allowances for D)ams.'" lingincer Circular

1-C

1110-2-27. LU.S. Arwy Corps or Engineer

s. I August

28. "iShore Protect iot.

!Il~amini*g and I)esign, Tedhnicil Relp)rt No.

4. U.S. Arauy

"Coastal Elngineering Research Cenler. 3rd edition. I906.

1.59-16