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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1973 August at.

August 1973 U.S. ATOMIC ENERGY COMMISSION

REGULATORY

DIRE"W"TORATE OF REGULATORY STANDARDS

GUIDE

REGULATORY GUIDE 1.59 DESIGN BASIS FLOODS FOR NUCLEAR POWER PLANTS

A. INTRODUCTION

TlThis guide describes a1n acceplahl' ntl lhod (it determinirng fOr siles aloi*g strealis tit riveis ilie design General Design Criterion 2. "-Design Bases for basis floods that nuclear power plants maust lie designed Protection Against Natural Phenomentia." of Appendix A to withstand without loss of saltety-related functions. It to 10 CFR Part 50. **General Design Criteria for Nuclear further discusses tlie phenomlena producing colmpar*able Power Plants." requires. in part. that structures. systems. design basis floods for coastal. estuary; and Gieat Lakes and components important to safety be designed to sites. It does not discuss the design requirements for withstand the effects of natural phenomena such as flood protection. The Advisory Committee on Reactor floods, tsunami. and seiches without loss of capability to Safeguards has been consulted concerning this guide and perform their safety functions. Criterion 2 also requires has concurred in the regulatory position.

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

B. DISCUSSION

historically reported for the site and surrounding region.

with sufficient margin for the limited accuracy and Nuclear poower plants must be designed itf prevent quantity of the historical data and the period of time ill the loss of safety-relat ed functions resulltig front the which the data have been accumulated. (2) appropriate most severe flood conditions thai call reasonably be combinations of the effects of normal and accident predicted to occur at a site as a result of sevele conditions with the effects of the natural plhenonlena. hydrometenrological conditions, seismic activity. or and (3) the importance of the safety functions to be both.

performed.

The Corps of Engineers for many years has studied Paragraph 100.10 (c) of 10 CFR Part 100,"Reactor conditions arid circumstances relating to floods and Site Criteria," requires that physical characteristics of flood control. As a result of these studies, it has the site, including seismology. meteorology, geology.

developed a definition for a probable niaxinmui 'lood and hydrology, be taken into account in determining the (PM F)' and attendant analytical techniques for acceptability of a site for a nuclear power reactor. estimating with an acceptable degree oft conservattsm flood levels on streatis or rivers resulting fromi Appendix A. "Seismic arid Geologic Siting Criteria hydromLeteorological conditions. For estimating for Nuclear Power Plants." was published in the Federal seismtiically induced flood levels. an acceptable degree of Register on November 25, 1971 (36 FR 22601) as a proposed amendment to 10 CFR Part 100. The proposed appendix would specify investigations required 'Corps ot tEngincecr Pribahltc Ma',intsni ItIodt definlililn for a detailed study of seismically induced floods and appears in many publication, of thait :g00ncy sch1is IEngineering water waves. Proposed Appendix A to 10 CFR Part 100 Circular EC-I 110-2-27, Change I. 'T"ngincering :snd would also require that (lie determination of design Design -Policies and Procedures Perlaining 10 t)eerminaition of Spillway Capalities and Frecboard Allowances fir t)jn<,. dated bases for seismically induced floods and water waves be 19 Feb. 1968. Ttie probahble niamimuni fhlood is atso direclly based on the results of the required geologic and seismic analogous to ftte Corps (if 1'ngineers "Spillway Design Itlod" as investigations and that these design bases be taken into used for darns whose failures would result in a significant toss of account in the design of tile nuclear power plant. 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

2. Research and Test Reactors 6. Tranportation the itauence or continuance of a permit or license by the Commitsion.

3. Fuels ard Materials racilitien

8. Occupational Health

4. Environmentall and Siting 9. Antitrust Review Published guides will be revised periodically, as appropriate, to accommodate 10. General comments end to reflect new information or experlence. 5. Materialt and Plant Protection

conservatism for evaluating the effects of lte initiating for the design of the nuclear plant. For instance, the event is provided by the proposed Appendix A to 10 analysis of floods caused by darn failures, landslides, or CFR Part 100. tsunami requires consideration of seismic events of the severity of the Safe Shutdown Earthquake occurring at The *onditions resulting I'rom the worst site-related the location that would produce the worst such flood at flood precHble at the nuclear power plant (e.g.. PMF, the nuclear power plant site. In the case of seismically seismically induced flood, seiche. surge. severe local induced floods along rivers, lakes, and estuaries which precipitation) with attendant wind-generatcd wave may be produced by events less severe than a Safe activily constitute the design basis flood conditions that Shutdown Earthquake, consideration should be given to safety-related structures. systems. and components the coincident occurrence of floods due to severe identified in Regulatory Guide 1.292 must he designed hydrometeorological conditions, but only where the ito withstand and remain functional. effects on the plant are worse, and the probability of such combined events may be greater, than the effects For sites along streams or rivers, a hypothetical on the plant of an individual occurrence of the most probable maximumiflood of the severity defined by the severe event of either type. For example. a seismically Corps of Engineers generally provides the design basis induced flood produced by an earthquake of flood. Ior sites alone lakes or seashores, a flood approximately one-hal f the Safe Shutdown severity Condition of cotinparahle severity could be produced by coincident with a runoff-type flood produced by tihe the most severe combination of hydrometeorological worst regional storm of record may be considered to parameters reasonably possible, such as may be have approximately the same severity as an earthquake protduced by a probable maxinmum hurricane" . or by a of Safe Shutdown severity coincident with about a probable matximum seiche. On estuaries. a probable 25-year flood. For the specific case of seismically inaxinitun rivet c lood. a probable maximum surge. a induced floods due it) dam failures, an evaluat ion should probable tuaximnuni seiche. or a reasonable combination be made of flood wave! which may be caused by of less severe phenomenologically caused flooding events domino-type darn failures triggered by a seismically should all he considered in arriving at design basis flood induced failure of a critically located dam and of flood conditions comparable in frequency of occurrence with waves which may be caused by multiple darn failur':s in a a probable ;naximum flood on streams and rivers. region where dams may be located close enough together that a single seismic event can cause multiple failutes.

Ini addition to floods produced by severe Ihy drometeorological conditions. Ihe most severe seismically induced floods reasonably possible should be Each of the severe flood types discussed above considered for each site. Along streams. rivers, and should represent the upper limit of all estuaries, seisinically induced floods may be produced phenomenologically caused flood potential combi- by dam failures or landslides. Along lakeshores, nations considered reasonably possible, and analytical coastlines, and estuaries. seismically induced or techniques are available and should generally be used for tst,namit-ype flooding should be considered. their prediction for individual sites. Those techniques Consideration of seismically induced floods should applicable to PMF and seismically induced flood include the same range of seismic events as is postulated estimates on streams and rivers are presented in Appendix A to this guide. Similar apperdices for coastal, estuary. and Great Lakes sites, reflecting comparable

2Regulatory Guide 1L29 (Safety Guide 29), "Seismic Design levels of risk. will be issued as they become available.

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 Analyses of only the most severe flood conditions remain funetionalt These structures. systems. and components may not indicate potential threats to safety-related are those necessary to assure (I) the integrity of the reactor coolant pressure boundary, (2) the capability to shut down the systems that might result from combinations of flood reactor and maintain it in a ,.afe shutdown condition, or (3) the conditions thought to be less severe. Therefore.

capability to prevent or mitigate the consequences of accidents reasonable combinations of less-severe flood conditions which could result in potential offsite exposures comparable to should also be considered to the extent needed for a the guideline exposures, of I1t CFR Part tI0O. These same structure%, systems, and components should also be designed to consistent level of conservatism. Such combinations withstand conditions resulting from the design basis flood and should be evaluated in cases where the probability of remain functional. their existing at the same time and having significant If is expected that safety-related structures, systemns. and consequences is at least comparable to that associated components of other types of nuclear power plants will be identified in future Regulatory guides. In the interim. Regulatory with the most severe hydrometeorological or seismically Guide 1.29 should be used as guidance when identifying induced flood. For example, a failure of relatively high rafety-related structures, systems, and components of other levees adjacent to a plant could occur during floods less types of nuclear power plants. severe than the worst site-related flood, but would

'See Corps of Engineers Coastal Engineering Research produce conditions more severe than would result during Center "Technical Report No. 4, Shore Protection, Planning and a greater flood (where a levee failure elsewhere would Design." third edition. 1966. produce less severe conditions a[ the plant site).

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Wind-generated wave activity may produce severe b. Along lakeshores. coastlines, and estuaries.

flood-induced static and dynamic conditions either eslimales of flood levels resulting frorn severe surges.

independent of or coincident with severe seiches. and wave action caused by hydronteteorological hydromelcorological or scisnmic flood-producing activity should he based on criteria cOluparahle in mechanisms. For example, along a lake. reservoir. river, conservatism to those used for probable maximum or seashore, reasonably severe wave action should he Ihoods. Criteria and analytical techniques providing this considered coincident with the probable maximum level of conservatism for the analysis of these events will water level conditions. 4 The coincidence of wave he summai'zed in subsequent appendices to ilbis guide.

activily with probable maximum water level conditions c. Flood Aronditions Ihat could be caused by should take into account the fact that sufficient time earthquakes of the severity used in thie design of the can elapse between the occurrence of the assumed nuclear facility should also be considered in establishing meteorological mechanism and the maximum water level the design hasis flood. A simplified analytical technique to allow subsequent meteorological activity to produce for evaluating the hydrologic effects of seismically substantial wind-generated waves coincident with the induced dam failures disctrssed herein is presented in high water level produced by the initial event. In Appendix A of this guide. Techniques for evaluating the addition, the most severe wave activity at the site that effects of tsunami will be presented in future can be generated by distant hydrometeorological activity appendices.

should be considered. For instance, coastal locations d. In addition to the analyses of the most severe may be subjected to severe wave action caused by a floods I hat may be induced by either distant storm that, although not as severe as a local hydrometeorological or seismic mechanisms. reasonable storm (e.g., a probable maximum hurricane), may combinations of less-severe flood conditions should also produce more severe wave action because of a very long be considered to the extent needed for a consistent level wave-generating fetch. The most severe wave activity at of conservatism, Such combinations should be evaluated tile site that may be generated by conditions at a in cases where the probability of their existing at the distance from the site should be considered in such same time and having significant consequenceL is at least cases. In addition, assurance should be provided that comparable to that associated with the most severe safety systems necessary for cold shutdown and hydrometeorological or seismically induced flood.

maintenance thereof are designed to withstand the static e. To the water levels associated with the worst and dynamic effects resulting from frequent flood levels site-related flood possible (as determined from coincident with the waves that would be produced by paragraphs a.. b.. c.. or d. above) should be added the the maximum gradient wind for the site (based on a effects of coincident wind-generated wave activity to study of historical regional meteorology). generally define the upper limit of flood potential. An acceptable analytical basis for wind-generated wave

C. REGULATORY POSITION

activity coincident with probable maximum water levels is the assumption of a 40-mph overland wind from the I. The conditions resulting from the worst site-related most critical wind-wave-producing direction, unless flood probable at a nuclear power plant (e.g., PNIF. historical windstorm data can be used to substantiate seismically induced flood, hurricane. seiche, surge. heavy that such an event (i.e., wind direction and/or speed) is local precipitation) with attendant wind-generated wave more extreme than has occurred regionally. However. if activity constitute the design basis flood conditions that the mechanism producing the maximum water level.

safety-related structures. systems, and compor.Ents such as a hurricane, would itself produce higher waves, identified in Regulatory Guide 1.292 must be designed then these higher waves should be used as the design to withstand and remain functional. basis.

a. On streams and rivers, the Corps of Engineers definition of a probable maximum flood (PMF) with 2. As an alternative to designing "hardened"

attendant analytical techniques (summarized in protection- for all safety-related structures. systems. and Appendix A of this guide) provides an acceptable level components as specified in regulatory position I . above, of conservatism for estimating flood levels caused by it is permissible to not provide hardened protection for severe hydrometeorological conditions. some of these features if:

a. Sufficient warning time is shown to be available to shut the plant down and implement adequate

4Probable Maximum Water Level Is deflined by the Corps of emergency procedures"

Engineers as "the maximum still water level (i.e.. exclusive of b. All safety-related structures. systems. and local coincident wave runup) which can be produced by the components identified in Regulatory Guide 1.29) are most severe combination or hydrometeorological and/or seismic parameters reasonably possible for a particular location. Such phenomena are hurricanes, moving squall lines, other cyclonic I tardened portection means structural provisions meteorological events. tsunami, etc., which, when combined incorporated in the plant design that will protect %afcty-related with the physical response of a body of water and severe structures, systems, and components from the static and ambient hydrological conditions, would produce a still water dynamic effects of floods. Examples of the types of flood level that has virtually no risk of being exceeded." (Sec protection to be provided for nuclear power plants will le the Appendix A to this guide) subject of a separate regulatory guide.

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designed to withstand the flood conditions resulting less-severe flood conditions are also considered to the i from a severe slorm such as tie worst regional storm of extent needed for the consistent level of conservatism:

record"' with attendant wind-generated wave activity and Ihl1 mw. lie produced by the worst winds of record and reiain functional:

d. In addition it) paragraph 2.b. above, at least c. In addition to the analyses required by those structutres, systems, and components necessary for paragraph 2.b. above, reasonable combinations of coldl shutdown and maintenance thereof are designed with "hardened" protective fealtures to withstand tlie For sites along streams and rivers thik event is characterized entire range of flo0d conditions up to and including the by the Corps of. Engineer! definition of a Standard Projcct worst site-related flood probable (e.g., PM F. seismically Flood. Such floods have been found to produce tlow rates induced flood. hutricane, surge, seiclhe, heavy local generally 40 wofill percenrtl tihte P.SIF. For sites along seahorc, this event m)*" le ch;taracterized b% the Corp, oit" :ineinctrs iercipitalion) with coincident wind-generated wave defiNition of j Standard Projecl Ilurricane. For other 'ijC a act ion as discussed in regulatory positiotn I. above and comparable level olf risk should le assumed. remain funictiolnal.

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• a

0 APPENDIX A

TABLE OF CONTENTS

A.I Introduction .......................... ...................... .5(1.5 A.2 Probable Maxinmum Flood (PMF) .......... .......................................................... I .q A.3 Hydrologic Characieristics ................ . . . . .. . . . . . . . . . . . . . . . . .5' .6 A.4 Hlood Hydrograph Analyses .............. I..,. I................... 1.59*.7 A.5 Precipitation Losses and Base Flow ......... ...................... 1.59-7 A.6 Runoff Model ......................... ...................... 59 -8 A.? Probable Maximum Precipitation Estimates .. .. . . .. . . . . . .. . .. . . . . ... 1.5- AS8 Channel and Reservoir Routing ............ ... ..................... 1.59-1 I

A.9 PNI F llydrograph Estimates ............... .................... 1.5 i. 1 2 A.10 Seismically Induced Floods .............. ..................... 1.59 -12 A.1 I Water Level Detei minations ............. .................... 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 historical storm and runoff data (fhood hydrograph analysis). the most severe precipitation reasonably This appendix has been prepared to provide possible (probable maximurn precipitation-.lPI

guidance for flood analyses required in support of riinimum losses. tnaximum base flow. channel and applications for licenses for nuclear power plants to be reservoir routing, the adequacy of existing and propetsed located on streams and rivers. Because of the depth and river control structures to safely pass a PMF. water level diversity of presently available techniques. this appendix determinations, and the superposition of potential summarizes acceptable methods for estimating probable wind-generated wave activity. Seismically induced Ihoods maximum precipitation, for developing rainfall-runoff such as may be produced by dam failures or landslides.

models, for analyzing seismically induced dam failures. may be analytically evaluated using many PMF

and for estimating the resulting water levels. estimating components (e.g.. routing techniques. water level determinations) after conservative assumptions of The probable maximum flood may be thought of as flood wave initiation (such as dam failures) have been one generated by precipitation, and a seismically made. Each potential flood component requires an induced flood as one caused by dam failure. For.many in-depth analysis. and the basic data and results should sites, however, these two types do not constitute the be evaluated to assure that the PMF estimate is worst potential flood danger to the safety of the nuclear conservative. In addition. the flood potential from power plant. Analyses of other flood types (e.g., seismically induced causes must be compared with the tsunami, seiches, surges) will be discussed in subsequent PMF to provideappropriate flood design bases. but the appendices. seismically induced flood potential may be evaluated by simplified methods when conservatively determined The probable maximum flood (PMF) on streams results provide acceptable design bases.

and rivers is compared with the upper limit of flood potential that may be caused by other phenomena to Three exceptions to use of the above-descrihed develop a basis for the design of safety-related structures analyses are considered acceptable as follows:

and systems required to initiate and maintain safe a. No flood analysis is required for nuclear power shu.tdown of a nuclear pow'er plant. This appendix. plant sites where it is obvious that a PMF or sismically outlines the nature and scope of detailed hydrologic induced flooding has no bearing. Examples of such sites engineering activities involved in determining estimates are coastal locations (where it is obvious that surges.

for the PMF and for seismically induced floods resulting wave action, or tsunami would produce controlling from dam failures, and describes the situations for which water levels and flood conditions) and hilltop or "dry"

less extensive analyses are acceptable. sites.

b. Where PNIF or seismically induced flood Estimation of a probable maximum flood (PMF) estimates of a quality comparable to that indicated requires the determination of the hydrologic response herein exist for locations near the site of the nuclear (losses, base flow, routing, and runoff model) of power planw, they may be extrapolated directly to the watersheds to intense rainfall, verification based on site, if such extrapolations do not introduce potential

1.59-5

errors of more than about a foot in PMF water level insofar as these are deemed reasonably possible of estimates. occurrence on the basis of hydrometeorological c. It is recognized that an in-depth PNF estimate reasoning." The PMP should represent the depth, time, may not le warranted because of the inherent capability and space distribution of precipitation that approaches of lihe design of some nuclear power plants to function tile upper limit of what the atmosphere and regional sofely with little or no special provisions or because the topography cani Iroduc

e. The critical PMP

time and costs of making such an estinate ate not meteorological conditions are based on an analysis of coninmensurate with the cost of providing protection. In air-mass properties (e.g., effective precipitable water, such cases, other nieans of estimating design basis flnois depth of inflow layer, temperatures, winds), synoptic are acceptable if it can he demonstrated that the situations prevailing during recorded storms in tile technique utiliied or the estimate itself' is conservative. region, topographical features, season of occurrence, and Similarly. conservative estimates of seisinically induced location oh the respective areas involved. The values thus flood potenti:al may provide adequate denmonstration of derived are designated as the PMP, since they are nuclear power plant safety. deterinited witthin Ilie limitations of current meteorological theory and available data and are based A.2. PROBABLE MAXIMUM FLOOD (PMF) on the most effective combinalion of critical factors con Iollinrg.

Probable maxir'inn Ilood sttid:,- should be coiripatible with the specific definitions and criteria A.3 HYDROLOGIC CHARACTERISTICS

summnnarized as follows:

a. The Corp; of Engineers defines the PMF as "the Hydrologic characteristics of the watershed and hyp.,thetical I1(x)d characteristics (peak discharge. sireani channels relative to the plant site should be Volmnc. arid hydroge? ih shape) that are considered to he duierniniied fromt the Iollowing:

the most severe reasonrabl\ possible at a particular a. A topographic map of the drainage basin location. haised on relatIively comprehensive showing watershed boundaries for the entire basin and hvdr ometeoro logic:' I analysis o f critical principal tributaries and other subbasins that are rt niill-producing precip tation (and snowmell. if pertinent. The mnap should include ; location of pertinent) and hydroltgic factors favorable for principal stream gaging stations and other hydrologically ima*inuirm fltiod ruinoff." Detailed PM F determinations related record collection stations (e.g., streamflow, are usuially prepared by estimating the areal distribution precipitation) and the locations of existing and proposed of *'prohbahe maximurn" precipitation (PNIP) over flie reseroirs.

subject drainage basin in critical periods of time. and b. The drainage areas in each of the pertinent computing the residual runoff hydrograph likely to watersheds or subbasins above gaging stations, reservoirs, result with critical coincident conditions of ground any river control structures, and any unusual terrain wetness and related factors. PMF estimates are usually features that could affect flood runoff. All major based un the observed and deduced characteristics of reservoirs and channel improvements that will have a hi St ori:al flood-producing storms anid associated major influence on streamfnow during flood periods hy dro log ic factors modified on the basis of should be considered. In addition, the age of existing hydronietecorological analyses to represent the most structures and information concerning proposed projects severe runoff conditions considered to be "reasonably affecting runoff characteristics or streamflow is needed possible" in the particular drainage basin under study. In to adjust streamflow records to "pre-project(s)" and addition to determining the PMF for adjacent large rivers "with project(s)" conditions as follows:

and strearims. a local PMF should be estimated for each (1) The term "pre-project(s) conditions" refers local drainae coUrSe that can influence safety-related to all characteristics of watershed features and facilities, including lie roofs of safety-related buildings. developments that affect runoff characteristics. Existing to assure that local intense precipitation cannot conditions are assumed to exist in the fiture if projects constitule a threat to tile safety of tlie nuclear power are to be operated in a similar manner during the life of plant. the proposed nuclear power plant and watershed runoff b. Probable maxinium precipitation is defined by characteristics are not expected to change due to tile Corps of Engineers and the National Oceanic and development.

Atnmospheric Administrat ion (NOAA) as "thie t liheret ically (2) The term "with project(s)" refers to the greatest depth of precipitation for a given duration that future effects of projects being analyzed, assuming they is nieleorologically possible over the applicable drainage will exist in the future and operate as specified. If area that would produce flood flows of which there is existing projects were not operational during historical virtually no risk of being exceeded. These estimates floods and may be expected to be effective during the usually involve detailed analyses of historical lifetime of the nuce.r, power plant. their effects on flood-producing storms in the general region of the historical floods should be determined as part of the drainage basin under study. arid certain nmodificalions analyses out lined in Sections A.5. A.6. and A.8.

and extrapolations of historical data and reflect more c. Surface and subsurface characteristics that severe rainfall-runoff relations than actually recorded. affecl runoff and streamiflow to a major degree, (e.g..

1.59-6

large swamp areas, noncontributing drainage areas, precipitation measurements are usua~ly distributed, in groundwater flow, and other watershed features of an time, using precipitation recorders. Areal distributions of unusual nature to the extent needed to explain unusual precipitation. for each time increment, are generally characteristics of streamflow). based on a weighting procedure in which tihe incremental d. Topographic features of the watershed and precipitation over a particular drainage area is computed hi-!orical flood profiles or high water marks. particularly as tile sum of tihe corresponding incremental in the vicinity of the nuclear power plant. precipitation for each precipitation gage where cacch e. Stream channel distances hetween river control value is separately weighted by the percL1ntage of the structures, major tributaries, and the plant site. drainage area considered to be represented by the rain f. Data on major storms and resulting floods of gage.

record in the drainage basin. Primary at tcntion should be b. The determination of base flow as the time given to those events having a major bearing on distribution( of the difference between gross runoff arnd hydrologic computations. It is usually necessary to net runoff.

analyze a few major floods of record in order to develop c. Computation of distributed (in time)

such things as unit hydrograph relations, infiltration differences between precipitation and net direct runoff.

indices, base flow relationships, information on flood the difference being considered herein as initial and routing relationships, and flood profiles. lxcept in inflitrafion losses.

unusual cases, climatological data available from the d. The determination of the combined effect of Department of Commerce. The U.S. Army Corps of drainage area. channel characteristics, and reservoirs on Engineers. National Oceanic and Atmospheric the runoff regimen, herein referred to as the "'runoff Administration and other public sources are adequate to model." (Channel and reservoir effects are discussed meet the data requirements for storm precipitation separately in Section A.8.)

histories. The data should include:

(I) Hydrographs of major historical floods for A.5 PRECIPITATION LOSSES AND BASE FLOW

pertinent locations in the basin, where available, from the U.S. Geological Survey or other sources. Determination of the absorption capability of the

(2) St o rmi precipitation records, basin should consider antecedent and initial conditions depth-area-duration data, and any available isohyetal and infiltration during each storm considered.

maps for the most severe local historical storms or floods Antecedent precipitation conditions affect precipitation that will be used to estimate basin hydrological losses and base flow. These assumptions should be characteristics. verified by studies in the region or by detailed storm-runoff studies. Tile fundamental hydrologic A.4 FLOOD HYDROGRAPH ANALYSES factors should be derived by analyzing observed hydrographs of streamflow and related stormis. A

Flood hydrograph analyses and related thorough study is essential to determine basin computations should be used to derive and verify the characteristics and meteorological influences affecting fundamental hydrologic factors of precipitation losses runoff from a specific basin. Additional discussion and (see Section A.5) and the runoff model (see Section procedures for analyses are contained in various A.6). The analyses of observed flood hydrographs' of publications such as Reference 2. The following streamflow and related storm precipitation (Ref. I) use discussion briefly describes the considerations to be basic data and information referred to in Section A.3 taken into account in determining the minimum losses above. The sizes and topographic freatures of the applicable to the PMF:

subbasin drainage areas upstream of the location of a. Experience indicates the capacity of a given soil interest should be used to estimate runoff response for and its cover to absorb rainfall applied continuously at each individual hydrologically similar subbasin utilized an excessive rate may rapidly decrease until a fairly in the total basin runoff model. Subbasin runof' definite minimum rate of infiltration is rcached. usually response characteristics are estimated from historical within a period of a few hours. Infiltration relationships storm precipitation and streamflow records where suchi are defined as direct precipitation losses such that the are available, and by synthetic means where no accumulated difference between incremental streamflow records are available. The analysis of flood precipitation and incremental infiltration equals the hydrographs (Ref. 2) should include the following: volume of net direct runoff. The infiltration loss relationships may include initial conditions directly, or a. Estimates of the intensity, depth, and areal may require separate determinations of initial losses. The distribution of precipitation causing the runoff for each order of decrease in infiltration capacity and the historical storm (and rate of snowmelt. where this is minimum rate attained are primarily dependent upon significant). Time distributions of storm precipitation the vegetative or other cover, the size of soil pores are generally based on recording rainfall gages. Total within the zone of aeration, and the conditions alfecting the rate of removal f"capillary water from the zone of

'Strcamflow hydrographs (of major floods) are available in aeration. The infiltration theory, with certain publications by the US. Geological Survey. National Weather Service, State agencies, and other public Sources. approximations, offers a practical means of estimating

1.59.7

the volume of surface runoll fronm intense rainlfall. A.6 RUNOFF MODEL

However. in applying tile method to natural drainage basins, tile following factors must be considered: The hydrologic response characteristics of the (I) Since the infiltration capacity of a given watershed to precipitation (such as unit hydrographs)

soil at the beginning of a storm is related to antecedent should be determined and verified from historical floods field moisture and the physical condition ofthe soil. the or by conservative synthetic procedures. The model infiltration capacity for the same soil may vary should include consideration of nonlinear runoff appreciably from storm to storm. response due to high rainfall intensities or unexplainable

(.2) The infiltration capacity of' a soil is factors. In conjunction with data and analyses discussed normally highest at the beginning of rainfall, and since above, a runoff model should be developed, where data rainfall frequently begins at relatively moderate rates, a are available, by analytically "reconstituting" historical substantial period of time may elapse before the rainfall floods to substantiate its use for estimating a PMF. The intensity exceeds the prevailing infiltralion capacily. It is raiitfall-runofftlime-areal distribution of historical floods gnerally accepted that a fairly definite quantity of should be used to verify that tile "reconstituted"

waler loss is required to satisfv initial soil moislture hydrographs correspond reasonably well with flood deficiencies before nnoff will occur, the amount of hydrographs actually recorded at selected gaging stations initial loss depending upon antecedent conditions. kRef. 2). In most cases. reconstil ut ion studies should he

(3) Rainfall does not normally cover the entire made with respect to two or more floods and possibly at drainage basin during all periods of* precipitation with two or more key locations, particularly where possible intensities exceeding infillration capacities. Futhermore. errors in the determinations could have a serious impact soils and infiltration capacities vary throughout a on decisions required in the use of* the runoff model for drainage basin. Therefore, a rational application of any the PMF. In some cases, the lack of sufficient time and loss.rate technique must consider varying rainfall areal precipitation definition, or unexplained causes.

intensities in various portions of the basin in order to have not allowed development of' reliable predictive de te rmine tile area covered by effective runoff models, and a conservative PMF model should be runolf-producing rainfall. assured by other means such as conservatively developed b. Initial loss is defined as thie maximnum amount synthetic unit hydrographs. Basin runoff' models for a of precipitation that can occur without producing PMF determination should provide a conservative runoff. Initial loss values may range from a minimum estimate of the runoff that could be expected during the value of a few tenths of an inch during relatively wet life of the nuclear power plant. The basic analyses used seasons to several inches during dry summer and fall in deriving thie runoff model are not rigorous, but may months. Tile initial loss conditions conducive to major be conservatively undertaken by considering the rate of floods usually range from about 0.2 to 0.5 inch and are runoff from a unit rainfall (and snowmelt. if pertincnt)

relatively small in comparison with the flood runoff of some unit duration and specific time-ae.ral volume. Consequently. in estimating loss rates from data distribution (called a unit hydrograph). The applicability for major floods, allowances for initial losses may be of a unit hydrograph. or other technique, for use in estimated approximately without introducing important computing the runoff from an e..'uiiated probable errors in the results. maximum rainfall over a basin may be partially verified c. Base flow is defined herein as that portion of a by reproducing observed major flood hydrographs. An flood hydrograph which represents antecedent runoff estimated unit hydrograph is first applied to estimated condition and that portion of the storm precipitation historical rainfall-excess values to obtain a hypothetical which infiltrates the ground surface and moves either runoff hydrograph for comparison with the observed laterally toward stream channels, or which percolates runoff hydrograph (exclusive of base flow-net ninoff),

into the ground, becomes groundwater, and is discharged and the loss rate, the unit hydrograph. or both. are into stream channels (sometimes referred to as bank subsequently adjusted to provide accurate verification. A

flow). The storm precipitation, reduced by surface study of the runoff response of a large number of basins losses, is then resolved into the two runoff components: for several historical floods in which a variety of valley direct runoff and base flow. Many techniques exist for storage characteristics, basin configurations, estimating thie base flow component. It is generally topographical features, and meteorological conditions assumed that base flow conditions which could exist are represented provides the basis for estimating the during a PMF are conservatively high. the rationale being relative effects of predominating influenm-i for use in that a storm producing relatively high runoff could PMF analyses. In detailed hydrological studies, each of meteorologically occur over most watersheds about a the following procedures may be used to advantage:

week earlier than that capable of producing a PMF. One a. Analysis of rainfall-runoff records for major assumption sometimes made for relatively large basins is storms;

that a flood about half as severe as a PMF can occur b. Computation of synthetic runoff response three to five days earlier. Another method for evaluating models by (I) direct analogy with basins of similar base flow relates historical floods to their corresponding characteristics and/or (2) indirect analogy with a large base flow. The base flow analyies of historical floods. number of other basins through the application of there"fore, may he readily utilized in PMF empirical relationships. In basins for which historical determinations. streamflow and/or storm data are unavailable, synthetic i .59.9

4 techniques are the only known means for estimating estimates are made of tile amount of increase in rainfall hydrologic response characteristics. However, care must quantities that would have resulted if condilions during be taken ito assure that a synthetic model conse.rvatively the actual storm had been as critical as those considered reflects tile runoff response expected froin precipitation probable of occurrence in tile region. Consideralion is as severe as thie estimated PMP. given to the modifications in meteorological conditions that would have been required IOr each of" the record Detailed flood hydrograph analysis techniques and storms to have occurred over the drainage haisin under studies fkor specific basins are available from many study. considering topographical features and locations agencies. Published studies such as those by tile Corps of of the respective areas involved.

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 The physical linimiations in meteorological comparable with that indicated herein. In particular, the mechanisms *orthe maximum depth. time. and space Corps of Engineers have developed analysis techniques distribution of precipitation over a basin are I )

(Rfs. 2, 3) and have accomplished a large number of humidity (precipitable water) in tile air flow over the studies in connection with their water resources watershed. (2) the rate at which wind may carty lhie development activities. humid air into tile basin. :ind (3) tile fraction of tile inflowing atmospheric water vapor that can be Computerized runoff models (Ref. 3) offer an precipitated. Each of these limitations is handled extremely efficient tool for estimating PMF runoff rates differently to estimate tile probable miaximum and for evaluating tihe sensitivity of PMF estimates to precipitation over a basin, and is modified further for possible variations in parameters. Such techniques have regions where topography causes marked orographic been used successfully in making detailed flood control (designated as the orographic model) as opposed estimates. to the general model (with little topographic effect}) 0

precipitation. Further details on the models and Snowmelt may be a substantial runoff component acceptable procedures ate contained in References 5 for both historical floods and the PMF. In cases where it and 6.

is necessary to provide for snowmelt in the runoff a. The PNIP in regions of limited t opographic

. model, additional hydrometeorological parameters must be incorporated. The primary parameters are the depth of assumed existing snowpack. the areal distribution of 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 assumed existing snowpack ( and in basins with distinct changes in elevation, the areal distribution of snowpack them to basins of interest. The increase in rainfall with respect to elevation), the snowpack temperature quantities that might have resulte! from maximizing and density distributions, the moisture content of the meteorological conditions during the rtcord storm and snowpack. the type of soil or rock surface and cover of tile adjustments necessary to transpose the respective the snowpack, the type of soil or rock surface and cover storms to the basin under study should be taken into in different portions of the basin, and the time and account. The maximum storm should represent tli.. most elevation distribution of air temperatures and heat input critical rainfall depth-area-duration relation for the during the storm and subsequent runoff period. particular drainage area during various seasons o" ithe Techniques that have been developed to reconstitute year (Refs. 7. 8. 9, 10). In practice. the parameters historical snowmelt floods may be used in both considered are (I) the representative storm dewpoint historical flood hydrograph analysis and PMF (Ref. 4) adjusted to inflow moisture producing the maximum determinations. dewpoint (precipitable water), (2) seasonal variations in parameters. (3) the temperature contrast. (4) thie A.7 PROBABLE MAXIMUM geographical relocation, and (5) thie depth-area PRECIPITATION ESTIMATES distribution. Examples of these analyses are explained and utilized in a number of published reports (Refs. 7.8.

Probable maximum precipitation (PMP) estimates 9. 10).

are the time and areal precipitation distributions This procedure, supported with an appropriate compatible with the definition of Section A.2 and are analysis. is usually satisfactory where a sufficient based on detailed comprehensive meteorological analyses number of historical intense storms have been of severe storms of record. The analysis uses maximized and transported to the basin and where at precipitation data and synoptic situations of major least one of them contains a convergent wind

"mechanism" very near the maximum that nature can be storms of record in a region surrounding the basin under study in order to determine characteristic combinations expected to produce in the region (which is generally the

. of meteorological conditions that result in various rainfall patterns and depth-area-duration relations. On the basis of an analysis of airmass properties and 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 synoptic situations prevailing during the record storms, adequacy of the storm sample, additional inaximizatioun

1.59-9

• .. .

steps are required in regions of more limiteid storm amenable to generalization for snowinell computations sanmples. (Ref. 14). The meteorological (e.g., wind, temperature, b. PMI1 determinations in regions of orograplhit dewpoints) sequences prior to, during, and after the influences generally are for hlie high mountain regions postulated PMP-producing storm should be compatible that lie in the path of Ithe prevailing moist wind. with the sequential occurrence of the PMIP, The user Additional maximization steps front paragraph A.77.a. should place the PNIP over the basin and adjust the above are required in the use of the orographic model sequence of olher parameters to give the most critical (Refs. 5, 6). The orographic moxlel is developed for the runof flor t(ie season considered.

orographic component of precipitation where severe precipitation is expected it) be caused largely by tire The meteorological parameters for snowniel lifting imparted to fie ait by' mounwains. This orographic comIpu tations associated with PNIP are discussed in more influence gives a basis for a wind model with maximized detail in References II 12, and 14.

inflow. Assuming laminar %low of air over any particular mountain cross section. one can calctlate Ihe liife" of Other items that need to be considered in the air. the levels at which raindrops and snowflakes are determining basin melh are optimntum depth. areal extent.

formed. and their drift with the air before they strike and type of snowpack. and other snowmuell factors (see lhe ground. Such mnodels are verified by reproducing the Section A.8). all of which must he compatible with the precipitation'in observed storms and are then used for most critical arrangement of the PMP and associated estimating PIMP by introducing maximum values of nueiiorological paramneters.

mtoisture and wind as inllow at thie foot of thie mountains. Maximum moisture is evaluated just as in Critical piobable maxiniuni storm estimates for very nonorogiaphic regions. In mnotntainous regions, where large drainage areas are determined as above, but may storms cannot readily be transposed (paragraph A.7.a. differ somewhat in flood-producing storm rainfall from above) because of !heir intimate relation to the those encountered in preparing similar estimates for immnediate tuderlying topography. historical stornits are small basins. As a general rule. the critical PMP in a small resolved into their convective and orographic basin results primarily from extremely intense small-area compnecnts and maximnized as follows: (I) mraximuim storms; whereas in large basins the PMP usually results moisture is assunied. (2) maxinmum winds are assumed. from a series of less intense, large-area storms. In very and finally (3) maximum values of tIle orographic large river basins (about 100,000 square miles or larger)

consponent and convective component (convective as in si.:h as the Ohio and Mississippi River basins, it may be nonorographic areas'l of precipitation are considered to necessary to develop hypothetical PMP storm sequences occur simultanretously. Some of the published reports (one storm period followed by another) and storm that ill ustr:ute the combination of orographic and tracks with an appropriate limte interval between storms.

convective components. including seasonal variation, are The type of meteorological analyses required and typical References II. 12, and 13. examples thereof are contained in References 9, 15, and

16.

In somne large watersheds. major floods ate often the result of melting snowpack or of snownilt combined with rain. Acco:dingly. the probable maxinmum The position of probable maximum rainfall centers.

precipitation (rainfall) and maximunt associated identified by "isolyetal patterns" (lines of constant runoff-producing snowpacks are both estimated on a rainfall depth), may have a very great effect on the seasonal and elevation basis. The probable maximum regimen of runoff from a given volume of rainfall excess.

seasonal snowpack water equivalent should be particularly in large drainage basins in which a wide determined by study of accumulations on local range of basin hydrologic runoff characteristics exist.

watersheds from historical records of the region.

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 Several methods of estimating the upper limit of determine the critical isohyetal pattern that produces ultimnate snowpack and rueling are summarized in the inaxiumtm rate of runoff at thie designated site. This References 4 and 5. The methods have been applied in may be accomplished by superimposing an outline of the Columbia River basin, the Yukon basin in Alaska. the drainage basin (above the site) on the total-storm the tipper Missouri River basin, and the upper Mississippi PMP isohyetal contour map in such a manner as to place in Minnesota and are described in a number of reports of the largest rainfall quantities in a position that would the Corps of Engineers. In many internmediate-latitude result in the maximum flood runoff (see Section A.8 on basins, the greatest flood will likely result from a probable maximuni flood runoff). Thi isohyetal pattern combination of critical snowpack (water equivalent) and should be reasonably consistent with the assumptions PMP. Thie seasonal variation in both optimum snow regarding the meteorological causes of the storm. A -

depth (i.e., the greatest water equivalent inl the considerable range in assumptions regarding rainfall snowpack) and the associated PMP combination should patterns (Ref. 11) and intensity variations can be made be meteorologically compatible. Temperature and winds in developing PMP storm criteria for relatively small associated with PMP are two important snowmelt factors basins, without being inconsistent with meteorological

1.59-10

L

,1 0. causes. Drainage basins less than a tew thousand square The Corps of Engineers arnd the miles in area (particularly if only one unit hydrograph is Hydrometeorological Branch of NOAA (under a available) may be expressed as average depth over tile cooperative arrane tientI since 19)39)) have made drainage area. However. in deoerntining the BilP pattern cor nprchlenrsive inet corological studies of extremno for large drainage basins (with varing basin hydrologic flood-producing storms ( Ref. I ) and have developed a characteristics, including reservoir etfects). runoff ntuimbe r o(f estimates of "probahle maximunm estimates are required for different storm pattern precipilation." The PMP estimates arc presented in locations and orientations to ohtain the final PMF. various unpublished mnemoranda and published reports.

Where historical rainfall patterns are not used for PMP, The series of' published reports is listed on the lyv sheet two other methods are generally employed as follows: of referenced Hydronietcorological Reports such as a. Average depth over the entire basin is based onl Reference I8. The published memoranda reports mtay he the maximized areal distribution of Ihe PMP. obtained from thie Corps of iEngineers or h. A hypothetical isohyclal pattern is assumed. HyJrometeorological Branch. NOAA. These reports and Studies of areal rainfall distribution from intense storms memoranda present pgneral techniques: included among indicate elliptical patterns may be assumed as the reports are several that contain "generalized"

representative of such events. Examples are the typical estimates of PM I' for different river basins. The patterns presented in References 8. 14. 17. and 18. generalized studies (Refs. 7. 12) usually assure reliable and consistent estimates for various locatlions in the To compute a flood hydrograph from the probable region for which they have been developed inasniuch as maximum storm, it is necessary to specify the time they 'are based on coordinated studies of all available sequence of precipitalion in a feasible and critical data. supplemented by thorough meteorological analyses. In sonic cases. however, additional detailed meteorological time sequence. Two meteorological analyses are needed for specific river basins (Refs. 7. 8)

factors must be considered in devising the time to take into account unusually large areas. storm series, sequences: ( I) the time sequence in observed storms and topography, or orientation of drainage basins not fully

(2) the manner of deriving the PMP estimates. The first reflected in the generalized estimates. In many river imposes little limitations: the lhetographs (rainfall time basins available studies may be utilized to obtain the sequences) for observed storms are quite varied. There is PMP without the in-depth analysis herein or in tihe some tendency for the two or three time increments referenced reports.

with thie highest rainfall in a storm to bunch together. as

0 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.

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 An example of 6-hour increments for obtaining a the characteristics determined represent historical critical 24-hour PMP sequence would be that the most conditions (which may be verified by reconstituting severe 6-hour increments should be adjacent to each historical floods) but ;dso that they would conservatively other in time (Ref. 17). In this arrangement the second represent conditions to be expected during a PMF.

highest increment should bc adjacent to the highest. the third highest should be immediately before or after this Channel and reservoir routing methods of many

12-hour sequence. and the fourth highest should be types have been developed to model the progressive before or after the 18-hour sequence. This procedure downstream translation of flood waves. Tihe same may also be used in the distribution of the lesser second theoretical relationships hold for both channel and

(24-48 hours) and third (48-72 hours) 24-hour periods. reservoir routing. However, in the case of flood wave These arrangements are permissible because separate translation through reservoirs, simplified procedures bursts of precipitation could have occurred within each have been developed that are generally not used for

24-hour period (Reference 7). The three 24-hour channel routing because of the inability of such precipitation periods are interchangeable. Other simplified methods to model frictional effects. The arrangements that fulfill the sequential requirements simplified channel routing procedures that have been would be equally reasonable. The hyclograph. or developed have been found useful in modeling historical precipitation time sequence. selected should be the most floods, but particular care must be exercised in using severe reasonably possible that would produce critical such models for severe hypothetical floods such as the runoff at the project location based on tihe general PMF because the coefficients developed from analysis of appraisal of the hydrometeorologic conditions in the historical floods may not conservatively rellect flood project basin. Examples of PMP time sequences fulfilling wave translation for more severe events.

the sequential requirements are illustrated in References I1, 12. and 17. For small areas. maximized local records Most of tihe older procedures were basically should be considered to assure that the PMP time attempts to model unsteady-flow phenomena using sequence selected is severe. simplifying approximations. The evolutiorn of computer

1.59-1 I

itnv olvedt. in flvw. out hllow, and pool elevat ion

- I

use has allowed development ,,ofIanalysis techniques that permit direct solutiontit' basic 'Instead% flow equations hydrographs should be prepared.

mlilizinig ntimerical analysis teclinitques adaptable to the digital comptuter (Ref. I19). In addition. most of' the Many existing and proposed dams and oilier river older techniques have been adapted for computer use control structures may niot be capaible of safely passing (Ref. 3). floods as severe as a PMF. Tile capability of river control structures to safely pass a PMF and local coincident In all rout ing techniques. care must be ,:xercised in wind.generated wave activity must be determined as part assurinig hat1 ijmiramet ers selectLed Jor model verification of' the PM F atnalysis. Where it is poissible that such are based on several hislorical floods (whenever possible) structures imaynitot safely survive Iloods as severe as a and that their applicationl Ith1 PMF will restilt in PM F. tile \vtwrst such conidition withi resipect to conserva.liVe est mates 1 l'h\ ata Cles. water levels. downstream nuclear lpower plants is assuimied (hut should velocities, and ilIpacM torceI . Theoretical discussions of1 be suhtsltanlialed hr analysis ohl lpsl eamn PNIF poi':litiall the many methods availahle for such analyses are to be their failuore during a PMF. and the PM F

contained in Refelences 2. 19).20.- I . mnd 22. detertminatiion should include the resuiltant effects. This analysis:also requires that tihe consequncces otflupsreamii dam failures on downtstreanm damis ( domtino effects) he A.9 PMF HYDROGRAPH ESTIMATES

considered.

PM F net runolf hydrograph estimates are made bh sequentially applying critically located and distributed A.10 SEISMICALLY INDUCED FLOODS

PM P estinmt tes using the runoff timodel. conservatively low%, estimates of prcipitalioti losses, and conservatively S.isinically induced bloods on streams and rivers hilh estimates (1' base Ilow z'nd antecedent reservoir may be caused hr landslides or dain failures. Where river levels. Coitrol structures are widely spaced, their arbitrarily as.suilied indiciduwil total.l instantaneous failure and lit PlMF determinationis it is cenerall v assumed that resul tinig downsttreailmi flotodl wave atltenuation (routing)

short-lerin reservoir flood control storage would be mliar be showII to coTIns6lcite lbi) threat to nuclear depleted by possible antecedent floods. An exception facilities. Where the relative size. location, and proximity would be whet it cat be demonstrated that tile of' dams !o ptentiial seismic generators indicate a threat occurrence oif a measonably seveie flood I say aboolu; to nuclear power plants. tite capability of suIch structures one-h:alf ofl a P1I\) less than a week (usually a tinitnrtni (cither singly or in combination) Ito resist severe oit' 3 to- 5 days prior :ii a lIFM c:nli be evacialetl frotil earthquakes (critically located) shimald he considered. Ili the reservoir helfre tile artival otf a PMVF. However, it is river basins where the flood aunoff season may unusual to use all antecedent storage level less than constitute a significant portion of' the year (such as the one-halftile flood control storage available' Mississippi. Columbia. or Ohio River basins). f'ull flood control reservoirs willi ai 25-year flood is assunied Time applicatiomn (i P\MP in bhasins whose hydrologic coincident with the Safe Shutdown t..artliquake. Also.

features vat fron llcation to location requires the cotnsideration should he given to the occurrence of' a detenriiimatit, that thie estimated PM F hydrograph flood of approximately one-half the severity of a PM F

represents the most critical centering of the PIMP storm with frill flood control reservoirs coincident wi\h the with respect to the site. ('are must be taken in basins maximumi earthquake determined on the basis of'

witlhi substantial headwater flood control storage to historic seismicity ito mainlain a consistent level of assure that maoire highly concentrated PMP over a analysis I'or Other combinations of such events. As with smaller area dowistireant of' the reservoirs would not failures dime to inadequiate flood control capacity, produce a greater PNIF tIan a total basin storm that is domino and essentially simultaneous multiple f'ailures partially controlled. In siich cases more than oCe P['NIP may also require consideration. If the arbitrarily runoff analysis maylhe required. Usually. only a few assumed total failure of the most critically located (from trials oft a total basin l.NI' are required to determine the a hydrolh.:,ic standpoint ) struct ures indicates flood risks at most critical centering. 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 The antecedent snowpack and its contribution to detailed geologic and seisunic investigations. the flood the PNIF are included when it is determined that potential at the nuclear power plant site is next generally snowrnell coilrihntions to thie flood Would produce a evaluated assuming the most probable mechanistic-type PNIF (see Section A.7). However. these typcs of failure of' the quest ioned struci tires. IfItile results of each hypothetical floods are generally the controlling events step of the above analysis cannot be safely only in the far west and northern United States.

acconmnodated at the nuclear power plant site in an acceptable manner, the seismic potential at tile site of Runoff hydrogruphs should be prepared at key hydrologic hlcations (e.g.. strcanigages and dams) as well each questioned structure is then evaluated in detail, the as at the site of mnclear facilities. For all reservoirs structural capability is evaluated in the same depth as for

1.59. 12

° nuclear power plant sites, and the resulting seismically floodplain georrit tv definition as steady-fiowv models.

induced flood is routed to the site of the nuclear power and thelrefore hit li use may allowv more accurate water plant. This last detailed analysis is not generally required surface level t"'caini;ws whiiere ýid'*'-t~i'w since intermediate investigalions usually provide approxinmatlions are inlle. ()n.e such iilwloidV-Iw sufficient conscrvalive inflormiation to allow coriputier 1t1odel is dicused ill Re*('tih. e 11).

determinalion of an adequate design basis flood.

All ieas.omahly i,'cnr:ile wvacr h'ct, c*{irnwilii'u A.11 WATER LEVEL DETERMINATIONS nlrdels reqmuire 11;1,lpl:1 &lfiminitiori l :11c.ts that cat1 inatetialklv affect w* ticl levels. I.ood wa%( t .l;:iriom .

All the preceding discussion has been concerned and c:litihratlini by lv rnr:henirl~ical iecii.,-iwii of primarily with determinations of flow rates. The Ilow hislorical l*d)ts (tit mte ,hcclioit of- c.1iblat:ioi rate or discharge must be converted to water level cocttficiellts based (itl the cil 'itsa,;li'c liallnIerl of elevation for use in design. This may involve information derived torll SAilr 'lildies -I'oilier iv,.r determination of' elevation-discharge relations Ifor natural reaches). Particular c:are should he cxercis-d it, asstiie stream valleys or reservoir conditions. The reservoir that corntrolling tlfomd lc.el est iniates tic tilwvayvs elevation estimates involv,: the spillway discharge conservatively high.

capacity and peak reservoir level likely to be attaiiied during the PMF as governed by the inflow hydrograph. A.12 COINCIDENT WIND-WAVE ACTIVITY

the reservoir level at the beginning of the 'M[:. and the reservoir regulation plan with respect to total releases The superposition tlt \n'd-wave :activitv on I'MF tir while the reservoir is rising to peak stage. Most river seismically induced wael! level dcte rnin ltions is water level deterininations involve the assumption of required to assure that. in 11le event Cilt hr coildit ito did steady, or nonvarying, flow for which standard methods occur, ambient nieteorological activityv would Inot cause are used to estimate flood levels. Where little floodplain a loss of safe ty-related tun t iotn due to wav, act ion.

geometry definition exists, a technique called

"slope-area" may be employed wherein the assumptions The selection of' wind spejeds andtI critical wind are made that the water surface is parallel to the average directions assu.med coincident with mnxiiniini I'MI: or bed slope, any available floodplain geometry seismically i.'duced water levels should provide :t,,n; i rincc information is typical of the river reach under study, and of virtually no risk to safety-reialed equipmientr icces.arnV

no upstream or downstream hydraulic controls affect to plant shutdowvn. The ('orps of' ngineecrs .uqiests the river reach fronting the site under study. Where such (Refs. 26. 27) that average rmaximum %%-itnd siced% of'

computations can be shown to indicate conservatively approximately 40 to (10 inph have occurred in miajor high flood levels, they may be used. However, the usual windstorms in most regions of the United States. For method of estimating water surface profiles for flood application to the safety analysis of nuclear facilities, the conditions that may be characterized as involving worst regional winds of record should le :ssnmned essentially steady flow is a technique called the coincident with the PMF. However. the postuhlted winds Itstandard-step method." This technique utilizes thle should be meteorologically compatible with the i- .grated differential equation of steady fluid motion conditions that induced tire PMF or with tlie flood commonly referred to as the Bernoulli equation conditions assunred coincident with seismically induced (References 22. 23, 24, and 25) where, depending on dam failures) such as the season of tfie year. the ntite whether supercritical or subcritical Rlow is tinder study, required for the PMP storon to 11r0%'e our of the area and water levels in the direction of flow computation are be replaced by meteorological conditions that could determined by the trial and error balance of upstream produce the postulated winds, ard the restrictions on and downstream energy, respectively. Frictional and wind speed and direction produced by topography. As other types of head losses arc usually estimated in detail an alternative to a detailed study of hitorical regional with the use of characteristic loss equations whose winds, a sustained 40-inph overland wind speed t'romr coefficients have been estimated from computational any. critical direction is an acceptable positulation.

reconstitution of historical floods, and from detailed floodplain geometry information. Application of the Wind-generated set up (or wind tide) atd wave

"standard-step method" has been developed into very action (runup and impact torces) may be estimated using sophisticated computerized models such as the one the techniques described in References 26 and 28. Tire described in Reference 23. Theoretical discussions of the method for estimating wave action is based on stutistical techniques involved are presented in References 22, 24, analyses of a wave spectrum. For nuclear power planrts.

and 25. protection against the maximuin wave, defincd in Refernce 28 as tire average of tire upper one percent ofl"

. 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 the waves in the anticipated wave spectrumI , should bIe assumed. Where depths of water ill tronitr0'

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 . In addition, assurance should be provided that

-tripped' and caused to break both before reaching and safety systems ncessary for cold shutdown and on safeiy.related structures, dynamic Irces may. be maintenance thereof are designed to withstand the static estimated from Reference 28. Where waves may induce and dynamic effects resulting from frequent flood levels surging in intake structure sumps. pressures on walls and coincident with the waves that would be produced by the underside of' exposed floors should be considered, the nmaximumn gradient wind for the site (based on a particularly where such sumps are not vented and air study of historical regional meteorology).

Colmpression call greatly increase dynamic forces.

1.59.14 I

V

6 4 REFERENCES

I. Precipitation station data and unpublished records 9. "Meteorology of Flood Producing Storms in the of Federal, State, municipal, and other agencies may Ohio River Basin," Hydronieteorological Report be obtained from the U.S. Weather Bureau (now No. 38. U.S. Weather Bureau (now NOAA). 196L.

called National Weather Service). In addition, studies of some large storms are available in the 10. "Probable Maximum and TVA Precipitation Over

"Storm Rainfall in the Un it ed States. the Tennessee River Basin Above Chltllanooea."

Depth.Area-Duration Data." summaries published Hydrometeorological Report No. 43, U.S. Weather by Corps of Engineers, U.S. Army. Bureau (now NOAA), 1965.

2. Corps of Engineers publications, such as EM 11. "Interim Report- -Probable Maximum Precipitation

1110-2-1405 dated 31 August 1959 and entitled, in California." Hydrometeorological Report No. 36.

"Engineering and Design-Flood Hydrograph U.S. Weather Bureau (now NOAA). 1961.

Analyses and Computations." provide excellent criteria for the necessary flood hydrograph analyses. 12. "Probable Maximuni Precipitation, Northwest (Copies are for sale by Superintendent of States," Hydrometeorological Report No. 43. U.S.

Documents. U.S. Government Printing Office, Weather Bureau (now NOAA), 1966.

Washington, D.C. 20402.) Isohyetal patterns and related precipitation data are in the files of the 13. "Probable Maximum Precipitation in the Hawaiian Chief of Engineering, Corps of Engineers. Islands," Hydrometeorological Report No. 39. U.S.

Weather Bureau (now NOAA). 19)63.

3. Two computerized models arc "Flood Hydrograph Package. HEC-I Generalized Computer Program," 14. "Meteorological Conditions for the Probable available from the Corps of Engineers Hydrologic Maximum Flood on the Yukon River Above Engineering Center, Sacramento, California, dated Rampart, Alaska," Hydronieteorological Report No.

October 1970 and "Hydrocomp Simulation 42, U.S. Weather Bureau (now NOAA), 1966.

Programming-HSP," Hydrocomp Intl.. Stanford, Calif. 15. "Meteorology of Flood-Producing Storms in the Mississippi River Basin." Hydrometeorological

4. One technique for the analysis of snowmelt is Report No. 34, U.S. Weather Bureau (now NOAA).

contained in Corps of Engineers EM 1100-2.406, 1965.

"Engineering and Design-Runoff From Snowmelt,"

January 5, 1960. Included in this reference is also 16. "Meteorology of Hypothetical Flood Sequences in an explanation of the derivation of probable the Mississippi River Basin," Hydrometeorological maximum and standard project snowmelt floods. Report No. 35, U.S. Weather Bureau (now NOAA),

1959.

5. "Technical Note No. 98-Estimation of Maximum Floods," WMO-No. 233.TP.126, World 17. "Engineering and Design-Standard Project Flood Meteorological Organization, United Nations, 1969 Determinations," Corps of Engineers EM

and "Manual for Depth-Area-Duration Analysis of 1110.2-1411, March 1965, originally published as Storm Precipitation," WMO-No. 237.TP.129, World Civil Engineer Bulletin No. 52-8.26 March 1952.

Meteorological Organization, United Nations, 1969.

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

6. "Meteorological Estimation of Extreme Minnesota," Hydrometeorological Report No. 44.

Precipitation for Spillway Design Floods", Tech. U.S. Weather Bureau (now NOAA). 1961).

Memo WBTM HYDRO-5. U.S. Weather Bureau (now NOAA) Office of Hydrology. 1967. 19. "Unsteady Flow Simulation in Rivers and Reservoirs," by J. M. Garrison. J. P. Granju and J.

7. "Seasonal Variation of the Probable Maximum T. Price. pp 1559-1576, Vol. 95. No. IIYS,

Precipitation East of the 105th Meridian for Areas (September 1969), Journal of the Ilyt'draulics from 10 to 1,000 Square Miles and Durations of 6, Division. ASCE. (paper 6771).

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. 20. "Handbook of Applied Hydrology." edited by Ven Te Chou, McGraw.Hill. 9)64. Chapter 25.

8. "Probable Maximum Precipitation. Susquehanna River Drainage Above Harrisburg, Pa., 21. "Routing of Floods Through River Channels." EM

"Hydrometeorological Report No. 40. U.S. Weather H 10-2-1408. U.S. Army Corps of Engineers. I

Bureau (now NOAA), 1965. March 1960.

1.59-15

.2. "'l~nLiti .'riig 1 yvdiauilics". e.'dited hy Hlu tier Rouse. 2o. "Compiitation of Freeboard Allowances,fr Waves John WViley & Sons. l1tc. 19Q50. in Reservoirs." I-ngineca Technic;al Leiter lTL

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

s. I Augist

1960.*

.. 1eW

c Sil face Plroilies. HI.I-2 Genraliued Co nipmiaUt Program.' available from( tie Corps of

1:-ni neers Hydrologic Engineering Center. 27. "Policies a nd Proceedures PerIaining to Sacrameilnito. C:ail. D)etermination of Spillway ('apaci ties anid Frecehoard Allowances for D)ams.'" lingincer Circular 1-C

_'4. "()pen Chalnel Ilydratlic'" by Ven Te Choli; 1110-2-27. LU.S. Arwy Corps or Engineer

s. I August

28. "iShore Protect iot. !Il~amini*g and I)esign, Tedhnicil

"lack%:%tlctr

-j (Cirves in River (Channels." EM

II1 40-).I4. U.S. Ariny Corps of Elpgineeis. Relp)rt No. 4. U.S. Arauy "Coastal Elngineering Dc),. a',:.cr "7. I*)g! Research Cenler. 3rd edition. I906.

1.59-16