MULTI-AGENCY FEDERAL WORKSHOP 

Probabilistic Flood Hazard Assessment

January 29-31, 2013

NRC, Rockville, MD

Combined Events in External Flooding Evaluation

for

Nuclear Power Plants

Kit Ng

Bechtel Power Corporation

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• Combined Events as part of Design Basis Flood 

Determination

• New Plants – Early Site Permit/Combined Operating License

• Operating Plants Flood Reevaluation – Fukushima Response

• Deterministic Approach

• Regulatory and Industry Guidance

• Combined Events Criteria

• Considerations for Probabilistic/Risk-Informed Approach

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Combined Events in External Flooding Evaluation

for Nuclear Power Plants
• ANSI/ANS 2.8-1992 “Determining Design Basis Flooding at 

Power Reactor Sites”

• NUREG/CR-7046 “Design-Basis Flood Estimation for Site 

Characterization at Nuclear Power Plants in the United 

States of America”

• IAEA Safety Standard SSG-18 “Meteorological and 

Hydrological Hazards in Site Evaluation for Nuclear 

Installations”

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Combined Events in External Flooding Evaluation

for Nuclear Plants
ANSI/ANS 2.8-1992

• No single flood-causing event is adequate as a design flood 

base for power reactors. 

• Dependency - In any combination of flood-causing events, 

distinction between dependent and independent events is not 

sharp. 

• Time sequential meteorological events, for example, are only 

partially and not invariably dependent and their magnitudes 

are only partially and not invariably dependent also. Seismic 

and meteorological events are independent. [Combined 

events can be independent, dependent or causal]

• Acceptable Probability – less than 1 x 10-6 is an acceptable goal

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Combined Events in External Flooding Evaluation

for Nuclear Plants
NUREG/CR-7046

• Because of their extreme nature, probable maximum events 

from two separate phenomena should not be combined unless 

they are clearly dependent or result from a common cause. 

• For example, seismic events should not be combined with 

precipitation events. 

• Exception: PMF and PMH for relatively small drainage basins 

in regions where the PMP may result from a hypothetical and 

maximized hurricane event. 

• Wind waves effects are almost always combined with other 

flood-causing mechanisms. 

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Combined Events in External Flooding Evaluation

for Nuclear Plants
Combined Events in External Flooding

Screening of Flooding Mechanisms

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Flooding 

Mechanism

Site Location

Coastal + 

Riverine Coastal Riverine Others

Local Intense 

Precipitation X X X X

River and Stream 

Flooding X NA X NA

Dam Failure X (upstream 

dams) NA X (upstream dams) NA

Storm Surge X X NA NA

Seiche X X X (onsite pond) X (onsite pond)

Tsunami X X X (landslide 

potential) NA

Ice C C C C

Wind Wave 

Actions X X X X

NA = Not Applicable; C=Cold Region
Combined Events Criteria Applicable to 

Open Coast/Riverine Sites

• Precipitation Flooding 

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Event Combination Alternative I Alternative II Alternative III

Primary Event PMP Probable 

Maximum 

Snowpack

Snow Season 

PMP

Antecedent or 

Subsequent Event

Lesser of 40% 

PMP or 500-

year Rain 

Coincidental

Event

100-year 

Snow Season Rain

100-year 

Snowpack

Base Flow Mean Monthly Mean Monthly Mean Monthly

Wind Wave* 2-year wind 2-year wind 2-year wind

* In critical direction
Combined Events Criteria Applicable to 

Open Coast/Riverine Sites

• Storm Surge Flooding

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Event 

Combination

Alternative

I

Alternative 

II

Alternative 

III

Alternative 

IV*

Primary 

Event

Surge & Seiche

from worst

regional hurricane

or windstorm

25-year 

Surge and 

Seiche

Probable

Maximum 

Surge and 

Seiche

Surge from 

PMH

Coincidental

Event

Lesser of 1/2 PMF 

or 500-year flood

PMF 25-year 

Flood

PMF

Antecedent

Sea Level**

10% 

high tide

10% 

high tide

10% 

high tide

10% 

high tide

Wind Wave Attendant Attendant Attendant Attendant

* in hurricane area and Drainage Area <300 mi2

** include sea level rise
Combined Events Criteria Applicable to 

Open Coast/Riverine Sites

• Seismic Dam Failure Flooding 

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Event 

Combination

Alternative I Alternative II

Seismic Event* SSE coincidental with 

flood peak

OBE coincidental with 

flood peak

Coincidental Event 25-year Flood Lesser of 1⁄2 PMF or 500-

year Flood 

Wind Wave 2-year wind speed in 

critical direction

2-year wind speed in 

critical direction

*Seismic SSE and OBE events in deterministic analysis vs exceedance probabilities from 

PSHA
Combined Events Criteria Applicable to 

Open Coast/Riverine Sites

• Tsunami Flooding

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Event 

Combination

Shore 

Location

Streamside (Alternatives I, II, III, IV)

Tsunami PMT Runup I Runup from PMT Bore

II Runup from Worst Observed Tsunami 

 Bore

III Runup from SSE Tsunami Bore 

IV Runup from OBE Tsunami Bore 

Antecedent 

or Concurrent

10%

High Tide

I 25-year Flood

II PMF

III Mean Monthly Baseflow + SSE Dam Break

 Flood

IV 25-year Flood + OBE Dam Break Flood

Wind Wave I – IV 2-year wind speed in critical direction
Combined Events Criteria Applicable to 

Open Coast/Riverine Sites

• Other Site Specific Combined Events

Potential Breach of Large Onsite Basin due to PMSS, PMF 

or Dam Break Flood Wave

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Combined Events in External Flooding

Deterministic Approach

• PMF

– PMP from NOAA HMRs

– HEC-HMS Rainfall-Runoff Model; HEC-RAS for Flood Level 

Routing

– Unit Hydrographs calibrated with 2 to 4 Largest Historical 

Storms

– Peaking and Reduced Lag of Unit Hydrographs

– Maximize Precipitation Depth over Basin

– Back to Back Storms (40% PMP or 500-year Rain) with 3 to 5 days Interval: Zero Initial Loss, Bankfull

– Sensitivity on Surface Roughness, Rainfall Loss Rate

– Basin Areas from 300 sq. miles to 20,000 miles

– 2-year wind – setup and wave runup

– Gridded runoff model considered

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Combined Events in External Flooding

Deterministic Approach

• Dam Break Flooding

– 1⁄2 PMP from NOAA HMRs; 25-year or 500-year rain from NOAA Atlas or TP

– Breach Models (Wahl, USBR, Froehlich, FERC) 

– HEC-RAS for Flood Wave Routing

– Sensitivity – Surface Roughness

– Upstream Dams - 1 to over 60

– Onsite Cooling Ponds

– Basin Areas up to 20,000 sq. miles

– 2-year wind – setup and wave runup

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Combined Events in External Flooding

Deterministic Approach

• Storm Surge Flooding

– PMH parameters from NOAA NWS 23 

– Maximize Surge Level by varying hurricane tracks

– 10% Exceedance High Tide from NOAA tide data

– Sea Level Rise included – 100-year projection

– SLOSH or ADCIRC 

– Sensitivity – Forward Speed, Radius of Maximum Wind

– Attendance wind-wave runup

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Combined Events in External Flooding

Deterministic Approach

• Tsunami Flooding

– PMT Seismic or Submarine Landslides

– Literature Review for Source Identification and 

Characterization

– Establish Conservative Initial Wave Form

– 10% Exceedance High Tide from NOAA tide data

– Sea Level Rise included – 100-year projection

– MOST (East Break Slump); FUNWAVE and NHWave (Cape Fear, Florida Escarpment and Great Bahamas Bank), Delft3D 

(1755 Lisbon)

– Sensitivity – Initial Wave Height and Shape, Surface Roughness in Inundation Phase

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Combined Events in External Flooding

Probabilistic Approach - Considerations

• Individual or Combined Hazard Curve

• Dependence among Combining Events 

• Associated Hazards besides Flooding Levels

– Flooding Duration, Time of Arrival

– Hydrostatic and Hydrodynamic Forces

– Debris, Scouring, Waterborne Missiles

• NUREG/CR-7046 “individual, worst-case flood hazards may 

result from different flooding mechanisms. Therefore, it may 

not be appropriate to focus on a single “design-basis flood;” 

rather, the design should consider the worst-case hazards 

resulting from all appropriate combined-effects floods that are 

relevant..”

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Combined Events in External Flooding

Probabilistic Approach - Considerations

• Acceptance Criteria: 1x10-4 – 1x10-6 (?)

• Combine deterministic and probabilistic approaches?

• Improvement on the uncertainties

• How will the quantified uncertainties be incorporated into 

Risk Assessment?

• New Guidance and Standard (ANS 2.31, ANS 2.8,?)

• Level of Effort

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