© 2017 Electric Power Research Institute, Inc. All rights reserved.

John E. WeglianSenior Technical Leader

NRC External Flooding Research Workshop1/24/2017

Local Intense Precipitation

Frequency Studies

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EPRI Local Precipitation-Frequency Studies

EPRI published a report on assessing the precipitation-frequency curve for an inland and coastal site– EPRI ID 3002004400, Local Precipitation-Frequency Studies:

Development of 1-Hour/1-Square Mile Precipitation-Frequency Relationships for Two Example Nuclear Power Plant Sites

– Publically released

Analysis used two-hour rainfall data to capture extreme events that may span between two separate hourly data collection intervalsExtended the data by using independent measurements of

precipitation from independent gauges that were applicable to the site

PMP: “Probable” Maximum Precipitation

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Limits to Statistical Extrapolation

Nathan and Weinmann, 2001 – Estimation of Large to Extreme Floods: Books VI

100 2000 106104 105

Large Rare Extreme

Floo

d P

eak

Recurrence Interval

Cre

dibl

e Li

mit

of E

xtra

pola

tion

Statistical Regression &

Observed Data Calibration

Extrapolation

Regional/Paleo Data Supplement

Simulation

Model Development and Parameter Variability

4© 2017 Electric Power Research Institute, Inc. All rights reserved.

NOAA Atlas 14 Precipitation Measurement Locations for Two Example Sites

Inland Site Coastal Site

Inland Site Example Utilized:- 116 Stations- 5,349 Station-Years of Record

Coastal Site Example Utilized:- 35 Stations- 1,635 Station-Years of Record

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Seasonality

Seasonality data can be used for modeling rainfall runoffProvides an estimate of soil-moisture conditionsProbability of a storm where precipitation annual maxima

exceeds the 20-year recurrence interval (inland site) or 10-year recurrence interval (coastal site) threshold is plotted vs the numerical date (e.g., July 20 = 7.65)The data is well described with a near-normal distribution

both sites

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Seasonality Data for Inland Site

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Numerical Storm Dates for Inland Site

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Seasonality Data for Coastal Site

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Numerical Storm Dates for Coastal Site

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Regional Probability Distributions

Regional L-moments were computed for annual maxima dataL-moment ratio diagrams were created with L-skewness and

L-kurtosis pairsGeneral Extreme Value (GEV) distribution was identified as

a suitable three-parameter probability distributionThe four-parameter Kappa distribution was selected to

describe the annual maxima data– Very flexible and capable of emulating GEV distribution– Allows for possibility of emulating other distributions in an uncertainty

analysis

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L-Moment Ratio Diagram for Inland Site

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L-Moment Ratio Diagram for Coastal Site

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Equivalent Independent Record Length (ERIL)

Applicable rain gauges in the vicinity of the site can be used if they measure independent storm events If the storms of interest have large areal coverage (relative to

the gauge density), the ERIL will be small– Significant correlation between gauges would reduce the independent

measurements

If the storms of interest have small areal coverage, the ERIL will be largeLIP analyses show that the storms of interest are small

relative to the density of gauge stations

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EIRL for Inland Site

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EIRL for Coastal Site

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Assessment of Uncertainty

Uncertainty was modeled considering the regional probability distribution to be a 3-parameter distribution represented by a form of the 4-parameter Kappa distribution with a fixed value of the 2nd shape parameter (h) In the uncertainty analysis, the parameter h was allowed to

vary around the regional h valueThis method preserves the correlation between L-kurtosis

and L-skewness and provides for variability in the shape parameter, hLatin-hypercube sampling used to select 1000 sample sets

17© 2017 Electric Power Research Institute, Inc. All rights reserved.

Precipitation-Frequency Relationships For Inland Site

PMP – Probable Maximum Precipitation

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Precipitation-Frequency Relationships For Coastal Site

PMP – Probable Maximum Precipitation

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Local Precipitation Summary Statistics

Description Inland Site Coastal Site

PMP for the Site 17.9 “ 18.4”

World Meteorological Organization Record Short Duration (1-Hour or Less) Rainfalls 12.0” – 15.8”

Study Area 1-Hour Maximum Rainfall 3.5” 4.0”

Study Area 2-Hour Maximum Rainfall 5.5” 6.5”

PMP AEP Estimated by Extending Mean Precipitation-Frequency Relationship <10-9 <10-9

10-6 AEP Precipitation Using Mean Precipitation-Frequency Relationship 8.9” 8.5”

AEP – Annual Exceedance Probability

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Together…Shaping the Future of Electricity

John E. WeglianSenior Technical Leaderjweglian@epri.com704-595-2763