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© 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
2© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
3© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
5© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
6© 2017 Electric Power Research Institute, Inc. All rights reserved.
Seasonality Data for Inland Site
7© 2017 Electric Power Research Institute, Inc. All rights reserved.
Numerical Storm Dates for Inland Site
8© 2017 Electric Power Research Institute, Inc. All rights reserved.
Seasonality Data for Coastal Site
9© 2017 Electric Power Research Institute, Inc. All rights reserved.
Numerical Storm Dates for Coastal Site
10© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
11© 2017 Electric Power Research Institute, Inc. All rights reserved.
L-Moment Ratio Diagram for Inland Site
12© 2017 Electric Power Research Institute, Inc. All rights reserved.
L-Moment Ratio Diagram for Coastal Site
13© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
14© 2017 Electric Power Research Institute, Inc. All rights reserved.
EIRL for Inland Site
15© 2017 Electric Power Research Institute, Inc. All rights reserved.
EIRL for Coastal Site
16© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
18© 2017 Electric Power Research Institute, Inc. All rights reserved.
Precipitation-Frequency Relationships For Coastal Site
PMP – Probable Maximum Precipitation
19© 2017 Electric Power Research Institute, Inc. All rights reserved.
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
20© 2017 Electric Power Research Institute, Inc. All rights reserved.
Together…Shaping the Future of Electricity
John E. WeglianSenior Technical [email protected]