Review of Probable Maximum Precipitation Procedures and Databases Used to Develop Hydrometeorological Reports (NUREG/CR-7131)

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Publication Information

Manuscript Completed: November 2018
Date Published: February 2020

Prepared by:
John F. England, Jr., Victoria L. Sankovich, R. Jason Caldwell

U.S. Department of the Interior
Bureau of Reclamation
Technical Service Center
Water and Environmental Resources Division
Flood Hydrology and Emergency Management Group
Denver, Colorado 80225

Elena Yegorova, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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This report presents a review of existing Probable Maximum Precipitation (PMP) methodologies, data bases, and estimates within the United States. The major focus is on a review of generalized PMP estimates in the southeastern United States, in order to subsequently assess the adequacy of existing PMP estimates and the need for potentially updating the PMP estimates in this region. The main objectives of the review were to: (1) review PMP procedures and databases used to develop Hydrometeorological Reports (HMRs); (2) examine storm databases in the Southeast and document the evolution in PMP methodologies and estimates over time; and (3); summarize extreme storm research and PMP work done since HMR 51 was published, that is germane to PMP estimates in the Southeast. Most of the review was limited to describing existing, generalized PMP reports (HMRs) and existing data related to those reports, including HMRs 51-53, and HMR 55A. Subsequent HMRs were reviewed for their treatment of PMP in orographic regions, and to summarize changes in methodologies. Comparisons were made between storm data and procedures used in HMR 33 and HMR 51 for the southeastern U.S. case study region. Studies related to PMP and recent research on extreme storm estimation were briefly reviewed and summarized.

Essentially, PMP methods as applied in the HMRs, are static and can be updated. A brief review of recent literature indicated some key areas for improvement. Research in several areas (e.g., incorporation of WSR-88D radar data into a PMP storm catalog; and studies for scientific understanding of extreme rainfalls, storm rainfall studies and extreme rainfall probabilities using numerical weather prediction models) has been completed by other organizations. Radar rainfall estimates can be very useful in extreme storm processing and PMP estimation. Several investigators have utilized advanced, 3-dimensional atmospheric models to replicate observed extreme storms, simulate them, and investigate the precipitation and other ingredients for extreme rainfalls and floods. Thus improvements to PMP and extreme storm estimation practice can be made by utilizing results of these investigations and associated methodologies. Most of this research has yet to be assimilated into operational estimates of PMP.

There are readily-available probabilistic alternatives to PMP for assessments and designs of critical infrastructure. Several methods, such as regional precipitation frequency with L-Moments, have been utilized in probabilistic assessments of major infrastructure. These methods should be considered, along with improvements on extreme storm rainfall estimation in lieu of or including PMP.

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