Application of Radar-Rainfall Estimates to Probable Maximum Precipitation in the Carolinas (NUREG/CR-7132)

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Publication Information

Manuscript Completed: June 2021
Date Published: June 2022

Prepared by:
R. J. Caldwell, J. England, Jr., V. Sankovich

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

NRC Job Code N6570

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

Availability Notice

Abstract

Probable Maximum Precipitation (PMP) is a widely-used concept in the design and assessment of critical infrastructure such as dams and nuclear facilities. In the Southeastern United States, PMP estimates are from Hydrometeorological Report 51 (HMR 51). The database of extreme storms used in HMR51 was last updated in the late 1970s. This study focuses on warm-season tropical cyclones in the Carolinas region of the Southeast United States, as these systems are the critical maximum rainfall mechanisms that result in extreme floods. We investigate ten tropical cyclones that impacted the Carolinas during the period 1996-2007. The major focus is to identify if these recent storms challenge the PMP values from HMR 51, in order to assess the adequacy of existing PMP estimates and the need for potentially updating the PMP estimates in a North Carolina-South Carolina pilot region.

The availability of modern, gridded datasets and increasing computing power provide the impetus to improve on existing PMP methods. We utilize the Multisensor Precipitation Reanalysis (MPR) dataset from NCDC, that covers a test region of the Carolinas for the period 1996-2007 and is available at high spatial and temporal resolution. During the period 1996-2007, many tropical cyclones impacted the two states, including Hurricane Floyd in 1999 and seven different storms in 2004, among others. Depth-Area Duration (DAD) calculations and in place storm maximization were performed for ten recent storms. Maximization of these storms also employed modern gridded datasets of moisture-related variables. Transposition, orographics, and envelopment were excluded from the analysis, in order to show in place impacts of new extreme storm data on existing PMP design estimates. Maximized DAD values from the new storms were compared with HMR 51 PMP and three of the largest events that are the basis for HMR 51.

The results suggest that Hurricanes Floyd (1999) and Fran (1996) approach or exceeded HMR 51 PMP at larger area sizes. Hurricane Floyd exceeded the PMP at durations of 24 and 72 hours, while Fran exceeded PMP at a 6-hour duration. The results of the current study should be considered preliminary but suggest an increase in HMR51 PMP estimates for large area sizes may be warranted along the Carolina coasts, based on in place maximization of Floyd and Fran over the Carolinas. The research also provides insight into the sensitivity of the method to several factors, including representative storm moisture, radar biases and grids, and precipitable water. Long-term trends in moisture availability were also investigated using Sea Surface Temperatures (SST) and dewpoints T_d as proxies. Under the pretense of climate change and variability, the potential exists for storm moisture availability and long-term moisture climatologies used in storm maximization factors to increase or change over time. In general, limited significant trends in SSTs were identified along the East Coast or in the Gulf of Mexico. Based on the initial analysis conducted as part of this pilot study, if current SST trends continue, there will likely be little impact on in place moisture maximization factors and PMP. Future work in the Carolinas should consider a focus on the development of methodologies for transposing storms and adjusting these storms based on orographics, and improving the methods developed and used in this project.