CS2C - Concurrent Session 02C: New Probable Maximum Precipitation (PMP) for Dam Safety – the case of North Carolina

Abstract Description: This study presents new Statewide PMP (Probable Maximum Precipitation) for North Carolina and adjacent regions that incorporated recent storms including Hurricane Helene. The study, which will improve Dam Safety practices in NC, was managed by NC Department of Environmental Quality (NCDEQ) Dam Safety Section with the technical analysis completed by Applied Weather Associates (AWA), and is a result of Collaborative State and Federal Efforts, in which the United States Department of Housing and Urban Development (HUD) awarded a Community Development Block Grant Program (CDBGP) funds to NC State.

This vital study provides updated PMP depths from 1-hr to 72-hr durations, which supersede those in Hydrometeorological Reports (HMRs) 51, 52, and 56 as well as new Temporal Distributions for hydrology modeling. Moreover, Annual Exceedance Probability (AEP) analyses for 18 different return periods for 6-hr & 24-hr durations are provided.

A PMP Evaluation Tool designed to run using ArcGIS Pro is developed to provide gridded PMP output for a user-designated basin and provides all outputs needed for modeling. Local Storms (typically covering narrower area, 1-hr to 24-hr durations), General, and Tropical Storms (both covering larger area, 1-hr to 72-hr durations) were analyzed.

The HMR51 which applied PMP as one storm type with minimum duration of 6-hr, and minimum watershed area size of 10 sq. miles has so far been widely used in NC. However, over 70% of the dams in NC have watershed area sizes of less than 1 sq. mile and over 90% have less than 5 sq. miles. For most dams with such small watersheds much shorter duration PMPs (less than 6-hr) are critical. Results of PMP depths when compared to HMR51 show a range of reductions in western and central NC and increases in eastern NC.

The Tool has been tested and used to produce PMP values for several specific sites in NC, revealing the difference in Tropical Storm PMPs pre- and post- Hurricane Helene (which caused significant damages particularly in the Asheville area of NC). HEC-HMS models run on a few of the sites using the new PMPs and new Temporal Distributions also reveal that the shorter duration 2-hr PMP is more critical for dams with relatively small watersheds. The results of this study will be necessary information for any water infrastructure, such as dams, culverts, and drainage networks to ensure safety and functionality during extreme rainfall events.