GS2A: Analysis of Historic Rainfall from Hurricane Helene-Does This Change PMP?

Abstract Description: Widespread heavy rain caused by a combination of the remnants of Hurricane Helene and the highly variable topography produced historic rainfall and catastrophic flooding over and near the southern Appalachian Mountains. Portions of western North Carolina bore the brunt of this storm’s fury, causing extensive damage to infrastructure and many fatalities. Many areas are still recovering a year later.

Applied Weather Associates (AWA) used its Storm Precipitation Analysis System (SPAS) to quantify the rainfall accumulation in across region. This storm's precipitation rivaled and, in some cases, exceeded the largest observed events in the region, including the July 1916 Alta Pass storm. SPAS outputs show average precipitation depths over a 10 square mile area greater than 30 inches, and over a 100 square mile area greater than 25 inches for both 48-hour and 72-hour durations.

This information has been evaluated against the numerous Probable Maximum Precipitation (PMP) studies AWA has completed in the region, including the Tennessee Valley Authority and several statewide and FERC studies. This information was compared to PMP depths which were developed without the storm included to see if the storm exceeded PMP development for dam design and evaluations. Many interesting findings resulted. It was often the largest tropical storm rainfall event in the region at small area sizes and for longer durations (48- and 72-hours), but other storm types exceeded the Helene rainfall for these area sizes and durations. This demonstrated the rarity of the storm but also showed that it did not exceed the PMP envelopment for the study location analyzed. This is important because it demonstrates the utility of the storm-based PMP process and the need to evaluate PMP for all relevant storm types.

This presentation will detail the importance of capturing the impacts of this type of rare event to help inform detailed reconstruction of extreme rainfall for dam safety and design. This adds another significant storm to the AWA’s extensive storm database of nearly 1000 extreme storm events that are used in PMP development. And most importantly, shows again that these storms have happened before and will happen again, and the dam safety community therefore needs to be properly prepared for these types of rare events.