CS2B - Concurrent Session 02B: Understanding Tropical Storm Risk in the Appalachian Mountains – An Assessment of Tropical Storm Areal Reduction Factors

Abstract Description: This presentation focuses on the development of watershed precipitation-frequency (PF) relationships and the assessment of areal reduction factors (ARFs) for Tropical Storms and Remnants (TSR) with storm tracks from the Atlantic Ocean and Gulf of Mexico. ARFs were computed for three watersheds within the Tennessee Valley tributary to the South Holston, Douglas, and Fort Loudoun dams. These watersheds span a range of sizes and drain the western slope of the Appalachian Divide where TSRs are one of the primary flood-producing mechanisms, making them ideal for assessing the spatial and statistical characteristics of extreme TSR rainfall events. The goal is to improve the understanding of watershed-scale TSR spatial and temporal storm patterns and ARFs, which are critical for hydrologic modeling and flood risk assessment.

The spatial distribution of TSR-driven rainfall was analyzed for historical storms, including the recent Hurricane Helene, using nearly 1,000 rain gauges in the region. This spatial information was then combined with previously derived point PF magnitudes to generate millions of TSR events in each watershed using a stochastic storm generation approach. The approach yielded watershed-specific PF relationships and uncertainty bounds that consider uncertainties in the L-moments for point PF, the regional probability distribution, and the spatial distribution of TSR rainfall.

Study findings demonstrated that ARFs for watershed PF relationships are not fixed values but vary with annual exceedance probability and watershed size. The results are compared to prior regional analyses and ARFs for other storm types – specifically exploring the assumption that TSRs and extra-tropical cyclones have spatial similarities resulting in equivalent ARFs, and the findings help.

The outcomes of this study provide new insights into the behavior of TSR-driven extreme precipitation to further the understanding of tropical storm risk in the Appalachian Mountains. This will enhance hydrologic risk assessments and support the development of more resilient infrastructure in the region.