THE REMNANTS OF HURRICANE IDA: A CASE STUDY OF THE USE OF UNIT HYDROGRAPH MODELING FOR PUNCTUATED RAINFALL EVENTS IN A FLOOD-PRONE APPALACHIAN WATERSHED IN WESTERN PENNSYLVANIA

The remnants of Hurricane Ida began dropping rain in western Pennsylvania on August 31, 2021 after achieving landfall in Louisiana on August 29 as a damaging category 4 hurricane. As the storm tracked inland to the northeast, post-landfall weakening was low (Zhu et al., 2022). Rainfall totals in Appalachian regions of Virginia and West Virginia still realized rain depths of 2.5 to 7.5 cm on August 31. On September 1 the tropical system encountered a cold front, triggering historic rains (12.5 - 25.0 cm), tornadoes, flooding, and over 50 deaths along the main track of the storm in the northeast.

Western Pennsylvania, closer to the periphery of the storm, generally received rains of less than 12.5 cm. This included the Little Conemaugh watershed in Cambria County, source of multiple floods in Johnstown. The catchment of 492 square km is rugged, forested, and flashy with clay/loam soils. This basin was under study when initial Ida remnants dropped 2.3 cm of rain on August 31. September 1 saw 8.1 cm in 12 hours, for a 24-hour total of 10.4 cm. Return periods were roughly 20 years and 10 years for 24- and 12-hour totals, respectively.

A unit hydrograph (UH) model already calculated for the basin using stage IV precipitation and stream gage data (from small, well-behaved storms in 2020) was applied to the Nexrad-based rain data from September 1. Using a priori abstractions (1.76 mm/hr) and elevated baseflow scenario, modeled peak discharge was 412 cumec, compared to 345 cumec estimated at gage. This placed estimated peak stage within 6% of measured, showing possible efficacy of calibrated UH modeling of high-amplitude events despite noted difficulties in flashy basins. Such modeling may, thus, also facilitate reliable return period estimates of discharges in the basin.