Flash Flooding in the Philadelphia Metropolitan Region

Abstract

Hydrometeorological analyses of rainfall and flood response are presented for urbanizing drainage basins in the Philadelphia metropolitan region. Research focuses on major flood events resulting from Tropical Storm Allison (June 16 and 17, 2001), Hurricane Floyd (September 16 and 17, 1999), an extratropical cyclone on October 19 and 20, 1996, and a squall-line snowmelt event on January 19 and 20, 1996. We also examine the rainfall distribution from an organized thunderstorm system (June 12 and 13, 1996), which produced extreme flooding in ungauged watersheds of the region. The largest flood peaks for many of the USGS stream gauging stations in the Philadelphia metropolitan region were produced by Hurricane Floyd. The most extreme flooding in the Philadelphia metropolitan region in small drainage basins was due to Tropical Storm Allison, which produced localized storm total rainfall accumulations greater than 300 mm. Hydrometeorological and hydrologic studies illustrate the important role of landfalling tropical cyclones for flood hazards for major urban regions of the northeastern United States. Analyses of high-resolution radar rainfall fields are presented, with special emphasis on radar rainfall error structure and the spatial and temporal variations of flood-producing rainfall. Diagnostic and hydrologic model studies in Pennypack Creek, Wissahickon Creek, and the Little Neshaminy Creek are carried out to examine the controls of land surface processes and space-time rainfall distribution on extreme flood response in urban watersheds. Land surface processes and the contrasting distribution of rainfall in space and time from the storm systems that affect the northeastern United States combine to shape the scale-dependent distribution of extreme floods in urban watersheds.