Effects of Flood Control Structures on Flood Response for Hurricane Floyd in the Brandywine Creek Watershed, Pennsylvania

Small watersheds in urbanizing and suburbanizing regions are an increasingly important element of the flood hazard problem in the United States and abroad. This paper presents a methodology to estimate flood response in urban watersheds, with the goal of assessing the performance of systems of small flood control reservoirs for extreme floods. The setting of this study is the Brandywine Creek watershed in Chester County, Pa. A number of flood control structures were built within the basin, and the availability of data from U.S. Geological Survey (USGS) stream gauging stations and rainfall estimates from National Weather Service weather radars provides the opportunity to study the performance of a small watershed flood control system. In this paper, the flood response to rainfall from Hurricane Floyd, which produced record flooding in Brandywine Creek on September 16, 1999, is examined. A distributed hydrologic model was developed and used to estimate flood discharges both with and without the existing flood control structures. The model was calibrated using observed discharge data from the USGS gauging stations, and model simulations were then performed with the flood control structures removed to estimate their impact on the watershed response to the storm. Results from the model showed that the flood control structures mitigated flood hazards to a greater degree than had been expected by the studies completed prior to construction. This paper shows how the USGS streamflow measurement program can be augmented with hydrologic modeling and WSR-88D weather radars to provide useful hydrologic information for analysis of extreme floods.