Abstract

An examination of major flooding events in Northern New Castle County, Delaware is presented in the context of the discrimination between flooding due to landuse change and flooding due to anomalous heavy rain events. The Christina River Basin encompasses the Red and White Clay Creek Watersheds. These basins are the focus of this study because of their short response time, shift to suburban landuse types, and impact on approximately 165,000 people in southeastern Pennsylvania and northern Delaware. The Christina basin has recorded many floods since record keeping began approximately 70 years ago, These floods were classified according to the synoptic weather situation that was responsible for the flooding. A qualitative synoptic classification identified four general synoptic types responsible for flooding including tropical storms, frontal systems, coastal storms/nor'easters and convective systems. Stream gages across the Christina Basin have seen an increase in the frequency of flooding and in some cases an increase in the strength of the flooding over time.

A detailed hydrologic model (HEC), developed by the Army Corps of Engineers, was used to investigate the role of landuse change from agricultural to single family residential landuse. The GIS-based preprocessor, HEC-GeoHMS, uses a digital elevation model, gage locations, landuse and soil to output basin characteristics to the model. Simulations are run for two diverse atmospheric forcings and various landuse scenarios. The resulting simulations yielded flood hydrographs which closely matched the observed discharge data. When actual hydrographs are compared to a landuse scenario representing pre-European settlement conditions (all forest), recent anomalous heavy precipitation still causes major floods but at levels that are 50-80% lower than that of the current landuse conditions. The results of this research provide a better understanding of the complex interaction between landuse changes and synoptic weather patterns. The results can also be used to forecast future flooding potential based upon changes in landuse across the Christina Basin.