Hydrologic analyses of the July 17-18, 1996, flood in Chicago and the role of urbanization

On July 17-18, 1996, two mesoscale convective systems (MCSs) passed through northeastern Illinois, causing a record 440-mm total storm rainfall within a 24-h period at Aurora, Illinois, with values exceeding 200 mm throughout a broad area of the region. The storm caused flooding with a return period larger than 100 years at different USGS regional stream gauging locations. The Davenport, Iowa, Weather Surveillance Radar-1988 Doppler (WSR-88D) radar coverage allows high-quality characterization of the storm at fine spatial and temporal scales. Of particular interest is the inter- and intravariability in watershed response to the two pulses of intense rainfall. Spatial distribution of rainfall and the degree of urbanization of the individual basins are the dominant factors determining the magnitude of runoff response. These properties are highly dependent on the extent and history of urbanization. Examination of the annual maximum instantaneous peak discharge and the peaks-over-threshold (POT) time series at three stream gauging stations in Illinois (Blackberry Creek, DuPage River, and Sawmill Creek) over the past 50 years points to the large effect of urbanization on the flood peak distribution in the greater Chicago metropolitan area.