Effects of flood controls proposed for West Branch Brandywine Creek, Chester County, Pennsylvania

Twenty-four-hour rainfall, distributed over time according to the U.S. Soil Conservation Service type II rainfall distribution, was used as input to calibrated rainfall-runoff models of three subbasins in the West Branch Brandywine Creek watershed. The effects of four proposed flood controls were evaluated by using these rainfalls to simulate discharge hydrographs with and without the flood controls and comparing the simulated peak discharges.

In the Honey Brook subbasin, 2-, 10-, and 100-year flood-discharge hydrographs were generated for station West Branch Brandywine Creek at Coatesville. For the 2- and 10-year floods, proposed flood controls would reduce the peak discharge from 1 to 8 percent. The combination of all three flood controls proposed for the Coatesville subbasin would reduce the 100-year peak discharge 44 percent.

In the Modena subbasin, 2-, 10-, and 100-year flood-discharge hydrographs were generated for station West Branch Brandywine Creek at Modena. A flood control proposed for Sucker Run, a tributary, would reduce the peak discharge of Sucker Run at State Route 82 by 22, 25, and 27 percent and the peak discharge of West Branch Brandywine Creek at Modena by 10, 6, and less than 1 percent for the 2-, 10-, and 100-year floods, respectively.

For the 2- and 10- year floods, flood control proposed for the Coatesville subbasin would have little effect on the peak discharge of West Branch Brandywine Creek at Modena. For the 100-year flood, the combination of all three flood controls proposed for the Coatesville subbasin would reduce the peak discharge at Modena 25 percent.

When flood control in the Modena subbasin was combined with flood control in the Coatesville subbasin, the 10-percent reduction in the 2-year flood peak of West Branch Brandywine Creek at Modena was due almost entirely to flood control in the Modena subbasin. For the 10-year flood, flood control in the Modena subbasin would reduce the peak discharge 6 percent, and any single flood control in the Coatesville subbasin would provide an additional 1 to 3 percent reduction. Although flood control in the Modena subbasin would have little effect on reducing the 100-year flood peak, it would provide an additional 5 percent reduction in the peak discharge, for a total reduction of 30 percent, when combined with the three flood controls in the Coatesville subbasin.