Watershed-scale evaluation of a system of storm water detention basins

The effectiveness of an existing system of storm water detention basins operating at the watershed scale is evaluated. Data utilized in the study were collected from Valley Creek watershed in Chester County, Pa., which has undergone rapid development from the westward spread of suburban Philadelphia. Since the late 1970s, more than 100 storm water detention basins have been constructed in this 62 km(2) (24 mi(2)) watershed, each designed on a site-by-site basis. The design objective of these detention basins is to limit a site's postconstruction peak flow rate to or below its predevelopment level for 2- through 100-year storms. To evaluate the watershed-wide effectiveness of the network of detention basins, all basins were surveyed and included in a hydrologic model of the watershed. The U.S. Army Corps of Engineers Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) model was calibrated by using measured rainfall and observed streamflow from a U.S. Geological Survey (USGS) stream gauge. Results from modeling six measured storm events show that the detention basins reduce watershed-wide peak storm flows by an average of only 0.3%, and can potentially increase peak flow rates. The model was also used to investigate the effects of alternate on-site stormwater management strategies, including outlet structure modification and runoff volume-based management. Results indicate that a runoff volume-based plan is the most effective means of attenuating watershed-wide peak flow rates.