A Flow Duration Analysis (FDA) was conducted for the Piscataway watershed next to Washington DC. The 67 sq. mi watershed was segregated into 86 subcatchments. SWMM 5.0.015 was used for this analysis. The aquifer option was used to simulate subsurface flow routing. This provided recession limb hydrographs that closely corresponded to flow observations in this highly permeable watershed.
To set up the model, each subwatershed was segregated into a directly connected effective impervious area (EIA) subarea, a disconnected impervious area (DIA) and its associated disturbed urban pervious subarea, and a natural pervious subarea. Each of these subareas had very different hydrologic parameters resulting in quite different responses. A groundwater aquifer was also allocated to each of the pervious subareas. Flows from these five sources are combined at the subwatershed outlet to develop the cumulative response of the subwatershed.
In this manner, the fabric of the landscape in each watershed was disaggregated to better model the runoff response of these very different categories. EIAs were identified as where either curbing exists and/or within 400 feet of where stormwater pipes are distributed. In contrast, low density subdivisions without curbs and divided highways with median strips were allocated as disconnected. The resulting FDA closely matched observations for the period evaluated.