Representation of non-connected impervious area in SWMM runoff modeling

The overland flow runoff algorithm used in the USEPA SWMM model has been a leading method for dynamic runoff simulation for over 30 years. This model divides drainage catchments into two compartments, one each for impervious and pervious surfaces. Runoff discharges via a non-linear reservoir discharge equation to a drainage inlet, from where it can be routed through a collection system. SWMM44H, developed in 2002, allowed routing onto another drainage catchment. SWMM5, released in 2004, introduced a new parameter that greatly improves representation of typical urban runoff. The new parameter allows partitioning of directly and non-directly connected impervious area (DCIA and NDCIA) within a single catchment. Explicit representation of DCIA and NDCIA in the model better represents the physical configuration of a catchment and enables more direct parameterization of physical measurements such as soil infiltration rates and imperviousness into the model. Those parameters were traditionally first estimated and then adjusted to represent DCIA and NDCIA in the model, particularly where impervious areas drain onto pervious areas, such as roofs that drain onto lawns.

Examples of this concept are presented for a hypothetical catchment and for calibrated watersheds from a study area near Philadelphia, Pennsylvania. The model was calibrated by only adjusting the impervious to pervious transfer coefficient. Comparisons between an estimated DCIA that is obtained from field data and a calibrated DCIA were conducted.