Representation of Non-Directly Connected Impervious Area in SWMM Runoff Modeling

Abstract

The overland flow runoff algorithm used in the USEPA SWMM model (Huber and Dickinson, 1988) has been a leading method for dynamic runoff simulation for over 30 years. The Runoff module in SWMM divides drainage catchments into two principal 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 or downstream drainage subcatchments (Figure 18.1). SWMM44H, developed in 2002, allowed routing onto another drainage subcatchment (Huber, 2001). SWMM5, released in 2004, introduced new parameters that improve representation of typical urban runoff. The new parameters partition directly-connected impervious area (DCIA) and non-directly-connected impervious area (NDCIA) within a single catchment (Figure 18.2). The traditional representation of an urban watershed can be quite effective in environments where impervious area dominates and runoff from pervious area is of minor importance. However, in other settings, this approach suffers from its failure to directly represent impervious areas that drain onto pervious areas, such as the cases that roofs drain onto lawns, or high impervious lands drain onto the low impact development areas or other BMP infrastructures. Explicit representation of DCIA and NDCIA better