Filter Strips

Filter Strips are grassed or vegetative areas through which runoff passes as sheet flow. They do not effectively reduce peak discharges but are effective in removing particulate pollutants for small storms (< 1 year storm) (Akan and Houghtalen, 2003). Flat slopes (<5%) and low to fair permeability (0.15 to 4.3 mm/h or 0.006 to 0.17 in/h) of natural subsoil are required for their effective operation (Sansalone and Hird, 2003). SWMM does not have a unique visual object to represent filter strips but they can be represented as a pervious subcatchment that receives runoff from an upstream subcatchment as illustrated in Figure 3.1. The two most important processes that must be simulated with filter strips are infiltration and storage. A filter strip can be simulated as a 100% pervious subcatchment whose geometry (area, width and slope) is directly obtained from the field. This subcatchment receives water from an upstream contributing area (impervious or semi-impervious) and drains to a conduit representing the gutter or street. The infiltration for a filter strip can be simulated using any of SWMM's infiltration options.