Equivalent Imperviousness for Grass Buffers

Over the years, many storm water best management practices (BMPs) have been developed to reduce the runoff volume and pollutant concentrations. In general, the concept of BMP takes the four following steps to dispose on-site storm water (USWDCM 2001): (1) to reduce runoff peaks and volumes by minimizing directly connected impervious areas (MDCIA), (2) to provide water quality capture volume (WQCV) for an on-site retention process, (3) to stabilize downstream banks and stream beds along the waterways, and (4) to implement BMPs for special needs for industrial and commercial developments within the tributary area. The principle behind MDCIA is twofold: to reduce impervious areas and to direct runoff from impervious surfaces over grassy areas to slow down runoff and promote soil infiltration. Draining paved areas onto porous areas can reduce runoff volumes, rates, pollutants, and cost for drainage infrastructure. Several approaches are used to reduce the effective imperviousness of a development site, including (1) the reduction of paved area, (2) an increase of porous pavement, (3) filtering runoff by grass buffers, and (4) infiltration by porous cascading planes.