A pretreatment vegetated filter strip is a uniformly graded, vegetated area (i.e., grass or close-growing native vegetation) that is used to treat sheet flow from adjacent pervious and impervious areas prior to entering a structural stormwater BMP. Pretreatment vegetated filter strips reduce runoff velocity and utilize vegetation to filter coarse sediment and debris. Pretreatment vegetated filter strips should span the entire width of the contributing area to ensure treatment of runoff from the entire area and are most effective if they receive uniformly distributed sheet flow. A level spreader is required if the filter strip receives concentrated flow or flow that could become concentrated because concentrated flows reduce the effectiveness of the practice. Figure 13-2 shows a schematic of a vegetated filter strip used for pretreatment of runoff from pervious and impervious areas prior to discharge to a structural stormwater BMP.

Unlike the vegetated pervious areas that are suitable for providing stormwater retention and treatment credit as described in Chapter 5 - Low Impact Development Site Planning and Design Strategies, pretreatment vegetated filter strips are not stand-alone treatment practices due to their relatively small size and should only be used immediately upgradient of another structural stormwater BMP. Pretreatment vegetated filter strips provide relatively limited runoff volume reduction, infiltration, and peak flow reduction.