Filter strip and grassed swale
(Source: LID Center)
Filter strips are bands of dense, permanent vegetation with a uniform slope, primarily designed to provide water quality pretreatment between a runoff source (i.e., impervious area) and another BMP. Filter strips are important components of a BMP treatment train.
A filter strip may be constructed with or without a permeable berm at the downstream end. The maximum berm height should be no more than one (1) foot and may be used to contain the water quality volume (WQv). Because it increases the contact time with runoff, a berm will reduce the required filter strip width.
Stormwater Management Objectives
Filter strips can significantly reduce the volume of runoff from small, frequently-occurring storms if:
- the soils are sufficiently pervious;
- sheet flow is maintained through the entire length and width of the strip; and
- contact time is long enough for infiltration to occur.
Infiltration and evapotranspiration are the means by which water is retained. Soil amendments can be used to enhance permeability if the existing soils are compacted.
Filter strips decrease the peak discharge by reducing the volume of runoff through ponding and infiltration and by reducing the velocity because of surface roughness.
As a general guideline, a filter strip can be expected to reduce TSS concentrations by 50 percent, total Phosphorus by 20 percent, total Nitrogen by 20 percent, and heavy metals by 40 percent. Essentially, filter strips are designed to fill with sediment. Filter strips achieve water quality improvements through infiltration and vegetative filtering and their effectiveness increases with runoff contact time and density of vegetation.
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