Stormwater filtration BMPs - overview

Stormwater filtration BMPs - overview
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Stormwater BMP	General Overview	Illustration
Dry swale or step pool	Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. Dry swales function primarily as a conveyance BMP but may incorporate check dams into the design, thus allowing water to be stored behind the check dam and filtered through the underlying soil or media. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Wet swale	Wet swales act as very long and linear shallow biofiltration or linear wetland treatment systems. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams.
Biofiltration Basin	Often called rain gardens, biofiltration basins use engineered or mixed soils and plantings to capture and filter runoff. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Permeable pavement with underdrain	Permeable pavements are paving surfaces that allow stormwater runoff to filter through surface voids into an underlying stone reservoir. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). All permeable pavements have a similar structure, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains and geotextile over uncompacted soil subgrade. Discharge of this stored runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Tree Trench/Tree Box (with underdrain)	A system of trees that are connected by an underground storage structure. The system consists of a trench lined with geotextile fabric with structural stone, gravel or soil boxes in which the trees are placed. Tree systems consist of an engineered soil layer designed to treat stormwater runoff via filtration through plant and soil media, and through evapotranspiration from trees. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Media filter (sand filter)	Runoff is captured and stored in a filter media consisting of a sand filter bed (~18 inches), with temporary runoff storage above the bed. Pollutants are trapped or strained out at the surface of the filter bed. Discharge of filtered runoff occurs primarily through an underdrain.
Green roof	Green roofs consist of a a layer or layers of media that filter suspended solids and pollutants associated with those solids. Water passes through the media to an underlying impermeable layer that conveys the filtered water from the roof.