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Filter strips and swales are vegetated surface features that drain water evenly off impermeable areas. Swales are long shallow channels whilst filter strips are gently sloping areas of ground.
Both devices mimic natural drainage patterns by allowing rainwater to run in sheets through vegetation, slowing and filtering the flow. Swales can also be designed for a combination of conveyance, infiltration, detention and treatment of runoff.
Swales are usually designed as conveyance systems, but can also be designed with check dams to increase attenuation and, where applicable, infiltration. Filter strips only attenuate the flow slightly but they can be used to reduce the drained impermeable area.
Swales and filter strips are effective at removing polluting solids through filtration and sedimentation. The vegetation traps organic and mineral particles that are then incorporated into the soil, while the vegetation takes up any nutrients.
Swales and filter strips are often integrated into the surrounding land use, for example public open space or road verges. Local wild grass and flower species can be introduced for visual interest and to provide a wildlife habitat. Care should be taken in the choice of vegetation as tussocks create local eddies, increasing the potential for erosion on slopes. Shrubs and trees can be planted but in this case the vegetated area will need to be wider and have a gentler slope.
(18 June 2001)