A grassed swale is a graded and engineered landscape feature appearing as a linear, shallow, open channel with trapezoidal or parabolic shape. The swale is vegetated with flood tolerant, erosion resistant plants.
When properly designed to accommodate a predetermined storm event volume, a grassed swale results in a significant improvement over the traditional drainage ditch in both slowing and cleaning of water.
In swales, stormwater is slowed by strategic placement of check-dams [ 446 KB pdf file], new window] that encourage ponding and these ponds in turn facilitates water quality improvements through infiltration, filtration and sedimentary deposition. Collected stormwater is expected to drain away through the soil within several hours or days.
Grassed Swales are an appropriate stormwater management practice for most regions of North America. Swales are a low cost low maintenance option to remove sediments, nutrients and pollutants. They increase stormwater infiltration and add a visually aesthetic component to a site.
Establishment of grassed swales is a potential solution wherever stormwater needs to be transported from impervious surfaces, slowed down and allowed to infiltrate into soils.
Exceptions to their use are in desert like areas where irrigation would be required for long-term maintenance and in colder regions with permafrost where infiltration is minimal.
In northern climates without permafrost, design and maintenance requirements should be modified with respect to a shorter growing season, management of meltwater and depth of frost in soil (see details in section on Tips and Wisdom).
Typically grassed swales are used as an environmentally preferential solution or sometimes as an enhancement to the more traditional curb and gutter based storm sewer system. The linear structure of swales favors their use in the treatment of runoff from highways, residential roadways and common areas in residential sub-divisions, along property boundaries and in and around parking lots.