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Bioswale

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Bioswale

Bioswales are landscape elements designed to concentrate or remove silt and pollution from surface runoff water. They consist of a swaled drainage course with gently sloped sides (less than 6%) and filled with vegetation, compost and/or riprap. The water's flow path, along with the wide and shallow ditch, is designed to maximize the time water spends in the swale, which aids the trapping of pollutants and silt. Depending upon the geometry of land available, a bioswale may have a meandering or almost straight channel alignment. Biological factors also contribute to the breakdown of certain pollutants.

Contents

A common application is around parking lots, where substantial automotive pollution is collected by the paving and then flushed by rain. The bioswale, or other type of biofilter, wraps around the parking lot and treats the runoff before releasing it to the watershed or storm sewer.

Contaminants addressed

There are several classes of water pollutants that may be collected or arrested with bioswales. These fall into the categories of silt, inorganic contaminants, organic chemicals and pathogens.

  • Silt - These structures slow the conveyance of Silt and reduce the turbidity of receiving waters.
  • Organics - Many organic contaminants including Polycyclic aromatic hydrocarbons will volatilize or degrade over time and Bioswales slow the conveyance of these materials into waterways, and before they can affect aquatic life. Although not all organics will be captured, the concentration of organics is greatly reduced by Bioswales.
  • Pathogens - are deprived of a host or from a nutrient supply long enough for them to become the target of a heterotroph.
  • Common inorganic compounds are macronutrients such as phosphates and nitrates. Principal sources of these nutrients are excess fertilization, which can cause eutrophication in receiving waters. Plants surrounding these ponds absorb these nutrients.
  • Metallic compounds such as mercury, lead, chromium, cadmium and other heavy metals are concentrated in the structures. Unfortunately, these metals slowly poison the surrounding soil. Regular soil removal is required in order to prevent metals from dissolving and releasing back into the environment. Some bioswales are designed to include hyperaccumulator plant species. These plants absorb but do not transform the metals. Cuttings from these plants often decompose back into the pond or are pruned by gardening services that do not know the compost they are collecting is poisonous.

    • Examples

    Two early examples of scientifically designed bioswales for large scale applications are found in the western US. In 1996 for Willamette River Park in Portland, Oregon a total of 2330 lineal feet of bioswale was designed and installed to capture and prevent pollutant runoff from entering the Willamette River. Intermittent check dams were installed to further abet silt capture, which reduced by 50% suspended solids entering the river system.

    A second example of a large scale designed bioswale is at the Carneros Business Park, Sonoma County, California. Starting in 1997 the project design team worked with the California Department of Fish and Game and County of Sonoma to produce a detailed design to channel surface runoff at the perimeter of a large parking area. Surface runoff consists of building roof runoff, parking lot runoff and overland flow from properties to the north of the project site. A total of two lineal miles of bioswale was designed into the project. The purpose of the bioswale was to minimize runoff contaminants from entering Sonoma Creek. The bioswale channel is grass-lined, and nearly linear in form. Downslope gradient is approximately 4% and cross-slope gradient is approximately 6%.

    References

    Bioswale Wikipedia
    (Text) CC BY-SA