One of the ways to reduce the amount of stormwater is to allow it to infiltrate--sit around in some form of structure that prevents most of the water from running off and giving the stormwater time to absorb into the local soils. When vegetation is used, this is referred to as Bioretention.

There are several types of bioretention, each with their own application based on the location of the facility. Some styles work better than others--and some styles look better than others, though if built properly for the site, all styles work well.

Bioretention Soil Research

Monitoring data from several studies conducted in the western Washington region (Herrera 2014a, 2015) indicate that some pollutants are exported from bioretention systems using the default bioretention soil media (BSM), most notably nitrogen (N), phosphorus (P), and copper (Cu). To address this concern, Herrera Environmental Consultants implemented a study in partnership with Kitsap County to improve BSM performance for the capture and retention of these pollutants. This study was funded by the Washington State Department of Ecology through the Municipal Stormwater Grants of Statewide and Regional Significance program (2013–2015 biennium).

Read the full report.

Bioretention Cells

These are shallow depressions that accept stormwater from small areas with plants and a specially designed soil media. The media is typically designed to provide a specific saturated hydraulic conductivity and pollutant removal characteristics and to support healthy plants. A variety of plants are used in bioretention areas, including trees, shrubs, grasses and/or other herbacious plants. Bioretention cells may or may not have an under-drain and are not designed as a conveyance system.

Bioretention Swales

Bioretention swales incorporate the same design features as bioretention cells. However, bioretention swales are designed as part of a conveyance system that can move stormwater when maximum ponding depth is exceeded.

Rain Gardens

A non-engineered, shallow landscape depression with native soil or a soil mix that also contains plants that are designed to capture stormwater from small areas such as sidewalks and front yards. It contains inlets from the contributing area, and outlets to other stormwater structures that can appear as a pipe or as attractive stone “riverbeds.” These can be shaped and sized to fit a particular site, and are typically landscaped with a variety of plants to fit the surroundings or be aesthetically pleasing. Ecology does not consider Rain Gardens capable of providing any level of stormwater quality treatment.

For more information on Rain Gardens, please visit the Washington Stormwater Center's Rain Garden webpage.