4.4 Bioretention

Bioretention is a stormwater management practice originating in Prince George’s County, Maryland in the early 1990s. The term bioretention describes a stormwater management practice that uses the chemical, biological, and physical properties of plants, soil microbes, and the mineral aggregate and organic matter in soils to transform, remove, or retain pollutants from stormwater runoff. Numerous designs have evolved from the original Prince George’s County application; however, there are fundamental design characteristics that define bioretention across various settings.

Bioretention facilities are:
  • Shallow landscaped depressions with a designed soil mix and plants adapted to the local climate and soil moisture conditions that receive stormwater from a small contributing areas.
  • Designed to mimic natural forested conditions, where healthy soil structure and vegetation promote the infiltration, storage, filtration, and slow release of stormwater flows.
  • Small-scale, dispersed, and integrated into the site as a landscape amenity.
  • Can be used as a stand-alone practice on an individual lot; however, best performance is often achieved when integrated with other LID practices.

The terms bioretention and rain garden are sometimes used interchangeably. However, for Washington State the term bioretention is used to describe an engineered facility designed and sized for specific water quality treatment and flow control objectives that includes designed soil mixes and often under-drains and control structures (see Figure 4.4.1).

The term rain garden is used to describe a nonengineered landscaped depression intended to capture stormwater from adjacent areas with less restrictive design criteria for the soil mix (e.g., compost amended native soil). Rain gardens typically do not have underdrains or other control structures.

Both bioretention and rain gardens are applications of the same LID technique and can be highly effective for flow control and water quality treatment. Guidance for design and installation of rain gardens is available in this Manual, as well as the Rain Garden Handbook for Western Washington (WSU, 2013).

The term bioretention is used to describe various designs using soil and plant complexes to manage stormwater. The following terminology is used in this manual:
  • Bioretention cells: Shallow depressions with a designed planting soil mix and a variety of plant material, including trees, shrubs, grasses, and/or other herbaceous plants. Bioretention cells may or may not have an under-drain and control structure and are not designed as a conveyance system. Side slopes are typically gentle; however, side slopes may be steep or vertical in urban areas with space limitations. Ponding depths are typically 6 to 12 inches.
  • Bioretention swales: Incorporate the same design features as bioretention cells; however, bioretention swales are designed as part of a system that can convey stormwater when maximum ponding depth is exceeded.
  • Infiltration planters: Designed soil mix and a variety of plant material, including trees, shrubs, grasses, and/or other herbaceous plants within a vertical walled container usually constructed from formed concrete, but could include other materials. Infiltration planters have an open bottom that allows infiltration to the subgrade. These designs are often used in urban settings (see Figure 4.4.2).
  • Flow-through planters: Designed soil mix and a variety of plant material, including trees, shrubs, grasses, and/or other herbaceous plants within a vertical walled container usually constructed from formed concrete, but could include other materials. A flow-through planter is completely impervious and includes a bottom and, accordingly, must include an under-drain and perhaps a control structure. These designs are often used in urban settings. To be considered an LID practice the flow-through planter must have a volume reduction, flow control, or treatment component to the design (see Figure 4.4.2).


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Fig4-4-1_TypBioSection_rev1
Figure 4.4.1

Bioretention with primary design elements (Under-drain is optional). Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012)



Fig4-4-2_4-4-1_BioretentionPlanter_rev1
Figure 4.4.2
Bioretention with primary design elements (Under-drain is optional). Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012)