Stormwater Management Fact Sheet: Bioretention

 

Description

Bioretention areas are landscaping features adapted to treat stormwater runoff on the development site. They are commonly located in parking lot islands or within small pockets in residential land uses. Surface runoff is directed into shallow, landscaped depressions. These depressions are designed to incorporate many of the pollutant removal mechanisms that operate in forested ecosystems. During storms, runoff ponds above the mulch and soil in the system. Runoff from larger storms is generally diverted past the facility to the storm drain system. The remaining runoff filters through the mulch and prepared soil mix. Typically, the filtered runoff is collected in a perforated underdrain and returned to the storm drain system. For more information see Bioretention as a Water Quality Best Management Practice, Article 110 in the Practice of Watershed Protection.

Applicability

Bioretention systems are generally applied to small sites, but can be applied to a wide range of development. Bioretention can be applied in many climate and geologic situations, with some minor design modifications.

Regional Applicability
Bioretention systems are applicable almost everywhere in the United States. In arid or cold climates, however, some minor design modifications may be needed.

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. Bioretention facilities are ideally suited to many ultra urban areas, such as parking lots. While they consume a fairly large amount of space (approximately 5% of the area that drains to them), they can fit into existing parking lot islands or other landscaped areas.

Stormwater Hotspots
Stormwater hotspots are areas where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater. A typical example is a gas station or convenience store parking lot. Bioretention areas can be used to treat stormwater hotspots as long as an impermeable liner is used at the bottom of the filter bed.

Stormwater Retrofit
A stormwater retrofit is a stormwater management practice (usually structural) put into place after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other objectives. Bioretention can be used as a stormwater retrofit, by modifying existing landscaped areas, or if a parking lot is being resurfaced. In highly urban watersheds, they are one of the few retrofit options that can be employed. However, it is very expensive to retrofit an entire watershed using bioretention areas since they treat small sites.

Cold Water (Trout) Streams
The species in cold water streams, notably trout, are extremely sensitive to changes in temperature. In order to protect these resources, designers should avoid treatment practices that increase the temperature of the stormwater runoff they treat. Bioretention is a good option in cold water streams because water ponds in them for only a short time, decreasing the potential for stream warming.

Siting and Design Considerations

Designers need to consider conditions at the site level and must incorporate design features to improve the longevity and performance of the practice, while minimizing the maintenance burden.

Siting
Some considerations selecting a stormwater treatment practice are the drainage area the practice will need to treat, the slopes both at the location of the practice and draining to it, soil and subsurface conditions, and the depth of the seasonably high groundwater table. Bioretention can be applied on many sites, with its primary restriction being the need to apply the practice on small sites.

Drainage Area

Bioretention areas should usually be used on small sites (i.e., five acres or less). When used to treat larger areas, they tend to clog. In addition, it is difficult to convey flow from a large area to a bioretention area.

Slope

Bioretention areas are best applied to relatively shallow slopes (usually about 5%). Sufficient slope is needed at the site to ensure that the runoff that enters a bioretention area can be connected with the storm drain system. It is important to note, however, that these bioretention areas are most often applied to parking lots or residential landscaped areas, which generally have gentle slopes.

Soils /Topography

Bioretention areas can be applied in almost any soils or topography, since runoff percolates through a made soil bed, and is returned to the stormwater system.

Groundwater

Bioretention should be separated from the watertable to ensure that the groundwater never intersects with the bottom of the bioretention area, which prevents possible groundwater contamination and practice failure.

Design Considerations
Specific designs may vary considerably, depending on site constraints or preferences of the designer or community, but some features, should be incorporated into all bioretention areas. These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information see the Manual Builder Category) (see Figure 1).

Pretreatment

Pretreatment refers to features of a bioretention area that capture and remove coarse sediment particles. Incorporating pretreatment helps to reduce the maintenance burden of bioretention, and reduces the likelihood that the soil bed will clog over time. Several different mechanisms are used to provide pretreatment in bioretention areas. Runoff can be directed to a grass channel or filter strip to settle out coarse sediments before the runoff flows into the filter bed of the bioretention area. Other features may include a pea gravel diaphragm, which acts to spread flow evenly and drop out larger particles.

Treatment

Treatment features enhance the ability of a stormwater treatment practice to remove pollutants. Several basic features should be incorporated into bioretention areas to enhance their pollutant removal rates. The bioretention system should be sized be between 5% and 10% of the impervious area draining to it. The practice should be designed with a soil bed that is a sand/ soil matrix with a mulch layer above the soil bed. The bioretention area should be designed to pond a small depth of water (6" to 9") above the filter bed.

Conveyance

Conveyance of stormwater runoff into and through a stormwater practice is a critical component of any stormwater treatment practice. Stormwater should be conveyed to and from the practice safely and minimize erosion potential.

Bioretention areas are designed with an underdrain system to collect filtered runoff at the bottom of the filter bed and direct it to the storm drain system. An underdrain is a perforated pipe in a gravel bed, installed along the bottom of filter bed. Stormwater management practices, and used to collect and remove filtered runoff. Designers should also provide an overflow structure to convey flow from large storms (that are not treated by the bioretention area) to the storm drain system.

Maintenance Reduction

In addition to regular maintenance, bioretention areas should incorporate design features to reduce the long term maintenance of a bioretention area. Designers should ensure that the bioretention area is easily accessible for maintenance.

Landscaping

Landscaping is critical to the function and appearance of bioretention areas. It is preferred that native vegetation is used for landscaping, where possible. Plants should be selected that can withstand the hydrologic regime they will experience (i.e., plants that tolerate both wet and dry conditions). At the edges, which will remain primarily dry, upland species will be the most resilient. Finally, it is best to select a combination of trees, shrubs, and herbaceous materials.

Design Variations

One design alternative to bioretention areas is the use of a "partial exfiltration" system, which promotes greater groundwater recharge (see below).

Partial Exfiltration

In this design variation, the underdrain of a bioretention area only is only installed on part of the bottom of the system. This design allows for greater infiltration of stormwater runoff, with the underdrain acting as more of an overflow. This system can be applied only when the soils and other characteristics are appropriate for infiltration (for more information see the Infiltration Trench and Infiltration Basin Fact Sheet in the Fact Sheet Category).

Arid Climates

In arid climates, bioretention areas should be landscaped with drought tolerant plant species.

Cold Climates

In cold climates, bioretention areas can be used as a snow storage area. When used for this purpose, or if used to treat parking lot runoff, the bioretention area should be planted with salt tolerant, and non-woody plant species.

Limitations

Bioretention areas have a few limitations. Bioretention areas cannot be used to treat large drainage areas, limiting their usefulness for some sites. Although bioretention areas do not consume a large amount of space, incorporating bioretention into a parking lot design may reduce the number of parking spaces available. Finally, the construction cost of bioretention areas relatively high compared with other stormwater treatment practices. (See Cost Considerations for a more detailed explanation).

Maintenance Considerations

Bioretention requires seasonal landscaping maintenance. In many cases, bioretention areas require intense maintenance initially to establish the plants, but less maintenance is required in the long term. In many cases, maintenance tasks can be completed by a landscaping contractor, who may already be hired at the site.

Table 1. Typical Maintenance Activities for Bioretention Areas

Activity

Schedule

  • Remulch void areas
  • Treat diseased trees and shrubs

As needed

  • Water plants daily for two weeks

At project completion

  • Inspect soil and repair eroded areas
  • Remove litter and debris

Monthly

  • Remove and replace dead and diseased vegetation

Twice per year

  • Add additional mulch
  • Replace tree stakes and wire

Once per year


Effectiveness

Structural stormwater management practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal. In general, bioretention areas can only provide pollutant removal.

Groundwater Recharge
Bioretention areas do not usually recharge the groundwater, except in the case of the partial exfiltration design (see Design Variations).

Pollutant Removal
Little pollutant removal data has been collected on the pollutant removal effectiveness of bioretention areas. In fact only one study has been conducted (Davis et al., 1998). The data from this study is presented in Table 2.

Table 2. Typical Pollutant Removal Rates of Bioretention Systems

Pollutant

Pollutant Removal (%)

TSS

81

TP

29

TN

49

NOx

38

Metals

51-71

Bacteria

-58


Assuming that bioretention systems perform similarly to swales, their removal rates are relatively high (for more information, see Comparative Pollutant Removal Capability of Stormwater Treatment Practices, Article 64 in The Practice of Watershed Protection).

Cost Considerations

Bioretention areas are relatively expensive. The following cost equation was developed by Brown and Schueler (1997), adjusting for inflation:


C = 7.30 V0.99

Where:
C = Construction, Design and Permitting Cost ($)
V = Volume of water treated by the facility (cubic feet)
This amounts to about $6.80 per cubic foot of water storage.

An important consideration when evaluating the costs of bioretention is that it often replaces area that would likely be landscaped anyway. Thus, the true cost of the bioretention area may be less than the construction cost reported. Similarly, maintenance costs for bioretention areas are not very different from normal landscaping maintenance. Land consumed by bioretention areas is relatively high compared with other practices (about 5% of the drainage area). However, this land should not be considered lost, since it is often fits with existing setbacks and landscaping requirements.

References

Brown, W. and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic Region. Prepared for: Chesapeake Research Consortium. Edgewater, MD. Center for Watershed Protection. Ellicott City, MD.

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1998. Maryland Stormwater Design Manual. Prepared for: Maryland Department of the Environment. Baltimore, MD.  http://www.mde.state.md.us/environment/wma/stormwatermanual/mdswmanual.html

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.

Center for Watershed Protection (CWP). 1996. Design of Stormwater Filtering Systems. Prepared for: Chesapeake Research Consortium. Solomons, MD. and US EPA Region V. Chicago, IL.

Davis, A., M. Shokouhian, H. Sharma, and C. Henderson. 1998. Optimization of Bioretention Design for Water Quality and Hydrologic Characteristics. Department of Civil Engineering, University of Maryland, College Park.

Engineering Technologies Associates and Biohabitats. 1993. Design Manual for Use of Bioretention in Stormwater Management. Prepared for: Prince George's County Government; Watershed Protection Branch. Landover, MD.

Prince George's County Department of Environmental Resources. 1997. Low Impact Development. Laurel, MD

Schueler, T. 2000. "Comparative Pollutant Removal Capability of Urban Stormwater Treatment Practices: A Reanalysis," Article 64 in the Practice of Watershed Protection.  Center for Watershed Protection.  Ellicott City, MD.