Best Management Practice Fact Sheet 9: Bioretention

This fact sheet is one of a 15-part series on urban stormwater management practices.

Please refer to definitions in the glossary at the end of this fact sheet.
Glossary terms are italicized on first mention in the text. For a comprehensive list, see “Urban Stormwater: Terms and Definitions,” Virginia Cooperative Extension (VCE) publication 426-119.

What Is Bioretention?

A bioretention cell, or rain garden, is a best management practice (BMP) designed to treat stormwater run-off from roofs, driveways, walkways, or lawns. They are a shallow, landscaped depression that receives, temporarily holds and and treats polluted stormwater with the goal of discharging water of a quality and quantity similar to that of a forested watershed (figure 1).

Where Can Bioretention Be Used?

Bioretention can be used in commercial areas, parking lots, and highways to treat roof, parking, and road runoff. Bioretention can also be used in residential landscapes or parks; however, steps should be taken to minimize sediment. Bioretention cells that receive sediment loading tend to clog and hold water rather than infiltrate it.

What Does Bioretention Do?

The practice of bioretention was developed to reduce pollution in runoff from urban impervious surfaces during moderate storms. A typical bioretention cell consists of a depression with a vegetated layer, a mulch layer, several layers of sand, soil, an organic media filter bed, an overflow, and an optional underdrain (figure 2). A small pretreatment basin known as a forebay is created with river rock to trap sediment prior to entering the bioretention cell. Within a cell, runoff is treated by a variety of physical, chemical, and biological processes.

Bioretention provides both quantity and quality control benefits. It enhances biodiversity by providing natural habitat and can be a beautiful addition to the urban landscape.

How Does Bioretention Work?

Stormwater storage is provided when water ponds in the cell. The collected stormwater is filtered through different layers of mulch, media, and compost inside the cell. Media, plants, and microorganisms in the soil treat the pollutants carried by the runoff through physical processes like filtration, infiltration, or adsorption and biological processes like biological uptake or microbial decomposition.

An underdrain consists of perforated pipe in a gravel layer installed along the bottom of the media filter bed. An upturned outlet promotes periodic anaerobic conditions within a fluctuating water table and facilitates removal of nitrogen. In soils with high infiltration rates, the underdrain can be omitted, thus increasing runoff reduction. Bioretention cells without underdrains should be avoided in commercial and industrial areas to prevent groundwater contamination.

Limitations

• Adequate sunlight and irrigation may be required.
• Because of the potential of bioretention cells to clog from sediment, installation of bioretention should always wait until upstream areas are stabilized.
• Typically limited to 5 percent of a small drainage area (2 acres). For example, for an 18,000-square-foot lot, a 900^-square-foot bioretention cell is likely needed.
• Minimum media depth of 1.5 feet is required; however, increasing to up to 3 feet improves performance.
• At least 2 to 4 feet of elevation drop from the top of the cell to groundwater and/or the outlet should be available.
• Plants must tolerate dry periods and potential submerged roots for as long as 24 hours. Specific plant lists are available from the Virginia Department of Conservation and Recreation (VA-DCR; 2011) and Virginia Cooperative Extension (VCE; Andruczyk and Fox (2018).

Maintenance

• Maintenance is similar to garden general landscape work.
• Inspect the forebay and mulch/media surfaces in the treatment area to avoid clogging and repair if necessary.
• Replace dead plants and replenish mulch layer — recommended annually as needed.

Performance

Bioretention can be very effective at reducing runoff and removing pollutants such as excess nutrients from incoming water flow. A typical bioretention cell has a media depth of 1.5 to 2 feet. An annual reduction of 25 percent for total phosphorus, 40 percent for total nitrogen, and 40 percent for runoff can be expected. Improving the media and its depth to 2 to 3 feet and providing a gravel underdrain and other enhancements can improve the estimated annual reductions to 50 percent for total phosphorus, 60 percent for total nitrogen, and 80 percent for runoff (VA-DEQ 2011).

Expected Cost

Construction costs of bioretention average approximately $32 per ft² of surface area of the practice. Operation and maintenance costs can be estimated at 5% of construction cost on an annual basis (Washington State Department of Ecology, & Herrera Environmental Consultants, 2012).

Additional Information

TheVirginiadepartmentsofConservationandRecreation (VA-DCR) and Environmental Quality (VA-DEQ) are the two state agencies that address nonpoint source pollution. The VA-DCR oversees agricultural conservation; VA-DEQ regulates stormwater through the Virginia Stormwater Management Program.

Additional information on best management practices can be found at the Virginia Stormwater BMP Clearinghouse website at https://www.swbmp.vwrrc.vt.edu/ (Permanent link: https://perma.cc/WC5L-KCZ8). The BMP Clearinghouse is jointly administered by the VA-DEQ and the Virginia Water Resources Research Center.

Online Resources

Chesapeake Stormwater Network – http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/VA_BMP_Spec_No_9_BIORETENTION_FINAL_Draft_v2-0_06Nov2013.pdf (Permanent link: https://perma.cc/42R9-EF65)

Low Impact Development Center – http://lid-stormwater.net/index.html. http://lid-stormwater.net/bio_benefits.htm (Permanent link: https://perma.cc/6QCR-G8PE)

North Carolina State University, Department of Biological and Agricultural Engineering, Stormwater Group – https://stormwater.bae.ncsu.edu/research-projects/bioretention-areas/ (Permanent link: https://perma.cc/35L2-QJST)

Pennsylvania Department of Environmental Protection – https://extension.psu.edu/rain-gardens-the-basics Permanent link: https://perma.cc/Z7TX-TVDA

Prince George’s County (Maryland) – https://www.princegeorgescountymd.gov/1478/Design-Manuals

U.S. Environmental Protection Agency – https://www.epa.gov/npdes/national-menu-best-management-practices-bmps-stormwater.edu (Permanent link: https://perma.cc/J34A-3KU6)

Virginia Stormwater BMP Clearinghouse – https://www.swbmp.vwrrc.vt.edu/ (Permanent link: https://perma.cc/WC5L-KCZ8)

Companion Virginia Cooperative Extension Publications

Andruczyk, M., and L. Fox. 2018. Urban Water Quality Management: Rain Garden Plants. VCE Publication 426-043.

Daniels, W., G. Evanylo, L. Fox, K. Haering, S. Hodges,

R. Maguire, D. Sample, et al. 2011. Urban Nutrient Management Handbook. Edited by J. M. Goatley. VCE Publication 430-350.

Fox, L. J., Sample, D. J., & Robinson, D. J. (2018). Stormwater Management for Homeowners Fact Sheet 5 Rain Gardens. VCE Publication SPES-5P.

Gilland, T., Fox, L., and Andruczyk, M. 2018. Urban Water Quality Management: What Is a Watershed? VCE Publication 426-041.

Acknowledgements

The authors would like to express appreciation for the review and comments provided by the following individuals: Brian Benham, associate professor, Virginia Tech; Jon Hathaway, assistant professor, University of Tennessee; Jia Liu, Ph.D. student, Biological Systems Engineering, Virginia Tech; Thomas Bolles, environmental educator, Virginia Tech; and Adria Bordas, Extension agent, Virginia Tech.

References

Andruczyk, M., and L. Swanson, L. Fox. 2018. Urban Water Quality Management: Rain Garden Plants. VCE Publication 426-043. http://pubs.ext.vt.edu/426/426-043/426-043.html.

Virginia Department of Environmental Quality (VA DEQ). 2011. VVirginia DEQ Stormwater Design Specification No. 9: Bioretention, Version 1.9. https://www.swbmp.vwrrc.vt.edu/wp-content/uploads/2017/11/BMP-Spec-No-9_BIORETENTION_v1-9_03012011.pdf.

Virginia Department of Forestry. 2014. Rain Gardens Technical Guide: A Landscape Tool to Improve Water Quality. http://dof.virginia.gov/infopubs/Rain-Garden-Technical-Guide-2014-05_pub.pdf.

Washington State Department of Ecology, and Herrera Environmental Consultants. 2012. PugetSound Stormwater BMP Cost Database.

Glossary of Terms

Adsorption – A process by which dissolved compounds separate from the liquid phase and become physically or chemically bound to solid materials and are removed via treatment.

Anaerobic – Chemical reactions that proceed without the presence of oxygen.

Baseflow – The portion of flow in a stream that continues even during extended dry periods.

Best management practice – Any treatment practice for urban lands that reduces pollution from stormwater. A BMP can be either a physical structure or a management practice. A similar but different set of BMPs is used to mitigate agricultural runoff.

Biodiversity – The number of different species and a measure of the health of the observed system.

Biological uptake – The process by which plants absorb nutrients for nourishment and growth.

Bioretention, bioretention cells – A best management practice that is a shallow, landscaped depression that receives and treats runoff with the goal of discharging water of a quality and quantity similar to that of a forested watershed. Bioretention cells typically consist of vegetation, and, optionally, an underdrain and an outlet structure. Bioretention is sometimes called a rain garden; normally the term bioretention implies the practice was designed specifically for a site.

Compost – Vegetative or organic matter that has been allowed to fully decompose, leaving a rich, organic medium that can be mixed with soils.

Erosion – A natural process by either physical process, such as water or wind, or chemical means that moves soil or rock deposits from one source and transports it to another. Excessive erosion is considered an environmental problem that is very difficult to reverse.

Filtration – A treatment process that removes pollutants by straining, sedimentation, and similar processes.

Forebay – A small basin within a best management practice that removes sediment by settling prior to other treatment processes, thus protecting those processes from excess sediment and potential clogging.

Groundwater contamination – The presence of unwanted chemical compounds in groundwater. In this case, we would normally be referring to dissolved compounds, such as nitrates. It could possibly include unwanted bacteria.

Habitat – The environment where organisms, like plants, normally live.

Impervious surface – A hard surface that does not allow infiltration of rainfall into it; not pervious.

Infiltration – The process by which water (surface water, rainfall, or runoff) enters the soil.

Media, media filter bed, filter bed – The topsoil that supports plant growth. Bioretention media typically has a high sand and low clay content and a low phosphorus content.

Microbial decomposition – The breakdown of compounds or organic matters into smaller one with the aid of microorganisms.

Mulch – An organic material applied on the surface above the media to protect vegetation and underlying media.

Nutrients – Substances that are required for growth of all biological organisms. When considering water quality, the nutrients of most concern in stormwater are nitrogen and phosphorus. Excessive amounts of these substances are pollution and can cause algal blooms and dead zones to occur in streams and estuaries.

Outlet – The point of exit of water from a BMP usually through a control structure.

Pervious – A ground surface that is porous and allows infiltration into it.

Rain garden –See bioretention. Often used interchangeably with bioretention, however it typically refers to a less formal design and installation process. Typically implemented in residential areas by homeowners.

Sediment – Soil, rock, or biological material particles that are formed by weathering, decomposition, and erosion. In water environments, sediment is transported across a watershed via streams.

Stormwater – Water that originates from impervious surfaces during rain events, often associated with urban areas; also called “runoff.”

Stormwater treatment practice – A type of best management practice that is structural and reduces pollution in the water that runs through it.

Underdrain – A perforated pipe in the bottom of a bioretention cell designed to collect water that does not infiltrate native soils.

Watershed – A unit of land that drains to a single “pour point.” Boundaries are determined by water flowing from higher elevations to the pour point. A pour point is the point of exit from the watershed, or where the water

would flow out of the watershed if it were turned on end.

Stormwater treatment practice – A type of best management practice that is structural and reduces pollution in the water that runs through it.

Underdrain – A perforated pipe in the bottom of a bioretention cell designed to collect water that does not infiltrate native soils.

Watershed – A unit of land that drains to a single “pour point.” Boundaries are determined by water flowing from higher elevations to the pour point. A pour point is the point of exit from the watershed, or where the water would flow out of the watershed if it were turned on end.