Bioretention is a vegetated shallow depression where stormwater collects and is allowed to slowly percolate down into the ground. Contaminants and nutrients are removed from the stormwater by special microbes that biologically remove these components or through a filter medium that mimics the natural process. The name refers to the fact that the removed components are held within the system after removal. Each site may feature multiple bioretention areas for the treatments and storage of stormwater runoff close to the sources. Not only are bioretention areas useful in removing contaminants from stormwater, they also add aesthetic value to a property.
- An increase in groundwater recharge because more water can infiltrate into the soil.
- A reduction in the volume of runoff water on a site by 35 to 50 percent.
- The addition of valuable wildlife habitat and, thus, an increase in wildlife.
- An increase in the aesthetic value of yards and neighborhoods.
- Better protection against floods.
- Improved water quality in lakes and canals.
When installing bioretention areas on a site, be sure to consider the following:
- Using an underdrain or perforated pipe system on the bottom of the filter bed may encourage water to be drawn further underground between storm events.
- To ensure long-term success, the bioretention site should be functional, maintainable and attractive.
- Multiple bioretention areas close to the sources of pollution may result in lower-impact designs.
- Bioretention cells are most beneficial on small sites less than 5 acres.
- Proper landscaping and maintenance is critical to the performance of a bioretention area:
- Select native vegetation with habitat value.
- Select species that can withstand frequent and substantial waterflow. Plants that tolerate both wet and dry conditions work best.
- Consider planting a combination of herbaceous materials and trees and shrubs.
- Design the planting layout to allow for easy access. Also, make sure it allows for periodic removal of sediments without significant disruption — or the removal — of plants.
- A lack of consideration for these details can lead to failed plantings and increased flooding.
See the Retrofit Considerations page for guidance when integrating bioretention into an existing water management system.
Bioretention areas can add aesthetic appeal to urban areas, while also providing havitat for wildlife. Courtesy of Nevue Ngan Associations.
Bioretention areas reduce flooding of sidewalks and roads by absorbing excess stormwater. Courtesy of Nevue Ngan Associations.
Internal bioretention works well in parking lots, where water can be directed to a central location. Courtesy of Center for Watershed Protection.