Weekly “What is it?”: Bioretention

 

 

 

Managing stormwater runoff is an ongoing issue for neighborhoods and municipalities nationwide. Preventing impacts of potential flooding is usually the most immediate concern. But more long-lasting are the legacies of water quality decline left by the contaminants that wash off the streets with the rainfall. While the state of Florida has utilized stormwater ponds for 40 years to collect and filter this polluted runoff, ongoing research has begun to show they are not the most effective means of management.

 

 

 

While traditional stormwater treatment has focused on centralized collection of runoff from curbs, gutters, drains, and large ponds, low impact development (LID) hinges on a decentralized method of treatment. LID, coupled with Green Stormwater Infrastructure (GSI), relies on strategically placed, often smaller, methods of capturing and treating stormwater runoff throughout a development or community.

 

 

One of those methods is bioretention. Bioretention cells are often small vegetated areas on roadsides or in parking lots that collect runoff from the immediate vicinity and treat it through soil filtration and nutrient uptake. As opposed to one or two large stormwater ponds, developments using LID/GSI would include numerous small bioretention areas. They are typically more aesthetically pleasing than your average stormwater pond, planted deliberately with native trees, grasses, or flowers that can handle being inundated by water for short periods. Bioretention areas are often less than a foot deep, and sometimes only dug down 6-8” and bordered with curbing. Runoff typically enters bioretention areas via sheetflow from the street, or curb cuts—smaller openings that funnel runoff into the planted area.

 

 

While they may look like small gardens, bioretention cells are actually highly engineered treatment systems designed to handle a calculated amount of runoff. To count as functioning stormwater treatment, they must be permitted and approved by state and local environmental regulatory organizations. Homeowner rain gardens are very similar to bioretention areas, but bioretention cells typically include a gravel bed, a drain placed below the soil, and an overflow device.