Rain Gardens

Definition

Photo of a rain garden.

Rain garden (Source: LID Center)

Rain gardens, also known as bioretention cells, are vegetated depressions that store and infiltrate runoff. Uptake into plants reduces runoff volume and pollutant concentrations. The soil media is engineered to maximize infiltration and pollutant removal. Rain gardens are typically designed to avoid ponding for longer than 24 hours.

Design Variations

Rain gardens function as soil and plant-based filtration devices that remove pollutants through a variety of physical, biological, and chemical treatment processes. They can resemble miniature ponds or long strips, and may be lined or unlined, depending on site requirements. Rain gardens are used to treat stormwater that has run over impervious surfaces in commercial, residential, and industrial areas. Use of rain gardens for stormwater management is ideal for median strips, parking lot islands, and swales.

Stormwater Management Objectives

Volume

Rain gardens allow for high-rate infiltration of stormwater runoff and provide storage and exfiltration capacity to surrounding soils. These mechanisms result in substantial volume reduction of generated stormwater. Volume reductions are also realized through plant uptake and evapotranspiration facilitated by the rain gardens.

Peak Discharge

Rain gardens effectively both reduce stormwater volume and increase the duration of stormwater discharge. Controlling these two hydrologic functions serves to diminish the peak discharge of the storm event. Volume reduction decreases the total amount of stormwater discharged and duration extension decreases the energy of the discharge.

Water Quality

Rain gardens are among the best BMPs for stormwater quality control incorporating physical and microbiological remediation processes. The removal effectiveness of rain gardens has been studied during field and laboratory studies conducted at the University of Maryland (Davis et al., 1998). Bioretention has been shown to be effective at removing 90 percent of bacteria, 90 percent of organics, 90 percent of total suspended solids, 70-80 percent of Total Kjeldahl nitrogen, 93-98 percent of metals, and 70-83 percent of total phosphorus.


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