What is a Rain Garden?
The term rain garden defies precise definition. Basically, a rain garden is a strategically located low area planted with native vegetation that intercepts runoff. Other terms include mini-wetland, storm water garden, water quality garden, stormwater marsh, backyard wetland, low swale, wetland biofilter, or bioretention pond. The variables include dimensions, design, engineering components, and plant selection.
Rain gardens are designed to direct polluted runoff into a low, vegetated area, where the pollutants can be captured and filtered. The features of a rain garden aid in this biofiltration process: a shallow basin depth, gentle side slopes, soil that allows infiltration, and vegetation that traps sediment and sediment-polluting runoff. Vegetation shields the soil surface from raindrop impact while the root mass holds the soil particles in place. Improved water quality results from the nutrient removal process as the water and pollutants come into contact with roots and microbes in the soil. Plants, trees, and groundcover absorb up to 14 times more rainwater than a grass lawn.
The design of a rain garden can be varied to accommodate soils, watershed hydrology, existing drainage patterns, aesthetics, microclimate, and purpose.
A rain garden should be placed near impervious surfaces so that rainwater and snowmelt will drain into the dip or depression. Locate the garden strategically near impervious surfaces, such as alleys, sidewalks, driveways, and under downspouts or gutters, to capture the rain as close as possible to the point where it falls. Rain gardens planted between two residential properties can channel runoff to front or back yard gardens, while simultaneously acting as a living fence between neighbors. In one instance, a rain garden located under a downspout to capture roof runoff captured approximately 14,000 gallons of water per year. Gardens should not be located over gas or water services.
Rain gardens and planted infiltration trenches have also been incorporated into parking lot designs. Look for areas where there is no curb and the drainage goes into a planted area!