Rain Garden Detains Stormwater
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
The Northern Virginia Soil and Water Conservation District entered new territory in summer 2004 by designing and overseeing the installation of a rain garden to address a community’s drainage problem.
Yorktowne Square Condominiums, home of a green roof and a certified backyard habitat, sought help from the district to deal with a marshy common area with extensive erosion.
Already considered experts in stream restoration design and implementation, the district has expanded its broad skills in stormwater management through projects to retrofit water quality ponds and, more recently, to engineer rain gardens.
Teaming up with the district on the Yorktowne project was the Virginia Department of Forestry and Fairfax County's Maintenance and Stormwater Management Division, which provided equipment and operators to do the construction. Yorktowne supplied the construction materials purchased with state grant funds.
The rain garden will filter pollutants from the first half-inch of rain from any storm. The rain garden also will help with drainage because it replaces poor soil and inadequate grading with permeable soil and a basin where stormwater can be collected and gradually filtered. The outfall for the rain garden is an ivy covered wooded area that lay between the homeowner association property and I-495. The rain garden drains an area of 0.6 acres, which includes rooftops and a parking lot.
In this engineered rain garden, the hole was dug 4 feet deep with a 600 square foot surface area. Across the bottom of the ponding area, on top of a 6 inch base of gravel (VDOT #57), lay a perforated drain pipe. Over the pipe is 6 more inches of gravel (VDOT #57), and then a 6 inch layer of pea gravel.
On top of the gravel layers is a manufactured soil (Biofilter ®) with 25 percent organic material. The soil is covered with mulch. The mulch is held down by netting to reduce the likelihood of it floating away after a storm. The rain garden has many plants, all native to the area and tolerant of moist soil. The plant roots and mulch are responsible for pollutant removal.
For more information about rain gardens, see: Rain Gardens, Green Roofs and Other LID Practices.