(Conservation Currents,
Northern Virginia Soil and Water Conservation District, October
2004)
The Northern Virginia Soil and Water Conservation
District entered new territory this summer 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 .6 acres, which is mostly 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, contact
Asad Rouhi via e-mail
or at 703-324-1427, TTY 711.
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