Bank Stabilization at Accotink Creek
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
Accotink Creek is the longest stream in Fairfax County, stretching about 32 miles from the City of Fairfax to the Gunston Cove. It begins with small feeder creeks and flows through Annandale and North Springfield and then passes through Lake Accotink.
Like many streams in Fairfax County, Accotink Creek has serious erosion. Development in the watershed has replaced absorbent soil and vegetation with impervious (nonporous) surface such as roofs, roads, and parking lots.
Below the dam of Lake Accotink is where the Virginia Department of Forestry (VDOF) and the Northern Virginia Soil and Water Conservation District (NVSWCD) demonstrated several stream bank stabilization techniques to 40 participants at the end of an intensive three-day workshop.
Jim McGlone is the assistant manager at Lake Accotink Park through which a stretch of Accotink Creek flows. He offered the site for the workshop because he is concerned about the ecosystem of the stream as well as the Potomac River into which the stream empties. He is particularly interested in maintaining the stream for its fish habitat. The park stocks the stream for trout fishing every spring and fall.
VDOF’s Judy Okay and NVSWCD’s Asad Rouhi showed the group several bioengineering techniques to protect the banks and improve habitat including biodegradable logs and erosion control matting, shrubs and live stakes, and cedar revetments. In addition, the group learned about structural practices including a-jacks and rock cross vanes. (Click here for definitions.)
The group donned its hip waders and jumped right in. Mike Blake of the John Marshall Soil and Water Conservation District in Fauquier County was impressed with what he saw and heard. “ We don’t do much with stream stabilization in Fauquier, so these techniques are new to me.”
Joining the workshop team was the Fairfax County Department of Public Works and Environmental Services, which operated the heavy equipment needed to lift the larger rocks in the cross vane.
An increasing number of local and county governments, homeowner associations, and watershed groups are seeking the services of the state forestry department and the conservation district for hands-on stream stabilization training.
A-jacks are three two-foot long porous cement stakes that fit together just like toy jacks. They are placed in a row along a trench at the toe of a stream so that each a-jack is interconnected with its neighbor. They add structural stability to the lower bank.
Biologs are tightly bound cylinders of coconut (coir) fiber held together by coir fiber netting. Generally, they come in lengths of 10-20 feet and diameters of 10-12 inches. They are installed at the toe of a bank. The material is tough, flexible, and absorbent. Once installed, the biolog becomes saturated with water, and vegetation can be planted directly in it. By the time the log degrades in seven or eight years, a root network of plants will have been established through and behind it.
A tree revetment consists of a row of trees that have been cut, moved into place, and anchored against the lower part of an eroding stream bank. Tree revetments protect the banks until other vegetation can grow. Cedar trees commonly are used for revetments.
Live stakes are branches cut from live but dormant trees and shrubs such as river birch, alder, red twig dogwood, or willow. They are stuck into the banks through biodegradable erosion control matting. The stakes will root and grow, holding soil in place and providing wildlife habitat. Fascines are tightly bound bundles of live stakes. Typically, fascines are placed in a shallow trench along the stream bank parallel to the stream.
Rock Cross Vane
A rock cross vane is a rock structure which extends upstream from both sides of the bank. Its purpose is to concentrate the flow in the middle of the stream, thus narrowing the flow path. As a result, it removes stress from the banks and prevents erosion. A rock cross vane increases the flow depth upstream from the structure, which improves fish habitat.