Design of second Colvin Run stream restoration project is underway

Plans to counter the ongoing degradation of Colvin Run at Lake Fairfax Park in Reston are in the works, but much like climate change and the development that have contributed to the stream’s erosion, it may take some time for them to become visible.

The Fairfax County Department of Public Works and Environmental Services (DPWES) is currently working on the final design for a second phase of its Colvin Run at Lake Fairfax Park stream restoration project, a department spokesperson confirmed to Reston Now.

Phase II of the project will focus on restoring approximately 5,000 feet of an unnamed tributary that feeds into Colvin Run. The project’s first phase addressed 2,219 feet of a channel downstream of the Lake Fairfax spillway and was completed on Aug. 8, 2017, according to the DPWES stormwater improvement projects map.

“The primary goal for both projects is to improve water quality,” DPWES spokesperson Sharon North said in an email.

According to DPWES, restoring Colvin Run is necessary to reduce stream bed and bank erosion, enhance the natural habitat, maintain channel connections within Lake Fairfax Park, and improve the water quality by removing nitrogen, phosphorus, and Total Suspended Solids.

The issues that the stream is experiencing stem from a combination of increasing development in the area and the growing intensity of storms.

“Colvin Run and its tributaries are downcutting, widening and re-aligning in response to hydrologic changes after upstream development and the increased intensity and frequency of storm events,” North wrote. “This channel evolution results in soil erosion, habitat degradation and decreased water quality.”

The two phases of the Colvin Run project were determined by a scoping team with input from engineers, ecologists, landscape architects, construction managers, arborists, and other experts who assessed the area’s current conditions as well as “the potential for ecological and water quality improvement,” North says.

Colvin Run Phase I involved raising the channel’s elevation, installing boulder grade control structures that imitate bedrock outcroppings to prevent future erosion, and adding native vegetation to help stabilize the soil and surrounding habitat.

For Phase II, North says the design will call for stone and wood grade control structures to create pools, riffles, and “a base-flow channel” to help the channel and floodplain capture excessive sediment that flows in from upstream.

In a May 26 update, the project website says completion of the final design and approval of construction for Phase II are expected to come this summer, but DPWES did not respond by press time when asked whether that is still the case.

Construction on the project isn’t scheduled to begin until the summer of 2023, depending on future budget availability.

“A gap in time between the completion of design and the start of construction for a project is normal,” North said, noting that stormwater management projects typically need to get separate authorizations for design and construction.

Construction on the project’s second phase carries an estimated cost of $3.2 million that would be supported by Fairfax County’s Stormwater Service District tax.

Anticipated impacts from construction include trail closures, trail detours, noise, and increased traffic from trucks delivering equipment and materials. Construction vehicles will access Colvin Run from Lake Fairfax Park off of Lake Fairfax Drive and Hunter Mill Road.

Map via Fairfax County Department of Public Works and Environmental Services