2018 was Northern Virginia’s wettest year on record since 1889, and was a soggy reminder that Fairfax County is susceptible to more frequent heavy rainstorms. With more than 800 miles of streams in Fairfax County, residents living near these waterways endured flooding and witnessed the resulting erosion of public and private property.
One area that has experienced extensive damage in recent years is a short stretch of Backlick Run in the Alexandria area of the Mason Magisterial District. Here the stream makes a sharp turn as it passes under a railroad bridge and then makes a wide arc past an office park, a residential neighborhood, and parkland. Aerial photos show the stream channel’s arc has been widening and cutting into its northern bank. The stream is estimated to have migrated by as much as 35 feet in the past year, and in the past 12 years the stream has washed away nearly two and a half acres of Fairfax County Park Authority property. Streams meander and change course naturally, but in an urban environment such a drastic change in channel alignment has pronounced impacts on water quality, existing infrastructure, and the surrounding community.
Fixing the problems at this location is complicated by the need to obtain access easements from multiple property owners and by the presence of sensitive transportation and wastewater infrastructure. Several projects are underway or being planned to build resiliency in this problematic stream corridor. The goals of these projects are to improve in-stream habitat and water quality, maintain public safety, limit potential environmental degradation, and to protect public and private property.
Mason District Supervisor Penelope (Penny) Gross recognizes the impact of urban development on the environment and the challenges it creates over time. “In normal years, stabilization might use standard infrastructure repairs, but the heavier damage, caused by more frequent and more intense storms, demands new and creative approaches,” Gross said. “The partnership between Fairfax County, property owners, and other entities brings together the vital resources needed to restore the stream environment, and protect the long-term public and private investment in this area.”
Backlick Run is located within the Cameron Run Watershed, which drains 42 square miles of central Fairfax County and small parts of Falls Church and Alexandria. Indian Run and Turkeycock Run flow into Backlick Run before it connects to Holmes Run, then Cameron Run, and finally to the Potomac River near the Huntington area. Two branches of Baclick Run originate in Annandale and Springfield, and the stream’s drainage area is 6.3 square miles, of which 36 percent is impervious surfaces that don’t absorb water into the ground. This includes stormwater runoff from the Springfield Interchange’s 50 ramps and bridges.
In December 2016, a Fairfax County resident reported extensive erosion behind his home on Bren Mar Drive. Stormwater Planning Division staff from the Department of Public Works and Environmental Services arrived after the holidays to find that the stream’s main channel had migrated to within 40 feet if the resident’s property and was also encroaching on his neighbors’ lots. County staff determined that if the erosion continued unabated, the resident’s shed would be at risk of falling into Backlick Run.
In June 2017, the county awarded Wetland Studies and Solutions, Inc. a contract to prepare only a concept to restore the stream due to limited funding. (The need for this project was included in the long-term management plan for the Cameron Run Watershed in 2011-2012.) In July 2018, the remaining work toward a final plan was authorized. When Stormwater Planning Staff visited the site during the summer of 2018 to assess the existing conditions, what they found was a situation that had the potential to be more catastrophic than previously defined.
Backlick Run had migrated to within 10 feet of the residents’ property during a recent storm, and a 40-foot section of an adjacent 33-inch sanitary sewer pipe had been exposed by the water’s erosive force. The pipe was in danger of being further undermined by future high-flow events. If the pipe were to rupture, wastewater would spill into the stream. Not only would this disrupt local residential and business service and lead to costly sewer line repairs, but such a discharge would cause environmental damage impacting wildlife dependent upon Backlick Run.
“Despite knowing this stream reach pretty well, I was still shocked to see the exposed sanitary line following the previous week’s three days of rain,” said project manager Dave Anglin. “The new erosion left behind an 18-foot cliff face.”
An emergency response was authorized to replace soil over the exposed sanitary sewer pipe and armor the stream bank with large boulders to protect it against further erosion. This was a temporary step to protect the pipe and stabilize the banks until the more comprehensive stream restoration project could be implemented. Patriot Construction Corporation completed the work and quickly reburied the pipe in August; however, Backlick Run was still migrating north.
DPWES staff determined that a second effort was needed to further fortify the stream bank and ensure the sanitary sewer pipe remained buried. Patriot, under the expert guidance of WSSI and Fairfax County staff, used heavy equipment to place massive boulders to reestablish the stream’s original channel. This step is also temporary, but it buys the team time until the long-term stream restoration project is designed and permitted.
Final restoration designs are due in May 2019, and the project will be advertised for bids in August. Restoring 2,366 linear feet downstream of the railroad bridge crossing will take approximately a year to complete once the project begins, at a cost of $4.4 million. A second project slated for 2020 will restore approximately 2,800 linear feet of Backlick Run’s main stem channel and a tributary located upstream from the railroad bridge at the current site.
Unfortunately, the severe erosion is threatening more than just private property and the county’s wastewater infrastructure. DPWES staff have observed erosion behind a retaining wall that protects a Norfolk Southern Corporation railroad used by freight trains and Virginia Railway Express commuter trains. The stream has scoured under the wall that protects the foundation of the railroad bed, and soil is slowly washing away through the newly formed pool. Fairfax County is collaborating with Norfolk Southern Corp. in developing the stream restoration plan.
Norfolk Southern has dealt with flood damage at this bend in Backlick Run before. The company repaired the foundation of its railway bridge after the remnants of a tropical storm sent raging stormwater rushing down the channel. It is estimated that during 100-year flood events, water flows under the bridge at a rate of 5,800 cubic feet (43,383 gallons) per second.
This volume of water is equivalent to nearly four Olympic-size swimming pools flowing under the bridge per minute!
In addition to the undermined retaining wall, frequent storms in recent years have exposed an old sanitary sewer line that runs under the bridge and parallel to the railroad tracks. The line was installed in the 1950s and decommissioned in the ‘80s, but for unknown reasons it was never sealed. DPWES is working with Norfolk Southern to obtain permission to fill the pipe with grout to reduce the risk of cave-ins that could potentially destabilize the ground supporting the railroad tracks. Fort Meyer Construction Corporation was awarded the contract and will proceed with the abandonment project once the permitting process is completed.
Finally, Backlick Run is also impacting commercial property. Bren Mar Office Park, located on Bren Mar Drive adjacent to the residential area, is home to several private businesses and a Federal Homeland Security operations center. A small unnamed tributary that traverses the office park property and feeds into Backlick Run is eroding the property with every storm. In recent years, the office park has addressed erosion problems to protect its fence line and parking lot, but more stabilization is needed. The office park’s erosion problems should be improved by the planned stream restoration project which will repair the stream banks along the southern edge of the property.
An Afghani proverb says, “A little water is a sea to an ant.” In this case, residents and businesses located near Backlick Run are the ants. And much like the way in which ants work together as a unit, DPWES is working with partners and property owners to build resiliency in the stream corridor.
The emergency projects have already protected the exposed sanitary sewer pipe from damage similar to what occurred in 2015 when a sanitary sewer line immediately downstream ruptured. The projects to seal the abandoned sewer line in the railroad embankment and to restore the stream using natural channel design principles will further protect the properties along Backlick Run, reduce the potential for damage from future storms, meet the requirements of the county’s Municipal Separate Storm Sewer Permit, and promote the ecological value of this stream reach for local wildlife and residents.
Completed and Planned Stream Restorations
The Department of Public Works and Environmental Services has completed 36 stream restoration projects and has 35 more planned or in construction. More than nine miles of streams have been restored since 2010, and more than five miles are in construction. An additional 1.2 miles are planned for construction in fiscal year 2019 (July 1, 2018 to June 30, 2019), and 13.3 miles are in the design phase for fiscal year 2020 and beyond. Stream restoration projects are funded through the Stormwater Service District.