The Snakeden Branch at Lake Audubon stream restoration project consisted of restoration of 739 linear feet of channel that, over time, had experienced significant geomorphologic change in terms of lateral bank erosion and incision, or deepening, of the stream bed. Bank erosion and channel incision ultimately led to exposure of the network of sanitary sewer laterals servicing the nearby community and the receiving 8” sanitary main running parallel with the stream.
Located in Reston, VA, the upper end of the project began at a 66” RCP Outfall running beneath South Lakes Drive, terminating into Lake Audubon at the downstream end. Sandwiched within a small, confined area between two neighborhoods, in a largely urbanized area, impervious runoff into storm sewers ultimately led to the degradation of the stream channel and was the primary driver for the restoration efforts ultimately undertaken.
Overall, the design approach for restoration of this channel was to raise the existing channel bed to provide connectivity to the floodplain with grade control structures to reduce sediment deposition within Lake Audubon while also protecting the sanitary infrastructure. Raising of the channel bed provided floodplain connectivity, but also raised the water table. Allowing for riparian vegetation along the stream banks access to water which it had previously not had due to the large elevation difference between the top of bank and water surface.
What made this project truly unique was the use of concrete blocks in lieu of the standard imbricated boulders to construct the grade control features. Use of concrete blocks was a new facet of restoration for the entire project team. Through a great degree of teamwork and collaboration with the client: Fairfax County Stormwater Planning Division, and the design engineer: Stantec Consulting Services, Inc., the hurdles of how to produce and install these uniformly sized & logistically challenging blocks in an approach largely based on the use of irregular, quarried stone was navigated smoothly and successfully.
Double Rock Step constructed using Concrete Blocks
Modified Cross Vane at the downstream extent of the project
From the block to block connectivity to the methodology in how to handle and place, construction crews had to reorient their thinking of how to build these structures. The irregularity of quarried stone has its own nuances that require a specialized eye and touch to comply with the geometry of the design. However, with use of concrete blocks, devoid of irregularity and deviations, the entire construction process was flipped on its head, seemingly merging the construction approach of typical grade control structures with the precision and delicacy associated with installation of precast concrete components.
In total 9 structures were built, all comprised entirely of concrete blocks in two distinct sizes in various configurations along the stream channel. Blocks were utilized in a linear sill to stabilize an exposed sanitary line at the top of the project, in several step structures to gain the necessary elevation drop to gradually tie into Lake Audubon, and in modified cross vanes to facilitate floodplain activation in storm events and stabilize banks along stream meanders.
Concrete blocks set this project apart, but the overall ecological uplift and long-term stability provided by the whole of the restoration efforts is where the benefits truly lie. The existing tree canopy removed to facilitate construction was largely retained on site for use in habitat assemblages, reducing the carbon footprint. Removed canopy was then re-vitalized with planting of native species’ in a volume than what had been there previously with the plant palette being designed to provide a dense riparian buffer.
Modified Cross Vane constructed using the Concrete Blocks surrounded by Native riparian seeding and planting
The confined nature of this project had its hurdles in terms of managing material flow into and out of the site. Primary construction access shared the driveway of one home, snaking in between another before reaching the primary staging area. Construction is not an inherently quiet process, especially when considering the 1,000+ Tons of stone delivered to the project and the 276 concrete blocks brought in by tractor trailer and transported down the access. However, the neighboring community was extremely understanding and accommodating of the construction process, making the project possible.
Extremely tight access with close proximity to adjacent homes and a treacherous slope to contend with for access of primary staging area.
Double Rock Step in action following a rain event
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