Little Pimmit Run Stream Restoration


(Conservation Currents, Northern Virginia Soil and Water Conservation District)

One of the greatest environmental problems throughout Fairfax County is stream degradation. Local stream channels downcut and widen as development of the upstream watershed increases the volume and speed of water entering our streams. The sediment eroded from stream channels and banks smothers in-stream habitat and degrades water quality. Stream restoration is expensive, but often is the only effective solution. These projects may involve re-creating or re-contouring the channel, enhancing riffles, adding rock structures, modifying stream banks and increasing streamside vegetation.

In summer 2007, a district-facilitated partnership restored a 775-foot segment of Little Pimmit Run in McLean. Homeowners adjacent to the stream initiated the primarily privately-funded project. Steep-walled and undercut stream banks and degraded riparian habitat characterized the stream prior to the project. Rapid erosion (~2 ft/yr) and bank instability threatened the Fairfax County parkland adjacent to the stream along with the backyards of several homeowners.

The district worked with the homeowners to assemble a team of professionals to design and build the restoration project. The project’s designers, Randy Sewell of Vanasse Hansen Brustlin Inc. and the district’s Asad Rouhi, applied the principles of natural channel design, an approach that takes into account naturally-occurring stream processes such as flooding and sediment transport as well as stream ecology. District and VHB staff took stream measurements, completed mathematical analyses and applied computer models to assess the stream, before drafting a restoration plan. The design they developed provides habitat, protects property, discourages erosion, protects water quality and facilitates recreation along and within Little Pimmit Run.

The newly stabilized stream section includes a stone stream crossing for trail users. It also includes several J-hooks, stone candy cane shaped structures that protrude into the stream to control and direct stream flow. The J-hooks constructed as part of the Little Pimmit Run project protect the stream banks by directing flowing water into the center of the stream channel. J-hooks can also encourage localized in-stream sedimentation because they change the level or grade of the stream. For this reason, they can effectively protect in-stream utility lines. A J-hook at Little Pimmit Run protects an 8-inch sanitary sewer line that crosses the stream.

Degraded stream pre-project
Eroding banks threaten trees and property prior to restoration and in-stream habitat is limited.
Restored Little Pimmit Run
Stacked stone walls stabilize the restored stream. New pools and riffles support aquatic wildlife.
J-hook under construction
A J-hook under construction.
Stone stream crossing
A stream crossing provides pedestrian access to trails along Little Pimmit Run.

 

Stacked stone walls now form the bank of the stream in several places. The attractive stone structures discourage migration of the stream channel and protect property adjacent to Little Pimmit Run. Within the channel, new riffles and pools provide habitat for fish and aquatic insects; vegetation along the banks and on sediment benches within the channel will absorb pollutants, provide habitat, prevent erosion and mitigate flooding.

A stream restoration project requires a skilled construction team to turn the design into a successful, functioning final project. Angler Environmental, an environmental consulting and construction firm, worked closely with the designers. Their skill with heavy machinery and their understanding of stream dynamics and ecology qualified them to place rock, shape the channels and banks, and plant new streamside vegetation. Their professionalism also ensured that the project was completed on time and within budget.

The Virginia Chapter of the Soil and Water Conservation Society recognized both the design and construction teams associated with the Little Pimmit Run project for their ecological sensitivity, their team approach and the excellence of their work. At their annual fall meeting, the chapter awarded VHB and the district the Ecological Excellence Award for Design. Angler Environmental received the chapter’s Ecological Excellence Award for Construction. The district’s $250 portion of the cash award will be used for future maintenance of the project.

Even with the skill and knowledge of the design and build teams, the Little Pimmit Run stream restoration could not have taken place without the cooperation of several agency partners, specifically the Fairfax County Park Authority, Fairfax County Wastewater Management and the Dranesville District Supervisor. These agencies contributed 1/3 of the funds for the $385,000 project because of the infrastructure protection improvements and recreation opportunities it provides. In the process, they created a special public-private partnership that demonstrated citizens and government can work together to solve environmental problems.

Since the project’s completion in June 2007, the restored stream channel has successfully withstood several large storms. The riffles host a growing abundance and diversity of aquatic insects, the building blocks of the food chain that includes fish, mammals and birds. On a recent visit, a great blue heron snagged a fish in a new stream pool.

Both the homeowners and the government partners are pleased with the project. The homeowners are relieved that their homes and properties are safe. For the government partners, the benefits include the water quality, property and infrastructure protection benefits the project provides. The project is also valuable as a demonstration site and reference for future stream restoration in Fairfax County.

“The Little Pimmit project features two techniques, the stone stream crossing and the J-hook that protects the sewer line, that have not been used in a Fairfax County stream restoration project,” testifies the district’s Rouhi. “This project is a model for stabilization in urban streams where there are a lot of technical and financial restrictions.”

In addition to the design elements that make the project significant, Rouhi is quick to recognize that the project’s success really depended upon human factors. “If all of the partners involved in this project had not come together, the Little Pimmit Run project would not have happened.”

District Administrator Diane Hoffman agrees, “Managing a project like this with many invested partners is challenging, but the project could not have succeeded without everyone involved. The success of the Little Pimmit Run project serves as a testament to the power and value of partnerships.”


Little Pimmit Run Drainage Channel Post-Construction

Little Pimmit Run Stream Bank and Drainage ChannelNVSWCD revisted Little Pimmit Run in Fall 2011. At Little Pimmit Run, runoff from a drainage ditch (left) and stream bank erosion threatened an exposed 21-inch sanitary sewer main. The stream restoration project solved the issue by stabilizing the stream bank (100 linear feet) and drainage channel (70 linear feet) using J-hook rock vanes, floodplain benching, step pools and re-vegetation. The project was funded by Fairfax County Dept. of Public Works and Environmental Services and NVSWCD managed the design, permitting, construction, as well as the outreach and coordination with the community that facilitated construction of the project.


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