Environmental - Award of Merit

Woodrow Wilson Bridge-Northwest Branch/Sligo Creek Stream Mitigation
Oxon Hill, Md.

Teamwork and innovation defined the Northwest Branch/Sligo Creek Stream Mitigation project, which restored these channels to their natural design to encourage the passage of fish.

The project was one part of a comprehensive environmental mitigation project developed to compensate for environmental impacts associated with the $2.4 billion construction of the new Woodrow Wilson Bridge over the Potomac River.

Northwest Branch and Sligo Creek are tributaries of the Anacosita River - which drains to the Potomac River in Washington, D.C. - and are located in highly developed, urban areas. Human influence over the years has caused blockages and barriers in the tributaries that affect natural fish passage and spawning habits.

Twelve existing stream blockages - eight in the Northwest Branch and four in Sligo Creek - were targeted for re-establishment. A high percentage of impervious surfaces in surrounding areas had resulted in significant stream channelization, causing once-meandering streams to form relatively straight flood-conveyance channels.

Sewer lines that were once buried below the stream bottom had become exposed as streams deepened and widened to accommodate increased stormwater flow. These sewer and other utility lines were the main source of fish migration blockages.

Additionally, grade-control structures installed in the 1960s and 1970s had created hydraulic drops that fish could not traverse.

Potomac Crossing Consultants and other project team members utilized in-stream structures to facilitate fish passage over the blockages. One type - the riffle-grade control structure - is an engineered application designed to allow fish passage. Compared to a traditional fish ladder, the RGC is designed to mimic natural stream riffles and allow for a more natural passage of the fish upstream to spawn.

This project was the first that used RGCs for the purpose of fish passage.

"The project was stunning in its straightforward success," one of the judges said. "It was a difficult reclamation and a creative solution."

Teamwork was important. A stream specialist and resident engineer from Potomac Crossing Consultants joined the contractor's design specialist onsite during construction, and the group held weekly progress meetings to document and solve problems as well as maintain open communication among all project stakeholders.

The design specialist was given the authority to approve design changes on the spot, saving time and money and avoiding delays associated with changing field conditions.

The Maryland Department of the Environment and the U.S Army Corps of Engineers also helped the project run smoothly by allowing the team to complete a limited amount of work "in the wet." While stream diversions were used to conduct work within a dry work area, the completed structures would often need to be tweaked to accommodate fish passage design parameters after the diversions had been removed.

The agencies allowed crews to shift boulders and riffle-grade material without diverting the stream to maximize the potential for fish passage.

The agencies also allowed limited work during the fish migration season - typically off-limits for construction activity. Because of this variance, the project team was able to ensure that the new habitat would be open to migratory fish by the following spring's spawning season.

Owner: Maryland State Highway Adminstration
Design: KCI Technologies Inc.
Construction Management: Potomac Crossing Consultants
Prime Contractor: Environmental Quality Resources

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