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February 5, 2024

National Finalist: Gold Award
Water Resources

Photo from ACEC Washington
The project successfully reconnected the Lower Dungeness River to 110 acres of floodplain.

Shannon & Wilson

Project: Lower Dungeness River floodplain restoration and levee realignment
Client: Clallam County

The Lower Dungeness River floodplain restoration and levee realignment project in Clallam County successfully combines habitat restoration, riverine flood protection and drainage improvements. Shannon & Wilson played a pivotal role as the primary engineering consultant and project delivery lead, overseeing geotechnical and environmental engineering, scour analysis, levee design, floodplain restoration and drainage design, among other aspects.

The project aimed to balance the needs of fish, flood management and the local community along the Lower Dungeness River. It included the construction of a 6,000-foot setback levee designed to retain 100-, 250- and 500-year flood events. Innovative techniques were employed, such as geosynthetic reinforcement, riprap installation, and large wood complexes to enhance stability, prevent erosion and encourage fish habitat development.

Following Army Corps design guidelines, the project removed 4,430 feet of existing levee and installed 6,000 feet of new setback levee to contain the 100-year flood event with 3 feet of freeboard. It was constructed using geosynthetics and an internal drainage blanket to allow steepened side slopes that limit negative environmental impacts.

Mechanically stabilized earth walls with fish-passable culverts and vegetated wrapped faces connect the new levee with residences and public streets while maintaining an organic appearance.

The project removed 2,950 feet of existing road and 6,785 tons of contaminated material from the reconnected floodplain. Large wood complexes, side channels and native plants were installed in the reconnected floodplain to encourage riparian ecosystem development, increase spawning habitat, reduce water velocities, improve sediment storage outside the main river channel, and further reduce flood risk. Community support was essential throughout as the project included unavoidable traffic detours and recreational trail impacts.

The project successfully reconnected the river to 110 acres of floodplain, promoting spawning and rearing habitats for threatened salmon species. Despite encountering challenges like unanticipated conditions and the need for extensive community outreach, the project was completed within budget and schedule.


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