Cape Fear Fish Passage Design
A historic lock and dam (L&D) structure in North Carolina had become an impediment to fish migration, resulting in significant population decline. The U.S. Army Corps of Engineers turned to Tetra Tech to design a unique fish passage that will allow a natural route for migration, improve fish populations, and strengthen the local economy.
The nearly 100-year-old L&D No. 1 on the Cape Fear River near Wilmington, North Carolina, was a rock-filled, timber crib structure with a one-foot-thick concrete cap historically used to support commercial navigation and freight transportation. The need for the dam, along with upstream L&D No. 2 and 3, declined as more available and efficient land-based transportation was introduced. The dam became an impediment to anadromous fish attempting to reach upstream spawning areas, resulting in significant population declines in species including striped bass, Atlantic sturgeon, shortnose sturgeon, river herring, and American shad.
Biologists from the U.S. Geological Survey and North Carolina Division of Marine Fisheries estimate that local fish species are present at small percentages of their potential population: striped bass, 10 percent; endangered Atlantic sturgeon, 12 percent; and shortnose sturgeon, 1 percent. Previous efforts to allow spawning fish access to upstream areas still allowed only about half of shad and striped bass to make it past L&D No. 1.
The U.S. Army Corps of Engineers, Wilmington District, selected Tetra Tech to design a new fish passage at L&D No. 1. The fish passage mitigates potential effects on the endangered shortnose sturgeon and other fish species due to rock removal and blasting for the proposed Wilmington Harbor deepening project.
The fish passage structure consists of a series of rock arched ramps with weir and pool configurations, allowing for a 3 to 5 percent variable design slope for fish passage. Tetra Tech also designed a steel sheet pile retaining wall, which is parallel with the existing lock structure to prevent proposed fish passage rock loads from destabilizing it.
This time-sensitive American Recovery and Reinvestment Act-funded project will improve fish habitat and increase fish populations, which in turn will support larger prey species and enhance the ecosystem. The project will also have a positive economic effect on the surrounding community. Improving the fishery in Cape Fear will bring anglers from around the region, boosting the local economy.