Case Study Contributors
South Fork Eel River Basin, Northern California, USA. MAP
Challenges and Lessons Learned
Design and Construction by Mendocino County Department of Transportation
The culvert lies at the confluence of Windom Creek with the South Fork Eel River. During summer months salmonid habitat conditions are poor within the South Fork Eel River adjacent to Windom Creek due to minimal flow and elevated water temperatures. Windom Creek was identified as a cold water refugia for juvenile salmonids since it maintains good flows and cold-water throughout the summer.
The existing concrete box culvert contains two 10 ft (3.0 m) wide bays sloping at 2.9%. The wide, flat concrete floor created a depth and velocity barrier for juvenile and adult salmon and steelhead. A 20 ft (6 m) long steep-sloping (1H:5V) rip rap apron extends from the culvert outlet to the South Fork Eel River, impeding passage at low to moderate flows. This site received a priority ranking of 13th in the Mendocino County Inventory and Fish Passage Evaluation Report.
A combination of low-flow walls and offset baffles were used to retrofit the culvert. A 5 ft (1.5 m) wide x 2 ft (0.6 m) high concrete low-flow wall was built within the outer half of each culvert bay. Steel offset baffles were installed within the inner half of each bay following guidelines in the California Department of Fish and Game “California Salmonid Stream Habitat Restoration Manual.” The baffles were secured with a steel tab and 1 inch (2.5 cm) threaded rod anchor. A steel sill plate half as tall as the baffles was installed at the outlet of each bay. Reduction in culvert size and capacity associated with the low-flow wall and baffles was not directly calculated since pre-project culvert capacity was considered more than adequate and because the culvert is backwatered by the downstream river during flooding.
The original design included installation of two boulder weirs below the culvert to create jump pools for low-flow passage. This feature was not constructed since the biologist and engineer present during construction believed high flows from the South Fork Eel River would backwater the rip rap apron and improve fish passage at migration flows.
Post Project Observations and Lessons Learned
The project failed to address passage of juvenile salmonids seeking thermal refugia during summer. Flow continues to plunge onto the rip rap cascade and there is no outlet pool for migrating fish to hold or leap from during low to moderate flows.
Monitors in 2005 observed swift water velocities (relative to swimming abilities of juvenile salmonids) throughout the offset baffles in both bays during winter base flow. Velocities were highest approaching the outlet. The sill plates across the outlet are too low to backwater the baffles and reduce velocities during winter base flow.
Recommendations include installing a taller outlet sill and reconfiguring the rip rap to form outlet pools below each bay.