Little Pilchuck Creek Fish Passage

Population levels of salmon, steelhead and other critical fish species have continued to drop across Washington State, impacting overall ecological health and the state’s lucrative fishing industry. Older culverts and other man-made infrastructure are important factors in fish declines. Physical barriers and narrow culverts with unnaturally rapid flow rates often prevent fish from accessing upstream spawning and rearing habitat. To address this important ecological issue, the Washington State Department of Transportation (WSDOT) is working to renovate or redesign hundreds of stream/road crossings across the state to meet more rigorous fish passage standards.

Little Pilchuck Creek, which crosses State Route 92 near Lake Stevens, Washington, was one of the earliest culvert-replacement projects the state pursued. The creek was a historic habitat for young salmon, but fish were partially blocked from upstream habitat by an undersized culvert. WSDOT needed a creative way to remove the fish barrier posed by the Little Pilchuck Creek culvert, while also making the process repeatable and sustainable for future fish passage projects.

GeoEngineers recommended and designed a 60-foot bridge that didn’t constrain flow and allows more natural stream behavior. The unusually large crossing was necessary to accommodate flow from the wider flood plain upstream of the crossing and minimize impacts to nearby wetlands. GeoEngineers’ natural resource protection staff worked closely with WSDOT’s landscape architect to retain and enhance floodplain wetland areas that provide refuge for juvenile salmon during high flow events. Avoiding and minimizing wetland impacts also streamlined project permitting and reduced overall project costs.

Approach

• GeoEngineers used hydraulic modeling to evaluate the crossing and recommend a design that would maintain a consistent velocity of water. Due to the site’s wide and shallow floodplain, the team determined that the proposed concept for a 40-foot-wide bridge was insufficient, and that a 60-foot-wide bridge was needed instead.
• Our geomorphologists planned for long-term sustainability by including impacts to downstream lowlands in their channel designs. In all, the team recommended that approximately 60 feet of the channel be regraded, helping restore natural stream processes of sediment transport and channel migration.
• GeoEngineers worked closely with WSDOT to avoid and mitigate environmental impacts while reducing the need for complex permits. The team expedited permitting by using a U.S. Army Corps of Engineers nationwide permit instead of a project-specific permit, reducing both permitting and construction costs.
• GeoEngineers determined the new crossing structure dimensions and designed approximately 500 linear feet of channel complete with large wood and other habitat enhancement elements. Integrating the new channel design into existing channel features at the upstream and downstream ends managed long-term geomorphic risk and reduced project construction cost.
• GeoEngineers developed floodplain grading plans that significantly reduced the need to fill adjacent wetland areas compared to the original design concept. By developing a stream design that re-aligned the channel along an adjacent path, GeoEngineers saved WSDOT money and time by avoiding the need for stream bypass and fish exclusion work.

Results

GeoEngineers’ team of hydrologists, biologists and engineers helped WSDOT replace an undersized culvert with a 60-foot bridge that drastically improved fish access to 29 miles of spawning and rearing habitat farther up Little Pilchuck Creek. By integrating existing channel features up and downstream from the Little Pilchuck Creek crossing, the team managed geomorphic risk, improved long-term sustainability and reduce project construction costs. GeoEngineers helped WSDOT restore a vital salmon habitat while giving them a roadmap that can help improve efficiency for similar future projects across the state.