July 29, 2004
Improving traffic flow for fish, people
By JON IVES, ANDY WONES and BRIAN HIGGINS
Jones & Stokes
The city of Mill Creek had a traffic flow problem not only affecting people, but also fish. At the Penny Creek-Ninth Avenue Southeast junction, fish faced an impasse with a fate far worse than a rush-hour delay coho and chinook salmon and cutthroat trout could not complete their voyage upstream due to an inadequate culvert.
The city called on a team of consultants and engineers that included local engineering firm Schaaf & Wheeler and environmental consulting firm Jones & Stokes to address the fish passage obstruction as well as bring the roadway up to city standards.
The project was a multipurpose undertaking that allowed anadromous fish movement up Penny Creek, improved safety for the residents, and restored native plant species to the area. Its design combined hydrologic, hydraulic, geomorphologic, biological, fisheries, traffic, and safety considerations into plans and specifications that addressed traffic flow problems for both fish and residents.
"Our primary focus was to improve fish passage and provide adequate conveyance where the existing culvert was seriously undersized," Jones & Stokes project manager Andy Wones said, "but, this also was an opportunity to bring this crossing up to city standards for road crossings and improve pedestrian safety."
Because the existing 48-inch culvert was undersized and perched above the downstream water surface, fish could not easily traverse Penny Creek and use a fish ladder 1,700 feet upstream. The project team replaced the culvert with a 17-foot-wide, 50-foot-long, 9-foot-high box culvert, which allowed the streambed to be returned to its natural channel dimensions and provided access to an additional two miles of spawning and rearing habitat.
Because of the undersized culvert and unstable wing walls, the roadway crossing was at risk of washing out and isolating residents on Ninth Avenue.
In addition to improving passage conditions for both salmon and trout, the project also reconstructed the Ninth Avenue roadway. The road was straightened and widened from 17 feet to 25 feet to match the road section to the north and to enhance traffic flow towards the downtown area. For foot and bicycle traffic, guardrails and sidewalks were added, including a 10-foot-wide sidewalk that extends the North Creek Trail.
The city places a high value on aesthetics, so revegetation of the construction area became an opportunity to restore ecological function by replacing invasive plants with appropriate native vegetation, and to create landscaping in keeping with the city's aesthetic values. Streamside and roadside plantings were installed and the guardrail on both sides of the road was painted green to blend into surrounding vegetation and to match other city guardrails.
An innovative approach was needed to maintain bed stability and minimize impacts to the stream channel. An inflatable rubber dam was installed upstream of the project site to impound the natural stream flow for diversion around the culvert without disturbing the channel morphology. Downstream of the culvert, a state Department of Fish and Wildlife-designed log sill was installed to provide grade control.
The project, which achieved the city's goals in a timely and cost-effective manner, provided a unique opportunity for the project team to combine its diverse expertise to address a variety of complex challenges. It recently won an Honor Award from the Seattle Section of the American Society of Civil Engineers.
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