Deep Creek Fish Passage Culvert Design
The Deep Creek fish passage culvert design addressed an undersized stream crossing along an unnamed tributary outside Ninilchik, Alaska. Trihydro partnered with the Kenai Peninsula Borough and U.S. Fish and Wildlife Service to design a replacement crossing that would restore fish passage, improve ecological connectivity, and accommodate design flows, including the 100-year flood level. The completed design replaced the existing 3-foot culvert with an 11-foot main culvert and a 5-foot floodplain culvert.
The Kenai Peninsula Borough needed to replace an undersized culvert that had become a significant fish passage barrier along an unnamed tributary to Deep Creek. Installed during previous emergency flood repairs, the existing 3-foot culvert had a steep outlet drop and significant ponding at the inlet. The Deep Creek watershed supports coho, chinook, and pink salmon, as well as dolly varden and steelhead trout.
Stream Assessment and Hydraulic Analysis
Trihydro collected topographic and geomorphic data and completed detailed stream assessments to support the fish passage culvert design. The team identified a reference reach, conducted pebble count and key piece analyses, and completed a full longitudinal survey to characterize stream conditions and inform design decisions.
The project team prepared a hydrologic and hydraulic design report that included design flow development, evaluation of vertical channel adjustment potential, computer-aided design (CAD) plan development, technical specifications, and a construction cost estimate.
Construction Oversight and Ecological Connectivity
Trihydro provided construction oversight and worked closely with the Kenai Peninsula Borough to maintain permitting deadlines during construction. The completed crossing enhanced ecological connectivity, and improved access to habitat for priority salmon species within the Deep Creek watershed.
