Overcoming Obstacles on Trabuco Creek with Physical Modeling

One of the barriers to Steelhead recovery on Trabuco Creek in San Juan Capistrano, CA is at the Metrolink Rail Crossing where a grouted rock drop structure has been installed to protect existing utility crossings and bridge piers and now impedes upstream passage for adult and juvenile fish. The elevation drop at the barrier has increased by 20 feet since 1940 and continues to grow, requiring frequent repairs to the grouted riprap. An engineered fishway and a replacement drop structure and flood channel have been proposed to provide sustainable protection to existing infrastructure and provide passage for the southern California Steelhead, a federally listed endangered species. The fishway design uses a combination of natural rock materials and concrete and because of its configuration, length, and drop height would be the first of its kind in California. The design of the fishway, flood channel, and diversion is critical to ensure fish passage over a wide range of creek flows, prevent sediment from clogging the fishway, and force large flood flows into the floodway. A 1:6 and 1:20 scaled physical hydraulic model were constructed of the proposed fishway and flood channel which allowed designers and stakeholders to verify the stability of the proposed channels over a wide range of flows, optimize the diversion between the fishway and floodway, and adjust the fishway design to improve passage characteristics for juvenile and adult fish. The physical models were used to identify modifications that resulted in significant improvements to the hydraulic performance of the design.