Sediment Transport through Road Culverts Retrofit for Fish Passage

A critical component of watershed restoration includes improved mobility within the watershed for fish and other aquatic organisms. At the large scale, this effort includes installation of fish ladders around dams and at smaller scales replacement or retrofit of road culverts or installation of roughened channels to mitigate steep channel slopes. A project to quantify changes in culvert hydraulic performance and hydraulic capacity for three culvert shapes and six fish passage retrofit designs was initiated in June 2005. In 2007, laboratory physical model experiments were conducted to evaluate sediment transport and trapping characteristics of these designs over a range of flows. Generally, experimental results indicate trapped sediment in culverts retrofit to improve fish passage decreases the effectiveness of the retrofit due to sediment deposition in areas with lower velocities (where fish can rest). Other observations include: 1. Trapped sediment reduced the effective culvert barrel roughness and, thus, decreased water depths and increased velocities through the culvert, compared to clear water experiments with the retrofit baffles. 2. High flows (culvert barrel water depth/culvert height greater than 0.5) successfully cleared trapped sediment under conditions of minimal transport from upstream 3. Preliminary results indicate moderate flows (culvert barrel water depth/culvert height between 0.25 to 0.5) in combination with moderate sediment feed rates caused the greatest accumulation of trapped sediment These experiments highlight the importance of including sediment accumulation in design and analysis, and potentially impact design recommendations for culverts retrofit for fish passage and other similar fish passage improvement structures.