Developing Cost-Effective Design Guidelines for Fish-Friendly Box Culverts, with a Focus on Small Fish

Low-level river crossings can have negative impacts on freshwater ecosystems, including blocking upstream fish passage. In order to restore upstream fish passage in culverts, we developed physically-based design methods to yield cost-effective culvert structures in order to maintain or restore waterway connectivity for a range of small-bodied fish species. New guidelines are proposed for fish-friendly multi-cell box culvert designs based upon two basic concepts: (1) the culvert design is optimised for fish passage for small to medium water discharges, and for flood capacity for larger discharges, and (2) low-velocity zones in the culvert barrel are defined in terms of a percentage of the wetted flow area where the local longitudinal velocity component is less than a characteristic fish speed linked to swimming performances of targeted fish species. This approach is novel and relies upon an accurate physically-based knowledge of the entire velocity field in the barrel, specifically the longitudinal velocity map, because fish tend to target low-velocity zone (LVZ) boundaries. The influence of the relative discharge threshold Q₁/Q_des, characteristic fish swimming speed U_fish, and percentage of flow area on the size of box culvert structures was assessed. The results showed that the increase in culvert size and cost might become significant for a smooth culvert barrel with U_fish < 0.3 m/s and Q₁/Q_des > 0.3, when providing 15% flow area with 0 < V_x < U_fish. Similar trends were seen for culvert barrel with recessed cell(s).