Author(s): J. Cabonce; H. Wang; H. Chanson
Keywords: Box culverts; Upstream fish passage; Small-bodied fish; Triangular corner baffles; Physical modelling; Fish testing; Fish-friendly culvert design guidelines
Abstract: Current culvert designs have little evolved since ancient designs. Some recognition of the ecological impact of culverts on natural streams and rivers led to changes in culvert design guidelines, too often associated with un-economical design recommendations. A simple small triangular corner baffle system may assist upstream passage of small body-mass fish in box culvert structures on very flat bed slope, while inducing little reduction in discharge capacity at design flow conditions and creating sizeable slow flow regions at less-than-design flow conditions. The system was tested systematically in a nearfull-scale physical model, 0.5 m wide and 12 m long. The present investigation delivered a detailed characterisation of the flow field in smooth and triangular baffled channels, at a scale comparable to a small standard box culvert barrel. Tests showed that small-bodied fish preferred to swim in slow-velocity regions, typically in the baffle corner. To be most effective, the corner baffle size has to be comparable with the fish dimensions, and strong flow reversal must be avoided, since it might confuse fish attempting upstream passage. Finally, design guidelines of fish-friendly culverts must be re-thought, with a focus on fish passage for less-than-design flows and maximising the discharge capacity at design flow. Current design practices must evolve from a semi-empirical approach based heavily on simplistic observations and educated guesses to advanced physics-based theoretical considerations and sound engineering guidelines.