Waterway culverts are very common structures along water systems, ranging from rural roads to national highways and urban drainage networks. Current design guidelines are inadequate for fish passage, especially small-bodied Australian native fish species. Physical modelling was conducted in the laboratory under controlled flow conditions with the aim to maximise slow flow and recirculation regions suitable to upstream passage of small fish. An asymmetrical boundary roughness configuration was tested, consisting of a rough invert and rough sidewall, and the results were compared to a smooth boundary configuration. The hydrodynamic measurements showed the marked effect of boundary roughness on the distributions of time-averaged velocity and velocity fluctuations. Experiments were also conducted on two small fish species (Silver perch, Duboulay's Rainbowfish). This roughness configuration appeared to provide secondary current regions next to the rough sidewall and at the corner between the rough sidewall and channel bed.
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