Comprehensive study of the hydrodynamic effects of weirs installed in the bypass of bar rack systems to better guide downstream migrating fish

Turbulent flows are investigated in the bypass of a bar rack system to understand the effects of weirs placed at the bypass entrance on hydraulic conditions used to guide fish moving downstream. Three test cases were studied: (a) no weir (reference case), (b) arc-shaped weir and (c) triangular weir. A stereo particle image velocimetry system was employed to acquire the three velocity components in the mid-plane of the bypass and examine various statistics including mean velocities, Reynolds stresses, turbulent kinetic energy as well as turbulence eddies and swirling strength. The weirs deflect the flow away from the channel bed due to the blockage effect and increase the bypass velocity significantly, though remains under the suggested critical value of 1 m s−1 when the approach velocity was 0.38 m s−1. In the wake region of the triangular weir, a distinct recirculation bubble forms which may negatively affect fish migration. There is an increase in the level of Reynolds stresses and turbulent kinetic energy in the upstream vicinity of the weirs and the shear layers issuing from their tips. It is shown that the topology of the turbulence eddies is greatly modified by the weirs which can more readily assist fish guidance toward the bypass. Also, the swirling strength of the turbulence eddies behind the triangular weir is more pronounced compared to the arc-shaped weir. For the reference case, the swirling strength of the eddies is the weakest.