A CFD-based investigation of hydraulic regimes in overflow weir fishways

Overflow weir fishways represent a basic type of pool-and-weir fishway structure. To investigate its hydraulic characteristics, this study conducts numerical simulations to obtain the flow field in a typical pool chamber. A total of nine cases are designed by varying the inlet discharge and pool depth, and the flow patterns, velocity distribution, turbulent kinetic energy, and dissipation rate are systematically analyzed. Three flow regimes are identified: plunging flow, streaming flow, and transition flow. The variation laws of these regimes with inlet discharge and pool depth are obtained. The distributions and evolution characteristics of velocity, turbulent kinetic energy, and dissipation rate are also summarized. The results show that the flow regime is jointly controlled by inflow discharge and vertical drop between adjacent pools. With the increase of inflow discharge, the flow regime shifts from plunging flow to transition flow and then to streaming flow. Under the same inflow discharge, a larger vertical drop tends to form plunging flow with lower turbulence intensity. This study reveals the fundamental hydraulic characteristics of overflow weir fishways and provides a reference for the structural design and hydraulic optimization of such fishways.