Evaluation of flow patterns in vertical slot fishways with different slot positions based on a comparison passage experiment for juvenile grass carp

Abstract Flow patterns (FPs) are different due to different fishway structures and different boundary conditions in a river. FPs can greatly affect fish passage efficiency along the entire fishway. In this study, experimental hydraulic modelling is combined with the study of fish behavior in a physical model of a vertical slot fishway (VSF) with different slot positions and varying pool lateral baffle length (P) to width (B) ratios. A three-dimensional (3D) computational fluid dynamics (CFD) model is used to help understand the detailed hydraulic structures. Three distinct FPs are studied. When P/B   0.4 and a long baffle was used, the jet had a curved form along the opposite side wall, and this was termed FP3. Comparison experiments are conducted on the three FPs, and 82 to 90 trials are conducted to compare juvenile grass carp ( Ctenopharyngodon idella ) reactions to each FP. An analysis of the fish behavior in the model clearly demonstrated that the fish tended to need less time to pass in FP2 than in the other two FPs. FP2, with the principal flow in the center position, tended to be more suitable for fish migration. In addition, using an analysis of the less efficient FPs, it was found that the velocity gradient may act as an important migration behavior motivator, and that a recirculation zone too close to the downstream slot could increase the risk of downstream flush. It is suggested that further studies should investigate the behaviors of different fish species in the three typical FPs.