The crucial hydrodynamic factors affecting fish passage efficiency and fish swimming behaviors in a multi-slot and pool-weir combined fishway

A novel multi-slot and pool-weir combined fishway (named as MSWF-5) was recently proposed by combining the merits of vertical-slot fishway and pool-weir fishway. This study aims to systematically compare the hydrodynamic properties between MSWF-5 and other technical fishways, and further reveals the effect of hydrodynamic properties on fish passage efficiency and fish-swimming behaviors in MSWF-5 by using grass carp (Ctenopharyngodon idella). The hydrodynamic properties are obtained by three-dimensional numerical simulations, and fish-swimming behaviors are acquired by fish-passage experiments in laboratory physical model. It is found that the passage success rate of grass carp in MSWF-5 is highly correlated with the spatial-averaged time-averaged velocity magnitude along the slot section (Us-mean; r=-0.892, p<.05). Besides, although similar maximum water velocity for slot and weir in MSWF-5, the vast majority of fish in the trials swam through the slot (92.7%~99.7%, a water-depth of 17.6~32.6 cm) compared to the weir opening (0.3%~7.3%, a water-depth of 2.6~17.6 cm). Moreover, both the time-averaged velocity magnitude (U) and the time-averaged Reynolds-shear-stress X-Y component magnitude (|τxy|) are significantly negatively correlated with the occurrence frequency of fish (r=-0.954~-0.473, p<.01). Concretely, U is more correlated with the occurrence frequency of fish in the MSWF-5 slot-passage channel (the velocity uniformity coefficient (β) =53%~64%, r=-0.879~-0.669), but |τxy| is more correlated with the occurrence frequency of fish in the MSWF-5 weir-passage channel (β=70%~73%, r=-0.954~-0.878). This study offers new knowledge on fish-swimming responses to complex turbulent flows, contributes to developing efficient multi-species fishways, and provides reference for fishway-design practices and regulations/guidelines formulation.