The crucial hydrodynamic factors affecting fish-passage efficiency and fish-swimming behaviours 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 behaviours in MSWF-5 by using grass carp (Ctenopharyngodon idella). The hydrodynamic properties are obtained using three-dimensional numerical simulations, and fish-swimming behaviours are acquired using 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 < 0.05). 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 to -0.473, p < 0.01). 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 to -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 to -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 formulation of regulations/guidelines.