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 ( U s ‐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 (| τ xy |) 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 | τ xy | 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.