Hydraulic characteristics of a novel orifice–Weir–pier combined fishway

Fishways are widely used to mitigate the disruption of river connectivity caused by dams and sluices. Conventional and combined pool-type fishway frequently suffer from concentrated main flow, excessive local velocities, and high turbulence, which reduce fish passage efficiency. This study proposes a novel orifice–weir–pier fishway (OWPF) by incorporating cylindrical piers into a pool-type configuration. Physical model experiments and computational fluid dynamics (CFD) simulations were conducted to investigate the effects of pier placement on velocity distribution, turbulent kinetic energy (TKE), and recirculation structures. Results indicate that the cylindrical piers transform the wall-attached L‑shaped jet into S-shaped mainstream, reducing average velocity and maximum velocity by 5.4%–14% and 17%–35%, near-orifice velocity by up to 45%. The average TKE and maximum TKE decrease by 21.2%–31.7% and 19.6%–36.6%, respectively. The recirculation area increases from 25% to 30%–31%, while average velocity decreases from 0.097 m/s to 0.069–0.089 m/s, thus significantly improving fish passage conditions. The pier arrangement case M2 achieves the most effective hydraulic regulation. These findings indicate that cylindrical piers are an effective and adaptable measure for enhancing fishway performance and provide a reliable reference for the future fishway design.