Numerical investigation on the hydraulic characteristics and optimal design of pool-weir stepped fishway

The fishway, as a crucial ecological facility for restoring river connectivity and facilitating fish migration, has its hydraulic characteristics significantly affecting fish passage efficiency. To address the challenges of steep slopes and constrained topography in fishway construction along the upper Yellow River, this study investigates the hydraulic characteristics of a novel pool-weir stepped fishway through numerical simulations. The effects of step height (Δh) and weir crest width (n) on flow velocity distribution, turbulent kinetic energy (TKE), and flow regime are examined. Results show that variations in step height significantly influence flow patterns. When Δh is 0.04 and 0.06 m, low-velocity recirculation zones form in the resting pools, providing a suitable environment for fish. At Δh = 0.15 m, plunging flow with high velocity occurs, with a maximum velocity of 2.7 m/s, which is unfavorable for migration. Although increasing weir crest width reduces flow velocities, its effect diminishes at n = 0.35 m. In contrast, at n = 0.15 m, high-velocity zones are effectively reduced. Considering flow velocity, water depth, and TKE distribution, the optimal design parameters are Δh ≤ 0.06 m and n = 0.15 m. Hydraulic performance under three discharges (Q = 0.5, 0.7, 0.9 m3/s) is further evaluated. The results show that with this configuration, the fishway achieves a uniform velocity distribution, and low TKE, supporting effective fish passage. This study provides a theoretical basis for optimizing pool-weir stepped fishway design in the upper Yellow River and offers engineering guidance.