Hydraulic Performance of Energy-Dissipation Configurations for a High-Dam Fish-Lift Collection System: A CFD-Based Comparative Study

In pipeline-replenished fish lifts, the replenishment flow enters the collection system at a high velocity and must first be dissipated in a stilling basin, but a conventional deep basin requires a large excavation volume and a high construction cost; whether an adjustable-resistance boundary at the basin top can replace part of the basin depth still requires systematic quantitative assessment. Here, a permeable cover plate at the basin top is compared quantitatively with two conventional measures, a deeper basin and internal baffles, under identical inflow, using a unified set of fish-related indices. A three-dimensional computational fluid dynamics (CFD) model, validated against a 1:5.33 physical model of the Fengman Hydropower Station fish lift, was used to examine three energy-dissipation configurations: stilling-basin depth (2–5 m), cover-plate permeability (72–50%), and baffle height (1–4 m). Performance was assessed by the coefficient of variation of velocity (CV) at the control section upstream of the fish grid (X = 14 m) and by the streamwise turbulent kinetic energy (TKE) in the fish bucket and residence pool. The results show that increasing the basin depth from 2 to 5 m reduced CV from 0.31 to 0.23. Reducing the cover-plate permeability from 72% to 50% reduced CV to 0.12 and the streamwise TKE by about 87%, confining the high-velocity and high-turbulence regions to the stilling basin, whereas baffle height acted non-monotonically with an optimum at 2 m (CV = 0.20). Overall, a 3 m deep basin with a 50% permeability cover plate gave the most uniform approach flow and the lowest turbulence (maximum TKE about 0.003 m2/s2), providing more favourable conditions in the collection zone for fish to hold position and migrate upstream, while requiring 2 m less excavation than the reference scheme; an adjustable-resistance boundary can thus replace part of the basin depth without sacrificing hydraulic performance, offering a new design route for high-head fish lifts with constrained abutment excavation.