Hydraulic characteristics of the vertical slit fishway structure with fish passage effect for sediment reduction

Vertical-slit fishways can often serve as the critical ecological passages for the fish migration past instream barriers like dams. However, its operational performance is severely compromised in the sediment-laden rivers. Siltation within pool chambers can obstruct the migration routes to degrade the essential hydraulic habitats, and cause functional failure. Previous research has focused on the sediment reduction and fish passage facilitation as conflicting objectives. Therefore, this study aimed to design, evaluate, compare and integrate the structural modifications for sediment reduction within a vertical-slit fishway. The explicit goal was set to synergistically improve both sediment management and fish passage performance. A systematic investigation was made to quantify the influence of these structures on internal hydraulic features, sediment deposition, and the upstream passage of fish. A three-dimensional (3D) numerical hydrodynamic model was constructed to integrate with a 1:6-scale physical hydraulic model using Froude similarity. The physical model also replicated a typical vertical-slit fishway prototype. The numerical model was developed using Flow-3D software. The standard k-epsilon turbulence model and the Volume of Fluid were employed for free-surface tracking. The validation test was conducted against velocity measurements from the physical model. The high accuracy was achieved to capture the complex flow patterns. Five sediment-reduction structures were tested: three variants in the position and aspect ratio of a bottom orifice into the baffle, and two variants in the placement of a cylindrical element in the pool chamber. All experiments maintained the identical upstream flow discharge, water level, and sediment concentration boundary conditions. Sediment deposition morphology and volume were measured after a 1.5-hour test duration. Concurrently, the live-fish trials were conducted on the juvenile common carp to assess passage effectiveness; The upstream success rate was calculated, where the fish successfully navigated from the release point past the first upstream slit within a 10-minute period. The experimental results demonstrated that there was a causal relationship between the modified hydraulic environment induced by the structures and the sediment deposition. In the standard fishway design (control case), the deposition was concentrated in the low-velocity recirculation zones adjacent to the main flow jet, with a total sediment mass of 4.7 kg in the pools. Five sediment-reduction configurations successfully modified the flow field to reduce the siltation, compared with the control. A dual-main-jet flow pattern was obtained in the bottom-orifice structures to lower the peak velocity and turbulent kinetic energy near the vertical slit. The vorticity maxima were redistributed to the sides of the jets, and the characteristic distribution of velocity was altered the locations of the sediment deposition. In contrast, the cylindrical structures were induced the prominent periodic vortices with the Ω-shaped flow distribution. The local vorticity and turbulent kinetic energy were significantly enhanced around the cylinder to intensify the sediment suspension and transport capacity. Among all configurations, the most substantial reduction of the sediment was found in the cylindrical structure with a diameter of 0.05 m, which was positioned with its centroid 0.10 m from the upstream baffle and 0.15 m from the right sidewall. The total deposition mass decreased by 48.9% to 2.4 kg. Fish passage data revealed that the structural modifications also influenced the fish behavior and success rates. The optimal cylindrical configuration increased the upstream passage success rate of test fish by 16 percentage points, from 64% (control) to 80%. Fish trajectory analysis via hotspot maps indicated that both bottom orifices and cylinders also provided the additional and alternative migration pathways. The spatial utilization then increased in the pool chambers. The Ω-shaped flow generated by the optimal cylinder into the hydraulically diverse conditions, thus reducing energy expenditure during ascent. In conclusion, the sediment-reduction structures were strategically integrated into the vertical-slit fishways. An effective strategy was provided to concurrently manage the siltation for the high fish passage. The superior performance of the cylindrical elements was achieved to reduce the sediment deposition in the fish passage, compared with the bottom-orifice types. The optimal cylindrical configuration was modified the internal hydraulic regime for the Ω-shaped flow pattern. Sediment transport was promoted for the favorable migration conditions. These findings can offer valuable practical insights for the engineering design of sustainable and efficient fishways in sediment-prone river systems. The parametric optimization of such structures, particularly cylinders, can be extended to long-term monitor the prototype in the durability and ecological benefits.