Response mechanism for start-up combinations on water-sediment flow pattern in the forebay of forward pumping station

Water-sediment flow patterns can greatly contribute to the operational efficiency and safety of the pumping station. Among them, the vortex flow patterns have frequently caused sediment deposition in the forebay of forward pumping stations in a sediment-laden river. The forebay of the pumping station is also characterized by a complex two-phase flow structure with water and sediment. It is still lacking in the impact of pump start-up combinations on the flow pattern and suspended sediment movement. In this study, the mixture multiphase flow and the Realizable k-ε turbulence model were utilized from an engineering perspective. Grid evaluation criteria were selected as the hydraulic loss of water sediment flowing through the forebay and the operational efficiency of the pump. The inlet of the diversion channel was set as a mixed velocity inflow boundary, while the pump outlet was designated as a free outflow boundary. Solid boundaries were treated using the wall function. The free boundary of the water surface was handled with the assumption of a free-sliding rigid cover. A mathematical model was then established for the forebay of the pump station. The flow and sediment movement were simulated under different start-up combinations. The characteristics and formation mechanisms of each vortex region were analyzed to predict the sediment deposition in the forebay. An optimal start-up combination was proposed to alter the flow field structure. The hydrodynamic dredging was achieved for the large sediment deposits within the forebay. The velocity and sediment deposition within the forebay were measured using the Acoustic Doppler Current Profilers velocity profiler. These measurements were also used to validate the numerical simulation. The results show that a "plane sudden expansion vortex" was formed in the water-sediment two-phase flow from the diversion channel to the inlet of the forebay, due to the separation of the boundary layer. The vortex depth was about 90% of the water depth. A "near bottom three-dimensional vortex" in the intake pond was formed, influenced by the sudden increase of local water depth and the plane vortex. The vortex depth was about 93% of the water depth. The suspended sediment migrated to both sides of the forebay wall under the action of the vortex, where the "fan-shape" sediment deposition was observed. The total sediment deposition accounted for about 60% of the total volume of the forebay. At the same time, the sediment content at the bottom of the intake pond was 8 to 9 times higher than that of the diversion canal. While the sediment content in the middle area was basically the same as that of the diversion canal. A total of five start-up combinations were proposed, including imported full open type, imported half-open type, intermediate focusing type start-up pump, two side decentralized type start-up pump, and fully open type pump. The comprehensive comparison showed that the flow pattern was significantly improved in the forebay after 91 days of operation of the pump station. Specifically, the flow velocity uniformity at the pump port of intermediate focusing type case 5 increased by 5.84 percentage pointss, whereas, the drift angle, the area of high sediment concentration area, and the sediment deposition decreased by 3.35°, 75%, and 58.60%, respectively. The findings can provide theoretical support to mitigate the harmful effects of vortex areas and sediment deposition in the forebay, thereby optimizing the operation of the pumping station. In addition, future research should consider optimizing and improving many more factors, including the separated forebay, in order to further optimize the sediment deposition and operation efficiency in the forebay of the pump station.