Effects of discharge flow and reservoir water level on intake water temperature with a front retaining wall

Following the construction and operation of hydropower stations in alpine and cold regions, the vertical thermal stratification structure of reservoirs is significantly altered. To mitigate the adverse ecological impacts of low-temperature water release and enhance the withdrawal of warm surface water, stratified intake measures are commonly adopted to satisfy downstream ecological protection requirements. In this study, the front retaining wall of a hydropower station located in the upper reaches of the Yellow River was selected as the research object. A three-dimensional hydrodynamic–thermal coupled model was established using FLOW-3D to simulate the effects of different discharge flows and operating water levels on intake water temperature and to investigate the response characteristics of vertical thermal stratification. The simulation results demonstrate that discharge flow is one of the dominant factors governing the vertical thermal structure and intake water temperature, while variations in operating water level further influence the withdrawal temperature. When the discharge flow remains below 75% of the rated load, the warm surface water is sufficient to satisfy the flow demand and gradually diffuses downward, maintaining the intake water temperature at or above 7.5 °C. In contrast, when the discharge flow exceeds 75% of the rated load, the surface warm water becomes insufficient to meet the increased discharge demand, resulting in the direct entrainment of low-temperature bottom water. Consequently, a significant negative correlation is observed between discharge flow and intake water temperature. After passing over the crest of the retaining wall, the vertically stratified water masses undergo mixing within the forebay section, leading to a temperature difference of less than 1 °C in the tunnel section. In addition, the side diversion tunnels exhibit a symmetrical temperature distribution along the central axis. Based on the simulation results, coordinated regulation of discharge flow and operating water level is recommended to optimize ecological operation by maximizing the withdrawal of warm surface water and mitigating the impacts of low-temperature releases on downstream aquatic ecosystems.