Simulation analysis of the improvement effect on downstream water temperature at tankeng hydropower station based on FLOW-3D

Abstract

[Objective] After the completion and operation of the Tankeng Hydropower Station, significant changes have occurred in the water temperature stratification structure of the reservoir area and the spatio-temporal distribution of the water temperature in the downstream river. To optimize the reservoir operation strategy and mitigate the impact of low-temperature water discharge on the river ecosystem, it is urgent to clarify the response relationship between reservoir operation factors and the discharged water temperature, and quantitatively evaluate the improvement effect of regulatory measures. [Methods] Taking the Tankeng Reservoir as the research object, a local three-dimensional numerical model of the water intake was constructed based on the FLOW-3D model. The discharged water temperature processes under different water level conditions in spring, summer, autumn and winter, and the combination of 6~8 layers of stop-log gates were simulated respectively. The response law of thermal stratification and the regulation mechanism were analyzed. [Results] The results show that:(1) The vertical thermal stratification structure of the Tankeng Reservoir presents significant seasonal differentiation characteristics. The water temperature stratification phenomenon begins to appear in March and basically disappears in October. Affected by the stop-log gates, the main vertical water temperature difference is concentrated within a water depth of 25 m.(2) In the strong stratification seasons(spring, summer, autumn), the effects of water level on water temperature are 0.1~0.2 ℃/m, 0.1~0.5 ℃/m, and 0.1~0.3 ℃/m respectively. The regulation ranges of a single-layer stop-log gate on the discharged water temperature are 0.4~0.8 ℃, 1.5~2.3 ℃, and 0.6~2.1 ℃ respectively. In the weak stratification season(winter), the impacts of unit water level and stop-log gate on the discharged water temperature are below 0.1 ℃ and 0.3 ℃ respectively.(3) When the stop-log gate is set below the thermocline, the intake water temperature is low. Adding one more layer of stop-log gate in spring, summer, autumn and winter can increase the discharged water temperature by 0.7 ℃, 2.2 ℃, 1.8 ℃ and 0.2 ℃ respectively. [Conclusion] The results indicate that during the strong stratification period, the stop-log gate can significantly improve the discharged water temperature by controlling the intake water depth, which is better than the single water-level regulation measure, and the regulation effect is better when it is set above the thermocline. In the seasons with weak thermal stratification, the regulation effects of both measures are significantly weakened, which is directly related to the flattening of the vertical water temperature gradient. Therefore, it is recommended that during the reservoir operation and regulation, the reservoir storage and the arrangement of stop-log gates should be reasonably planned in combination with the seasonal changes of the thermocline to ensure water intake above the thermocline.

Keywords

FLOW-3D / local part of the water intake / stop-log gate / water level / discharged water temperature / influencing factors / hydropower station / spatiotemporal distribution