Thermal-hydrodynamic circulations and water fluxes in a tributary bay of the Three Gorges Reservoir

Abstract

The impoundment of a large riverine reservoir creates a series of tributary bays in its original tributaries. The increase in the water level and decrease in the velocity may result in the degradation of the water quality in the tributary bays. The thermal conditions can also change in the mainstream of the reservoir and tributary bays. Density flow induced by temperature differences often occurs in tributary bays, which likely influences the water exchange between the tributary bays and mainstream of the reservoir and the nutrient concentration in the tributary bays. To examine the seasonal variation in the circulation pattern and water exchange in the tributary bay, a three-dimensional numerical model was established for the lower reach of the Xiangxi River, a tributary of the Three Gorges Reservoir. The model was calibrated and validated using field-measured data and the results indicate that it can be used to simulate the thermal-hydrodynamic circulations in the tributary bay. The model-simulated results show that the water circulation patterns vary depending on the season. The bottom-layer intrusive currents from the Three Gorges Reservoir generally occur in the normal-running period and early discharging period, while surface-layer intrusive currents are dominant in the rest of the year and middle-layer intrusive currents occasionally occur in the flooding period and early impounding period. Based on the model-simulated results, the water fluxes in the cross sections in the tributary bay were calculated and the factors controlling the fluxes were examined. The results show that the thermal conditions in the tributary bay and mainstream of the reservoir are the predominant factors controlling the water exchange between the tributary bay and mainstream of the reservoir. Although the inflow from the tributary is relatively small, the water exchange between the tributary bay and mainstream of the reservoir is significant. The results show that that the water within the intrusion limit is renewed more than five times in an operation period. However, the water exchange in the upper reach of the tributary bay is weak, especially during the discharging period of the Three Gorges Reservoir. This may be due to the combined effects of small inflow from the upper reach of the Xiangxi River and weak intrusive currents from the mainstream of the Three Gorges Reservoir in the upper reach of the tributary bay.