Water Exchange in a Tributary Bay of a Large Reservoir

The density flows induced by temperature difference often occurs in the tributary bays. These will influence water exchange between the tributary bays and the mainstream of the reservoir and the nutrient concentration in the tributary bays. To examine the seasonal variation of the water exchange in the tributary bay, a three-dimensional numerical model was set up for the lower reach of the Xiangxi River, a tributary of the Three Gorges Reservoir. The model was calibrated and validated against field-measured data and the results indicated its capability of simulating the thermal-hydrodynamic circulations in the tributary bay. The model-simulated results showed that the water circulation patterns varied with seasons. The bottom-layer intrusive currents from the Three Gorges Reservoir are usually in the normal-running period and the early time of the discharging period, while the surface-layer intrusive currents are dominant in the other time of the year with the middle-layer intrusive currents occasionally occurring in the flooding period and early time of the impounding period. Based on the model-simulated results, the water fluxes at the cross sections in the tributary bay were calculated. Although the inflow from the tributary was relatively small, the water exchange between the tributary bay and the mainstream of the reservoir was significant with the result showing that the water within the intrusion limit could be renewed at more than 5 times in an operation period.