Abstract:

[Objective] With the release of low-sediment water, the middle and lower reaches of the Yangtze River will undergo long-distance and long-duration river channel scouring. Optimizing the operation strategy of the Three Gorges Reservoir to reduce scouring downstream of the dam is of great significance. [Methods] Based on the statistical analysis of measured data, the response mechanisms of erosion and deposition in the downstream river channel were analyzed. Using mathematical modeling, a preliminary study was conducted on reservoir operation strategies aimed at reducing downstream scouring. The sediment regulation concept of “regulating sediment to reduce scouring, regulating water to regulate sediment, and achieving sediment regulation through water regulation” was proposed. [Results and Conclusion] At representative hydrological stations along the middle and lower reaches of the Yangtze River, sediment load exhibits a good power-law relationship with flow, indicating a strong correlation between sediment transport and flow. Therefore, sediment regulation in the middle and lower reaches can be achieved by controlling corresponding flow processes. Typical years of 2012 and 2013 were selected to study the effects of regulating sediment peaks during the flood season and different regulation modes for small and medium-sized floods on the reservoir’s sediment flushing ratio and downstream river channel scouring. Downstream scouring is influenced by both the sediment released from the upstream reservoir and the volume and process of the incoming flow. Increasing the reservoir’s maximum flow during sediment peak periods and shortening its duration are both beneficial for reducing downstream scouring. Among these, increasing the maximum flow has a more significant effect. Therefore, to mitigate downstream scouring, the flow during flood seasons should not be too low. Based on typical years from 2008 to 2017, the effect of optimized operation schemes on reducing scouring between Yichang and Datong was studied. In terms of total scouring volume across this reach, the implementation of an optimized operation scheme aimed at reducing downstream scouring resulted in increased sediment discharge from the reservoir and a total reduction of 24.47 million m³ in scouring volume, with an average annual reduction of 2.447 million m³. Overall, considering sediment peak regulation during the flood season can reduce downstream river channel scouring to a certain extent. Using hydrological and sediment data from 1991 to 2000 and considering both the joint operation of upstream cascade reservoirs and the optimized operation of the Three Gorges Reservoir for scouring reduction, the long-term water and sediment outflow processes of the reservoir were predicted and used as boundary conditions for long-term simulations of downstream scouring. A one-dimensional hydrodynamic and sediment transport model for the Yichang-Datong reach of the middle and lower Yangtze River was applied to predict the long-term evolution of downstream river channel scouring under optimized operation. The research findings can provide a reference for the optimized operation of the Three Gorges Reservoir.

Key words: downstream of the Three Gorges Dam, riverbed scouring, operation optimization, Three Gorges Reservoir