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Preliminary Study on Optimizing the Operation Mode of the Three Gorges Reservoir to Reduce Riverbed Scouring Downstream of the Dam
YUAN Yuan, WANG Xiang, FENG Zhi-zhou, HUANG Ren-yong, GUO Xiao, WANG Min
Journal of Changjiang River Scientific Research Institute ›› 2025, Vol. 42 ›› Issue (8) : 27-37.
PDF(9137 KB)
PDF(9137 KB)
Preliminary Study on Optimizing the Operation Mode of the Three Gorges Reservoir to Reduce Riverbed Scouring Downstream of the Dam
[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.
downstream of the Three Gorges Dam / riverbed scouring / operation optimization / Three Gorges Reservoir
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Chongqing reach is located in the fluctuating backwater zone of the Three Gorges Reservoir (TGR). Sediment deposition and erosion of Chongqing reach is related to flood control, shipping and wharf operation. It has become one of the key problems the TGR faces. In this paper, based on the prototype observation data, the sediment scouring and silting law of Chongqing reach before and after the 175 m experimental impoundment of the TGR was investigated for the first time. In addition, combined with the analysis of river bed composition and the one-dimensional mathematical model, the scouring conditions of suspended sediment in Chongqing reach were calculated and analyzed. Furthermore, the basic conditions of sediment transport in the study reach were applied to the practice of sediment reduction of the TGR. The following conclusions can be drawn as follows: (1) From 2008 to 2012, when the water storage level of TGR reached 175 m, the erosion intensity of the river bed in Chongqing reach started to decline, and the main souring period of suspended load was postponed from the water-impoundment stage to the pre-flood stage. After 2013, Xiluodu and Xiangjiaba hydropower stations in the lower reaches of Jinsha River was completed and put into operation. The sediment of Jinsha River was intercepted and the sediment entering the Three Gorges was greatly reduced, which led to another increase of the river bed scouring intensity of Chongqing reach. (2) When the inflow in front of the Three Gorges Dam (TGD) is greater than 4000 m 3/s and the water level is lower than 167 m, the suspended load on the river bed in Chongqing reach begins to scour. When the inflow exceeds 5000 m 3/s, the water level in front of the TGD drops to 163 m, and the scouring intensity of suspended load increases obviously. Increasing the drawdown rate of water level from 163 m before the TGD to 30 cm per day can avoid the cumulative sedimentation in the Chongqing reach. |
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Under the action of impoundment and regulation of large water conservancy projects, especially reservoirs, the incoming flow and sediment conditions of the downstream channel change remarkably, which will lead to unbalanced sediment transport in the channel, and induce scouring and remaking of the riverbed. The riverbed scour and remaking process in the downstream channel of water conservancy project is characterized by the coupling of multiple processes (inclusive of sediment scour, coarsening, exchange and suspended sediment recovery) and the macroscopic morphological changes of multi-scale complex responses (such as sediment scouring and silting, bed surface morphological changes, longitudinal slope changes, as well as river pattern and river regime adjustment). The related researches in China and abroad are mainly carried out from two aspects, namely, microscopic mechanism and macroscopic law, by means of field measurement data analysis, theoretical research, physical model test and numerical simulation. In this paper the research results of river bed scour and remaking process in the downstream of water conservancy projects in China and abroad are summarized; the research progress of key technologies in physical model test and numerical simulation of river bed scour and remaking are expounded; and the key points for further research and the scientific and technological problems to be solved are presented.
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Under the background of a large reduction in the sediment into the Three Gorges Reservoir (TGR), dispatching of medium-and-small-scale flood in flood season has exacerbated the risks of sediment deposition and flood control in TGR and reduced the chances of large discharge of the reservoir. Studying the dynamic dispatching of “storing clear water and releasing muddy water” in flood season is conducive to further optimizing the dispatching mode. Using one-dimensional unsteady flow mathematical model of the mainstream and tributaries of TGR, we put forward the dynamic operation scheme of “storing clear water and releasing muddy water” in flood season of TGR through calculations. The calculation results illustrate that the dynamic operation mode of “storing clear water and releasing muddy water” in flood season takes into account both sediment discharge, power generation and flood control. A water level at 150 m when “storing clear water” is superior to 155 m; the water level can fluctuate between 145 m and 150 m in the period of “storing clear water”. We recommend to start the releasing of muddy water when the sediment concentration of Cuntan station reaches 2.0 kg/m<sup>3</sup> and meanwhile the incoming flow of Cuntan station on that day exceeds 25 000 m<sup>3</sup>/s; on the contrary, when the sediment concentration of Cuntan station falls to about 0.1 kg/m<sup>3</sup>, we suggest to end the releasing of muddy water and begin to store clear water. The research findings provide reference for the optimal operation of TGR in flood season.
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