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减少坝下游河道冲刷的三峡水库调度方式优化初步研究
Preliminary Study on Optimizing the Operation Mode of the Three Gorges Reservoir to Reduce Riverbed Scouring Downstream of the Dam
通过三峡水库调度方式优化减少坝下游河道冲刷是一项非常有益的研究工作。在实测资料统计分析基础上,通过数学模型计算,对有利于减少坝下游河道冲刷的三峡水库调度方式进行了初步研究,提出了“以减冲为目标进行调沙,以调沙为目标进行调水,以调水为手段实现调沙”的泥沙调度思想,研究结果表明:长江中下游各河段代表性水文站输沙量均与流量有着较好的幂指数关系,建库后不同时期枝城、沙市、监利站水流输沙能力显著降低,枯水期小流量对应输沙量减小程度大于大流量。2008年以来,螺山、汉口、九江站大流量与小流量下输沙量的差距减小;汛期采用不同中小洪水调度方式时,对坝下游河道的冲刷有一定的影响,控泄流量相对较大时,可一定程度上减少河道的冲刷,预测了优化调度条件下坝下游河道冲刷长期演变趋势。研究成果可为三峡水库优化调度提供参考。
[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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重庆主城区河段河道泥沙冲淤事关防洪、航运及码头作业等,是三峡水库泥沙问题的重点内容之一。本文依据原型观测资料,以三峡水库175 m试验性蓄水前后河段的泥沙冲淤规律为基础,结合河床组成分析和一维数学模型,计算提出河段悬移质泥沙走沙基本条件,并应用于减淤调度实践中。结果表明：① 三峡水库175 m试验性蓄水后至2012年重庆主城区河段河床冲刷强度下降,主走沙期推迟至汛前消落期,2013年后上游来沙减少使得河床冲刷强度再次增大;② 当寸滩站流量大于4000 m <sup>3</sup>/s、坝前水位低于167 m时,河段开始走沙;当寸滩站流量增大至超过5000 m <sup>3</sup>/s、坝前水位下降至163 m时,河段走沙能力增强;加大水库自163 m水位的消落速度,能够避免库尾河段产生累积性淤积。
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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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在入库沙量大幅减少的背景下,三峡水库持续开展汛期中小洪水调度,增大了汛期库区泥沙淤积和防洪风险,减少了水库下泄大流量的机会。在三峡水库汛期,开展“蓄清排浑”泥沙调度方式动态运用研究有助于进一步优化三峡水库汛期调度方式。利用三峡水库干支流河道一维非恒定流数学模型,探索开展了三峡水库汛期 “蓄清排浑”动态运用方式计算研究,并提出了汛期“蓄清排浑”动态运用方案。计算结果表明:三峡水库汛期“蓄清排浑”动态运用方式可以同时兼顾排沙、发电和防洪,“蓄清”水位150 m要优于155 m,“蓄清”运行期间库水位可选择在145~150 m之间浮动运行;建议将寸滩含沙量达到2.0 kg/m<sup>3</sup>且当日寸滩站入库流量≥25 000 m<sup>3</sup>/s的时间作为水库增泄“排浑”的起始时间,将出库含沙量降至约0.1 kg/m<sup>3</sup>作为“排浑”调度结束重新进入“蓄清”调度的泥沙参考因素。研究成果可为三峡水库汛期优化调度提供参考。
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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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