长江上游水库群拦沙与长江中下游河道冲刷的关联性

[Objective] After the operation of the Three Gorges Reservoir, the ratio between siltation volume in the Three Gorges Reservoir and channel scour volume in the middle and lower reaches of the Yangtze River does not conform to the value (1∶0.5) determined in previous studies, mainly due to the substantial reduction in sediment inflow into the Three Gorges Reservoir. This study aims to analyze the characteristics of sediment deposition in the Three Gorges Reservoir and the scouring variations in the middle and lower reaches of the Yangtze River, and to investigate the correlation between sediment retention by the Three Gorges and other reservoirs and the resulting channel scouring in the middle and lower Yangtze River. [Methods] Measured hydrological data and topographic data from 2003 to 2021 were utilized. The sediment transport method, cross-sectional method, and low-water-level verification were comprehensively adopted for the analysis. [Results] The ratio between basin-scale sediment retention and downstream channel scour was 1∶0.48, which was generally consistent with that of typical reservoirs in China such as Sanmenxia Reservoir and Danjiangkou Reservoir. 1) Regarding siltation volume in the Three Gorges Reservoir, the cumulative siltation from 2003 to 2021 was calculated as 2.381×109 t, which was equivalent to 2.238×109 m3, using the sediment transport method. The cross-sectional method estimated 1.784×109 m3 of siltation in the mainstream of the reservoir area. The difference between the two methods was 25.4%, due to differences in calculation principles, but the results were generally consistent. Siltation was mainly concentrated in the perennial backwater zone, totaling 1.853×109 m3. Siltation within the flood control storage accounted for only 0.67%, indicating a relatively small loss of effective storage capacity. 2) Regarding channel scour in the middle and lower reaches of the Yangtze River, the cumulative scour in the Yichang-Datong reach was calculated as 8.99×108 m3 using the sediment transport method. The cross-sectional method yielded 3.146×109 m3. The minimum scour volume inferred from the decline in water level under low-flow discharge conditions was 2.160×109 m3, demonstrating that the cross-sectional method results were more reasonable. The discrepancy was mainly attributed to the underestimation of fine-grained sediment in the sediment transport method and the exclusion of human activities, including sand mining of 6.64×108 m3 and channel dredging of 3.08×108 m3. The cumulative scour of the entire river reach reached 5.03×109 m3. Continuous scour persisted in the Jingjiang reach with no reduction in intensity, and scour in the downstream reaches significantly intensified after 2013. 3) At the basin scale, the total reduction in sediment volume from 2003 to 2021 was 8.497×109 m3. After excluding the influence of human activities, the ratio between sediment retention and scour was 1∶0.48, which was consistent with the sediment retention-scour pattern of large reservoirs. Siltation in the Three Gorges Reservoir itself was 2.238×109 m3, which directly caused 1.074×109 m3 of downstream scour and accounted for 26.5% of the total scour. The contribution in the first decade was significantly higher than that in the second decade, indicating that its impact gradually weakened with the enhanced sediment retention effect of upstream reservoir groups. [Conclusion] It is necessary to optimize sediment release scheduling of reservoir groups to improve the sediment flushing ratio. Measures such as dredged sediment resource utilization and sediment replenishment should be combined to mitigate channel scour, while strengthening sand mining management and long-term monitoring of scour and deposition.