The Change Law and Formation Cause of Sediment Erosion and Deposition in the Three Gorges Reservoir in the Past 20 Years

During the past 20 years of the Three Gorges Reservoir impoundment , the incoming water and sediment conditions have changed constantly in the upper reaches of the Three Gorges, resulting in the continuous evolution of the process of incoming water and sediment and the characteristics of sediment erosion and deposition in the reservoir area. Based on the water and sediment series from 1961-1970 (60 series) and 1991-2000 (90 series), this paper makes a comparative study on the variation rules of the incoming water and sediment and erosion-deposition in the past 20 years (2003-2022) of the Three Gorges Reservoir. The results show that: (1) Under the influence of successive storage operation of the Wudongdong, Baihetan, Xiluodu and Xiangjiaba cascaded reservoirs in the lower reaches of the Jinsha River, the incoming sediment from the main stream of the Jinsha River decreased sharply, resulting in the annual average of 1.9 billion tons of sediment from 2003 to 2012 to 0.8 billion tons from 2013 to 2022; (2) From 2013 to 2022, the proportion of sediment from Jinsha River directly decreased to 1.8%, and the proportion of sediment from Jinsha River was no longer the main source of sediment from the Three Gorges reservoir, while the proportion of sediment from Jialing River, Minjiang River and the region increased to 42.0%, 28.8% and 9.3%, respectively, becoming the main production areas of sediment from the Three Gorges reservoir; (3) Due to the influence of the upstream reservoir blockage, the coarse sediment content in the Three Gorges reservoir decreased to a certain extent. During 2013-2022, the proportion of coarse sediment (d>0.125mm) in Cuntan Station decreased to 3.0% from 10.3% during 1987-2002 and 6.5% during 2003-2012, respectively; (4) In the past 20 years of impoundment of the Three Gorges reservoir, the average annual sediment accumulation is 1.03 billion tons, which is only 30.8% of the total in the demonstration stage. The sediment accumulation mainly occurs in the perennial backwater area, which is characterized by discontinuous and flaky "Spaced and banded" distribution. The sediment accumulation in the broad valley reach of the main stream in the reservoir area accounts for 94.4% of the total sediment accumulation, while that in the valley reach only accounts for 5.6%; (5) Before the construction of the Three Gorges Reservoir, the sediment carrying capacity is obviously greater than the sediment concentration in the canyon and the wide valley reach. After the operation of the reservoir, the sediment carrying capacity of the wide valley reach is significantly reduced, and the sediment carrying capacity is less than the sediment concentration. The sediment carrying capacity of the canyon reach decreases slightly, and the sediment carrying capacity is still generally greater than the sediment concentration.