Transport Characteristics and Driving Mechanism of Coarse and Fine Grained Sediment from Three Gorges Reservoir to Downstream of Dam

The enhancement of human activities in the basin has profoundly affected the evolution process and development trend of river hydrology and geomorphology.As the largest hydropower project in the world,the operation of the Three Gorges Dam has caused widespread concern on the downstream water and sediment flux,sediment transport and topography.In this study,the influence of impoundment of the Three Gorges Reservoir on sediment sorting and downstream sediment transport from 1990 to 2021 were studied,which deepen the understanding of human activities on the evolution of river water-sediment-geomorphology system.Results:The Three Gorges Reservoir impounded 76.33% of the sediment in the upstream basin into the downstream of the dam,and the sediment discharge ratio of fine particles (d≤0.125mm) and coarse particles (d>0.125mm) was less than 25.0% (24.26% and 11.69%);The total sediment and fine-grained sediment in the lower reaches of the Three Gorges Dam have been recovered since 2003,and the recovery value is lower than the average value before impoundment (1990~2002);the operation of the Three Gorges Reservoir changes the water level and flow process in the reservoir area,and the rise of water level in the reservoir area reduces the total sediment volume,coarse particle and fine particles sediment discharge ratio,the sediment discharge ratio of fine particles increases and the ratio of coarse particles remains stable in the years with long days of inflow greater than 30 000m³/during the flood season (May~October).After the operation of the Three Gorges Reservoir,the sediment volume in the downstream of the dam from Yichang to Hukou is reduced causing long-distance cumulative scouring of the riverbed,and the fine-grained sediment recovered along the way.The recovery of coarse-grained sediment caused by the gradual coarsening effect of the riverbed is in a weakened state.