Influence mechanism of erosion and deposition evolution of Dongting Lake confluence channel on river-lake relationship under new water and sediment conditions

Abstract: Under the combined operation of upstream cascade reservoirs with the Three Gorges Project as the core and the comprehensive impact of climate change, the water and sediment conditions in the middle and lower reaches of the Yangtze River have undergone significant changes, which have had a profound impact on the evolution and pattern of river-lake system. This paper takes the confluence reach of the Yangtze River Mainstream and Dongting Lake as the research object. Through the physical model(plane scale 1:400, vertical scale 1:100) experiments, the confluence process of flow and sediment and the evolution rule of erosion and deposition of the Yangtze River Mainstream and Dongting Lake outlet channel under different silt-discharge and river lake boundary conditions were explored, and the influence mechanism of different erosion and deposition conditions on the river-lake relationship was analyzed. The results indicate that considering the further scouring development of the main stream of the Yangtze River in the future, under the condition of controlling the water level at the model outlet (Luoshan station) to drop by 3m, the longitudinal gradient of the water surface in the confluence channel of Dongting Lake increased by about 20%, showing a scouring trend, and the emptying effect of the Yangtze River Mainstream on the Dongting Lake was enhanced. In addition, under the condition of controlling the flood flow of 50000m³/s at the outlet, with the increase of the confluence ratio between Dongting Lake and the Yangtze River Mainstream, the scouring intensity in the river and lake confluence area increased, and the elevation difference between the main stream and tributary riverbed increases, resulting in the increasing jacking effect of Dongting Lake outflow on the main stream. The relevant achievements can provide new insights and decision-making references for maintaining the healthy river-lake system pattern and interaction relationship in the middle and lower reaches of the Yangtze River.

Key words: flow and sediment variation; river-lake relations; physical model experiments; Dongting Lake; Middle reaches of the Yangtze River