Study On Evolution Law Of The Braided River With Sharp Bend In The Three Gorges Reservoir Area Based On Two-dimensional Numerical Simulation

Movement of water and sediment in the braided river with sharp bend was more complex. After operation of Three Gorges Reservoir, waterway conditions of typical curves in the Yangtze River changed much, and part of the river channel conditions were even worse, so the problem about the waterway now is much more serious, which restricts the development of shipping. Huanghuacheng river was a typical sharply curved braided river in Three Gorges Reservoir area. Since operation of Three Gorges Reservoir, the water conditions had varied widely, and massive sediment deposited in the river, so it became one of the areas which had most serious siltation in the Three Gorges Reservoir area.Through analyzing the key deposition area,water level change, and the change of split ratio of Huanghuacheng river during the past 10 years, evolution law and characteristics were summarized. The characteristics of evolution process were as follows:1. After operation of Three Gorges Reservoir, Huanghuacheng river was located in perennial backwater zone. The water level was increased greatly,and the flow velocity reduced. The river was in cumulative deposition.2. During flood period,the split ratio had obvious changes,and sediment depositedmuch in curved and extensive area of non-mainstream zone.3. After impoundment, water level was increased and water flow was more straight.The left branch becamesubcritical flow area, which caused a great quantity of sediment deposition; In the confluence area, interaction between the tow water flows leads to the decrease of current energy and the sediment deposition at the end of the central bar.4. The entrance of the left branch along the head of the central bar did not have obvious deposition.The state was relatively in balance.Based on a two-dimensional mathematical model,this paper mainly studied the evolution law of the braided river with sharp bend. For the moment, much of the sediment deposition in the Three Gorges Reservoir Waterway was fine sediment, and this phenomenon was not consistent with the traditional knowledge and research, therefore, this paper tried to explore the initial movement characteristics of fine sediment in the reservoir area through measured data and flume experiments.The study did not adopt the method that charge if deposited by comparison between the sediment concentration and sediment carrying capacity, but by the flow rate.Based on the flow rate, sand movements were divided into the deposition zone, sediment transport zone and erosion zone. When the flow flow velocity was less than 0.5m/s, sediment deposited; When the flow velocity was more than 1.1m/s, sediment were washed away; when the flow flow velocity was between 0.5m/s and 1.1m/s, neither erosion nor deposition occurred. According to the method, the Three Gorges Reservoir sediment movement equation and bed deformation equation were derived.The rationality of the equations were confirmed through mathematical model,and they were applied to the study on the braided river with sharp bend.Through mathematical model, the reasonableness of the application of the equation was confirmed.According to the results, the accumulative deposition amount, change of typical sections, deposition strength along the river, development of talweg and contour maps of deposition thickness of Huanghuacheng river in the next 20 years were provided in this paper.Through the results we could know that the left branch of Huanghuacheng river was still the main part where sediment deposition was serious, and the focus gradually transferred towards export of the left branch.The deposition rate of the right branch were relatively slow. The deposition state of the river would still remain, but the deposition rate would slow down until balance.