Experimental Studies On Erosion-deposition Processes Of Sand Bars In Anabranching River

Anabranching river refers to the river pattern in which there is a stable sand bar,and it is widely distributed in the middle and lower reaches of the main stream of the Yangtze River and the middle and lower reaches of Xiangjiang and Yuanshui rivers.In recent years,the construction of a large number of water conservancy hubs on rivers in China has been completed,which has changed the natural hydrological process of the rivers.After the storage of the reservoirs,the discharge of clear water flushes the sand bars in the river channels,causing the erosion of the continent and shrinkage of the continent.phenomenon.Understanding the evolution process,development trend and influencing factors of the river channel under the new situation is of great significance to the channel regulation of the downstream channel of the hub and flood prevention and disaster reduction.A generalized moving bed experiment was used to observe the morphological changes of sand bars and side beaches,and to study the erosion and siltation processes of single and multi-level river channel sand bars under clear water erosion,and analyze the impact of flow changes on the evolution of sand bars.Combined with the vegetation coverage of the multi-level river channel,the effects of vegetation coverage and the evolution of the multi-level river channel were analyzed.The main conclusions of the study are as follows:(1)For continuous river channels,there are differences in the shape of sandbars and channels between inside and outside flow ranges.When the flow is within the interval flow,the shape of the sand bar and the channel changes significantly,and the rate of change increases with the increase of the flow.When the flow is outside the interval flow,due to the widening of the channel and the decrease of the flow velocity,the change in the shape of the sandbar is less affected by the flow,and the rate of change is slowed.In the multi-level river channel,the flow rate in the upstream sand bars B1-B3 is small-medium flow(Q=1.5-3.5 L/s),and the flow rate in the middle reaches of the sand bars B4-B6 is medium-large flow(Q=4.5-5.5 L/s)(2)There are three types of erosion in the top of sand bar:top erosion,unilateral erosion and bilateral erosion.Among them,the way that water flows directly to the top of sand bar is top erosion.The way to realize the erosion of the continent by continuously washing the unilateral continent is unilateral erosion.By continually scrubbing the two sides of the top of sand bar,the sides of the top of sand bar are sunk inwards to achieve erosion on both sides of the top of sand bar.The erosion pattern of the top of sand bar is influenced by the channel shape.The top erosion occurs in all types of sub-channels,the bilateral erosion occurs in single-level sub-channels,and the unilateral erosion occurs in multi-level sub-channels.(3)Under the action of the same flow,the river sections with different vegetation coverage rates change differently.Within the stepped flow(Q=1.5-5.5 L/s),the area of the sand bar without vegetation coverage decreased by 7.30%-25.96%,and the area of sand bar with vegetation coverage decreased by 3.12%-15.54%.In the area without vegetation coverage,the channel width is 60%-169.85%,and in the area covered with vegetation,the channel width is 5.4%-34.25%.In the area covered by vegetation,the erosion of sand bars weakens,and the rate of broadening of the channel slows down.