Experimental Study On The Scour Of Bend Branch Of The Yangtze River Under New Water And Sediment Condition

The branching type is widely found in alluvial rivers.Due to the complexity and variability of sediment conditions and boundary conditions,the evolution mechanism and law of the branching type are complex.After the impoundment of the Three Gorges Project sediment reduced significantly,resulting in long-distance and long-time erosion of downstream river.It has a great influence on the flood control,slope stability and ecological environment of the downstream river.Therefore,it is of great significance to study the characteristics of scouring and silting in the downstream river after the reservoir construction,make clear the complex coupling between the flow conditions and the topographic erosion adjustment and predict the trend of the river evolution.In this paper,a combination method of physical model test and theoretical analysis is used,to explore the water flow characteristics,scouring mechanism and evolution trend of the bend branching river in the middle reaches of the Yangtze River under the new water and sediment condition.The main conclusions are as follows:Under the water and sediment condition in each group,the typical bend branching channel is generally in the state of scour.,but the specific scour position and range are different.In the aspect of transverse distribution,the scour of the convex bank is greater than that of the right bank.When the discharge and the time are constant,the erosion amount of left branch decreases with the increase of sediment concentration.When the sediment of flow is constant,the erosion amount of left branch increases first and then decreases with the increase of the discharge,due to different duration of different flow-level.The erosion amount of left branch is the largest in the case of large flow for a long time.The relationship between the condition of water and sediment and the adjustment of topography is not independent or unidirectional,but the coupling effect of mutual influence.Under the action of water and sediment the scouring-silting adjustment occurs,and the change of the topography would affect the distribution of flow velocity and the sediment carrying capacity,resulting in the continuation of the change of the topography.The results of the evolution trend prediction in the series are as follows:channel is generally in the state of scour,the shaped meanderingchannel of left branch is developing,and the right branch is deposited.Changes of velocity distribution:the velocity in the left branch increases and the velocity in the right branch decreases in the branched section.The flow direction of the junction of the outlet section is obviously deflected.The location of maximum velocity changes frequently in interannual.It is not conducive to stability of flow.The width of the navigation channel is decreasing.The scour adjustment of branching reach with bend is not only unfavorable to the stability of channel and the condition of channel,but also through the swing of flow dynamic axis affect the flow structure of downstream,thus affecting river regime stability of the downstream reach.