Research On Riverbed Deformation Measurement And Three-dimensional Numerical Simulation In Ningxia Branch Of The Yellow River

The input water and sediment are the driving force and source of riverbed composition,while the riverbed is also the constraint factor affecting the change of river state,and the river is in a dynamic balance of water and sand conditions and riverbed changes all the time.The research section of the river is located in the typical alluvial plain of the Yellow River,Yinchuan Alluvial Plain,which is adjacent to the Yellow River Bund National Wetland Park and close to the Yinchuan metropolitan area,and the change of river potential of the river is closely related to people’s production and life.In order to research the influence of the deployment of the lead wire gabions on the hydraulic characteristics and riverbed deformation of Ningxia Branch of the Yellow River,this paper analyzes the underwater topography and water flow structure of the river section based on the field measurement method and calculates the amount of riverbed siltation;in order to investigate the influence of the upstream flow conditions and erosion time on the riverbed deformation of Ningxia Branch of the Yellow River,this paper adopts the numerical simulation method,based on the incoming flow conditions during the dry,flat and abundant water periods.The hydraulic characteristics and bed deformation of the branch river section under different conditions,including flow velocity distribution,water level distribution,bed deformation and sediment content distribution of the section,were analyzed by using numerical simulation.The main research results are as follows:(1)During the measured research period,the underwater topography of the river was low on the left and high on the right,the right bank was protected by the shore protection,the shoreline was basically stable,and the riverbed was silted up near the bank;the left bank and the main stream line kept shifting left and oscillating strongly,the left bank was seriously scoured,and the bank collapse phenomenon occurred;the cross-sectional shape along the course evolved from W-shaped to V-shaped;the cross-sectional flow field of the river section was basically oriented to the left bank,and no spiral movement was formed;the overall suspended sand content of the river section varied along the course The overall suspended sand content of the river section has a small variation of about 0.1 kg/m3 along the course;after 10 months of development in the research river section after the deployment of bank protection measures,the river channel in the vicinity of 100 m showed a siltation state,with a total siltation volume of 569.4 million m3 and an average siltation thickness of 1.20 m.(2)Flow velocities were low and low velocity zones persisted in the part of the research area before the channel narrowing in the middle upstream of the branching river section,and in the area of the diversion zone.The high velocity areas in the river section occur in different areas such as after the narrowed channel section in the middle of the upstream part of the river section,the confluence area in the downstream part of the branching river section,and the outlet of the downstream part of the river section.Backwater phenomenon was observed in some locations,and even eddies were formed,with the backwater eddies rotating in a counterclockwise direction.(3)According to the elevation change of the diverging section and confluence section in the research area,the gravitational potential energy of the water in the right branch theoretically does more work and the flow velocity increases more,however,due to the main influence of the river width,the river section shows the change result that the flow velocity increment in the left branch is large,while the flow velocity increment in the right branch is small.The difference between the divergence ratio of the left and right branches decreases with the increase of the upstream mainstream flow.The average water level in the upstream part of the diversion area decreases slightly along the course,and the average water level in the section near the head of the river center is slightly increased due to the effect of water congestion;the average water level along the left branch of the diversion section decreases slowly and then rapidly,while the average water level along the right branch decreases slowly and then rapidly and finally slowly.The average water level in the confluence area showed a trend of accelerated decline followed by a slow decline.(4)The state of river sediment flushing and siltation is obviously related to the flow velocity of the river channel,and the sand-carrying capacity of the river section increases with the increase of the average flow velocity of the water.The sand content of the river is smaller in the area with larger bed siltation height and larger in the area with larger bed scour depth in the research river section.The change of upstream incoming flow in different periods affects the riverbed scouring and siltation intensity;the extension of erosion time only changes the scouring depth and siltation height of riverbed deformation,and does not affect the form of riverbed deformation scouring and siltation.