Study On Riverbed Evolution And Two-Dimensional Numerical Simulation For Flow In Braided Reach Of Lower Yellow River

The Huayuankou to Gaocun reach in the Lower Yellow River(LYR)is the most typical braided reach.Due to its wide and shallow characteristics,scattered water flow,unsteady main channel,and large erosion and siltation of the river bed,it is difficult to control in braided reach,and the change of upstream inflows and sediments and the evolution of riverbed morphology have a certain impact on the prevention and control projects of the LYR.Therefore,based on the measured water and sediment data and topographical data of the Huayuankou to Gaocun reach of the LYR from 1990 to 2017,this paper statistically analyzes the changes in water and sediment conditions and the evolution of the riverbed morphology of the braided reach from Huayuankou to Gaocun before and after the operation of the Xiaolangdi Reservoir.The response relationship between the change of section shape and the water and sediment factors is discussed.Based on the shallow water equation and the finite element discretization method,a two-dimensional numerical model of water flow is established to simulate the hydrodynamic characteristics of the braided reach of the LYR under different scenarios.The results obtained are as follows:(1)Since the Xiaolangdi Reservoir played an important role in the implementation of the water-sediment regulation scheme,the conditions of water and sediment entering the LYR have fundamentally changed.Compared with before the completion of Xiaolangdi Reservoir,the water volume has not changed much,only increased slightly,but the amount of sediment inflow is sharply decreasing.The amount of water and sand is distributed more evenly throughout the year.The amount of water in non-flood seasons increases.The braided reach changes from a continuous silting period to a continuous erosion period.The median particle size of the main trough and floodplains has increased,and the sediment gradation along the section The proportion of medium-coarse sand increases,and the riverbed becomes coarser,but the median diameter of the main channel,the variation range of the median diameter,and the proportion of coarse sand are all larger than the change in floodplains.(2)With changes in water and sediment conditions from the upstream and changes in river regimes,the shape of the river bed in the braided reach has also been adjusted accordingly.From the perspective of changes in the lateral morphology of the riverbed,the cross-sectional change trends of the hydrological stations along the way are not different.From the perspective of the longitudinal shape of the river channel,the average elevation of the cross section along the braided reach showed a continuous parallel downward cutting trend from 2000 to 2017.The swing amplitude and width of the main trough of the cross section change from upstream to downstream,and the bankfull area in the middle reach of the braided reach is smaller than the upper and lower reach,and the bankfull area gradually decreases with time.(3)The adjustment law of section shape in the main trough is closely related to the water and sediment conditions.By studying the response relationship between the section shape and the water and sediment factors,it can be seen that the bankfull area of the main channel is negatively correlated with the sediment content and the sediment incoming coefficient;the river cross-sectional geomorphic coefficient of the main channel is positively correlated with the sediment content and the sediment incoming coefficient;in contrast,the year average incoming sediment coefficients can better reflect the change law of the cross section;the relationship between the adjustment law and intensity of topography in the main trough and the water and sediment conditions is very complicated,and the main trough section shape adjustment law can’t be accurately described by he single influencing factor(4)The two-dimensional numerical simulation of the braided reach of the LYR can better meet the actual situation.Through the numerical simulation of of the different scenarios in the typical river reaches of the braided reach,it is obtained:when the flow is less than 900m~3/s,the hydrodynamic characteristics of the river main channel from Jiahetan to Xiezhaizha reach have a large change with the increase of flow;When the flow is more than 900m~3/s,,the degree of change of the flow field in the river channel is relatively low.In the same river channel area,due to the serious erosion in the river channel,the increase of the bankful area,and the increase of the branching of the river channel,the mainstream range of the river channel was larger in 2016 than in 2005.The upstream flow field is more scattered,and the hydrodynamic characteristics in the river channel change more with the flow rate.It shows that the cross-sectional shape of the river channel and the scale of the guide flow project have an important impact on the hydrodynamic characteristics of the river channel.