Neck Cutoff Processes Of Meandering River And Seepage Change Under The Vegetatoin Condition

Cutoff is a staged process in the evolution of meandering river,and has the effect of adjusting the shape of the river.The characteristic of the neck-cutoff is that under the influence of river bank erosion and bank collapse,the width of the neck in the?-shaped channel continues to decrease until the upstream and downstream channels on both sides of the neck are connected.The prototype of the meandering river channel in the indoor flume experiment is the Qigongling bend in the middle reaches of the Yangtze River,with a geometric scale of 1:2500,to study the evolution and development process of the meandering river under the conditions of flat beach discharge and vegetation.Realize and capture the bending phenomenon,analyze the change process of the neck and the response of the river to the cutoff,and simulate the seepage of the neck during the bending process according to the experiment.i)Under bankfull discharge,observe the development process of neck cutoff,and design a total of 4 sets of water flow conditions,namely RUN1(S=1.67‰,Q=2.0-3.0L/s),RUN2(S=1.67‰,Q=2.3-2.7 L/s),RUN3(S=3.0‰,Q=2.3-2.6 L/s),RUN4(S=3.0‰,Q=3.1-3.5 L/s).The cutoff phenomenon appeared in the 2-4 groups of experiments,and the time required for the bending was 46 h,28 h and 47 h,respectively.According to the rate of neck width reduction,the change process of neck before cutoff is divided into three stages,and the change of neck witch rate is high-low-high.After the neck is connected,the widening of the new channel will go through three stages of connection-widening-gradual stabilization.In the 3 sets of experiments(RUN2-RUN4)where the cutoff phenomenon occurs,the position and time required for the cutoff are different.ii)In the experiment of neck-cutoff under the condition of vegetation,tall fescue was planted on both sides of the river in advance,and the flat beach flow was started after the vegetation had stabilized.There are two groups of experiments under vegetation conditions,the initial gradient is 3‰,the bankfull discharge intervals are(4.7-5.2 L/s)and(4.2-5.0 L/s),and the two groups of experiments were cut after 61 h and 41 h,respectively.The comparison of neck-cutoff experiments with and without vegetation shows that the vegetation increases the bankfull discharge of the river channel,makes the cutoff position more upstream,prolongs the time required for the neck cutoff,and inhibits the widening of the new channel.iii)Based on the flume experiment,theSEEP/W module of GEOSTUDIO was used to establish the steady state seepage model of the neck during the cutoff process,and the relationship between the width of the neck and the water level difference with the seepage index(seepage flow,seepage velocity and seepage gradient)was calculated.Before the width of the neck is reduced to 5%of its initial width,the seepage flow is only affected by the water level difference.After the width of the neck opening is reduced to 5%of its initial width,the seepage flow is affected by the reduction of the width of the neck,showing increasing trend.When the upstream sediment deposits at the the neck and the river bed is high,the seepage cross-sectional area is increased,or when the cross-sectional area of the seepage flow decreases too fast when the neck is reduced to a certain width,the change of seepage velocity and seepage flow presents an opposite trend,In other cases,the change law of seepage velocity and seepage flow is the same.The change of seepage gradient is the same as the change of seepage velocity.