Bank Collapse Process Of Qigongling Bend And Its Hydrodynamic Numerical Simulation Of Given Neck Cutoff In The Lower Jingjiang River

The lower Jingjiang of the middle reaches of the Yangtze River is a typical curved river channel.The riverbed and channel planform in the middle Yangtze River(Jinjiang reach)was significantly changed as the four systematic cutoffs,including two natural cutoffs(Lianziwan and Shatanzi)and two artificial cutoffs(Zhongzhouzi and Shangchewan)occurred during 1949-1979.As a consecutive bend(i.e.,Qigongling)in the confluence of lower Jingjiang River and Eastern Dongting Lake,its bank collapse rate of upstream and downstream inbend neck was accelerated after the impoundment of the Three Gorge Reservoir.Neck width of Qigongling bend is much less than average,and neck cutoff triggered by overbank flooding or bank collapse is very likely to occur.Based on remote sensing imagery,hydrological data,field observations and numerical simulations,this study focuses on the problem of the Qigongling bend,and discusses the river regulation after the four cutoffs in the Jingjiang river.The collapse of the Qigongling bend and the potential neck-cutting process may affect the water level of the Chengingji.This is of great significance for understanding the issues of revetment,flood prevention and prevention of sudden cutoffs.The main results of this study are as follows:(1)The amount of erosion of the new river channel after the artificial cut of Zhongzhouzi and Shangchewan in Jingjiang River is respectively about 35 million m3 and 1900-4200 million m3.The sedimentation volume of the old river channel is respectively about 160 million m3 and 15400 m3.The rapid decline of the four oxbow lakes in Jingjiang River is mainly affected by local human activities.The channel gradient of four cutoffs along the Jinjiang reach increased by 0.02×104-0.11×10-4 so as to cause the adjustment of stream power along the channel.The results indicate that the erosion of channel bed in long term was induced by four cutoffs.The channel bed of the Jinjiang reach was not approximately approaching the erosion-deposition equilibrium until 1990s,but slight deposition in the lower Jinjiang reach.(2)In the more than 60 years of the Qigongling Bend,the plane shape changed from violent to gradual,and the river bend moved to the downstream.The upstream bank collapse rate is 12.5 m/yr,which is higher than that in other locations.The bank of the long-term revetment project is no large-scale bank collapse.Under the same conditions,the bank collapse rate in the constant discharge is larger than that using the actual discharge process.(3)Using MIKE21 hydrodynamic model,two-dimensional hydrodynamic model of the upper and lower reaches of Qigongling Bend(Jianli-Luoshan reach)was established,and the simulation was carried out under the preset cutting conditions of different flow rates and different depths of the neck.The water level at Chenglingji station tends to increase by at least 0.3 m after neck cutoff due to the top-off effect of the Eastern Dongting Lake confluence.Although the top-off effect has different amplitude of water level,the discharge difference of new cutoff channel and previous main channel is always less than 1%of incoming discharge.The larger the angle between the new and old river troughs,the smaller the sub-flow rate;the influence of different opening directions on the high water level diversion is slightly larger than the middle water level.