七弓岭弯道颈口裁弯条件及其对城陵矶水位影响

下荆江与东洞庭湖交汇段的七弓岭弯道，其颈口宽度已远小于河道宽度，而且三峡水库蓄水后清水冲刷河岸，加速颈口河道缩窄，存在漫滩冲刷或崩岸贯穿作用下颈口裁弯的可能性，故研究七弓岭弯道的颈口裁弯临界条件较为迫切。采用水文资料、地形数据和MIKE21水动力模型，建立七弓岭弯道上下游（监利-螺山）的二维水动力学模型，在不同流量和颈口设置不同深度的浅槽条件下，开展颈口裁弯前后的河道水动力数值模拟，分析七弓岭弯道颈口裁弯条件和预测七弓岭裁弯对城陵矶水位的影响。研究表明，七弓岭颈口可能在高水位条件下发生冲切型裁弯和较窄颈口宽度条件下发生崩岸型裁弯，裁弯将导致城陵矶水位至少增加0.3 m，这主要是七弓岭下游的洞庭湖入汇长江对七弓岭弯道水位的顶托作用，而且在不同强弱顶托作用下裁弯后新河道与老河道的分流量差值小于上游来流量的1%。

Threshold for neck cutoff occurrence in Qigongling bend and its influence on Chenglingji stage of lower Jingjiang River

The neck of Qigongling bend, located in the section of lower Jingjiang River connecting the East Dongting Lake, is much narrower than the average mainstream channel width, and its narrowing is speeded up together with a riverbank scour under the existing conditions of this special location in combination with the effect of Three Gorges Reservoir. Hence the occurrence of its cutoff to be triggered by overbank flooding or bank collapse is quite possible, and the threshold of this event must be understood. In this work, we develop a two-dimensional hydrodynamic model of the upper and lower reaches of the bend (i.e. Jianli-Luoshan reach) based on the hydrological data, topographic data, and MIKE21 hydrodynamic model, and conduct simulations and analysis for different inflow conditions and different flow depths in the cutoff channel to reveal the influence of the cutoff event on the mainstream stage at Chenglingji. Results show that being triggered by overbank flooding and bank collapse, the possibility of the neck cutoff is very high as a result of either a high flood discharge or neck narrowing. Chenglingji stage tends to be raised by at least 0.3 m by the event under the lake’s top-off effect on the confluence. However, the difference between the discharges through the new cutoff channel and old main channel will keep less than 1% of the inflow, though this top-off produces quite different effects on the mainstream stage.