Abstract: 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.

Key words: Qigongling bend, neck cutoff, water level, hydrodynamic simulation, lower Jingjiang River