Two-dimensional morphodynamic model and its application in the sharp bend of the middle Yangtze River

深入研究急弯段水沙输移及河床变形的数值模拟方法，能为长江中游河段河势预测提供技术支撑，具有重要意义。本文在基于单元中心格式有限体积法的二维水沙数学模型基础上，考虑整治工程和二次流的影响，构建了适用于急弯段的二维水沙数学模型。将改进后的模型应用于长江中游监利-城陵矶河段。本研究主要结论如下：（ⅰ）考虑整治工程影响时对计算河段中有工程的区域进行节点代码标记，计算过程中有整治工程且前期模拟未形成可冲层的区域不发生冲刷，考虑二次流修正时计算了扩散应力项，用于反映二次流对垂线平均流速横向分布的影响；（ⅱ）采用2014年和2018年监利-城陵矶河段的实测水沙及地形资料对模型进行率定及验证，计算结果表明：广兴洲站水位过程的纳什效率系数在0.97以上，且急弯段典型断面水深、垂线平均流速及含沙量横向分布的计算值与实测值符合较好，故改进后的模型可以较准确地模拟急弯段水沙输移过程；(ⅲ) 下荆江急弯段弯顶上游的主流线偏靠凸岸，在弯顶下游才向凹岸摆动，比已有研究报道的急弯段的主流摆动过程更滞后，且观音洲弯道凹岸弯顶上游和凸岸弯顶下游均存在回流区。七弓岭和观音洲弯道含沙量均呈现凸岸高于凹岸的特点，且河床总体呈现凸冲凹淤的特点。;It is of great significance to establish a numerical model of flow-sediment transport and riverbed deformation in the sharp bends to provide technical support for the prediction of river regime in the middle Yangtze River (MYR). A two-dimensional model based on the finite volume method of element center format was established to simulate the flow-sediment transport and riverbed deformation processes in the sharp bends, considering the effects of river regulation projects and secondary flow. The improved model was applied to the Jianli-Chenglingji sub-reach of the MYR. The results indicated that: (ⅰ) when considering the influence of regulation projects, the nodes were marked with codes for the area protected by revetment works, which cannot be scoured in the calculation, and the diffusion stress term was calculated to consider the effect of secondary flow on the transverse distribution of depth-averaged flow velocity; (ⅱ) the model was calibrated and verified based on the measurements of flow, sediment and topographic data of the Jianli-Chenglingji reach in 2014 and 2018, respectively, and the results indicated that the NSEs of section-averaged water level at Guangxingzhou were more than 0.97, and other hydrological parameters of depth, depth-averaged flow velocity and sediment concentration at several cross-sections were in agreement with the measured data. Therefore, the improved model can accurately simulate the flow-sediment transport process in the sharp bend; (ⅲ) the mainstream line was located near the inner bank upstream of the bend apex, and shifted towards the outer bank downstream of the bend apex in the sharp bends of Qigongling (QGL) and Guanyinzhou (GYZ), which was more delayed than the shift process of mainstream line in the sharp bends reported in previous studies. Flow recirculation zones can be observed near the outer bank upstream of the bend apex and near the inner bank downstream of the bend apex of GYZ bend. The depth-averaged sediment concentration near the inner bank was higher than outer bank in the QGL and GYZ bends, and the riverbed deformation was characterized by erosion of inner bank and deposition of outer bank.