江心洲平衡形态水动力条件的理论分析

江心洲在何种水动力条件下具有稳定的形态一直是河床演变学中的核心问题，采用河道形态变分分析方法，综合分析分汊型河流在水流连续方程、阻力方程、推移质输沙方程和总流能量方程控制下的自动调整机理。以长江中下游两汊不对称的江心洲为研究对象，将其形态概化为规则的等腰三角形，详细数理分析发现，主汊的分沙比大于分流比是这类江心洲达到平衡的必要条件。当分汊河道达到平衡状态时，江心洲的形态会随着汊道间水沙分配比例的调整而变化，且分流比与分沙比越接近，江心洲越窄长。另外，利用长江中游监利汊道的实测水沙资料对理论分析进行验证，结果表明，江心洲形态的理论计算值与相应的实测值符合程度较好，相对误差为9.0%左右。

A theoretical investigation of the hydrodynamic conditions for equilibrium island morphology in anabranching rivers

The equilibrium condition of island morphology is of great importance to the understanding of the self-adjusting mechanism of anabranching rivers. Based on the basic flow relationships of continuity, resistance, sediment transport and energy conservation, the equilibrium condition of a two-channel anabranching system with the island planform of approximately a triangular form is derived theoretically by applying a variational approach. The results show that when the anabranching river achieves stationary equilibrium, the main channel with a larger carrying capacity takes a larger proportion of sediment load than the proportion of flow discharge. The shape of island morphology in equilibrium varies with the proportions of flow discharge and sediment load passing into the two channels. The smaller the discrepancy between the proportions of flow discharge and sediment load, the slenderer the island shows. In addition, for Jianli anabranching reach located in the middle Yangtze River, it is demonstrated that the theoretically computed morphologies of the island are highly consistent with the field observations, with a relative error of about 9.0%.