长江中下游阻隔性河段作用机理

Mechanism of barrier river reaches in the middle and lower reaches of the Yangtze River

阻隔性河段能够阻隔上游河势调整向下游的传递,对稳定河势起到关键性作用。本文以长江中下游34个单一河段为研究对象,在系统总结长江中下游河道演变规律的基础上,归纳出阻隔性河段控制要素包括:单一微弯的河道平面形态、河段中上部无挑流节点;河相系数小于4;河道纵比降大于1.2?;凹岸黏粒含量高于9.5%;床沙中值粒径大于0.158 mm等。从Navier-Stokes方程出发,推导出河湾水流动力轴线弯曲半径的表达式,进而分析了各控制要素对水流动力轴线摆动及阻隔性河段形成的作用。阻隔性河段的判别条件为:不同流量级下水流动力轴线摆动力与河道边界条件约束力的比值始终小于1;阻隔性河段作用机理在于:即便上游河势发生调整,本河段的河道边界始终能约束主流摆动幅度,归顺上游不同河势条件下的主流平面位置,为下游河道提供了相对稳定的入流条件,从而阻隔上游河势调整向下游传递。

The channel in the middle and lower reaches of the Yangtze River is characterized by staggered distribution of the bifurcated channel and the single-thread channel. The amplitude of change of river regime is stronger in a bifurcated channel than in a single-thread channel. The adjustment of the river regime in the upper reaches usually propagates to the lower reaches through the migration of the main stream line and causes the riverbed evolution of the lower reaches. Whether the adjustment of the river regime in the bifurcated channel can pass through a single-thread channel and propagate to the lower reaches will affect the stability of the overall river regime. Studies show that the barrier river reach can block the upstream channel adjustment from propagating to the lower reaches; therefore, it plays a key role in stabilizing the river regime. This study investigates 34 single-thread channel reaches in the middle and lower reaches of the Yangtze River. On the basis of the systematic summary of the regularities of riverbed evolution in the middle and lower Yangtze River, the control factors of barrier river reach are summarized and extracted： the planar morphology of single-thread and meandering; no flow deflecting node in the upper or middle part of the river reach; the hydraulic geometric coefficient is less than 4, the longitudinal gradient is greater than 1.2%0; the clay content of the concave bank is greater than 9.5%; the median diameter of the bed sediment is greater than 0.158 mm. Derived from the Navier-Stokes equation, the calculation formula of the bending radius of flow dynamic axis is deduced, and then the roles of these control factors in restricting the migration of the flow dynamic axis and shaping the barrier river reach are analyzed. The barrier river reach is considered as such when the ratio of the migration force of the flow dynamic axis to the constraining force of the channel boundary is less than 1 at different flow levels. The mechanism of the barrier river reach is such that even when the upstream river regime is adjusted, the channel boundary of this reach can constrain the migratory amplitude of the main stream line and centralize the planar position of the main stream line under different upstream river regime conditions, providing relatively stable incoming flow conditions for the lower reaches, thereby blocking the adjustment of upstream river regime from propagating to the lower reaches.