下泄洪水峰值增加时糙率降低和河床冲刷的相对关系

Comparative Roles of Roughness Reduction and Bed Erosion on Downstream Peak Discharge Increase

黄河下游河道下泄高含沙洪峰流量沿程增值的现象和所谓的人造洪水,引起了水力工程师和科学家的浓厚兴趣。下泄流量增大涉及防洪系统负荷的增加和洪峰向下游移动引起的洪水风险的增加。但是,隐含在这个现象下的基本原理仍然未得到阐述。这种现象的复杂性要求对其有综合的理解。就高含沙水流的紊流和糙率降低,2004年8月我们在小浪底到夹河滩河段作了一个数值模型研究。在这个模型中,我们用了一个关于曼宁系数n和体积含沙量间的简单幂律公式。为了集中在现象本质的研究上,河流被概化为一维顺直河流,河流断面为规则不变的矩形断面,而且,河床介质为均质泥沙,构建的方程和数值解解释了河床变形和浊流之间的关系。考虑含沙水流引发的河床糙率减小因素,模型成功地再现了下游洪峰增值的现象。由于高含沙水流落后洪峰很短时间,洪水波的后部分经历了较少的摩擦,可以超过前面的洪水波。模型结果同样表明,河床变形对巨大的洪峰流量和下游流量演变影响很小。

Highly silt-laden flow in the Yellow River （China） tends to exhibit abnormal features during floods, among which the phenomenon of a downstream increasing discharge peak is observed in the Lower Yellow River. This has become an issue of significant interest to hydraulic engineers and scientists, also in relation to the so-called man-made floods. The increasing discharge entails an increasing load on the flood defense system and an increasing flood risk as the flood peak moves downstream. Yet, the basic mechanisms underlying this phenomenon remain to be unraveled, in order to be able to mitigate its potential damage. The many existing explanations and the complexity of this phenomenon call for a comprehensive understanding. Following the perspective of turbulence and effective bed roughness reduction in hyperconeentrated flow, we have conducted a numerical model study for the hyperconcentrated flood of August 2004 in the Xiaolangdi--Jiahetan reach. In that model, we used a simple power law relation between Manning＇ s n and the volumetric sediment concentration. In order to focus on the essentials of the phenomenon, the river was schematised to a 1 D straight channel with a rectangular cross-section of constant width and with the bed consisting of uniform sediment. The feedback between the bed deformation and the turbid flow, however, is fully accounted for, in the constituting equations as well as in the numerical solutions. The model successfully reproduced the phenomenon of downstream flood peak increase when considering the hyperconcentration-induced bed roughness reduction. As the hyperconcentration lags shortly behind the flood peak, later parts of the flood wave experience less friction and can overtake the wave front. The model results also suggest that the contribution of bed deformation is much less important to the magnitude of the flood peak and its downstream evolution, at least for ＇ normal＇ hypereoncentrated floods without extreme bed erosion and sediment concentrations.