通过黄土高原一些河流资料的分析,发现了宽变幅水沙两相流冲淤过程中的 2个临界值。位于非高含水流区域中的临界点,可称之为冲淤下临界 ;位于高含沙水流区域中的临界点,可称之为冲淤上临界。水沙两相流冲淤的双临界现象,对于多沙河流河道泥沙的输移有重要意义。随着含沙量的增大,排沙比先减小而后增大,2次与代表排沙比等于 1的直线相交,这 2个交点即分别对应于挟沙水流冲淤的下临界和上临界。运用所揭示的水沙两相流冲淤过程的双临界现象,可以对冲积河流河型的形成进行新的解释。以我国近百条冲积河流的资料为基础,点绘了年均悬移质输沙率与年均流量的关系,并以不同的符号来区分弯曲河型、游荡河型和高含沙曲流河型。结果表明,上述 3种河型可以很好地被代表特征含沙量分别为 C=60 kg/m3和 C=3.3kg/m3的 2条直线所区分。这 2个特征含沙量即反映了河道水沙两相流冲淤的上、下临界值。

Through an analysis of the data from some rivers on the Loess Plateau of China, the phenomenon of double thresholds in scour fill process of wide range water sediment two phase flows has been found. The threshold located in non hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double threshold phenomenon plays an important role in sediment transportation of heavily sediment laden rivers. With the increase in sediment concentration, the channel sediment delivery ratio shows an increase first, followed by a decline, intersecting twice with the straight line representing the sediment delivery ratio of 1.0. The two intersection points correspond the lower and upper thresholds, respectively. A new explanation for alluvial channel pattern formation can be reached considering the double thresholds in scour fill process of wide range water sediment two phase flows. Based on the data from nearly 100 alluvial rivers in China, the mean annual sediment transport rate has been plotted against water discharge, with different channel patterns, i.e., meandering at low concentrations, braided and meandering at hyperconcentrations represented by different symbols. The three patterns can be well differentiated by the straight lines represented by suspended sediment concentrations of C=60 kg/m 3 and C=3.3 kg/m 3. The two characteristic concentrations can be regarded as a reflection of the double thresholds in scour fill process of wide range water sediment two phase flows in rivers.