Study on channel variation in mountain rivers

山区河流断面宽，产沙活跃，沉积物量大。在那里形成的河道即使在高洪水中也是辫状的，因为洪水宽度太宽而不能形成稳定的河流。本研究的主要目的是探讨河床演变过程及其相关的推移质变化、泥沙分选等现象，主要研究成果如下：1.几个流通常产生在一个水槽床选择本研究。从而形成编织的通道系统。各支流的流向和宽度在时间和空间上都是变化的，这就导致了辫状河道系统中支流数量的变化以及整个水流宽度的变化.级配河床中水流的这种变化比均匀河床中更为活跃，这是因为级配河床中泥沙分选导致的水流分叉更为频繁.输沙率随水流宽度的减小而增大，随断面上水流条数的减少而增大。换句话说，它增加在收敛过程中的流，反之亦然，在分叉阶段。推移质分布预测需要明确各河段的流宽、系统内形成的河段数等参数。根据交替栅条形成的判据，推导出了水流宽度的计算公式。这对于估算时间平均的沉积速率是非常有用的。

A large amount of sediment is commonly deposited in the regions with a wide section of mountain streams where sediment yields are very active. Stream channels formed there are braided even in a high flood because the flot width is too wide to form a stable stream. The project of this study is to investigate the processes of channel variation and their associated phenomena such as variation of bed load, sediment sorting and so on. The results obtained from the research are summarized as follows :1. Several streams are commonly produced in a flume bed chosen for the present study. resulting in a braided channel system. The flow direction and width of each stream change in time and space, which brings about varieties of the number of stream as well as of the whole flow width in the braided channel system.2. Such variations of streams in the system are more active in the bed with graded material than with uniform one, because a stream bifurcation occurs more frequently due to a sediment sorting in the former case.3. The sediment discharge rate increases with the decreases of the flow width and the number of stream in the cross section under consideration. In other words, it increases in a converging process of stream and vice versa in a bifurcating phase.4. It is necessary to specify the parameters such as the flow width of each stream and the number of stream formed in the system for the prediction of bed load distribution. A formula of the flow width is developed in terms of a criterion for alternating bar formation. It is very useful for evaluating a temporally averaged sediment rate.