Origin of Bedforms in a Scale of Valley Width and Their Effect on the Sediment Transport in Mountain Rivers

在山区河流中，可见到几倍于谷宽的大尺度底形，这与河道的分聚密切相关。利用动床试验，研究了不同粒径泥沙的床面形态及其对下游河道输沙的影响。研究结果表明：（1）在充分扩散水流作用下，初始河道在全槽宽（谷宽）范围内形成的双排沙与天然大尺度底形十分相似，因此可以认为大尺度底形是大洪水作用下形成的非常双排沙。(2)大规模底形的分叉集中河道在弯道部分受中小流量冲刷较深。这意味着大尺度底形可以成为中小洪水的泥沙来源。(3)在上游河道集中和下游河道分叉处观察到节点 ...更多信息 am.为了研究分流河道水流交替变化的机理，采用Y形渠道对分流河道水流进行了抽流试验研究。结果表明：（1）这一现象是由上游河道中的交替沙坝作用引起的。它不会发生在任何情况下没有酒吧。(2)主流的交替变化不是由于上游拦门沙的迁移，而是由于拦门沙使上游河道水流向一侧严重偏斜。当大量泥沙向某一侧槽排沙时，会堵塞进口，导致水流向另一侧槽改道。(3)这些机理可以用一维床面演变微分方程组来描述，并归结为一个倾倒振荡方程。从方程中找到了振荡条件。

In a mountain river, large scale bedforms to be in a scale of several times of the valley width can be seen, which closely correlate with the channel branching and gathering. Movable bed experiments were conducted by using sediment materials with mixed grain size to find the origin of the bedforms and their effects on the sediment transport in the downstream channels. The results showed :(1) Double row bars generated in the initial channel under a fully spreading flow in the whole channel width (valley width) closely resembles a natural large scale bedform, then it can be concluded that the large scale bedforms are very double row bars generated by a large flood.(2) Bifurcated and concentrated channels with large scale bedforms are deeply eroded in the portion of bends by a medium or small flow discharge. This means that large scale bedforms can be sediment resources for medium or small floods.(3) Node points were ovserved where upstream channels concentrate and bifurcate just downstre … More am. At the points, alternating change of main flow occur from one bifurcated channel to the other one to play an important role for chenge of sediment transport rate in the downstrem channels.Extracted experimental tests for bifurcated channel flows were carried out by using a Y-shape channel, in order to elucidate the mechanisms of alternating change of main flow. As the result, the followings ware found :(1) The phenomenon is caused by the effect of an alternating bar in the upstream channel. It does not occur in any case with no bars.(2) The alternating change of main flow does not arise from the migration of upstream bar, but from the storong deviation of flow toward one side bank in the upstream channel due to bar. While much sediment discharge toward one side channel drives to block the entrance, that induces change of flow course to another channel.(3) These mechanisms could be formulated by simultaneous 1-D differential equations for bed evolution, which summarized to a dumping oscillation equation. From the equation, oscillation condition was found. Less