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

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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