The Sabo plan did not consider the difference in the timing of flooding and sediment discharge at the confluence of a tributary and the main river. We postulated that the water surface and riverbed deformation at the confluence vary greatly with the timing of flooding and sediment discharge. We carried out basic flume experiments to show the influence of the timing of a flood at the confluence on riverbed deformation. In addition, we performed experiments in which the sediment discharge from the branch river was controlled by a slit]type Sabo dam. The trunk flume was 6.0 m long and 0.3 m wide with a 1/80 slope, and the branch flume was 3.0 m long and 0.2 m wide and had slopes of 1/40 ]1/25. The angle of the two flumes at the confluence was 60 degrees. The model hydrographs of the trunk and branch were the same shape. There were two experimental patterns: in one, the peaks of the branch and trunk rivers were at the same time, and in the other, the peak of the branch river preceded the peak of the trunk river. As a result, the timing of the flooding of the trunk and branch rivers and sediment discharge were shown to deform the riverbed markedly. Therefore, in runoff analyses, it is necessary to remember that in nature, there is often a difference in the timing of the sediment discharge of trunk and branch rivers in mountainous districts, where the sediment discharge is very large.


Key wordsFRiver confluence, flume experiment, bed deformation