Abstract

Rocks are exposed on bed in mountainous rivers. Furthermore, alternate layers with cohe sive and non-cohesive materials can be seen in rivers flowing in delta area. Layers of rocks and cohesive material are exposed at scoured areas in these rivers and wave height of bars tends to be suppressed, comparing to wave height of bars on non-cohesive material. It is expected that the number of transverse mode of bed configuration increases there and streams are bifurcated. In the present study, the effect of layers of rocks and cohesive material on the number of transverse mode of bed configuration is discussed using the results of two dimensional bed deformation analysis and flume tests. The results show that the bed degradation at the eroded area is suppressed because of the expose of the rigid bed and the wave height of bars decreases. Furthermore, the transverse mode of multiple row bars develops. When thin cohesive material layer exists in non-cohesive material layers, transverse mode of multiple row bars develops. After exposing the non-cohesive layer under the cohesive layer, bed degradation is accelerated and deep scoured holes in the circle are formed. As a result, a stream with longer wavelength from the bank to the opposite bank is formed. Flume tests show that the bed is decreased at the same place in the analysis and the mode of multiple row bars is developed.

Keywords: alternate bar, cohesive material, mode of multiple row bar, rock, bed load layer