Vertical Structure on Riverbed Composed of Materials with Extremely Wide Range of Sediment Grain Sizes

The objective of this study is to clarify sediment transport phenomenon on riverbed composed of materials with an extremely wide range of sediment grain sizes. Such a phenomenon can be seen in the river reach just downstream of a dam. The riverbed materials are generally classified into the following three grain size groups: (a) larger particle which cannot move at all by water flow, (b) medium particle which can move as bed-load and (c) smaller particle which can move as suspended load. In the first stage of this study, the experiments were conducted that each particle size group was represented by one grain size. The vertical structure of the riverbed was measured in the state after attaining the static equilibrium. We briefly explain the significance of considering when larger particles are contained in riverbed materials. As the degree of exposure of large particles increases, the shear force acting on medium and small particles exposed on the riverbed surface decreases. This is called the hiding effect by larger particles, and this effect must be taken into account when considering the deformation process of riverbed. In other words, it is necessary that larger particles are treated as part of riverbed materials but as mere boundaries. As a result of this stage, it was found that a layer consisting only of medium particles was formed on the riverbed surface in addition to larger particles. The thickness of this layer was within the range of about one to 1.5 times the medium particle size.

Based on these results, the following experiment was conducted as the second stage of this study. Since the materials of the actual river have continuous grain size distribution, we investigated under riverbed with medium grain size group composed of several grain sizes. As a result, a layer consisting only of the medium particle group was formed on the riverbed surface as in the case where the medium particle group was consisted of one grain diameter. When we observed this structure in the layer in detail, it was found that the height of the top surface of sediment appeared in descending order of the sediment grain sizes, and the bottom of each material is at the uniform level on averaged.