Hiding effect among sediment particles exposed on surface of riverbed with an extremely wide range of sediment grain sizes

The objective of this study is to clarify sediment transport phenomena on riverbed composed of materials with extremely wide range of sediment grain sizes. The riverbed materials are generally classified into the following three grain size groups: (a) larger particles which cannot move at all, (b) medium particles which can move as bedload, and (c) smaller particles which can move as suspended load. In the first stage of this study, the experiments were conducted in which each grain size group was represented by one grain size. The vertical structure of the riverbed was measured in the state after attaining the static equilibrium. As a result, it was found that a layer consisting of only medium particles was formed on the riverbed surface. In such a field, the hiding effect of larger particles needs to be fully considered. This hiding relationship was proposed based on our experimental results. The experiment in the second stage was conducted as follows. Since the materials of the actual river have continuous grain size distribution, we also investigated the vertical structure under the condition of riverbed with medium grain size group composed of several grain sizes. Based on the experimental results, it was found that a layer consisting of only medium particles was also formed in the static equilibrium state. In this layer, the bottom elevation of each size of medium particles was at the uniform level on averaged.