For sediment management from an ecological point of view, it is essential to evaluate the quantitative and qualitative changes of riverbeds. Particularly the importance of assessing the change in void structure of bed material has recently been pointed out. Some bed variation models are available to the numerical analysis on topographical features of rivers, but they are not useful for such an assessment at all. This study aims at developing a framework of bed variation model considering porosity change in riverbed material. Relating the porosity to the grain size distribution of a sediment mixture and installing the relation into a standard bed variation model, we have developed a framework of advanced bed variation models able to assess changes of porosity. Then, a simple analytical model for a mixture of two particle groups with much different grain sizes is presented in this framework. A flume experiment was conducted to realize the transformation processes of void structure for two situations; one is that only fine sediment is removed from a sediment mixture and another is that fine sediment deposit into a coarser bed material. After the validity of the presented model was verified using a data set provided by the experiment, the model was applied to a calculation of reservoir sedimentation and its performance was examined in detail for these two situations. The simulation results have showed the model can produce a reasonable distribution of porosity of the riverbed material in the longitudinal and vertical directions for both situations.

Key wordsFriverbed variation model, grain size distribution, porosity, particle packing, eco]hydraulics