Abstract
In the flood of the Akatani River in July 2017, it was observed that the huge amounts of fine sediment and driftwood coming from upstream was having an influence on the flow of floodwater flows in downstream areas. In this study, a method is proposed to analyze behavior of fine sediment and driftwood within the flow of floodwater. According to this method, the behavior of driftwood is described using an convection-diffusion equation, which enables the analysis of driftwood transportation, erosion and deposition processes. As a result of the numerical simulation conducted on the Akatani River by means of this method, about 70% of the fine sediment coming from upstream is deposited in the analyzed area, thus causing a violent change in the direction of flow. The computation results show that the sediment sorting process, which took place downstream, is reproduced well by the present method. As a result of the numerical simulation, the accumulation of driftwood around the bridge strongly affected the flow of floodwater; this is also shown by the aerial photo and field survey.
