2019 Volume 75 Issue 1 Pages 1-14
In recent years, the frequency of localized torrential rainfall is increasing, resulting in the occurrence of debris flow disasters that have a big influence on society. Since there are massive numbers of dangerous streams where debris flow is expected to occur, there is great significance in undertaking risk assessment, in order to implement efficient and effective countermeasures. It is very important to calculate the amount of sediment discharge and the movable sediment volume defined as the volume of unstable sediment, which are the basic factors in this evaluation. Additionally, it is crucial to estimate the erosion depth and width which are practical pieces of information. In this research, field surveys were carried out on 1,106 mountainous streams crossing the expressways in the Chugoku region. The distribution characteristics were evaluated based on a statistical analysis of the movable sediment volume, erosion depth and erosion width obtained in the field survey, and a standard value of the estimated erosion depth and width for calculating the amount of unstable sediment in dangerous mountain stream surveys was proposed.