SPATIAL ANALYSIS ON OCCURRENCE FACTORS OF MULTIPLE SLOPE FAILURES USING TOPOGRAPHIC AND RAINFALL INDICES WITH HIGH SPATIAL RESOLUTIONS

In this study, the spatial characteristics of the factors influencing the occurrence and scale of multiple slope failures in areas with smaller grid sized than the unit size for the current emergency alert system for sediment disasters in Japan were investigated by using high-resolution data for the spatial distribution of topography and rainfall. The study areas were located in southern Hiroshima Prefecture and southeastern Ehime Prefecture of Japan, where multiple slope failures occurred due to heavy rainfall in July 2018. With an increase in slope gradient, the frequency of smaller-scale slope failures increases significantly in southeastern Ehime Prefecture, whereas this tendency is not as noticeable in southern Hiroshima Prefecture. This indicates that other indices (e.g., geological feature, large total precipitation) also influence the occurrence of slope failure. A slope-facing direction is not seem to affect the occurrence of slope failure in both study areas. Total precipitation (i.e., long-term rainfall index) influence the frequency of the occurrence of slope failure in southern Hiroshima Prefecture. Whereas, maximum hourly precipitation (i.e., short-term rainfall index) relatesto the occurrence of slope failure in both study areas. These indicate that the occurrence of slope failures depend on different rainfall indices in each study area. Combining the topographic indices (e.g., slope gradient) and rainfall indices (e.g., hourly precipitation, total precipitation) may lead to a highly accurate estimation of the occurrence of slope failure on a smaller grid size than the unit size for the current system.