Rainfall Indices at Estimated Occurrence Times of Sediment Disasters Triggered by the July 2018 Heavy Rainfall

The July 2018 heavy rain, which was actually a series of intermittent downpours instead of a short-term continuous heavy rainfall, triggered a large number of sediment disasters. This study was conducted to evaluate sediment disaster triggers. In the study, an interview-based survey was conducted on the occurrence times of the sediment disasters caused by the heavy rain and a rainfall analysis was completed using analyzed rainfall data from the Japan Meteorological Agency. These were followed by an analysis of estimated occurrence times of the sediment disasters and the temporal changes in rainfall indices determined through the rainfall analysis. An analysis of disasters at 36 sites examined for the purposes of this study showed that many occurred when the soil water index (SWI) during the study period (June 28, 2018, to the estimated occurrence time of a sediment disaster) was maximized. The analysis also indicated that slope failures tended to occur when hourly rainfall was relatively low and the SW/ was high and debris flows occurred when the SW/ was high and hourly rainfall was relatively high. Examination of the data, considering the alert level of the SWI, showed that in cases where the SW/ continued to increase after exceeding the alert level, 75% of the sediment disasters analyzed occurred within approximately 19 h.