Unprecedented rainfall index for reducing fatalities caused by sediment-related disasters

This study evaluated effectiveness of the unprecedented rainfall index, T, proposed by Kosugi (2022) in achieving appropriate evacuation actions by municipalities and residents for reducing victims caused by rainfall-induced landslides and debris flows. T is defined as the time stamp representing the situation that “current rainfall is the largest since T”; namely, if we go back to time before T, the rainfall at the current time is of a magnitude that has been experienced in the past (i.e., the situation is within the historical range) in every evaluation criterion used in analyses. In other words, the rainfall at the current time is of a magnitude never experienced in the period from T through the present for at least one criterion. T was computed for all disasters that caused death and missing of people in Japan in 2021: the Atami, Unzen, and Okaya disasters. In every disaster, at the time of the landslide and debris flow occurrences, T reached the beginning of rainfall record, indicating that the rainfall reached a magnitude never experienced in the past. Based on these results, it was confirmed that (1) T time series has a large potential to reduce the normalcy bias in all 3 disasters studied, (2) for evaluating anomaly in rainfall, rainfall features should be analyzed based on various evaluation criteria, and (3) the time of past land alteration at an individual location, such as an occurrence time of historical storm event and a construction time of embankment slope, might be set as a T threshold.