現行の土砂災害警戒システムの枠組みから導出される確率年による土砂災害発生危険度の評価について

抄録

In this paper, we propose the methodology of applying extreme value analysis to the current Japanese early warning system (JEWS). With this method, a return period associated with the probability of sediment disaster occurrence (RP_SL) is obtained by applying extreme value analysis to the distance from the coordinate origin to an x-y coordinate point on the snake line widely used in the JEWS. This distance is defined as the normalized distance, the x-axis of this coordinate is the soil water index divided by the 2-year return period value, and the y-axis is the 1-hour cumulative rainfall divided by the 2-year return period value. The results of applying RP_SLs to the three recorded sediment disasters in the city of Kure, Hiroshima Prefecture, between 1921 and 2018, excepting the 5-year period between 1945 and 1949, are as follows : 1) The RP_SLs of all three sediment disasters were above the 50-year return period and rank in the top three. 2) These sediment disasters occurred at a time when the RP_SL exceeded the past maximum RP_SL that was not associated with sediment disaster. These results reaffirm the effectiveness of the snake line widely used in the current JEWS, demonstrating the validity of the RP_SL. The superiority of the proposed RP_SL is that it is a statistical value (i.e., return period), which leads to the possibility of intuitive recognition of the temporal risk fluctuation associated with sediment disaster occurrences.