Evaluation of sediment‐related disasters risk with a single rainfall index based on a return period: Case study of Izu‐Oshima, Tokyo, Japan

Yasuhiro SHUIN, Norifumi HOTTA, Yosuke YAMAKAWA, Masakazu SUZUKI

Abstract

In this paper, we propose a new single index for estimating the magnitude of heavy rainfall events that have a high probability of causing sediment‐related disasters（SRD）and examine the efficacy of an early warning system（EWS）based on this index. The single index is derived from a snake line connecting plots of heavy rainfall events on an x‐y plane, with the common logarithm of return period at a given soil water index（SWI）on the x‐axis and hourly rainfall（HR）on the y‐axis. The index（IRPI）is defined as the distance from the coordinate origin to x‐y points on the snake line. The IRPI is integrated with SWI, representing the long‐term rainfall effect, and HR, indicating the short‐term rainfall effect on SRD using the same units（i.e., years）as return period. EWS using IRPI were applied to the western region of Izu‐Oshima Island, Tokyo, Japan, where typhoons Wipha（２０１３）and Ida（１９５８）caused severe SRDs. The results of this study were as follows：１）The difference between heavy rainfall causing SRD and heavy rainfall that does not occur is to be recognized as a difference of IRPI；２）IRPI can be displayed along a time axis based on the single index, making estimation of the temporal fluctuation of SRD risk during each heavy rainfall event simpler than estimation using an x‐y plane without a time axis; and３）Employing IRPI in an early warning system allows intuitive understanding of the meaning of the critical value, since IRPI uses the same units as return period.

Key words

early warning system, return period, sediment‐related disaster, single rainfall index