Rainfall thresholds for deep-seated rapid landslides

Abstract

Soils and weathered bedrock have been known to slide simultaneously, with the resulting landslides sometimes moving rapidly and triggering debris flows. In this study, we refer to these landslides as deep-seated rapid (catastrophic) landslides (hereafter, DCLs). DCLs can result in serious damage, although the frequency of such disasters is generally low. Therefore, early warning systems and the construction of countermeasures for DCLs are important tools for disaster risk reduction. We analyzed the characteristics of recent storms that triggered DCLs in Japan. We found that several of these storms triggered multiple DCLs (multi-DCL events), although most triggered only a single DCL (single-DCL events). For short-term (<24 h) rainfall intensity, there was no significant difference in maximum rainfall intensities between storms that triggered single and multiple DCLs. Conversely, for long-term (48 or 72 h) rainfall amounts, storms that triggered multiple DCLs exhibited considerably higher rainfall amounts than storms that triggered no DCLs or a single DCL. In particular, more than 90% of storms that triggered multiple DCLs recorded rainfall of more than 600 mm per 48 h. Our results suggest that the 48-h or 72-h rainfall amounts were effective for assessing temporal changes in DCL susceptibility, but not the 1-h to 24-h rainfall amounts. This indicates that the occurrence of DCLs is strongly controlled by long-term rainfall amounts but less strongly by short-term rainfall intensity