Hofu mountainous areas in Yamaguchi Prefecture have weathered Cretaceous granite and gentle stream beds characterizing dissected landform features. During the heavy rainfall period on 21 July 2009, many slope failures and debris flows occurred in Hofu region. Unstable debris still remains on slopes and stream beds, where debris flow cones progressively formed due to past debris flows. Based on our investigation, the biggest sediment movement around Tsurugi River occurred in the 2009 disaster allowed to understand that debris flows of similar magnitude occurred three times or more in the past. A similar situation was confirmed in the case of Ueda-minami River where a custodial care facility for the elderly suffered from such disaster. The formation of the sediment production areas was affected by the recurrence of flooding along Sabagawa River. In addition, the interval of debris flow recurrence is estimated to be between 60 and 346 years, based on radiocarbon dating tested on carbide collected from the accumulated colluvium layers. Comparing with the historical documents of Yamaguchi Prefecture, it is demonstrated that debris flows of similar magnitude occurred more than eight times since 700 CE. It is very important to estimate the recurrence cycle of debris flow and to assess risk of stream having much sediment production in order to prevent disasters in the future. This paper reported that many aspect analysis using the observation of soil profile, the radiocarbon dating of carbide, and the reading and interpretation of historical documents are effective tools to inspect the history of disasters and to assess long-term frequency of disasters in a particular area.

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