Analyses of storm event triggering debris flows in Nagiso-Town, Nagano Prefecture, on July 9, 2014

Ken’ichirou KOSUGI and Takahisa MIZUYAMA

This study investigated storm event which triggered debris flows in Nagiso]Town, Nagano Prefecture, on July 9, 2014, by using antecedent precipitation indices (APIs) with various half]life times (HLTs). The storm was characterized by a large amount of precipitation during anteceding 2 days as well as a large rainfall intensity after a cumulative rainfall of more than 100mm. At the occurrences of debris flows, combinations of APIs with HLTs of13\173h and APIs with HLTs of 0.1\2.0h exceeded their past maximal values, and the storm was proved to be the biggest event for recent 30years. We proposed a method to compute the minimal hourly rainfall which is required to cause the excesses of APIs over their past maxima. The value was estimated to be 34mm at 40minutes before the debris flow occurrences. Thus, the method was expected to provide useful information for early warning and evacuation against debris flow disasters