Abstract
The present study discusses the setting half-life of effective rainfall as a standard of debris flow occurrence. Three debris flow disasters in Osumi district, Nagiso-town and Hiroshima-city are chosen for investigation, and the rainfall index R', which expresses rainfall history by one value that combines long-term and short-term effective rainfalls, is calculated. According to the results, around 20% of R' increase and decrease by considering a geologic difference for setting of the half-life, but we need attention when the long-term half-life is less than 12 hours (e.g., volcanic ashes). Geology greatly influences the effective rainfall and R' through the half-life. Precise of the prediction of debris flow occurrences is expected by adding geological feature information to rain indexes.
