Abstract
Large-scale debris flow disasters take place due to heavy rainfall recently. The present study discusses the relationship between the trigger and the inherent factor of debris flow occurrence. Three debris flow disasters in Izuoshima, Nagiso-town and Hiroshima-city are chosen for investigation. Debris flow occurrence due to rainfall is greatly dependent on two factors: previous prolonged rainfall and the last short strong rainfall. Rainfall index R' can express with one value combining the long-term effective rainfall Rw and the short-term effective rainfall rw. In addition, it is necessary to clarify the inherent factors of a natural place, such as topography, geology and vegetation. Especially, slope gradient has strong influences on the sediment runoff characteristics, such as flow depth, velocity, discharge and sediment concentration. We should learn more about formation in land, land-use history and the history of disasters as preparations to a natural disaster.
