Mechanisms of Landslide Triggered Debris Flows

Debris flows are often caused by heavy rainfalls. Warning of debris flows is issued by rainfall pattern, rainfall intensity and cumulative rainfall(or duration) in the countries such as Japan, USA et al. However, some debris flows are not regulated by rainfall. Two big debris flow disasters occurred in Japan. One (Gamahara debris flow) in Nagano Prefecture in 1996 killed 14 persons, one (Harihara debris flow) in Kagoshima Prefecture killed 21 persons in 1997. Both took place without any rainfall intensity at the time of occurrence, and especially the Gamahara debris flow occurred in the day of almost no precipitation in a dry season, and cumulative rainfall was not high. The field investigation and dynamic loading undrained ring shear tests for these two debris flows presented two mechanisms of landslide triggered debris flows which do not need full saturation of debris for the high speed motion of debris with a very low shear resistance.