Abstract
Debris flows occur when enough water supplied into materials on torrential streams. The occurrence criteria of debris flow has been evaluated with rainfall. However, the critical rainfalls triggering off debris flow may be differ by the supply process of water into materials. Material has been as riverbed deposit before the rainfall, or is supplied by slope failures and landslides during the rainfall. The authors selected Nishinogaito torrent for the test field, where debris flows has occurred frequently in these several years. The authors analyze the critical rainfalls based on the supply process of materials. In the Nishinogaito torrent, the material has been supplied before rainfall. Analyzing the accumulated rainfall within the time of concentration, it was estimated to be ten minutes and the critical rainfalls triggering debris flow in the Nishinogaito torrent was found to be 17 mm per ten minutes. From the result, we described a possibility to estimate the supply process of materials and or its occurrence process of debris flow from analyzing the critical rainfalls.
