ABSTRACT
The debris flow that occurred in the upper stream of the Gamahara-zawa killed 14 construction workers and injured 9 others. The debris flow was triggered by a small slope failure. It flowed down 2.7 km downstream with a grade of 20 degrees. The volume of the slope failure was about 33,000m3. According to the chemical analyses of the water, calcium and sulfate iron components are predominant. It showed a lot of groundwater discharged through the volcanic products. The precipitaion of the day before disaster was comparatively small even if the melting snow was considered. Conclusively, it is supposed that the slope failure was easy to occur due to the existence of big slope failures caused in the preceding year and due to the geologic conditions.
