1984年御岳山岩屑なだれの流動状況の復元と流動形態に関する考察

Abstract

A great slope failure was caused by the Western Nagano Prefecture Earthquake, 1984, ona slope of Mt. Ontake, at a valley head of the Denjogawa Valley, a left tributary of the Nigoriga-wa River. A large amount of debris ran down along the river about ten kilometers as a debrisavalanche with high speed burying ten persons and reached the main stream of the OhtakiRiver and damed up it.We investigated the serial processes of slope failure, flow and deposition of this debris massfrom the field survey, analysis of topographic change and eyewitness evidences, to restore theoriginal motion of the debris avalanche, and to reveal the physical mechanism of the rapid move-ment of the debris mass. From the investigation referring to some other data of similar rapidmass movements, we infered that the bottom layer of debris mass flowed taking a state of debrisflow saturated with water and that dry debris mass moved riding on this lower layer.