Debris Avalanches and Debris Flows in Urakawa River Basin

This report interprets the physical and mechanical properties of the unstable debris deposits in the Urakawa river basin. The Urakawa river basin locates along the great tectonic line (Fossa Magna) in Nagano prefecture in Japan and is well known by frequent occurrence of debris flows initiated by slope failures in the upper part of the basin area. Also in April 1992 a relatively large debris flow with the volume of 1 million m^3 has occurred. Various geotechnical investigations were performed in order to analyse the mechanism and the process of debris flows. Especially the shear strength parameters of debris flow deposits on the river bed were determined by using a ring shear apparatus