Debris Slopes on Forested Mountain Side

Abstract

This paper is designed to examine the characteristics and formation processes of debris slopes on forested mountain sides. Two study areas were chosen. One is in the Mikouchi River Basin, Kanto District, Japan, and the other in the Muko River Basin, Kinki District, Japan. Both areas are composed of rhyolitic rocks overlayed by soil and tree growth.
The slopes studied are characterized by their concave profile and steep inclination which is comparable to or exceeds the angle of a talus slope. From a point of view of debris movement, the slope is, as a rule, split into five slope segments, from higer position to lower position : free face, debris-transported slope on which debris can not cease its motion due to steepness and small roughness, high-positioned steep clitter, talus and low-positioned gentle clitter. Two different processes are suggested to the cause of the reverse grading phenomena of debris deposits. One is the sieving process through the grain flow. The other is the removal of the surficial fines of weathered material to the inner coarser at upslope followed by their deposition at downslope in that order through one-at-a time movement such as rockfalls. The fact that the study areas are covered by a mantle of tree growth and soil is thought to suggest that the built-up material on the slopes in question might have been deposited over a relatively short period of time, although a small amount of deposition still continued at present.