Flow behavior of debris flows in the upper stream on mountainous debris torrent is described. Data are presented from a measurement and monitoring study at the sites in Ichinosawa upper stream of the Ohya-Kuzure, the initiation area of debris flow, Shizuoka prefecture, between 1997 and 2002. Debris flow was monitored using video cameras with the specification of interval shooting, and the surface level of debris flows measured using a sonic sensor, the water level using the capacitive water depth probe, the water pressure using semiconductor pressure sensor. As the result of observation, debris flows in the study area can be classified into two types. One consists mainly of cobbles and boulders of 20cm and larger, and the mud water do not appear in the matrix of the debris flow surface. The other consists of muddy turbulent water with a little coarse debris, sometimes including few cobbles or boulders. The upper part of the former debris flow is under unsaturated conditions. The appearance of such unsaturated part is supported by the analysis based on the past studies. The latter debris flow is saturated by muddy water as a whole. In addition, both erosion and deposition are observed alternately during the event of a debris flow on the steep torrent, having 28to 30degrees in gradient.

Key words:debris flow, initiation area, debris-flow observation, unsaturated debris flow, saturated debris flow, Influence of riverbed roughness on debris flows