Initiation condition and flow-behavior of debris flow on the mountainous torrent

Debris flows in mountainous torrents cause severe natural hazards due to their immense velocity, large volumes and high destructive power. Many detailed field observations of debris flows have been carried out in many countries of the world, e.g., Japan, China and Italy, as a way to enhance our knowledge of these hazardous phenomena. However, only few observations have been made in the initiation areas because of the difficulty setting up monitoring system. As a result, limited data are available regarding the debris flow formations in initiation areas. The knowledge of debris-flow initiation and development is important for the improvement of attenuation measures, such as warming systems and structural measures. In 1998, debris-flow monitoring system was installed in the upper Ichinosawa catchment of Ohya collapse, Japan. The aim of this paper is to consider and present the process of initiation and development of the debris flow based on the observed data. The Ohya collapse, triggered by a large earthquake in 1707, is located at the source of the Abe River in the middle part of Shizuoka prefecture, central Japan. It is one of Japan's huge collapses with total volume of missing material of approximately 120 million m3. The upper Ichinosawa catchment is located in the northern part of the Ohya collapse appeared to be suitable for the installation of a debris-flow monitoring system, because it has experienced a high frequency of debris flows, commonly about four events per year. The main geologic unit of the upper Ichinosawa catchment is tertiary strata represented by highly fractured shale and alternation of strata composed of sandstone and shale. Most of the basin slope is bare area, occupies 70% of channel area Unconsolidated debris material, sand to boulder sized, has accumulated on the channel bed, and many talus slopes are generated on the foot of the slopes. The thickness of debris deposits reach several meters in some sections of the channel Erosion in the channel during debris flow events suggests that the debris flows may have been initiated by bed fluidization or by intense bedload transport transforming rapidly into a debris flow due to high solid concentration at the steep channel gradient. Thus, the initiation mechanism can be defined as an in-channel mobilization. The monitoring system was installed in the early spring of 1998, and includes video cameras, water pressure sensors, and rain gauge. Other observation instruments, i.e., ultrasonic sensors and a capacitive water depth probe, were installed after that, and certain instruments were reinstalled at other sites for the purpose of improving the quality of observation data.