Okihiro Ohsaka

Field observations of debris-flow initiation processes on sediment deposits in a previous deep-seated landslide site

Although information regarding the initiation processes of debris flows is important for the development of mitigation measures, field data regarding these processes are scarce. We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide, central Japan. On 19 June 2012, our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits (nine surges) and talus slopes (eight surges). The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow, movement of deposits as a mass, and upward development of the fluid area. The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits. The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits; a continuous water supply from the upper stream by the surface flow might have induced this saturation. The third type was associated with changes in the downstream topography caused by erosion. The flow velocity of most surges was less than 3 m s−1 and they usually stopped within 100 m from the initiation point. Surges with abundant pore fluid had a higher flow velocity (about 3 – 5 m s−1) and could travel for a longer duration. Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes.

Japanese Attitudes Towards Forests According to Comparative Opinion Surveys

The aim of this study was to discover what the residents of various countries think and feel about their forests. Large scale opinion surveys had been conducted throughout the world since 1979 to 1996 in various Japanese cities, four German cities, one French city, three Brazilian cities, and in three Peruvian cities. Results of the opinion surveys were analyzed, and attitudes towards forest and nature summarized as follows; 1. Many people believed that a spirit exists in natural things animism and had a quite deep emotional affinity with forest and nature except in Tokyo. 2. On the ethical behavior of forest management, positive responses were most frequent in Germany and France, followed by Brazil and Peru, and then Japan.