[1] Many debris flows initiate in steep channels (>20 degrees), yet studies have focused on lower-gradient streams where failure is controlled by water height above channel deposits. Multiple debris flows in a steep channel in Ohya landslide, central Japan, were linked to infilling processes (i.e., freeze-thaw and dry ravel) and partial saturation of accumulated sediment. Because of very active geomorphic processes in this catchment, 39 debris flows were observed in the past 6 yr. Pre-and post event imagery indicates selective transport of finer materials during most debris flows that comprise saturated and unsaturated flows; however, fluvial sediment processes, important in relation to debris flow initiation in gentler channels, were minimal. Critical conditions for the movement of such unsaturated materials are derived by equating shear stress with solid friction of the channel bed.

Although information on the behaviour of debris flow in the initiation zone is important for the development of mitigative measures, field data regarding this behaviour are scarce. This research examines the behaviour of debris flow in the initiation zone, based on field observations in the upper Ichinosawa catchment of the Ohya landslide in Japan. In spring 1998, a monitoring system, consisting of video cameras, ultrasonic sensors, capacitive water depth probes, and water pressure sensors (WPS), was installed to assess the behaviour of debris flows in the initiation zone. On the basis of video image analysis, we found that main flow phases during debris-flow events consisted of flow containing largely muddy water and flow containing largely cobbles and boulders. Data obtained from ultrasonic sensors and WPS show that the former flow type (muddy flow) has large amounts of interstitial water throughout its mass, whereas the latter flow type has an unsaturated layer in the upper portion. Results indicate that the concentration of solids in debris flows differs from flow to flow. Debris flows in the upper Ichinosawa catchment cause both erosion and deposition and exhibit changes in their concentration of solids.Key words: debris flow, Ohya landslide, flow behaviour, observation, initiation zone.

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Although it is important to understand the behavior of debris flows in the initiation zone for the development of mitigative measures, data are scarce due to difficulties in field monitoring. To clarify debris flow behavior within the initiation zone, we established a monitoring system in the upper Ichinosawa catchment within the Ohya landslide, central Japan. In the Ohya landslide, loose sediments, previously deposited on steep channel bed, is the main source of debris flow material. Video image analysis of six debris flows revealed that the largest boulders in the debris flows were usually smaller than those in the channel deposits. Thus, debris flows appear to facilitate the selective transport of channel deposits in the upper Ichinosawa catchment. Flows that occur during debris flow surges can be classified as either i) flows comprising mainly cobbles and boulders, or ii) flows comprising mainly muddy water. The duration of each flow type is different amongst debris flow events. Flows mainly composed of cobbles and boulders accounted for most of the surges when channel deposits, which were the main source of debris flow material, were abundant. In contrast, flows were mainly composed of muddy water in surges when channel deposits were scarce. The particle size of the boulders had no clear relationship with flow height, with the size of the largest boulders generally ranging from 15 to 40 cm regardless of flow heights (ranging 0-5 m). The particle size of the material entrained by the debris flow differed among debris flow events. Coarse particles were frequently found on the flow surface when the particle size of the channel deposits was larger. Therefore, the characteristics of boulders in debris flows within the debris-flow initiation zone were affected by the volume and the size of sediment at the source of the debris flow material.