Information on the behavior of debris flow in the initiation zone is important for the development of counter measures. However, only a few observations have been conducted in the initiation zones of debris flows owing to monitoring difficulties. To detect the behavior of debris flow in an initiation zone, we established a monitoring system in the upper Ichinosawa catchment within the Ohya landslide, central Japan. By analysis of video images obtained from field monitoring, flows that appear during sequences of debris flow surges were classified into two primary types: flows comprising mainly cobbles and boulders, and flows comprising mainly muddy water. The velocity of the muddy flows can be evaluated by Manning's equation. Flows comprising mainly cobbles and boulders had higher flow resistance compared to muddy flows, and cannot be evaluated their velocity by Manning's equation. Furthermore, in the case of muddy flows, it seems the flows were turbulent, whereas flows comprising mainly cobbles and boulders were laminar and their boulders slide. Flows comprising cobbles and boulders usually appeared at the front of a surge and are followed by the muddy flows. In each typical debris flow surge, the flow depth was highest during passage of flow comprising cobbles and boulders and the flow velocity was highest at the front of the muddy flow. However, some surges were comprised only one flow type. Grain size of boulders was different amongst debris flow events. Longitudinal grain size segregation of debris flow was not clear in the upper Ichinosawa catchment.


Key wordsFdebris flow, initiation zone, field observation