Behavior of Boulders within a Debris Flow Initiation Zone

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.