土砂・洪水氾濫対策に資する集合運搬流れの粒度偏析メカニズム解明に向けた実験的試み

抄録

This paper reviews the proposed models and previous experimental results focusing on elucidating the mechanism of particle size segregation within the collective sediment-transport flows such as a debris flow and a two-layer flow composed of water and collective transport sediment. These are the foundations of addressing the urgent disaster prevention issues of countermeasures toward a simultaneous inundation by sediment and flood in Japan. The constitutive laws governing the flows composed of mixed particle sizes and the mechanisms of particle size segregation are often considered as independent factors mutually influencing by changing of particle size distribution. Many proposed theories on particle size segregation are based on reverse grading, but there is no decision on whether its formation is caused by the upward movement of coarser particles and the downward movement of finer particles in the flow’s interior (or both). Based on the proposed models and previous experimental results, we suggested that the dimensionless shear stress, τ_＊, which can simply represent the flow characteristics, serves as an indicator for evaluating particle size segregation in a debris flow. Using τ_＊ as an indicator for the flow composed of mixed particle sizes may also contribute to understand the sediment-transport mechanisms at the connection area between the steep stream where collective sediment-transport flows are formed and the river where individual sediment-transport flows are formed.