Debris flows are characterized by coarser particles being concentrated at their frontal segment during flow. To determine the underlying mechanism of this effect, we carried out flume experiments with sediment mixtures, in which the flume length, bed roughness, and flume inclination were varied. In our experiments, we investigated the phenomenon of frontal segment concentration of courser particles using two lengths of flume, which were considered movable, and two fixed beds at different levels of bed roughness. The flume inclination was varied systematically in the 3‒18° range. The experiments were conducted under conditions that have not been examined sufficiently in previous flume experiments. We obtained several useful findings regarding the relationships between the various experimental conditions and the mechanism underlying the concentration phenomenon and provide a qualitative analysis of the mechanism based on our findings. The analysis indicates that even when coarser particles do not rise to the upper layer in the interior of a debris flow, they tend to concentrate at the frontal segment as the finer particles fall and migrate backward from the frontal segment.

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