Session6: Water-related Disaster Risk Reduction
Title: EFFECTS OF PARTICLE-SIZE DISTRIBUTION AND STREAM GRADIENT ON SEDIMENT SORTING OF A DEBRIS FLOW
Takashi Wada, Jumpei Maeda, Hiroshi Miwa
We conducted flume experiments with sediment mixtures to reveal the effects of particle-size distribution and stream gradient on sediment sorting of a debris flow, focusing on the concentration of coarser particles that appeared at the flow front. In our experimental results, as the particle sizes of the sediment material became coarser or the flume gradient became lower, the sediment sorting progressed more remarkably. A possible reason for this is the decreasing of debris flow velocities, considering that the lowering of flume gradient and the enlargement of particle sizes of the materials decrease them. Since their changes influence the movement of materials in the depth direction, Middleton’s suggested mechanism (1970), which is a falling mechanism of finer particles through the interstice between the materials in the flow’s interior, may explain sediment sorting of a debris flow. In consideration of these results, we developed a numerical model to describe the sediment sorting based on the conventional one-dimensional model. In our model, the debris flow depth is divided into the several layers with the same thicknesses. The migration velocity of materials and the sediment concentration of each sized particle are considered in each divided layer. The volume of downward movement of each sized particle is also incorporated. The calculated results using our model indicate the concentration of coarser particles at the debris flow front. In addition, our model could explain temporal changes in the proportions of each sized particle of the flow on various particle-size distributions of the materials and various slope gradients.