Understanding the flow characteristics of a debris]flow front is essential for evaluating the impact force and predicting the attainable time of the debris flow. However, the fluidity of the front has not been examined experimentally for either mono] or multi]granular debris flows. This study conducted flume tests of debris flows consisting of sediment particles that were either monogranular or a mix of two diameters and compared the results with numerical simulations. Particle]size segregation was examined in the mixed]diameter experiments using a high]speed video camera and five debris]flow samples obtained from the front edge of the debris flow at ca.0.5]second intervals. The experimental and calculated results showed good agreement for the monogranular debris flows in terms of flow depth, front velocity, and fluxes, whereas disagreement was seen in the mixed]particle]size debris flows. The disagreement was thought to arise from the dominant shear stress resulting from the concentration of smaller sediment particles in the lower layer of the flow due to inverse grading detected in the body of the debris flows. Additionally, the calculated velocity of the front was underestimated compared with the actual velocity, as the particle]size segregation at the front of multi]granular debris flows did not affect the fluidity in the initial and developing phases.

Key wordsFdebris flow, particle]size segregation, flume test, numerical simulation