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It is essential to consider the fluidity of a debris flow front when calculating its impact. Here we flume-tested mono-granular and bi-granular debris flows and compared the results to those of numerical simulations. We used sand particles with diameters of 0.29 and 0.14 cm at two mixing ratios of 1:1 and 3:7. Particle segregation was recorded with a high-speed video camera. We evaluated the fronts of debris flows at 0.5-second intervals. Then we numerically simulated one-dimensional debris flows under the same conditions and used the mean particle diameter when simulating mixed-diameter flows. For the mono-granular debris flows, the experimental and simulated results showed good agreement in terms of flow depth, front velocity, and flux. However, for the bigranular debris flows, the simulated flow depth was less, and both the front velocity and flux were greater than those found experimentally. These differences may be attributable to the fact that the dominant shear stress was caused by the concentration of smaller sediment particles in the lower flow layers; such inverse gradations were detected in the debris flow bodies. Under these conditions, most shear stress is supported by smaller particles in the lower layers; the debris flow characteristics become similar to those of mono-granular flows, in con-trast to the numerical simulation, which incorporated particle segregation with gradually decreasing mean diameter from the front to the flow body. Consequently, the calculated front velocities were underestimated; particle segregation at the front of the bi-granular debris flows did not affect fluidity either initially or over time. [ABSTRACT FROM AUTHOR]

Abstract Even though dry granular flow can cause well-known impact hazards to retaining structures, studies rarely have been conducted to determine the influence of particle-size segregation on the impact of dry granular flow. Based on the existing experiments concerning the impact of dry granular flow, we calibrated a code of the Discrete Element Method and designed three numerical simulations. Two types of degree of particle-size segregation N s and N l were defined to show the extent of particle-size segregation in the depth and flow directions. The results of the simulation indicated that a dry granular flow initiated with different degrees of particle-size segregation (N s) could develop into very distinctive conditions in terms of the relative positions of different groups of particles in the depth and flow directions. The initial deposition with a higher N s could make it easier for the coarser particles to be located in the front and top of a dry granular flow, with the finer particles being in the tail and bottom of the flow. In the impact process, the greater the values of N s and N l are, the more the coarser particles impact the retaining wall at a higher position at an earlier time during the impact. However, a higher degree of segregation of the particle sizes does not necessarily correspond to a higher impact force and action point. It is more rational to use the change of N s with respect to its initial value as the index to show the influence of particle-size segregation, and a greater change in N s corresponds to a higher impact force and a higher action point. The greater change of N s corresponds to a lower energy dissipation, which is accounted for by the friction among the particles in the flow process, i.e.,less contacts among the particles correspond to the less dissipation of energy, which means that more energy can be converted into impact force. Graphical abstract Unlabelled Image Highlights • New index N s defined as the extent of particle-size segregation in depth direction. • New index N l defined as the extent of particle-size segregation in flow direction. • A greater change in N s corresponds to a higher impact force and a higher action point. • A greater change of N s corresponds to a lower energy dissipation. [ABSTRACT FROM AUTHOR]