How does particle-size segregation affect the fluidity of multi-granular debris flows?

Abstract

It is essential to consider the fluidity of a debris-flow front when calculating its impact. Here, we flume-tested monogranular and bigranular 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 50% and 50% (5:5), and 30% and 70% (3:7), respectively. Particle segregation was recorded using a high-speed video camera. We evaluated the fronts of debris flows at 0.5-s intervals. We then numerically simulated one- dimensional debris flows under the same conditions, and we used the mean particle diameter when simulating mixed-diameter flows. For monogranular debris flows, the experimental and simulated results were in good agreement in terms of flow depth, front velocity, and flux, but the bigranular debris flows were not well-simulated; the simulated flow depth was less than that found experimentally, and the front velocity and flux were greater. The 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. In this situation, most shear stress is supported by smaller particles in the lower layers; the debris-flow characteristics become similar to those of monogranular flows. Consequently, the calculated front velocities were underestimated; particle segregation at the front of bigranular debris flows did not affect fluidity either initially or over time.