Some factors affecting mass-front velocity of rapid dry granular flows in a large flume

Tests were carried out to investigate some propagation mechanisms involved in rapid, dry, granular flows and to identify factors influencing mass-front velocity in a large flume with two slopes by analyzing image records, as a step in understanding the behavior of rock avalanches. The varied test conditions were gradation and volume of materials, shape and initial arrangement of blocks, consecutive releases, obstacles, and bottom roughness. Flume test results show that the mass-front velocity varied with the characteristics and concentration of particles. The velocity was high and its fluctuation was also great along the upper slope for the cobbles. The velocity increased with the volume for mono-materials. The composite of cubes and gravel with a large volume displayed a rather lower mobility than the mono-material of cubes and of gravel with a small volume. The velocity for the composite of cobbles and gravel with a large volume was almost the same as the mono-material of cobbles and of gravel with a small volume. The velocity also depended on macro- and micro-topography. The velocity decreased by about 25% when the granular flow changed the direction of movement near a concavity for all cases. Two peak velocities were showed in those cases with a convexity. The materials decelerated when it encountered the convexity, and then accelerated greatly after they took a ballistic trajectory from the vertex of the convexity and briefly lost contact with the base of the flume. The mean velocity along the lower slope reduced by 29.4% due to the resistance of a model forest, and increased by 26.2% without matting.