Mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradation

Abstract

The mass-front velocities of granular flows results from the joint action of particle size gradations and the underlying surfaces. However, because of the complexity of friction during flow movement, details such as the slope-toe impedance effects and momentum-transfer mechanisms have not been completely explained by theoretical analyses, numerical simulations, or field investigations. To study the mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradations we conducted model experiments that recorded the motion of rapid and long-runout rockslides or avalanches. Flume tests were conducted using slope angles of 25°, 35°, 45°, and 55° and three particle size gradations. The resulting mass-front motions consisted of three stages: acceleration, velocity maintenance, and deceleration. The existing methods of velocity prediction could not explain the slowing effect of the slope toe or the momentum-transfer steady velocity stage. When the slope angle increased from 25° to 55°, the mass-front velocities dropped significantly to between 44.4% and 59.6% of the peak velocities and energy losses increased from 69.1% to 83.7% of the initial, respectively. The velocity maintenance stages occurred after the slope-toe and mass-front velocity fluctuations. During this stage, travel distances increased as the angles increased, but the average velocity was greatest at 45°. At a slope angle of 45°, as the median particle size increased, energy loss around the slope toe decreased, the efficiency of momentum transfer increased, and the distance of the velocity maintenance stage increased. We presented an improved average velocity formula for granular flow and a geometrical model of the energy along the flow line.