Laboratory measurements of impact forces of supercritical granular flow against mast-like obstacles

Impact forces by snow avalanches on narrow obstacles need to be taken into account in the design of many constructions in avalanche prone terrain, such as masts of electrical power lines, ski lifts and cable cars. An important question in connection with such impact forces on high obstacles that extend through the flow is how they depend on the width and cross-sectional shape of the obstacle for a given velocity and thickness of the oncoming flow. Widely used engineering guidelines imply that a significant fraction of the dynamic pressure of the avalanche impacts the obstacle simultaneously over a substantial part of the full height range corresponding to the run-up of the avalanche. A series of laboratory experiments with granular material was conducted in a 7.5 m long and 0.35 m wide chute in order to investigate impact forces on narrow rectangular and cylindrical obstacles for rapid supercritical granular flow (Fr = 13). Obstacle heights varied from about twice the flow depth to more than 20 times the flow depth, which was higher than the observed run-up, and the width of the obstacles varied from about twice the flow depth to about 7 times the flow depth. It was found that the total force on the obstacle was largely unrelated to the obstacle height for heights that exceeded about 3 flow depths, which is much lower than the run-up on the highest obstacles. For a wide range of obstacle heights exceeding about 3 flow depths, the total force on the rectangular obstacles was of similar magnitude to the dynamic pressure, 12ρfu2, acting over an area corresponding to the width of the obstacle and the upstream depth of the flow. The total force was about 30% lower for cylindrical compared with rectangular obstacles. The variation of impact forces on obstacles with the flow velocity for thin, free-surface granular flows may be significantly different from confined flows where a free surface is not present. The presence of the free surface may lead to much lower impact forces at very high velocity than would be expected by comparison with confined flows.