T. H. Luong, J. Baker, I. Einav
Dec 19, 2019
Understanding how granular flows interact with obstacles is important in the design of mitigation measures for avalanches and other geophysical hazards. Such measures often involve building new rigid structures from artificial materials, which can be expensive to erect and unattractive on the eye. However, natural forests can also be adopted as effective protective measures, with collections of trees acting as obstacles that dissipate energy, thus reducing the damage caused by flowing material. This paper investigates such resistive effects using small-scale experiments of granular flows through model forests. Arrays of rigid cylinders, or ‘trees’, are attached to an inclined plane at different tree densities, and a mass of granular material is released from rest from a variable-height hopper. A high-speed camera is used to capture the flow progression as material moves downslope and interacts with the obstacles. It is found that, compared to experiments on an empty plane, the presence of trees causes the flow to spread out in the lateral direction and, crucially, to slow down considerably. This effect is quantified into a single empirical law accounting for the slope inclination, release height and tree spacing, which is motivated by considering equivalent steadily propagating flows on empty planes. We find that the bulk slow-down through forested regions can be modelled in this simplified framework using a tree-spacing-dependent effective friction coefficient, although capturing the full spread-out dynamics would require a spatially-resolved approach.