The flow rate of avalanches could be reduced by up to two-thirds by covering vulnerable slopes with trees planted every three metres. This is the conclusion of researchers in France, who used experiments and a new theoretical model of millimetre-sized grains flowing down a sloping “forest of cylindrical obstacles” to gain insights into damaging snow slides.
“Snow avalanches are a threat for humans in mountain areas with steep slopes,” explains study team leader Philippe Gondret of the FAST laboratory at Paris-Saclay University. “There is still the need for a better knowledge of these phenomena…to help develop possible protections against such a danger.”
Pillars reduce the flow in a non-trivial way
While the researchers expected that forest cover would slow the flow of avalanches to some extent, they found that the presence of pillar-like obstacles on a slope reduces the flow in a non-trivial way. “While in the absence of pillars, the average granular flow velocity increases rapidly with layer thickness, at high pillar density the granular flow rate becomes almost independent of the thickness of the granular layer,” Gondret tells Physics World.
In their experiments, the researchers created a slope from a plank 1 m long and 50 cm wide that can be inclined at well-controlled angles. Such set-ups are routinely employed to study the rheological properties of flowing granular material, but Gondret and colleagues added a twist: they covered the plank with a regularly spaced “forest” of pillars around 2 mm in diameter. These pillars mimic the presence of natural obstacles, which are known to reduce the destruction avalanches can cause. By varying the spacing between them, the researchers were able to zero in on how pillar density affects flow.
Glass beads substitute for snow
With their “forested slope” in position, the researchers were ready to simulate some avalanches. “The grains we employed are glass beads around 0.5 mm in diameter, which are just like sand grains but with a spherical shape,” explains Gondret. “We deduce the instantaneous flow rate using a scale connected to a computer that provides the weight of falling grains at the end of the plane as a function of time.”
To control the flow thickness upstream, Gondret and colleagues fine-tuned the aperture of their bead reservoir and measured the results downstream using an inclined laser sheet. They then developed a mathematical model that predicts how the presence of the pillars affects the rate of the flow and calculated the minimum density of pillars required to significantly reduce the energy the flow carries.
Slab avalanches resemble strike-slip earthquakes
“Our study provides a quantitative criterion for avalanche flow reduction as a function of pillar density and slope angle,” Gondret says. “With our experimental results and from the theoretical modelling we developed, we can infer that the flow rate could be reduced by a factor of two thirds for natural avalanches through a forest with one tree every three metres.”
Applications beyond avalanches
As well as being important for snow avalanche prediction, the results could have implications for other dangerous geophysical granular flows, such as volcanic lahars and pyroclastic flows, and for the flows of materials used in industry.
The researchers now plan to study how randomly arranged pillars affect flow rates. “We also hope to characterize the effect of cohesion of the grains on the overall flow,” Gondret reveals. “These two types of experiments aim to better mimic real-life situations, such as the spatial distribution of trees in forests and the cohesive properties of snow.”
They report their work in Phys. Rev. Fluids.
