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How do forests influence avalanches?


For people in mountain regions, forests play a very important protective role. They act as a safeguard for communities and infrastructure against landslides, rockfalls, debris flows and avalanches. Compared with man-made barriers, mountain forests constitute a much less costly and more ecologically friendly means of protection. But how does a forest influence snow masses and thus prevent them from reaching the valley bottom and causing damage? Both standing and fallen trees stabilise the snowpack and are thus capable of preventing avalanches or at least reducing the size of the slab that is released. In addition, the canopy gives rise to a different snowpack structure in the forest than in the open field. In the forest, snow falls from the trees, and the canopy changes the energy balance of the snow surface. As a consequence of these factors, the forest snowpack is subject to small-scale inconsistencies, and the weak layers that play a critical role in the release of avalanches can form less easily. Furthermore, the forest is able to stem and stop small avalanches.


The forest as an avalanche inhibitor

When a large avalanche is released high above the tree line, its generates so much energy that it can destroy an entire forest and reduce large trees to splinters. For this reason, breaks or aisles are often cut in the forests of mountain regions that have a lot of snow. If the trees withstand the tremendous compressive force exerted by the avalanche, on the other hand, it loses energy and its progress towards the valley bottom is inhibited. But how can the influence of the forest on avalanches be quantified? How do different forest structures affect avalanche flow behaviour? How can physical models be used to characterise the forest and forecast avalanche runout distances? With a view to answering these questions, numerous forest avalanches have been examined, documented and simulated in recent winters.


Developing forest avalanche models

Based on field studies of the interaction between forests and avalanches, scientists have formulated new theories concerning the energy balance and mass balance of avalanches, and integrated their suppositions for testing purposes in the Rapid Mass Movements (RAMMS) computer program developed at the SLF.

By showing how avalanches tumble towards the valley bottom in the three-dimensional terrain, RAMMS helps local decision-makers, structural engineers and foresters to calculate the appropriate dimensions for avalanche defences, produce zoning plans, and devise silvicultural measures.

In order to improve the model further, additional field measurements and laboratory experiments need to be performed. To simulate avalanche snow, tiny glass beads are released down a steep chute to collide with a variety of forest structures.

In future, it is hoped that RAMMS will be able to calculate the effect of the forest on avalanche runout distances, velocities and pressures. Such information enables the safety authorities to establish the most economical protective measures, and to decide whether a settlement or road needs to be protected by either man-made defences or simply a replanted or denser forest.


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