Chapter 5 Forest Avalanche Control

Protective forests can prevent the triggering of avalanches, their formation, size, and sometimes even the reach of avalanches. This chapter explains how a forest stand fulfills its protective role in reducing the risk of avalanches.

Avalanches are mass movements of snow that can contain rocks, soil or ice (McClung and Schaerer 2009). Depending on their size, avalanches can have devastating effects on human facilities and activities. Most avalanches occur above the timberline or in non-forested areas. Thus, strictly speaking, the term forest avalanche only applies when an avalanche has been triggered within a forest stand.

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Figure 5.1: Distinction between forest avalanches and non-forest avalanches. Modified according to (Meyer-Grass and Imbeck 1987)

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In general, the effect of a forest in minimising the risk of an avalanche is greatest in the potential release area. In contrast, forest in the transit area of a fully formed avalanche is often completely cleared, with much less or no effect at all – depending on the flow dynamics and flow height of the avalanche. However, in the case of avalanches of low flow heights and flow velocities, sufficiently strong trunks can significantly influence the range due to the reduction of the flow velocity and the retention of moving snow masses (detrainment) (Feistl et al. 2014).

5.1 Influence on climatic effects

The stabilising effect of the forest on avalanche triggering with regard to climatic effects, is related to its influence on the distribution of the snow cover and the snow depth as well as snow layering.

5.1.1 Snow cover distribution and snow height in forest stands

Snow deposition occurs in a forest at different heights and intensity, leading to a snowpack that varies spatially in terms of depth and water equivalent. Snow depth normally decreases with decreasing distance to the stem.

https://doi.org/10.1016/j.coldregions.2022.103587 https://www.scopus.com/record/display.uri?origin=citedby&eid=2-s2.0-85134231549&citeCnt=2&noHighlight=false&sort=plf-f&src=s&st1=Snow+cover+distribution+in+forest+stands&sid=3e1aa352542e81d0f54ed976378af3a9&sot=b&sdt=b&sl=55&s=TITLE-ABS-KEY%28Snow+cover+distribution+in+forest+stands%29&relpos=1

5.1.2 Snow layering in forest stands

References

Feistl, T., P. Bebi, M. Teich, Y. Bühler, M. Christen, K. Thuro, and P. Bartelt. 2014. “Observations and Modeling of the Braking Effect of Forests on Small and Medium Avalanches.” Journal of Glaciology 60 (219): 124–38. https://doi.org/10.3189/2014JoG13J055.

McClung, D., and P. Schaerer. 2009. The Avalanche Handbook. 3rd ed., 4th print. Mountaineers Bks. Mountaineers Books.

Meyer-Grass, M., and H. Imbeck. 1987. “Waldlawinen Und Deren Abhängigkeit von Standorts- Und Bestandesverhältnissen: Erste Ergebnisse Des Forschungsprojektes 351.” Interner Bericht 645. Eidgenössisches Institut für Schnee- und Lawinenforschung.