Unequal avalanche protection afforded by forests depends on type of avalanche

Forest protects against avalanches primarily by impeding their release in the starting zone. In addition, the extent to which a forest affords protection against avalanches depends on the ability of the trees to withstand the impact of descending snow masses. If the trees break or are uprooted, the avalanche can continue unhindered towards the valley floor.

As a consequence of researches conducted for a doctoral thesis, scientists at the SLF can now more accurately quantify the protective effects of forests which are hit by varying types of avalanches. For each avalanche type they developed mathematical equations defining the critical density, velocity and height of an avalanche which causes trees to break. Using as references two large avalanches in Monbiel near Klosters (Grisons) and Täsch (Valais) and six avalanches in Germany, they examined whether their theoretical calculations actually reflected the avalanche events. The researchers’ conclusions are set forth below.

Lone spruces break most readily

Dry powder avalanches exert pressure not only on the tree trunk, but also on the crown. As a consequence, trees can break even if the density of the air-snow mixture is relatively low and the avalanche is sliding at a rate of only 20 metres per second. The larger the surface area of the crown, the greater the danger for the tree. Lone and/or evergreen trees thus break more readily than deciduous broadleaf trees or deciduous conifers, such as larch or birch, or trees situated in the middle of the stand.

Given that the velocity of wet-snow avalanches is often less than 5 m/s, the most effective way of recording their effect on trees was by way of static pressure calculations. In contrast to the effects recorded from dry powder avalanches, it was demonstrated that other factors besides snow mass and tree shape influence the point at which the trunks break. In the case of wet-snow avalanches, consideration also had to be given to the properties of the terrain, roughness of the substratum and spacing of the trees.