Forest

It is possible in RAMMS::EXT to introduce forest shape files describing forests of different types and age (Fig. 1). The height of the tree with the diameter at breast height is then used to compare the calculated stress with the  minimum breaking stress to determine which area of the shape file is destroyed by the avalanche. Comparing predicted and actual destroyed forest is an ideal way to calibrate the return period of a specific avalanche, since the age of the trees is known. Trees can be destroyed by the avalanche core, or the powder cloud. Core pressures are higher, but because of they impact trees at lower levels do not lead to large overturning moments.  Conversely, the lower magnitude powder cloud load extends over the entire height of the tree, leading to large overturning moments. 

Figure 1: A mixed flowing/powder avalanche destroyed a patch of trees at the Masura avalanche track, Klosters, 2018. The exact extent of the destroyed forest (a mixed birch and alder stand) was used to calibrate the extended model.
Figure 2: Different forest types in the RAMMS::EXT model. The K-value defines the mass stopping effectivity of each forest type. The braking effect of the forest can be removed if the trees are destroyed.

The forest module applies the detrainment method developed by Feistl et al. (2014, 2015). Avalanche mass is stopped behind dense clusters of trees, leading to a loss of avalanche momentum. If the trees break from the avalanche impact pressure, the braking effect of the trees is removed from the calculation.

Figure 3: The Masura avalanche track in 2018 at the time of the avalanche. RAMMS::EXT calculation (maximum core velocities). Both the runout and lateral extent of the core agree with observations.
Figure 4: Calculated powder pressure distribution (>5kPa) and the calculated location of forest destruction. The location of destruction corresponds to the observations. See photo.