Conclusions
The lower boundary of the stagnic soil layer and soil depth were regionalised and proved to be a good estimation for the depth of the failure plane. However, terrain parameters explained the spatial distribution of soil bulk density and the depth of the failure plane only to a relatively small extent which is reasonable due to frequent translocation of soil material by landslides. Nevertheless, their prediction uncertainty still allowed for a reasonable prediction of unconditionally unstable sites while considering the depth of the failure plane at the lower boundary of the stagnic soil layer and an internal friction angle of c 22°.
A first estimation of landslide susceptibility was provided and tested for plausibility by comparison with landslide scars on a recent aerial photograph. On these landslide affected sites, the reduced shear strength caused by reduced soil cohesion (3 kPa) in a small zone of water saturation within the soil is enough to cause a slide. No further destabilisation due to an increase in shear stress (Eq. (3)) by increasing the total sliding unit weight (Eq. (7)) by complete water saturation within the entire soil profile is necessary. We therefore assume that preferential flow paths cause a heterogeneous moisture distribution and therefore lead to small zones of water saturation which are enough to trigger landslides. The high stone content within many soil profiles has shown that the occurrence probability for these preferential flow paths is high. It is, however, not necessary that the whole soil compartment burdening the failure plane is saturated with water. In soils which are already landslide affected, we assume that heavy rain may widen preferential flow paths to gaps until finally a slide occurs as was observed during soil investigation. We assume that a small failure zone is enough to cause a slide affecting a much larger area.
On the other hand, the total sliding unit weight caused by the soil compartment burdening the failure plane is enough to cause the observed landslides. In conclusion, forest biomass is not of major importance for natural landslide initiation within the research area as was postulated by Zimmermann and Elsenbeer (2008). Terrain form, soil mechanics preferential water flow paths and soil total sliding unit weight are sufficient to explain recent landslide scars.