Functional soil-landscape modelling to estimate slope stability in a steep Andean mountain forest region

Landslides are a common phenomenon within the Ecuadorian Andes and have an impact on soil-landscape formation. Landslide susceptibility was determined in a steep mountain forest region in Southern Ecuador. Soil mechanical and hydrological properties in addition to terrain steepness were hypothesised to be the major factors in causing soil slides. Hence, the factor of safety (FS) was calculated as the soil shear ratio that is necessary to maintain the critical state equilibrium on a potential sliding surface. Regression tree (RT) and Random Forest (RF) models were compared in their predictive force to regionalise the depth of the failure plane and soil bulk density based on terrain parameters. The depth of the failure plane was assumed at the lower boundary of the stagnic soil layer or soil depth respectively, depending on soils being stagnic or non-stagnic. FS was determined in dependence of soil wetness referring to 0.001, 0.01, 0.1 and 3 mm h−1 net rainfall rates. Sites with FS ≥ 1 at 3 mm h−1 (complete saturation) were classified as unconditionally stable; sites with FS < 1 at 0.001 mm h−1 as unconditionally unstable.Bulk density and the depth of the failure plane were regionalised with RF which performed better than RT. 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.

Research highlights
► Landslide risk was predicted from soil parameters in dependence on soil wetness.
► Landslide occurrence leads to high uncertainty in soil regionalisation.
► Landslide risk depends on the weight burdening the failure plane and slope steepness.
► Unconditionally unstable sites were successfully tested for plausibility.
► Small zones of water saturation trigger landslides.