Improved rockfall susceptibility maps - examples from Switzerland and Norway

Rockfall susceptibility maps depict, at a regional scale, areas potentially affected by fragmental rockfalls; they are therefore an essential tool for land-use planning by highlighting exposed infrastructure. Traditionally, susceptibility maps are binary (exposed or not), whereas novel maps combine high-resolution datasets and advanced computer models to define different susceptibility levels for both failure and runout modelling.

Here, we use digital elevation models (DEMs) created from airborne LiDAR data, vectorized geological and tectonic maps, land-use and forestry data along with the numerical model Flow-R for novel rockfall susceptibility maps in Switzerland and Norway. For homogeneous geological regions, we determine critical slope angles (derived from high-resolution DEMs) for the release of rockfalls from cliffs, but also remobilization of blocks. In the Vaud canton in Switzerland, for example, critical angles for detachment from cliffs range from 39° for molasse (weakly consolidated sandstone) to 48° for massive limestone, while block remobilization might occur at slope angles as low as 33°-35°. These critical slope angles allow defining classes of susceptibility for rockfall initiation (i.e. low, medium, high).

The Flow-R software is a spatially distributed empirical model for the propagation modelling of mass movements at regional scale and used here for the runout modelling of rockfalls. Flow-R combines a spreading algorithm controlling the path and the spreading of mass movements with a simplified friction-limited model determining the runout distance. The reliable and well-known reach angle approach is chosen as friction model, as it can easily be calibrated at regional scale using inventories of past rockfalls. Using a range of model parameters along with the sources' susceptibility allows distinguishing between extreme events (with low reach angles) and rare or current events (with higher reach angles). The effect of protective forest can also be included in the runout models.

Finally, the improved resolution and level of detail of these new rockfall susceptibility maps greatly improves their practical use for land-use planning and in natural hazard and risk management by focusing costly field investigation and mitigation measures to critical areas.