Towards an improved rapid assessment tool for rockfall protection forests using field-mapped deposited rocks

In steep, mountainous terrain, protection forests play a key role in rockfall risk prevention, because trees reduce the energy of falling blocks or even stop them. The simple but robust tool RockForNET (RFN) models the protective effect of forests in order to assess the residual rockfall hazard. It uses the energy line principle with a fixed energy line angle (ELA) to derive the rockfall energy that has to be dissipated by the forest. The objective of this study was firstly to empirically reconstruct the ELA and initial fall heights of field-mapped rockfall deposits on 16 forested slopes in Switzerland. The second objective was to assess to what extent RFN can be improved by estimating trajectory-specific ELAs as well as better representative initial fall height values for rock faces. The analysis showed that the prediction of the protective capacity of a forest could substantially be improved by using transect-specific ELAs and more specific initial fall height values, especially for block volumes between 0.2 and 1 m3. Furthermore, we found a strong relationship between the retro-calculated ELAs and the normalized area below the rockfall trajectories, indicating that the normalized area is a promising method for deriving trajectory specific ELAs.