Using trajectory modeling for delineation and valuation of protection forests against rockfalls and landslides

Protection forests can significantly contribute to mitigating gravitational natural hazards such as rockfalls and landslides. A quantitative risk assessment, including areal delineation and economic valuation, is essential for an efficient hazard and forest management. The trajectories of gravity-driven rapid mass movements are a central element of such a risk assessment in order to link hazard processes and risk elements with forest stands. A quantitative framework is introduced for this purpose, which is based on trajectory modeling using the energy line principle. The trajectories enable a novel method for a comprehensive mapping, including protection forest maps and risk reduction maps. This method includes a novel tool that integrates the protective effect of forest stands against rockfall in the energy line principle, which is introduced as proof of concept and predicts plausible values for kinetic energy reduction up to 905 kJ. Furthermore, the method includes a novel concept for spatially resolving the risk reduction due to the protection forest and assigning it to the forest stand, whereby plausible values up to 175’269 €$\cdot$y⁻¹ are estimated for the risk reduction. Two case studies at slope-scale in northern Italy demonstrate the application of this framework for rockfalls and landslides, and compare the outcomes with other studies from the Alpine region. This conceptual application of combining trajectory modeling with several other models contributes to a more informed and spatially explicit risk assessment for protection forest against gravitational natural hazards, which can serve as a basis for management decisions. This framework introduces the method on a conceptual level that is largely deterministic yet and shows ways for thorough validation and further development towards an applied level that could be fully probabilistic.