Field investigations of the interaction between debris flows and forest vegetation in two Alpine fans

A key objective in debris-flow hazard mitigation is the reduction of the potential depositional area in the fan. From this point of view, forested areas are able to provide a protective function hindering the flow motion and promoting the surge deposition. Despite extensive research on Alpine forests and their protective functions, relatively few studies in the literature have quantitatively focused on the relationship between debris-flow depositional features and vegetation. In light of the above, our research investigates how vegetation characteristics in the fan area interact with debris-flow deposition. Field investigations were carried out in two Alpine fans where debris-flow events occurred in the summer of 2012. By recording the characteristics of 1567 involved trees and the associated deposit thicknesses, this paper provides a data set that contributes to the improvement of the knowledge of these interaction processes. The integration of literature findings and the analysis of the collected dataset adds insights into the relationships between tree characteristics and the dynamics of debris flow during the runout path. The main results prove the capacity of the forest of uniformly promoting flow-energy dissipation, presence of high species diversity in debris-flow deposits when comparing disturbed and undisturbed forest stands, tree mortality largely affecting small diameters < 10 cm in the upper/medium part of the cone, and deposit thicknesses depending on the trunk diameter only if the debris-flow regime was collisional. Based on the field results and their analysis, some criteria for the management of protection forests in alluvial fans are suggested.