Wind-induced tree collapse along railways: An approach to mesoscale mapping of risk index with application

Wind-induced tree collapse on critical infrastructures, such as railway lines, results from the interaction between wind in the Atmospheric Boundary Layer, tree aerodynamics and mechanics, and specific features of the infrastructure. Wind-induced tree collapse may affect railway capacity and safety. The resulting losses may be related to delays, cancellations, or even damages caused to the infrastructure or the rolling stock. In order to face the potential adverse events above, risk analysis provides a sound methodological framework to infer critical railway segments referring to the main risk determinants: wind hazard, tree and railway exposure, and tree vulnerability to wind. Each risk determinant has and can be modelled at multiple scales in space and or time. The risk assessment should effectively model hazard, exposure, and vulnerability at a selected scale that is consistent across all three determinants and relevant to the context of interest. In this study, a mesoscale approach is proposed to assess and map the relative risk level of different railway segments along a line or within a network. Wind hazard index is grounded on the extreme wind speed mapping obtained by the Authors by means of a reanalysis-based approach. Tree exposure index is defined on the basis of land cover characteristics. Tree vulnerability index is defined with reference to the critical wind speed for tree collapse. Each index and the resulting risk is mapped by Geographic Information System tools. The Calabria region in Southern Italy is selected as a challenging benchmark due to its variable orography and due to the wide presence of railway lines surrounded by tree canopies both in coastal and mountainous zones. The proposed mesoscale approach allows to identify in quantitative relative terms the most endangered railway segments over the region of interest.