Post-wildfire debris flow in the Northwestern Italian Alps: description and numerical analysis of the June 2018 Bussoleno event

On June 7, 2018, a mud-debris flow occurred in the Comba delle Foglie basin, Susa Valley, Northwestern Italy. The event followed an unusually wet winter and spring with high cumulative rainfall, though no intense rainfall was recorded immediately prior to the flow. Approximately 20,000 m3 of sediments and woody debris were transported to an urbanized alluvial fan, causing severe damage, including the destruction of two residential buildings and widespread flooding. Key predisposing factors include extensive wildfires in Autumn 2017, which are atypical for this region characterized by an alpine climate. Following the event, significant mitigation measures were implemented. This study presents an analysis of the event, integrating data on fire damage, rainfall distribution, flow runout, and deposit thickness from post-event surveys. A numerical back-analysis of the flow dynamics is conducted using two approaches to define the triggering area. The simplified triggering approach applies a method frequently used in the literature, simulating the runout of a concentrated mass to evaluate the flow’s rheology. Two rheological laws are tested by comparing simulated flow depths and path with field data. The susceptibility-based triggering approach incorporates a preliminary susceptibility analysis, integrating fire severity mapping and literature findings to account for wildfire-induced impacts on soil stability. A comparison of the simulations shows that the susceptibility-based analysis better reproduces the flow path, enhancing back-analysis accuracy. However, the simplified approach remains a reliable tool for the case study. Finally, the effectiveness of the implemented mitigation measures is assessed through numerical simulations, providing insights into the settlement’s potential response to future debris-flow events.