DYNAMIC LANDSLIDE HAZARD EVALUATION IN THE CONTEXT OF WILDFIRES AND CLIMATE CHANGE IN THE MEDITERRANEAN AREA

The Mediterranean area is known to be a hotspot for Climate Change, especially regarding the precipitation and temperature extremes projected for the future in the different scenarios. Along with these changes characterized by longer dry periods, wildfires hazard dramatically rises, exposing wide areas to weathering and increasing the potential soil erosion. The evaluation of the landslide hazard cannot disregard such projections, leading to the study of new methodologies for a dynamic hazard evaluation under climate change conditions. Sardinia island is particularly prone to landslides, with almost the 27% of territory falling in landslide hazard (6474.4 km2 over 24099 km2), the 25% of which (1648,3 km2) falling in high and very high landslide hazard. The studied area, named Montiferru (West Sardinia), in July 2021 was involved in a dramatic wildfire that devastated 13,000 hectares, falling in the category of Extreme Wildfire Events (EWEs). After a significant rainfall event in November 2021, for the first time, rockfalls were recorded in a previously forested area, posing the attention for a new approach to landslide hazard evaluation. A multi-model approach combined with field analysis was used for this study, aimed at evaluating how the wildfire (EWE) changed the soil properties and, by consequence, the landslide hazard. Soil properties from laboratory tests, such as granulometry, particle weight, density, specific gravity, Atterberg limits and shear test, were compared between burned and undamaged areas. In addition, in-site soil infiltration tests were carried out. Physical data were implemented in SWAT (Soil and Water Assessment Tool) for the simulation of runoff and erosion and Rockyfor3D was used for the rockfall simulations, comparing scenarios before and after the wildfire. Soil properties in burned areas, such as liquid and plastic limit, were found to be generally worst if compared to undamaged areas, increasing the susceptibility to mud flows. Infiltration rate is lower in burned soils, due to the formation of a thin layer of ashes in the surficial soil profile, which reduces hydraulic conductivity of soils, possibly causing an increase of surface runoff. Accordingly, results of SWAT simulations showed that surface runoff increased after the EWE, together with erosion and sentiment transport, mainly occurring via channel erosion. Rockfall simulations showed that, in the burned areas, the lack of vegetation could increase the kinetic energy, runout and, by consequence, the hazard. Such effect is more evident in smooth slopes than in rough slopes interested by scree and boulders. The next challenge will be to implement this setup with future climate data, to simulate how Climate Change could further affect the landslide hazard and wildfire risk in the future, evaluating mitigation scenarios and focusing on the changes in hazard patterns. Results of the study highlight that assessment of landslide hazard should be a dynamic process, requiring a priori evaluation of different scenarios, considering climate and land cover changes (i.e., the EWE), mostly where soil properties are crucial features.