Multidisciplinary approach on the preparatory effect of wildfires in shallow landslides

Impacts of wildfires in forest areas can lead to new avalanche-prone slopes with a higher risk of landslides (e.g., shallow landslides, debris flows, rock falls), soil erosion, and water quality problems. The scarce maintenance of vegetated lands can increase the risk that fires may affect areas closer to human settlements, raising both the risks associated with the fire and those linked to landslides. Wildfires constitute one of the most relevant preparatory factors for shallow landslides, that are investigated, among other preparatory processes, in PE3 RETURN. Wildfires are responsible for changes in watersheds' hydrologic and geomorphic response, and they also lead to the denudation of hillslopes and the consequent reduction of the root strength, which mainly contributes to the soil cover stability (Abdollahi et al., 2023). Depending on vegetation resistance and resilience, the effects of the wildfires on the territory can last for years, during which heavy rainfall or earthquakes can more likely trigger shallow landslides. The unpredictability of wildfires phenomena is one of the main challenges in studying their characteristics, as on field measurements of its effects are difficult to collect. Future scenarios linked to the effect of climate change predict that forest fire frequency and severity will likely increase, as will the extreme rainfall events. As both probability of occurrence and magnitude of wildfires can change over time in relation with climate changes, more efforts should be made to deepen knowledge of interacting disturbances and to understand how to mitigate the related risks. In this work, a multidisciplinary approach to quantify time-dependent scenarios of shallow landslides prepared by wildfires is presented, evaluating the environmental conditions before, during, and after the wildfires occurrences. This allows understanding how wildfires can affect slope stability and how long this effect persists over time as well as how the ground reacts. All these aspects are very important to be defined since they constrain the quantification of how the soil can change in its chemical and physical characteristics as well as in evaluating the heat transfer at different depths below the ground (Chicco et al., 2023). The used approach involves specific in-situ and laboratory geotechnical investigations on surface soil layers to determine the physical, hydraulic, and mechanical interactions between soil, vegetation, and fire, together with back-analysis approaches as well as numerical models aimed at reproducing how wildfires can affect soil depth and slope stability. To calibrate these models, controlled burning, simulating a small-scale fire, was used, allowing the evaluation of the relationship between the combustion of biomass on the ground and the heat transfer in the subsoil, its duration, and the thermophysical characteristics of geological materials within which the heat pulse propagates.