Post-wildfire monitoring for hazard mitigation in Alpine area

In a climate change scenario, natural disasters and their consequences are expected to increase. In particular, it is proven that the raise of global temperature will drive a higher occurrence of wildfires, leading to a wide range of problems in the mountain areas, such as slope instabilities. As confirmed by many authors, in addition to the disruption of vegetation, wildfires have indeed severe effects over the natural slopes, linked to the hydrological changes provoked by burning, which may cause further economic losses and casualties.

The risen probability of flash flooding and debris flows after wildfires is recognized to depend on an alteration of the soil hydrological properties, and in particular of the soil infiltration capacity. Many studies in literature focused on the trends of soil infiltration recovery after fire, but none of them regards Alpine case studies and only a few are European, even if wildfire phenomenon is relatively common nowadays in the Alps. Furthermore, rainfall thresholds for possible landslide triggering have not been defined for wildfire-affected areas yet.

This work investigates the impact of a wildfire occurred in 2019 in the Southern Alps, starting from the data collected during three years of monitoring activity at different spatial scales and after laboratory rainfall simulations. The investigation of the burnt area was conducted both remotely, by the analysis of Copernicus Sentinel-2 imagery, and throughout field surveys, by performing falling-head infiltration tests. The monitoring activity was distributed over three different sub-areas, taking into account the different fire severity (burnt or unburned sub-area) and the original vegetation type (pine woods or grassland). Moreover, soil samples were collected inside those sub-areas for further laboratory permeability tests and rainfall simulations.

Results were used to retrieve recovery trends for the calibration of a simple 1D hydrogeological model.

In particular, the remote sensing analysis helped to evaluate a recovery time of seven years of the site to pre-fire conditions. On the other hand, field monitoring suggested the recovery to depend mostly on the restoration of the canopy protection, as preventing factor for direct responses to rainfall and soil erosion.