Sediment generation through thermal spalling during the 2021 montiferru planargia wildfire and its contribution to postfire debris flows

Wildfires are increasing in both magnitude and frequency because of climate change and anthropogenic factors. Fire-induced changes lead to a sudden increase in the movement of sediment, making it more likely for soil particles to reach channels. As wildfire frequency increases, we are learning more about related erosional and depositional phenomena, including thermal spalling. Thermal spalling happens when rocks crack due to sudden temperature spikes. Dramatic temperature changes cause pore pressure increases and mineral dilation resulting in fracturing and spalling from rock exposures and fragmentation of existing debris. In this study, we observed a rocky Mediterranean area with a dense oak forest and shrubberies that was partially burnt during the 2021 Montiferru-Planargia wildfire. Before the fire, rockfalls had deposited debris under the forest canopy. Thermal spalling eroded exposed rocks and boulders, generating new deposits. This loose sediment increases debris flow, especially during rainstorms after fires. In the burnt scar, 33 debris flows occurred in the two years postfire showing clear impacts of thermal spalling whose signs are observed in the clasts transported by flows. Our results indicate that thermal spalling is a significant sediment source for post-fire debris flows and should be integrated into hazard modelling.