Fire-to-debris flow sequences in small catchments: sediment dynamics and impacts on urban settlements

Wildfires are among the natural or anthropogenic disturbances affecting mountain catchments. Wildfire’s role as a geomorphic agent has been recognized in many landscapes worldwide, especially where sediment transport increased in response to post-fire intense rainfall. Vegetation removal, changes to soil hydraulic properties, and degradation of outcropping rocks have been identified as direct effects of fires on hillslopes. These effects can lead to rain-induced enhanced runoff and soil erosion processes, with consequent formation of overland flows entraining sediments and ash progressively downstream. According to a cascading mechanism, overland flows can then generate high-magnitude debris flows. This kind of hydro-geomorphic response has been commonly observed in burned catchments, sometimes together with shallow landslides. Notably, when catchments include urban settlements, post-fire debris flows pose significant hazards to life and property.

Every year, the Mediterranean basin is affected by thousands of wildfires that spread through different topographic settings, from lowland to steep mountains. Accordingly, post-fire debris flows are more likely to occur in catchments with high relief and hillslope-to-channel connectivity, where severe wildfires burn dry vegetation over steep hillslopes covered by erodible soils. These conditions are quite common in southern Italy, where intense rainstorms associated with convective cells occur in the late summer-autumn period, such as after wildfires of the summer season. In the Campania region, more than a hundred post-fire debris flows have been documented in the last two decades. This contribution focuses on one of the last events that occurred on August 27, 2024. The scientific relevance of this event is due to new insights on both mechanisms controlling the hydro-geomorphic response, and a better comprehension of the impacts on urban settlements. Specifically, a preliminary analysis of the following points is presented: 1) predisposing and triggering conditions of debris flows; 2) sediment source areas; 3) the role played by human modifications of the natural drainage network; 4) impacts on people and urban structures. In addition, a series of weak points that are hampering the implementation of effective strategies for risk reduction are discussed. This and other minor events recorded during the Autumn of 2024 in the region suggest that accurate prediction tools need to be developed, together with in-depth analyses of natural factors that control the post-fire sediment cascade. This is crucial to protect people living in post-fire settings that, according to the climate change scenarios, may be exposed to more severe geo-hydrological risk conditions in the next years.