Rainfall conditions leading to runoff-initiated post-fire debris flows in Campania, Southern Italy

Wildfires increase surface runoff and erosional processes from short and intense rain bursts. In steep mountain watersheds, the progressive entrainment of material induced by surface runoff can lead to the formation of post-fire debris flows (PFDFs) downstream. Campania region, in Southern Italy, is heavily affected by wildfires, mostly in the period between June and September, and is also exposed to geo-hydrological hazards due to geological, topographic and climatic conditions. In this work, a catalog with spatial and temporal information on the occurrence of 113 PFDFs, that hit this region in the period 2001-2021, is presented and analyzed. The average number of events per year is 5, with a peak in 2017 (38 PFDFs). The rainfall conditions associated with the initiation of the PFDFs are reconstructed and analyzed, investigating their structure and magnitude. In particular, the return times of the peak cumulative rainfall of the triggering bursts in 10, 20, and 30 min have very short values (median values from 1.3 to 1.4 years, 75° percentiles from 2.7 to 3.2 years), indicating that these rainfall events are neither rare nor extreme, as also observed by other authors worldwide. Differences are observed in return times of the rainfall amounts and spatial distribution of the PFDFs that occurred in two sub-periods, such as "July-September" (higher rainfall magnitudes and longer return times) and "October-December" (PFDFs concentrated in the inner part of the region rather than the coastal sector). Moreover, the rainfall thresholds for the initiation of PFDFs in the study area are calculated. Interestingly, the minimum triggering conditions are similar to those observed in different geographical contexts. Finally, the triggering conditions identified in the present work are compared with the currently operating warning system in the region, finding that PFDFs cannot be correctly predicted with this system. Research outcomes reported in this work can provide reliable and effective support to reduce the risk posed by such events in the Campania region, which might increase in the near future.