Wildfires Effect on Debris Flow Occurrence in Italian Western Alps: Preliminary Considerations to Refine Debris Flow Early Warnings System Criteria

Abstract

Rarely, a close correlation between wildfires and the occurrence of channelized debris flows has been observed in the Western Italian Alps. Only two cases in history have been reported, after brief and localized rainfall events of moderate intensity in Italy's Piemonte region (NW Italy) caused debris flows, on 18 July 2005, in Verbania province (Pallanzeno municipality), and on June 2018 in Turin province (Bussoleno municipality). These phenomena occurred after a large portion of the catchments were affected by wide wildfires in the preceding months. Debris flow deposits showed an unusually large number of fine-grained particles, forming dark-brown mud-rich deposits associated with burnt wood deposits. Rainfall analysis related to the period between the wildfires' occurrence and the debris flow events, using both raingauge and weather radar data, pointed out that the debris flows triggered in July 2005 and June 2018 were characterized by greater magnitude but associated with less precipitation intensity rates as compared with previous mud flows occurring just after wildfires. These behaviors can be explained by the presence of burned organic material and fine-grained sediment, generated from the soil's thermal reworking, which formed a thick layer, centimeters deep, covering a large percentage of catchments and slopes. Most of this layer, generated by wildfires’ action were winnowed by rainfall events that had occurred in the months before the debris flow events, of significant magnitude, exhuming a discontinuous hydrophobic soil surface that changed the slopes’ permeability characteristics. In such conditions, runoff increased, corrivation time shortened, and, consequently, discharge along the two catchments’ channels-network increased as well. Consequently, the rainfall effects associated with rainfall events in July 2005 and June 2019 were more effective in mobilizing coarse sediments in channel beds than was typical for those catchments.

Predisposing Factors for Shallow Landslides in Alpine and Hilly/Apennines Environments: A Case Study from Piemonte, Italy

“…As previously disclosed, the landslide and wildfire density heatmaps do not show a high correlation. According to [22], in the Western Alps, a close correlation has been found for debris flow events in small alpine catchments that were previously affected by wildfires. However, the correlation of the shallow landslides with wildfires is verified only for some local cases in the Biella and Verbano provinces (north Piemonte) and Lanzo valleys (north-western Piemonte).…”

“…In the investigated area, wildfire occurrence also plays a remarkable role in modifying the slopes' surficial characteristics. The effect of wildfires on slope stability is highlighted by many authors [19][20][21][22]. All cited works draw attention to the need for integrating post-fire hazard into shallow landslide prediction systems and, in broader terms, into risk mitigation studies.…”

The Territorial Debris Flow Early Warning System of Piemonte (North-western Italy)

“…It has been demonstrated that the area ratio of outcropping bedrock vs. plant cover, and consequently the capacity of a catchment to generate debris flow, is https://doi.org/10.1051/e3sconf/202341503030 , 03030 (2023) E3S Web of Conferences 415 DFHM8 strongly influenced by the occurrence of wildfires that largely involve a given catchment [4]. In that case, the debris flows' triggering thresholds should be lowered from 20% to 60% compared with the standard values in Tab.…”

Exploring the Application of a Debris Flow Likelihood Regression Model in Mediterranean Post-Fire Environments, Using Field Observations-Based Validation

“…In addition, reduced protective ground cover, lack of vegetation canopy [17], changes in soil properties [21], decreased infiltration and increased runoff [1,17,22] can intensify erosion processes. Moreover, other landscape characteristics, beyond the fire, can influence debris flow generation and mass movement, including geological attributes [23], or the presence of fine-grained material [24,25]. Despite their significance and frequency, there is no systematic diagnosis of debris flow risks after forest fires in the Mediterranean region [26].…”