Analyzing debris flow and rockfall interactions: A case study in surrounding of Alpe di Succiso, Northern Apennines (Italy)

Debris flows represent one of the most prevalent and impactful natural hazards in mountainous areas, posing significant risks to both human life and infrastructure. Alpe di Succiso (2017 m a.s.l.) located in Northern Apennines, Italy, is an area where numerous occurrences of debris flows have been identified in this area encompassed by a National Park, thus densely traversed by touristic routes and infrastructures. Understanding the spatial and temporal patterns of these debris flows is critical for assessing the hazard and managing the associated safety risks for mountaineers, hikers, tourists, and, more generally, for the communities and infrastructure in the area.

This research integrates field observations of debris flows, dendrochronological analysis, geomorphological mapping (Rashid et al., 2024), and satellite imagery to reconstruct the history of debris flow events and is partially comprised in the DECC project (2023). A key focus is the 1987 debris flow, triggered by an intense rainfall event on August 25, which recorded 179 mm of rainfall in a single day, including 133 mm within a 6-hour period.

This study investigates the dynamics of debris flows, through channel-specific analysis, GIS-based zonation, and statistical evaluation. Slope angle and elevation data were analyzed to delineate source, transport, and deposition zones across four channels of debris flows. Channel 1 was identified as a debris flood channel, while Channels 2, 3, and 4 exhibited typical debris flow characteristics.

To account for the wide variation in grain sizes in the study area (0.0005 mm to 5 meters), four techniques were employed. Sieve analysis was used for grains between 2 mm and 32 mm, while laser granulometry measured finer particles below 2 mm. Direct field measurement was applied to intermediate grains (32 mm to 1000 mm), and particle counting was used for large particles above 1 meter. This multi-method approach ensured accurate representation of sedimentary material across the broad grain size spectrum.

Geomorphological analysis indicates that rockfalls and rock weathering significantly contribute to the material on the slopes. During debris flow events, these deposits are triggered at the first stage like debris/rock slide and then as flow creating channels and deposits. A geological survey of rock outcrops feeding these debris flow channels revealed that the Rock Quality Designation (RQD) ranges from 44 to 65 (poor to fair quality), while the Rock Mass Rating (RMR) falls between 45 and 56 (fair quality).

Using the RAMMS Debris Flow software (RAMMS, 2017), a scenario-based modeling approach was employed to better understand the interactions between debris flow and material constituting the slope deposits. Simulation results are currently being compared with the geometry of levees and lobes to refine the model and ensure accuracy. This comprehensive approach aims to improve the understanding of debris flow dynamics and the influence of rockfalls, thereby aiding hazard assessment and management.