Areal landslide hazard assessment: case study of landslide-prone area covered by pyroclastic deposits

Large mountainous areas of Campania (southern Italy) are frequently hit by rainfall-triggered shallow landslides, which often cause significant damage to buildings and infrastructures. Specifically, they involve steep slopes covered with unsaturated air-fall pyroclastic deposits, formed by alternating layers of ashes and pumices of variable thickness, lying upon heavily fractured limestone bedrock. The main triggering factor of these catastrophic events is the rainfall. Nonetheless, there are other causes linked to the hydrological conditions predisposing the slopes to failure (Roman Quintero et al., 2025), often associated with soil moisture conditions prior to the onset of rainfall (Greco et al., 2021). Predicting the occurrence of these landslides is highly challenging due to the significant spatial and temporal variability of the factors driving them. Thus, landslide hazard assessment needs attention and remains a critical task, especially in terms of reliably predicting the triggering location. In this work, a method for the preliminary assessment of landslide hazard in a sloping area covered by pyroclastic deposits is proposed, based on available historical precipitation records and considering only slope inclination and soil thickness as geomorphological controlling factors while assuming soil characteristics as homogeneous. The study area is located on the Cornito slope near the town of Cervinara, around 40 km northeast of the city of Naples, which belongs to the north-facing part of the Partenio Massif in the southern Apennines of Campania. Specifically, a small catchment of 0.4 km² was investigated, where on 16 December 1999 a rain event of approximately 300mm in 48h triggered several landslides evolving in the form of fast debris flows. The largest one travelled nearly 2 km downslope toward the town of Cervinara, causing destruction and killing five people. The natural landforms of the catchment were considered using the Digital Elevation Model (DEM) with a resolution of 10 m grid cell, downloaded from the dataset TINITALY/01. This DEM was obtained by simple linear interpolation of contour lines digitized from the 1:25000 maps of the Istituto Geografico Militare (IGM) before the landslides of 1999 (Tarquini et al., 2007). Grid cells were grouped into fifteen classes of slope inclination and corresponding soil thickness, ranging from 33.5° to 47.5°, for simulating the hydrological processes of rainwater infiltration. Specifically, the 1D Richards’ equation model was run to simulate soil saturation profile at hourly resolution for each cell, considering the hourly rainfall recorded during the event of 1999. The model has been calibrated with both laboratory measurements (Roman Quintero et al., 2024) and field data collected during previous hydrological monitoring activities (Marino et., 2020). Then, based on the results obtained with the unsaturated flow model, the landslide hazard map is generated by looking at cells with a Factor of Safety, calculated under the infinite slope hypothesis, smaller than 1. The generated areal landslide hazard map was validated by comparison with the documented landslide inventory, showing agreement with the spatial distribution of reported landslides, especially with the location of the scarp of the largest one recorded, with an estimated mobilized volume of 30000 m³.