Rockfall data: collection methods, analysis and use for hazard and risk assessments

On 25 December 2022, a rockfall of approximately 6,000 m³ detached from the east wall of the Mont de Nona rocky crest (Pré-Saint-Didier, Aosta Valley, NW Italy), impacting the mountain road connecting the La Thuile ski resort and the international route to France via the Piccolo San Bernardo Pass. The event triggered an emergency response by the Aosta Valley Geological Survey to support rapid, provisional risk-mitigation measures, including the construction of a rockfall embankment at the road and stabilization arrangements on the slope aimed at limiting the consequences of potential subsequent collapses.

This contribution presents an integrated, multi-sensor surveying workflow designed to document the post-event morphology and the final state of the emergency works in a steep, partly inaccessible alpine environment, and to provide an accurate topographic basis for subsequent hazard and risk evaluation. The survey combined: (i) Unmanned Aircraft System (UAS) photogrammetry supported by RTK GNSS ground control; (ii) a scanning total station acquiring high-resolution point clouds and imagery, particularly effective in areas with limited aerial visibility; and (iii) a high-performance GNSS receiver to precisely determine the occupied scanning-station positions within a global reference system, enabling rigorous georeferencing of the terrestrial dataset through a traditional traverse approach.

Post-processing integrated terrestrial and aerial point clouds into a single 3D dataset and applied classification tools to separate vegetation and bare ground, producing a Digital Elevation Model (DEM) of the site. The DEM was subsequently used to extract targeted 2D cross-sections along the slope–road system to support verification of the embankment geometry and to frame scenario-based assessments of residual rockfall hazard.

The case study demonstrates how complementary survey technologies can be effectively combined to deliver rapid, accurate, and operationally robust terrain models for alpine mass-movement emergencies. UAS mapping provides efficient coverage of large and impervious areas, while scanning total station data ensures high spatial resolution and completeness where aerial viewpoints are limited. GNSS-based georeferencing ensures that products are immediately interoperable with regional geodata and suitable for follow-up analyses, supporting decision-making in time-critical risk management contexts.