Field monitoring of the recent reactivation of the large dormant Ca’ di Sotto earthflow

The reactivation of earthflows in fine-grained geological media represents a complex phenomenon characterised by transitions from prolonged dormant phases to sudden accelerations.  While dormant stages typically exhibit slow movements (less than 1 m/year), critical rainfall conditions may trigger rapid surges in which the landslide mass can attain velocities up to several meters per hour within a limited time frame. Despite the extensive literature on the subject, the mechanisms and dynamics underlying this peculiar behaviour remain incompletely understood, largely due to challenges in acquiring direct field data that accurately capture these episodic events.This study presents field data documenting the October 2024 reactivation of the large, dormant Ca’ di Sotto earthflow, located in the Northern Apennines (Italy) within the municipality of San Benedetto Val di Sambro. During the initial stages of reactivation, adverse weather conditions, including persistent fog and rainfall, severely hindered direct visual observation and aerial monitoring of the landslide's evolution. To overcome these challenges, a GNSS-based monitoring system was promptly deployed, comprising 31 evenly distributed periodic measuring points (surveyed daily) as well as three dual-frequency permanent GNSS stations.GNSS data revealed an exceptionally rapid reactivation of the Ca’ di Sotto earthflow. The initial failure quickly propagated from the source area through the entire 2-km-long landslide body within a few days irreversibly compromising the functionality of a water bypass system built at the toe of the earthflow along the Sambro Stream after a previous reactivation in 1994. The failure of this bypass caused a critical water level rise in an upstream impoundment that had formed during the 1994 event.In the following weeks, as precipitations significantly subsided, the landslide mass progressively decelerated, transitioning from peak velocities of 100 – 150 m/day recorded during the initial phase to rates of a few cm/day. At this stage, the monitoring system was enhanced with periodic drone surveys and a robotic total station, providing hourly measurements with millimetric precision across 24 regularly distributed monitoring prisms. Particularly, two transverse prism arrays were strategically installed at different elevations to serve as early warning systems for potential future reactivations.Additionally, emergency hydraulic risk assessments were conducted, examining plausible scenarios of river blockage, impoundment water level fluctuations and management with contingency water pumping systems. These scenarios were evaluated considering ad hoc impoundment characteristic curves and hydrographs derived for design rainfall events, following the standardised NRCS (Natural Resources Conservation Service) unit hydrograph methodology.