A Decade of 4D Object-Based Monitoring of Cliff Hazard Dynamics

The Red Beach in Santorini, Greece, is a dynamic landscape formed by the rapid erosion of unstable volcaniclastic cliffs. This study presents a comprehensive, decadal analysis of cliff instability activity using a Multi-Temporal Object-Based Image Analysis (MT-OBIA) framework. Driven by a systematic collection of Unmanned Aerial System (UAS) high-resolution imagery, we developed a time series of high-resolution Digital Surface Models (DSMs) and orthomosaics. Our OBIA workflow was specifically designed to segment and classify features unique to this environment, including scarps/sources, deposits, and cracks. The results quantify a mean annual cliff retreat rate of 0.45 m/year, with significant spatial and temporal variability, including a major collapse event in the winter of 2019 that resulted in over 1 meter of instantaneous retreat. The OBIA-derived inventory, comprising over 1,200 individual objects, reveals a strong seasonal pattern linked to intense storm surges and coastal erosion. This research establishes a robust and transferable methodology for high-frequency geohazard monitoring in coastal environments, providing critical data for the safety management of one of Greece's most visited tourist destinations.