Using 3D modelling to identify cliff erosion processes and their controls and to map and predict geohazards: a case study of Pakri Cliff, Estonia

Coastal zones, particularly those with low-lying areas, are increasingly affected by anthropogenic pressures and climate change. Rocky and cliffed coasts, slow-evolving geomorphic systems, experience erosional processes that pose significant geohazards. Understanding the controls on cliff retreat is critical for predicting coastal evolution, assessing geological risk, and supporting land-use and heritage management. This study integrates 2D orthophoto interpretation with high-resolution 3D unmanned aerial vehicle-based structure-from-motion (UAV-SfM) photogrammetry to link structural geological mapping with quantitative change detection, thereby improving the precision and spatial coverage of cliff-change analysis. Three 3D models of Pakri Cliff, derived from datasets collected in 2015, 2022, and 2024, were used to quantify cliff retreat and characterise key geological features, including strata, joint systems, and coas tal notches. Results indicate a mean retreat rate of 18 cm/year. While erosion rates vary with lithology, their relative distribution remains stable over time, maintaining a distinct vertical profile. Coastal notches are initiated along joints and progressively modified by marine dynamics. These findings highlight that cliff erosion at Pakri is a multi-mechanism process controlled by geological structure, weathering, mass movement, and wave action, all of which exhibit high spatiotem poral variability. Ultimately, this integrated approach demonstrates how coupling structural geology with high-resolution monitoring enhances the detection of geohazards and provides a more comprehensive understanding of long-term cliff evolution mechanisms.