Rockfall hazard assessment using UAV photogrammetry and 3D modelling at Selmun Cliff in Malta

Rockfalls represent a major hazard along the coastal cliffs of the Maltese Islands, where litho-structural settings dominated by Upper Coralline Limestone (UCL) overlying Blue Clay (BC) promote progressive slope instability. This study presents an integrated workflow for rockfall hazard assessment at the Selmun promontory (northeastern Malta), combining high-resolution UAV-based photogrammetry, diachronic analysis of historical aerial imagery, and 3D probabilistic trajectory modelling using RocPro3D. UAV-SfM surveys provided centimetric-resolution digital elevation models and 3D surface reconstructions, enabling the detailed mapping of detachment niches, fracture-bounded blocks, and debris accumulations. Historical images spanning more than six decades were used to reconstruct past rockfall events, calibrate restitution parameters, and validate numerical simulations. The block inventory identified a predominance of medium-sized boulders (< 130 m3), with several large blocks exceeding 500 m3 and one semi-detached boulder (ID 0034) estimated at ~ 1224 m3. Morphological indicators suggest a temporal gradient in slope activity, with fresh, angular blocks reflecting recent detachment and rounded, karstified boulders representing older events. Simulation of historical collapses reproduced observed runout patterns with high fidelity, confirming rebound heights up to 20 m, velocities approaching 30 m/s, and impact energies near 700,000 kJ. Forward modelling of Block0034 demonstrated the disproportionate hazard of large-volume failures, with bifurcated runout paths, maximum velocities of 36 m/s, and impact energies exceeding 700,000 kJ, in some cases reaching the coastline and the sea. The results underline the multi-scalar nature of rockfall processes at Selmun, where frequent low-magnitude falls coexist with rare, high-energy events capable of long-range propagation. Despite geometric simplifications—including the spherical approximation of blocks—and minor terrain filtering applied during surface reconstruction, the adopted methodology proved robust and reliable. Its application provides essential tools for hazard zoning, monitoring strategies, and risk mitigation in Malta and demonstrates potential applicability to other Mediterranean coastal environments with similar geological and morphological conditions.