Numerical modelling of static and dynamic factors controlling the progressive retreat of rocky coastal cliffs

Abstract This study investigates instability mechanisms in soft rocky coastal cliffs through an integrated numerical framework that couples computational fluid dynamics (CFD) with the finite-discrete element method (FDEM), applied to the tuffaceous cliffs of Ventotene Island (Italy). The modelling strategy systematically explores the role of structural predisposition, marine erosion, wave-induced loading, thermal forcing, and time-dependent strength degradation in controlling cliff stability and failure evolution. The results show that neither gravity loading nor basal undercutting alone can reproduce realistic failure mechanisms, even in the presence of a DFN. Progressive rock failure (PRF) emerges only when basal erosion is explicitly coupled with inward-migrating strength degradation, implemented using a simplified fatigue-based S–N approach. Under the considered lithology and the present Mediterranean climatic conditions, wave impacts and thermal forcing induce predominantly elastic or sub-critical mechanical responses and do not directly trigger failure. However, wave-induced dynamic perturbations concentrate displacements in fractured cliff sectors, suggesting a preparatory role through fatigue and stress redistribution, with potential triggering effects once critical degradation thresholds are approached. Thermal forcing becomes mechanically effective only under amplified temperature excursions or in lithologies with higher thermal sensitivity. The coupled CFD–FDEM approach is therefore interpreted as a process-based, exploratory framework that captures the dynamic response of coastal cliffs to realistic environmental boundary conditions, rather than as a direct predictor of wave-driven collapse. The methodology provides a physically grounded and transferable framework for investigating multi-forcing interactions, damage accumulation, and PRF in coastal cliffs, offering new insights into the mechanisms governing coastal instability and long-term cliff evolution.