Multi-temporal evolution analysis of Maronti Cliff (Ischia Island, Italy) derived from multi-perspective photographic datasets

Abstract

Coastal cliff erosion is a significant hazard for the safety of people, buildings, utilities and infrastructure, given the sudden and episodic nature of the retreat process in time and space. Thus, understanding past retreat trends and a regular diagnosis of the cliff face condition is fundamental for risk management. In this study we show preliminary results of a project aimed at the definition of the coastal erosion (in terms of temporal and spatial scales) of a small portion of cliff located along the Maronti beach in Ischia, Italy. Drone-based and terrestrial acquisition approaches offered the opportunity to exploit photogrammetric techniques to estimate the spatial and temporal rate of change of the investigated portion of cliff. In this study, cliff face topographical evolution is evaluated out by comparing the results obtained from three photographic datasets (variable approach either boat and UAV-based) obtained in 2011, 2021 and 2023. Spatial distribution of two main instability events dated back to 2020 and 2022 were well represented in the maps of change generated with the M3C2 algorithm. The comparison of point clouds for the period 2011-2021 and 2021-2023 produced max retreat rate of the cliff face of approximately 12 and 25 m, respectively. Results highlighted the need for robust co-registration methods to accurately estimate erosion rates. Critical issues are discussed to highlight potentials and limitations encountered with the abovementioned multi-temporal cliff evolution assessment techniques.