Multi-technique approach for the reconstruction of rocky coast evolution

Coastal areas are high dynamic environments which, especially considering the present climate conditions, are undergoing huge morphological changes mostly in terms of erosion. The retreat of coastal slopes, either progressive or sudden, is the result of the interaction between marine and terrestrial processes acting on specific litho-structural contexts.

The analyses of retreat style and relative rate can be lead combining field measurements and high-resolution remote sensing techniques. These approaches allow the quantification of erosion trend and the identification of the key factors driving the observed changes over time. The integration of multi-techniques measurement strengthens the evaluation of the interplay between terrestrial and marine processes and litho-structural factors, such as lithological variability, enabling a detailed understanding of how different coastal typologies respond to these processes.

Within the framework of the extended partnership RETURN (multi-Risk sciEnce for resilienT commUnities undeR a changiNg climate - Italy’s National Recovery and Resilience Plan), our study focuses on the evolution of a segment of the southern coast of the Lazio region (Italy). The study area is characterized by a soft-rocky cliff and shore platform system, partially emerged and partially submerged, where a high cliff retreat rate has been observed. To this end, multitemporal surveys were conducted using various remote sensing techniques, including optical photogrammetry via Unmanned Aerial Vehicle (UAVs), LiDAR surveys using UAV-mounted laser scanners, imagery captured with a MicaSense RedEdge-P multispectral camera equipped on a UAV, and portable laser scanner with Simultaneous Localization and Mapping (SLAM) technology (FJD TRION P1 model). Optical photogrammetry and LiDAR, both conducted via drones, enabled us to produce high-resolution 3D point clouds, orthophotos (<2 cm/pixel), and Digital Terrain Models (DTM, <5 cm/pixel). Through repeated surveys over two years, a multitemporal change detection analysis was conducted, revealing significant changes in response to storm events and providing rates of cliff retreat up to 1 m in localized sectors. SLAM technology allows to examine outcrop portion, less visible from UAV surveys, as the bottom of the rocky cliff. Here, the impact of storm waves was monitored, and the specific SLAM results were useful for unravelling the role of extreme event on the cliff retreating and associated rock-fall triggering along the cliff wall. The use of the multispectral sensor, particularly through the Green and Blue bands, provides useful data for better understanding the morphodynamics along the submerged portion of the shore platform. In particular, the submerged platform exhibits the same rock fracturing patterns observed in the emerged section and is composed of blocks that detach and partially slide into the sea, contributing to the retreating trend of the cliff and shore platform system.

The integration of multi-techniques not only enabled the quantification of the retreat rates of the cliff under analysis, but also allowed their correlation with predisposing and triggering factors, providing the foundation for the comprehension of potential future evolution in a changing climate context.