Geophysics and geomatics methods for coastal monitoring and hazard evaluation

Abstract

The Maltese archipelago's northern region is experiencing a massive lateral spreading landslide. The region is distinguished by a coastal cliff environment with a hard coralline limestone outcropping layer sitting on a thick layer of clay. This geological formation causes coastal instability that results in rockfall and lateral spreading. This research has developed a methodology for high-precision monitoring of coastal cliff erosion using the integration of geomatics and geophysical techniques. From novel data collected by unmanned aerial vehicle (UAV) digital photogrammetry, a 3D digital model of the Selmun promontory was reconstructed and used to map and measure important geological features such as fractures, joints, and large boulders. Then, geophysical techniques such as electrical resistivity tomography and ground penetrating radar have been used for the identification and mapping of vertical fractures affecting the hard coralline limestone plateau and to validate the 3D geological model. In addition to this, high-precision orthophotos from UAV have been compared with aerial and satellite images captured between 1957 and 2021 and have been georeferenced into a GIS. The movement of boulders and cracks in rocks was then vectorised to highlight and quantify movement in time. The resulting data are then used to derive a qualitative assessment of the coastal variations in the geometric properties of the exposed discontinuity surfaces, to evaluate the volumes and the stop points of the observed rockfalls. Subsequently, a quantitative analysis was carried out through the use of the numerical simulation software RocPro 3D to reproduce the paths followed by the blocks and estimate different values such as speed, energy and stop points of the fallen blocks. Consequently, the source areas of past events were identified as well as potential source areas to simulate future events of unstable blocks on the cliff. The outcomes of this research were finally implemented in a GIS to offer a new approach for the collection and processing of coastal monitoring data which, ultimately, drives the local authorities to address social, economic and environmental issues of pressing importance and facilitates effective planning and mitigation of risks.