The use of Geomatics for the analysis of mass wasting processes in volcaniclastic coastal cliffs of the Flegrean area (Italy)

Many coastal cliffs forming part of the Italian coastlines are affected by retreat processes, mainly occurring through landslides and surface weathering. Coastal landslides represent a serious risk for population living in littoral settlements, where a massive urbanization occurred in the last decades. New tourist settlements, railway networks and roads were built in these areas, altering in several cases the natural evolution of coastal dynamics. Actually, 28.4% of Italians live here with an average density of 400 inhabitants per square kilometer. The coastal sector of the Campi Flegrei volcanic area (southern Italy) is partially formed by sea cliffs. These are made by volcaniclastic deposits and are affected by rapid recession since their origin. Several cliffs consist in remnants of ancient volcanic edifices formed in the last 15 ka, during explosive eruptions of the Campi Flegrei volcanic activity. The lacking of entire flanks of edifices located in the actual coastal area highlights the rapid action characterizing dismantling processes. In order to achieve a quantitative knowledge about mass movements that are contributing to the geomorphic evolution of the volcaniclastic coastal cliffs, a multidisciplinary approach mainly based on the use of geomatic techniques and secondarily on geotechnical laboratory tests is applied in this thesis. The study is articulated in a large-scale analysis, aimed at characterizing slope failures occurring in the whole Flegrean area and along the coastline, as well as in site-specific analyses focused on two representative coastal cliffs: the “Baia dei Porci” and “Torrefumo” cliffs. Both sites are located in the municipality of Monte di Procida, in the western sector of Campi Flegrei. The longterm geomorphic evolution of the Torrefumo cliff (1956 - 2008) was investigated by comparing digital elevation models (DEM) derived from aerial images and airborne lidar data. This activity allowed quantifying volumetric changes, retreat rates and maximum retreat of the cliff top. Shortterm geomorphic changes (2013 - 2016) that affected this cliff were detected by comparing 3D point clouds acquired with terrestrial laser scanning technique (TLS). In this way, a cliff failures inventory map was developed and the magnitude-frequency distribution of events was calculated. Further statistical analyses of the inventory data, together with field and topographic observations, showed that different geomorphic processes are contributing to the cliff retreat, and vary according to the outcropping lithotypes. The TLS-based analysis was also used to develop a cliff failure susceptibility map through the application of a bivariate statistical method. In this site, laboratory tests were also performed on representative rock samples of the outcropping lithologies, collected at the base of the cliff. These tests allowed verifying mechanical strength of rocks, resistance to weathering and mineralogical composition of the altered rock surfaces. At the Baia dei Porci cliff site, a large landslide displacing about 40,000 m3 of volcaniclastic deposits occurred in October 2013. The landslide was monitored until January 2016 to assess its geomorphic evolution. In this case, the “Structure-from-Motion” (SfM) photogrammetry technique was used by performing four surveys with a boat and a remotely piloted aircraft system (RPAS), as well as a topographic survey of the ground control points. Comparison of the 3D point clouds derived from processing of photogrammetric data, allowed accurate identification of areas where geomorphic changes occurred, and to quantify volumes of material mobilized after the 2013 failure. This case study highlighted the high potential of the used techniques for accurate monitoring of inaccessible coastal cliffs. This research provides a valuable contribution to fill the research gap concerning a quantitative understanding of erosion affecting volcaniclastic coastal cliffs, and it can be considered among the first comprehensive studies dealing with monitoring of cliff retreat processes in the Campi Flegrei area. Insights emerged from monitoring activities and laboratory analyses demonstrate that behavior of these cliffs is controlled by a complex set of factors that need further in-depth analyses. In addition, more efforts should be carried out to assess risk conditions affecting urban settlements built close to the retreating cliff tops