Recent findings documenting retreat rates and failure mechanisms of sea cliffs along the coastline of the Campi Flegrei caldera, Italy

The coastline of the Campi Flegrei active volcanic district is one of the rocky coastal zones more densely urbanized of Italy. Here, coastal cliffs are made by volcaniclastic deposits and include remnants of ancient volcanic edifices formed in the last 15 ka. Due to petrographic, geotechnical and geostructural properties of volcaniclastic deposits, these cliffs have been affected by rapid recession processes since their origin. A review of the first analysis and monitoring activities, aimed at achieving a quantitative knowledge about retreat processes that are contributing to the geomorphic evolution of the Campi Flegrei rocky coast, is presented. The research is based on the use of geomatic techniques and focuses on some coastal cliffs: Coroglio, Punta Epitaffio, Torrefumo and Baia dei Porci cliffs, located in different sectors of the Campi Flegrei area. Mass wasting processes are effectively contributing to the dismantling of coastal cliffs in the Campi Flegrei volcanic area. Episodic cliff failures can displace huge masses of rock that induce sudden and localized cliff recessions, even if Terrestrial Laser Scanner monitoring activities show that small-scale failures, between 10^-2 and 1 cubic meter, are more frequent than larger collapses. Findings gained by the multi-temporal monitoring of landslides demonstrated that after the main failure, landslide scars can be affected by further collapses, as well as deposits eventually formed at the cliff toe can be rapidly eroded by the sea wave action and by rainfall-induced surface erosion. Measured short-term retreat rates range from 0.001 to 0.025 m/yr during 2013-2016, while mid-term rates vary between 0.17 to 1.2 m/yr during 1956-2008.