Mechanisms and frequency-size statistics of failures characterizing a coastal cliff partially protected from the wave erosive action

Landslides and surface erosion are major processes controlling the progressive recession of many rocky cliffs along the Italian coastline. Nevertheless, many coastal settlements were built along cliffed sectors prone to rapid collapses. This represents a serious risk for tourists and living people, as well as for buildings, roads and railway networks. The densely urbanized coastline of the Campi Flegrei active volcanic district is one of the rocky coastal areas of South Italy mostly exposed to the recession. 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 since their origin. This research focuses on a cliff of the Campi Flegrei coastaline (Torrefumo, Monte di Procida) which, although currently protected from the sea waves by a seawall, is still retreating. We assessed the ongoing recession using a change detection analysis, based on accurate topographic data acquired with two terrestrial laser scanning surveys executed in 2013 and 2016. The quantitative comparison of 3D point clouds datasets allowed detecting 191 cliff failures. We verified that the frequency-magnitude distribution of the detached blocks followed an inverse power law, and most of the involved volumes were between 0.01 and 1 m(3). Retreat rates of different cliff sectors varied from 0.001 to 0.025 m/year. Our analysis also allowed us to recognize slope failure mechanisms and distinguish rock falls from grain-by-grain surficial erosion.