Geothematic maps as a supporting tool for the choice of engineering works in complex volcanic cliffs: the Ponza case study, central Italy

The island of Ponza belongs to the Pontine Islands, located in the Gulf of Gaeta (Latium region, Central Italy). It is almost completely made up of volcanic products, deriving from two different magmatic associations starting from the Lower Pleistocene, in turn characterized by rhyolitic-rhyodacitic and trachytic products, respectively. The present-day geomorphological setting at Ponza is the result of interactions among the lithological features of the volcaniclastic cemented and loose rocks, the various volcano-tectonic events and the weathering processes, the latter caused by the exogenous agents (sea wave action, wind abrasion and erosion due to runoff and infiltration water). All the above processes, combined with the lithological characters of the rocks, favour the development of instability phenomena, affecting both the rocks and the loose deposits. At a greater detail, the rock masses, showing well-developed joint systems and landforms produced by differential erosion, are involved in rockfalls, which locally may even reach large size. The debris and infilling materials covering the volcanic bedrock, on the other hand, are affected by shallow slides and flows, mostly triggered by intense rainfalls. Long stretches of the coastline, where in the past an intense quarrying activity was carried out, show nowadays rock cliffs produced by the anthropogenic activities, which present instability features similar to those affecting the natural cliffs. The diffuse erosional processes, and the frequent slope movements, represent the main factors of risk for the local population, as well as for the tourists, which occupy during the summer season the long stretches of beaches bounded by rock cliffs. In such a setting, i.e. an island where tourism plays a crucial role in the social-economical activities, production of landslide susceptibility maps is a fundamental step in the process of risk mitigation, through identification of the priority areas where to concentrate the first and necessary engineering works. Here a methodology for the rapid production of landslide susceptibility maps is described, based upon a geological and geomorphological criterion, and tailor-made for complex geological settings characterized by presence of loose deposits and hard rocks, and by several different landforms, including some of anthropogenic origins as man-made cavities. Beside obtaining a zonation of the areas most susceptible to slope movements, such an approach allows: - to define intervention priorities, based upon the elements at risk therein present; and, - to choose, and to properly design (also in terms of an economic estimation of the costs), the most feasible surveys and engineering stabilization works, of both the active and passive types.