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The coastal domain of central western Sicily is characterized by the presence of rocky coasts, which mainly consist of pocket beaches situated between bedrock headlands that constitute ecological niches of great touristic and economic value. In this peculiar morphodynamic system, the sedimentary contributions are mainly derived from the rockfall that affects the back of nearby cliffs or the sediment supply of small streams that flow into it. In this study, we investigated the geomorphological processes and related landforms that contribute to the evolution of a pocket beach area located in a coastal sector of NW Sicily Island. The cliffs in this are affected by several rockfalls, and deposits from these rockfalls also add to the rate of sedimentary contribution. The analysis was conducted through the application of traditional approaches and contemporary methods that have previously been used to forecast the collection of input data in the field, often under difficult conditions due to the accessibility of the sites, and which have been supported by UAV surveys. Through the analysis of the digital models of terrain and orthophotos, geometrical and multitemporal analyses of landforms were carried out. A dedicated software was utilized for the detection of rockfall runout zones and block trajectories and for defining the automatic extraction of rock mass discontinuities. The data were compared with those derived from traditional geomechanical surveys. The availability of the existing and acquired remote sensing data proved essential for this study for both defining the reference geological model and for performing the site-specific analysis of rockfall.

A map derived by rockfall analysis at Mount Pellegrino is presented herein. The study area is affected by several phenomena of rockfall which caused numerous damage and a strong social and economic impact. Official reports and maps that give a general assessment of rockfall hazard are available in this respect, however, it would be advisable to provide a more specific cartographic support useful for land management and planning. The drafting of new maps showing the rockfall runout areas is an additional tool that may be used in conjunction with the existing maps as a means of risk mitigation and reduction. On the basis of geological, geomorphological, and geomechanical analysis and exploiting the information relating to a landslides inventory obtained by using both analytical and empirical methods, two different rockfall propagation areas were reconstructed. The final thematic map permit to appreciate the differences and similarities between the obtained runout areas.

La cittadina di Cefalù, ubicata a circa 70 km da Palermo, è una delle perle del Mediterraneo, per la bellezza dei luoghi, dominati dalla Rocca, imponente rupe carbonatica alta circa 250 m s.l.m., e per i beni architettonici (Figg. 1 e 2). Fondata dai greci con il nome di Κεφαλοίδιον tra il XIV e il XIII a.C., conserva sulla Rocca il palazzo santuario ciclopico-megalitico, noto come Tempio di Diana. Sul ciglio della Rocca e per il suo intero sviluppo si snoda la cinta muraria di tipo megalitico risalente alla fine del V secolo a.C. (periodo pre-ellenico), in parte ricostruita nel periodo bizantino (VII÷IX secolo d.C.). Nel periodo normanno (secolo XII d.C.) furono costruiti i principali monumenti, tra i quali la cattedrale, sito UNESCO dal 2015 (Fig. 2). La Rocca è caratterizzata da un’elevata suscettibilità ai fenomeni di crollo che, spesso, si verificano in concomitanza di eventi sismici o meteorici particolarmente intensi. Tali crolli determinano condizioni di rischio molto elevato per gran parte dell’abitato, compresa la Cattedrale, ubicata ad alcune decine di metri dalla Rocca, e possono coinvolgere le sovrastanti preesistenze storico-archeologiche. In ogni caso inibiscono lo sviluppo socio-economico della cittadina normanna, ad evidente vocazione turistica.

This study concerns the assessment of the rockfall risk for two strategic sectors of the Mount Pellegrino, one of the most relevant place in the city of Palermo. During the last two decades, several rockfalls affected the study area setting up a serious threat to the people, structures and infrastructures preventing the economic and social development. For these landslides a detailed GIS inventory was realized; the most dangerous rockfalls occurred within the two study sectors. In both areas we have carried out detailed field surveys and the geo-mechanical characterization of the rock slopes. These analyses allowed to define the rock mass parameters, the set of discontinuities and the kinematics. The back analyses for the past events were conducted permitting to define the blocks trajectories using a numerical model for rockfall simulation. Through this procedure the hazardous areas have been identified and the map of the landslide risk was produced considering the possible spread of the blocks both on land and at sea. The results were exploited to compare this map with the existent map in the framework of the Sicilian Master Plan for hydrogeological risk (PAI), showing that the new risk areas are larger than those previously mapped.

In the last 20 years the natural oriented reserve of Monte Pellegrino is affected by several rockfall events. This sector of the Northern Sicily is a strategic urban area and represent the most important element of both the religious and cultural tradition and landscape for the city of Palermo.The rockfalls are here the major natural threats and represent a relevant risk of people, structures and infrastructures and prevents the economic and social development that could be made by high tourist potential of the area. For the above mentioned reasons a detailed geological and geotechnical study in order to define a quantitative risk analysis is now being carried out, concurrently with the implementation of the landslide inventory essential for analysis and monitoring. Monte Pellegrino, located along the Alpine orogenic belt (Catalano et al., 2013) in the emerged Sicilian fold and thrust belt, is an isolated carbonate massif characterized by the presence of poor rock masses and steeply sloping hillsides. The quantitative risk analysis was performed through several steps and taking into account the provisions of directive which is in force in the local institutions. The spread of a rockfall depends on many control factors such as geological setting and geomechanical features for both source area and below area, it is therefore necessary to define different input elements: an inventory of landslides, a database of factors and a dataset that contains the results of the on-site inspections like the geostructural and geomorphological data. The archive of landslides occurred over a period of 20 years was created; the spatial database (constructed in accordance with the standards) contains information on the identification code and date of the event, location, type, involved lithology and related thematic maps. Other thematic maps are those requirements deriving from the factors layers as tectonic features, morphological characteristics, geometric attributes of the slope, type of coverage, structures and infrastructures, trajectory of the block and the stop point. The above mentioned steps allow the implementation and calibration of the model for rockfall analysis; in particular, by means back-analysis stage it is possible to determine the restitution and friction coefficients through a comparison of the points where the rock blocks stop in the simulation with the rockfall history stop points. The next step we took is to produce the map of those areas with different degree of risk defined through the density of the trajectories reconstructed through the model. Finally, are presented here two between the cases studied needed to set up the forecasting model for the rockfall trajectories. Catalano R., Valenti V., Albanese C., Accaino F., Sulli A., Tinivella U., Gasparro Morticelli M., Zanolla C., Giustiniani M., 2013 – Sicily fold-thrust belt and slab roll-back: the SI.RI.PRO. seismic crustal transect. Journal of the Geological Society, 170, 451–464.