Landslide hazard zonation of slopes susceptible to rock falls and topples

Abstract. A landslide hazard zonation is a division of the land surface into areas, and the relative ranking of these areas according to degrees of actual or potential hazard from landslides on slopes. Zonation from scientific research does not generally imply legal restrictions, but can be useful to those people who are charged with the land management, by providing them with information that is indispensable for planning and regulation purposes. This paper presents a zonation of rock slopes in carbonate mountains on the boundary to the east of the valley of the Sele River (Campania, southern Apennines of Italy). The mountains are severely affected by rock falls and topples, and the related hazard is, therefore, very high; the presence of inhabited areas (the towns of Valva, Colliano and Collianello) and other human infrastructures at the slope foothills make these phenomena extremely dangerous to the anthropogenic environment. The area is highly seismic, as experienced on the occasion of several moderate to strong earthquakes that have hit this sector of the Apennines. According to the zonation proposed here, the ridge of Mount Valva and Mount Marzano is subdivided into four main areas on the basis of the processes which take place in the different sectors of the mountains: the source area, the talus slope, the rockfall shadow (where scattered outlying boulders are present), and the safe area (outside of the reach of fallen blocks). The four sectors were identified through air-photo interpretation and detailed field surveys, aimed in particular at characterizing and interpreting the main rock mass joint patterns, and their relative orientation with respect to the local slope direction. Geological, morphological and structural analyses permitted one to evaluate and classify those parts of the slope that are more susceptible to detachment of rocks, and to identify the more diffuse types of failure. Due to high seismicity of the study area, particular attention was given to the evaluation of the seismic susceptibility to rock falls, by applying two methods recently proposed in literature. Results from this phase of the study were then integrated by additional information from historical research on slope movements occurred previously in the area. The landslide hazard zonation, shown on large-scale cartography, could be compared to maps depicting the distribution and typology of the anthropogenic activities, and thus constitutes a useful tool for administrators and planners, in order to evaluate the hazards related to slope movements, and the vulnerability of settlements, roads, and other man-made infrastructures.