A multi-sensor surveying approach supporting landslide and rock cliff evolution analyses at the Temple of Juno, UNESCO Valle dei Templi site (Italy)

A multi-sensor remote surveying approach is presented in this study to characterize the landslide activity at a UNESCO world heritage site, where the low invasiveness must be ensured during technical surveys. The sacral complex of temples in the Valle dei Templi (Italy) is set in a peculiar geological setting, where different types of slope failure in terms of lithology, kinematics, distribution and velocity occur. Aerial photogrammetry was used to build digital rock mass models that were analysed from geostructural, kinematic and morphological points of view. Results show that block toppling, wedge and plane sliding are the most recurrent failure patterns, and that the rock mass weathering enhances the cliff instability. The digital model processing allowed also mapping numerous previously fallen blocks, currently laying along the slope at the foot of the cliff, and measuring their volume. By taking into account fragmentation, their original volume was calculated to achieve information on the cliff recession magnitude. A differential rockfall activity was outlined. Furthermore, the presence of slow gravitative movements was found by a modern InSAR analysis based on the use of algorithms, which allow identifying stable reflection from distributed targets. This approach shed light on the current activity of a slope sector where landslides were mapped in the past. The retrogression of these movements could enhance the rock cliff instability by causing undercutting and triggering further rockfalls. In the light of achieved results, three major instability models were defined for the studied site. These explain the widespread instability of the cliff that represents a threat for the safe fruition of the world heritage Temple of Juno standing nearly close to its edge.