GNGTS 2016 - Atti del 35° Convegno Nazionale

358 GNGTS 2016 S essione 2.2 movements. Then, the second step was focused on the evaluation of the hazard associated with the preceding movements. Using a probabilistic approach, according to the standard procedure R.H.A.P. (Rockfall Hazard Assessment Procedures), the authors tried to estimate the “maximum potential advancement run out” through the assessment of the maximum length of the potential paths of the blocks. This calculation enables to detect the areas characterized by the same probability of being reached by the boulders. To undertake the numerical study, a GPS campaign has been carried out to have a census of blocks and to preliminary estimate their volumes. The laser scanner technology allows to overpass the limits in the accuracy of the aerial photos . Thus, the frames obtained from the laser scanner acquisitions were then analyzed by the software Image Master (Topcon, 2012) in order to obtain three-dimensional models for performing analyses, like calculating the precise magnitude of the boulder volumes. A preliminary back analysis has then been performed by using different values of the input parameters within the following formula used to estimate the velocity of the rocky boulders during the rolling movement after their detachment from the rocky wall: (1) Keeping constant the values of the initial velocity V 0 and gravitational acceleration g , the angle of the slope b and the dynamic rolling friction angle δ rol were derived from GIS spatial analysis tools and from literature data, respectively. In fact, the length of the slope l has been derived as raster file through the function cost distance belonging to the ArcGIS (Esri Italia, 2010) spatial analyst. The resulting velocity values calculated by means of Eq. 1 were used within the formula of the kinetic energy: (2) enabling to calculate a raster file with the kinetic energy values drawn from each boulder. This information layer has been mapped to provide preliminary information on the degree of LS hazard related to the run-out of the blocks. The parameter obtained by Eqs. 1 and 2 represents the intensity of the phenomenon (block rolling and falling) and it can be directly related to the seismically induced landslide hazard. Although the preceding analysis is only “qualitative” estimates it enables to assess the extension of the area of Casentino urban territory where the blocks are likely to roll and fall, Fig. 3 – Acceleration spectra for each investigated points that have been calculated by numerical computations compared to the building code spectra (according to NTC08) for a reference return period of 475 years.