Application of a Statistical Approach for the Assessment of Design Block in Rockfall: A Case Study in Sesia Valley (Northern Italy)

Rockfall is one of the most dangerous and unpredictable slope instabilities. Modelling this phenomenon is a very complex issue because of the high number of variables influencing the triggering and the propagation phases. In particular, the choice of the design block volume can affect the results of the propagation analysis as well as the reliability of the risk mitigation structures (Hungr et al. 1999). Thus, De Biagi et al. (2017b) proposed a methodology based on a statistical approach for building a fallen blocks volume-frequency law. The approach takes into account both the volumes of fallen blocks measured onto the slope and the historical events, providing a relationship between the volume and the recurrence period. A subsequent note (De Biagi 2017) suggests how to take into account the errors that can be made with this methodology due to the usual limited number of recorded events and surveyed blocks. In this paper, the statistical method is applied to a case of recurrent rockfall events that affect Nosuggio village, located in the Sesia Valley (North Piedmont, Italy). The study shows how it is possible to obtain a reliable volume-frequency law even in the case in which a well-structured survey of the blocks is not easy to perform.