Quantitative risk assessment in a rockfall-prone area: the case study of the Crolles municipality (Massif de la Chartreuse, French Alps)

Abstract

The rockfall phenomenon is defined as a rock block that detaches from a vertical or sub-vertical cliff and travels down the slope in rapid motions. Every year, some rockfalls reach urbanized areas, causing damage to structures and injuring people. In this context, a precise rockfall risk analysis has become essential for authorities and stakeholders. Given the importance of characterizing the risk in an objective and reproducible manner, this study proposes a quantitative risk analysis (QRA) procedure accounting for the specificities of the rockfall process. Here, the risk for buildings is calculated at the scale of the municipality of Crolles (southeastern slopes of the Chartreuse Mountains, French Alps), by considering a wide range of volumes (1–20 m³) and the 300-m-high sub-vertical cliff overhanging the village. The results, expressed as the mean surface destroyed per year, demonstrate (i) the protective effect of the forest stand for block volumes up to 7 m³, and (ii) the surprisingly high proportion of total risk due to intermediate volume classes (7–12 m³). On a spatial plan, these results make it possible to identify the areas most prone to risk and show that a large proportion of the total risk is concentrated on a few buildings located in the Le Coteau neighborhood. From a practical point of view, and despite epistemic uncertainties related to the rockfall magnitude/frequency relation, the parameterization of the trajectory model and the absence of a site-specific vulnerability curve, this study clearly demonstrates the value of QRA as a reproducible and objective method for risk zoning and risk management.