Assessing the rock failure return period on an unstable Alpine rock wall based on volume-frequency relationships: The Brenva Spur (3916 m a.s.l., Aosta Valley, Italy)

• Published in: Engineering Geology
• Main Authors: Fei, Li, Jaboyedoff, Michel, Guerin, Antoine, Noël, François, Bertolo, Davide, Derron, Marc-Henri, Thuegaz, Patrick, Troilo, Fabrizio, Ravanel, Ludovic
• Other Authors: Institute of Earth Sciences Lausanne, Université de Lausanne = University of Lausanne (UNIL), Struttura Attivit`a Geologiche, Regione Autonoma Valle d'Aosta, 11020 Quart, Italy, Fondazione Montagna sicura, Environnements, Dynamiques et Territoires de Montagne (EDYTEM), Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: Rockfall Photogrammetry SLBL Failure scenario Return period; [SDE]Environmental Sciences; Mont Blanc; permafrost
• Online Access: https://hal.science/hal-04312440; https://hal.science/hal-04312440/document; https://hal.science/hal-04312440/file/Fei-et-al_EngGeol_Brenva.pdf; https://doi.org/10.1016/j.enggeo.2023.107239

International audience Defining the relationship between volume and return period is critical when estimating the risk of rockfalls and/ or rock avalanche, especially during continued global warming at high altitudes that threatens rock wall stability. Characterizing the volume-frequency relationship based on historical datasets is, however, limited by observation and quantification biases, which have not received enough attention. Here, to monitor recent activities for the Brenva Spur (Mont-Blanc massif, Italy) that is also a rock avalanche scar and estimate the return period of future rock failures based on the volume-frequency relationship (and the corresponding uncertainty), a structure-from-motion photogrammetric survey was conducted from 2017 to 2021. 39 rockfall sources with volumes ranging from 11 to 13,250 m 3 were identified within the scar. The total failure volume is 22,438 m 3 , with an associated erosion rate of 15.5 mm/year, indicating very active morphodynamics possibly linked to the permafrost evolution in the spur. The volumes were characterized by a negative power-law that fits significant two events in 2016 (3.4 × 10 4 m 3) and one in 1997 (2.0 × 10 6 m 3) remarkably well, and the randomness of the fit was evaluated by a Monte Carlo approach. 7 potential failure scenarios ranging from 3.1 × 10 4 m 3 (S 1) to 4.8 × 10 6 m 3 (S 7) were defined according to a structural analysis and the sloping local base level concept. Their extrapolated return periods derived by the power-law fit indicate a longer return period for the maximum failure scenario than for the smaller scenarios. S 1 has a 50% chance of occurring every 3 years, while S 7 has a 50% chance of occurring every 31 years. Though the median return period of S 7 is 31 years, the 95% and 68.2% confidence intervals range from 8 to 399 years and 14 to 93 years, respectively, which reflects a high level of uncertainty but is realistic when considering global warming, progressive rock failure, etc. In addition to characterizing recent rock ...