Estimating rockfall release frequency from blocks deposited in protection barriers, growth disturbances in trees, and trajectory simulations

voir hal-03580281 Correction to: Estimating rockfall release frequency from blocks deposited in protection barriers, growth disturbances in trees, and trajectory simulationsInternational audienceThe spatial and temporal quantification of rockfall frequency remains a major challenge in mountain environments, especially also in terms of rockfall management. Approaches that have been used traditionally to quantify rockfall frequency include historical records, remote sensing, or in situ monitoring, but have been shown repeatedly to suffer from a lack of completeness or to rely on rather short time series (of a few years) that are, in addition, limited to small areas. As such, they normally cannot meet the stringent requirements of hazard and risk analyses. Here, we propose a new procedure coupling field analysis of rockfall deposits in mitigation structures and growth disturbances in tree-ring series with three-dimensional (3D), process-based rockfall modeling to estimate rockfall frequencies for individual cliff compartments. This procedure has been tested on a slope in the Swiss Alps (La Fory, Sembrancher VS), where rockfall triggered from a 38 hm(2) cliff to threaten a 1-km-long section of the road to the Grand St. Bernard tunnel and the local railway line. Based on 84 rockfall deposits retrieved from 420-m-long protection barrier and growth disturbances in trees over the 1994-2017 period, we estimate rockfall hazard to between 0.043 and 0.348 events yr(-1) at various locations on the slope. In total, we used 68,680,000 rockfall simulations to translate hazard values into rockfall frequencies at the cliff level. Converging release frequencies between tree-ring analyses (0.991.35 events yr(-1) hm(-2)) and values obtained from deposits in the protection barrier (0.82 events yr(-1) hm(-2)) confirm the reliability of our procedure. Despite remaining limitations, our approach enables precise quantification of rockfall release frequency in a holistic and reproducible manner both in space (a few hm(2)) and time (several decades), and thus yields the data needed for hazard and risk mapping. We therefore recommend, where applicable, to include this procedure in future rockfall management strategies