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Understanding the dynamics of slope instabilities is critical to mitigate the associated hazards but their direct observation is often difficult due to their remote locations and their spontaneous nature. Seismology allows to get unique information on these events, including on their dynamics. However, the link between the properties of these events (mass and kinematics) and the seismic signals generated are still poorly understood. we conducted a controlled rockfall experiment in the Riou-Bourdoux torrent (South French Alps) to try to better decipher those links. We deployed a dense seismic network and inferred the dynamics of the block from the reconstruction of the 3D trajectory from terrestrial and airborne high-resolution stereo-photogrammetry. We propose a new approach based on machine learning to predict the mass and the velocity of each block. Our results show that we can predict those quantities with average errors of approximately 10 % for the velocity and 25 % for the mass. These accuracies are as good as or better than those obtained by other approaches, but our approach has the advantage of not requiring to localize the source and an a priori knowledge of the environment, nor of making a strong assumption on the seismic wave attenuation model. Finally, the machine learning approach allows us to explore more widely the correlations between the features of the seismic signal generated by the rockfalls and their physical properties, and might eventually lead to better constrain the physical models in the future.

International audience; Rockfall propagation models are routinely used for the quantitative assessment of rockfall hazard. Their capacities and limitations remain difficult to assess due to the limited amount of exhaustive experimental data at the slope scale. This article presents experiments of block propagation performedin a quarry located in Authume (France). This study site was chosen for its complexity, related with the presence of topographical discontinuities and ofconfigurations promoting block rolling. A total of more than one hundred blocks were released on two propagation paths. The propagation of the blockswas assessed by measuring the block stopping points as well as their kinematics at specific locations of the paths, called evaluation screens. Significant variability of the stopping points and of the block kinematics at the evaluation screens was observed and preferential transit and deposit zones were highlighted. The analysis of the results showed predominant effect of topography, in particular that related to topographical discontinuities. Significant influence of local and small scale parameters (e.g. block orientation, local topography) was also highlighted. These conclusions are of particular interest for researchers or practitioners who would like to assess the relevance of propagation modelling tools considering this complex study site. In this configuration, the quality of block propagation simulations should notably rely on the accuracy of digital terrain models, and on the integration of local conditions effects using physically based approaches.

International audience

The choice of a natural risk prevention strategy must be considered at the scale of a territory in order to take into account all its components. Since 2015, France has developed integrated natural risk management (INRM) approaches in Alpine territories. The challenge of INRM lies in the definition and implementation of innovative projects for initiating synergies with respect to natural risks while seeking to increase resilience through the new and different involvement of the territorial actors. The Baronnies Provençales Regional Nature Park is one of the pilot territories for the operational implementation of this approach, with a particular focus on forest-based solutions. For this reason it has been chosen as the French Pilot Action Region (PAR) of the Interreg Alpine Space project GreenRisk4Alps. In this article we present an example of good practice related to the benefit of large-scale rockfall risk modeling, the analysis of potential cascading effects and the added value of a territorial perspective.

International audience; Due to intense urban sprawl in rockfall-prone areas, a precise rockfall risk assessment has become a crucial issue for public authorities and stakeholders. In this context, quantitative risk analysis (QRA) procedures, accounting for the specificities of the rockfall process, have been developed. For the last few decades, several studies have examined the impacts of global warming on rockfall activity, especially at high-altitude sites. However, the influence of land-use and land-cover (LULC) changes, very frequent at lower altitudes and in the vicinity of urbanised areas, on rockfall propagation and associated risks has received little attention.This study proposes a holistic QRA on a municipality scale (the municipality of Crolles, in the French Alps) that includes both all the potential release areas at the whole cliff scale and a wide spectrum of rockfall volumes randomly extracted from volume classes distributed between 1 and 20 m(3). In addition, to quantify precisely the effect of LULC changes on rockfall risk, four characteristic scenarios representative of LULC changes observed in the municipality and more generally in the Alps since the mid-19th century, have been included in the analysis.The results demonstrate the significant impacts of landscape reorganisation on the spatial distribution of risk with increasing forest cover, which can be counterbalanced by evolving LULC in a transition unit located between the forest strip and the urban front. They also evidence that a large proportion of the risk is explained by small block volumes, which are the most affected by landscape structure and evolution. From a practical point of view, and despite several uncertainties related to different modelling assumptions, the results reported herein clearly demonstrate the applicability and the value of QRA for rockfall risk management at a municipality scale in a context of rapid and intense environmental changes.

International audience; To overcome the lack of historical archives at active rockfall environments, dendrogeomorphic techniques have been used extensively on forested slopes since the early 2000s and several approaches developed to extract rockfall signals from tree-ring records. Given the unpredictable nature of rockfall, these reconstructions are, in principle, of great help when it comes to relate fluctuations in rockfall activity to meteorological variables. Yet, so far, dendrogeomorphic time series were only rarely compared with meteorological records. Here, we ascribe this shortfall to the absence of clear guidelines on how to optimize the sampling strategy. In order to test this hypothesis, we capitalize on the extensive dataset of rockfall impacts recorded in trees growing in a mixed forest plot within the French Alps. We designed six different scenarios retrospectively and compared rockfall reconstructions with meteorological records. Our results demonstrate that reconstructions that include trees located in the most active segments of the plot (i.e. close to the cliff and over periods lacking multidecadal trends in the reconstruction) capture summer precipitation as the main driver of rockfall activity more clearly. This result is in line with monitoring studies from calcareous cliffs in the Alps located outside periglacial environments thus confirming the robustness of our approach.

International audience; This article proposes a procedure for developing tools to quantify the on-site efficiency of any rockfall barrier. This procedure relies on meta-modeling techniques to predict the barrier ability in arresting rock blocks, whatever their trajectory. For demonstration purpose, a specific low-energy barrier for which a finite element model was available is considered. The barrier response is simulated varying six parameters describing the rock block kinematics. Six different methods are used to create meta-models predicting the simulated barrier response. The ability of each method in creating meta-models with good prediction capacities is evaluated. Meta-models created utilizing the best methods are then used to quantify the efficiency of the barrier in arresting rock blocks in two real situations. These situations exhibit very similar 95% percentiles of the block passing height and kinetic energy but very different distributions for the other parameters describing the kinematics of the rock blocks. The predictions reveal that the barrier efficiency is extremely site-dependant. The discussion addresses the meta-models performance and highlights the benefits in using such meta-models for quantifying the barrier efficiency, in particular with respect to more classical barrier design approaches. Last, the proposed eight-step procedure for generating meta-models to be used in operational contexts is described.

International audience; Forest covers 40% of the European Alpine region and contributes to the protection of human beings and infrastructures against natural hazards such as rockfalls. However, despite the recognition of this ecosystem service, most mountain territories do not have a map of protection forests. When a map exists, it generally depends on data restricted to a limited extent, which prevents any replication or comparison on other areas. The aim of this study is to develop a method using harmonized and open data to produce the first map of protection forests against rockfalls at the European Alpine region scale. Based on these data, we first identified potential rockfall release areas and calibrated the model according to 2812 real rockfall events located around the Alps. Second, 46.5 billion 3-D rockfall propagation simulations, taking into account topography, land use and human assets, were computed on the entire area. Protection forest is defined as being located on at least one rockfall trajectory that had impacted human assets. Our results show that 14% of the forests have a potential protection function against rockfalls in the Alpine Space region. This proportion goes up to 21.5%, if we consider only the core of the Alpine area. 80% of the protection forest area contributes to mitigate rockfall hazard on road network, 55% on buildings and only 6% on railways. This work provides a robust, objective and reproducible method for locating protection forests on a large geographical scale. Such a map may serve as a basis for national and European risk management policies.

The paper presents a new approach to assess the effecctiveness of rockfall protection barriers, accounting for the wide variety of impact conditions observed on natural sites. This approach makes use of meta-models, considering a widely used rockfall barrier type and was developed from on FE simulation results. Six input parameters relevant to the block impact conditions have been considered. Two meta-models were developed concerning the barrier capability either of stopping the block or in reducing its kinetic energy. The outcome of the parameters range on the meta-model accuracy has been also investigated. The results of the study reveal that the meta-models are effective in reproducing with accuracy the response of the barrier to any impact conditions, providing a formidable tool to support the design of these structures. Furthermore, allowing to accommodate the effects of the impact conditions on the prediction of the block–barrier interaction, the approach can be successfully used in combination with rockfall trajectory simulation tools to improve rockfall quantitative hazard assessment and optimise rockfall mitigation strategies.

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. L’aléa chute de bloc est lié au détachement d’un bloc rocheux depuis une paroi verticale ou sub-verticale, qui se propage rapidement vers l’aval sous l’effet de la gravité par rebonds successifs. Cet aléa cause chaque année de nombreux dommages aux biens ou aux personnes. Dans ce contexte, une évaluation précise du risque chutes de blocs est devenue cruciale pour les gestionnaires et les pouvoirs publics. Afin de caractériser le risque de manière précise et reproductible, une procédure d’évaluation quantitative du risque adaptée à l’aléa chute de bloc a été utilisée dans cette étude. Le risque pour le bâti a ainsi été calculé à l’échelle de la commune de Crolles, située sur le versant sud-est du Massif de La Chartreuse dans les Alpes Françaises. Cette procédure prend en compte l’intégralité de la distribution des volumes rocheux entre 1 m³ et 20 m³ et inclut les 300 m de parois subverticales qui surplombent le village. Nos résultats, exprimés comme la surface moyenne du bâti détruite chaque année, démontrent (i) l’effet de protection du couvert forestier pour des volumes de 1 m³ à 7 m³, et (ii) la part importante du risque total due aux classes de volume intermédiaires (7-12 m³). Sur un plan spatial, nos résultats permettent d’identifier les secteurs les plus exposés et démontrent qu’une large proportion du risque total est localisée sur quelques habitations situées dans le quartier du Coteau. D’un point de vue pratique, et malgré des incertitudes d’ordre épistémique liées à la magnitude/fréquence des événements rocheux, au paramétrage du modèle trajectographique et à l’absence de courbe de vulnérabilité spécifique au site, notre étude démontre clairement l’intérêt d’une approche quantitative de type QRA pour le zonage et la gestion du risque rocheux.

International audience; The invasive tree species Ailanthus altissima increasingly occupies forests in the southern parts of the Alps. Many of these forests grow on steep slopes and protect settlements and infrastructure from natural hazards, such as rockfall. It is feared that the mechanical properties of Ailanthus are less favourable for energy reduction of falling rocks due to lower wood strength as well as higher prevalence of heart rot. Therefore, the question has arisen whether the spread of Ailanthus will substantially change rockfall risk. The objective of this study is to analyse the influence of the spread of Ailanthus trees in forest stands on their protective effect against rockfall. We first quantify the block energy reduction capacity of single Ailanthus trees based on a block-tree impact model and compare it to that of other species. Subsequently, we analyse the effect of Ailanthus on rockfall risk for different forest scenarios with a varying proportion of Ailanthus at the stand scale. The capacity of Ailanthus to reduce the block energy was quantified using a model of the block impact on a tree based on the Discrete Element Method. We then integrated the obtained results in the rockfall trajectory model Rocky For 3D. Based on rockfall simulations, we finally calculated rockfall risk for different forest scenarios representing current forest conditions and an increasing spread of Ailanthus. The energy reduction capacity of Ailanthus lies in the range of the species predominantly present in the study area, as well as other species that are typically found on rockfall slopes in the Alps. Rockfall risk for houses and roads does not increase with an increasing proportion of Ailanthus without change in the forest structure. Assuming a decrease in tree diameters with an increasing proportion of Ailanthus trees, rockfall risk, however, critically increased. Consequently, whether or not Ailanthus changes rockfall risk in the long term, strongly depends on its influence on the forest structure. To anticipate the evolution of protection forests invaded by Ailanthus, more long-term ecological data on growth and succession of Ailanthus at stand scale is required.

International audience; In mountain environments, precise rockfall risk evaluation is crucial to reduce death tolls and costs. However, to date, existing rockfall risk assessment procedures remain scarce, as they focus only on existing elements at risk and with the damage expectation as sole risk measure. Here, we propose an approach to evaluate the distribution of damages on an individual basis as a continuous function of space. Furthermore, rockfall risk is evaluated through (i) the damage expectation and based on (ii) the value-at-risk (VAR) and (iii) the expected shortfall (ES). VaR and ES risk measures allow better assessing the risk due to extreme events and accounting for various shortterm/long-term constraints faced by stakeholders. This procedure is applied to Le Brocey slope (municipality of Crolles, French Alps), frequently affected by rockfall events, illustrating its potential for land-use planning. Notably, obtained individual risk values can be confronted with acceptability thresholds to perform legal zoning on sound basis. Also, they allow comparison of risk management strategies as function of different behaviors towards risk, budgetary constraints and/or temporal horizons. Hence, the approach provides valuable tools for future land-use planning and decision-making. It could easily be transferred to other hazards as a wider contribution to the determination of the best balance between safety and sustainability.

International audience; Rockfall propagation simulation models are widely used for assessing rockfall hazards as well as for the design of rockfall protection structures. This research work investigates the relevance of rockfall propagation models to assess rockfall hazard in the vicinity of embankments. In this article, particular focus is placed on the development of engineering oriented rebound models taking into account the shape of the block. Two different block impact modelling approaches are considered and compared. Small scale experiments involving blocks with different shapes impacting a small-scale embankment were conducted to provide data for calibrating the models. The trajectory of the blocks was tracked using a high speed camera (1000 fps) and an image processing algorithm was developed to extract the experimental trajectories. The two different approaches considered for modelling the impact of the block in the embankment vicinity were a probabilistic block impact model and a deterministic approach accounting for the block shape. In this latter case, the impact was calculated using an elasto-plastic contact model between the block and the slope surface. Both approaches succeed in simulating experimental results. Nevertheless the probabilistic model is limited by the introduction of the block shape parameter in the calculation while the deterministic approach seems to be limited in terms of computational efficiency.

International audience; The paper addresses the response of a semi-rigid rockfall protection barrier using numerical models. The study show a large dependence of the barrier response to the impact conditions. The block size and impact position, rather than the velocity direction and magnitude induce different modes of failure of the fence, which in turn result in different values of failure energy. As a result, the barrier capacity cannot be established in a deterministic way. The effectiveness of structures as such can be more successfully evaluated through a reliability probabilistic approach. Results can be used to create a meta-model of the barrier response which can be incorporated into rockfall simulation models, enabling a reliable and comprehensive design of rockfall mitigation interventions performed with this type of structure.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; Long-term records of rockfalls have proven to be scarce and typically incomplete, especially in increasingly urbanized areas where inventories are largely absent and the risk associated with rockfall events rises proportionally with urbanization. On forested slopes, tree-ring analyses may help to fill this gap, as they have been demonstrated to provide annually-resolved data on past rockfall activity over long periods. Yet, the reconstruction of rockfall chronologies has been hampered in the past by the paucity of studies that include broadleaved tree species, which are, in fact, quite common in various rockfall-prone environments. In this study, we test the sensitivity of two common, yet unstudied, broadleaved species — Quercus pubescens Willd. (Qp) and Acer opalus Mill. (Ao) — to record rockfall impacts. The approach is based on a systematic mapping of trees and the counting of visible scars on the stem surface of both species. Data are presented from a site in the Vercors massif (French Alps) where rocks are frequently detached from Valanginian limestone and marl cliffs. We compare recurrence interval maps obtained from both species and from two different sets of tree structures (i.e., single trees vs. coppice stands) based on Cohen's k coefficient and the mean absolute error. A total of 1230 scars were observed on the stem surface of 847 A. opalus and Q. pubescens trees. Both methods yield comparable results on the spatial distribution of relative rockfall activity with similar downslope decreasing recurrence intervals. Yet recurrence intervals vary significantly according to tree species and tree structure. The recurrence interval observed on the stem surface of Q. pubescens exceeds that of A. opalus by N20 years in the lower part of the studied plot. Similarly, the recurrence interval map derived from A. opalus coppice stands, dominant at the stand scale, does not exhibit a clear spatial pattern. Differences between species may be explained by the bark thickness of Q. pubescens, which has been demonstrated to grow at twice the rate of A. opalus, thus constituting a mechanical barrier that is able to buffer low energy rockfalls and thus can avoid damage to the underlying tissues. The reasons for differences between tree structures are related to the clustered coppice-specific spatial stem distribution in clumps that could result on one hand in bigger gaps between clumps, which in turn decreases the probability of tree impacts for traveling blocks. On the other hand, data also indicate that several scars on the bark of coppice stands may stem from the same impact and thus may lead to an overestimation of rockfall activity.

Classical rockfall simulation models do not integrate the effect of coppice forest on bocks propagation. Although field observations and experimental tests have shown that coppice forest can play an important role of protection against rockfall hazard. In order to integrate coppice forest in rockfall modeling two models will be developed. The first one will generate a virtual coppice forest stands, the other will model the impact of a block against a coppice stem. To characterize the spatial organization of coppice forest and to develop a methodology to generate virtual coppice forest stands, dendrometric data have to be collected in forest with protection function. In a second time spatial point pattern analyses have to be carried out on the collected data in order to develop a model which automatically generates coppice forest structures using key dendrometic parameters. Second, the key issues in a numerical model of a block that impacts a clump in coppice forest are the definition of a “green wood” rheology material as well as the modeling of the contact between a block and a stem under dynamic loading. These issues will be tackled using laboratory experiments and a mechanical model of impact based on the discrete element method (DEM). Finally the two modules developed in this work: virtual coppice forest stand generation and impact modeling of a rock on a coppice will be integrated and validated in a 3D rockfall model.

International audience; & Context One eighth of Europe's forests are still managed as coppice. In some European countries, more than half of the forest exhibits coppice structures from the past or present coppice management. Many of these forests grow in the broadleaf zone of mountainous regions, often on steep slopes. Here, they play an important role in rockfall protection. However, it remains unclear how coppice forests should be structured for optimal rockfall protection or how the protection effect changes during the aging of the coppice. & Aim A few studies have applied rock trajectory analyses, but so far, no process-based model has been used to quantify the protective effect of differently structured coppice forests. The present study compared 40 coppice patches from two chronosequences in South Tyrol, North Italy, regarding their protective effect against rocks of two sizes using the rockfall simulation model Rockyfor3D. & Results The results indicate that coppice stands older than 30 years better protect against rockfall than medium-aged and young stands, although the old ones have lower stem densities. Surprisingly, a random stem distribution had a better protective effect than the clumped stem distribution typical for coppice stands. & Conclusion Implications for future management are discussed in detail, including the relevance of standards in coppice forests.

International audience; Trajectory simulation codes are widely used for assessing and zoning rockfall hazards in view of land use planning. Theses tools may also be used to estimate the benefit in terms of risk reduction derived from the construction of protection structures such as embankments. This paper investigates the relevance of currently used trajectory analysis tools for this purpose through a real case study. The trajectory analysis is first used to assess the hazard and define the embankment geometry. In a second time, it is used to assess the embankment protection efficiency after its insertion in the digital terrain model. Results reveal abnormal trajectories in the vicinity of the embankment suggesting limitations related to both the spatial resolution of the terrain model and to the rebound models. To cope with these limitations, an innovative approach is proposed and evaluated for improving the ability of the simulation code in assessing the residual hazard down the embankment.