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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 us 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 is still poorly understood. We conducted a controlled rockfall experiment in the Riou Bourdoux torrent (southern 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 stereophotogrammetry. 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 in that it does not require the source to be localised, nor does it require a high-resolution velocity model or 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 it might eventually lead to better constraints on the physical models in the future. [ABSTRACT FROM AUTHOR]

Over large regions exposed to natural disasters, cascading effects resulting from complex or concatenated natural processes may represent a large portion of total risk. Populated high‐mountain environments are a major concern, and methods for large‐scale quantitative risk analyses are urgently required to improve risk mitigation. This article presents a comprehensive quantitative rockfall risk assessment over a large archetypal valley of the Andean mountains, in Central Chile, which integrates a wide spectrum of elements at risk. Risk is expressed as an expected damage both in monetary terms and casualties, at different scales relevant for decision making. Notably, total rockfall risk is divided into its main drivers, which allows quantifying seismically induced rockfall risk. For this purpose, the local seismic hazard is quantified and the yield acceleration, that is, acceleration required to initiate rockfall, is determined at the regional scale. The probability of failure is thereafter derived in terms of annual frequency of rockfall initiation and integrated in the quantitative risk assessment (QRA) process. Our results show the significant role of seismic activity as the triggering mechanism of rockfalls, and highlight elements at risk that have a major contribution to the total risk. Eventually a sensitivity analysis is conducted to (i) assess the robustness of obtained risk estimates to the data and modeling choices and (ii) identify the most influential assumptions. Our approach evidences the feasibility of large‐scale QRAs in sensitive environments and opens perspectives for refining QRAs in similar territories significantly affected by cascading effects and multihazards. [ABSTRACT FROM AUTHOR]

The use of dynamic computational methods has become indispensable for addressing problems related to rockfall hazard. Although a number of models with various degrees of complexity are available, model parameters are rarely calibrated against observations from rockfall experiments. A major difficulty lies in reproducing the apparent randomness of the impact process related to both ground and block irregularities. Calibration of rigorous methods capable of explicitly modeling trajectories and impact physics of irregular blocks is difficult, as parameter spaces become too vast and the quality of model input and observation data are insufficient. The model presented here returns to the simple 'lumped-mass' approach and simulates the characteristic randomness of rockfall impact as a stochastic process. Despite similarities to existing approaches, the model presented here incorporates several novel concepts: (i) ground roughness and particle roughness are represented as a random change of slope angle at impact; (ii) lateral deviations of rebound direction from the trajectory plane at impact are similarly accounted for by perturbing the ground orientation laterally, thus inducing scatter of run-out directions; and (iii) a hyperbolic relationship connects restitution factors to impact deformation energy. With these features, the model is capable of realistically accounting for the influence of particle mass on dynamic behaviour. The model only requires four input parameters, rendering it flexible for calibration against observed datasets. In this study, we calibrate the model against observations from the rockfall test site at Vaujany in France. The model is able to reproduce observed distributions of velocity, jump heights, and runout at observation points. In addition, the spatial distribution of the trajectories and landing points has been successfully simulated. Different parameter sets have been used for different ground materials such as an avalanche channel, a forest road, and a talus cone. Further calibration of the new model against a range of field datasets is essential. This study is part of an extensive calibration program that is still in progress at this first presentation of the method, and focuses on fine-tuning the details of the stochastic process implemented both in two-dimensional (2D) and three-dimensional (3D) versions of the model. Key words: rockfall, run-out simulation, stochastic methods. Il est devenu indispensable d'utiliser des methodes de calcul dynamiques pour resoudre des problemes lies aux risques d'eboulement. Bien qu'il existe actuellement un grand nombre de modeles aux degres de complexite varies, les parametres de ces modeles sont rarement ajustes en fonction des observations faites lors d'experiences d'eboulements. Une difficulte majeure reside dans la reproduction du caractere apparemment aleatoire du processus d'impact des particules, lequel est lie aux irregularites des sols et blocs rocheux. Il est difficile de calibrer des methodes rigoureuses capables de modeliser clairement les trajectoires et impacts de blocs irreguliers d'un point de vue physique, etant donne que l'etendue des parametres devient trop importante et que la qualite des donnees d'observation et de celles entrees dans le modele est insuffisante. Le modele presente ici revient a l'approche simplifiee de la << masse unique >> et simule le caractere typiquement aleatoire de l'impact des pierres lors de l'eboulement sous forme d'un processus stochastique. Malgre les similitudes avec les approches existantes, le modele presente inclue plusieurs concepts novateurs : (i) la rugosite du sol et des particules est representee apparait comme etant une variation aleatoire de l'inclinaison de la pente lors de l'impact; (ii) les deviations laterales de la direction dans laquelle rebondissent les particules par rapport au plan de leur trajectoire lors de l'impact s'expliquent par la variation d'inclinaison du sol sur les cotes, ce qui provoque un eparpillement des particules dans toutes les directions; et (iii) une relation hyperbolique existe entre les facteurs de restitution et l'energie de deformation a l'impact. Grace a ces caracteristiques, le modele est en mesure d'expliquer de maniere realiste l'influence de la masse des particules sur la dynamique de l'eboulement. Du fait qu'il ne requiert que quatre parametres de depart, le modele est a calibrer en fonction des ensembles de donnees recueillies. Dans la presente etude, nous calibrons le modele en fonction d'observations faites sur le site de Vaujany, en France, ou l'on procede a des essais d'eboulement. Grace a ce modele, il est possible d'obtenir les memes courbes de distribution des vitesses et les memes hauteurs et trajectoires de projection des particules que celles observees a divers endroits sur le terrain. En outre, on a reussi a simuler la distribution spatiale des trajectoires et destinations des particules. Differents groupes de parametres ont ete utilises divers types de terrains tels qu'un canal d'avalanche, une route forestiere et un cone d'eboulis. Il est indispensable d'effectuer une calibration plus poussee du nouveau modele en fonction d'un large eventail de donnees collectees sur le terrain. La presente etude s'integre dans un programme de calibration poussee, toujours en cours au moment oU l'on presente le nouveau modele pour la premiere fois, et a pour objectif de preciser les details du processus stochastique mis en pratique dans les versions bidimensionnelle (2D) et tridimensionnel (3D) du modele. [Traduit par la Redaction] Mots-cles : eboulement, simulation des trajectoires de projection, methodes stochastiques., Introduction Rockfall is a type of landslide consisting of detachment, fall, rolling, and bouncing of rock fragments (Hungr et al. 2014). It may occur as single events or in clusters, [...]

Flexible barriers can be used to trap woody debris or debris flows. However, their small scale modelling is challenging because of their possible deformation. This article addresses how to meet the partial mechanical similitude of manufactured flexible barriers. Relevant dimensionless parameters are defined from flow velocity, barrier geometry, and component mechanical properties. These similitude criteria are validated using numerical simulations of barriers exposed to a hydrodynamic loading at various scales. The simulations also confirm the importance of accounting for the mechanical characteristics of the barrier components when designing model barriers in view of achieving realistic deformations. Next, a real barrier with complex features is scaled to conduct flume experiments. This scaled barrier is 3D-printed with material selected to achieve the mechanical similitude criterion. Another validation of this approach is performed considering hydrostatic loading and checking that simulated and measured deformations are similar. As an application case, the deformations measured during the experiments performed with woody debris are also compared to the hydrostatic loading. [ABSTRACT FROM AUTHOR]

In mountainous regions, global warming will likely affect the frequency and magnitude of geomorphic processes. This is also the case for rockfall, one of the most common mass movements on steep slopes. Rainfall, snowmelt, or freeze-thaw cycles are the main drivers of rockfall activity, rockfall hazards are thus generally thought to become more relevant in a context of climate change. At high elevations, unequivocal relationships have been found between increased rockfall activity, permafrost thawing and global warming. By contrast, below the permafrost limit, studies are scarcer. They mostly rely on short or incomplete rockfall records, and have so far failed to identify climatically induced trends in rockfall records. Here, using a dendrogeomorphic approach, we develop two continuous 60-year long chronologies of rockfall activity in the Vercors and Diois massifs (French Alps); both sites are located clearly below the permafrost limit. Uncertainties related to the decreasing number of trees available back in time were quantified based on a detailed mapping of trees covering the slope across time. Significant multiple regression models with reconstructed rockfalls as predictors and local changes in climatic conditions since 1959 extracted from the SAFRAN reanalysis dataset as predictants were fitted to investigate the potential impacts of global warming on rockfall activity at both sites. In the Vercors massif, the strong increase in reconstructed rockfall can be ascribed to the recolonization of the forest stand and the over-representation of young trees; changes that are observed should not therefore be ascribed to climatic fluctuations. In the Diois massif, we identify annual precipitation totals and mean temperatures as statistically significant drivers of rockfall activity but no significant increasing trend was identified in the reconstruction. All in all, despite the stringency of our approach, we cannot therefore confirm that rockfall hazard will increase as a result of global warming at our sites. [ABSTRACT FROM AUTHOR]

Many examples of rockfall simulation software provide great flexibility to the user at the expense of a hardly achievable parameter unification. With sensitive site-dependent parameters that are hardly generalizable from the literature and case studies, the user must properly calibrate simulations for the desired site by performing back-calculation analyses. Thus, rockfall trajectory reconstruction methods are needed. For that purpose, a computer-assisted videogrammetric 3D trajectory reconstruction method (CAVR) built on earlier approaches is proposed. Rockfall impacts are visually identified and timed from video footage and are manually transposed on detailed high-resolution 3D terrain models that act as the spatial reference. This shift in reference removes the dependency on steady and precisely positioned cameras, ensuring that the CAVR method can be used for reconstructing trajectories from witnessed previous records with nonoptimal video footage. For validation, the method is applied to reconstruct some trajectories from a rockfall experiment performed by the WSL Institute for Snow and Avalanche Research SLF. The results are compared to previous ones from the SLF and share many similarities. Indeed, the translational energies, bounce heights, rotational energies, and impact positions against a flexible barrier compare well with those from the SLF. The comparison shows that the presented cost-effective and flexible CAVR method can reproduce proper 3D rockfall trajectories from experiments or real rockfall events. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

At present, a quantitative basis for delimiting realistic rockfall runout zones on the basis of trajectory simulation data is generally missing. The objective of this study is to come up with standardized reach probability threshold values (RPTV) to separate "realistic" from "unrealistic" simulated rockfall runouts. We therefore compared reach probability values (Preach) simulated with Rockyfor3D for 458 mapped, fresh rockfall blocks (silent witnesses SW) on 18 different sites with a volume >= 0.05 m³ and estimated occurrence frequencies up to 300 years. We analysed which block, slope and forest characteristics influenced Preach of the SW based on a linear mixed effects model. The results indicate that the limit of a realistic runout zone lies in the range where simulated Preach values are between > 1 % and approximately 3 %. We conclude that RPTV can be defined to values lying in the range from 1.2 % to 2.5 % depending on the defined block volume and the encountered cumulative basal area in a forested transit zone. Where possible, the defined RPTV should be compared and validated by field recordings of SW. [ABSTRACT FROM AUTHOR]

Bending moment–curvature relations (M-χ) are useful in many beam modelling approaches. It allows describing in a simple way the mechanical behaviour at the cross-section scale and can be used to model continuous beams as single degree of freedom systems dedicated to perform stochastic analysis. This paper focused on tree mechanical charaterisation and presents an identification method for obtaining bending moment–curvature relationships up to failure from Digital Image Correlation (DIC) and experimental three points bending tests. Due to wood intrinsic variability, the proposed identification method has been validated from a Finite Element (FE) model which aims at simulating precisely the bending response of a cylindrical stem. The refined FE model has been developed based on the geometry and the boundary conditions of the experimentally tested stems. M-χ curves are derived from the FE model and used within a Single Degree Of Freedom (SDOF) approach. At the stem scale, the prediction of the SDOF model (Force-Displacement) is compared to the FE model and lead to a good agreement. Then the approach is used to characterise fresh European beech samples allowing deriving their mechanical properties (longitudinal Young modulus, modulus of elasticity and modulus of rupture). [ABSTRACT FROM AUTHOR]

Wooden structures made of felled trees are used in forested slopes as protection structures against rockfall. A model based on the Discrete Element Method is developed to analyse their response to normal impacts of blocks. The interaction between the blocks and the stems and the stem response are explicitly integrated. After the model calibration using impact tests, a wide range of impact scenarii representative of real configurations is explored. The influence of three parameters (stem diameter, impact velocity and block diameter) is analysed. Three impact types were identified for different ratios R D between the block and stem diameters. For small R D , the impact is limited to a brief contact. For intermediate R D , successive contacts of similar duration are observed. After the last contact, the block is sent back with a large velocity. For large R D , two contacts are observed. The second contact can be assimilated to a quasi-static loading of the stem. In addition, structure damage increases for increasing R D ranging from almost no damage for the first impact type to rupture of the stem for the third type. The largest R D associated with the first impact type is the target configuration that favours block energy decrease and limits structure damage. [ABSTRACT FROM AUTHOR]

A number of studies have shown the ability of the vibration resonant method (VRM) to measure the modulus of elasticity (MOE) and the damping ratio (ξ) of calibrated wooden samples of various dimensions. This method based on free transverse vibrations was also studied to assess the decay extent in small stakes. Nevertheless, a need for investigations was identified in order to apply the method to decayed logs. Both decay and geometrical singularities brought heterogeneity not taken into account by the VRM. The aim of this study was to examine the effects of fungal decay on these two mechanical properties of small-diameter silver fir logs using the VRM. Fifty-five small-diameter logs were monitored over a decay process taking place in a greenhouse located near Grenoble (France). The MOE and ξ were measured in intact and decayed states. The results showed the reliability of the measurements of both properties. ξ was found to be independent of moisture content for wood above the fibre saturation point. The results validated the loss in MOE and the gain in ξ due to fungal activity. Thus, indicators based on the MOE and ξ were proposed and compared to quantify the decay extent. Indicators calculated from the first mode of vibration appeared to be relevant for that purpose. Future work is needed to validate indicators on structural size logs and to compare this quantitative assessment to those obtained by the standard EN 152 that currently serves as a benchmark. [ABSTRACT FROM AUTHOR]

The management of ecological engineering structures making up a timber structure requires periodical evaluations, including the level of decay of the constituent parts of the timber structure. Methods exist to measure the level of decay in the laboratory or in the field. However, they are rarely suitable for the conditions of ecological engineering structures, or give partial information. The aim of this study was to predict two mechanical degradation indicators ( $$Dw_{MOE}$$ and $$Dw_{MOR}$$ ) of silver fir ( Abies alba) wooden strips during microbial decomposition using near-infrared spectroscopy (NIRS). For 1.5 years, the degradation of 180 squared wooden strips, 30 mm wide and 500 mm long, buried in a greenhouse near Grenoble, France (Altitude: 200 m) was monitored. $$Dw_{MOE}$$ and $$Dw_{MOR}$$ were set from the normalized losses in modulus of elasticity (MOE) and modulus of rupture (MOR), two mechanical properties classically used for timber-structure design. A calibration set of 109 samples was selected to build two separate predictive models of $$Dw_{MOE}$$ and $$Dw_{MOR}$$ using partial least square regression. The NIR-based models applied to a validation set of 47 samples indicated good prediction performance. The model has a root mean square error of prediction (RMSEP) of 0.15 and a coefficient of determination ( $$r^2_p$$ ) of 0.79. The $$Dw_{MOR}$$ model has a RMSEP of 0.13 and a $$r^{2}$$ -value of 0.91. These results highlight the considerable potential of NIRS in assessing the extension of decay in wooden logs. [ABSTRACT FROM AUTHOR]

Seismic monitoring of mass movements can significantly help to mitigate the associated hazards; however, the link between event dynamics and the seismic signals generated is not completely understood. To better understand these relationships, we conducted controlled releases of single blocks within a soft-rock (black marls) gully of the Rioux-Bourdoux torrent (French Alps). A total of 28 blocks, with masses ranging from 76 to 472 kg, were used for the experiment. An instrumentation combining video cameras and seismometers was deployed along the travelled path. The video cameras allow reconstructing the trajectories of the blocks and estimating their velocities at the time of the different impacts with the slope. These data are compared to the recorded seismic signals. As the distance between the falling block and the seismic sensors at the time of each impact is known, we were able to determine the associated seismic signal amplitude corrected for propagation and attenuation effects. We compared the velocity, the potential energy lost, the kinetic energy and the momentum of the block at each impact to the true amplitude and the radiated seismic energy. Our results suggest that the amplitude of the seismic signal is correlated to the momentum of the block at the impact. We also found relationships between the potential energy lost, the kinetic energy and the seismic energy radiated by the impacts. Thanks to these relationships, we were able to retrieve the mass and the velocity before impact of each block directly from the seismic signal. Despite high uncertainties, the values found are close to the true values of the masses and the velocities of the blocks. These relationships allow for gaining a better understanding of the physical processes that control the source of high-frequency seismic signals generated by rockfalls. [ABSTRACT FROM AUTHOR]

Highlights: [•] Using a finite element method, we developed a 3D slope stability model. [•] We modelled the impact of 5 mountain forest management scenarios on slope stability. [•] Slope geomorphology plays an essential role in slope stability. [•] The distribution of roots with soil depth significantly affects slope stability. [Copyright &y& Elsevier]

Abstract: The analysis of the influence of the roots on the shear resistance of the soil requires identifying the effect of the different root–soil interaction processes depending on soil and roots properties. For that purpose, a numerical model of direct shear tests of non-rooted and rooted granular soils based on the Discrete Element Method was developed. The soil is modeled as an assembly of locally interacting spheres and the roots are modeled as deformable cylinders in the soil matrix. The model allows accounting for the root tensile loading until breakage, the root bending loading, the root–soil adhesive links until adhesion breakage, the root slippage associated with a frictional resistance at the root–soil interface. The study focuses on identifying the different root–soil interaction mechanisms depending on the soil type. Both frictional and cohesive granular soil types were used in the simulations. The effects of the roots mechanical properties – tensile, bending modulus and root–soil interfacial friction angle – and of the root number were also analyzed for the different soil types. The results first show that the effect of the roots strongly depends on the shear strain for any soil type. For frictional soils, an increasing shear strain induces progressively a pure tensile loading of the roots until slippage of the root–soil interface. For cohesive granular soils, the pure tensile loading of the roots is followed by a progressive breakage of the adhesive root–soil links and by a complete slippage of the roots. The results show that the influence of the root number is significant if the prevailing processes are root tensile loading combined with slippage whereas it is less important if root loading is combined with progressive breakage of the adhesive links for the root configurations explored. Finally, the results show that the shear strain range associated with the different processes strongly depends on the relative rigidities of the roots and soil matrix. The model developed was shown of great interest to analyze the shear resistance of the rooted soil assemblies depending on the shear strain. Such an approach could therefore be used to test the different assumptions done in the analytical models. Developing analytical models of slope stability based on the calculation of the shear resistance of rooted soil depending not only on soil and root properties but also on shear strain intensity also constitutes a perspective for the use of the model developed. [Copyright &y& Elsevier]

Abstract: The accuracy of rockfall trajectory simulations depends to a large extent on the calculation of the rebound of falling boulders on different parts of a slope where rockfalls could occur. The models commonly used for rebound calculation are based on restitution coefficients, which can only be calibrated subjectively in the field. To come up with a robust and objective procedure for rebound calculation, a stochastic impact model associated with an objective field data collection method was developed and tested in this study. The aims of this work were to assess the adequacy of this approach and to evaluate the minimum amount of field data required to obtain simulation results with a satisfactory level of predictability. To achieve these objectives, the rebound calculation procedure developed was integrated into a three-dimensional rockfall simulation model, and the simulated results were compared with those obtained from field rockfall experiments. For rocky slopes, the simulations satisfactorily predict the experimental results. This approach is advantageous because it combines precise modelling of the mechanisms involved in the rebound and of their related variability with an objective field data collection procedure which basically only requires collecting the mean size of soil rocks. The approach proposed in this study therefore constitutes an excellent basis for the objective probabilistic assessment of rockfall hazard. [Copyright &y& Elsevier]

• Hydrological and mechanical reinforcement from vegetation was assessed. • Roots in high elevation forests are slow to recover after soil disturbance. • Roots in gaps in low elevation forests recover quickly after soil disturbance. • Hydrological reinforcement fluctuates more strongly with season than mechanical reinforcement. • Mechanical reinforcement is more reliable than hydrological reinforcement. Natural hazards such as shallow landslides are common phenomena that disturb soil and damage forests. Quantifying the recovery of forest vegetation after a hazard is important for determining the window of susceptibility to new disturbance events, especially at high elevations, where extreme weather events are frequent and the growing season is short. Plant roots can reduce the size of this window on unstable hillslopes, by adding mechanical reinforcement (c r) to soil and increasing its matric suction, termed here as hydrological reinforcement (c h). These data are used in landslide models to calculate the Factor of Safety (FoS) of a hillslope. We calculated temporal variations in c r and c h in naturally regenerated mixed, montane forests in the French Alps. In these closed-canopy forests, open-canopy gaps were present, with understory vegetation comprising herbs, forbs and shrubs. At three altitudes (1400, 1700 and 2000 m), we dug small trenches as proxies for shallow landslide events and calculated c r before soil disturbance in both open gaps and closed forests. Then, using monthly tree root initiation and mortality data measured in rhizotrons, we calculated monthly c r for four years after the disturbance. To compare results with c r , c h was estimated using matric suction data that were measured in trenches at 1400 m for >1 year. Temporal FoS was then calculated using an infinite slope stability model. Results showed that finer, short-lived roots contributed little to soil reinforcement compared to thicker, long-lived roots. After disturbance, mean c r (over the entire soil profile) never fully recovered to the initial value at any site, although >90% recovery was observed in open gaps at 1400 m. Mean c r was slow to recover in closed forests, especially at 2000 m, where only 19% recovery occurred after 41 months. The c h in closed forests was considerable during the summer months, but marked increases in soil water moisture resulted in lower FoS, especially during December to April, when soil was near saturation. As c r changed little throughout the year, it was a more reliable contributor to slope stability. Our results show therefore, that particular attention should be paid to high elevation forests after a disturbance. Also, during the process of recovery, the highly variable soil water dynamics in closed forest can result in seasonal hotspots of vulnerability. Therefore, when tree transpiration is low, our results highlight a need for careful monitoring on steep or unstable slopes, especially in disturbed closed-canopy forests. [ABSTRACT FROM AUTHOR]