Franck Bourrier - Academia.edu (original) (raw)

Papers by Franck Bourrier

Quelle forêt pour les hommes ?

Dans ce livre, le Cemagref, institut de recherche en sciences et technologies pour l'environn... more Dans ce livre, le Cemagref, institut de recherche en sciences et technologies pour l'environnement, fait le point de ses travaux scientifiques menés sur la forêt. Pourquoi une recherche sur la forêt ? D'abord parce qu'elle occupe plus d'un quart de la surface de notre territoire métropolitain. Ensuite, parce qu'elle fournit tout un ensemble de services variés et, pour certains, essentiels à notre société : la production de bois, la protection contre les aléas naturels, la chasse et la promenade en sont de très bonnes illustrations. Enfin, parce qu'elle constitue un milieu naturel riche en biodiversité et occupe une place notable dans les débats sur les enjeux environnementaux. Forts de ce constat, les chercheurs et experts du Cemagref apportent leur contribution en abordant des problématiques majeures pour la gestion durable des forêts : état et dynamique de la forêt, de sa biodiversité, vulnérabilité des forêts aux changements globaux et stratégies d'ada...

International audienceRockfall propagation simulation models are widely used for assessing rockfa... more International audienceRockfall 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 ...

At present, a quantitative basis for delimiting realistic rockfall runout zones on the basis of t... more 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 (RP T V) to separate "realistic" from "unrealistic" simulated rockfall runouts. We therefore compared reach probability values (P reach) simulated with Rockyfor3D for 458 mapped, fresh rockfall blocks (silent witnesses SW) on 18 different sites with a volume >= 0.05 m 3 and estimated occurrence frequencies up to 300 years. We analysed which block, slope and forest characteristics influenced P reach 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 P reach values are between >1% and approximately 3%. We conclude that RP T V 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 RP T V should be compared and validated by field recordings of SW .

Exploring root reinforcement mechanisms using numerical simulations of rooted soils

In this study, we use a numerical model of direct shear tests of rooted and non rooted soils to a... more In this study, we use a numerical model of direct shear tests of rooted and non rooted soils to analyze the relative importance of these four root failure modes depending on the root and on the soil types. The numerical simulations were held using the Discrete Element Method (DEM). The DEM is an efficient modelling approach extensively used in the field of soil mechanics. This approach allows modelling the soil as an assembly of rigid locally deformable spheres interacting at their contact points. A specific approach for modelling the roots was also developed.

A discrete element modelling approach for block impacts on trees

Although recognised since centuries, the protective function of forests against rockfall continuo... more Although recognised since centuries, the protective function of forests against rockfall continuously gains importance in densely populated areas and along traffic ways throughout the Alps. Maintenance of these forests, in order to optimise or sustain this protective function, which accounts for all types of protective measures, has become an important task for those responsible for managing those forests. Common practices in the European Alps show that a considerable part of the trees felled in rockfall (and also avalanche) protection forests are left in place, in oblique position to the steepest slope direction. This is being done to compensate for the temporary loss of protection due to the reduced forest in the felling area. These fellings are, however, necessary to promote natural regeneration of the, in many cases, over-aged protection forests. Although being a widely used technique, there is little objective information on the efficacy of these trees felled across the slope (...

Combining spatial and thematic uncertainty and sensitivity analysis for mountain natural hazard assessment

1 Université Grenoble Alpes, Irstea, UR ETGR, Snow avalanches Engineering and Torrent Control Res... more 1 Université Grenoble Alpes, Irstea, UR ETGR, Snow avalanches Engineering and Torrent Control Research Unit – 2 rue de la papeterie, BP 76, F-38402 Saint-Martin-d’Hères Cedex, France. 2 Université Grenoble Alpes, Irstea, UR EMGR, Mountain Ecosystems Research Unit – 2 rue de la papeterie, BP 76, F-38402 Saint-Martin-d’Hères Cedex, France. 3 Probayes – 82 allée Galilée, F-38330 Montbonnot, France. 4 Sintegra – 11 chemin des Prés, 38241 Meylan, France.

Block propagation models are routinely used for the quantitative assessment of rockfall hazard. I... more Block propagation models are routinely used for the quantitative assessment of rockfall hazard. In these models, one of the major difficulties is the development of physically consistent and field applicable approaches to model the interaction between the block and the natural terrain. For most of propagation models, a thorough calibration of the input parameters is not available over the wide range of configurations encountered in practice. Consequently, the parameters choice is strongly depending on expert knowledge. In addition, most of models exhibit substantial sensitivity to some parameters, i.e. small changes of these parameters entail large differences in the simulation results.

To compensate for the temporary loss of protection due to the reduced forest stand density in the... more To compensate for the temporary loss of protection due to the reduced forest stand density in the felling area, a considerable part of the trees felled in rock fall protection forests are left in place. Although being a widely used technique, there is little objective information on the efficacy of these trees felled transverse to the slope. To quantify the protective function of such structures, full scale rock fall tests on a protective structure made with trees felled transverse to the slope were carried out. The results show significant energy losses when impacting the structure. However, the rock energy significantly increases again after the structure if the rock is not stopped. Consequently, a larger number of transverse tree trunks is required to stop all rocks and, to increase the number of stopped rocks, the structure height has to be determined in relation to the rock size which promotes, in most cases, structures made of stacked trees.

Rock Mechanics and Rock Engineering, 2021

Block propagation models have been used for years for rockfall hazard assessment. However, the ca... more Block propagation models have been used for years for rockfall hazard assessment. However, the calibration of model parameters that allow the simulations to accurately predict rockfall trajectories for a given study site remains a key issue. This research aims at investigating the predictive capabilities of block propagation models after a preliminary calibration phase. It is focused on models integrating the shape of blocks since, despite their sound physical bases, they remain less used than lumped-mass approaches due to their more recent popularisation. Benefiting from both a recently built model integrating block shape, usable in 2D and 3D, and from recent experimental results at the slope scale, we first performed a calibration based on the use of the 2D model, and then we evaluated the predictive capabilities of the calibrated model in 2D and in 3D using the remaining part of the experimental results. The calibrated model simulations predict the main characteristics of the propagation, that is the preferential deposit zones and the ranges of velocities at specific locations. Good matches between simulations and experimental results in both the calibration and validation phases emphasizes the wide applicability of the model: after a calibration phase on a sufficient number of different soil types, the model may be used in a predictive manner. The good match between 2D and 3D simulations also highlights the ease-of-use of the model for field applications, as the 2D

Rock Mechanics and Rock Engineering, 2017

In order to model rockfall on forested slopes, we developed a trajectory rockfall model based on ... more In order to model rockfall on forested slopes, we developed a trajectory rockfall model based on the discrete element method (DEM). This model is able to take the complex mechanical processes at work during an impact into account (large deformations, complex contact conditions) and can explicitly simulate block/soil, block/tree contacts as well as contacts between neighbouring trees. In this paper, we describe the DEM model developed and we use it to assess the protective effect of different types of forest. In addition, we compared it with a more classical rockfall simulation model. The results highlight that forests can significantly reduce rockfall hazard and that the spatial structure of coppice forests has to be taken into account in rockfall simulations in order to avoid overestimating the protective role of these forest structures against rockfall hazard. In addition, the protective role of the forests is mainly influenced by the basal area. Finally, the advantages and limitations of the DEM model were compared with classical rockfall modelling approaches.

Revue Française de Géotechnique, 2020

Une étude comparative de résultats d’essais de lâcher de blocs et de simulations prédictives four... more Une étude comparative de résultats d’essais de lâcher de blocs et de simulations prédictives fournies par 18 participants a été menée dans l’optique d’évaluer les capacités prédictives des études trajectographiques. La propagation d’une centaine de blocs rocheux sur deux zones de propagation, comportant des discontinuités topographiques et des configurations favorisant la propagation des blocs selon des modes assimilables à du roulement, a été analysée au niveau de zones spécifiques, dénommées écrans d’évaluation. Une variabilité significative des vitesses de passage des blocs a été mesurée au niveau des écrans d’évaluation et des distributions bimodales de ces vitesses ont été observées pour les écrans situés en aval de discontinuités topographiques. L’analyse comparative des résultats expérimentaux et des études trajectographiques met en évidence une grande variabilité des résultats de simulation fournis et, par conséquent, de fortes incertitudes associées aux prévisions, pour ces...

European Journal of Environmental and Civil Engineering, 2018

Wooden structures made of felled trees are used in forested slopes as protection structures again... more 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 RD between the block and stem diameters. For small RD , the impact is limited to a brief contact. For intermediate RD, successive contacts of similar duration are observed. After the last contact, the block is sent back with a large velocity. For large RD, 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 RD ranging from almost no damage for the first impact type to rupture of the stem for the third type. The largest RD associated with the first impact type is the target configuration that favors block energy decrease and limits structure damage.

Applied Geography, 2020

Forest covers 40% of the European Alpine region and contributes to the protection of human beings... more 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.

Global and Planetary Change, 2019

Due to intense urban sprawl in rockfall-prone areas, a precise rockfall risk assessment has becom... more 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-19 th 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.

Geomorphology, 2017

The use of dynamic computational methods has become indispensable for the assessment of rockfall ... more The use of dynamic computational methods has become indispensable for the assessment of rockfall hazards and the quantification of uncertainties. Although a substantial number of models with various degrees of complexity has become available over the past few years, models have only rarely been parameterized against observations, especially because long-term records of rockfalls have proven to be scarce and typically incomplete. On forested slopes, tree-ring analyses may help to fill this gap, as they have been shown to provide annually resolved data on past rockfall activity over long periods. In this paper, a total of 1495 rockfall scars recorded on the stem surface of 1004 trees have been studied at a site in the Vercors massif (French Alps) to calibrate the 3D process based simulation model RockyFor3D. Uncertainties related to the choice of parameters accounting for energy dissipation and surface roughness have been investigated in detail. Because of the lack of reliable data, these parameters typically are estimated based on expert judgments, despite the fact that they have significant impacts on runout distances and bounce height. We demonstrate that slight variations in roughness can indeed strongly affect the performance of runout modeling and that the decreasing downward gradient, observed in field data, is properly reproduced only if reduced roughness (b10 cm) enables blocks to reach the distal parts of the study plot. With respect to the height of impacts, our results also reveal that differences between simulations and observations can indeed be minimized if softer soil types are preferred during simulation, as they typically limit bouncing. We conclude that field-based dendrogeomorphic approaches represent an objective tool to improve rockfall simulations and to enhance our understanding of parameterization, which is of key importance for process dynamics and thus hazard zoning.

Forest Ecology and Management, 2016

The role of forests in the mitigation of natural hazards has been repeatedly demonstrated. The pr... more The role of forests in the mitigation of natural hazards has been repeatedly demonstrated. The protective effect of mountain forests against rockfalls has especially been pointed out because it can constitute a natural and cost-effective protection measure in many situations. However, this particular ecosystem service may substantially differ according to the structure and the composition of the forest. Until now, the rockfall protection capability has always been studied at a local scale with only few forest types. Moreover, the comparison of the protective effect of the different forest types studied remains difficult because different methods and indicators were used. For the same reasons, it is not possible to draw conclusions about the influence of biological and structural diversities on the protection capabilities of forests from former works. The aims of this study were (1) to quantitatively assess the protective effect of forests at the French Alps scale and build a classification based on the protection capability, (2) to compare the protective effect of the different forest types present in the French Alps and (3) to analyse the relations between the protective effect and the forest diversity in terms of stand structure and tree composition. For this purpose, the model Rockyfor3D was used to simulate the propagation of rocks on 3886 different forest plots spread over the whole French Alps. Quantitative indicators characterizing the protective effect of each forest plot were then calculated from the simulation results and used to perform the different analyses. Our results emphasized the importance of taking into account the length of forest in the maximum slope direction for an accurate assessment of the protective effect. Thus, the minimum length of forest to get a reduction of 99% of the rockfall hazard was chosen as indicator to compare protective effect between forests. Using this indicator, half of the French Alpine forests presented a high level of protection after a short forested slope (190 m). A decreasing gradient in the protection capabilities was observed from forest types dominated by broadleaved species to those dominated by conifer species. Moreover, considering an equivalent proportion of conifers, stands dominated by shade-tolerant tree species showed better ability to reduce rockfall hazard. Finally, our study highlighted that a high biodiversity and a structural heterogeneity within the forest have a positive effect on the reduction of rockfalls hazard.

Ecological Indicators, 2016

Natural hazards are frequent in mountain areas where they regularly cause casualties and damages ... more Natural hazards are frequent in mountain areas where they regularly cause casualties and damages to human infrastructures. Mountain forests contribute in mitigating these hazards, in particular rockfalls. Assessing the protective effect of a forest against rockfall is a difficult task for both forest managers and rockfall experts. Accurate and simple tools are therefore required to efficiently evaluate the level of protection that results from the presence of forest. This study defines three novel indicators to quantify the protective effect of forests against rockfalls, regarding 1) the reduction of the frequency of rockfalls, 2) the reduction of their maximum intensity, and 3) the combination of the reduction of the frequency and the energy of the rocks. The first two indicators are relevant for rockfall experts whereas the third is mostly interesting for foresters as it summarizes the protective effect of forest. The Rockyfor3D model was adapted and used to simulate rockfalls propagation on 3886 different forest stands located in all the French Alps. The results of the simulations were used to calculate the three indicators for each forest stand. Finally, the relations between the forest structures and compositions and the indicators values were investigated. Our principal result shows that only three forest characteristics are required to accurately predict the indicators and evaluate the protective level of a forest against rockfall. The two first variables correspond to the basal area and the mean diameter at breast height (DBH) of the forest stand which are two parameters commonly used by forest managers. The third characteristic is the length of forest in the maximum slope direction which can be computed with a geographic information system (GIS). The method proposed in this study is easily reproducible and is suitable to evaluate the protective effect of European mountain forests at different scales. At local scale, the proposed indicators can enrich rockfall studies in which forests are usually set aside to simplify the evaluation. Moreover, the indicators may find direct applications with foresters by allowing them to identify the protective level of their forest and consequently to adapt their management. Finally, the indicators are convenient to perform spatial analysis and produce maps of the protective effect of mountain forests that could find many applications in land settlement or evaluation of ecosystem services.

Risk, Reliability and Safety: Innovating Theory and Practice, 2016

Natural phenomena in mountains such as rockfalls cause severe damage to exposed population and as... more Natural phenomena in mountains such as rockfalls cause severe damage to exposed population and assets. Numerical modelling is widely used to assess the hazard level (combination of phenomenon's intensity and frequency). This paper describes how innovative methods and tools are used to assess both thematic (related to the characteristics of the rockfalls) and spatial (related to the local topography) data uncertainties considering rockfall propagation. Those uncertainties are propagated in rockfall simulation models using classical Monte Carlo (probabilistic) and hybrid (possibilistic) approaches. Uncertainties related to the altimetric information, through Digital Elevation Model (DEM), are modelled by random fields using the new ModTer software, that can produce terrain simulations. This paper compares both uncertainty propagation approaches, taking into account spatialized uncertain variables, and proposes a sensitivity analysis describing the contribution of DEM variability on the global uncertainty.

Landslides, 2017

The objective of this research was to use numerical models based on mechanical approaches to impr... more The objective of this research was to use numerical models based on mechanical approaches to improve the integration of the protective role of forests against rockfall into block propagation models. A model based on the Discrete Element Method (DEM) was developed to take into account the complex mechanical processes involved during the impact of a block on a tree. This modelling approach requires the definition of many input parameters and cannot be directly integrated into block propagation models. A global sensitivity analysis identified the leading parameters of the block kinematics after impact (i.e. block energy reduction, trajectory changes and rotational velocity): the impact velocity, the tree diameter and the impact point horizontal location (i.e. eccentricity). Comparisons with the previous experimental and numerical studies of block impacts on trees demonstrated the applicability of the DEM model and showed some of the limitations of earlier approaches. Our sensitivity analysis highlights the significant influence of the impact velocity on the reduction of the block's kinetic energy. Previous approaches usually also focus on parameters such as impact height, impact vertical incidence and tree species, whose importance is only minor according to the present results. This suggests that the integration of forest effects into block propagation models could be both improved and simplified.

Quelle forêt pour les hommes ?

Exploring root reinforcement mechanisms using numerical simulations of rooted soils

A discrete element modelling approach for block impacts on trees

Combining spatial and thematic uncertainty and sensitivity analysis for mountain natural hazard assessment

Rock Mechanics and Rock Engineering, 2021

Rock Mechanics and Rock Engineering, 2017

Revue Française de Géotechnique, 2020

European Journal of Environmental and Civil Engineering, 2018

Applied Geography, 2020

Global and Planetary Change, 2019

Geomorphology, 2017

Forest Ecology and Management, 2016

Ecological Indicators, 2016

Risk, Reliability and Safety: Innovating Theory and Practice, 2016

Landslides, 2017

The use of cable yarding systems constitutes an interesting solution for steep-slope harvesting i... more The use of cable yarding systems constitutes an interesting solution for steep-slope harvesting in mountain forests. However, it requires many specific skills for both forest managers and operators. The purpose of this study is to provide a numerical tool to help managers in planning cable yarding projects and to facilitate the set-up of a cable line by operators. A model was developed and coded in Python language which allows fast calculation of the load path and tensile forces along the cable for different 3D configurations. This model uses theoretical equations initially developed for the mechanical design of cable structures. Its applicability in forestry was validated by field experiments. An additional module using high resolution digital terrain model optimizes the cable layout and automatically places required intermediate supports. If geographic information about timber volume is available, it also allows crossing technical possibility with harvesting potential. From the manager point of view, this tool presents significant advantages for the forest operation planning. Moreover, productivity of operators can be improved with the technical outputs of this model which enable to shorten the time spent in setting-up a cable line and so to reduce the total cost of a cable project.