14 research outputs found

    Changements climatiques et risques naturels dans les Alpes

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    Sous l’effet des changements climatiques, les territoires de montagne alpins enregistrent des évolutions rapides et manifestes qui suscitent l’attention croissante des populations, des scientifiques et des gestionnaires. Pour mieux faire face aux aléas et aux vulnérabilités spécifiques à ces territoires, l’adaptation de la prévention des risques naturels aux changements climatiques est aujourd’hui prescrite par les politiques publiques nationales et communautaires en vigueur dans l’arc alpin. Cet article passe en revue les progrès récents dans la connaissance des changements perçus, mesurés et projetés à l’échelle des Alpes françaises dans 1) les paramètres climatiques, 2) la cryosphère, les hydrosystèmes et la morphodynamique des versants, 3) les aléas naturels et 4) les risques induits. Cette synthèse renouvelée fournit un aperçu des résultats des projets de recherche, de coopération et de capitalisation sur ces thématiques, acquis pour la plupart au cours de la période de programmation 2007-2013, en s’appuyant sur des bases de données et plates-formes de connaissances développées par différents opérateurs scientifiques et techniques dans le cadre de l’arc alpin. Elle est illustrée par une méta-analyse d’exemples déjà publiés de chroniques d’activité d’aléas hydro-gravitaires, accompagnées des paramètres climatiques identifiés comme « prédicteurs ».Under the effects of climate change, Alpine mountainous regions are undergoing fast and well-perceptible evolutions, which are attracting the growing attention of people, scientists and managers. To cope better with the hazards and vulnerabilities specific to these territories, the current national and European public policies in the Alpine countries now prescribe adapting natural hazard prevention to climate change. This paper provides a review of recent advances in knowledge on the perceived, measured and projected changes in i) climate patterns, ii) the cryosphere, hydrosystems and geomorphological dynamics on Alpine slopes, and iii) natural hazard evolution and induced risks at the scale of the French Alps. We give a brief overview of new results achieved by research, cooperation and capitalisation projects in these thematic fields during the programme period 2007-2013, which are available on databases, thematic knowledge platforms and observatories developed by different scientific and technical operators in the larger framework of the European Alpine arc. We illustrate this renewed synthesis by published examples of hydro-gravitational hazard activity chronicles, along with climate patterns identified as “predictors”

    Déformation et rupture des versants en terrain métamorphique anisotrope. Apport de l' étude des Ruines de Séchilienne.

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    The break or rock slopes is causing major risks in all mountainous regions. The anisotropic metamorphic rocks show all the variety of strain mechanisms, which are analysed in the first part of that work, from European examples. The study of the “Ruines de Sechilienne” case, that mobilises several scientific disciplines and is presented in the second part, shows particular interest because of the phenomenon's complexity and because of the importance of the investigation and monitoring actions realised during one decade (1988-1998). The geological approach permitted a precise description of the slope's structure at various scales. The geo-mechanical approach consisted in elaborating a simplified structural model and testing numerically various hypotheses with the distinct elements methods. This permitted us to propose a strain mechanism qualitatively fitting with the geological observations and the displacements measured. This strain mechanism, controlled by the main pre-existing discontinuities families, induces stress concentrations and tensile stresses that may cause a progressive and non reversible damage of the slope (“progressive break”). Finally, the hydro-geological approach focused on studying the outflow, the temperature, the geo-chemical and isotopic properties of the surface and of the underground water sources, in order to determine their origin and their type of transfer. Coupling these three approaches made it possible to propose a global hydro-mechanical model able to explain the influence of the rain regime on the strain rate of the slope. Two research directions are proposed to investigate the possible break geometry. Nevertheless, forecasting the date of the beak remains out of reach. Finally, the observation of the south slope of la Toura, at Saint-Christophe-en-Oisans, suggests that this “progressive break” mechanism might fit to other slopes than the “Ruines de Sechilienne”La rupture des versants rocheux est une source de risque majeur dans toutes les régions montagneuses. Les terrains métamorphiques anisotropes présentent tout l'éventail des mécanismes de déformation, lesquels sont analysés dans la première partie de ce travail à partir d'exemples européens. L'étude pluridisciplinaire des" Ruines de Séchilienne ", présentée en seconde partie, revêt un intérêt particulier du fait de la complexité du phénomène et par l'ampleur des moyens d'auscultation et de surveillance mis en oeuvre pendant une décennie (1988-1998). L'approche géologique a permis une description fine de la structure du versant à toutes les échelles. L'approche géomécanique a consisté à élaborer un modèle structural simplifié et à tester numériquement différentes hypothèses par la méthodes des éléments distincts. Ceci nous autorise à proposer un mécanisme en accord qualitatif avec les observations de terrain et les mesures de déplacement. Ce mécanisme de "rupture interne ", contrôlé par les grandes familles de discontinuités préexistantes, induit des concentrations de contrainte et des efforts de traction pouvant provoquer un endormmagement progressif et irréversible du massif. Enfin, l'approche hydrogéologique s'est attachée à analyser le débit, la température ainsi que la composition chimique et isotopique des eaux du massif, pour préciser leur origine et leur mode d'écoulement. En couplant les trois approches, on a pu proposer un modèle hydromécanique global expliquant l'influence de la pluviométrie sur le rythme de la déformation du versant. Des pistes sont avancées pour étudier la géométrie probable de la rupture. Par contre, la prévision de la date de la rupture reste hors d'atteinte. Enfin, l'observation du versant sud de la Toura, à Saint Christophe- en-Oisans, suggère que le mécanisme de " rupture interne" pourrait s'appliquer à d'autres sites que Séchilienne. Mots clés Versants - massifs rocheux - déformatio

    Rock fall predictibility - A probabilistic approach combining historical and geomechanical studies.

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    20 pages; Abstract and figure captions in English.International audienceForecasting the date of a rock fall is sometimes possible in the case of monitored moving rock masses. But for most potentially unstable slopes, mechanical methods generally fail to estimate their probability of rupture as a function of time. This limitation arises mainly from ignorance of the temporal evolution of the factors which determine the stability. Nevertheless, natural risk management for land use demands an attempt to quantify rock fall hazards like other natural hazards (earthquakes, floods...), for which the probability of occurrence of given intensity as a function of time is estimated. Recent studies show that statistical analysis of historical rock falls permits, under certain conditions, an estimation of the probability of occurrence of a rock fall in a given volume class, during a given period. The HGP method (Historical, Geomechanical and Probabilistic) associates this estimation of the future rock fall activity on the study zone with a hazard grading of detected potentially unstable rock masses. It is a first step toward the evaluation of the individual probability of rupture of a specific instability, as a function of time. Examples of historical and geomechanical studies are presented and the meaning of the computed probabilities is discussed in terms of a fictious example

    Rock fall hazard assessment: from qualitative to quantitative failure probability

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    5 pages.International audienceA new method (HGP) is proposed to estimate the failure probability of potentially unstable rock masses in a homogenous area, as a function of time. It is based on both geomechanical and historical approaches: the first one is aimed to classify the rock masses according to their relative failure probability, but the time factor can not be approached by a mechanical analysis; the second one to estimate the mean expected rock fall number in the studied area, for the considered period and for different volume classes. This rock fall frequency can be estimated from a rock fall inventory, directly or indirectly using a power law for the volume distribution. A relation between these frequencies and the erosion rate is established, which allows to approach the former from a paleogeographical study. The failure probabilities can be calculated from the expected total rock fall number in the area, and the numbers of potential rock falls with different relative probabilities. The method is applied to the case of the Grenoble area. Development of rock fall data basis is needed to improve both geomechanical and historical approaches

    Rock fall hazard: from expert opinion to quantitative evaluation

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    8 pages; Abstract and figure captions in English.International audienceA new method is proposed to estimate the failure probability of potentially unstable rock volumes, as a function of time [1]. It is based on both geomechanical and morphodynamical approaches: the first one is aimed to classify the rock volumes according to their relative failure probability [2]; the second one to estimate the mean expected rock fall number in the whole slope, for the considered period and for different volume classes. This number can be obtained from a rock fall inventory [3] or from the slope erosion rate. The method is applied to calcareous cliffs in the Grenoble area

    Application of geophysical methods for the investigation of the large gravitational mass movement of SĂ©chilienne, France

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    International audienceSeveral geophysical techniques (electromagnetic profiling, electrical tomography, seismic refraction tomography, and spontaneous potential and seismic noise measurement) were applied in the investigation of the large gravitational mass movement of Séchilienne. France. The aim of this study was to test the ability of these methods to characterize and delineate the rock mass affected by this complex movement in mica schists, whose lateral and vertical limits are still uncertain. A major observation of this study is that all the zones strongly deformed (previously and at present) by the movement are characterized by high electrical resistivity values (>3 kΩ·m), in contrast to the undisturbed mass, which exhibits resistivity values between a few hundred and 1 kΩ·m. As shown by the surface observations and the seismic results, this resistivity increase is due to a high degree of fracturing associated with the creation of air-filled voids inside the mass. Other geophysical techniques were tested along a horizontal transect through the movement, and an outstanding coherency appeared between the geophysical anomalies and the displacement rate curve. These preliminary results illustrate the benefits of combined geophysical techniques for characterizing the rock mass involved in the movement. Results also suggest that monitoring the evolution of the rock mass movement with time-lapse geophysical surveys could be beneficial
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