Characterization of an internal slope movement structure by hydrogeophysical surveying

International audienceA hydrogeophysical study was carried out by a water controlled injection within a landslide situated on an active part of the La Clapière landslide foot (Alpes Maritimes, France). Coupling of both real-time geophysical and hydrological follow ups allowed the representation and quantification of the surface water drainage in space and time within the slipped mass. Thus, 30% of the injected water is quickly drained by a complex slipping surface meanly situated at 10-m depth. The transit time between injection and outflow of the water allowed an overloading of about 10 m3 (i.e. 10 tons) comparable with classical rain events in the area. This weight and the associated interstitial pressures increase have not led to any movements asking for the origin of the water volumes which could induce destabilizations. This experiment enabled an accurate redefinition of the internal slope structure and the understanding of the dynamics of the slipped mass with a surface hydraulic request

Geophysical survey to estimate the 3D sliding surface and the 4D evolution of the water pressure on part of a deep seated landslide

International audienceGeophysical surveys were conducted on the very unstable front part of the La Clapière landslide in the French Alps (Alpes Maritimes). The electrical resistivity survey was carried out to obtain, for the first time on this deep-seated landslide, 3D information on the slipping surface and the vertical drained faults. Moreover, we planned to follow within time (6 months) the evolution of the saturated zones (presence of gravitational water) and their percolation into the shearing zones. Our 4D results showed the importance of the complex water channelization within the slope and relation to geological discontinuities

Experimental analysis of groundwater flow through a landslide slip surface using natural and artificial water chemical tracers.

International audienceArtificial and natural tracer tests combined with high accurate electronic distancemeter measurements are conducted on a small landslide with a well known slip surface geometry. Outflow yields and chemical contents are monitored for all the experiment duration and they analyzed to estimate the slip surface hydraulic parameters. The main result is that the slip surface acts as a drain for groundwater flows that evacuates interstitial pressures in the slope and brings the sliding mass to be more stable one

Combined three-dimensional electric and seismic tomography study on the Aknes rockslide in western Norway

We present a combined 3-D geoelectric and seismic tomography study conducted on the large Aknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (<1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 k Omega m in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths. (C) 2009 Elsevier B.V. All rights reserved

AD-VITAM : analyse de la vulnérabilité des territoires alpins méditerranéens aux risques naturels(mouvements de terrain)

International audienc

De la crise Messinienne aux forçages climatiques de l’Holocène, l’histoire des mouvements gravitaires de grande ampleur dans la Basse vallée du Var (06) FRANCE

National audienceLes glissements de terrain de grande ampleur (ʋ > 106m3) sont des objets géomorphologiques complexes dont la cinématique propose des contraintes temporelles à mi-chemin entre les temps géologiques et les temps Humains. Le principal défi scientifique repose sur la compréhension des temps de récurrences et d’accélération des masses, et les dernières études (Zerathe, 2013) montrent qu’il existe un intervalle de temps qui fluctuent entre du temps long (400ka) et du temps court (4ka). Ces mouvements de masse montrent aussi une certaine complexité dans leurs dynamiques, avec pour une même configuration géologique et géomorphologique des glissements lents et aussi des « collapse » rocheux. En effet, l’inertie d’un versant géologique est une fonction complexe de facteurs et autres sollicitations plus ou moins déterminantes. Ces facteurs sont de deux ordres, internes et externes, et les problématiques liées à ces glissements viennent de leur forte variabilité respective. Dans le cadre de ce travail, nous avons étudié les glissements de grande ampleur du front subalpin méridional qui représente un territoire à enjeux car densément peuplé (région Provence- Alpes-Côte-d’Azur), avec notamment un focus sur les secteurs de Carros et du Broc situés dans labasse vallée du Var. Cette étude est basée sur l’articulation d’une approche pluridisciplinaire mettant en inter corrélation l’utilisation de système d’information géographique (SIG), de la géomorphologie de terrain et de campagnes de mesure de résistivité électrique. Le territoire d’étude, n’a jamais été analysé pour les glissements qui s’y produisent, et l’ensemble des investigations menées montrent que les secteurs de Carros et du Broc correspondent à un secteur extrêmement perturbé, non pas depuis l’Holocène, mais depuis le Messinien (5.5Ma). Les dernières études menées sur le secteur ont permis l’identification d’une vingtaine de glissements de terrain tous datés de l’évènement 4.2 (Zerathe, 2013), avec des volumes compris entre 10*106 et 30*106 m3, sauf que ce nouveau secteur présente des masses comprises entre 30*107 m3 et 10*109 m3. En effet, l’ensemble des travaux présentés dans ce rapport ont mis en évidence la présence d’une succession de glissements de terrain dans la zone de Carros et ont abouti à une interprétation chronologique de son versant depuis la crise Messinienne. Ils ont également permis de définir plus précisément les limites latérales du glissement voisin du Broc. Enfin en mettant en relation le coefficient d’inertie du glissement de Carros et le rôle des facteurs interne et externes notamment le climat, nous proposons une analyse sur le risque lié à ce glissement. Cette étude permet la mise en place de nouvelles problématiques (comportement de versant, climat…) qui peuvent s’étendre à l’ensemble du front subalpin méridional et dégager de nouvelles perspectives dans l’étude des glissements de terrain de grande ampleur

Alpine giant landslide and the Messinian Salinity Crisis (MSC) (Southern alps, France)

International audienceThe research work we have been doing in the laboratory has allowed us for more than a decade to map and study tens of large landslides in the southern part of the Alps. During this cartographic research, we identified a new zone which revealed the presence of a landslide of incomparable size and which uses a triggering process linked to the processes of eustatic variations and fluvial over-erosion. In the southern branch of the Castellane arc in the S-E of France the large-scale landslides are numerous. This conditions are due to the particular tectonic and geological conditions but none of them has an impact as visible in the landscape than that of the Sinne small valley's DSGSD located in the lower valley of the Var. It is characterized in the current time by (1) a deep and large double crested ridge which is typical of DSGSD (2), the presence of 3 DSLs on the slope and (3) the Var (river) deviation in the bottom of the slopes. These observations have been revealed by field campaigns, geological maps analysis and the use of different GIS tools (DEM, slope and roughness maps and topographic profiles). We related the triggering of this DSGSD and the annexed gravitational phenomenon (DSLs) to the Messinian Salinity Crisis (MSC 5.96-5.33 My) area due to the marine regression (canyon creation) and transgression (impoundment of valleys) it engendered in the Mediterranean rivers. Indeed, the first elements related to gravitational objects in the zone are the Carros breccia formation, a consolidated slopes formation. The Carros breccia are synorogenic of the Castellane arc setting up during the Miocene and its upper level contain a Pliocene ages microfauna, so their deposits are contemporaneous to the Messenian crisis (MSC). From this knowledge we propose in this abstract paper to trace the implementation of the Sinne small valley's DSGSD from the MSC to present days, focusing on the main stages and their implications in the lower Var valley

Conceptual Model Explaining Territorial Interactions Based On Core Vulnerability Components and GIS Based Methodology To Map Territorial Vulnerability

International audienceThe area of risk management in geo-hazards is fairly complex due to extreme and unpredictable climatic events. Studies of natural hazard, risk and vulnerability have increased significantly in the last 20-25 years leading to a marked improvements in our understanding of core issues at the heart of these concepts. Emergence of vulnerability is the core concept for understanding what it is about the condition of people and their socio-economic and environmental interactions that enables hazard to become a disaster. This notion entails the predisposition of specific system, society, land, community or even region to be affected and do not conquer with disaster. Being dynamic concept vulnerability assessment is varying across temporal and spatial scale. Generally the term used to assess and manage risk that a system undergoes from negative change owing perturbation. This study is conducted in Bar-Sur-Loup located in south-east of France. The aim of study is to identify territorial components/issues that play significant role in formulating territorial vulnerability, realize territorial vulnerability by developing conceptual model to risk of rainfall induces shallow landslide and application of GIS based methodology to map vulnerability of teritorial elements at risk. The choice of this territory is vital due to its geographic location, increased urbanization, industrialization and limited access to transportation system. In this regard the realization of territorial vulnerability seems very complex and interrelated and needs to be considered different factors based on their quantitative and qualitative analysis. We have identified and realized 11 territorial issues and map each issue with respect to risk of rainfall induced shallow landslide. These issues include, issues of population (density of population), vulnerability of built environment (residential building and industries), and object oriented critical infrastructure (hospitals, emergency services, mayor office, indication of safe zones), network oriented infrastructure (roads, tunnels, bridges, electricity etc)