Using geotypes for landslide hazard assessment and mapping: a coupled field and GIS-based method

Switzerland is exceptionally subjected to landslides; indeed, about 10% of its area is considered as unstable. Making this observation, its Department of the Environment (BAFU) introduces in 1997 a method to realize landslide hazard maps. It is routinely used but, like most of the methods applied in Europe to map unstable areas, it is mainly based on the signs of previous or current phenomena (geomorphologic mapping, archive consultation, etc.) even though instabilities can appear where there is nothing to show that they existed earlier. Furthermore, the transcription from the geomorphologic map to the hazard map can vary according to the geologist or the geographer who realizes it: this method is affected by a certain lack of transparency. The aim of this project is to introduce the bedrock of a new method for landslide hazard mapping; based on instability predisposition assessment, it involves the designation of main factors for landslide susceptibility, their integration in a GIS to calculate a landslide predisposition index and the implementation of new methods to evaluate these factors; to be competitive, these processes have to be both cheap and quick. To identify the most important parameters to consider for assessing slope stability, we chose a large panel of topographic, geomechanic and hydraulic parameters and tested their importance by calculating safety factors on theoretical landslides using Geostudio 2007®; thus, we could determine that slope, cohesion, hydraulic conductivity and saturation play an important role in soil stability. After showing that cohesion and hydraulic conductivity of loose materials are strongly linked to their granulometry and plasticity index, we implemented two new field tests, one based on teledetection and one coupled sedimentometric and blue methylen test to evaluate these parameters. From these data, we could deduce approximated values of maximum cohesion and saturated hydraulic conductivity. The hydraulic conductivity of fractured rocks was obtained from the analysis of their geometrical properties (fractures density, aperture size and orientation). The other factors were extracted from DEM and hydrologic mapping. Then, we merged these parameters into three groups corresponding to three families of factors (gravitational, hydrodynamic I based on hydraulic conductivity contrast between superficial loose material and hard rock substratum and hydrodynamic II related to susceptibility to suffosion). We added a fourth factor related to the predisposition of the geotype (new classification for geologic formations, based on genetic standards for loose material and on lithologic standards for hard rock) to slope instability processes: the latter enabled us to integrate attributes proper to each geotypes (over-consolidation for ground moraines, stratifications for glaciolacustrine deposits, etc...) and which would be long and complex to integrate to a GIS. Afterward, we implemented an ArcGis® toolbox allowing to obtain automatically cohesion, hydraulic conductivity, saturation and slope from field parameters (granulometry, plasticity, fracturation, geomorphology and drainage) and DEM and to calculate geotype, gravitational, hydrodynamic I and hydrodynamic II factors; combining them, we led to a landslide susceptibility index. To know the relative importance of these four factors, we tried different weightings on four areas in different geologic contexts of Switzerland (flysch, molasse, crystallin) and of different sizes (from 0.1 to 2.5 km2). Finally, we applied this methodology (from field survey to GIS operations) to five other sites in Switzerland to check its validity: in flysch and molassic contexts, more than 90% of actual landslides were classified as “very susceptible to slope instability”. In Triassic area, more than 50% of the actual unstable areas were classified as "stable", which is not satisfying:in this case, it should be necessary to integrate another layer of data to take into account dissolution and plasticity of Triassic formations. The main assets of this new method are that the integration of field measurements in GIS provides for a good transparency in the landslide susceptibility index attribution and that the use of geotypes classification enables to use this method in most of European contexts

Apport des SIG pour la modélisation géologique tridimensionnelle: application à l'évaluation de la prédisposition aux instabilités de versants

The aim of this project is to introduce the bedrock of a new method for landslide hazard mapping; based on instability predisposition assessment, it involves the designation of main factors for landslide susceptibility, their integration in a GIS to calculate a landslide predisposition index and the implementation of new methods to evaluate these factors; to be competitive, these process will have to be both cheap and quick

Modelling the behaviour of a large landslide with respect to hydrogeological and geomechanical parameter heterogeneity

Thanks to a sophisticated transient hydrogeological modelling allowing the determination of the pore pressure fields in La Frasse landslide mass during a crisis, it has been possible to model the mechanical behaviour of the slide and obtain results that prove to be similar to the monitored data, in terms of peak velocity, distribution of velocity with time and space and total displacements. Such results are reached only when appropriate constitutive modelling laws are used, and when geotechnical tests supply all the required parameters. The main results concern the potential effect of a drainage system during a crisis, like the one experienced in 1994. It can include vertical boreholes equipped with pumps or drains drilled from a gallery. The draining system reduces horizontal displacements down to 5% of the values modelled during the crisis. This effect, which appears to extend over a large width, will be even more significant if the boreholes discharge the drained water into the gallery, due to its extension in the presently stabilised landslide mass below the active zone. The modelling tools developed for La Frasse landslide thus provide all the necessary information to optimise the drainage schem

Spécificités hydrogéologiques des hautes vallées alpines: Exemple de la Haute-Sarine (Suisse)

Resume.: La vallée alpine de la Haute-Sarine est à plus d'un titre représentative des aquifères alluviaux de montagne, de par son environnement comme ses caractéristiques hydrogéologiques. Les investigations et les analyses réalisées dans le cadre de cette étude nous ont permis d'améliorer les connaissances sur le remplissage quaternaire meuble de cette vallée, ainsi que de mettre en évidence l'alimentation préférentielle de la nappe non seulement par l'infiltration de la rivière et des précipitations mais aussi par les apports souterrains des karsts évaporitiques. Nous avons également pu constater la richesse des ressources en eaux souterraines de la Haute-Sarine, tant au point de vue qualitatif qu'au point de vue quantitatif. La connaissance de ces caractéristiques typiques des nappes alluviales de haute altitude a par la suite permis l'élaboration d'un concept de gestion et de protection des eaux souterraines adéquat qui s'inscrit dans une politique de développement durable des ressources dans le contexte alpi

The combined use of radio-frequency electromagnetics and radiomagnetotellurics methods in non-ideal field conditions for delineating hydrogeological boundaries and for environmental problems

Radio frequency geophysical methods are known for being very versatile tools in ground- and groundwater investigation at shallow depths. They are fast and easy to use and allow a high density of information over large surfaces, which makes them very suitable for geological mapping sensu lato (faults, lithological contacts, groundwater-bearing structures, vulnerability maps, and contaminant plumes) and for selecting borehole locations. Significant improvement concerning 2D and 3D modelling of the data has occurred in recent decades. However, field surveys are very seldom performed in “ideal conditions”—the lack of necessary transmitters, in the convenient direction, in order to catch the structures in E- and H-pol for modelling purposes, is not an unusual situation. The present paper shows how the use of RMT and RF-EM is nevertheless of great help and suggests different ways to explore qualitative data in different geological settings