Tag Archives: 2010

Rockfall susceptibility mapping

Advanced Susceptibility Mapping for Natural Hazards in the Swiss Alpine Valley of Bagnes

Alpine municipalities are exposed to numerous natural hazards, such as snow avalanches, rockfalls, landslides and debris flows. The Bagnes and Vollèges municipalities in Valais (Switzerland) lie between 600 m and 4200 m m.s.l. with an area of 300 km2. The anthropization is rapid because of the fast growing ski resort of Verbier. In such situation the municipalities needs to have global overview of the natural hazards for landplaning purpose and decision making. The susceptibility mapping at regional scale allows the detection of the areas that are exposed to natural hazards, without considering the intensity and the frequency of the phenomena.

The aim of this study is to provide susceptibility maps at 1:25’000 for the following natural hazards: landslides, shallow landslides, rockfalls, debris flows, snow avalanches, flooding and river overflowing.

The present method was first developed for the Canton of Vaud (2’800 km2). Because it is applied to a smaller area, more numerical models on High Resolution DEM and field investigations were performed. In addition historical event were included in the study.

  1. The landslide mapping identifies deep-seated slope gravitational deformations, landslides and shallow landslides. It is based on the observations of geomorphological criteria on HR-EM, orthophotos and field work. Finally, the activity of each landslide is described by the knowledge of local guides.
  2. The shallow landslide susceptibility mapping is realized thanks to the software SInMap, calculating Security Factor (FS) and Stability Index (SI) according to the land use, the topography and the climatic conditions. The model is calibrated on the basis of the 67 shallow landslides already identified for the first map.
  3. The rockfall susceptibility mapping is a two steps process. First, the potential source areas of blocks are detected using a statistical analysis of the slope angle distribution, including external knowledge on the geology and land cover. Then the run-out is assessed with numerical methods based on the shallow angle method (software Conefall) and on an energy-based run-out calculation (software Flow-R).
  4. The debris flow susceptibility mapping is based on Flow-R to map debris flow sources and spreading. Slope, flow accumulation, contributive surfaces, plan curvature, geological and land use dataset are used to detect the source areas. The spreading is simulated by a multiple flow algorithm (rule the path that the debris flow will follow) coupled to a run-out distance calculation (energy-based).
  5. The snow avalanches susceptibility mapping is again based on Flow-R, to map sources areas and spreading. Slope, altitude, land use and one minimum surface are needed to detect the sources areas. The spreading is simulated with the “Perla” methodology using Flow-R. A second simulation of the spreading with RAS is performed by means of the alpha-beta methodology.
  6. Regarding to the river overflowing along the Dranse de Bagnes, the hotspots which could create blockages (bridges, pipes, etc.) are identified on the field. The propagations of the overflowing are simulated with Flow-R from the spots recognized earlier.

Finally, results show good concordances with past events and the knowledge of the local geologist and guides. The susceptibility maps will help the decision-makers of the Bagnes valley to prioritize area of interest for the creation of more expensive hazard maps.

For more information, please read Jaboyedoff M., Choffet M., Derron M.-H., Horton P., Loye A., Longchamp C., Mazotti B., Michoud C. and Pedrazzini A.: Preliminary slope mass movements susceptibility mapping using DEM and LiDAR DEM. In: Terrigenous Mass Mouvements, Pradhan and Buchroithner (Eds.), Springer-Verlag Berlin Heidelberg, 109-170, 2012

Featured image: rockfall susceptibility mapping (hillshade: copyright swisstopo)

Benjamin Rudaz: Evolution Géomorphodynamique d’un bassin versant torrentiel

Benjamin Rudaz
Supervisor: Prof. Michel Jaboyedoff
Expert: Alexandre Loye

This master thesis is focused on the Saint-Barthélémy stream, in the canton of Valais, Switzerland. It drains a catchment cut in three different geological units forming the Dents du Midi massif. Its massive torrential fan fills the whole width of the Rhône Valley.

Historically, the Saint-Barthélémy has on numerous occasions affected the human activities in the region, by cutting communications, damming the Rhône river, or simply by overflowing its banks on the debris cone. Thus, this work starts with a historical reconstitution of its past activities, with the goal of finding a pattern. The best documented crisis happened between 1926 and 1930. The different crisis recorded are mainly caused by rock avalanches in the upper part of the catchment. They result in a huge sediment supply, in a steep and snow-filled gorge. Debris flows initiate there, amplify in volume in the middle-part of the catchment, and reach the alluvial fan in multiple waves. To stabilize the terrain, and thus avoid future debris flows, the 1930 crisis was followed by an intense correction campaign. The different dams are hereby described, and their current status is evaluated.

The steady-state regime theory, applied on longitudinal profiles, states that for a stream to be in such state, its long profile must be upward concave, and follow a mathematical form, be it exponential, logarithmic or that of a power law. The streams present in the catchment are evaluated on this basis, using theorical longitudinal profile, of the exponential type. Their concavity is calculated, thus giving information about the maturity of said profiles. The longitudinal profiles are compared in their normalized form. The results identify a global pattern of potential sedimentation at the center of the catchment, while the external parts are expected to erode. The desequilibrium is stronger in the northern lateral streams.

A longitudinal profile in steady-state regime should be smooth. Therefore, profile irregularities, or knickpoints, are another sign of unsteady regime. To identify those knickpoints, three methods are used. First, using normalized profiles, the knickpoints are subjectively identified. Two numerical methods are then used to detect brutal change of local slope, relative to the neighbouring segments. The results show more precisely where future erosion is expected. The upper Saint-Barthélémy stream is particularly disturbed. The knickpoints are finally caracterized between geological, geomorphological and human origin.

By orthorectifying aerial photographs dating between 1969 et 1992, the activity of landslides connected to the main stream is measured. Lastly, using a sloping local base level method, the volume trapped behind the check dams is calculated, allowing to estimate mechanical denudation rates for different time frames. These rates (0.14 up to 0.84 mm/year) are in the range of similarly obtained rates in the Swiss Alps.

Anthony Monnier: Etude du danger de chute de blocs le long de la ligne ferroviaire Aigle – Les Diablerets

Anthony Monnier
Supervisor: Prof. Michel Jaboyedoff
Expert: Andrea Pedrazzini

This Master memory focuses on the risk of rockfall along the Aigle – Sépey – Diablerets railway line, in the Swiss Alps.

This memory breaks up into four main parts: Identification of the rockfall sources, risk definition by trajectometric analyses and field observations, the creation of hazard maps, the calculation of the damages cost and the individual risk, protection measures proposals and a calculation cost/effectiveness of these measures.

The most dangerous zones along this railway line have already been protected a few years ago, on the basis of a detailed study and general studies. But it remains three zones classified in 1st priority which were not secured. These three zones are the subject of this memory.

We can find indicative hazard maps in Switzerland, generated by computer based on topographic and geological criteria, which do not have the force of law. They have only an advisory role for the landscape planning.

Securizing the Swiss railway lines against the rockfalls consists in protection measures and monitoring measures in rare cases. There are four principal types of protection against rockfall : the rockfall screen, the rockfall net, the barrage and the gallery.

The hazard identification consists in a consultation of the preceding studies and an investigation of the people directly concerned by the rockfall hazzard, field observations as well as localization of the already fallen blocks and the discontinuities families so being able to determine the mechanisms of rupture. Treatment of the DEM by Matterocking© can confront the field observations with the data-processing modelings. The use of the DEM requires an error evaluation, by confronting it with altimetric measures taken with a differential GPS.

Once the rockfall sources located, it’s time to determine the hazzard, consisting in the propagation and the intensity at each place of the study perimeter. The size of the blocks is estimated according to the fallen blocks and the scanline method. Rotomap3D©, a trajectometry software allows to determine the main rockfall corridors and then extract the profiles which are processing by Rocfall©, a two dimensions trajectometry software.

The results of these simulations make it possible to define the intensity of the danger according to the energy mobilized by rockfalls as well as the propagation and the hazzard of the scenarios which can occur on the three studied zones.

Two types hazard maps are produced: One is based only on the energy mobilized the rockfall simulations, the other uses the same the criterias than the preceding but it includes some conventions and its polygons are smoothed in order to allow a better legibility.

In order to calculate the cost of the potential damage generated by rockfalls in these areas and the individual risk of a person travelling by train, it is necessary to define some scenarios, their consequences, their recurrence time and the localization of their occurrence, according to the characteristics of the rockfalls and topographic criterias.

The protection measures suggested here consist only of rockfall nets. Two alternatives are proposed: a total protection and a protection decreasing to the maximum the risk for a minimum investment. The utility of these alternatives will be evaluated using a calculation cost/effectiveness.

Lastly, a railway line as that studied does not have a much traffic, which makes protection measures less or not profitable. A study way would be the installation of a rockfalls monitoring system. These systems are mainly developed in Canada. The most elaborate of them uses some geophones in order to be able to detect several natural phenomenas as the rockfalls but also exploitation incidents like a derailment or a broken rail.

Fabian Eggertswyler: Analyse de l’évolution d’une commune soumise à des dangers naturels au moyen de techniques numériques. Etude de site: La commune D’ormont-Dessus.

Fabian Eggertswyler
Supervisor: Prof. Michel Jaboyedoff
Experts : Prof. Jean Ruegg, Alexandre Loye, Marj Tonini

Alpine valleys represent a significant living environment and are decisive for economic activities. Construction of buildings and roads in these environments where the soil is not easily workable is increasingly important. However, these regions are highly vulnerable to natural hazards and infrastructure development contributes to increasing the extent of damage caused by natural disasters (OFS & OFEV 2008).

Since 1997, management of natural hazards is part of territory planning. Obviously, the observation of natural hazards in the tenure of the soil affects our security. It is therefore essential to plan the land in order not to aggravate the potential damage in high-risk areas or even, to reduce them on the long range (OFEV & OFS 2007).

Located in the Prealpes, Ormont-Dessus, due to its geomorphology, is subject to various and multiple risks. Mainly by agricultural necessity, these high-risk areas have been long-occupied and exploited (STUCKI E. & ROGNON P. 1998).

The study of this localitie’s evolution affected by natural hazards provides a report indicating how infrastructures and communication channels have been built and developed over the past 60 years. The analysis of their spatial distribution reveals a sharp increase in the number of buildings in the village of Les Diablerets. On the other side, on the slopes Chaussy Para, exposed in the past to many avalanches, the building density has decreased significantly. Furthermore, the proportion of buildings located in areas of potential hazards and mainly affected by landslides, has changed little since bâtiments A deeper investigation for the analysis of territorial development and high-risk.

A deeper investigation for the analysis of territorial development and high-risk areas is possible.

Benoit Fragnol: Le LiDAR et ses applications pour l’étude des chutes de blocs

Benoit Fragnol
Supervisor: Prof. Michel Jaboyedoff

In mountainous areas, rockfall hazards cause many damages to the infrastructures and have a major socio-economic impact. These phenomena are still not well-understood and that is why new technologies are required in order to improve the characterization of the rock mass condition and the failure mechanisms.

Field methods as structural measures, scanline, GSI, JRC are used to characterize geometrically and geomechanically the unstable area but they required a high number of measures to be reliable and on a substantial number of outcrops, which is not always possible.

The LiDAR technology (Light Detection and Ranging), terrestrial and airborne, appears to be a powerful tool for imaging study areas and to work in 3D instead of 2D. It is particularly well-adapted to the analysis of unstable areas because measurements can be taken from a distance up to 1 km to the cliff and limits exposure of the geologists during the field work. It can even overcome the field measurements when the study area is too dangerous or the outcrops cannot be reached. The softwares processing LiDAR data are necessary tools for the identification of the different discontinuity sets. They allow performing a structural analysis of the area based on spatial orientations [Dip direction/Dip] for each point of LiDAR point cloud (Coltop-3D), to give a relative probability of presence of instability depending on geometric relationship between facets of DEM and spatial orientations of discontinuity sets (Matterocking). It is also possible to compare different temporal series of LiDAR data in order to identify rockfalls and to estimate their volumes (Polyworks).

The contribution of LiDAR imaging and data processing are illustrated by the structural analysis performed on the cliffs in front of Huteggen (VS, Suisse) along the main road leading to many ski resorts. This analysis aims to identify potential instabilities and define their failure mechanisms. An estimation of the instable volumes can be made based on LiDAR data combined with field observations. The results of this study allowed writing recommendations for the geological survey of the district in charge of protecting the road against natural hazards.

Florence Delasoie: Études des instabilités de versant de la Barmasse (Valais, Suisse) par une approche pluridisciplinaire.

Florence Delasoie
Co-supervisors: Prof. Michel Jaboyedoff, Dr. François Baillifard
Rapporteur: Prof. Jean-Luc Epard

The DSGSD (Deep Seated Gravitational slope Deformations) phenomena occur in most mountainous landscapes. Despite this phenomen has been recognized worldwide as a major processes of mass wasting, the mechanisms of most mapped DSGSD are not well understood.

In the Bagnes Valley (Valais, Switzerland), 14 DSGSD have been detected. Geologically, the field is situated in metamorphic rocks : quartzite-schists, prasinite. This study focuses on the Barmasse area, which is the seat of a 1.5 square-km-large DSGSD. The deep-seated movements cause shallower landslides with important displacements that produce deformations on the cantonal road connecting the lower Bagnes valley to the Mauvoisin hydroelectric power plant. Moreover, frequent rockfalls are originating from the scarp of these landslides or from their scree slope. Due to the economical importance of the object at risk, the municipality of Bagnes and the canton of Valais set up a monitoring system to monitor the movements of the shallow landslide, and build protection dams to reduce the rockfall risk.

The aim of this study are 1) to characterize the mechanisms of the DSGSD in order to evaluate its activity and its future evolution and 2) to better understand the link between superficial and deep movements.

A multidisciplinary approach is used, containing field mapping, structural analysis, measurements displacement (Extensometers, DGPS and Terrestrial Laser Scanning), dendrochronology, geophysics, hydrogeology,…

The results show that the mechanisms of the DSGSD as well as the shallow landslides is mainly driven by large scale toppling of subvertical sets. 6 families of discontinuities have been detected in the field and in Coltop 3D : J3 (parallel to the foliation S0), J2/J1, J4/J5 (subvertical) and J6 (horizontal). In the main scarp, J3=S0 couple with J4 or J5 makes a wedge mechanism rupture possible. Toppling is the most important mechanism failure along J2, J3 and S0, when the foliation dip toward the South.

The movements measured on the DSGSD itself are low (mm/year), but the displacements of the shallow landslides are very important : they reach about 3-5 m/year in the scree slope. As indicated by dendrochronological analysis, datation on trees located on the area and wood in the borehole, the activity of these landslides is recent (between 2’000 BP and 200 BP).

Postglacial debustering and gravity are the main triggering factiors for the deep-seated movement. The shallow landslides are controlle by rainfalls and snow melting. Today, the blocs deposit creep along the valley and the fracturing continues.

Raja Mastouri: Modelisation 3D des bassins sedimentaires du Sahel Tunisien: Implication Géodynamique et Intérêt Pétrolier

Raja Mastouri
Co-supervisors: Prof. Samir Bouaziz, Prof. Michel Jaboyedoff
Experts: Merj Tonini, Chedly Abbes, Mohamed Moncef Sarbeji

The Tunisian Sahel domain is characterized by geological outcrops that range from Miocene to Quaternary. This area has been considered as a vast plain and as collapsed block. Its morphology shows gradients of up to 200 m, as hills, detailed reliefs, plains and depressions. However, it’s marked by a gently tectonic frame work showing in subsurface more complexity in horst and graben with kilometric scale bounded by systems of branching faults and fold structures.

To better understand the layout of these structures in relation to the sedimentary series, the 3D modeling allows visualizing in three dimensions the geodynamic evolution of Mesozoic and Cenozoic basins of the Tunisian Sahel. It is to build and use a more simplified model closer to reality. This building 3D models and their use remains uncommon in the field of geology and represents a new tool in the study of sedimentary basins.

This study aims to develop a procedural guide that assembles a logical step for achieving 3D geological model surface and subsurface using geographic information systems (GIS) and in particular the software “Arc-GIS” to create a digital model land and a 3D surface model and the software “Surfer” to digitize the basis of subsurface data. This method describes the application of modeling software “GéoShape” that facilitates obtaining a more detailed 3D model of surface and subsurface volume for oil exploration.

Nicolas Gendre: Caractérisation de deux événements d’éboulements rocheux dans la région des Préalpes fribourgeoises. Le cas de la Dent de Lyss et celui de Varvalanna (Fribourg, Suisse).

Nicolas Gendre
Supervisor: Prof. Michel Jaboyedoff
Expert: Andrea Pedrazzini

Les éboulements de la Dent de Lyss et de Varvalanna se situent dans les chaînes des Préalpes fribourgeoises. Ils ont eu lieu respectivement fin avril 1992 et le 15 mai 2006. La niche d’arrachement de l’éboulement de la Dent de Lyss se trouve dans des calcaires massifs et siliceux du Jurassique. Dans le cas de Varvalanna la zone de l’éboulement est plus complexe, il s’agit de calcaires et schistes du Lias et de dolomie du Trias.

Ce travail vise à étudier la géomorphologie, la géologie structurale et la qualité du massif
rocheux. Le but est de comprendre les mécanismes de ruptures, les facteurs déclenchants ainsi que les conditions de propagation de ces deux événements.

La méthodologie consiste en une étude de terrain détaillée en lien avec des MNT (Modèle
Numérique de Terrain) à haute résolution. L’étude de terrain permet de récolter des données très précises, mais locales de la qualité du massif rocheux et des paramètres structuraux. En particulier une analyse quantitative des caractéristiques des discontinuités comme la persistance, l’espacement, le remplissage etc. et les conditions du massif rocheux comme l’altération ont été étudiés sur le terrain. Cette analyse de terrain a été comparée au MNT 1 m pour avoir un point de vue plus global. Dans le but de définir clairement les domaines structuraux et l’influence de changements lithologiques dans la zone de l’éboulement.

L’acquisition de scan LIDAR sur le site de la Dent de Lyss a permis une meilleure compréhension des structures dans la niche d’arrachement. Finalement des modèles géomécaniques des deux éboulements ont été créés pour valider les résultats obtenus avec les méthodes utilisées précédemment lié entre autres aux mécanismes de ruptures.