Tag Archives: 2009

Emmanuel Wick: Etude détaillée d’un bassin versant à laves torrentielles le long de la Route Internationale n° 7, Courbe de Guido, Province de Mendoza, Argentine

Emmanuel Wick
Supervisors: Prof. Dr. Michel Jaboyedoff, Valérie Baumann

The International Road 7 crosses Argentina from East to West, linking Buenos Aires to the Chile border. While crossing the Andes Cordillera, it puts itself at numerous natural risks, such as avalanches, rock falls and debris flow.

This study will be part of a larger one that will eventually characterize the hazard along the mountainous portion of this road. In this study we will focus on the catchment of the debris flow that swept a car away in January 2005. All of this happened in the northern part of the Guido’s curve, between Potrerillos and Uspallata, in the Mendoza Province.

The Guido debris flow’s catchment measures 4.7 km2 and is constituted of three main torrents that meet a few meters ahead of the International Road. Each one of these torrents started during the evening of January 11, 2005, reaching the road apparently at a very short interval of time. Three years later, signs of them are still perfectly visible, frozen by the arid climate of the region.

The study has been mainly realized from satellite Quickbird imagery and field data collected during a three-week field survey.

It has been established from meteorological data, particle size and mineralogy the conditions that lead to a start of debris flow in this catchment. It depends on strong rainfall combined with numerous material mainly constituted of sands produced by the erosion of a very altered granite. The debris flows have been classified as being a granular matrix that has a collisional-frictional behavior.

From various criteria and with the help of a digital elevation model, it has been shown that the starts happen mainly at the top of the catchment.

Various calculations of volumes, peak discharges and velocities have been realized from geomorphologic methods, empiric formulas and from the analysis of satellite imagery. It appears that an important potential of volume that can be mobilized exists, especially for the longest torrent. A new event could move more than 65.000 m3 of material; the last event reached an estimated velocity of approximately 7 m/s.

A detailed geomorphologic study gave the opportunity to emphasize the activities of anthropogenic origin that led to the deviation of the torrents towards a more adequate place for the construction of a mitigation work. Identically three propagation scenarios illustrating the vulnerability of the road have been proposed.

The present state of the work where underneath the debris flows must cross is not optimum: it is underdimensioned and the last event deposits have not been completely evacuated. The road has been considered as vulnerable even in case of much lighter debris flows.

The results gave the opportunity to put forward several realistic protection measures for the Argentinean context in descending order of importance. They are simple and should be feasible at reasonable cost.

Clément Michoud: Application de l’interférométrie différentielle Radar dans la surveillance du glissement de La Frasse, Suisse

Clément Michoud
Supervisors: Prof. Dr. Michel Jaboyedoff, Andrea Pedrazzini

Spaceborne Differential Interferometric Synthetic Aperture Radar (DInSAR) is a remote sensing technology based on the measure of the phase of a radio wave reflected by ground scatterers. Calculate the phase’s difference of those ground scatterers detected by two radar satellite’s acquisitions in two distinct times allows, in theory, to detect the small deformations of the topography which occurred between the temporal baseline. Limited by atmospheric artifacts and decorrelation problems (changes in electrical properties and moisture of the soil), DInSAR is more effective in urban, arid or rocky areas than in forest or cultivated areas. But it can detect quasi vertical movements covering very large areas in a continuous way. That is why DInSAR can be considered as an efficient tool to detect and monitor slope instabilities.

This Master Thesis in Engineering Geology was done in the Institute of Geomatics and Risk Analysis of the University of Lausanne to bring theoretical and technical requirements to use DInSAR technology in studies of slope surface processes.

The La Frasse landslide is studied for this work. It is located in the State of Vaud (Switzerland). With a length of 2 km, a larger of 500 m, his surface of rupture is located between 60-10 0m of depth. With high velocities (presently at 40 cm/y in the active lower part), La Frasse’s landslide is in highly hazardous area according to the Swiss legislation. According to Varnes classification, it is a complex slide composed of tertiary flysch material and flowing over flysch and limestone bedrock. The landslide area is occupied by forests, pastures and some habitations. Today, the instability is maintained by the river “Grande-Eau” erosion of the foot.

To acquire the Radar dataset, the project « Differential Synthetic Aperture Radar Interferometry (DInSAR) in monitoring large landslide in La Frasse (Vaud, Switzerland) » was concluded with the European Space Agency. To validate DInSAR results, we dispose of data provided by an optical detection of ground movements system, with prisms on the landslide localized every 2h with a Laser station (Robovec™ System, Soil Mechanics Laboratory, Swiss Federal Institute of Technology of Lausanne).

First of all, two interferometric pairs are constructed with data provided by the satellite Alos. The large wavelength (24 cm) of ALOS’ PALSAR sensor gives results in the foot of the landslide, where the movements are the most important. Afterwards, fifty-three interferometric pairs are constructed with twenty-two images acquired with the European satellite Envisat. With the lower wavelength (5.6cm) of the Envisat’s ASAR sensor but the higher number of interferograms, the analysis provides the mean velocities on landslide’s scaterrers by using the SBAS methodology.

The results of this study are coherent with the amplitude of the deformations monitored by Robovec™. Moreover, the computation of the mean velocities shows that today, total displacements are measured in the active lower part of the landslide; the data complete Robovec™ results. But the accuracy of the measured displacements and the number of scatterers could be improved by synthetizing the topographic phase from an aerial laser scanning DEM instead of the STRM DEM. The number of scatterers can also be increased by installing corner reflectors in the fastest-moving area of the landslide.

Featured image: Mean annual displacement velocities of scatterers detected by SBAS-InSAR between 2002 and 2008 (hillshade: copyright swisstopo)

Aurélie Pannatier: Méthodologie de détection et d’analyse d’instabilités rocheuses aux échelles régionale et locale. Applications à la vallée de Yosemite (Californie/Etats-Unis) et à l’éperon du Sex Frei (Valais/Suisse).

Aurélie Pannatier
Co-supervisors: Prof. Michel Jaboyedoff, Thierry Oppikofer
Expert: Frank Philippossian
Rapporteur: Prof. François Marillier

Rock instabilities have been studied for a long time, but no strict methodology has been developed to ease their analysis. In this research paper, it has been decided to create two types of methodology related to the scale of the area under study.

The first type of methodology is applied to the regional scale. Its goal is to determine areas that might produce the rockfalls solely using a digital elevation model. It uses the interaction of the following parameters: slope, planar or wedge slidings, and thickness of the erodible mass. A susceptibility map of possible rockfalls is finally obtained focusing field investigations where the susceptibility is the highest.

This methodology has been applied to the Yosemite Valley (CA, USA). This region has been studied for a long time because it is very sensitive to rockfalls. Therefore, it is possible to compare the results of this method with the pre-existing studies in order to judge its coherence and precision.

The second type of methodology is applied to the local scale. It is, in a way, the extension of the regional methodology. After having identified the areas in needs of further investigation, the next method is applied using field observations, digital elevation model or modeling programs. Finally, geomorphologic and structural analysis, a monitoring using the terrestrial LIDAR, a geomechanic analysis, a modeling of the failure mechanisms, and a prospective analysis are performed in order to create a propagation map and an intensity map.

This methodology has been applied to the cliff of the Sex Frei located above the small village of Pralong in the Heremence Valley (Canton of Valais, Switzerland). This cliff has been studied for over thirty years by the “Bureau d’Etude Geologique”. Movements of about 1 cm/year have been detected using a differential GPS.

A study of the entire Heremence Valley indicates that the area under study has a high susceptibility to produce rockfalls. Putting the local methodology into practice showed that the cliff is at limit of stability and that a planar sliding prevails in the mechanism of failure. The propagation and intensity maps created permit to tell with certainty that no isolated block can reach the bottom of the valley, putting the Village of Pralong at risk. However, it is not excluded that a more serious event could occur after an earthquake, then implying a rock avalanche that could possibly reach the bottom of the valley.

These methods can, of course, be completed and adapted according to the performed study, like for example the creation of a hazard map.

Alain Breguet et Jérôme Dubois: Landslide risk mapping in Pakistan and Nepal based on field survey and modeling techniques.

Alain Breguet et Jérôme Dubois
Co-supervisors: Prof. Michel Jaboyedoff, Karen Sudmeier-Rieux
Expert : Andrea Pedrazzini
Rapporteur : Prof. François Marillier

Himalayan region is characterized by an important density of landslides related to earthquake, monsoon rains and the important weathering of the bedrock. Several mountain communities are affected by these phenomena. A complete geotechnical hazard assessment is often difficult to complete in developing countries due to low available funding. In this study, we propose a risk mapping methodology that can be quickly applied in order to have a first overview of human risk associated with landslides in the Pakistani Kashmir and Dharan region of eastern Nepal.

This methodology proposes a quick field survey coupled with an intensive use of different types of remote sensing data. The main morphological information is collected in the field in order to have an objective analysis of the phenomena. Each landslide is mapped with its location, size and state of activity based on cracks and vegetation investigation. Participatory mapping with local communities is required to gain information and to understand which problems are caused by each landslide and to locate relict cracks and landslides. Secondly, modeling based on a digital elevation model (DEM) provides other information like a stability index (SINMAP), distance calculations and slope analysis.

The risk is assessed for buildings and roads with two different calculations. Each building receives a susceptibility level and a potential damage value based on the characterization of each landslide (frequency, volume, activity). The risk is the multiplication of these two values by the number of inhabitants per building. The roads are separated into sections below and above landslides. The calculation includes the frequency and probability of spatial occurrence of the landslide and different characteristics of the road including its length, the number of vehicles per day, vehicle speed and occupancy. Four colors are defined to show the amount of risk of each building and road section.

This methodology is quick, relatively cheap and can be applied in places with incomplete datasets. This simple methodology can be integrated into development projects by development agencies and local development authorities to consider risk due to landslide.

Guillaume Favre-Bulle: Instabilités rocheuses le long de la route nationale 7 Secteur Guido – Uspallata (km 1118 à 1133) Provincia de Mendoza – Argentina.

Guillaume Favre-Bulle
Co-supervisors: Prof. Michel Jaboyedoff, Valérie Baumann, Andrea Pedrazzini

The National Road 7, important link between Argentina and Chile, is subject to many natural hazards, threatening the vehicles travelling along. This work aims to classify the risk associated with falling blocks, to intervene as a priority on the most dangerous places to avoid investing in useless protective measures.

The study area is located west of the city of Mendoza in the Andes. The road passes several times along rocky outcrops that may be highly fractured and let blocks free. The analysis was conducted on two points: a GIS analysis of the DEM (Digital Elevation Model) and a field study. The GIS analysis has called for the creation and the search for a DEM of good quality. After several trials with satellite images and the SRTM, pairs of stereoscopic aerial photographs of 1966 have given the least bad outcome. These DEM are strongly deformed in places and do not reproduce accurately the topography.

The GIS analysis is based entirely on the study of the geometry of the DEM, the result is related to quality. Several factors are summed together : rock instabilities for the break susceptibility, slope, slope orientation, kinematic tests of the main discontinuities and SLBL. The results are used for propagations simulations of falling blocks. The results are not very good because of the poor quality of the DEM. Areas with high failure susceptibility are numerous and extensive, making it impossible to establish priority areas. The Conefall propagation can not be calibrated with field data and the results are indicative only.

On the field, discontinuities outcrops were measured and used to calculate several classification methods: RMR, SMR, and GSI. The results give rock masses of good quality, but can be very unstable following the geometry of the discontinuity and the slope of the outcrop. Then, two RHRS methods were calculated on each outcrop to classify them in order of priority for intervention. Both methods (RHRS Budetta and RHRS Colorado) take into account the characteristics of the slope, climate, geology and exposure of vehicles to danger. Both methods were compared and it is the Colorado RHRS which was recommended for future studies.

The RHRS allowed to classify outcrops for priority of intervention. Sites with the highest scores will be subject to further study to determine what protective measures will be most suitable and the money will be managed in the best possible way.

Sébastien Lévy: Analyse de l’évolution morphologique d’une partie du bassin versant de la rivière Chacoura, Québec, Canada.

Sébastien Lévy
Co-supervisors: Prof. Michel Jaboyedoff, Professeur Jacques Locat
Expert: Alexandre Loye

The large plains of Eastern Canada sensitive clays are cut by many rivers, in a way that their slopes have been and are still affected by landslides. They play a major role in the modelling of the landscape of these regions, in addition to induct a risk for human infrastructures. The role of erosion as a trigger of landslides is also important.

On the Chacoura River, north of Louiseville (Quebec), several large scars of landslides, more or less recent, are visible. A first inventory of landslides and areas of erosion was carried out in 1984 on some series of aerial photographs, covering a period from 1948 to 1979. It suggests that the majority of recorded landslides are located in areas that were previously affected by erosion. In this Master Thesis, a detailed analysis of aerial photographs, dating from 1948 to 1997 and a LiDAR-Digital Elevation Model (DEM) dating from 2007, has been done, using four different approaches: (1) a map of the phenomena on a portion of the river, which is considered as very active, was drawn by identifying various elements such as landslides, limits of the slope, position of the river, area covered by the forest and agricultural drainage structures. (2) A quantitative analysis based on the previous map has shown the temporal evolution of landslides in terms of area and volume, using statistical relationships. (3) The study focused on two flows, which happened in 1976 and 1990; it consisted of a detailed analysis of the evolution of their perimeter, area and the initial conditions of the slope. The comparison of DEMs was used to estimate volumes eroded in these flows, and in the entire zone of interest. (4) The last part focused on the general evolution of the river valley, and particularly the identification of areas where erosion could be more active in the future. This study was based on transverse and longitudinal profiles, and using the Sloping Local Base Level (SLBL) method.

The results show that the location of landslides is firmly linked to the presence of typical specific topographic features, such as (1) the shape of the meandering river, (2) the flow of agricultural drainage, or (3) the erosion at the toe of the slope. The study of landslides over a period of 59 years shows also that (4) the major landslide scars in this region are in fact the sum of several events of lesser importance. The analysis of the development of Chacoura River valley indicates that (5) it is still at an early stage of development. Besides (6) the lower part of the river seems to be relatively stable, however the upper part and the tributaries will probably face significant changes in the next 10,000 years. Finally hazard maps, based on the measured distances of flow retrogression have been created for the St-Léon-le-Grand region.

Micaela Isabel Vargas Garnica: Modèle pour l’analyse spatiale des risques environnementaux et approche sociale de la zone de Tablas Montes et El Sillar, province de Colomi-Cochabamba-Bolivie.

Micaela Isabel Vargas Garnica
Co-supervisors: Prof. Michel Jaboyedoff, Rafael Tapia
Expert : Karen Sudmeier-Rieux

Whilst managing a territory, several considerations should be accounted for in order to establish an appropriate use of the land. Proper soil use is thereby conditioned by environmental risks which include instability of slopes.

This instability that soil slopes are subjected to, is mostly due to a combination of natural factors including its morphology, topography of the area, geology, soil characteristics (vegetation, rainfall or surface runoff) as well as key factors like the anthropic deforestation and human interface like construction works performed in a particular soil.

This study presents a methodology for determining the sensitivity of stable and unstable areas to environmental risks (principally to landslides) using different variables such as slope analysis, vegetation index, classification of land and a water flow accumulation study. The analysis was carried out in two areas in the province Colomi (Cochabamba-Bolivia), two areas that have been characterized in recent years to be sensitive to the repetitve hazard of landslides.

With the use of SIG tools, a spatial analysis model was developed designed to obtain maps of stability-instability of the affected area. This model will help identify the presence of environmental hazards in the areas of study and it also constitutes a tool to prevent and control associated environmental risks. Moreover, this model could be useful for other soil locations with similar characteristics.

Recommendations were made to reduce the environmental risks in the areas of study, taking into account the socio-economic reality of the area.

A brief introduction is also presented regarding the perception of environmental risks by the local population. Some of the most important social factors that have to do with environmental risk are herein detailed also.

Marc Choffet: Evaluation de la vulnérabilité du bâti existant face aux inondations dans un contexte d’assurance immobiliére. Etude de cas du village de Le Pâquier, Fribourg.

Marc Choffet
Supervisor: Prof. Michel Jaboyedoff
Experts: Dr. Olivier Lateltin, Pr. Jean Ruegg

The vulnerability has a direct influence on the concept of risk. Yet, in the literature, its definition may vary. The theoretical part of this Master Thesis will seek to define and explain the underlying concepts.

In a context of building insurance, it is a determining factor heavily influencing the amount of damages. It is also a frequent topic for insurance companies because it is among the causes of the increase in damages related to natural disasters. The theoretical part of this writing will seek to understand how this concept affects the amount of damages and which factors make it evolve. It will issue its representativeness as well as factors that are changing. The concept of vulnerability will be developed, like other elements of risk, after searching through bibliography, described through a state of the art.

The second part of this Master Thesis will seek to illustrate the theoretical vulnerability concepts learned in the first part and apply them in a context of flooding of buildings. For that, a study area with highly diversified buildings was chosen, namely the village of Le Paquier in the canton of Fribourg, Switzerland. The field investigations will consist in visits of buildings and will be documented by numerous photographs. An entry interface, including all vulnerable parts of the buildings will be developed to facilitate data collection and to allow the assessment of risk. Hazard, exposure and the value of property are included provided by existing hazard map and building insurance data in this reflection to consider an integrated management of risk. A series of flood scenarios will be made to the field study.

As part as an opening to a larger scale to offer a rigorous assessment of vulnerability, this Master Thesis focuses on identifying the difficulties caused by such an approach. These observations are described in the results of the work, as well as prospects. They are also a strong interest for future development of the proposed methods.

The amount of potential damages from the three scenarios presented in the result part of the Master Thesis cannot be taken as absolute values, but as a possible investigation of the fragility of buildings.

Valentin Métraux: Quantification de l’érosion et de la dynamique sédimentaire dans un bassin versant torrentiel à l’aide de la télédétection. Application au bassin versant du Merdenson (Valais/Suisse).

Valentin Métraux
Supervisor: Prof. Michel Jaboyedoff
Expert: Alexandre Loye

The watershed of Merdenson dominates the villages of Vollèges and Cries. Many major events of debris flows are recorded on this torrent and threaten not only road and rail infrastructure, but also the villagers of Cries.

The damage caused by debris flows have led the municipal and cantonal authorities to intervene in the watershed to limit the spread of debris flows. This work aims to characterize the erosion in the catchment, both in terms of volume and distribution.

The study of the distribution of erosion and its growth has called for the creation of digital
elevation models by stereoscopic techniques. A methodology for processing the results of stereoscopy has also been created to make the models usable in a GIS environment. In a second step, the evolution of the topography was compared to a set of variables, morphological, geological or related to existing structures. Finally, an erodibility index as well as prior results have led to hypotheses about possible developments in the medium to long term in the watershed of Merdenson.

The average volume of sediment produced has been estimated (150,000 m3/year) for the entire watershed of Merdenson. A comparison with estimates from previous studies validates the results of this study and gives a ratio between the volumes of sediments carried by debris flows and those by the normal flow. The volumes have been separated between the different parts of the study area. The evolution of the erosion has been built from the time sequence of digital elevation models created by stereoscopy.

In a second time, erosion and deposition of the 47 years data have been correlated with different variables such as lithology, the shape of the channel and its slope. All these results revealed some probable trends of the watershed and make assumptions about changes in the dynamics of the stream thereof.

The majority of the results are confirmed by observations from the field in this work and in previous studies. The links made between erosion and morphological variables are confirmed by other studies and the creation of stereoscopic document is validated.

Benoît Mazotti: Création d’une carte de danger lave torrentielle. Le cas du bassin versant Nord du Mont-Rogneux (Val de Bagnes, Valais).

Benoît Mazotti
Co-supervisors: Prof. Michel Jaboyedoff, Alexandre Loye
Expert: François-Joseph Baillifard

The aim of this work is to diagnose the main hazard for debris flows in the North watershed of Mt-Rogneux which is located in the Val de Bagnes in Valais / Switzerland. The village of Versegères is on the fan, which is partly in danger.

The methodology has been divided into four parts which are: 1. A general analysis of the watershed that responds to the questions where? and what? 2. A specific analysis of the watershed that responds to the questions why? how? and How much ? 3. A diagnosis of debris flow hazard answering the questions when? how far? which intensity? and 4. An analysis of the influence of the permafrost as an aggravating factor triggering debris flows (secondary purpose).

Regarding to the general analysis of the watershed, the two main goals were to create a map of phenomena and makes hypotheses on the dynamic of the watershed. This approach has allowed to identify signs of previous torrential activities and thus to make some assumptions on the dynamics of the torrent. The conclusions were that the watershed is composed by two types of dynamic (high frequency and low to medium intensity or low frequency and exceptional intensity) and that the role of the deposition area (accumulation – remobilisation) is important. It was possible to detect sectors that are clearly more dangerous than others. The frequency analysis of the watershed permit to show the importance of seasonality on the probability of occurrence of debris flows. A short chapter will introduce the influence of climate change on this phenomenon.

For the specific analysis, the main purposes were to characterize the dangerousness of the sources areas, to know the type of debris flows, to estimate their sizes and to perform simulations of spread within the watershed. First, the analysis of the dangerousness of the sources areas indicates that the central corridor has the higher probability of debris flows trigger. The granulometry analysis identified that the debris flows were from granular type. Maximum volumes of debris flows for frequent, rare or extreme frequencies have been determined. From this, it was possible to calculate potential discharge from which we will figure out the speeds and the heights of the overflow cone droppings. To conclude this chapter, seven spread scenarios from the sources areas were modelled to find the spatial extent of such events.

The assessment of debris flow hazard begins with an estimation of the danger of the debris flow for the fan and the village of Versegères. From the calculations of discharge and speed it was possible to detect areas of supposed overflows. Six scenarios of events have been established for the three probabilities of occurrence: High (frequent scenario), medium to low (rare scenario) and very low (extreme scenario). The conclusions highlighted more dangerous areas and some conflicts with human infrastructures. In the end, all the limits of propagation have been grouped to form a single map, easier to use to inform people.

A method was created for the responsible of natural hazards (specialists) in order to qualitatively estimate the debris flows hazard on the time axis. Coupled with an intensity of rain expected or real, this qualitatively estimation can be linked to one or more scenarios of those kind of events.