Tag Archives: 2015

Jean?Marie Vuignier: Caractérisation de la source sédimentaire et estimation du budget sédimentaire dans le bassin versant de Jatún Mayu (Cochabamba, Bolivie)

Jean?Marie Vuignier
Director: Prof. Michel Jaboyedoff
Co?director: Dr. Ivanna Penna
Experts: Prof. Stuart Lane, Dr. Karen Sudmeier?Rieux

Natural and human-induced erosive processes shape landscape by transferring masses from the mountain to downstream areas. They also impact population both located in the source areas of sediments as well as urban areas settle on the depositional area. Mountain areas in Bolivia present high surface dynamics and high rates of rural migrations, causing e.g. a significant increase of population in Cochabamba city in the last 20 years. This work aims to estimate the sediment production on the Jatún Mayu watershed in Cochabamba department taking into account the different origins of sediments.

The region of study consists of a mountain area situated in the Andes with altitudes ranging from 2500 to 4600m. Fieldwork on July 2014 and high-resolution satellite image
interpretation (2004 & 2009) allowed mapping and measuring landslides and gullies. A
hundred of landslides are recorded mostly around the river channel. Most of the gullies are
situated in the upper part of the valley where the vegetation is less abundant on low-sloping agricultural lands.

Photogrammetric reconstructions using camera and drone were the main method used to
characterize some strategic points along the river in order to get dimensions of landslides,
gullies, as well as the riverbed roughness, as the final goal was to model floods at the mouth of the watershed, where migrants have been settling for the last years. A total of 9 points of interests along the riverbed were surveyed and for each of them a square 5x5m surface was analyzed. Approximately 250 pictures by area were needed to estimate roughness along the channel. A flood model has been performed, by using the Riverflo-2D Plus software, to produce a model of the downstream region.

Nicolas Emery: Evaluation de l’influence de la hauteur d’eau et de l’inclinaison de la pente sur le comportement des glissements de terrain en laboratoire

Nicolas Emery
Director: Prof. Michel Jaboyedoff
Expert: Dr. Marc-Henri Derron
Supervisors: MSc. Dario Carrea, Dr. Antonio Abellan

Landslides are complex phenomena. Despite the number of studies that have been conducted on this topic, they still remain unpredictable. It is particularly difficult to predict them because of the many factors that can impact the stability of a slope. This thesis will focus mainly on two of these factors: water and the slope inclination. Its goal is then to identify their impact on the characteristics of a landslide such as its geometry, its volume, its speed and its response time. For this purpose we have created an analogue model of slope in laboratory, made with sand of a diameter smaller than 2 millimeters. The level of water injected into the model has at first been varied whilst keeping the slope inclination steady, then we changed the slope inclination whilst maintaining a stable water level. The slope surface has been scanned thanks to the scanner Konica Minolta Vivid 9i with time intervals of one minute. For each experiment, the volume has been determined based on the failure surfaces visible through the transparent glasses that form the outside borders of the model. A MATLAB code has been developed to allow their upload from pictures of failure surfaces taken sideways as well as for the calculation of the landslide volume for each experiment. The MATLAB code developed by Carrea et al. (2012) has been used to follow the trajectories of the small balls set on the slope in order to calculate their speed based on LiDAR scans. As last, an analysis of the geometry and of the calculation of response time have been realised thanks to video recordings during the experiments.

The results show a significant impact of the water height on the extent of the landslide resulting in a Iinear increase of the landslide volume as well as its speed with the water height. As regards the geometry, a variation of the water implies little difference. Indeed, it has been observed that a landslide with a low water height is comparable to a landslide with a higher water height, but that it interrupts at an early stage, which implies that the final scarp is located further down the slope. Finally, the proximity of the toe of the slope from the water input has certainly prevented the determination of the real influence of the water height over the response time.

The change in slope inclination has shown major differences regarding to the geometry as the experiments with a weak slope inclination have mainly been characterised by a viscous gravity flow, whereas when the slope inclination is high, topples phenomena and rock falls show up. It has also been demonstrated that the landslide volume increases with the slope inclination. However, for the velocity, no trend has been identified due to the high uncertainty of the results. As response time strongly depends on the distance between the toe of the slope and the water input, and as this declines with the slope inclination in our experiments, it hasn’t been possible to highlight any relationship between these two variables.

Liliane Nguyen: Identication de précurseurs synoptiques aux évènements de précipitations extrêmes pour des situations de Sud dans les Alpes par l’analyse des trajectoires inverses

Liliane Nguyen
Supervisors: Dr. Pascal Horton, Prof. Michel Jaboyedoff

One of the most expensive natural disasters in Switzerland consists of floods related to heavy precipitation. Moreover, the occurrence of heavy rains may induce landslides and debris flows as it was observed during the three major precipitation events that occurred recently in the Swiss Alps (August 1987, September 1993 and October 2000). Even though all these inclement weather conditions took place under a southerly circulation, especially in autumn, not all southerly circulations lead to heavy precipitation. Although many studies have been carried out to understand them, they are still very difficult to forecast, due to the complexity of the phenomena involved. In consequence, the forecasting of extreme events still contains important uncertainties, especially in an alpine environment. The numerical models struggle to take into account the complexity of this environment strongly influence by different local-scale specific behaviors. Therefore, this work aims to identify simple synoptic precursors to such events throughout backward trajectories of the air masses.

Backward trajectories can be modeled with different methods and along two or three dimensions. The 2D trajectories are calculated and projected on an invariant parameter such as the pressure or the temperature, while the 3D trajectories indicate the height position of the air masses. Backward trajectories are calculated by using wind fields provided by different meteorological datasets, such as atmospheric reanalysis. In order to process trajectories, various tools already exist. Nowadays, backward trajectories have become very popular in the atmospheric science. In consequence, multiple tools and
datasets are provided by different organizations.

Therefore, this work is separated into two parts. Part one test and compare as many combinations of tools, datasets and methods as possible in order to gain knowledge about trajectories in the case of heavy precipitations in the Alps and to reduce the number of models to be assessed for the second part. As a result, we removed models yielding to similar results with an absolute horizontal transport deviation measure (ATEH). Among them, we tested tools such as the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), a simple Matlab script developed at the University of Lausanne by Pascal Horton named HorTraj and the METeorological data Explorer (METEX). The implemented methods in these models are different: while HYSPLIT and METEX use a Lagrangian Particle Dispersion Model, HorTraj uses the fully implicit algorithm of Merril (1986). Since these tools can be used with various datasets, the trajectories are processed with different ones such as the NCEP/NCAR Reanalysis (R1, R2 & C20r), the ECMWF reanalysis (ERA40 & ERAinterim), the Japanese Reanalysis (JRA-55) and the NASA Reanalysis (MERRA2). Moreover, for each tool and dataset, various methods can be used to calculate the altitude of the air masses. In this work, three-dimensional, isobaric, isentropic, isosigma, constant density and from divergence trajectories have been used. As a result, 21 trajectory models have been tested and compared, and 9 of them were selected. Results show that the larger differences between trajectories have been mainly from the dataset used rather than the model.

Then in part two, the 9 selected models were used to search simple precursors leading to heavy precipitations. 10 days backward trajectories were processed for the Binn station (which is a gauging station that often measures big amount of rain) on four levels pressure between 1000 and 500 hPa. As for the validity domain, we selected all the days included between 1961 and 2014 that were characterized by a southerly circulation in autumn. Based on those trajectories, part two suggests four analysis for the identification of precursor. First, the ATEH is used to assess similarities between extreme events and non-extreme events. Then a Lagrangian moisture source diagnostic is suggested to determine the origin of moisture contributing to precipitation. A residence-time probability analysis is also suggested to highlight area where the residence-time probability is greater than the average probability associated with typical meteorological patterns. Finally, last analysis presents an identification of special type of flow conditions, such as the stagnation, recircultation and ventilation that can enhance extreme precipitations.

Results show that trajectories leading to heavy precipitation tend to be similar than trajectories leading to dry or small events. Region of interest that highly contributes to heavy precipitations have also been highlighted, such as the western Mediterranean and North Africa.

Remo Gygax: GPU accelerated numerical simulations and laboratory experiments on viscous avalanches

Remo Gygax
Director: Prof. Y. Podladchikov
Jury: Prof. M. Jaboyedoff, Prof. F. Herman, L. Räss, C. Longchamp

Numerical simulations are an eective tool in natural risk analysis. They are useful to determine the propagation and the runout distance of gravity driven movements such as rock avalanches. A numerical model based on fluid mechanics is proposed to simulate such events. In absence of an accepted mathematical explanation of the interactions inside a rock avalanche bulk, a numerical viscous framework is established. Several studies have shown that the numerical implementation of physical processes of viscous flow produces a good t to actual observations of rock avalanche propagation. The use of a continuum approach enables to implement the equation of motion and the mass balance equation of viscous flow on a fully staggered grid. These equations are solved using a finite difference scheme.

The anticipated numerical results on viscous ow are well documented. Nevertheless, analogue laboratory experiments with glycerine on an incline are conducted to find the dependencies between the major parameters. The fingering of the viscous ow was successfully reproduced, at the same time in the laboratory as well as in the numerical implementation. Nowadays, High Performance Computing hardware is available at reasonable costs. Using parallel GPU computing with the Nvidia C-CUDA programming architecture allows to accelerate the simulation. The difference in performance is compared to the widely used scientific modelling software MATLAB. We obtain a nearly 158 times faster simulation while running the developed code on the parallel High Performance Computing architecture.

Alizée Vioget: Analyse de l’évolution morphostructurale des falaises littorales du Bessin, Basse-Normandie, France

Alizée Vioget
Director: Prof. Michel Jaboyedoff
Co-directors: Prof. Olivier Maquaire
Expert: MSc. Clément Michoud

The coasts’ recession is a major issue for the management of littoral regions. In this context, two coastal areas in “Calvados” and “Pays de Caux”, French Normandy, are studied by the University of Caen for several years.

The studied section of the cliffs of Bessin is about 5 km long and lies between the World War II artillery batteries of Longues-sur-Mer and Arromanches-les-Bains. The site’s lithology is mainly made of two formations: the limestones of Bessin that lie on top of the marls of Port-en-Bessin. On the coastline, the cliff’s height varies between 10 and 75 meters above sea level. The marl formation acts like an aquitard, as it is semi-impermeable. Therefore, more or less important water outflows are observable at the point of contact between the marls and the limestones.

First, the study aimed to create an up to date geomorphological map as well as a kinematic classification of the existing instabilities of the different cliff’s profiles. This part was realized with on site field measurements. Several profiles were observed depending on the type of cliff studied: sinking of limestone panels due to creeping marls at the base, overhang limestone formation, wave-cut notch, detachment, tilt, rotational rockslide, superficial rockslide etc. These several behaviors depend on the cliff’s exposure to the Channel Sea and weathering factors, morphology, presence of pebble beach etc. Thus, the coastline section was classified depending on the different morphological types observed, which influence the stability and erosion rates. Principal morphological types here are: overhang limestone formation near Cape Manvieux and rotational rockslide by creeping marls near le Chaos.

Then, the cliffs’ condition was compared to the diachronic analysis of the shoreline evolution supported by different photographic documents. This part of the study allowed to refine the spatiotemporal occurrence of the different ground movements. However, cliffs’recession evolves in successive leaps and bounds. Thus, results would not be significant unless there are long observation periods. Therefore, the documents used consisted of orthophotos and oblique aerial pictures that cover a period from 1947 to 2012. This step revealed a recession velocity of about 0.06 to 0.30 m/yr.

Finally, a complex rockslide that happened in May 2013 near Cape Manvieux has been investigated. Its kinematic and the mass volume that has moved were determined. For that purpose, a terrestrial LiDAR acquisition of the instability was performed in July 2013. LiDAR data and field observations let think that the current state of the instability was created by multiple events and is a complex mix of creeping marls and toppling of limestone destabilized by a back subvertical discontinuity which is parallel to the coastline.

The site geomorphological study associated with the diachronical analysis gave lots of information regarding the mechanisms shaping the cliffs of Bessin and their evolution. Thus, the section of interest could be cut into several transects having their own mechanisms and erosion rhythm. Hence, the event of May 2013 could be replaced in its context and described as a major magnitude event which displaced about 30’000 m3.

Susanna Büsing: Analyse spécifique de l’activité récente des éboulements au Piz Lischana (Basse-Engadine)

Susanna Büsing
Supervisor: Prof. Michel Jayboyedoff
Internal Experts: MSc. Antoine Guerin, Dr. Marc-Henri Derron
External Expert: Dr. Marcia Phillips

On 31 July 2011, a rockfall with an estimated volume of 7000 m3 occurred at an altitude of 3050 meters on the southeastern side of the Lischana mountain, located in the Lower Engadin valley (Grisons, Switzerland). Luckily the rockfall event was filmed and ice could be observed on the failure plane after analysis of the images. Due to the fact that another crack was opened next to the Lischana summit and to protect the about 1500 mountaineers who climb the mountain in-between the months of July to October, the access to the summit was prohibited by the municipality of Scuol and the official mountain peak displaced of 50 meters. In autumn 2014 the expected rockfall occurred, as well as two other rockfalls situated more north and about 300 meters lower.

In order to characterize and analyze the two events situated under the summit, three 3D high density point clouds have been made by photogrammetry and LiDAR, one before and two after the rockfall of September 2014. 120 photos were taken during a helicopter flight in July 2014 to produce the first photogrammetric point cloud, and more than 400 terrestrial photos were taken at the end of September to produce the second photogrammetric point cloud. In July 2015 a third point cloud was made with three LiDAR scans.

The point clouds were georeferenced with a 2-meter digital elevation model (DEM) and compared to each other in order to calculate the volume of the two past rockfalls. A structural analysis of the two rockfall was made and compared to the geological structures of the whole eastern face in order to increase the understanding of the past rockfalls and their failure mechanisms and the probability of future rockfalls. Moreover, valuable information about the velocity from the filmed rockfall event using a Particle Image Velocimetry method (PIV) could be extracted. These analyses combined with two thermal panoramas and the analyses of triggering factors (permafrost, freeze-melt cycles, thermomechanical processes, rainfall, radiation, glacier decompression and seismic) improved the understanding concerning the recent rockfall activity on the Lischana summit and underline the influence of permafrost warming on rock instability.

In a second time, a comparison of the LiDAR and photogrammetry techniques was made in order to increase the knowledge concerning the limits and the advantages of the photogrammetry technique. The point clouds have been analyzed regarding their general quality, the quality of their meshes, the quantity of instrumental noise, the point density of different discontinuities, the structural analysis and the cinematic tests. Results show that the photogrammetric technique is of adequate quality to make a structural analysis and that a good choice of the parameters allows to almost reach the quality of the LiDAR point cloud, but several factors (focal length, variation of distance to object, image resolution) may increase the uncertainty of the photo alignment. This study confirms additionally that the coupling of the two techniques is possible and provides reliable results.

Battista Matasci: Rockfall susceptibility assessment and remote geological mapping with LiDAR point clouds

Battista Matasci
Director: Prof. Michel Jaboyedoff
Jury: Dr. Marc-Henri Derron, Dr. Greg M. Stock, Dr. Brian Collins, Prof. Giovanni B. Crosta, Dean François Bussy

Characterizing the geological features and structures in three dimensions over inaccessible rock cliffs is needed to assess natural hazards such as rockfalls and rockslides and also to perform investigations aimed at mapping geological contacts and building stratigraphy and fold models. Indeed, the detailed 3D data, such as LiDAR point clouds, allow to study accurately the hazard processes and the structure of geologic features, in particular in vertical and overhanging rock slopes. Thus, 3D geological models have a great potential of being applied to a wide range of geological investigations both in research and applied geology projects, such as mines, tunnels and reservoirs. Recent development of ground-based remote sensing techniques (LiDAR, photogrammetry and multispectral / hyperspectral images) are revolutionizing the acquisition of morphological and geological information. As a consequence, there is a great potential for improving the modeling of geological bodies as well as failure mechanisms and stability conditions by integrating detailed remote data.

During the past ten years several large rockfall events occurred along important transportation corridors where millions of people travel every year (Switzerland: Gotthard motorway and railway; Canada: Sea to sky highway between Vancouver and Whistler). These events show that there is still a lack of knowledge concerning the detection of potential rockfalls, making mountain residential settlements and roads highly risky. It is necessary to understand the main factors that destabilize rocky outcrops even if inventories are lacking and if no clear morphological evidences of rockfall activity are observed. In order to increase the possibilities of forecasting potential future landslides, it is crucial to understand the evolution of rock slope stability. Defining the areas theoretically most prone to rockfalls can be particularly useful to simulate trajectory profiles and to generate hazard maps, which are the basis for land use planning in mountainous regions. The most important questions to address in order to assess rockfall hazard are:

  • Where are the most probable sources for future rockfalls located?
  • What are the frequencies of occurrence of these rockfalls?

I characterized the fracturing patterns in the field and with LiDAR point clouds. Afterwards, I developed a model to compute the failure mechanisms on terrestrial point clouds in order to assess the susceptibility to rockfalls at the cliff scale. Similar procedures were already available to evaluate the susceptibility to rockfalls based on aerial digital elevation models. This new model gives the possibility to detect the most susceptible rockfall sources with unprecedented detail in the vertical and overhanging areas. The results of the computation of the most probable rockfall source areas in granitic cliffs of Yosemite Valley and Mont-Blanc massif were then compared to the inventoried rockfall events to validate the calculation methods. Yosemite Valley was chosen as a test area because it has a particularly strong rockfall activity (about one rockfall every week) which 2 leads to a high rockfall hazard. The west face of the Dru was also chosen for the relevant rockfall activity and especially because it was affected by some of the largest rockfalls that occurred in the Alps during the last 10 years. Moreover, both areas were suitable because of their huge vertical and overhanging cliffs that are difficult to study with classical methods. Limit equilibrium models have been applied to several case studies to evaluate the effects of different parameters on the stability of rockslope areas. The impact of the degradation of rockbridges on the stability of large compartments in the west face of the Dru was assessed using finite element modeling. In particular I conducted a back-analysis of the large rockfall event of 2005 (265’000 m3) by integrating field observations of joint conditions, characteristics of fracturing pattern and results of geomechanical tests on the intact rock. These analyses improved our understanding of the factors that influence the stability of rock compartments and were used to define the most probable future rockfall volumes at the Dru. Terrestrial laser scanning point clouds were also successfully employed to perform geological mapping in 3D, using the intensity of the backscattered signal. Another technique to obtain vertical geological maps is combining triangulated TLS mesh with 2D geological maps. At El Capitan (Yosemite Valley) we built a georeferenced vertical map of the main plutonic rocks that was used to investigate the reasons for preferential rockwall retreat rate. Additional efforts to characterize the erosion rate were made at Monte Generoso (Ticino, southern Switzerland) where I attempted to improve the estimation of long term erosion by taking into account also the volumes of the unstable rock compartments.

Eventually, the following points summarize the main out puts of my research:

  • The new model to compute the failure mechanisms and the rockfall susceptibility with 3D point clouds allows to define accurately the most probable rockfall source areas at the cliff
    scale.
  • The analysis of the rockbridges at the Dru shows the potential of integrating detailed measurements of the fractures in geomechanical models of rockmass stability.
  • The correction of the LiDAR intensity signal gives the possibility to classify a point cloud according to the rock type and then use this information to model complex geologic structures.

The integration of these results, on rockmass fracturing and composition, with existing methods can improve rockfall hazard assessments and enhance the interpretation of the evolution of steep rockslopes.