Tag Archives: 2016

Céline Longchamp: The propagation of unconstrained dry granular flows: from laboratory to numerical modelisation

Céline Longchamp
Director: Prof. Michel Jaboyedoff
Jury: Prof. Giovanni Crosta, Prof. Yury Podladchikov, Dr. Irene Manzella, Prof. Suren Erkman

As rock avalanches are rare catastrophic events in which granular masses of rock debris flow at high speeds, commonly with unusually long runout distances, analog and numerical modeling can provide important information about their behavior. This thesis is composed of three main contributions: (1) laboratory experiments in order to demonstrate that the basal roughness and the grainsize as well as the volume and slope angle are important parameters of the motion of a dry granular mass; (2) the analysis of rock avalanche dynamics by means of a detailed structural analysis of the deposits coming from data of 3D measurements of mass movements of different magnitudes, from decimeter level scale laboratory experiments to well-studied rock avalanches of several square kilometers magnitude; (3) development of a numerical model to simulate the laboratory experiments.

Laboratory experiments are performed with a tilting plane. Granular material is released, chutes down a slope, propagates and finally stops on a horizontal surface. Different grainsizes (115, 545 and 2605 ?m) and substratum roughness (simulated by sandpapers with grainsize from 8.4 to 269 ?m) are used in order to understand their influence on the motion of a granular mass. This work shows that there is a logarithmic relation between the substratum roughness and the motion of the granular flow. For same volume, slope angle and fall height, the runout of the mass is comprised between 4.5 and 11 cm. The influence of the volume and the slope angle is also investigated. The runout increases from 8 to 11 cm with volumes from 300 to 600 cm3. Contrarily to the volume, the slope angle (from 35° to 60°) influences greatly the runout of the mass front (from 5 to 20 cm).

In order to emphasize and better detect the fault structures present in the deposits, we applied a median filter with different moving windows sizes (from 3×3 to 9×9 nearest neighbors) to the 3D datasets and a gradient operator along the direction of propagation. The application of these filters on the datasets results in: (1) a precise mapping of the longitudinal and transversal displacement features observed at the surface of the deposits; (2) a more accurate interpretation of the relative movements along the deposit (i.e. normal, strike-slip, inverse faults) by using cross-sections. Results show how the use of filtering techniques reveals disguised features in the original point cloud and that similar displacement patterns are observable both in the laboratory simulation and in the real scale avalanche, regardless the size of the avalanche.

To simulate the analog granular flow, a numerical model based on the continuum mechanics approach and the solving of the shallow water equations was used. In this model, the avalanche is described from a Eulerian point of view within a continuum framework as single phase of incompressible granular material. The interaction of the flowing layer with the substratum follows a Mohr-Coulomb friction law. Within same initial conditions (slope, volume, basal friction, height of fall and initial velocity), results obtained with the numerical model are similar to those observed in the analog model. In both cases, the runout of the mass is comparable and the size of deposits matches well. Moreover, both analog and numerical modeling provide velocities of same magnitudes. In this study, we highlighted the importance of the friction on a flowing mass and the influence of the numerical resolution on the propagation. The combination of the fluid dynamics equations with the frictional law enables the self-channelization and the stop of the granular mass.

Zeynabou Sy: Analyse de risques hydrologiques et modélisation de la nappe de Thiaroye (Dakar, Sénégal)

Zeynabou Sy
Directors: Prof. Michel Jaboyedoff, Serigne Faye

The Dakar region experiencing recurrent floods in the years 1989 to 2012. These recurrent flooding particularly affect the urban area of Pikine and Guediawaye. Previous studies show that these floods have a positive development in the sector. The various complex causes of these events can be summarized in urban causes, demographic, topographic and hydrogeological. The departments of Pikine and Guediawaye have depressions high endoreic character. These interdunals depressions are called Niayes. They are outcrop areas of the aquifer of Quaternary sands of the Cap-Vert peninsula. So these localities where floods occur are naturally areas for the development of this hazard. These agglomerations of Pikine and Guediawaye were born following the drought of the 1970s. During the drought the rural exodus has led to migration and therefore the urbanization of these areas. They were areas of wet and swampy depressions seasonally. The drought led to believe that they had become dry and therefore they became apt for human habitat. They were so rapidly urbanized, the Dakar region saw its population in the peri-urban area increased from 23’000 in 1960 to 1’044’814 inhabitants in 1998. On the other hand the area of the Cap-Vert Peninsula home under the dune sands of the Quaternary, a free aquifer : the aquifer of Thiaroye. Thus Niayes are outcrops of this aquifer. The aquifer of Thiaroye was exploited in 1950, contact with the ocean has created a risk of causing salt intrusion and stop its pumping between 1959-1961. After the withdrawal of the bevel, the pumping restarted with a lower rate. Thus the lower pumping and the rarity of rainfall in the 1970s, have maintained the level of the low table. These two factors contributed to drought wetland making think that these areas have become dry. Uncontrolled urbanization in towns of Pikine and Guediawaye was done anarchic way without respect of any construction standard. This has led to the return of the aquifer wastewater. Very high levels of nitrate in groundwater have trained to the decline in pumping rate and eventually stop pumping. The population is growing, the region of Dakar imported its water from the Guiers lake in addition to operating in the aquifer of infrabasaltiques sands (confined aquifer) located at the head of the Cap-Vert peninsula. These imported wastewater is an additional recharge for the aquifer. The decrease in the flow of rates, returns to the aquifer of wastewater; makes the most superficial aquifer. The low permeability of the sands as that the surface of the aquifer becomes closer to the ground. The recovery of rainfall since the 1995-1999 in a context of aquifer very close to the ground and urbanization in depression areas mean that floods have become recurrent in the Dakar region. The total cessation of drilling operations is a decision that may reinforce the scale of such disasters.

Caroline Lefeuvre: Influence des effets thermiques dans l’e valuation de l’ale a chute de blocs

Caroline Lefeuvre
Co-supervisors: Prof. Michel Jaboyedoff, Dr. Marc-Henri Derron

Les aléas liés aux mouvements gravitaires sont des phénomènes très présents en zone de montagne. Parmi ceux-ci, il est possible de nommer les glissements de terrain, les laves torrentielles, les avalanches rocheuses, ou encore les chutes de blocs. Un aléa est caractérisé par sa probabilité, ou fréquence d’occurrence, son intensité et sa localisation.

Ce travail se concentre sur l’étude des chutes de blocs. Le terme chute de blocs désigne en général un aléa de faible intensité, mettant en jeu la chute de faibles volumes de blocs rocheux inférieurs à 10000m3 (Dussauge-Peisser et al, 2002). C’est un aléa qui peut potentiellement menacer des enjeux tels que des habitations ou des réseaux routiers. C’est donc la raison pour laquelle le risque chute de bloc est étudié.

Si l’intensité de l’aléa chute de blocs est plus ou moins connue, de par sa définition, il est important d’étudier la quantification de la probabilité d’occurrence de l’aléa afin de bien comprendre l’aléa. De nombreuses études ont été menées sur les facteurs pouvant provoquer des chutes de blocs. Le déclenchement d’un événement de chute suite à des conditions météorologiques particulières a été intensément étudié. Par exemple, Delonca et al. (2014) et D’Amato et al. (2016) ont analysé l’influence des précipitations. L’influence des variations de température a été examinée à travers les cycles gel dégel (Frayssines & Hantz, 2006) et les cycles thermiques (Collins & Stock, 2016 et Vargas et al., 2013). Les secousses sismiques sont également des facteurs déclenchant de chute (Bakun-Mazor et al., 2013). Enfin, des déformations tectoniques peuvent causer des fractures dans la roche (Scholz, 1968), ce qui génère des zones de faiblesse dans la roche.

Dans le cadre de ce travail, l’aspect fréquence de l’aléa chute de bloc sera principalement étudié, notamment à travers l’influence des variations de températures au sein de la roche, étant susceptibles de créer une fatigue thermique des roches. En effet, de récents travaux ont montrés que les variations journalières de température génèrent des déformations cycliques, mesurées au niveau d’écailles rocheuses (Collins & Stock, 2016). Ce travail a pour but de modéliser l’impact des conditions thermiques sur la déformation et la fatigue des roches, pouvant conduire à la déstabilisation de petits blocs.

Le site d’étude utilisé pour ce travail est une paroi rocheuse située dans la commune de Bourg-en-Lavaux, à environ 10km de Lausanne (Vaud, Suisse). Les chutes de blocs provenant de cette paroi ne menacent pas d’enjeux. Cependant l’activité élevée de chutes en fait un site d’étude idéal. Les blocs sont en moyenne de la taille du mètre cube. De plus, la zone d’étude bénéficie de nombreuses instrumentations. En effet, le site est équipé d’une station météo, d’un capteur thermique dans la roche et d’un appareil photo. Les données recueillies avec ces derniers ont servis de comparaison avec les résultats de la modélisation.

Cindy Vulliez: Apports des méthodes d’imagerie 3D pour la caractérisation et le monitoring du glissement rocheux de Séchilienne (Vallée de la Romanche, Isère, France)

Cindy Vulliez
Director: Prof. Michel Jaboyedoff
Co-directors: Dr. Antonio Abellán, MSc. Antoine Guerin
External Expert: Marie-Aurélie Chanut

The Sechilienne landslide, which is located in the Romanche Valley (France) is mass movements of about 650 m high and 250 m wide, with a potential volume of 3 million cubic meters of material in the active part. The slope, which is mainly composed of micaschist, is characterized by the presence of a NE-SW sub-vertical fracturing system involved in the destabilization of the area. Several investigations are being performed by different research groups on this landslide, including fieldwork investigations, remote sensing, seismic acquisitions, geochemistry, deformation analysis by extensometers, etc. The rock slope has been continuously moving since the eighties decade, with a growing acceleration during the period 2009-2013 followed by a progressive stabilization during the last years.

In this work, we used different 3D techniques in order to monitor the whole rock slide
displacements. First of all, nine different fieldwork campaigns using Terrestrial Laser Scanning were performed during the last six years to obtain high-resolution point clouds (8 cm point spacing) of the rock slope topography. In addition, we used three Helicopter-based Laser Scanning campaigns carried out in between 2011 and 2014 acquired by the Cerema. Finally, more than 600 photos were taken in April 2015 in order to build a photogrammetric model of the area using Structure-from-Motion (SfM) workflow in Agisoft PhotoScan software. All types of data allowed us having a good spatial vision of the evolution of the whole slope. 

A detailed structural analysis was performed from both LiDAR and SfM point clouds using
Coltop3D. Eight joint sets were detected, allowing us to estimate the susceptibility of the slope to three main failure mechanisms: planar sliding, wedge sliding and flexural toppling. Moreover, we carried out the monitoring of the active area by three approaches: comparisons among all the point clouds, tracking of several points in the frontal area, 3D tracking of several homogenous rocky compartments of the rear part using the roto-translation matrix. Large-scale movements (from several dm to more than 10 m) were observed in the active area with a coupling between subsidence and toppling oriented towards the valley. Lateral structures that cut de rear active part also seem to be affected by a clockwise rotation around the topple axis. Once the individual blocks fell during the period 2009-2014, we individualize their volumes and calculated them. We
also determined a frequency-volume relation of events in the slope and estimated the erosion rate of. Finally, we compared the two methods of imaging based on the results obtained.

The acquisition of accurate topographic data taken from LiDAR and SfM has been useful to
characterize the instability, quantifying the displacements and clarifying the failure mechanisms involved in the complex dynamic of the Sechilienne rockslope active part.

Two more PhD students completed their theses at the Risk Analysis group

On last 8th July Friday, two PhD students (Raja Mastouri and Zar Chi Aye) from the Risk Analysis group have successfully finished their public defenses and obtained their doctoral degrees in Geosciences. Congratulations!

Raja Mastouri applied 3D seismic in combination with terrestrial laser-scanning and photogrammetry techiques to analyse faults and fractures in Eocene carbonate reservoirs, and to investigate basin tectonics in the Gulf of Gabes (Tunisia).

Zar Chi Aye developed a prototype web-based collaborative decision support platform for risk management of natural hazards. This platform aims to assist risk managers in analyzing impacts of natural hazards as well as authorities and decision makers in the decision-making process for selection of risk management measures in a collaborative and interactive manner.

If you are interested, check out the summary of their doctoral theses here and here, for more information.

Zar Chi Aye: A collaborative web-GIS based decision support platform for risk management of natural hazards

Zar Chi Aye
Directors: Prof. Michel Jaboyedoff and Dr. Marc-Henri Derron
Jury: Prof. François Golay, Pr. Paola Reichenbach, Dr. Ivana Ivánová, Vice-Dean Suren Erkman

Photo Credit: Civil Protection, FVG region (Italy)

One of the main problems in risk management is the lack of good communication as well as efficient and effective collaboration between the agencies, services and organizations in charge of risk prevention, mitigation and management. The involvement of various stakeholder groups is an important component of risk prevention and mitigation. This calls for an integrated and coordinated approach which helps responsible stakeholders in managing risk, starting from risk identification to the decision-making process for achieving the best combination of risk reduction strategies. As natural hazards and associated risks are spatial in nature, web-based decision support tools integrated with Geographic information systems (GIS) have been increasingly considered as useful instruments for providing decision support. Taking the advantages of modern web, spatial and open-source technologies to achieve a centralized and integrated framework, in this research, a web-GIS based collaborative decision support platform is proposed for risk management with involvement of various stakeholders. The principal purposes of this research are: (1) to conduct a systematic and integrated risk management approach with diverse involvement of different stakeholders; (2) to explore the possibility and application of interactive web-GIS decision support tools for the analysis, communication and exchange of decision support information between risk management stakeholders and (3) to propose an innovative approach to potentially enhance collaboration activities between stakeholders through interactive and participatory approaches.

The conceptual inputs of this study are based on the initial feedback, semi-structured interviews and observations obtained from the field visits and stakeholder meetings carried out in three case studies of Europe: the Ma?opolska Voivodeship of Poland, Buz?u County of Romania and the Friuli-Venezia-Giulia region of Italy. Even though some platforms exist in study areas, no single case has a platform at hand which enables as flexible and collaborative approach for the formulation and selection of risk management measures as attempted in this study. Moreover, most platforms have focused mainly on inventory of events, risk visualization and dissemination of information. In this research, a prototype is realized and focused on the risk analysis, formulation and selection of potential measures through the use of an interactive web-GIS based interface integrated with a Multi-Criteria Evaluation (MCE) tool. This platform is regarded not only as a web platform for centralized sharing of risk information but also for ensuring an integrated framework where involved stakeholders can analyse risk and evaluate risk reduction measures. For the prototype development, a three-tier client-server architecture backed up by Boundless (OpenGeo) was applied with its client side development environment. This developed prototype was presented to the local and regional stakeholders of the study areas and feedback was collected to understand their perspective in determining whether the platform is useful and applicable for their activities in risk management. The prototype was also further evaluated with students to obtain feedback on different aspects of the platform as well as to analyse how the application of interactive tools could assist students in studying and understanding risk management.

The main part of this research was carried out within the Marie Curie Research and Training Network “CHANGES: Changing Hydro-meteorological Risks as Analyzed by a New Generation of European Scientists” funded by European Commission’s 7th framework program (www.changes-itn.eu, 2011-2014, Grant No. 263953).

The demonstration of the prototype application can be found on YouTube. You can also download the PhD manuscript here.

Raja Mastouri: Application of offshore 3D seismic combined with onshore terrestrial laser-scanning and photogrammetry to analyse faults and fractures in Eocene carbonate reservoirs and to investigate basin tectonics in the Gulf of Gabes (Tunisia)

Raja Mastouri
Director: Prof. Michel Jaboyedoff
Jury: Prof. Samir Bouaziz, Prof. François Marillier, Dr. Robin Marchant, Vice-Dean Suren Erkman

The detailed knowledge of the tectonic processes affecting the Gulf of Gabes, Eastern Tunisia domain, is essential in attempting to fully understand the controls of fracture development.

The 3D seismic interpretation provides a good opportunity to analyze the subsurface images better. In fact, the geometrical characteristics of the different associated fault systems and the basin individualization are well established using the 3D techniques.

Two principal systems (NW-SE and NE-SW trending extensional faults) were active during the Cenozoic and Quaternary in different phases. These faults form a series of grabens that vary in length from a few to several hundred kilometers. The structuring of NE-SW en-echelon faults indicates a strike-slip type of bordering faults. In this work, we focused on the evolution of the tectonic structures in the basin that shows the continuous extension phases of the Upper Cretaceous to Pliocene. Outcrop analogues were used to better understand fracture systems in petroleum reservoirs. In the last years, new developments in 3D high-resolution technology such as Terrestrial Laser Scanning (TLS), photogrammetry digital and GigaPan provided precise 3D high-resolution data and opened new prospects for structural studies.

In this study, TLS scan data and photorealistic model were used to analyze density, spacing, and orientation of fractures that characterize the carbonate analogue of the Eocene El Garia Formation (Ousselat cliff and Jebil anticlinal) and Reineche Formation (Damous quarry), exposed in Eastern Tunisia. The analysis of fracture and fault spacing provides provided significant information on their geometric distributions and orientations. In addition, the 3D model obtained by Structure from Motion (SfM) was used to analyze density and orientation of fractures that characterize the carbonate of the Eocene Bou Dabbous Formation (Foundek el Jedid quarry). The synthesis of both sites would underline the regional features. The two main regional fracture systems trending NW-SE to NNW-SSE and NE-SW to NNE-SSW were identified. Furthermore, these systems also affected the main reservoir formations of the El Garia and Bou Dabbous reservoirs in the Eastern Tunisia. Besides, these fractures influenced fluid flows in this reservoir.

The data obtained from analogue outcrops and the 3D seismic highlight the similarity in geometry structures and fracture orientations. This combination of analogue outcrops and the 3D seismic data also shows that this basin is affected by extensional faults (N160 and N040) in offshore. In contrast, the Eocene reservoirs in onshore are affected by extensional faults (N160 and N040), and the compressive fault N030.

Caption-PhD-Michoud

Clément Michoud: From Regional Landslide Detection to Site-Specific Slope Deformation Monitoring and Modelling Based on Active Remote Sensors

Clément Michoud
Directors: Prof. Michel Jaboyedoff and Dr. Marc-Henri Derron
Jury: Dr. François J. Baillifard, Prof. Lars H. Blikra, Prof. Jacques Locat, Dean François Bussy

Landslide processes can have direct and indirect consequences affecting human lives and activities. In order to improve landslide risk management procedures, this PhD thesis aims to investigate capabilities of active LiDAR and RaDAR sensors for landslides detection and characterization at regional scales, spatial risk assessment over large areas and slope instabilities monitoring and modelling at site-specific scales.

At regional scales, we first demonstrated recent boat-based mobile LiDAR capabilities to model topography of the Normand coastal cliffs. By comparing annual acquisitions, we validated as well our approach to detect surface changes and thus map rock collapses, landslides and toe erosions affecting the shoreline at a county scale. Then, we applied a spaceborne InSAR approach to detect large slope instabilities in Argentina. Based on both phase and amplitude RaDAR signals, we extracted decisive information to detect, characterize and monitor two unknown extremely slow landslides, and to quantify water level variations of an involved close dam reservoir. Finally, advanced investigations on fragmental rockfall risk assessment were conducted along roads of the Val de Bagnes, by improving approaches of the Slope Angle Distribution and the FlowR software. Therefore, both rock-mass-failure susceptibilities and relative frequencies of block propagations were assessed and rockfall hazard and risk maps could be established at the valley scale.

At slope-specific scales, in the Swiss Alps, we first integrated ground-based InSAR and terrestrial LiDAR acquisitions to map, monitor and model the Perraire rock slope deformation. By interpreting both methods individually and originally integrated as well, we therefore delimited the rockslide borders, computed volumes and highlighted non-uniform translational displacements along a wedge failure surface. Finally, we studied specific requirements and practical issues experimented on early warning systems of some of the most studied landslides worldwide. As a result, we highlighted valuable key recommendations to design new reliable systems; in addition, we also underlined conceptual issues that must be solved to improve current procedures.

To sum up, the diversity of experimented situations brought an extensive experience that revealed the potential and limitations of both methods and highlighted as well the necessity of their complementary and integrated uses.

Download the PhD manuscript

Ecosystems Protecting Infrastructure and Communities (EPIC): Nepal-Reducing risk from landslides and flash floods

Large and small hazard events are already the main cause of mortality – second only to epidemics – for mountain populations in Nepal and a major impediment to rural development. Due to the dispersed nature of mountain hazards, especially landslides and flash floods, little attention has been paid by NGOs or government agencies to reducing such risks. Also, in parallel with the decentralization of power and budgets, new road construction is booming, often being undertaken by communities themselves who lack any technical knowledge. Bio-engineering measures, which are cost-effective and easily adapted to the local context, could significantly reduce landslides along roads but are rarely incorporated in road construction in Nepal.

This initiative builds on an existing baseline of research and links with specific partners and communities. Support from EPIC will enable ecosystem-based approaches to become integrated within planning and decision-making services in Nepal, ultimately making a positive and lasting contribution towards community security and welfare.

The project was undertaken in the Panchase region of Western Nepal. Selected communities are from three districts: Kaski, Parbat and Syangja district.

For detailed information, please refer to EPIC’s website.