Category Archives: Theses_completed

Jérémie Voumard: “Natural hazards affecting the Swiss transportation networks: from their characterization to their low cost survey”

Jérémie Voumard
Director: Prof. Dr. Michel Jaboyedoff
Jury: Dr. Marc-Henri Derron, Dr. Andrea Pedrazzini, Dr. Nicolas Pollet, Prof. Dr. Suren Erkmann

Geological and hydro-geological natural hazards, as landslides and floods, are a threat to many transport networks built in mountainous areas. These risks, that are often small in intensity, are poorly evaluated or unknown.
This doctoral thesis focuses on the characterization and quantification of natural hazards that impact Swiss communication tracks. It also explores various remote sensing techniques dedicated to the survey of areas around traffic lanes.
A database has been created to characterize the many small events that regularly affect roads and railway lines in order to compensate the lack of knowledge. The number of events and their trends – such as their spatio-temporal distribution, the weather, the geology, the direct damages or the types of affected tracks- are thus known at the national level over a period of five years (2012-2016). It shows that a natural event, on average, disrupts the traffic every 2.1 days and that the events occur mostly during the months of June and July, late afternoon. Direct costs were estimated at CHF 4 million per year, with an average cost per event estimated at 23 400 francs.
In order to characterize the approaches to roads and railways, we have developed and tested the photogrammetric technique “on-motion Structure from Motion”, whose cost is reasonable. This remote sensing technic makes possible to obtain colorized and georeferenced 3D point clouds from images taken by four action cameras placed on a moving vehicle. Its accuracy has been evaluated in laboratory conditions and on many sites. It was also compared with seven other traditional surveillance techniques to identify advantages and disadvantages.
This work highlights the impact and consequences of small-scale natural hazards that, taken as a whole, are not negligible for society. In addition, this study demonstrates that low-cost survey technic can compete with more expensive traditional survey techniques.

Keywords: Natural hazards, transportation networks, on-motion Structure for Motion, topographical survey.

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 μm) and substratum roughness (simulated by sandpapers with grainsize from 8.4 to 269 μm) 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.

Amenan Agnès Kouame: Apport de la modélisation hydrogéologique dans l’étude des risques de contamination de la nappe d’Abidjan par les hydrocarbures: Cas du benzène dans le District D’Abidjan

Amenan Agnès Kouame
Directors: Prof. Michel Jaboyedoff and Prof. Albert Goula Bi Tié
Jury: Prof. François Zwahlen, Dr. Marc-Henri Derron, Dr. Jean Kouame Kan, Prof. Erkman Suren

This study on the aquifer of the Continental Terminal is carried out in the Abidjan District located on the coastal sedimentary basin in southern Côte d’Ivoire (West Africa). This unconfined aquifer of the city of Abidjan of Mio-Pliocene age is called “Abidjan groundwater”. The water quality of this aquifer is facing with diverse sources of anthropogenic pollution such as scattered deposits of solid and liquid wastes of all kinds. Indeed, the inadequacy of sanitation and drinking water supply systems increases the pollution risk of the Abidjan’s groundwater. Besides, the proliferation of petrol stations, including tank breaking, needs to be considered in the event of an accident, which poses a real threat to groundwater given the complex hydrogeological structure of the region. In order to ensure the effective protection and management of the Abidjan water table, this work proposes to evaluate the risk of contamination of groundwater in the Abidjan aquifer by hydrocarbons such as benzene for the purpose of the implementation of protective measures. To achieve such an objective, a model of underground flow and contaminant transfer was designed from field data and literature. Soil analysis showed two types of soils: sand and silty sand. Using the FEFLOW software, the predictive simulation of underground flow coupled with the transport of dissolved benzene deposited on the soil surface at the N’Dotré and Anador station was implemented. Dissolved benzene initial concentrations are 43.12 and 14.17 mg/l for the two sites respectively for variably saturated zone. The results revealed that a borehole named ZE11 of the “Zone Est”capture field is polluted after 44 years and 2 months because the threshold concentration of 0.001 mg/l is reached. A maximum concentration of 0.011 mg/l is reached at this drilling at 47 years and 2 months. In this zone, 5 other wells in this Zone, ZE1, ZE7, ZE8, ZE13, ZE14 are threatened by pollution because the dissolved benzene was detected after a time ranging from 39 to 46 years. The ZE10 has even recorded a maximum concentration of 0.0008 mg / l in 46 years. The average distance from Anador site these is 4 km. At an average distance of 2 km from the N’Dotré service station, 8 boreholes belonging to the Anonkoua Kouté, AK15 AK5, AK6, AK7, AK8, AK10 AK4 and AK17, are also threatened by pollution, dissolved benzene has been detected. for a time between 38 to 47 years.

With the software, HYDRUS, for an initial concentration of 2 mg/cm3, the dissolved benzene sets 105, 112 and more than 365 days to cross the 100 cm cylinder of sand, silty sand and clay respectively, taking into account the delay. Without delay, these times are reduced to 100, 80 and 50 days respectively for clay, silty sand and clay. Considering the sand material, the pollutant takes more than 7 years to cross the average thickness of the unsaturated zone 56 m.

During its journey, if the pollutant crosses channels, these global times could be 10 to 100 times shorter. Therefore, special attention should be given to the sites of N’Dotré and Anador in case of rupture of the tank especially as this tablecloth of Abidjan is the only source of drinking water in the District of Abidjan. The method of decontamination of the unsaturated zone proposed is “bioventing” taking into account the high percentage of sand that constitutes the soil of the District of Abidjan geological formations.

Roya Olyazadeh: Natural hazards and open source development: adoption and application of mobile and Web-GIS technologies in risk management

Roya Olyazadeh
Director: Prof. Michel Jaboyedoff 
Jury:  Dr. Marc-Henri Derron, Dr. Christian Kaiser, Dr. Anne Puissant, Prof. Suren Erkman

Impacts of natural disasters have increased worldwide in the past decades. Risk assessments and analysis have been effectively pursued by research institutions, national and local governments, NGOs and different stakeholders for some time and a wide range of methodologies and technologies are proposed consequently. Nowadays, risk and disaster management including risk reduction mitigation and alternative selections are becoming more useful with World-Wide-Web and Geospatial Technologies. Web-GIS technologies offer a wide range of solutions to map the disasters, damages, analyze data and publish the results on the web. Open-source Web and Mobile GIS tools can help to improve the analysis of data and reduce the time and cost required. Open-source platforms and data have been widely used today because they stay free and facilitate access to data especially significant in developing countries. This research explores the applications and adoptions of Mobile and Web GIS technologies in the field of risk and disaster management. The purpose of this research is: (l)To review different open source web and mobile GIS systems related to Risk and Disaster management; (2) to explore the adoption and application of web and mobile GIS technologies for the collection, analysis, and decision making in disaster management; (3) to manage risk analysis using open data such as OpenStreetMap (OSM); (4) to apply advanced visualization, central database, fast and easy data and information acquisition. This research was carried out in 3 major phases: risk identification, assessment, and evaluation, which includes developing different platforms using open-source Web-GIS technologies such as OpenLayers, Leaflet maps, Cordova, GeoServer, PostgreSQL as the real DBMS (Database Management System) and PostGIS as its plugin for spatial database management. The first study presents a Web-GIS prototype using OSM data to evaluate the rapid impact of naturally produced disasters like an earthquake for the estimation of total damages. For this purpose, expert knowledge such as earthquake intensifies and vulnerability inputs are imported into the system. The second study uses the same methodology as the first platform and is applied in a teaching project (FIP) at University of Lausanne. Its purpose is to discover the relation between an open-source platform/data and students. The third application reviews the implementation and selected results of a secure Mobile map application called ROOMA (Rapid Offline-Online Mapping Application) for the rapid data collection of landslide hazard in Nepal. This prototype assists the quick creation of landslide inventory maps (LIMs) by collecting information on the type, feature, volume, date, and patterns of landslides. This application comprises Leaflet map coupled with satellite images as a base layer, drawing tools, geolocation (using GPS and the Internet), photo mapping and events clustering. The fourth study presents an improved version of ROOMA for Canton Vaud, Switzerland to collect all events and hazards such as landslide, floods, avalanches, and etc. The last platform reviews an implementation of a decision analysis module based on Multi-Criteria Decision Analysis (MCDA) in the platform of a Natural Risk Management Spatial Decision Support System (SDSS) called “RiskChanges”. This platform involves changes in hydro-meteorological risk and delivers tools for indicating the best risk reduction alternative. This module assists the users by importing the essential factors, such as risk and cost-benefit values from other modules. Besides the combination of the multi-criteria analysis and possible future scenarios in group sessions and comparison the results by means of a numerical and graphical view are carried within the system. The results of these different platforms clarify how open- source GIS technologies can be easily adopted on different levels of risk and disaster management such as post-disaster management, teaching, decision support systems and, etc. Moreover, the results present the technical difficulties to adapt these technologies to real-life situations.

Pierrick Nicolet: Quantitative risk analysis for natural hazards at local and regional scales

Pierrick Nicolet
Director: Prof. Dr Michel Jaboyedoff
Jury: Prof. Dr Paolo Frattini, Dr Alexander Garcia Aristizabal, Dr Markus Imhof, Dr Farrokh Nadim, Prof. Dr Suren Erkman, Prof. Dr Jean-Luc Epard

Natural hazards can have damaging consequences for human activities, causing death or economic losses. This PhD thesis concentrates on the risk of natural hazard, where risk is defined as the combination of the likelihood of a damaging event and its negative consequences. The types of hazards considered in this work are mostly geological hazards such as landslides and sinkholes, but hail is also investigated.

The first part of this work focuses on the local scale, where local stands for a small group of objects, typically a few houses or a road. First, in order to improve the characterisation of the hazard, the potential of the photogrammetric method, which consists in retrieving the 3D position of objects from a set of 2D pictures, is tested. Together with this promising method, Unmanned Aerial System (UAS) are presented, since they permit to carry a camera and thus to take pictures for a photogrammetric analysis. Then, a tool to calculate the risk at local scale is presented. This tool is designed in Microsoft Excel an aims at calculating rapidly the risk using hazard maps produced according to the Swiss guidelines. A particular aspect of this model is presented in the next chapter; it reviews the methods used to calculate the conditional probability for a falling object, such as a rock block, to impact a moving vehicle, taking into account the dimensions of the block and of the vehicles. Then, prospective aspects of such a risk model are presented and deal with the addition of multiple risk scenarios and the inclusion of uncertainty in the risk analysis using a Monte-Carlo approach. To conclude this part, a method which aims at taking the protection measures into account in the hazard maps without losing the initial hazard level is presented.

The second part of this work presents risk analyses at regional scale, where the region varies from the size of a canton to the size of a (small) country. The first study concerns the risk induced by evaporite sinkholes on a building portfolio. An inventory of damaged buildings is built from different sources and projections are made to estimate the losses that the public building insurance company could face if this type of hazard was insured. Then, a stochastic model which aims at modelling shallow landslides with regard to a precipitation event, and at calculating the probability of impact with buildings is presented. It shows that the location of the landslides which occurred during the precipitation event considered in this study is positively correlated with the building location, suggesting a human influence in the landslides. To conclude this part, an analysis based on a hail event is presented and aims at relating the radar-derived maximum hailstone size reached during an event with the losses, at calculating the mean annual risk using this relation and at modelling random event to refine the risk analysis.

Although the basic principles of risk analysis are relatively simple, this work highlights the diversity of the risk analyses procedure and the need to adjust the procedure to the research question. In addition, it highlights the need for good inventories of events and consequences, since these inventories are crucial to perform a good risk analysis.

Download the PhD manuscript here

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 Małopolska Voivodeship of Poland, Buzău 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.

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.

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

Alexandre Loye: Budgeting rockfall and modeling sediment delivery in torrent systems

Alexandre Loye
Director: Prof. Michel Jaboyedoff
Jury: Prof. Klaus Holliger, Prof. Michel Jaboyedoff, Dr. Marc-Henri Derron, Dr. Frédéric Liébault, Dr. Francesco Brardinoni, Prof. Oldrich Hungr

This thesis is a compilation of projects to study sediment processes recharging debris flow
channels. These works, conducted during my stay at the University of Lausanne, focus
in the geological and morphological implications of torrent catchments to characterize
debris supply, a fundamental element to predict debris flows. Other aspects of sediment
dynamics are considered, e.g. the coupling headwaters – torrent, as well as the development of a modeling software that simulates sediment transfer in torrent systems.

The sediment activity at Manival, an active torrent system of the northern French Alps,
was investigated using terrestrial laser scanning and supplemented with geostructural investigations and a survey of sediment transferred in the main torrent. A full year of sediment flux could be observed, which coincided with two debris flows and several bedload transport events. This study revealed that both debris flows generated in the torrent and were preceded in time by recharge of material from the headwaters. Debris production occurred mostly during winter-early spring and was caused by large slope failures. Sediment transfers were more puzzling, occurring almost exclusively in early spring, subordinated to runoff conditions, and in autumn during long rainfall events. Intense rainstorms in summer did not affect debris storage that seems to rely on the stability of debris deposits.

The morpho-geological implication in debris supply was evaluated using DEM and field
surveys. A slope angle-based classification of topography could characterize the mode
of debris production and transfer. A slope stability analysis derived from the structures
in rock mass could assess susceptibility to failure. The modeled rockfall source areas
included more than 97% of the recorded events and the sediment budgets appeared to be
correlated to the density of potential plane failure. This work showed that the analysis
of process-related terrain morphology and of susceptibility to slope failure document the
sediment dynamics to quantitatively assess erosion zones leading to debris flow activity.

The development of erosional landforms was evaluated by analyzing their geometry with
the orientations of potential rock slope failure and with the direction of the maximum
joint frequency. Structure in rock mass, but in particular wedge failure and the dominant
discontinuities, appear as a first-order control of erosional mechanisms affecting bedrockdominated catchments. They represent some weaknesses that are exploited primarily by mass wasting processes and erosion, promoting not only the initiation of rock couloirs and gullies, but also their propagation. Incorporating the geological control in geomorphic processes contributes to better understand the landscape evolution of active catchments.

A sediment flux algorithm was implemented in a sediment cascade model that discretizes
the torrent catchment in channel reaches and individual process-response systems. Each
conceptual element includes in a simple manner geomorphological and sediment flux information derived from GIS complemented with field mapping. This tool enables to simulate sediment transfers in channels, considering evolving debris supply and conveyance, and helps reducing uncertainty inherent to sediment budget prediction in torrent systems.

This thesis aims in a modest way to shine light on some aspects of sediment dynamics of
torrent systems.