All posts by Clément Michoud

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

Marc Choffet: Cost of Natural Hazards to Building and Influence of Risk Factors in the Context of Building Insurance

Marc Choffet
Director: Prof. Michel Jaboyedoff
Jury: Prof. Michel Jaboyedoff, Prof. Eric Verrecchia, Prof. Franco Romerio, Prof. Jean Ruegg, Dr. Markus Imhof

In Switzerland, the annual cost of damage by natural elements has been increasing for several years despite the introduction of protective measures. Mainly induced by material destruction, building insurance companies have to pay the majority of this cost. In many European countries, governments and insurance companies consider prevention strategies to reduce vulnerability. In Switzerland, since 2004, the cost of damage due to natural hazards has surpassed the cost of damage due to fire; a traditional activity of the Cantonal Insurance company (ECA). Therefore, the strategy for efficient fire prevention incorporates a reduction of the vulnerability of buildings. The thesis seeks to illustrate the relevance of such an approach when applied to the damage caused by natural hazards. It examines the role of insurance place and its involvement in targeted prevention of natural disasters.

Integrated risk management involves a faultless comprehension of all risk parameters. The first part of the thesis is devoted to the theoretical development of the key concepts that influence risk management, such as: hazard, vulnerability, exposure or damage. The literature on this subject, very prolific in recent years, was taken into account and put in perspective in the context of this study.

Among the risk parameters, it is shown in the thesis that vulnerability is a factor that we can influence efficiently in order to limit the cost of damage to buildings. This is confirmed through the development of an analysis method. This method has led to the development of a tool to assess damage to buildings by flooding. The tool, designed for the property insurer or owner, proposes several steps, namely:

  • Vulnerability and damage potential assessment;
  • Proposals for remedial measures and risk reduction from an analysis of the costs of a potential flood;
  • Adaptation of a global strategy in high-risk areas based on the elements at risk.

The final part of the thesis is devoted to the study of a hail event in order to provide a better understanding of damage to buildings. For this, two samples from the available claims data were selected and analysed in the study. The results allow the identification of new trends. A second objective of the study was to develop a hail model based on the available data. The model simulates a random distribution of intensities and coupled with a risk model, proposes a simulation of damage costs for the determined study area.

New publication: ESPL state of science (terrestrial LiDAR on rock slopes)

Abellán, A., Oppikofer, T., Jaboyedoff, M., Rosser, N. J., Lim, M. and Lato, M. J. (2014), Terrestrial laser scanning of rock slope instabilities. Earth Surf. Process. Landforms. DOI: 10.1002/esp.3493

This manuscript presents a review on the application of a remote sensing technique (terrestrial laser scanning, TLS) to a rock slope characterization and monitoring. Key insights into the use of TLS in rock slope investigations include: (a) the capability of remotely obtaining the orientation of slope discontinuities, which constitutes a great step forward in rock mechanics; (b) the possibility to monitor rock slopes which allows not only the accurate quantification of rockfall rates across wide areas but also the spatio-temporal modelling of rock slope deformation with an unprecedented level of detail.

Further investigation on the development of new algorithms for point cloud filtering, segmentation, feature extraction, deformation tracking and change detection will significantly improve our understanding on how rock slopes behave and evolve.

Perspectives include the use of new 3D sensing devices and the adaptation of techniques and methods recently developed in other disciplines as robotics and 3D computer-vision to rock slope instabilities research.

More information and full paper on the ESPL website.

New publication in NHESS about dynamic risk computation along roads

by Jérémie Voumard, Olivier Caspar, Marc-Henri Derron and Michel Jaboyedoff: “Dynamic risk simulation to assess natural hazards risk along roads”.

Risk generated by natural hazards on roads is usually calculated with equations integrating various parameters related to hazard and traffic. These are static variables, like an average number of vehicles crossing this section every day and an average vehicle speed. This methodology cannot take into account dynamic variations of traffic and interactions between vehicles such as speed modifications due to windy roads, slowdowns resulting from saturated traffic or vehicle tailbacks forming in front of traffic lights. Here we show, by means of a dynamic traffic simulator, that traffic variations may greatly influence the risk estimation over time. The risk is analysed on several sections of an Alpine road in Switzerland using a dynamic vehicles approach, and compared with the results of the static methodology. It demonstrates that risk can significantly increase on sinuous sections because of decreasing vehicle speed. For example, along an 800 m-long section of road containing two hairpin bends, the dynamic risk is about 50 % higher than the static one. Badly placed signalization, slowing down, or stopping the vehicles in a hazardous area may increase the risk by about 150 % (i.e. 2.5 times higher) along a straight road section where vehicles speed is high. A more realistic risk can thus be obtained from a dynamic approach, especially on mountain roads. The dynamic traffic simulator developed for this work appears to be a helpful tool to support decision-making in reducing risk on mountain roads and it shows the importance of keeping the traffic moving as freely as possible.

More information and full paper on the NHESS website.

New publication in NHESS about landslide early-warning systems

by Clément Michoud, Sara Bazin, Lars Harald Blikra, Marc-Henri Derron and Michel Jaboyedoff: “Experiences from site-specific landslide early warning systems”.

Landslide early warning systems (EWSs) have to be implemented in areas with large risk for populations or infrastructures when classical structural remediation measures cannot be set up. This paper aims to gather experiences of existing landslide EWSs, with a special focus on practical requirements (e.g., alarm threshold values have to take into account the smallest detectable signal levels of deployed sensors before being established) and specific issues when dealing with system implementations. Within the framework of the SafeLand European project, a questionnaire was sent to about one-hundred institutions in charge of landslide management. Finally, we interpreted answers from experts belonging to 14 operational units related to 23 monitored landslides. Although no standard requirements exist for designing and operating EWSs, this review highlights some key elements, such as the importance of pre-investigation work, the redundancy and robustness of monitoring systems, the establishment of different scenarios adapted to gradual increasing of alert levels, and the necessity of confidence and trust between local populations and scientists. Moreover, it also confirms the need to improve our capabilities for failure forecasting, monitoring techniques and integration of water processes into landslide conceptual models.

More information and full paper on the NHESS website.

 

Rockfall susceptibility mapping

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

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

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

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

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

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

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

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

SafeLand – Living with landslide risk in Europe

SafeLand was a Large-scale integrating Collaborative research project funded by the The Seventh Framework Programme for research and technological development (FP7) of the European Commission. The project team, composed of 27 institutions from 13 European countries, was coordinated by Norwegian Geotechnical Institute (NGI).

SafeLand aimed at developing generic quantitative risk assessment and management tools and strategies for landslides at local, regional, European scales. It also established the baseline for the risk associated with landslides in Europe, and improved our ability to forecast landslide and detect hazard and risk zones.

During this 3-years project, our group mainly contributed to the following deliverables:

  • D 1.6: Analysis of landslides triggered by anthropogenic factors in Europe
  • D 2.10: Identification of landslide hazard and risk “hotspots” in Europe
  • D 4.1: Review of Techniques for Landslide Detection, Fast Characterization, Rapid Mapping and Long-Term Monitoring (as Editor)
  • D 4.4: Guidelines for the selection of appropriate remote sensing technologies for monitoring different types of landslides
  • D 4.8: Guidelines for landslide monitoring and early warning systems in Europe – Design and required technology
  • D 5.1: Compendium of tested and innovative structural, non-structural and risk-transfer mitigation measures for different landslide types

All deliverables and more information about the SafeLand project can be found on www.safeland-fp7.eu.

Featured image: Landslide in Namsos, Norway (copyright SafeLand)

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.