Ryan Kromer is PhD graduate of Queen’s University and a post doctoral researcher at the Colorado School of Mines. He was a visiting PhD student at the University of Lausanne during 2015 and 2016 and is now visiting the Risk group from April to June 2019. During his visit, he will be conducting research on automated monitoring of landslides using terrestrial LiDAR and photogrammetry. The research visit is supported by the Herbette Foundation. Ryan is looking forward to another fruitfull visit with the group.
The Risk Analysis Group (C. d’Almeida, F.Noël) spent a week in the principality of Andorra to carry out lidar survey. The city of Andorra-la-Vella, located in the hearth of the Pyrenees is a very dense city surrounded by landslides and active cliffs.
Lidar and photographs survey of 12 sites were performed. These sites were previously monitored by the Risk Analysis Group (Antonio Abellan) in 2009 and 2012. Exploiting the lidar scans will permit to quantify the erosion activity of the cliffs and detect potential instabilities. The Risk Analysis Group thanks the geo-hazard expert Joan Torrebadella from Georisk-international for his warm welcome, and accompanying during this week.
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
- 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.
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
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.