On 11 August 2019, a debris flows stroke the village of Chamoson, in Wallis (Switzerland). One car with two passengers was washed away and disappeared in the debris flows.
On August 30, some members of the group risk went in Chamoson (1) to understand the processes going on in the source area up in the valley and (2) to map with a UAV-LiDAR the deposit zone. Our colleague of geophysics, Ludovic Baron, had been spending several days searching the disappeared car with a magnetometer. However the method proved to be very efficient, the car and its passengers were not found in the inspected area.
The national Swiss television was present and produced a short report of these techniques for the evening news:
The Material Point Method (MPM), which is an extension of the particle-in-cell method , provides a promising numerical framework which can naturally handle classical problems involving severe deformations, such as landslides. Interested readers may refer to a general introduction by Fern et al. about MPM here. We are currently developing an MPM matlab solver capable of solving a variety of elastoplastic problems, ranging from granular impacts to granular collapse.
We here only intend to show some preliminary results of a typical slumping process governed by a Coulomb type material with a non-associated flow rule, considering i) a weak layer with a lower internal frictional angle, (ii) an heterogeneous cohesion field and, iii) a strain weakening behavior of the material.
The initial geometry of the problem is described in the Figure below. We can set different value for the basal friction, i.e., mu is ranging from 0 (free slip boundary) to 0.5 (increasing stick-slip behavior at the interface). The mass instantly deforms under self-weight loading, i.e., the solver does not iterate to converge toward a force equilibrium before plastic loading to occur.
The two following numerical solution after 6 seconds shows naturally an important contribution of the basal friction both on the strain localization and the runout distance of the slumping mass. This also suggest different modes of deformation regarding the frictional behavior at the basal interface.
Deeper investigations are now required to fully understand the complex nonlinear behavior of a slumping mass.
This new project, started in June 2019, is a pilot study to develop techniques in order to (1) harmonize the 1:25000 geological maps and (2) to improve the Quaternary mapping of the region of “canton de Vaud” in Switzerland. This region includes 3 geographic zones: the Jura mountain, the Pre- Alpes and the molasse Plateau. During the XX century, geological maps have been produced by different authors over decades. Then, limits or stratigraphical attributions may differ from one map to another. In addition, new types of data are now available, in particular a high resolution LiDAR digital elevation model over the entire canton, and geophysical or boreholes data. Three pilot zones have been selected (one by geographic zone) to test new approaches to improve the mapping of geological formations and Quaternary deposits. Field surveys are also planned for the selected zones. Valérie Baumann, a new post doctoral researcher at UNIL, has an extensive experience in geological and Quaternary mapping. She will be in charge of the project, with the contributions of Michel Jaboyedoff, Jean-Luc Epard and Marc-Henri Derron from the Institute of Earth Sciences.
Detail of the union (red line) between the geological maps Monthey (1960; left side) and Diablerets (1990; right side) at scale 1:25000. Note the mismatch between the same geological units.
3D shaded relief from the summits and three old glacial cirques located in the Préalpes obtained from high resolution LiDAR data
View of the old glacial cirque located between the Tour d´Aï and Tour de Mayen
Once again the field course for master students in risk took place in the beautiful valley of Ubaye this year. During 11 days (end of May – beginning of June 2019), 12 students from environmental risk or geological risk had the opportunity to study the landslide of Lavalette, work on rockwall stability and discover the processes leading to debris flows. We had the pleasure to be hosted at the scientific center of Seolane in Barcelonnette.
Like every year, The European Geosciences Union (EGU) General Assembly in Vienna 2019 (7 – 12 April) was a great opportunity for our Risk Group to present recent researches and to interact with the international and multidiscipline platform of experts and scientists.
This assembly was a record-breaking meeting, with 16,250 presentations: 5,531 orals, 9,432 posters, and 1,287 PICOs. The program featured 683 unique scientific sessions together and received 16,273 scientists from 113 countries.
The Risk Group members presented 10 contributions as first authors whose titles are below.
The usual fieldtrip for bachelor students in risk took place in Les Diablerets . About twenty students took part to this course on 1-3 May 2019. The Pont Bourquin landslide was used for a mapping exercise and as a starting point for a risk analysis.
Example of a landslide map produced by students during 1 day mapping
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.
François Noël, Teresa Gracchi and Emmanuel Wyser of the Risk Analysis Group (UNIL) went to Barcelonnette (France) from the 23rd to the 27th of September for an amazing experimental work in the field dealing with rock fall trajectories and associated topics (e.g. impact response, energy transfer, etc.), amongst other French researchers from Active Deformation Group of the University of Strasbourg (EOST, leaded by Jean-Philippe Malet) and the IRSTEA Institute of Grenoble (leaded by Franck Bourrier).
Several (30) rocks were thrown down into a short but steep gully while seismic signals (EOST) and high-speed imaging (IRSTEA & UNIL) were acquired. High resolution 3D imagings were acquired thanks to TLS and SfM.
Accelerometers (ISTE & EOST) were also included in few rocky blocks to monitor and acquire data to better understand impact response, angular velocity changes and other exciting data.
In collaboration with the Fondazione Montagna Sicura, Michel Jaboyedoff, Antoine Guerin and Li Fei went to Entrèves (Aosta Valley, Italy) on 23 October 2018 to investigate the 1997 Brenva rockslide scar (3’870 m, Mont-Blanc massif), which reactivated in September 2016. A helicopter flight of about 25 minutes allowed acquiring hundreds of pictures (digital and thermal) of the rock mass in exceptional conditions, as the high mountain was dry in late autumn 2018. A high-resolution Structure-from-Motion model was then generated using these pictures, allowing us to analyze in detail the structural features and rockfall activity of the Sperone della Brenva.
Part of the Risk Analysis Group participated to the International Symposium on Rock Slope Stability 2018 in Chambéry, France, on 13 to 15 November 2018. It was a great opportunity to exchange about many topics related to natural hazards and rockfalls. The group presented two contributions as first authors whose titles are below:
About the “rockfall” failure hazard assessment. Michel Jaboyedoff et al.
Coupling 3D rockfall propagation to the spatio-temporal frequency for a realistic rockfall hazard mapping. Cécile D’Almeida et al.
The symposium concluded with very interesting field visits where we did learn about the history of the Mont Granier and its previous rock collapses and about the different instabilities that affects the gorges of Arly and how the local institutions manage them.