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