Barcelonnette field trip 2018

The field trip on gravitational slope movements for master students took place for the fifth year at Barcelonnette (French Southern Alps) at the beginning of June. During two weeks, the students had the opportunity to study the landslide of Lavalette, rockfalls around Meolans and debris flows in the Riou Bourdoux catchment. The quite intensive program was composed of mapping and terrestrial LiDAR in the field during day times, data analysis and numerical modelling the evening.

Hugo Collomb from RTM giving explanations on the debris flows mitigation measures in the Riou Bourdoux catchment

Once again we benefited from all the facilities provided by the Seolane center (center dedicated to host scientific stays at Barcelonnette), and we had the opportunity of a visit guided by Hugo Collomb of the French Office of Forest (ONF-RTM).

Séolane, Pôle d’accueil universitaire

7th Canadian Geohazard Conference

Part of the Risk Analysis Group participated to the 7th Canadian Geohazard Conference from the Canadian Geotechnical Society (CGS) in Canmore, Canada, on 3 to 6 June 2018. It was a great opportunity to exchange about many topics related to natural hazards, geotechnics and rockfalls.  The group presented four contributions whose titles are below:

2 Orals

  • Real-size rockfall experiment: How different rockfall simulation impact models perform when confronted with reality? Noël F., Wyser E., Jaboyedoff, Derron M.-H., Cloutier C., Turmel D. and Locat J.
  • Using Average Velocities Of Deep-Seated Landslides To Develop Intensity-Frequency Scenarios. Jaboyedoff M., Aye Z.A., & Derron M.-H., Artigue V. and Gerber C.

2 Posters

  • Automated decision sight distance evaluation based on airborne topographical data for risk management along linear infrastructures. Cloutier C., Locat J., Noël F. and Jaboyedoff M.
  • Comparison between three rock slope hazard assessment methodologies based on the Åknes case study from Norway. Oppikofer T., Hermanns R.L., Jaboyedoff M., Derron M.-H., Brideau M.-A., Jakob M., Sturzenegger M.

The conference also included a very interesting field visit where we did learn about the flooding that happened in Canmore in 2013 and how the local institutions did respond them. The trip continued with the visit of local sites with mitigation measures and concluded with a dinner on the Sulphur Mountain where we could enjoy a gorgeous view on the Rockies near Banff while exchanging with the other participants.

Risk group @ EGU 2018 in Vienna, Austria

The Risk Group participated in the European Geosciences Union (EGU) General Assembly in Vienna, Austria, on 08 April to 13 April 2018. This meeting was a great success, with 4,776 oral, 11,128 posters, and 1,419 PICO presentations. 15,075 scientists from 106 countries participated.

As usual, attending The European Geosciences Union (EGU) General Assembly in Vienna is an amazing opportunity for our group to be an interactive part with the huge platform of experts and scientific researchers from over the world and from different fields. The Risk Group people presented 10 contributions as first authors whose titles are below.

1 Oral

  • Optimizing rockfall simulations by combining high-resolution gridded digital terrain models with 3D point clouds. François Noël et al.

1 Pico

  • Using average velocities of deep-seated landslides to develop intensity-frequency scenarios. Michel Jaboyedoff et al.

8 Posters

  • Coupling 3D rockfall propagation to the spatio-temporal frequency for a realistic rockfall hazard mapping. Cécile D’Almeida et al.
  • SFM photogrammetry for rockfall hazard evaluation in a zero data site (acase study of a touristic area of northern Tunisia). Mariam Ben Hammouda et al.
  • Urbanized areas, natural hazard and risk in Rwanda. Emmanuel Nduwayezu et al.
  • Axisymmetric granular collapse: underlying invariance of granular dynamic? Emmanuel Wyser et al.
  • Inventory of shallow landslides in regard with their frequency in the Canton of Vaud (Switzerland). Cedric Meier et al.
  • Finding the best locations of monitoring devices based on visibility analysis with 3D point clouds. Teresa Gracchi et al.
  • Practical application of an open-source, web-GIS platform (RISKGIS) in learning risk management of geohazards. Zar Chi Aye et al
  • Assessment of potential pollution of an unconfined aquifer in Abidjan by hydrocarbons. Amenan Agnès Kouamé et al.




Tunisian Geological Days on mapping georisks

The Journées de la Géologie Tunisienne was organized by the office national des mines in Hammamet from 23 to 25 March. This year, this conference was dedicated to the mapping of georisks. Mariam Ben Hammouda and Marc-Henri Derron from the group Risk took part to this conference, presenting advances in point cloud processing. It was also the opportunity to visit the Cap Bon area where Mariam is doing her PhD thesis.

Collapsed road at Cap Bon

Although blue sky, that was a chilly week of March

Field trip and visit in Taiwan

Beginning of March was a great opportunity for Marc-Henri Derron to visit sites and colleagues in Taiwan for the first time. Invited by Prof. C.W. Lin (National Cheng Kun Univ. in Tainan) and Prof. R.F. Chen (Chinese Culture Univ. in Taipei), Marc-Henri had the opportunity to visit large landslides in central Taiwan, as well as giving 3 presentations on various aspect of landslide investigation techniques.

Slope conditions, steep and weathered, are drastically different from those encountered in the Alps. This visit was the first one for a group Risk’s member and we are confident it will lead to further cooperation.

Emmanuel Wyser: Investigations phénoménologiques et numériques de l’impact de gouttes d’eau sur un milieu granulaire et du processus de diffusion

Emmanuel Wyser
Director: Prof. Michel Jaboyedoff
Supervisor: MSc. Benjamin Rudaz

Water erosion phenomenons are increasingly studied and understood but raindrop erosion is far more complex. Raindrop erosion includes subprocesses such as impact, cratering, rim formation, daughter drop splash and soil particle splash. This work is focused on modeling complex ballistic trajectories of soil particle splashes and particle dispersion process. The general purpose is first to recreate the splash effect in laboratory and second to provide optimal numerical models and a better understanding of the soil particle splash.

Since the complex multiphase interactions are difficult to model, it is easier to compute numerically the dispersion process. Physical based models are the most common approach in this investigation field. The first assumption is that the crater shape might be the controlling factor in the dispersion process governed by the average splash distance. Moreover, complex physical based models may govern ballistic trajectories. These assumptions have now to be proven.

Phenomenological observations are given by experiments in laboratory, on a setup inspired by Furbish et al. (2007) study. Fine soil samples are used in this work and advanced grain size analysis is performed using laser diffraction technique. High-speed camera acquisitions and micro-LiDAR records are used throughout experimental investigations. Then, impact velocities are measured as well as crater shape or particle dispersion. Measured velocities tend to be close to those computed by numerical simulations. High-speed photography analysis shows that the mean initial splash angle is dependent on the drop penetration depth. Moreover, the mean splash angle seems to be dependent on the slope at the crater edge.

Numerical computation is then performed to model the dispersion phenomenon. Using a
probabilistic algorithm, the grain size distribution can be taken into account throughout numerical simulation. The initial splash angle mean value is derived from previous assumption about the key role of the crater shape. Gravity, drag and buoyant forces are also taken into account. Model validation is performed by comparisons between experimental and numerical results using digitalized experimental dispersion photography and LiDAR scanning. Several differences between numerical and empirical results are noticed. The shape of the particle splashing distance distribution is found to be not similar. LiDAR acquisition analysis also shows a non-spherical shape for the crater. But a statistical trend exists for the mean crater gradient -mean splash distance relationship. However this has more to do with dierential initial velocities at the crater’s edge than with mean splash angle.

Further perspectives should be oriented in multiphase interactions ( fluid to soil particle) for a better understanding of the whole phenomenon.

Jaccaud Léonard: Étude de l’instabilité rocheuse du Kilchenstock, Glaris (Suisse)

Jaccaud Léonard
Directors: Prof. Michel Jaboyedoff and Prof. Stefan Schmalholz

The Kilchenstock peak is located in the Swiss Alps about 15 km south of the city Glarus. Many rockfalls from the Kilchenstock have been reported since the 19th century. The first study of this rock slope instability is done by Albert Heim in the 30s. The area is mainly composed of folded flysch with a stratigraphy predominantly dipping towards SSE. Heim reveals a sliding mass near the top of the mountain. This represents an unstable volume of 2.5 million cubic meters. At the foot of this area blocks break off and fall towards the town of Linthal 1000m below.

Heim measured velocities of the sliding mass from 1927 to 1932. On two occasions the displacement’s velocity has accelerated (up to 40mm per day) suggesting an imminent large rock-collapse. No catastrophic failure has occurred so far, however such an event remains possible and its characterization consists of the main objectives of the present study.

A detailed structural study is performed based on digital elevation model (DEM) as well as field investigations. The results show that the activity isn’t confined to the area described by Heim, but rather extends to the whole slope, suggesting a larger and deeper potential rock slope deformation at the Kilchenstock. This is supported by different signs of activity and geomorphic gravitation features such as rockfalls, scarps, double ridge, cracks. The existence of several contiguous or imbricated landslides constitute a complex instability who started with the retreat of the glacier

The final results of the study is a detailed geological map of the Kilchenstock and a cross-section in order to better explain which structural parameters constrain the unstable system.

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.

Jean‐Marie Vuignier: Caractérisation de la source sédimentaire et estimation du budget sédimentaire dans le bassin versant de Jatún Mayu (Cochabamba, Bolivie)

Jean‐Marie Vuignier
Director: Prof. Michel Jaboyedoff
Co‐director: Dr. Ivanna Penna
Experts: Prof. Stuart Lane, Dr. Karen Sudmeier‐Rieux

Natural and human-induced erosive processes shape landscape by transferring masses from the mountain to downstream areas. They also impact population both located in the source areas of sediments as well as urban areas settle on the depositional area. Mountain areas in Bolivia present high surface dynamics and high rates of rural migrations, causing e.g. a significant increase of population in Cochabamba city in the last 20 years. This work aims to estimate the sediment production on the Jatún Mayu watershed in Cochabamba department taking into account the different origins of sediments.

The region of study consists of a mountain area situated in the Andes with altitudes ranging from 2500 to 4600m. Fieldwork on July 2014 and high-resolution satellite image
interpretation (2004 & 2009) allowed mapping and measuring landslides and gullies. A
hundred of landslides are recorded mostly around the river channel. Most of the gullies are
situated in the upper part of the valley where the vegetation is less abundant on low-sloping agricultural lands.

Photogrammetric reconstructions using camera and drone were the main method used to
characterize some strategic points along the river in order to get dimensions of landslides,
gullies, as well as the riverbed roughness, as the final goal was to model floods at the mouth of the watershed, where migrants have been settling for the last years. A total of 9 points of interests along the riverbed were surveyed and for each of them a square 5x5m surface was analyzed. Approximately 250 pictures by area were needed to estimate roughness along the channel. A flood model has been performed, by using the Riverflo-2D Plus software, to produce a model of the downstream region.