Aron Somazzi: Rockfall hazard and risk analysis in the Pichoux Gorges (Jura, Switzerland)

First, the potential sources of rockfalls are detected. These locations are determined using the slope histogram method. Then, one passes to the analysis and classification of the discontinuities present in the delimited zones. Density, direction, dip, spacing and persistence can give an idea of the state of the rock, the mode of rupture and the potentially mobilizable size. For this part, we link the measurements made in the field with those obtained digitally by remote sensing (terrestrial LIDAR, structure from motion from car or drone, handheld laser scanner). Then we go on to modeling of rockfall propagation. Four different simulation models were used: Eline, RocFall, Trajecto 3D and Rockyfor 3D. Results are then compared and analyzed.

Finally a hazard map is proposed and the risk assessed. In this study, we focus on the risk of direct block-car impact and the risk of collision with a block that is on the roadway. The sum of the risks gives us a value expressed in deaths per year or loss of francs per year.
After quantifying the value of the current risk, scenarios are proposed to reduce the risk. For this risk management part, cost-benefit analysis was used. This is an economic evaluation of the feasibility of the works of the protections proposed in relation to the costs.

Hydrogeochemical characterization of alpine spring waters

With Hans-Ruedi Pfeifer (Hon. Prof. at the University of Lausanne), I had the pleasure to publish a paper in the Bulletin de la Société Vaudoise des Sciences Naturelles on the chemical composition of alpine spring waters. This paper is a review of water analysis (major and trace elements) according to the type of bedrock forming the catchment.

Although it is published in French, an extended abstract in English is available and reproduced below, with references to the most significant figures.

Derron M.-H., Pfeifer H.-R (2017) : Caractérisation hydrogéochimique des eaux de source alpines. Bull. Soc :Vaud. Sc.Nat, 96, 5-29.

Extended abstract
In order to investigate the influence of bedrock on the chemical composition of alpine spring waters, more than 700 chemical analyses for major and trace elements have been collected from regional reports or thesis. All these waters are from shallow aquifers (no deep or geothermal circulation), where water is cold and oxic, with pH neutral to basic. Five types of bedrock have been distinguished: granite, mafite, ultramafite, limestone and gypseous rocks (mostly gypseous dolomite). Classical physicochemical parameters (pH, temperature and electrical conductivity), major elements and, depending on the authors, about 15 trace elements are usually provided. The concentration ranges of each element in solution, for each type of bedrock, are provided as percentiles in annexes (online). These values are indicators of common water compositions encountered in moderate to high altitude alpine environment.
Results for major elements show that the total dissolved load depends directly on the nature of the bedrock: silicated, carbonated or sulfate-bearing rocks (Figure 1).

Figure 1: Total dissolved solid vs electrical conductivity for alpine waters from silicated rocks, limestones and gypseous rocks (N=696).

Classical diagrams of Schoeller (Figure 2) and Piper (Figure 3), as well as the hydrogeochemical facies of JAECKLI (1970), are used to characterize each water type, corresponding to the five types of rocks considered.

Figure 2: Schoeller’s diagram for the five types of alpine waters considered in this work (median concentrations for each type).

Figure 3: Piper’s diagram of alpine spring waters for granite (N=98), mafic (61) and ultramafic (36) types on the left, limestones (294) and gypseous rocks (207) on the right.

Two types of water are well differentiated from the others. Waters of gypseous rocks are strongly enriched in Ca, Mg and SO4, with SO4/HCO3 >1. Waters from ultramafic rocks are enriched in Mg, with usually Mg/Ca>1. In all the other types of water (from granites, mafites or limestones), Ca and HCO3 strongly dominate. This convergence of compositions towards an undifferentiated calco-hydrogenocarbonated facies is known in metamorphic rocks. It can be attributed to traces of calcite in the silicate rocks and that metamorphic silicate minerals are much less reactive than calcite. In order to improve the discrimination of these water types, a new ternary diagram is proposed, using relative Ca, Mg and Si concentrations (Figure 4).

Figure 4: Ca-5Mg-10Si ternary diagram (mMole/L) for alpine spring waters (N=442), with indicative isolines of electrical conductivity. The positions of main rock forming minerals are in the upper figure.

It appears from these analyzes that dissolution properties of minerals (i.e. solubility and dissolution rate) strongly control the content in major elements of these spring waters (Figure 9). In particular, a low amount of a highly soluble and rapidly dissolved mineral may play the main role: gypsum or anhydrite in gypseous rocks, brucite in ultramafites, or calcite in the other rock types.
Dissolved contents of trace elements are highly variables, several orders of magnitude for most of them. Median values and overall distributions, by type of rocks, are displayed in Figure 5 and Figure 6 respectively.

Figure 5: Median concentrations of trace elements in alpine spring waters by type of rocks and by valence. Speciation according to Stumm & Morgan 1996 (cmplx = aqueous complexe).

Figure 6: Wheel of trace elements in solution (inside a slice , the points are spread randomly on the radius that corresponds to the concentration).

For most of trace elements, there is no obvious relationship between rock contents and concentrations in solution. Some exceptions are: 1) water from gypseous rocks are enriched in Li, Rb and Sr; 2) concentrations of U, Mo, As are higher in water from granite. In order to interpret these data and to identify the processes regulating the concentrations of trace elements in solution, the valence, the speciation and a mobility index are used (Figure 10). Dissolution properties of minerals seem to control the concentrations of alkaline elements (Li, Rb, Sr, Ba). Very low concentrations of dissolved Fe, Al, Mn and Ti may be due to precipitation as oxy-hydroxides. Adsorption of transition metals (Co, Ni, Cu, Zn, Cd, Pb) on mineral surfaces or suspensions can regulate their concentrations in these basic waters. Higher valence elements (Si, U, Mo, Cr, As) form anionic complexes in natural waters. If they are present in soluble minerals, these anionic complexes may explain the observed enrichment of these elements in some specific types of water (granitic and gypseous).

Figure 9: Solubility vs dissolution rate for main rock forming minerals (pure water at 25°C and equilibrium with atmospheric CO2 and O2). The dissolution rate is expressed as lifetime of a 1mm diameter spherical grain. Both Goldich’s sequences are shown for silicates. The upper figure illustrates a typeical dissolution experiment, with: m = mass of dissolved mineral, kcin = dissolution rate at the beginning of the reaction [g/m2/s], S = solubility [g/L], Areac= reactive surface of the mineral [m2/L].

Figure 10: Ratio of the median concentration in water on the concentration in rock (molar concentrations for both). The higher is this ratio, more the element is mobile in the system.

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

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.