Tag Archives: 2018

José Pullarello: First characterization of the “Rumi-Pana” rock avalanche deposits (Famatina Range, La Rioja, Argentina)

Active mountain fronts are subject to large scale slope collapses which have the capacity to run long distances on piedmont areas. Over time, fluvial activity and other gravitatory processes can intensively erode and mask primary features related to the collapses. Therefore, to reconstruct the history of their occurrence, further analyses are needed, like sedimentologic analyses. This work focuses on the occurrence of large rock avalanches in the Vinchina region, La Rioja (28°43’27.81” S / 68°00’25.42” W) on the western side of the Famatina range (Argentina). Here, photointerpretation of high-resolution satellite images (Google Earth) allowed us to identify two rock avalanches, with their main scarps at 2575 and 2750 masl. There are no determined absolute ages for these deposits, however by comparing their preservation degree with those dated further north (in similar climatic and landscape dynamics contexts), we can suggest these rock avalanches took place during the Pleistocene.We carried out a fieldwork survey in this remote area, including classical landslide mapping, structural analysis, deposits characterization and sampling. The deposits reach the valley bottom at around 1700 masl. with runouts about 5 km and 5.3 km long. In one of the cases, the morphology of the deposit is well preserved, allowing to accurately reconstruct its extension. However, in the second case, the deposits are strongly eroded by sources draining from the mountain front, therefore further analyses should be done to reconstruct its extension. In addition to morphologic interpretations, a multiscale grainsize analysis was done to di↵erentiate rock avalanches from other hillslope deposits: first 3D surface models of surface plots (5x5m) were built with SfM (structure from motion) photogrammetry; then classical sieving and finally laser grain-size analysis of deposits were performed. Samples were collected on different parts of the slope, but also along cross sections throughout the avalanche deposit. This deposits characterization was combined with results from mapping and image analysis to provide a first description of the sequence and the extension of events related to the evolution of this mountain front. The collected data helped to create a series of propagation models with the software DAN3D, developed by Hungr (2009). We chose a Voellmy rheology for the model with f = 0.10 ± 0.05 and ξ = 100 (m/s2) ± 50 (m/s2) for the rock avalanches and f = 0.10 ± 0.05 and ξ = 500 (m/s2) ± 200 (m/s2) for the debris flow. The results show a good propagation with more dispersion that we can see on the field. Part of the cover on the numerical model is not visible on the field, probably due to erosion and transport having moved the material, resulting in the current landscape.

Author: José Pullarello
Director: Marc-Henri Derron, Michel Jaboyedoff.
External Supervisor: Ivanna Penna.

Jérémie Voumard: “Natural hazards affecting the Swiss transportation networks: from their characterization to their low cost survey”

Jérémie Voumard
Director: Prof. Dr. Michel Jaboyedoff
Jury: Dr. Marc-Henri Derron, Dr. Andrea Pedrazzini, Dr. Nicolas Pollet, Prof. Dr. Suren Erkmann

Geological and hydro-geological natural hazards, as landslides and floods, are a threat to many transport networks built in mountainous areas. These risks, that are often small in intensity, are poorly evaluated or unknown.
This doctoral thesis focuses on the characterization and quantification of natural hazards that impact Swiss communication tracks. It also explores various remote sensing techniques dedicated to the survey of areas around traffic lanes.
A database has been created to characterize the many small events that regularly affect roads and railway lines in order to compensate the lack of knowledge. The number of events and their trends – such as their spatio-temporal distribution, the weather, the geology, the direct damages or the types of affected tracks- are thus known at the national level over a period of five years (2012-2016). It shows that a natural event, on average, disrupts the traffic every 2.1 days and that the events occur mostly during the months of June and July, late afternoon. Direct costs were estimated at CHF 4 million per year, with an average cost per event estimated at 23 400 francs.
In order to characterize the approaches to roads and railways, we have developed and tested the photogrammetric technique “on-motion Structure from Motion”, whose cost is reasonable. This remote sensing technic makes possible to obtain colorized and georeferenced 3D point clouds from images taken by four action cameras placed on a moving vehicle. Its accuracy has been evaluated in laboratory conditions and on many sites. It was also compared with seven other traditional surveillance techniques to identify advantages and disadvantages.
This work highlights the impact and consequences of small-scale natural hazards that, taken as a whole, are not negligible for society. In addition, this study demonstrates that low-cost survey technic can compete with more expensive traditional survey techniques.

Keywords: Natural hazards, transportation networks, on-motion Structure for Motion, topographical survey.

4th International Symposium on Rock Slope Stability 2018

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:

1 Keynote

  • About the “rockfall” failure hazard assessment. Michel Jaboyedoff et al.

1 Oral

  • 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.

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.

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.

Amenan Agnès Kouame: Apport de la modélisation hydrogéologique dans l’étude des risques de contamination de la nappe d’Abidjan par les hydrocarbures: Cas du benzène dans le District D’Abidjan

Amenan Agnès Kouame
Directors: Prof. Michel Jaboyedoff and Prof. Albert Goula Bi Tié
Jury: Prof. François Zwahlen, Dr. Marc-Henri Derron, Dr. Jean Kouame Kan, Prof. Erkman Suren

This study on the aquifer of the Continental Terminal is carried out in the Abidjan District located on the coastal sedimentary basin in southern Côte d’Ivoire (West Africa). This unconfined aquifer of the city of Abidjan of Mio-Pliocene age is called “Abidjan groundwater”. The water quality of this aquifer is facing with diverse sources of anthropogenic pollution such as scattered deposits of solid and liquid wastes of all kinds. Indeed, the inadequacy of sanitation and drinking water supply systems increases the pollution risk of the Abidjan’s groundwater. Besides, the proliferation of petrol stations, including tank breaking, needs to be considered in the event of an accident, which poses a real threat to groundwater given the complex hydrogeological structure of the region. In order to ensure the effective protection and management of the Abidjan water table, this work proposes to evaluate the risk of contamination of groundwater in the Abidjan aquifer by hydrocarbons such as benzene for the purpose of the implementation of protective measures. To achieve such an objective, a model of underground flow and contaminant transfer was designed from field data and literature. Soil analysis showed two types of soils: sand and silty sand. Using the FEFLOW software, the predictive simulation of underground flow coupled with the transport of dissolved benzene deposited on the soil surface at the N’Dotré and Anador station was implemented. Dissolved benzene initial concentrations are 43.12 and 14.17 mg/l for the two sites respectively for variably saturated zone. The results revealed that a borehole named ZE11 of the “Zone Est”capture field is polluted after 44 years and 2 months because the threshold concentration of 0.001 mg/l is reached. A maximum concentration of 0.011 mg/l is reached at this drilling at 47 years and 2 months. In this zone, 5 other wells in this Zone, ZE1, ZE7, ZE8, ZE13, ZE14 are threatened by pollution because the dissolved benzene was detected after a time ranging from 39 to 46 years. The ZE10 has even recorded a maximum concentration of 0.0008 mg / l in 46 years. The average distance from Anador site these is 4 km. At an average distance of 2 km from the N’Dotré service station, 8 boreholes belonging to the Anonkoua Kouté, AK15 AK5, AK6, AK7, AK8, AK10 AK4 and AK17, are also threatened by pollution, dissolved benzene has been detected. for a time between 38 to 47 years.

With the software, HYDRUS, for an initial concentration of 2 mg/cm3, the dissolved benzene sets 105, 112 and more than 365 days to cross the 100 cm cylinder of sand, silty sand and clay respectively, taking into account the delay. Without delay, these times are reduced to 100, 80 and 50 days respectively for clay, silty sand and clay. Considering the sand material, the pollutant takes more than 7 years to cross the average thickness of the unsaturated zone 56 m.

During its journey, if the pollutant crosses channels, these global times could be 10 to 100 times shorter. Therefore, special attention should be given to the sites of N’Dotré and Anador in case of rupture of the tank especially as this tablecloth of Abidjan is the only source of drinking water in the District of Abidjan. The method of decontamination of the unsaturated zone proposed is “bioventing” taking into account the high percentage of sand that constitutes the soil of the District of Abidjan geological formations.