Monthly Archives: November 2013

New publication in NHESS about dynamic risk computation along roads

by Jérémie Voumard, Olivier Caspar, Marc-Henri Derron and Michel Jaboyedoff: “Dynamic risk simulation to assess natural hazards risk along roads”.

Risk generated by natural hazards on roads is usually calculated with equations integrating various parameters related to hazard and traffic. These are static variables, like an average number of vehicles crossing this section every day and an average vehicle speed. This methodology cannot take into account dynamic variations of traffic and interactions between vehicles such as speed modifications due to windy roads, slowdowns resulting from saturated traffic or vehicle tailbacks forming in front of traffic lights. Here we show, by means of a dynamic traffic simulator, that traffic variations may greatly influence the risk estimation over time. The risk is analysed on several sections of an Alpine road in Switzerland using a dynamic vehicles approach, and compared with the results of the static methodology. It demonstrates that risk can significantly increase on sinuous sections because of decreasing vehicle speed. For example, along an 800 m-long section of road containing two hairpin bends, the dynamic risk is about 50 % higher than the static one. Badly placed signalization, slowing down, or stopping the vehicles in a hazardous area may increase the risk by about 150 % (i.e. 2.5 times higher) along a straight road section where vehicles speed is high. A more realistic risk can thus be obtained from a dynamic approach, especially on mountain roads. The dynamic traffic simulator developed for this work appears to be a helpful tool to support decision-making in reducing risk on mountain roads and it shows the importance of keeping the traffic moving as freely as possible.

More information and full paper on the NHESS website.

New publication in NHESS about landslide early-warning systems

by Clément Michoud, Sara Bazin, Lars Harald Blikra, Marc-Henri Derron and Michel Jaboyedoff: “Experiences from site-specific landslide early warning systems”.

Landslide early warning systems (EWSs) have to be implemented in areas with large risk for populations or infrastructures when classical structural remediation measures cannot be set up. This paper aims to gather experiences of existing landslide EWSs, with a special focus on practical requirements (e.g., alarm threshold values have to take into account the smallest detectable signal levels of deployed sensors before being established) and specific issues when dealing with system implementations. Within the framework of the SafeLand European project, a questionnaire was sent to about one-hundred institutions in charge of landslide management. Finally, we interpreted answers from experts belonging to 14 operational units related to 23 monitored landslides. Although no standard requirements exist for designing and operating EWSs, this review highlights some key elements, such as the importance of pre-investigation work, the redundancy and robustness of monitoring systems, the establishment of different scenarios adapted to gradual increasing of alert levels, and the necessity of confidence and trust between local populations and scientists. Moreover, it also confirms the need to improve our capabilities for failure forecasting, monitoring techniques and integration of water processes into landslide conceptual models.

More information and full paper on the NHESS website.

 

Rockfall susceptibility mapping

Advanced Susceptibility Mapping for Natural Hazards in the Swiss Alpine Valley of Bagnes

Alpine municipalities are exposed to numerous natural hazards, such as snow avalanches, rockfalls, landslides and debris flows. The Bagnes and Vollèges municipalities in Valais (Switzerland) lie between 600 m and 4200 m m.s.l. with an area of 300 km2. The anthropization is rapid because of the fast growing ski resort of Verbier. In such situation the municipalities needs to have global overview of the natural hazards for landplaning purpose and decision making. The susceptibility mapping at regional scale allows the detection of the areas that are exposed to natural hazards, without considering the intensity and the frequency of the phenomena.

The aim of this study is to provide susceptibility maps at 1:25’000 for the following natural hazards: landslides, shallow landslides, rockfalls, debris flows, snow avalanches, flooding and river overflowing.

The present method was first developed for the Canton of Vaud (2’800 km2). Because it is applied to a smaller area, more numerical models on High Resolution DEM and field investigations were performed. In addition historical event were included in the study.

  1. The landslide mapping identifies deep-seated slope gravitational deformations, landslides and shallow landslides. It is based on the observations of geomorphological criteria on HR-EM, orthophotos and field work. Finally, the activity of each landslide is described by the knowledge of local guides.
  2. The shallow landslide susceptibility mapping is realized thanks to the software SInMap, calculating Security Factor (FS) and Stability Index (SI) according to the land use, the topography and the climatic conditions. The model is calibrated on the basis of the 67 shallow landslides already identified for the first map.
  3. The rockfall susceptibility mapping is a two steps process. First, the potential source areas of blocks are detected using a statistical analysis of the slope angle distribution, including external knowledge on the geology and land cover. Then the run-out is assessed with numerical methods based on the shallow angle method (software Conefall) and on an energy-based run-out calculation (software Flow-R).
  4. The debris flow susceptibility mapping is based on Flow-R to map debris flow sources and spreading. Slope, flow accumulation, contributive surfaces, plan curvature, geological and land use dataset are used to detect the source areas. The spreading is simulated by a multiple flow algorithm (rule the path that the debris flow will follow) coupled to a run-out distance calculation (energy-based).
  5. The snow avalanches susceptibility mapping is again based on Flow-R, to map sources areas and spreading. Slope, altitude, land use and one minimum surface are needed to detect the sources areas. The spreading is simulated with the “Perla” methodology using Flow-R. A second simulation of the spreading with RAS is performed by means of the alpha-beta methodology.
  6. Regarding to the river overflowing along the Dranse de Bagnes, the hotspots which could create blockages (bridges, pipes, etc.) are identified on the field. The propagations of the overflowing are simulated with Flow-R from the spots recognized earlier.

Finally, results show good concordances with past events and the knowledge of the local geologist and guides. The susceptibility maps will help the decision-makers of the Bagnes valley to prioritize area of interest for the creation of more expensive hazard maps.

For more information, please read Jaboyedoff M., Choffet M., Derron M.-H., Horton P., Loye A., Longchamp C., Mazotti B., Michoud C. and Pedrazzini A.: Preliminary slope mass movements susceptibility mapping using DEM and LiDAR DEM. In: Terrigenous Mass Mouvements, Pradhan and Buchroithner (Eds.), Springer-Verlag Berlin Heidelberg, 109-170, 2012

Featured image: rockfall susceptibility mapping (hillshade: copyright swisstopo)

SafeLand – Living with landslide risk in Europe

SafeLand was a Large-scale integrating Collaborative research project funded by the The Seventh Framework Programme for research and technological development (FP7) of the European Commission. The project team, composed of 27 institutions from 13 European countries, was coordinated by Norwegian Geotechnical Institute (NGI).

SafeLand aimed at developing generic quantitative risk assessment and management tools and strategies for landslides at local, regional, European scales. It also established the baseline for the risk associated with landslides in Europe, and improved our ability to forecast landslide and detect hazard and risk zones.

During this 3-years project, our group mainly contributed to the following deliverables:

  • D 1.6: Analysis of landslides triggered by anthropogenic factors in Europe
  • D 2.10: Identification of landslide hazard and risk “hotspots” in Europe
  • D 4.1: Review of Techniques for Landslide Detection, Fast Characterization, Rapid Mapping and Long-Term Monitoring (as Editor)
  • D 4.4: Guidelines for the selection of appropriate remote sensing technologies for monitoring different types of landslides
  • D 4.8: Guidelines for landslide monitoring and early warning systems in Europe – Design and required technology
  • D 5.1: Compendium of tested and innovative structural, non-structural and risk-transfer mitigation measures for different landslide types

All deliverables and more information about the SafeLand project can be found on www.safeland-fp7.eu.

Featured image: Landslide in Namsos, Norway (copyright SafeLand)

Naples MWL – Mediterranean Workshop on Landslide (Napoli-21 and 22 October, 2013)

Landslides in hard soils and weak rocks – an open problem for Mediterranean countries

Vesuvio

During this workshop the communications addressed the problems describing the special features of hard soils and weak rocks (HSWRs). In addition, the mechanisms of earthflows were discussed. Several researchers from Europe and Mediterranean region were present (see program for people).

It shows clearly that new characterizations of the HSWRs are necessary, several attempts are ongoing. In addition the mechanisms of deformation have to be clarified. The risk-Group presented with the group of ISTERRE (UJF) new developments about the fluidization of HSWRs and their precursory signs using seismic noise, applied to Pont Bourqin Landslide (VD, CH) and Harmalière (Fr).

Michel Jaboyedoff

References:

Emmanuel Wick: Etude détaillée d’un bassin versant à laves torrentielles le long de la Route Internationale n° 7, Courbe de Guido, Province de Mendoza, Argentine

Emmanuel Wick
Supervisors: Prof. Dr. Michel Jaboyedoff, Valérie Baumann

The International Road 7 crosses Argentina from East to West, linking Buenos Aires to the Chile border. While crossing the Andes Cordillera, it puts itself at numerous natural risks, such as avalanches, rock falls and debris flow.

This study will be part of a larger one that will eventually characterize the hazard along the mountainous portion of this road. In this study we will focus on the catchment of the debris flow that swept a car away in January 2005. All of this happened in the northern part of the Guido’s curve, between Potrerillos and Uspallata, in the Mendoza Province.

The Guido debris flow’s catchment measures 4.7 km2 and is constituted of three main torrents that meet a few meters ahead of the International Road. Each one of these torrents started during the evening of January 11, 2005, reaching the road apparently at a very short interval of time. Three years later, signs of them are still perfectly visible, frozen by the arid climate of the region.

The study has been mainly realized from satellite Quickbird imagery and field data collected during a three-week field survey.

It has been established from meteorological data, particle size and mineralogy the conditions that lead to a start of debris flow in this catchment. It depends on strong rainfall combined with numerous material mainly constituted of sands produced by the erosion of a very altered granite. The debris flows have been classified as being a granular matrix that has a collisional-frictional behavior.

From various criteria and with the help of a digital elevation model, it has been shown that the starts happen mainly at the top of the catchment.

Various calculations of volumes, peak discharges and velocities have been realized from geomorphologic methods, empiric formulas and from the analysis of satellite imagery. It appears that an important potential of volume that can be mobilized exists, especially for the longest torrent. A new event could move more than 65.000 m3 of material; the last event reached an estimated velocity of approximately 7 m/s.

A detailed geomorphologic study gave the opportunity to emphasize the activities of anthropogenic origin that led to the deviation of the torrents towards a more adequate place for the construction of a mitigation work. Identically three propagation scenarios illustrating the vulnerability of the road have been proposed.

The present state of the work where underneath the debris flows must cross is not optimum: it is underdimensioned and the last event deposits have not been completely evacuated. The road has been considered as vulnerable even in case of much lighter debris flows.

The results gave the opportunity to put forward several realistic protection measures for the Argentinean context in descending order of importance. They are simple and should be feasible at reasonable cost.

Lauren Gavillet: Dynamique d’érosion et étude de stabilité d’un petit bassin versant alpin: le cas du Courset (VD, Switzerland)

Lauren Gavillet
Supervisor: Prof. Michel Jayboyedoff
Internal Experts: MSc. Pierrick Nicolet, MSc. Benjamin Rudaz
External Expert: Marcel Burri

The landslide triggered on August 26, 2005 by a heavy rain event, in the watershed of the Courset is the origin of this master thesis. To determine if this event was exceptional or if we can expect an intensification of erosion processes, the activity of the watershed is analyzed through its dynamic erosion and through analysis of the stability of slopes.

To collect interesting and useful informations on the watershed, a history of developments and events that have affected the watershed of the Courset is established. Geotechnical analyzes are performed to characterize the soils of the watershed. These results are then used to model the stability of slopes. Rainfalls in the region are also analyzed to be used in this modeling.

After a description of the watershed dynamics, longitudinal profiles and their knickpoints are identified in order to locate areas of the profiles that will be affected by erosion. The erosion potential of the entire watershed is also estimated. Past erosion is studied in light of the observed current activity.

A synthesis of the controversial quaternary filling hypothesis of the Lavey-Le Châtel glacial trough is established, and a new reconstitution is proposed.

Short visit: Collaboration Univ. Barcelona and Univ. Lausanne

The PhD student Manuel Jesus Royan, from RISKNAT Group (University of Barcelona) is going to visit our institute for 10 days, from 4 to 13 November 2013. We will collaborate on the study of rockfall precursory indicators. The interest of this part of his research is on analysing how certain predisposing factors as fractures can be linked with the existence of small scale rockfalls.

We will validate this hypothesis with a long term TLS monitoring dataset at Puigcercos cliff (Catalonia, Spain). More information available here: Royan M.J., Abellán, A., Jaboyedoff, M., Vilaplana, J.M.., Calvet, J. (in press). Spatio-temporal distribution of rockfall pre-failure deformation using terrestrial LiDAR. Landslides