Tag Archives: 2014

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.

Geoffroy Leibundgut: Analyse de mouvements de versants rocheux et quantification des risques induits le long de la vallée de la Grande-Eau, Suisse: Le phénomène de débâcle

Geoffroy Leibundgut
Supervisor: Prof. Michel Jaboyedoff
Co-supervisor: Jérôme Tixier

La nuit du 23 février 2014, un bloc rocheux de 100 tonnes se décroche et percute un chalet de montagne dans le village d’Isola (Alpes Maritimes, France). Deux enfants sont tués sur le coup. Le 22 mars 2014, un glissement de terrain spontané survient à Oso (Washington, USA) tuant 41 personnes et détruisant près de 49 foyers et autres infrastructures. Le 2 mai 2014, le village d’Abi Barik (province de Badakhshan, Afghanistan) est complétement détruit par deux glissements de terrain consécutifs. Environ 350 personnes sont tuées, 2 500 disparues et plus de 1 000 foyers sont affectés (chiffres tirés du blog internet de l’équipe risque de l’institut ; http://www3.unil.ch/wpmu/risk/).

Les dangers naturels sont au coeur de notre quotidien. Les chiffres énoncés plus haut montrent à quel point l’homme est vulnérable à ces évènements bien souvent aléatoires et difficilement prévisibles. La liste n’est pas exhaustive et on décompte déjà plus de 400 personnes décédées depuis le début de l’année 2014 à cause de ces 3 événements naturels. Dans un contexte où l’on s’attend à voir venir des changements climatiques importants influençant encore plus les mécanismes déclencheurs de mouvements de terrain (précipitations intenses dans certaines parties du globe, fonte des glaciers, etc.), il est plus que nécessaire de prendre en compte les risques liés aux dangers naturels dans nos sociétés et politiques. Le risque fait l’objet d’avancés importantes dans le domaine de la recherche. Des outils de mesures et d’analyses toujours plus performants et la vitesse accrue du développement informatique permettent une analyse et une caractérisation des processus dangereux de qualité à l’heure actuelle. Si la recherche s’attache à développer le savoir scientifique autour des mécanismes naturels dangereux pour mieux les prévenir, les politiques actuelles intègrent, de plus en plus, la notion du risque dans leur gestion du territoire. On voit apparaitre la mise en place de plans de prévention et de plans d’alertes dans les communes. Des politiques de sensibilisation voient également le jour. Autant d’éléments qui permettent, de nos jours, d’avoir une vision intégrée des risques naturels afin de mieux les prévenir et de mieux les gérer en cas de crise.

C’est en ce sens que s’inscrit ce projet. Il s’attachera à étudier deux types de dangers naturels imbriqués sur une zone d’étude particulière le long de la vallée de la Grand Eau, Suisse. A savoir, seront caractérisés, l’aléa d’instabilités rocheuses, autrement dit, la production de chutes de blocs et compartiments rocheux en zone de forte déclivité, puis l’aléa de création d’un barrage naturel dû au dépôt de ces blocs rocheux dans le lit de la rivière (appelé un embâcle) et la rupture potentielle de cette retenue d’eau (appelé débâcle). L’étude se proposera enfin de quantifier les risques d’inondation induits sur la ville d’Aigle en aval.

Ce travail s’articule donc en deux temps. D’abord, on caractérise l’aléa d’instabilités rocheuses. Plusieurs approches sont utilisées pour essayer de quantifier le danger présenté par la zone active. Une place importante sera laissée à la discussion. Ensuite, dans une approche prévisionnelle, il convient de créer des scénarios d’instabilités rocheuses pour simuler des cas de création d’embâcle. L’aléa embâcle/débâcle sera analysé pour plusieurs scénarios et enfin seront quantifiés les risques induits en aval pour les événements de débâcles significatifs.

CHANGES – Changing Hydro-meteorological Risks as Analyzed by a New Generation of European Scientists

The CHANGES network was a Marie Curie Initial Training Network funded by the European Community’s 7th Framework Programme FP7/2007-2013 under Grant Agreement No. 263953. The overall strategy of the network was to bring together a group of researchers with diverse backgrounds, and disciplines. The network included 11 partner institutions hosting one or more researchers and 6 associate partners that co-supervised research projects, offered internships and participated in CHANGES network events.

CHANGES intended to develop an advanced understanding of how global changes (environmental, climate change and socio-economical change) affect the temporal and spatial patterns of hydro-meteorological hazards and associated risks in Europe; how these changes can be assessed, modeled, and incorporated in sustainable risk management strategies, focusing on spatial planning, emergency preparedness and risk communication.

Risk Analysis group of UNIL was involved in WP-4 of the project and hosted two Early Stage Researchers (ESRs) related to the development of an integrated web-based Decision Support System of how to use available risk information in risk reduction. The developed platform was based on open source software and technologies, and the stakeholders can analyze different risk scenarios, evaluate possible risk reduction alternatives and make appropriate decisions using Multi-Criteria Decision Making approaches with the support of other modules and resources provided by the different components of the project.

For more detailed information, deliverables and publications, please refer to the CHANGES website.

Featured image: Super-Sauze, Barcelonnette (copyright CHANGES)

Characterizing and analyzing 3D temporal slope evolution

This was a project carried out at the Group Risk of the University of Lausanne, in close collaboration with experts from the University of Barcelona, the Geological Survey of Norway, the University of Alicante and the UPC. Please find below the summary of the project:

Although the use of new remote sensing techniques, either terrestrial, aerial or satellite-based, is shedding light into how landslides behave and evolve, still many questions need to be solved regarding the treatment of these datasets, more specifically LiDAR point clouds and its application to a better modelling and forecasting of landslides in 4D (X,Y,Z and Time).

Our research was focused on the development of new algorithms for the modeling and quantification of the geometrical variation of different failure mechanisms (e.g. toppling, falls, slides, etc) along time. The project was conceived through a threefold strategy: in a first step, we simulated mass movements at analog scale using a sandbox, in order to acquire high resolution 3D temporal data. Then, we exploited these datasets for the development of new algorithms aiming to better modelling and quantify the landslide geometrical variation along the different phases of the rupture. In the third and final step, we applied these algorithms to the study of more complex landslides in well instrumented pilot study areas, aiming to a better modelling and understanding of the 3D evolution suffered by complex mass movements during the pre-failure and failure stages.

This project was conceived as the logical continuation of the one year FNS project 138015 “Understanding landslide precursory deformation from superficial 3D data”. The outputs of the project will improve future implementation of 3D remote sensing techniques in early warning systems, a great challenge in current risk management strategies.

Funded by: Swiss National Science Foundation (SNSF).

More detailed information can be found at the project’s website.

Sustainable land management in mountain regions of Bolivia and Nepal in the context of outmigration, climate change and disaster risk reduction

This project was carried out at the University of Lausanne, in collaboration with: University of Bern-Centre for Development and environment; University of Neuchatel-Institute of Geography; International Union for Conservation of Nature; South Asia Regional Coordination Office-National Centre of Competence in Research; Helvetas Swiss Intercooperation; and Agroecología Universidad de Cochabamba.

In addition to poverty, food security and unsustainable land management practices, climate change and extreme hazard events are expected to place additional strains on the sustainable management of natural resources. Migration from mountain areas is already on the rise leaving behind women, children and elderly to deal with adapting to changing climate conditions alongside managing daily life: agricultural practices, maintaining terraces, irrigation canals, stabilizing slopes, etc. Although many studies have focused on migration from mountains to urban areas, few studies have analyzed the consequences of changing mountain demographics on land management.

Unsustainable livelihood strategies and land use management can, in turn, reduce access to resources, increase environmental degradation and intensification of hazard events, especially in fragile environments such as mountains. The concept of Sustainable Land Management (SLM) is the most promising response to land degradation that has emerged in the past two decades. It addresses technical and ecological aspects as well as economic and socio-cultural dimensions. Effective and sustainable natural resource management depends on suitable technologies and associated implementation approaches, and on flexibility and responsiveness to changing complex ecological and socio-economic environments.

This project focuses on the issues related to the “left behinds” and those “half-families” who either choose or are forced to stay in their place of origin, and how to make livelihoods options more sustainable and resilient. The goal is to explore the interface between people and mountain dynamics under a context of climate change in order to develop policy recommendations for land management and livelihood strategies, disaster risk reduction and implementation of climate adaptation plans. This trans-disciplinary approach which combines social and physical sciences, will address the following questions: How is land management in mountains being affected by three main factors: migration, climate change (temperature and rainfall patterns) and hazard events. How to increase the resilience of mountain populations facing such issues? Which measures are needed to increase the resilience of livelihoods and land management practices?

For this comparative study, two pilot areas (Quillacollo District, Cochabamba department) in Bolivia and Nepal (Dhankuta District) were selected.

The results of our study will be used to develop practical tools for guiding decisions on promising sustainable land management practices for local land management experts, disaster risk and water managers, local NGOs and policy makers in order to effectively support female-headed households and other land users and finally lead to more sustainable livelihood outcomes.

Funded by: Swiss Network for International Studies (SNIS).