Marc-Henri Derron

Inheritance of ductile and brittle structures in the development of large rock slope instabilities: examples from western Norway

Abstract The high density of slope failures in western Norway is due to the steep relief and to the concentration of various structures that followed protracted ductile and brittle tectonics. On the 72 investigated rock slope instabilities, 13 were developed in soft weathered mafic and phyllitic allochthons. Only the intrinsic weakness of such rocks increases the susceptibility to gravitational deformation. In contrast, the gravitational structures in the hard gneisses reactivate prominent ductile or/and brittle fabrics. At 30 rockslides along cataclinal slopes, weak mafic layers of foliation are reactivated as basal planes. Slope-parallel steep foliation forms back-cracks of unstable columns. Folds are specifically present in the Storfjord area, together with a clustering of potential slope failures. Folding increases the probability of having favourably orientated planes with respect to the gravitational forces and the slope. High water pressure is believed to seasonally build up along the shallow-dipping Caledonian detachments and may contribute to destabilization of the rock slope upwards. Regional cataclastic faults localized the gravitational structures at 45 sites. The volume of the slope instabilities tends to increase with the amount of reactivated prominent structures and the spacing of the latter controls the size of instabilities.

Preliminary Slope Mass Movement Susceptibility Mapping Using DEM and LiDAR DEM

Hazard mapping in mountainous areas at the regional scale has greatly changed since the 1990s thanks to improved digital elevation models (DEM). It is now possible to model slope mass movement and floods with a high level of detail in order to improve geomorphologic mapping. We present examples of regional multi-hazard susceptibility mapping through two Swiss case studies, including landslides, rockfall, debris flows, snow avalanches and floods, in addition to several original methods and software tools. The aim of these recent developments is to take advantage of the availability of high resolution DEM (HRDEM) for better mass movement modeling. Our results indicate a good correspondence between inventories of hazardous zones based on historical events and model predictions. This paper demonstrates that by adapting tools and methods issued from modern technologies, it is possible to obtain reliable documents for land planning purposes over large areas.

Complex landslide behaviour and structural control: a three-dimensional conceptual model of Åknes rockslide, Norway

Abstract Åknes is an active complex large rockslide of approximately 30–40 Mm3 located within the Proterozoic gneisses of western Norway. The observed surface displacements indicate that this rockslide is divided into several blocks moving in different directions at velocities of between 3 and 10 cm year−1. Because of regional safety issues and economic interests this rockslide has been extensively monitored since 2004. The understanding of the deformation mechanism is crucial for the implementation of a viable monitoring system. Detailed field investigations and the analysis of a digital elevation model (DEM) indicate that the movements and the block geometry are controlled by the main schistosity (S1) in gneisses, folds, joints and regional faults. Such complex slope deformations use pre-existing structures, but also result in new failure surfaces and deformation zones, like preferential rupture in fold-hinge zones. Our interpretation provides a consistent conceptual three-dimensional (3D) model for the movements measured by various methods that is crucial for numerical stability modelling. In addition, this reinterpretation of the morphology confirms that in the past several rockslides occurred from the Åknes slope. They may be related to scars propagating along the vertical foliation in folds hinges. Finally, a model of the evolution of the Åknes slope is presented.

Automatic Rockfalls Volume Estimation Based on Terrestrial Laser Scanning Data

Terrestrial Laser Scanning (TLS) technique allows acquiring high density and high precision measurements for landslide surface monitoring. Thanks to this 3D data, it is possible to increase accuracy in collapsed volumes estimation between two temporal acquisitions, from small events (dm3) to large rock slope failures (1000 m3). Some examples include, performing accurate on magnitude-frequency estimation as well as rockfall distribution along cracks or discontinuities and spatial and temporal distribution of rockfall events. This information can thus be used for a reliable understanding of rockfall dynamic in term of space and time. We purpose semi-automatic procedure, which uses two different TLS acquisitions (inputs) that would be able to provide identification of fallen rocks with their respective volume and localization (outputs). The script is divided in 3 main steps: 1. Point cloud segmentation 2. Rockfall clustering 3. Volume computation. The method has been applied on the pilot study area of La Cornalle cliff (Vaud, Switzerland).

Use of targets to track 3D displacements in highly vegetated areas affected by landslides

Monitoring landslides with terrestrial laser scanning (TLS) is currently a well-known technique. One problem often encountered is the vegetation that produces shadow areas on the scans. Indeed, the points behind a given obstacle are hidden and thus occluded on the point cloud. Thereby, locations monitored with terrestrial laser scanner are mostly rock instabilities and few vegetated landslides, being difficult or even impossible to survey vegetated slopes using this method with its classical non-full wave form. The Peney landslide (Geneva, Switzerland) is partially vegetated by bushes and trees, and in order to monitor its displacements during the drawdown of the Verbois reservoir located at its base, an alternative solution has been found. We combined LiDAR technique with 14 targets made of polystyrene placed at different locations inside and outside the landslide area. The obtained displacements were compared with classical measurement methods (total station and extensometer), showing good resemblance of results, indicating that the use of targets in highly vegetated areas could be an efficient alternative for mass movements monitoring.

Geological mapping and fold modeling using Terrestrial Laser Scanning point clouds: application to the Dents-du-Midi limestone massif (Switzerland)

Abstract Geological mapping in vertical rock faces is extremely challenging because of access difficulties and limited possibilities of recognition, localization and measurement of features at large distance with traditional tools. Moreover, vertical areas can be of primary interest since they often display good quality outcrops and relevant geological information. This study focuses on the detailed remote identification of rock types and fold structures using intensity values acquired by Terrestrial Laser Scanning. A correction of the intensity is proposed proportional to the square of the range and to the cosine of the incidence angle. Furthermore, two methods of remote lithological mapping in 3D have been developed using a manual and a semi-automatic approach. Both delivered good results that are consistent with the spatial distribution of rock-types and allowed us to generate an accurate 3D lithological map of Dents-du-Midi massif (Valais, Swiss Alps). The bedding orientation near the hinge of a Km scale fold was measured on LiDAR data in order to define the fold axis. Then, the result was used to build a model of the hinge in 3D.

Slope Instability Detection Along the National 7 and the Potrerillos Dam Reservoir, Argentina, Using the Small-Baseline InSAR Technique

The Argentina Road 7 that crosses the Andes Cordillera within the Mendoza province to connect Santiago de Chile and Buenos Aires is particularly affected by natural hazards. In order to produce landslide susceptibility maps, we aim to detect large deep seated instabilities by applying small baseline interferometric SAR processing. Although topographical- and atmospheric-related trends were not totally removed, 3 unstable zones were emphasized. Further studies focused along shores of the Potrerillos’ dam reservoir highlighted 2 large instabilities with mean velocities up to about 2 cm/year that may affect the N7.

Velocity Prediction on Time-Variant Landslides Using Moving Response Functions: Application to La Barmasse Rockslide (Valais, Switzerland)

Landslides are complex natural systems with non-linear and time variant response to a given input rainfall rate. Although landslide response (e.g. rate of displacement) is normally assumed uniform along time for a constant input (e.g. rainfall rate), we show how the use of adaptive moving windows for parameter’s calibration may lead to a better prediction of the displacement rates. The model is based on the computation of the displacement rates at each time lapse (e.g. one day) as a convolution of a given response function times daily rainfall. The response function was deduced from physically based infiltration laws, being the values of their parameters optimized in order to minimize the error between the real observations and the modeled velocities. The model was then applied to a long-term landslide deformation time series at La Barmasse landslide (Valais, Switzerland). Model performance was significantly improved using moving windows, showing the modeled rates of displacements close resemblance to real observations.

A Neglected Disaster: Landslides and Livelihoods in Central-Eastern Nepal

Landslides have an underestimated impact on rural livelihoods and food security in Nepal, with little attention received from government, the international community, or researchers. Landslides are the leading natural hazard after epidemics, killing over 100 persons per year and are predicted to increase with more intense monsoon rains (Ministry of Home Affairs (MoHA), Government of Nepal and Disaster Preparedness Network (DPNet) (2009) Nepal Disaster Report 2009: The Hazardscape and Vulnerability, Kathmandu). This paper explores current landslide trends in Nepal and impacts on rural development. Six communities in Central-Eastern Nepal were studied; four of landslide-affected rural communities, and for comparison’s sake, two are flood-affected urban areas. The research links geological knowledge about landslides with a social analysis of affected populations. The study’s methods are interdisciplinary, combining a geological assessment of landslides, with participatory social science research methods. Results show that landslides are very costly for households, often obliging them to convert or abandon rice fields, reconstruct their houses, or migrate abroad, considerably affecting food security. The goal of this research is to contribute to the literature on integrated approaches to landslide risk reduction.

Prototype of an open-source web-GIS platform for rapid disaster impact assessment

Abstract Impacts of natural disasters have increased worldwide in the past decades. Earthquakes are one of the disasters that have been studied for real-time analysis and crisis management. Disaster-related losses have been examined by the damage extent of the houses, infrastructures, fatalities and injuries converted to financial losses. Web-GIS technologies provide a wide range of solutions to map these damages, analyze data and publish the results on the web. Open-source tools and data have been widely used today because they stay free and facilitate access to data especially significant in developing countries. This research presents a web-GIS prototype using open-source geo-spatial technologies such as Postgis, GeoServer, Geoexplorer and OpenStreetMap (OSM) to evaluate the rapid impact of naturally produced disasters like earthquake for the estimation of total damages. For this purpose, expert knowledge such as earthquake intensities and vulnerability inputs are imported into the system. Moreover, OSM data for building information are also extracted for the analysis and the loss of the damage is then rapidly estimated and visualized in the platform. This work is part of a project for catastrophe modeling based on open-source data and software. We hope that applying open-source data, techniques and solutions will decrease the time and efforts needed for rapid disaster and catastrophe management.