Thierry Oppikofer

Systematic Mapping of Large Unstable Rock Slopes in Norway

Historically, large rock slope failures impacting into a fjord and causing a several tens of metre high displacement wave have been one of the natural hazards in Norway claiming most lives. In the last 7 years, the Geological Survey of Norway has implemented a systematic mapping approach to characterize unstable rock slopes prone to catastrophic failures, so that future events can be recognized beforehand and society can adapt to the hazard. Systematic mapping has been carried out in three countries and more than 285 unstable slopes have been found. Of these sites, 62 are monitored periodically and 4 have been characterized as high risk objects with continuous monitoring systems installed. In order to classify the likelihood of a future event, rock slope mapping of each object includes the analyses of slide kinematic, velocity of the slide accompanied with other indicators of slide activity and an analysis of recurrence of previous events along the slope.

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.

Hazard and Risk Classification for Large Unstable Rock Slopes in Norway

We present a classification system for hazard and risk that is posed by unstable rock slopes in Norway that might undergo catastrophic failure in future and can cause loss of life. The system is scenario-based as the intensity and rate of displacement, as well as the geological structures activated by the sliding rock mass vary significantly on the slopes. In addition, for each scenario the secondary effects, such as genera- tion of displacement waves or landslide damming of valleys with the potential of later outburst floods, are evaluated. The hazard analysis is based on two types of criteria: 1) Structural site investigations including analysis of the development of a back-scarp, lateral boundaries and basal sliding surface. This includes a kinematic analysis for sliding and toppling based on slope orientation, persistence of main structures and morphologic expressions of the sliding surface. 2) Analysis of slope activity primarily based on slide velocity, change of deformation rates, observation of rockfall activity, and historic or prehistoric events. The analysis of consequences focuses on the potential fatalities to the rock slide scenarios and secondary ef- fects. Based on the hazard and consequence analysis each scenario is classified in a risk matrix into cat - egory low, medium or high risk.

Preliminary Global Catalogue of Displacement Waves from Subaerial Landslides

Displacement waves generated by subaerial landslides can have devastating effects and extend impacts of a landslide tens of kilometres or more. Most research has involved characterization of individual events and, in some cases, comparison of a few events. We have begun to broaden understanding of the phenomenon by compiling a preliminary global catalogue of landslide-generated displacement waves that provides insights into their geographic distribution, size, frequency, and range of mechanisms and impacts. The catalogue is based on a review of published case studies, tsunami catalogues, and the Norwegian national landslide database. It contains 254 events from the fourteenth century AD to 2012. Greater event density in regions with longer written histories, in areas with more settlement, and in the more recent part of the record suggests spatial and temporal biases in documentation and recording. Despite these biases, the spatial pattern of events suggests settings—likely determined by geologic, physiographic and tectonic controls—with elevated hazards that can help focus hazard investigation and mitigation. The diversity of events helps identify differences in displacement wave processes and their resulting impacts, such as wave run-up and travel distance.

Approach for Systematic Rockslide Mapping of Unstable Rock Slopes in Norway

Systematic mapping of unstable rock slopes has been carried out in Norway since 2005. More than 300 unstable or potential unstable rock slopes have been detected and characterized so far. This utilises a standardized hazard and risk classification system that was established in 2012. The determination of the hazard and risk level follows a new standard approach for the systematic mapping of the analyzed sites that is iterative, starting with simple assessments. However the higher the hazard/risk level of a site is, the larger the amount of geological information collected, and the more detailed the run-out models and consequence analyses that will be carried out. This approach allows mapping resources to be focused on sites with higher risk level, delivering products with different levels of detail. Rock slope failures that would not result in any loss of life, as there is no life line or building in the run-out area, are mapped without a probability assessment. These analyses thus have no scale for the hazard class. Rock slope failures that can result in loss of life are analyzed using qualitative hazard analyses, thus the mapping products are hazard maps with qualitative probability classes. The work on this mapping approach is still ongoing; methods for assessment of the occurrence and consequences of secondary processes (e.g., triggering of displacement waves in water bodies, river damming and outburst floods) need still to be defined. An iterative approach will also be developed to analyze those processes.

Database of Unstable Rock Slopes of Norway

The Geological Survey of Norway is currently developing a database for unstable rock slopes in Norway. The database is intended to serve three main purposes: (1) to serve as a national archive for potential unstable slopes for use in hazard and risk analysis, aerial planning and mitigation work as well as research; (2) to serve as a robust and easy to use database during field mapping of unstable rock slopes; (3) provide a public available database accessible through an online web map service. The database structure contains several feature classes storing both raw and processed data, including structures, lineaments, displacement measurements, run-out areas and other observations. All feature classes are linked to one main point which holds a general summary for each unstable slope.

Hazard Assessment of Unstable and Potential Unstable Rock Slopes in Storfjord (Western Norway)

To perform quantitative hazard assessments of unstable rock slopes based on magnitude-frequency relations detailed information about the specific unstable rock slopes is needed. Systematic field mapping in the Storfjord area in western Norway led to an inventory of 13 unstable rock slopes and 31 potential rock slopes. This study presents necessary input data for such assessment, namely volume estimates using the sloping local base level technique and a qualitative hazard ranking based on the hazard and risk classification system for unstable rock slopes in Norway. This hazard ranking is based on a series of geomorphological and structural criteria and signs of activity. The hazard ranking shows clearly different cumulative frequency distributions for unstable rock slopes and potential ones with average scores of 5.2 and 3.4, respectively, out of 12 maximum points. These differences are explained by the insufficient development of delimiting structures in potential unstable rock slopes due to the absence of past displacements.

Catalogue of Historical Displacement Waves and Landslide-Triggered Tsunamis in Norway

Due to Norway’s more than 100,000 km long coast line and tens of thousands of lakes, Norwegian society is strongly exposed to displacement waves caused by landslides impacting into water bodies and to tsunamis caused by submarine landslides. In a database that goes back to the fourteenth century 270 such events are reported, related to rock falls, rock avalanches, debris flows, quick clay slides, snow avalanches and submarine landslides. By far the most severe effects are related to rock avalanches impacting into water bodies setting off waves with near-field heights of several tens of meters and far-field heights locally exceeding 10 m. Consequently, the highest numbers of fatalities (283) are related to those events reaching up to 73 fatalities in a single event. Our database has the potential to aid prediction of future landslide-generated displacement waves and tsunamis in Norway. However, it became evident that detailed data comprising landslide type, landslide volume, wave heights around the water body, only exist for few events and it is needed to study in detail future events in order to build better data sets that might be used for empirical prediction of those secondary effects.

Earthquake-Triggered Subaerial Landslides that Caused Large Scale Fjord Sediment Deformation: Combined Subaerial and Submarine Studies of the 2007 Aysén Fjord Event, Chile

On 21 April 2007 (Mw 6.2) an earthquake triggered more than 500 landslides near the epicenter along the Aysen fjord, Chile. One of the major failures occurred at the Punta Cola Valley involving a volume of 20.9 million cubic meters of rock. The main rockslide was followed by a rock/debris avalanche involving talus and glacio-fluvial deposits in the slope toe and valley floor that added a volume of 7.3 million cubic meters as entrained material. About half of the material involved in the rockslide-debris avalanche reached the shoreline and entered the fjord pushing deltaic deposits offshore while inducing a shoreline retreat of 100 m. The impact of the debris avalanche deformed an area of 7.6 km2 of the otherwise featureless and smooth sedimented fjord floor. The central part of the deformed area is currently deeper with respect to the undeformed floor, which suggests that between 1 and 10 m of sediment were eroded from an area of 1.85 km2 due to the direct impact of the avalanche. The combination of debris avalanche impact of this and other landslides, subaqueous failures and fjord floor deformation generated a series of displacement waves within the fjord with several meters to tens of meters high run-up along the shoreline.

The Role of Inherited Structures in Deep Seated Slope Failures in Kåfjorden, Norway

From studies of orthophotos and through field work, a complex deformation pattern has been recognized in the Lyngen area, Troms, Norway. The area is among the most alpine in Norway and contains a strong clustering of rock slope failures. The rock slope failures are characterized by two different deformation styles, and the difference in style is geographically separated by a fjord and valley lineament. Field studies suggest that two directions of tension oriented almost perpendicular to each other, utilize pre-existing brittle to brittle/ductile fabrics inherited from much older deformation events. The NE-SW direction of tension is parallel to the average displacement vector pointing down-dip along inherited faults. This vector is gravitationally controlled. The NW-SE displacement vector trends strike-parallel along the inherited faults. The presence of the latter appears to be confined geographically.