Martina Böhme

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.

Rock slope instabilities in Sogn and Fjordane County, Norway: a detailed structural and geomorphological analysis

Abstract More than 250 rock slope failures have occurred in Sogn and Fjordane County in historical times. So far, 28 sites are known where open cracks indicate that rock slope failures may occur in the future. Detailed structural and geomorphological analyses of these sites have been conducted, and form the basis for an evaluation and comparison of the unstable rock slopes. Four of these sites are described in detail herein. The main characteristics for rock slope instabilities in Sogn and Fjordane are: (1) a preferred location within relatively weak rock units, such as phyllites and weathered mafic gneisses; and (2) the development of most instabilities at convex slope breaks, which are evident as knick-points in the slope profile. Sogn and Fjordane is compared with other Norwegian regions, particularly Møre and Romsdal County, with respect to the spatial distribution of past and current rock slope instabilities. Sogn and Fjordane shows the greatest number of historical slope failures, whereas in Møre and Romsdal a larger amount of potential instabilities is observed. We propose that the larger amount of unstable rock slopes in Møre and Romsdal may be controlled by a locally high gradient of ongoing post-glacial uplift and a higher rate of neotectonic activity.

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.

Rock-avalanche activity in w and s Norway peaks after the retreat of the scandinavian ice sheet

We have compiled recently published and unpublished cosmogenic 10Be exposure ages of rock-avalanche deposits and break away scars in western and southern Norway in order to compare those to the retreat of the Scandinavian ice sheet. In total 22 rock-avalanche events were dated by their deposits (19) or break away scars (3). Sampling of rock-avalanche deposits and failure surfaces was not systematic over the region but with few exceptions we sampled all deposits within the same valley. All ages were recently calculated using the CRONUS online calculator and the geochronology ensemble reveal five late Pleistocene events, eight Preboreal events, and nine younger events. The decay of the Scandinavian ice sheet was not spatially synchronous but differed regionally and lasted over several thousand years in places, hence the requirement for widespread dating targets. One rock avalanche (at Innerdalen at 14.1 ka) occurred when ice existed in the valley, which is in agreement with the latest deglacial models. Depositional characteristics of ten (44%) of the rock avalanches suggest ice free conditions although they occurred within the first millennia following local deglaciation. Five events (22%) occurred between 9 and 7.5 ka at a time when climate was warmer and moister than today. Finally seven events (30%) appear to be relatively evenly distributed throughout the rest of the Holocene. Although limited in number we interpret that the dated events are representative of the temporal distribution of post-ice sheet rock avalanches in western Norway. However, the number of rock avalanches occurring onto the decaying ice sheet is likely underrepresented as those deposits are reworked and can be difficult to distinguish from moraine deposits. Our widespread data reveal a rapid rock slope instability response to the initial local decay of the Scandinavian ice sheet followed by a lower and constant frequency following the climate optimum (ca. 8.5 ka) in the Holocene.

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.

Large-Scale Rockslope Deformations in Sogn Og Fjordane County (Norway)

Large rockslope deformations are characterized by distinctive geomorphic signatures such as up-facing ridges, grabens, open cracks, etc. which extend along large sections of valley flanks. They often present relatively low displacement rates (up to tens of mm/y). Among the different factors that condition their development, local relief, structural conditions, rock mechanic behavior, and time are the key factors. In Norway, large scale rockslope deformation are widely represented. In this work we give an overview of the conditioning factors of four unstable slopes in the Sogn og Fjordane county, and their current degree of activity by using field mapping, remote sensing and different monitoring techniques such as dGPS and InSAR.