We present a new method to determine the average propagation speed of avalanches using seismic techniques. Avalanche propagation speeds can reach 70 m/s and more, depending on a wide range of factors, such as the characteristics of the avalanche track (e.g. topography) and the snowpack properties (e.g. density). Since the damage produced by the avalanche depends primarily on the size and on the speed of the avalanche, the knowledge of the latter is therefore crucial for estimating avalanche induced hazard in inhabited mountain areas. However, our knowledge of this basic physical parameter is limited by the difficulty of conducting various measurements in the harsh winter weather conditions that often accompany this natural phenomenon.The method of avalanche speed determination presented in this paper is based on cross-correlation and time-frequency analysis techniques. The data used in this study come from the Ryggfonn (Norway) avalanche experimental site operated by the Norwegian Geotechnical Institute (NGI), and recorded by an array of 6 geophones buried along the main avalanche path during the 2003-2004 and 2004-2005 winter seasons. Specifically, we examine the speeds of 11 different events, characterized by size and snow type. The results obtained are compared with independent speed estimates from CW-radar and pressure plate measurements. As a result of these comparisons our method was validated and has proved to be successful and robust in all cases. We detected a systematic behaviour in the speed evolution among different types of avalanches. Specifically, we found that whereas dry/mixed type flow events display a complex type of speed evolution in the study area with a gradual acceleration and an abrupt deceleration, the speed of the wet snow avalanches decreases with distance in an approximately linear fashion. This generalization holds for different size events.In terms of time duration and maximum speed of the studied events, dry/mixed type avalanches lasted between 8 to 18 s and reached speeds up to 50 m/s, whereas the duration of wet avalanches ranged between 50 and 80 s and their maximum speeds were 10 m/s.

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Abstract. Various types of slope processes, mainly landslides and avalanches (snow, rock, clay and debris) pose together with floods the main geohazards in Norway. Landslides and avalanches have caused more than 2000 casualties and considerable damage to infrastructure over the last 150 years. The interdisciplinary research project "GeoExtreme" focuses on investigating the coupling between meteorological factors and landslides and avalanches, extrapolating this into the near future with a changing climate and estimating the socioeconomic implications. The main objective of the project is to predict future geohazard changes in a changing climate. A database consisting of more than 20 000 recorded historical events have been coupled with a meteorological database to assess the predictability of landslides and avalanches caused by meteorological conditions. Present day climate and near future climate scenarios are modelled with a global climate model on a stretched grid, focusing on extreme weather events in Norway. The effects of climate change on landslides and avalanche activity are studied in four selected areas covering the most important climatic regions in Norway. The statistical analysis of historical landslide and avalanche events versus weather observations shows strong regional differences in the country. Avalanches show the best correlation with weather events while landslides and rockfalls are less correlated. The new climate modelling approach applying spectral nudging to achieve a regional downscaling for Norway proves to reproduce extreme events of precipitation much better than conventionalCorrespondence to: C. Jaedicke (cj@ngi.no) modelling approaches. Detailed studies of slope stabilities in one of the selected study area show a high sensitivity of slope stability in a changed precipitation regime. The value of elements at risk was estimated in one study area using a GIS based approach that includes an estimation of the values within given present state hazard zones. The ongoing project will apply the future climate scenarios to predict the changes in geohazard levels, as well as an evaluation of the resulting socioeconomic effects on the Norwegian society in the coming 50 years.

Abstract. This paper proposes a model for assessing the risk posed by natural hazards to infrastructures, with a focus on the indirect losses and loss of stability for the population relying on the infrastructure. The model prescribes a three-level analysis with increasing level of detail, moving from qualitative to quantitative analysis. The focus is on a methodology for semi-quantitative analyses to be performed at the second level. The purpose of this type of analysis is to perform a screening of the scenarios of natural hazards threatening the infrastructures, identifying the most critical scenarios and investigating the need for further analyses (third level). The proposed semi-quantitative methodology considers the frequency of the natural hazard, different aspects of vulnerability, including the physical vulnerability of the infrastructure itself, and the societal dependency on the infrastructure. An indicator-based approach is applied, ranking the indicators on a relative scale according to pre-defined ranking criteria. The proposed indicators, which characterise conditions that influence the probability of an infrastructure malfunctioning caused by a natural event, are defined as (1) robustness and buffer capacity, (2) level of protection, (3) quality/level of maintenance and renewal, (4) adaptability and quality of operational procedures and (5) transparency/complexity/degree of coupling. Further indicators describe conditions influencing the socio-economic consequences of the infrastructure malfunctioning, such as (1) redundancy and/or substitution, (2) cascading effects and dependencies, (3) preparedness and (4) early warning, emergency response and measures. The aggregated risk estimate is a combination of the semiquantitative vulnerability indicators, as well as quantitative estimates of the frequency of the natural hazard, the potential duration of the infrastructure malfunctioning (e.g. depending on the required restoration effort) and the number of users of the infrastructure.Case studies for two Norwegian municipalities are presented for demonstration purposes, where risk posed by adverse weather and natural hazards to primary road, water supply and power networks is assessed. The application examples show that the proposed model provides a useful tool for screening of potential undesirable events, contributing to a targeted reduction of the risk.

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