ABSTRACT:
A massive rockslide at Åknes in the Stranda municipality in western Norway would have dramatic consequences, as the tsunami triggered by the slide would endanger several communities around Storfjorden. Site investigations, monitoring and warning system for the potentially unstable rock slopes were implemented to reduce the hazard and risk. As part of hazard and risk assessment, event trees were constructed by pooling the opinion of engineers, scientists and stakeholders. The objective was to reach consensus on the hazard, vulnerability and elements at risk (consequences) associated with a rockslide and tsunami, quantify the hazard (probability of a rockslide and tsunami occurring) and the potential losses (human life and material and environmental damage). The probability of occurrence and the risk were obtained through a consolidation of all the branches of the event tree. The event tree analysis results in a map of the risk for the residents for the municipalities close to Åknes. The paper presents the event tree analysis process followed and some of the preliminary results achieved on the hazard associated with failure of the Åknes rock slope.
1. INTRODUCTION
Rock falls and rockslides are among the most dangerous natural hazards in Norway, mainly because of their tsunamigenic potential. The three most dramatic natural disasters in Norway in the 20th century were tsunamis triggered by massive rockslides into fjords or lakes (Loen in 1905 and 1936 and Tafjord in 1934), causing more than 170 fatalities. As public attention on natural hazards increases, the potential rockslides in the Storfjord region in western Norway have earned renewed focus. A massive rockslide at Åknes could be catastrophic as the tsunami triggered by the rockslide represents a threat to the communities around the fjord. The Åknes/Tafjord project was ini-tiated in 2005 by the municipalities, with funding from the Norwegian government, to investigate rockslides, establish monitoring systems and implement a warning and evacuation system to prevent fatalities, should a massive rockslide take place. The potential disaster associated with a rockslide and tsunami involves many parties, with differing opinions and perceptions. As part of the on-going hazard and risk assessment and validation of the early warning system, event trees were prepared by pooling the opinion of engineers, scientists and stakeholders. The objective was to reach consensus on the hazard and risk associated with a massive rockslide at Åknes. The paper describes the potential hazards, the event tree approach and preliminary examples of the results.
2. THE ÅKNES ROCK SLOPE
Åknes is a rock slope over a fjord arm on the west coast of Norway (Figure 1). The area is characterised by frequent rockslides, usually with volumes between 0.5 and 5 millions m3. Massive slides have occurred in the region, e.g. the Loen and Tafjord disasters (Figure 2). Bathymetric surveys of the fjord bottom deposits show that numerous and gigantic rockslides have occurred many thousands of years ago Åknes/Tafjord project (www.aknes-tafjord.no) includes site investigations, monitoring, and an early warning system for the potentially unstable rock slopes at Åknes in Stranda County and at Hegguraksla in Norddal County. The project also includes a regional susceptibility and hazard analysis for the inner Storfjord region, which includes Tafjord, Norddals-fjord, Sunnylvsfjord and Geirangerfjord (Figure 1).
