Vulnerability and risk seminar November 2005

The 17th of November 2005 a seminar on risk and vulnerability analysis was arranged at NGI.  The speakers and participants were sought to represent the different steps in risk analysis, see Figure 1.

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Figure 1: Stakeholder in risk assessment steps

The aims of the seminar were to:

  • Form an arena where specialists, authorities, engineers and researchers can openly exchange experience and ideas about risk assessment
  • Get to know each others - needs and problems
  • Increase the understanding for vulnerability and risk analyses

The seminar consisted of five main parts:

  1. Scope of analysis
  2. Hazard assessment
  3. Vulnerability assessment and consequence analysis
  4. Risk estimation
  5. Risk assessment and mitigation

There were speakers from ICG (NGI and NORSAR), Directorate for Civil Protection and Emergency Planning, County Governor of Sør Trøndelag, County geologist of Møre and Romsdal, National Office of Building Technology and Administration, Scandpower, Standards Norway, Middle East Technical University and Norwegian Water Resources and Energy Directorate

Below you find ppt.-presentations given in the different parts of the seminar. Clicking the blue links opens the documents.

Part I: Scope of risk analysis

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Figure 2: Example that several actors are involved in landslide risk management

Part II: Hazard assessment

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Figure 3: Qualitative hazard map

Part III: Vulnerability assessment and consequence analysis

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Figure 4: Population vulnerability for 1980-1990

Part IV: Risk estimation

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Figure 5 (left): Localisation of the Ormen Lange Gas Field, and Figure 6 (right): Risk acceptability

Part V: Risk assessment and mitigation

Åknes Risk workshop October 2007

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Figure 1 A workshop was held at NGI October to discuss and assess the risk associated with the Åknes rock slope.

Åknes is a rock slope over a fjord arm on the west coast of Norway. 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. Bathy­me­tric surveys of the fjord bottom deposits show that nume­rous and gigan­tic rockslides have occurred many thousands of years ago. The Åknes/Tafjord Project 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, Norddals­fjord, Sunnylvsfjord and Geirangerfjord.

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

  • Bring in all stakeholders into the discussion of hazard and risk at Åknes
  • Reach a consensus on the hazard and risk associated with a rockslide triggered-tsunami
  • Discuss and evaluate mitigation measures
  • Experiment with ETA as communication tool.

The following event trees were constructed during the three-day meeting:

  • rockslide due to seismic trigger
  • rockslide due to high pore pressure trigger
  • rockslide due to weathering and creep trigger
  • tsunami wave against Hellesylt
  • consequences of tsunami
  • optimum observations for early warning.

The participant list for the results shown below included the following representatives:

  • Manager for Åknes/Tafjord project
  • Mayor of community
  • Social scientist from community
  • City planner from community
  • Policeman working on emergency plans and evacuation
  • Local politician
  • Representative from community office
  • Directorate for safety and emergency preparedness
  • Journalist/media
  • Officer from ministry of highways
  • Meteorologist
  • Physical geographer
  • Social geographer
  • Geologist
  • Engineering geologist
  • Rock mechanics specialist
  • Geotechnical engineer
  • Tsunami specialist
  • Instrumentation specialist
  • Earthquake engineer
  • Seismologist
  • Mathematician
  • Statistician
  • Risk analysis specialist

Rockslide Triggered by Earthquake

For simplicity, the steps for the event tree in Figure 8 (seis­mic trigger) are shown for earthquake magnitude of 6 only. Earthquake magnitude (M) of 4 and M between 4 and 6 are treated in the same fashion as magnitude 6, but the probability estimates are different.

The steps for the event tree in Figure 8 include:

  1. earthquake occurs; 
  2. magnitude of earthquake (M = 4 to 6);
  3. distance from earthquake epicenter to rockslide scarp (D=less or greater than 50 km);
  4. earthquake acceleration (Amax < 0.1g to Amax > 0.25g);
  5. pore pressure (PP less or greater than normal);
  6. rockslide occurs co-seismically, i.e. at the same time or within 10 minutes of earthquake, or earthquake may lead to a degradation process leading to slope failure at a later stage (co-seismic , yes or no).


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Figure 2 Event tree for earthquake triggered rockslide

The failure probability is the summation of the failure proba&shy;bi&shy;&shy;lities, Pf, in all the branches of the tree. The aggre&shy;gated annual fai&shy;lu&shy;re probability for Magnitude 6 and larger in the figure above is Pf = 4 x 10-5/yr. To find the total probability of an earthquake triggered rockslide, the aggregated failure probability from the branch with Magnitude between 4 and 6 need to be added.

The event tree approach can quan&shy;ti&shy;fy ha&shy;zard and risk, and indicate the most critical situations. The ETA approach is especially useful for geo&shy;tech&shy;nical problems that involve large uncertainties. The examples given for the Åknes/Tafjord project illustrated the method. One should refer to the Åknes/Tafjord Project for site-specific quantitative estima&shy;tes.

The consensus process with a group of scientists from se&shy;ve&shy;ral fields of expertise, including the geo&shy;scientific, politi&shy;cal, social and public arenas, enabled the participants to quan&shy;tify the probability of occurrence of a catastrophic rockslide and tsunami, examine the required parameters for effective early warning and discuss possible mitigation mea&shy;su&shy;res. Work is still underway on these aspects of the analysis.

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Figure 3 The Åknes rock slope