International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
Extreme runout distance of snow-avalanche transported
boulders linked to hazard assessment;
some case studies in Northwestern and Northern Iceland
Armelle Decaulne 1,2,* , Þorsteinn Sæmundsson 2 and Helgi Páll Jónsson 2
1 University Clermont 2, CNRS UMR6042 Geolab, 4 rue Ledru, F-63057 Clermont-Ferrand cedex 1, FRANCE
2 Natural Research Center of Northwestern Iceland, Aðalgata 2, IS-550 Sauðárkrókur, ICELAND
*Corresponding author, e-mail: armelle.decaulne (at) gmail.com
ABSTRACT
We propose and quantify an α/β ratio for boulders transported by snow avalanches based on
the proportional property of two topographic parameters (i) runout distance α and (ii) path
steepness β. These two parameters were determined from geomorphological investigations in
back country areas where no human infrastructures and major activities have occurred. We
suggest that the application of the proposed α/β ratio within inhabited areas helps to determine
the expected runout of potential maximum magnitude snow avalanches and thus potentially
improves hazard zoning for the mitigation of avalanche danger in populated areas. We tested
this approach in areas with towns and villages exposed to avalanche danger. Our results
confirm that large snow avalanches have already occurred in some areas, while such
avalanches have to be expected in other areas where no or only smaller events have yet been
recorded.
1. INTRODUCTION
Among all terrestrial natural hazards occurring in Iceland, including earthquakes, volcanic
activity, floods, storms and mass movements, snow avalanches caused the main toll in human
lives and economic loss during the last century (Jóhannesson, 2001; Jóhannesson and
Arnalds, 2001). Thus, snow avalanches represent a significant hazard in Iceland for
settlements and vital infrastructures (e.g. transportation corridors). However, since 1995,
numerous mitigation measures have been undertaken to protect the threatened areas and
extensive modeling approaches have been developed (see the numerous work available for
several sites at http://www.vedur.is/ofanflod/haettumat/) (Magnússon, 1996, 2003; Bernharðsdóttir,
2001).
Based on our observations from back country field investigations, we were able to recognize
the maximum downhill extension of snow avalanches. Boulders deposited by snow
avalanches with extreme runout distances were selected to determine the minimum extreme
runout of the avalanches.
Topographic parameters such as the extreme runout and the path steepness are used to
establish a proportional relationship that enables the transfer of the observed data from the
back country sites into areas where human activities (e.g. agriculture, construction sites) have
displaced or removed valuable geomorphic indicators (mainly boulders deposited by
avalanches). We suggest that our approach will contribute to enhance the safety of people in
threatened areas by supplying pertinent information for hazard zoning, because (i) even if
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