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
around the structures was similar to that predicted by Tabler (1991) and snow was not being
eroded from the lee side of the fences due to high wind speeds. The design of the fences used
in the test were however not optimal according to Tabler (2003) and Swiss experience
(Margreth, 1997) and can therefore not be used to conclude on optimal design of the
structures under Icelandic weather conditions.
Figure 2: Snow fences to the left and wind baffles to the right in use in Switzerland (photos:
Stefan Margreth).
Nothing has been written about the design of wind baffles for Icelandic conditions. Such
structures consist of one or two cross-shaped boards with one or two posts and are widely
used elsewhere to break up large continuous cornices or to separate avalanche starting zones
(see Figure 2). A wind baffle forms a discontinuity in the snow distribution by increased snow
erosion around the structure. The snow erosion is caused by an increase in the wind speed in
the vicinity of the baffle and the effectiveness of the baffle is not affected by the exact wind
direction. If several wind baffles are built side by side the formation of a large continues
cornice can be prevented.
One of the biggest challenges in the use of snow drift measures in Iceland is designing
durable structures, especially when integrating such structures into avalanche protection plans
for inhabited areas. The uncommonly high wind speed and sudden changes in wind direction
on mountaintops in Iceland might also affect the optimal design of the structures and pose
problems regarding snow erosion on the lee side of the structures. Complicated wind patterns
on mountaintops could be modeled with high resolution computer simulations in order to
evaluate the possibilities of installing useful snow fences, such as was done in the
Hafnarhyrna starting zone in Siglufjörður in northern Iceland (Þórðarson and Jónsson, 2005).
3. SNOW DRIFT MEASURES ABOVE PATREKSFJÖRÐUR
The town of Patreksfjörður is located on the Westfjords peninsula. The town sits under the
mountain Brellur which rises up to 400−500 m a.s.l. The avalanche prone area Urðir is
located above the harbour (see Figure 3). The main starting zone for the avalanches is a
relatively shallow but wide depression at 200−340 m a.s.l. Above the bowl there are 10−20 m
high steep cliff bands. The mountaintop is flat and extensive.
Hákonardóttir, Margreth, Tómasson, Indriðason and Þórðarson 87