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
snow avalanche events are reported since 1118 in the country (Björnsson, 1980), no village
nor town possesses a reliable record of yearly snow avalanche occurrence before c. 1950-
1970; (ii) the recorded maximum extension of avalanches was fairly approximate before these
dates; (iii) several locations where settlements developed were considered to be safe before a
dramatic snow avalanche event occurred (Arnalds and others, 2004), and (iv) recent
catastrophic events followed new, unexpected paths or reached a larger runout distance than
previously recognized.
In the present paper, we test the pertinence of the field data, and discuss the validity of the
application of the proposed α/β ratio in inhabited areas based on known runout distance of
long reaching snow avalanches.
2. BACK COUNTRY REFERENCE PATHS
In all Icelandic snow avalanche prone areas, numerous scattered rock debris and boulders are
visible in the lowland, revealing a recurrent activity up to several tens of meters downhill
from the foot of the talus slope (Decaulne and Sæmundsson, 2006). Most of the time, such
boulders are the only lasting indicators for avalanche runout after snow melt. Boulders
transported by snow avalanches represent the minimum farthest downhill runout of the
avalanches, since the snow deposit generally reaches farther downhill than the boulders (Fig.
1). Field observations of such boulders therefore allow an estimation of the spatial extent of
large snow avalanches.
Figure 1 Scattered snow avalanches transported boulders downhill along snow avalanche
paths: A − above the village of Hnífsdalur, Northwestern Iceland, B − in the
remote part of the valley Fnjóskadalur, Northern Iceland (photos: Armelle
Decaulne).
It is important to recognize the original boulder deposits as deposited by the snow avalanches
without their subsequent perturbation (i.e. displacement, destruction). A further point which
has to be taken into account is the fact that boulders deposited by different snow avalanches
may have accumulated within the same area. However, it appeared to be possible to
distinguish boulders deposited by different avalanches based on the vegetation cover.
Six snow avalanche paths were selected, in four different back country areas (Botn í
Dýrafjörður, Reykjaströnd, Fnjóskadalur and Bleiksmýrardalur (Fig. 2). The six paths were
132 Extreme runout distance of snow-avalanche boulders