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
trajectory of a point mass on the upstream facing sloping side of the dam. A fundamental problem
with the point mass view of the impact of an avalanche with a deflecting dam is caused by the
transverse width of the avalanche, which is ignored in the point mass description. As a consequence
of this simplification, the lateral and longitudinal interactions between different parts of
the avalanche are ignored. Point mass trajectories corresponding to different lateral parts of an
avalanche that is deflected by a deflecting dam must intersect as already deflected material on its
way down the dam side collides with material heading towards the dam farther downstream.
Similarly, it is clearly not realistic to consider the flow of snow in the interior of an avalanche
that hits a catching dam without taking into account the snow near the front that has already
been stopped by the dam. The effect of this interaction on the run-up cannot be studied based on
point mass considerations and a more complete physical description of lateral and longitudinal
interactions within the avalanche body during impact with an obstacle must be developed.
Figure 1 A catching dam at Brún in Bjólfur in Seyðisfjörður, eastern Iceland. An avalanche
that fell on the 9 th of February 2008 and stopped on the dam face can be seen. The
dam is 20 m high with a 10 m high very steep upper part. (Photo: Emil Tómasson.)
2. DAM HEIGHT DETERMINATION
New dam height criteria have been developed based on the concepts of supercritical overflow
and flow depth downstream of a shock (Hákonardóttir, 2004; Hákonardóttir and Hogg, 2005;
Jóhannesson and others, 2008a,b). A dry-snow avalanche will typically flow towards a dam in
a supercritical state, that is with a Froude number greater than 1 (or perhaps greater than some
other limit larger than 1, depending on the rheology). The first determining factor for the
height of both catching and deflecting dams is, that uninterrupted, supercritical flow over the
dam must be prevented. If supercritical overflow is impossible, shallow fluid dynamics predicts
the formation of a shock upstream of the dam. This theoretical prediction has been confirmed
for fluid and granular flow in several chute experiments, and may have been observed
in natural snow avalanches. The second criterium for the design height of avalanche dams is,
that the flow depth downstream of the shock must be smaller than the dam height. These two
requirements in combination constitute the main part of the new design requirements. Furthermore,
the estimated snow depth at the location of the dam is to be added to the dam height.
Figure 2 shows the dam height as determined from the new dam design procedure for a dam
with side slope 1:1.5, corresponding to loose materials.
Jóhannesson and others 201