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
A comparison of predicted dam heights between the traditional design and the design based
on the shallow-layer equations is shown in Figure 1 for dams with a steep upstream face.
From the Figure, one notes that the shallow-layer theory results in similar heights of steep
catching dams as the traditional theory (λ ≈ 2). In the case of deflecting dams at low
deflecting angles (γ = 10°) the shallow-layer theory predicts higher dams than the traditional
design and at large deflecting angles (γ = 30°) the dams become lower. One further notes that
at low Froude numbers, the theory predicts higher dams than the traditional design.
3. RESULTS AND DISCUSSION
3.1 Braking mounds below Drangagil in Neskaupstaður
Neskaupstaður is located in eastern Iceland. A large part of the residential area is threatened
by avalanches from several well defined avalanche paths. A large avalanche from Drangagil
reached into the current residential area in 1894. The design of avalanche defence structures
for the Drangagil area was initiated in 1997 and they were built in 2002 (Sigurðsson and
others, 1998).
A total of 13 mounds were placed in two staggered rows in front of a catching dam to reduce
the speed of an avalanche which would finally be arrested by the catching dam, see Figure 2.
The geometry of the mounds resembles small dams. The mounds have a steep front facing the
mountain, are 10 m high and each mound is approximately 10−12 m wide at the top.
Figure 2 Braking mounds and catching dam in the Drangagil area in August 2002.
The design avalanche had a return period of 1000 years with a 3 m thick dense core, a speed
of 38 m s -1 , and thus a Froude number of 7 upon hitting the upper row of mounds. The
shallow-layer theory predicts that a dam with an effective height of 32 m is needed such that a
shock will form upstream of the mounds (hcr given by Equation 2). The theory therefore
predicts that the flow will be launched in a supercritical flow state over the mounds. Parts of
the avalanche will be deflected between the mounds, also in a supercritical flow state. The
two rows of mounds were therefore spaced such that an avalanche could be launched
ballistically over the first row of mounds and would land upstream of the lower row and of
the catching dam downstream of the mounds. With this design it is guaranteed that both rows
of braking mounds will effectively participate in dissipating energy from the avalanche before
it hits the downstream catching dam.
The new design guidelines (Jóhannesson and Hákonardóttir, 2003) also lead to a mound
geometry which was quite different from the more common cylindrically shaped mounds.
Hákonardóttir, Tómasson, Indriðason and Sigurðsson 81