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
Figure 2 The design dam height H above the snow cover depth hs as a function of the component
of the velocity normal to the dam axis, u1 sinϕ, where u1 is velocity
upstream of the dam and ϕ is the deflecting angle (ϕ = 90° for catching dams), for
several different values of the depth of the oncoming flow h1. A momentum loss
factor, k = 0.85, corresponding to a dam built from loose materials, is assumed in
the impact with the dam. The figure shows curves derived from both supercritical
overflow (red) and shock dynamics (green) labelled with the flow depth h1. The
design dam height should be picked from the higher of the two curves corresponding
to the estimated design flow depth. The part of each family of curves
corresponding to the higher dam is drawn with solid, thick curves. The labelled
axes at the top of the figures show velocity corresponding to the deflecting angles
ϕ = 15, 25 and 35°. The dam height is measured in the direction normal to the
terrain and needs to be transformed to vertical dam height for dams on sloping
terrain. (Note the logarihmic scale on the y-axis.)
The dam height determined according to the new criteria is generally similar to dam height
determined from traditional criteria. Slightly lower dams are recommended in some cases but
considerably higher dams are required for low deflecting angles. As an example, deflecting
dams with ϕ = 10−20° corresponding to typical Froude numbers need to be built approximately
one third higher according to the new criteria compared with the traditional formulae.
This is, however, not as significant a change as it seems at first sight, because the run-up
component of the dam height is much smaller in this case than for larger deflecting angles.
The difference between the new and old criteria may, for example, lead to an increase in runup,
above the snow cover from 6−8 m to 9−10 m.
202 The design of avalanche protection dams