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
2. SNOW AVALANCHE LOADING
The risks of snow avalanches falling along the line route were extensively investigated by
avalanche specialists through examination of the area, analysis of meteorological data,
reviewing historical evidences of avalanches in the region, modelling snow drift and calculating
runout distances and avalanche velocities see Jónsson and others (2005) and Margreth
and Ammann (2004). Avalanche risk and loading was evaluated for each tower site.
The basic force at a given height on an obstacle caused by an avalanche can be expressed as:
F = p · Cf · A = (0,5 · ρ · V 2 ) · Cf · A
where Cf is a unit force coefficient, p is dynamic pressure, A is the projection of the obstacle
area perpendicular to the avalanche load direction, ρ is density and V is avalanche velocity.
The avalanche loading is divided into three layers; a
dense avalanche core, a saltation layer and a snow
cloud. The saltation layer is an intermediate layer between
the fairly rigidly flowing dense core and the
turbulent snow cloud, physically representing the
transition between the two. Rolling particles on top
of the core can thus be found at the bottom of the saltation
layer, whereas the lesser dense upper part of it
more resembles a snow cloud. Density and flow
velocity are the two variables that govern the avalanche
pressure. Given a fixed velocity in all layers at a
given site, the pressure becomes a function of density.
Intuitively, the pressure reaches its highest value
within the dense core and subsequently reduces with
Figure 2: Definition of snow avalanche loading.
increased elevation. After accounting for the presence
of snow layers and previous avalanches, the upper boundary of the avalanche core reached up to 5 −
8.5 m above ground level. The form of the pressure distribution diagram is shown in Figure 2.
Figure 3 shows the distribution of towers in terms of avalanche core pressure, excluding form factor.
The corresponding avalanche velocities lie in the range of 5 − 49 m/s for the towers in FL3 and FL4.
Following parameters were given for each tower site: thickness of existing snow layer and
previous avalanche, thickness of avalanche core, thickness of snow cloud, avalanche velocity
and direction. Same parameters were given for each span between towers. Some basic numbers
defining the avalanche loading are as follows:
• Avalanche velocity (V): 5 – 49 m/s
• Density of dense core (ρ): 300 kg/m
170 Towers for snow avalanches in 420 kV transmission lines in Iceland
3
• Thickness of core: 2 – 3 m
• Upper boundary of core (due to snow on
ground and previous avalanches): 5 – 8.5 m
• Density at boundary between saltation layer
and snow cloud: 15 kg/m 3
18
16
14
12
10
8
6
4
2
0
• Thickness of saltation layer: 0.10 sec. * V
0 - 50 50 - 100 100 - 150 150 - 200 200 - 250 250 - 300
Core pressure [kPa]
300 - 350 350 - 400
•
•
Finite pressure at top of snow cloud: 150 Pa
Thickness of snow cloud = 15 – 35 m
Figure 3: Distribution of core pressure.
• Unit force coefficient Cf = 1.5 for circular and elliptical tower shapes within the dense
core and saltation layer, Cf = 1.2 in the snow cloud.
Towers [qua.]