Influence of snow cover properties on avalanche dynamics
1,2
Walter Steinkogler ,
1
Betty
1
Sovilla ,
Michael
1,2
Lehning
WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
2 École Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Introduction
Fig. 1: Location and overview of VDLS field site
Data
Avalanches with similar initial conditions and topography but
different dynamics selected from Vallée de la Sionne (VDLS) field site in
Western Swiss Alps.
VDLS field site characteristics (see Fig. 1):
• ~1200 vertical meters (2695 m a.s.l. - 1491 m a.s.l.)
• avalanche path length: ~ 2300 m
Fig. 5: Avalanche reaches
road multiple times a
winter and also endangers
houses (Austria).
# 816
Snow avalanches with the potential of reaching traffic routes and
settlements are a permanent winter threat for many mountain
communities. Snow safety officers have to take the decision whether
to close a road, a railway line or a ski slope.
Those decisions are often very difficult and demand the ability to
interpret weather forecasts, stability and structure of the snow cover
and to evaluate the influence of the snow cover on avalanche run-out
distances.
We focus on the effects of snow cover on avalanche dynamics and
thus run-out distance.
# 5274
Corresponding author: [email protected]
In cooperation with:
Regione Valle d‘Aosta, Dipartimento Difesa del Suolo e Risorse Idriche
Regione Lombardia, Agenzia Regionale per la Protezione dell‘Ambiente
Arpa Lombardia, Direzione Generale Protezione Civile, Prevenzione e Polizia Locale
Arpa Piemonte,Agenzia Regionale per la Protezione Ambientale
Kanton Graubünden, Amt für Wald Graubünden, Naturgefahren
Canton Valais, Service des forêts et du paysage (SFP), section des dangers naturels
Fig. 6: Avalanche reached
railway and caused train to
derail (Switzerland).
# 628
# 509
Results
In a first step all avalanches are investigated for the amount of snow they
entrained along the track and their flow dynamical characteristics. Then the
prevailing snow cover conditions are reconstructed by using data from local
snowpits or modelled stratigraphy from SNOWPACK.
• Separation of avalanches in release, erosion and run-out zones
• All avalanches significantly increase their mass (see Table 1) along the
path due to entrainment
• Entrained mass along the track was calculated by taking erosion
depth from FMCW radar B into account
• Run-out is limited by counterslope
Selected avalanches
• All avalanches released from same area with similar initial mass
(see Table 1)
• Extent and run-out reconstructed with airborne laser scans,
photogrammetry, pictures and numerical avalanche dynamics
model RAMMS
• All overflew FMCW radar B and hit the mast (see Fig. 1.)
Fig. 7-10: Extent and run-out of investigated avalanches.
• Erosion depths were identified for all avalanches (see Table 1)
FMCW radar
• Located in avalanche track of VDLS test site
• Allows to identify erosion depth and flow regimes
• Identification of erosion mechanisms (step entrainment,
frontal plowing)
• FMCW B (1892 m a.s.l.) located halfway down the track (see Fig. 1)
Different flow regimes can be identified:
• highly turbulent flow in front of avalanche
• followed by more distinct flow regime, associated with the dense
flow part
• (Powder cloud is not seen in images)
Fig. 11-14: FMCW Radar data located in avalanche track (location B, 1892 m a.s.l.).
Mast Data
• Significant differences in intensity and peak values
• Flow regimes can be identified (compare to FMCW radar pictures in
Fig. 11-14)
• Located in track (1628 m a.s.l.) before run-out zone (see Fig. 1)
• 20m high mast (see Fig. 4)
• Identification of flow dynamical characteristics
• Impact pressure, velocity and flow depth are measured
at multiple heights (see Fig. 4)
Fig. 15-18: Pressure measurements at mast for different heights.
Numerical snow cover model SNOWPACK
• Snow cover properties (e.g. density, temperature) of release and
entrained snow identified by taking erosion depth from FMCW radar
into account
• Average values are calculated (see Fig. 21 and Table 1)
• Input: data from automatic weather stations
• Output: e.g. snow temperature, snow density (Lehning et al.(2002))
• Allows to assess snow cover properties when avalanche occured
Fig. 19-20: SNOWPACK output for density and grain type (aval. # 5274).
Fig. 21: Density and temperature profiles.
Discussion
Fig. 2: Release area of
avalanche 629 and 628 (in
background). © SLF
Fig. 3: Avalanche 816 (2006
03 06) before hitting the
mast. © SLF
Fig. 4: Mast with
measurement
devices. © SLF
References
Lehning, M., P. Bartelt, B. Brown, and C. Fierz, 2002: A physical SNOWPACK model for the Swiss
avalanche warning. PART III: Meteorological forcing, thin layer formation and evaluation. Cold
Regions Science and Technology, 35, 169–184.
E.g. avalanche # 816:
 Highly turbulent part from 0-20 s with values jumping
from 0-1000 kPa
 Followed by more continous flow (20-30 s) with
max. values of 400 kPa
• Very different dynamics can be observed for similar initial conditions and topography
• Similar flow regimes can develop for short vertical distances
• Avalanches with small initial mass can develop distinct flow dynamics (e.g. large impact pressures) if sufficient snow is entrained
• Snow density defines released and entrained mass but doesn‘t seem to have a direct influence on flow dynamics
• Temperature effects seem to play an important role
• OUTLOOK: More detailed investigations on spatial and vertical temperature distributions (SNOWPACK and ALPINE3D)
• OUTLOOK: Influence of temperature on granular formation
Table 1: Summary of investigated avalanches.
Nr.
816
5274
628
509
Date
20060306 20030205 20040119 20030207
Release mass (t)
7906
5635
4118
12095
Entrained mass (t)
79942
59077
19428
9061
Deposition Mass (t)
87848
64712
23546
21156
Dep. Mass / Rel. Mass ()
11.1
11.5
5.7
1.7
Snow density (kg m-3)
216
151
191
204
Erosion depth d0 @ FMCW B (m)
1
1.3
0.8
0.4