Introduction
Avalanche release is the result of a series of mechanical actions involving
terrain, snow cover and meteorological conditions, and the understanding of
avalanche release at the level of the single mechanical processes is very
complex . Terrain is the only constant, and therefore
often serves as a basis for release area delimitation in avalanche hazard
management and mitigation .
While basing release area estimation on terrain analysis may be valid for
extreme avalanches where only coarse-scale terrain features such as major
ridges are relevant to delimit potential release areas, it reaches its limit
when smaller, more frequent avalanches have to be assessed, as for example
the optimisation temporal mitigation measures for road and ski resort
protection.
The potential release area size of such avalanches is often influenced by
smaller terrain features, which may hinder a fracture to propagate and
delimit the release area. However, during and after snowfall events, wind
, snow gliding and avalanches
redistribute snow and smooth the
fine-scale morphology of the terrain accordingly by filling irregularities. These
processes can have a significant impact on size and location of avalanche
release areas.
To understand the formation of avalanches, one has to recognise that the
winter snowpack consists of layers of different density or cohesion as a
result of intermittent snowfall periods and changing meteorological
conditions. Slab avalanches form due to the failure of a cohesive layer
(slab) overlaying a less cohesive layer (a so-called weak layer).
Accordingly, the bed surface, defined as the sliding plane of the slab (just
below the weak layer), may either be the ground or, most often, an underlying
snow layer. In a shallow snowpack, terrain roughness present at bed surface
can have a stabilising function, hindering the formation of continuous weak
layers as well as providing mechanical support to the
slab . As a result, predominately small and
localised release areas form. With increasing snow accumulation, surface
roughness is progressively smoothed out , and terrain
features buried in the snow cover below the bed surface reduce the mechanical
support of a slab . At the same time, variability in the
surface layers is reduced and the formation of continuous
weak layers and slabs is facilitated . Under these
conditions, wider release areas may form.
These observations raise the question of whether snow depth is related to avalanche
release area size. As surface roughness generally decreases with increasing
snow accumulation, snow depth could serve as a useful parameter to define
avalanche release area scenarios as a function of snow distribution. This
could be an important step forward towards a more snow-cover-dependent
avalanche hazard assessment, as for example, for transport routes or ski
resorts.
Therefore, in this study, the relation between release area size and surface
roughness of artificial triggered avalanches at the Vallée de la Sionne
field site is evaluated. Further, snow depth at a nearby weather station and
– when available – snow depth measured by laser scanning and photogrammetry
in the release area before and after avalanche release is compared to
potential release area size. We investigate in particular how the local snow
distribution affects location and extent of observed avalanche release areas.
Methods
Surface roughness
The main aim of this study is to evaluate whether changing surface roughness with
increasing snow accumulation is related to avalanche release area size. For
this purpose, the vector ruggedness measure developed by
, which has been shown to capture the influence of snow
depth on surface roughness , is used to compute the
irregularity of the terrain. It is a so-called vector dispersion method
, which compares the orientation of normal vectors of a
given patch of surface with the orientations of neighbouring surface patches.
The degree of variability in the orientations serves as a measure of the
irregularity of the topographical surface. Figure illustrates
the principle of vector dispersion methods.
In more detail, the vector ruggedness measure is based on changes of slope
and aspect in a given neighbourhood around a centre pixel of a digital terrain model (DTM).
Biquadratic polynomials are used as a basis for the computation of slope and
aspect:
z=ax2+by2+cxy+dx+ey+f,
where z corresponds to the elevation estimate at a point (x,y) and
a–f are the coefficients that define the quadratic surface
.
Direction (aspect) and magnitude (slope) of the steepest gradient at the
central grid cell of the fitted surface are determined by calculating the rate
of change in x and y direction:
dzdxy=dzdx2+dzdy2.
The partial derivatives for x and y are noted as
dzdx=2ax+cy+d
and
dzdy=2by+cx+e.
In order to obtain the parameter at the central point of the surface
(x=y=0), Eqs. () and () are integrated into Eq. (), and note
dzdxy=d2+e2.
Slope (α) is thus given as
α=arctand2+e2.
Likewise, aspect is defined as
β=arctaned.
Based on these definitions, roughness is computed as follows.
Normal unit vectors of every grid cell of a digital elevation model (DEM) are
decomposed into x, y and z components (Fig. ):
z=1⋅cos(α)dxy=1⋅sin(α)x=dxy⋅cos(β)y=dxy⋅sin(β).
A resultant vector |r| is then obtained for every pixel by
summing up the single components of the centre pixel and its eight neighbours
using a moving window technique.
|r|=∑x2+∑y2+∑z2,
as shown in Fig. b. The magnitude of the resultant vector is
then normalised by the number of grid cells and subtracted from 1:
R=1-|r|9,
where R is the vector ruggedness measure.
The graphic shows the measurement principle of vector dispersion
methods. The dispersion of vectors perpendicular to the terrain surface
defines the degree of surface roughness. Graphic from
.
Decomposition of normal unit vectors of a DTM grid cell into x,
y and z components using slope α and aspect β. Graphic from
.
It is evident that measured surface roughness depends on the resolution of
the DEM and the size of the neighbourhood window. In this study, a resolution
of 0.5 m and a window size of 3 × 3 pixels are used to compute roughness. In
this way, it is ensured that roughness elements down to the size of single
rocks or boulders, which can affect avalanche release propensity, are
captured.
Study area and data acquisition methods
The site of Vallée de la Sionne (VdlS) is located in the south-western part
of Switzerland in the Canton of Valais, near Sion (Fig. ). The
area upon which we focus in this study corresponds to typical locations of
avalanche release areas, and is characterised by elevations between
2460 and 2679 m a.s.l., whilst orientation ranges from E to SE.
The VdlS field site can be divided into three
different basins characterised by distinct topography: Crêta Besse 1 (CB1)
is steepest and roughest with a mean slope of 42.4∘, whereas Crêta
Besse 2 (CB2) is less steep with a mean slope of 36.2∘ and a
homogeneous terrain surface without major ridges or cliffs. CB1 is separated
from CB2 by a prominent rocky ridge. The Pra Roua (PR) basin has the smoothest
terrain surface with an average steepness between the one of CB1 and CB2 and
a mean slope of 37.7∘.
Avalanches are artificially released by explosives from a helicopter.
Generally, experiments were only performed after a significant snowfall (> 0.8 m)
where large avalanches can be expected. Snow depth data are obtained
from a nearby weather station providing half-hour data about snow depth (HS)
and other meteorological parameters such as wind and temperature. Artificial
avalanche release is generally performed at several locations ranging from
Pra Roua to CB2. Until the winter season 2004/2005, snow depth distribution
before and after avalanche release was recorded by photogrammetry
. Measurements were mostly restricted to the area around
the crown fracture, with an average point spacing of 5 m. The accuracy (rms; root mean square)
of the measurements is around 0.25 m when compared to manual measurements of
fracture depth. Since the year 2005/2006, photogrammetry was replaced by a
helicopter-based, airborne laser scanning (ALS) system, providing continuous,
high-resolution (0.5 m) snow depth data. The vertical accuracy of the data
is 0.10 m. A detailed description of the method and the precision of the
measurements can be found in .
Selection and analysis of existing release area data
In this paper, we focus our study on avalanches that were triggered at two
different locations. One is the rather smooth area of CB2 and the second is
the south-western part of CB1, consisting of a steeper, more irregular
surface than CB2. Figure shows the release areas of all
selected avalanches between the years 1998 and 2015 and the approximate
locations of the trigger points. Frequently, spontaneous releases occurred in
the PR basin prior to artificial avalanche release in CB1, which prevented
most release areas to extend into the PR basin. Therefore, for release area
no. 103, only the part that is located in the CB1 basin is considered. All
avalanches are dry slab avalanches. Very small avalanches that did not show
any fracture propagation after the initial explosion were not considered in
the analysis.
In a next step, we collected and calculated snow cover parameters from the
available photogrammetry and laser scanning measurements. In the case of
photogrammetry, values were obtained from and technical
reports issued by SLF
, indicating mean
snow depth before (HS1‾) and after avalanche release
(HS2‾), as well as the vertical distance between the two
(fracture depth d‾) along the fracture line or within
the entire release area when available. It has to be noted that snow
distribution at the crown is often not representative of the entire release
area and restricts the comparison between the two measurements.
In the case of the laser scanning measurements
, snow depth parameters were derived from
difference maps subtracting raster maps of the winter terrain before and
after avalanche release from the summer terrain (or by subtracting the two
snow surfaces in the case of fracture depth). Mean snow depth before
(HS1‾) and after avalanche release
(HS2‾), as well as mean fracture depth
(d‾), was calculated by computing the mean over all
raster cells within the release area from the corresponding difference maps.
Snow depth, HS, and new snow depth, HN, were derived from the weather station,
where new snow depth is defined as the difference in snow depth between
avalanche release and the start of the preceding snowfall period. Release
area width, W, is defined as the maximum horizontal distance between the two
flanks of the release area.
Resultant vector, r, is obtained by summing up the x, y
and z components of all pixels, n, within the neighbourhood window. Graphics
from .
Field site Vallée de la Sionne near Sion. In red, the
locations of the analysed basins, PR, CB1 and CB2, are marked.
Pixmaps© 2016 swisstopo (5 704 000 000).
Further, to take full advantage of the high-resolution snow depth
measurements since the season 2005/2006, we prepared another dataset that
covers all avalanches retrieved with ALS measurements independent of their
location . This also includes avalanches
that are released in the PR basin as well as avalanches that are released all over CB1. The
dataset consists of two sets of 6 dry slab avalanches that were artificially
triggered on 8 March 2006, and on 3 February 2015 (Fig. ).
For these avalanches, a more in-depth analysis was performed. Beside mean
fracture depth (d‾), mean snow depth before
(HS1‾) and after avalanche release at the bed surface
(HS2‾), as well as mean roughness of the snow-free terrain
(RT‾), the snow surface before avalanche release
(R1‾) and the bed surface after avalanche release
(R2‾), was calculated for all triggered avalanches
(Table ). Surface roughness was derived on a pixel basis
using the methodology as described in Sect. , followed by the
computation of the mean over all surface roughness pixels within the extent
of the release area.
Number, date, release area width (W), snow depth
(HS) and new snow depth (HN)
measured at the weather station of Donin du Jour; mean snow depth before
(HS1‾) and after avalanche release
(HS2‾) and mean fracture depth
(d‾) for avalanche release areas occurring before the
winter season 2005/2006. Further, the spatial measurement extent is
provided.
Aval no.
Date
W (m)
HS (m-2)
HN(m-2)
HS1‾ (m-2)
HS2‾ (m-2)
d‾ (m-2)
Extent
CB1
no. 103
10 Feb 1999
165
350
140
200
98
102
Crown
no. 509
07 Feb 2003
285
360
90
291
146
145
Area
no. 628
19 Jan 2004
120
300
110
314
195
119
Area
CB2
no. 301
30 Jan 1999
220
260
120
304
141
163
Crown
no. 200
24 Feb 1999
760
440
100
295
100
195
Crown
no. 506
31 Jan 2003
470
290
60
×
×
60
Crown
no. 629
19 Jan 2004
500
300
110
330
153
176
Area
Number, date, release area width (W), snow depth (HS)
and new snow depth (HN) measured at the weather station of Donin du Jour;
mean snow depth before (HS1‾) and after avalanche release
(HS2‾) and mean fracture depth
(d‾) for avalanche release areas occurring from the
winter season 2005/2006 onwards.
Aval no.
Date
W (m)
HS (m-2)
HN (m-2)
HS1‾ (m-2)
HS2‾ (m-2)
d‾ (m-2)
CB1
no. 816
8 Mar 2006
105
290
120
334
190
144
no. 917
26 Mar 2008
110
350
80
–
–
–
no. 20150016
3 Feb 2015
90
210
110
221
104
118
CB2
no. 726
17 Feb 2005
280
220
70
–
–
170
no. 817
8 Mar 2006
480
290
120
366
229
137
no. 918
26 Mar 2008
310
350
80
–
–
–
no. 20150020
3 Feb 2015
170
210
110
236
56
180
Number, date, release area size (A, mean snow depth before
(HS1‾) and after avalanche release
(HS2‾), mean fracture depth (d‾),
mean roughness of the snow-free terrain (RT‾), the snow
surface before avalanche release (R1‾) and the bed surface
after avalanche release (R2‾) for all avalanche release areas
with ALS measurements in VdlS.
Aval no.
Date
Size A (m2)
HS1‾ (m-2)
HS2‾ (m-2)
d‾ (m-2)
RT‾
R1‾
R2‾
no. 816a
8 Mar 2006
21 874
319
154
165
0.0014
0.0004
0.0009
no. 816b
8 Mar 2006
6944
334
190
144
0.0024
0.0005
0.0015
no. 2006003
8 Mar 2006
1906
296
142
162
0.0019
0.0013
0.0027
no. 2006004
8 Mar 2006
1265
343
148
195
0.0018
0.0006
0.0016
no. 2006005
8 Mar 2006
2885
261
112
149
0.0024
0.0009
0.0023
no. 817
8 Mar 2006
78 390
366
229
137
0.0019
0.0003
0.0007
no. 20150016
3 Feb 2015
10 816
190
74
116
0.0012
0.0003
0.0005
no. 20150017
3 Feb 2015
3508
221
104
118
0.0035
0.0009
0.0015
no. 20150019
3 Feb 2015
622
234
47
187
0.0016
0.0003
0.0018
no. 20150021
3 Feb 2015
341
156
31
125
0.0036
0.0010
0.0035
no. 20150022
3 Feb 2015
974
185
49
136
0.0032
0.0025
0.0035
no. 20150020
3 Feb 2015
10 909
236
56
180
0.0016
0.0002
0.0010
Relating release area size to surface roughness and snow depth
The relation between surface roughness and release area size is explored
using all avalanches where surface roughness computations are available
(Fig. ). Release area size is plotted on the one hand
against mean surface roughness of the snow surfaces before and after
avalanche release as well as against terrain roughness. On the other hand,
surface roughness maps of different snow covers are produced. We investigate
qualitatively whether varying surface roughness with different snow covers is
related to avalanche release area size.
Extent of all selected release areas in
(a) the southwestern part of CB1 and (b) CB2.
Snow depth before avalanche release on (a) 8 March 2006 and
(c) 3 February 2015. Difference of snow depth before and after
artificial avalanche release obtained from the scans of (b) 8 March
2006 and (d) 3 February 2015. The release zones and their avalanche
tracks are clearly visible. Further, release area numbers according to
Table are also shown.
Release area size as a function of its surface roughness at the
level of the snow-free terrain (red), the bed surface (light blue) and the
snow surface prior to avalanche release (dark blue).
Images of the artificially triggered avalanches at the CB2 basin on
(a) 8 March 2006 (avalanche no. 817) and (b) 3 February
2015 (avalanche no. 20150020).
Surface roughness of (a) the summer terrain,
(b) the winter terrain for the snow distribution of 8 December 2010,
(c) the winter terrain for the snow distribution of 3 February 2015
and (d) the winter terrain for the snow distribution of 8 March
2006. In black, the outlines of avalanche release areas at CB2 are shown. The
red circles show critical terrain features with different degrees of
smoothing due to varying snow depth.
Further, release area size is directly compared to several snow cover
parameters. In a first step, release area size of avalanches occurring at the
same locations is plotted against mean snow depth at bed surface measured by
ALS. As laser scanner measurements are not available for most of the
avalanches, release area width was then compared to snow depth measured at
the weather station. To this end, a linear regression between release area
width, W, and snow depth, HS, was performed. To account for the non-linear
increase of release area size with snow depth, logarithmic transformations of
release area width were used in the regression. In this way, the impact of
large, non-linear differences on the correlation is reduced. As avalanche
release areas are often confined by terrain features such as ridges and
gullies, we explore the local snow distribution around terrain
breaks in a last step. To this end, representative snow depth profiles, across locations
where release areas are typically arrested, have been created. The exact
locations of the snow depth profiles are shown in
Fig. .
Results and interpretation
Assessment of release area size
Table shows an overview of all released avalanches
measured with photogrammetry. Snow depth measurements before and after
avalanche release were either performed in the entire release area or along
the avalanche crown. An overview of all released avalanches measured with
ALS can be observed in Table . Using ALS, continuous snow
depth measurements were performed over the entire release area.
It can be observed that avalanche release areas in the CB2 basin are
typically large, with release area widths ranging from 170 to 500 m,
covering large areas of the basin surface. Normally, the release area does
not extend into the CB1 basin and is confined by the clear topographical
break between CB1 and CB2. In one case, during the catastrophic winter of
1998/1999 , an avalanche was released simultaneously over the
entire CB2 and CB1 basin over a width of 760 m. In contrast, release areas
in the CB1 basin are smaller, with release area widths between 90 and
285 m. Release areas mostly occur in sub-areas of the basin, which is
subdivided by many gullies and ridges.
Table describes release area properties of all avalanches
that were captured with ALS measurements, comprising all possible locations
ranging from the PR to the CB2 basin. Four large slabs (>10 000m2) were
observed where the fracture propagated over a larger distance within the very
smooth basins of CB2 and PR (no. 816a and no. 817, no. 20150016 and no. 20150020).
Four medium-sized slabs (1500m2–10 000 m2) were observed on the
southern end of CB1 (no. 816b and no. 20150017) and on CB1 (no. 2006004,
no. 2006005). The other slabs within CB1 were rather small (< 1500 m2) with
only very little fracture propagation (no. 2006003, no. 20150019, no. 20150021,
no. 20150022). Avalanche no. 816a and no. 816b also triggered deeper layers of the
snowpack. Interestingly, avalanche no. 816b released due to the detonation,
whereas avalanche no. 816a was triggered remotely by avalanche no. 816b.
Release area size and roughness
Figure shows release area size as a function of mean
roughness of summer terrain, RT‾, bed surface,
R2‾ and snow surface prior to avalanche release
R1‾ based on avalanches shown in
Table . However, we restricted our analysis to medium- and
large-sized release areas (>1500 m2), as no fracture propagation was
observed for smaller release areas after the initial explosion.
It can be observed that the avalanche bed surface is generally less rough
when compared with the underlying terrain. At the same time, the snow surface
prior to avalanche release is always smoother than the underlying bed surface
(and the terrain). This illustrates the progressive terrain smoothing within
avalanche release areas. The results further show a clear tendency of
decreasing surface roughness with increasing release area size. This tendency
is more pronounced for the bed surface and the snow surface before avalanche
release; to a lesser extent for terrain roughness. This suggests that the
winter terrain appears to be more explanatory of potential release area size
than the summer terrain.
Release area size as a function of snow depth at the bed surfaces
for the avalanches released in 2006 and in 2015.
Release area width in the basins CB1 and CB2 as a function of snow
depth measured at the weather station.
Release area width in the basins CB1 and CB2 as a function of
(a) new snow depth of the snowfall period previous to avalanche
release and (b) mean fracture depth.
Locations of snow depth profiles in the CB1 basin (profiles 2 and 3)
and at the border between CB1 and CB2 (profile 1).
Snow depth profile 1 along the crown fracture of avalanche no. 200
for snow distributions of avalanches no. 200, no. 817 and no. 20150020 at the
area separating CB1 from CB2. The numbers indicate the corresponding snow
depth measured at the weather station. It is important to note that
measurements along the crown fracture of avalanche no. 200 only exist for
data points; at the line in between data points, no measurements are
available.
Elevation profiles (a) 2 and (b) 3
(Fig. ) in snow-covered terrain before avalanche
release. The numbers indicate the corresponding snow depth measured at the
weather station.
However, the decrease of average snow surface roughness is limited to a
certain avalanche size. Release areas exceeding the critical size of
approximately 10 000 m2 show similar surface roughness, as for example
release areas no. 816, no. 817, no. 20150016 and no. 20150020. As an example,
Fig. shows images of release areas no. 817 and
no. 20150020. Surface roughness inside the release area was similar for both
avalanches (Fig. ). However, for release area no. 817,
the fracture could fully propagate through the entire basin and the fracture
stopped at the basin boundary (Fig. a), where larger
boulders and a ridge delimit the basin entity. In contrast, for release area
no. 20150020 (Fig. b), terrain features at the crown and the
lateral boundaries of the slab are observed in the centre of the basin,
potentially hindering the fracture to fully propagate. This suggests that
once a critical minimum value of average surface roughness is reached, a
fracture self-propagates until it is arrested by major terrain features or
major changes in snow cover.
This is supported by Fig. , which shows surface roughness
for the summer terrain and the three winter terrain surfaces of 8 December 2010,
3 February 2015 and 8 March 2006. The winter surfaces are characterised
by mean snow depths, calculated over the entire VdlS basin area, ranging from
1.2 m in 2010 over 2.4 m in 2015 to 3.7 m in 2006. It can be observed that
increasingly large and connected areas of low surface roughness are formed
with increasing snow depth. Moreover, the roughness patterns caused by
different snow covers appear to be well suited to demarcate the observed
release areas. Release area no. 20150020 can be associated with the roughness
pattern in between the summer terrain and the one with little snow depth in
the year 2010, whereas release area no. 817 corresponds rather to the snow
cover of 2006 and 2015. This suggests that surface roughness corresponding to
a snow cover similar to the one at avalanche release may be suited to delimit
the potential size of avalanche release areas.
Interestingly, there is only little difference in surface roughness in CB2
for 2006 and 2015, despite a significant deeper snowpack in 2006. The
difference in snow depth only has effects on the border of the basin, such as
the ridge linking CB1 to CB2 and several features within the CB1 basin (red
circles in Fig. ). These features are more attenuated in
2006 with additional snow depth. It can be assumed that in a further
increasing snowpack, they would also be cancelled out, enlarging the
continuous areas of low surface roughness. This highlights the important role
of snow depth for potential release area size, which will be explored in the
following section.
Release area size and snow distribution
Release area size and snow cover parameters
Figure shows mean snow depth at bed surface as a
function of avalanche release area for the two sets of avalanches in 2006 and
2015 (Table ). It is observed that avalanche release areas
triggered in the deeper snowpack of 2006 are always larger compared to their
similarly located avalanches of 2015. Both snow depth at bed surface and
before avalanche release is consistently deeper for every single avalanche
released in 2006 when compared to its counterpart in 2015. These observations
support our hypothesis that a deepening snow cover forms increasing areas of
low surface roughness, which could have resulted in larger release areas in
2006.
This is confirmed by Fig. , comparing release area
width of all avalanches with snow depth measured at the weather station. A
significant correlation between snow depth and release area width is observed
(R2= 0.78, p=0.023) in CB2 – to a lesser extent also in CB1 (R2= 0.70, p=0.120), which again supports the hypothesis that larger avalanche
releases can form in a thicker snow cover. Moreover, smaller avalanches are
observed in CB1 compared to CB2, supporting the hypothesis that rough
terrain, such as CB1, requires more snow to form sufficiently smooth winter terrain
conditions to produce large release areas. However, smaller avalanches in CB1
could also be explained by the steeper terrain of CB1 compared to CB2,
generally leading to increased avalanche and sloughing activity, preventing
the formation of a thick continuous snow cover.
At the same time, no correlation is observed between new snow depth and
release area width (Fig. a) as well as between mean
slab depth and release area width (Fig. b), suggesting
that these variables alone cannot explain the differences in release area
size.
Release area size and local snow distribution
The results in the previous section showed a clear relation between snow
depth at the weather station and observed release area size. Further, Sect. 3.2
suggests that snow depth at critical terrain features is particularly
relevant for potential release area size. This is in line with snow depth
profiles at several locations in the release area
(Fig. ).
Figure shows snow depth at times of avalanche release for
the release areas no. 200, no. 817 and no. 20150020 along profile 1 following the
crown fracture of release area no. 200 at the border between CB1 and CB2. We
observe that the snowpack of release area no. 200 is, in most locations, deeper
compared to no. 817, which is itself consistently deeper throughout the entire
profile than avalanche no. 20150020. In particular at the ridge separating CB1
from CB2, the snowpack of avalanche no. 200 appears to be significantly deeper.
This shows that snow depth of release area no. 200 was not affected by the
previous release of avalanche no. 301, triggered about a month earlier.
Avalanche no. 301 released below the crown fracture of avalanche no. 200 and did
not fully propagate through CB2. As a result, the thick snow cover at the
borders of the basin remained, possibly facilitating full fracture
propagation through CB1 in the case of avalanche no. 200, whereas the fracture
of no. 817 was arrested at the prominent ridge separating CB1 from CB2. The
lower snowpack for no. 20150020 resulted in an even smaller release area and
did not even fully propagate through CB2. These snowpack differences are
clearly shown by the weather station, which can be considered representative,
at least for the border between CB1 and CB2, where previous avalanches did
not occur.
This is confirmed in CB1. Two representative snow depth profiles (profiles 2
and 3, Fig. ) show a generally good agreement between snow
depth measured at the weather station and snow depth in the release area.
Further, profile 2, located across the prominent gully where the avalanches
no. 628 and no. 103 stopped, but no. 509 propagated through, shows a
significantly larger snow depth in the gully for no. 509 than for no. 628 and
no. 103. At the same time, snow depth outside the gully was similar or even
lower. Again, this confirms our observations in Sect. 4.1. that snow depth
at specific terrain features, which serve as delimiting borders for avalanche
release, may be decisive for potential release areas size. However, in
contrast to the snow covers of avalanches no. 628, no. 816b and no. 20150016, snow
depth in the gully for avalanches no. 103 and no. 509 deviates significantly from
the weather station measurements. Snow depth was significantly larger for
release area no. 509 and significantly lower for release area no. 103 compared to
the weather station measurements. Snow was probably locally removed from the
gully (e.g. avalanching) in the case of no. 103, whereas it had rather
accumulated for avalanche no. 509 (Fig. ).
These observations suggest that the weather station is, in many cases,
indicative of potential release area size, as it is representative of snow
conditions in the release area, in particular around critical features such
as ridges and rocky outcrops, which are often less affected by frequent
avalanches as they require a very thick snow cover before avalanches may
form. However, exceptions are possible, in particular when important snow
redistribution processes occur which de- or increase fracture propagation
propensity relative to snow depth measured at a nearby weather station.
Discussion
The results revealed the connexion between increasing release area size and
decreasing surface roughness at the bed surface with snow accumulation. This
is in line with the current understanding of terrain smoothing processes,
which reduce the mechanical support of a slab and favour
the formation of continuous slabs and weak layers ,
which can subsequently lead to potentially larger release areas. However, we
also observed large release areas where snow depth at the bed surface is low
and surface roughness is still present, such as no. 726. These avalanches were
characterised by large slab thickness, meaning that surface roughness at bed
surface was covered by a thick layer of snow, levelling out the
irregularities and forming a smooth surface at the top of the snowpack. In
other words, the overlaying slab was thick enough to form a continuous layer
and, ultimately, produce a wide release area. This suggests that surface
roughness at the top of the snowpack may be more representative of potential
release area size than roughness of the bed surface. In this way, terrain
smoothing may also be relevant for slab avalanches that are released on deeper
layers in the snowpack where surface roughness is still present. This is
supported by field observations, where wide release areas are not only
observed for avalanches running on upper layers of the snowpack, but also for
slab avalanches running on weak layers near the terrain surface (so-called
deep slabs). They are known to reach important release area widths, e.g. up
to 700 m, as reported in , even propagating across terrain
features that generally arrest fractures. This is also consistent with recent
simulations of the slab weak layer system , which show that
slab thickness influences fracture propagation propensity, due to its
smoothing effect on weak layer heterogeneity such as topographical
irregularities.
However, the relation between surface roughness and release area size has so
far only been established for very high DTM resolutions of less than 1 m and
may not be present at coarser resolutions. This suggests that the fine-scale
roughness, such as single rocks and boulders, is important for avalanche
formation. It could also be one reason – beside neglecting the winter terrain
– why previous studies (e.g. ) found no relation between
low surface roughness and avalanche release areas, as they were mostly based
on DTM resolutions of 2 m and larger.
Further, it is obvious that snowpack stability and the spatial variability of
snowpack properties affect release area size in a
given situation. As an example, observed release area sizes in our dataset
could have been affected by previous avalanches, significantly disturbing the
layering of the snowpack, especially in very steep areas where frequent
avalanches are regularly observed. Nevertheless, as we only selected
avalanches that occurred under clearly unstable conditions, and considering
the fact that in homogeneous terrain rather homogeneous terrain, stability patterns
prevail , terrain and its alteration as a result of snow accumulation
can be considered, with good reason, as the main constraint for potential
release area size in our dataset. This is further supported by recent
mechanically based statistical modelling of the slab–weak-layer system,
which emphasises the importance of changes in terrain or snow cover
distribution rather than snow properties, for potential release area size
.
Conclusions
In this study, the relation between surface roughness, snow depth and release
area size in a high-alpine field site was evaluated. Lidar and photogrammetry
measurements before and after avalanche release were used to characterise
snow distribution and surface roughness of artificially triggered avalanches.
The comparison between mean surface roughness of the release area and its
size showed a decrease of surface roughness with increasing release area both
for the bed surface and the snow surface before avalanche release. The trend
was less pronounced on the snow-free terrain, suggesting that the
snow-covered winter terrain is more relevant for potential release area size
than the underlying snow-free terrain.
However, the results also showed that the relation of release area size and
mean surface roughness is restricted towards a minimum value of surface
roughness. Reaching this value, release area size increased without
significant change of mean surface roughness. This suggests that fracture
propagation over large distances is facilitated once mean roughness reaches a
certain minimum. At this point, a fracture will most likely self-propagate
until it is arrested by major changes in the snow cover or terrain such as
terrain breaks, ridges or major boulders. This is supported by surface
roughness patterns in snow-covered winter terrain that appeared to be well
suited to demarcate release areas. Patches of low roughness enlarged with
increasing snow depth due to the levelling out of single terrain features
that initially served as terrain breaks. This is in line with snow depth
profiles in the release area showing that snow depth around terrain breaks,
which are critical for fracture propagation, controls potential release area
size. In this vein, an increase in snow depth leads to an increase in
potential release area size if it is large enough to attenuate terrain
features that currently delimit the release area. These findings clearly
support our hypothesis of an increase of potential release area size with
larger snow depth.
Snow depth – due to its link to surface roughness – could therefore serve
as an important parameter to define potential release area size. Furthermore, in
our study, snow depth measured at a nearby weather station was, to a
considerable extent, related to observed release area size. That is because it
was often representative of snow depth around critical terrain features that
are able to accumulate significant quantities of snow before they become
susceptible to avalanche release. This highlights the potential of a
representative weather station in the process of snow cover–avalanche
scenario definition. However, it has also been shown that important local
snow depth differences can form for similar snow accumulation at the weather
station, mainly due to avalanching and sloughing, which can both increase and
decrease potential release area size in a given situation. This limits the
explanatory power of snow depth for potential release area definition, in
particular in real-time hazard assessment.