When should a hazard map show the risk of small avalanches or snow gliding?
Stefan Margreth*
WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
ABSTRACT: Avalanche hazard maps describe the extent and intensity of snow avalanches. In Swit- zerland red, blue and yellow zones are used which are based on scenarios with return periods varying between 30 and 300 years. In addition, hazard maps should show areas prone to snow gliding. The procedure for the assessment of extreme avalanches is relatively well defined. However, the criteria whether the hazard on a small hillside should be handled as an avalanche area, as a snow gliding ar- ea or whether the hazard can even be neglected at all are not well defined. Winter 2011-2012 when snow gliding was a widespread threat in the Swiss Alps was the catalyst to elaborate a leaflet on the thematic. Typically, an avalanche hazard is specified in a hazard map, if the dynamic pressure of an avalanche is greater than the static snow pressure. We propose a procedure based on seven factors how the hazard of snow gliding can be assessed. The snow gliding hazard on hillsides can often be mitigated by relatively simple structural measures.
KEYWORDS: hazard map, snow gliding, glide snow avalanche, small avalanche
1 INTRODUCTION
Avalanche hazard maps describe the extent and intensity of snow avalanches. In Switzerland red, blue and yellow zones are used which are based on scenarios with return periods varying between 30 and 300 years (BFF/SLF, 1984).
This three-colour system is used in Switzerland for all types of natural hazards. The main criteri- on for the definition of the danger level is the impact pressure for a certain return period (Fig.
1). If the impact pressure of a 300 year ava- lanche is for example larger than 30 kPa a red zone is assigned.
In addition, hazard maps should show areas prone to snow gliding. The methodology for as- sessing extreme avalanches is relatively well defined. However, the criteria whether the haz- ard on a small hillside should be handled as an avalanche area, as a snow gliding area or whether the hazard can even be neglected at all are not well defined. Snow gliding areas in a hazard map are mostly assessed by expert evaluation. The winter 2011-2012 when snow gliding caused a widespread threat in the Swiss Alps (Mitterer and Schweizer, 2012) triggered a study to define a procedure how snow gliding areas should be established in a hazard map.
2 SNOW GLIDING
Snow gliding is the slip of the entire snow cover on the ground without essential defor- mation within the snow cover. Glide rates can vary from millimetres to more than one metre per day. A typical sign of fast snow gliding are half-moon to sickle-shaped cracks which are the result of tensile failures (Fig. 2). The gliding snow cover is limited at its lower end by the stauchwall, which is the snow cover zone below the glide zone. The snow cover at the stauch- wall is not or only slightly gliding and therefore carries all the weight of the gliding slab in shear (Bartelt et al., 2012). The stauchwall is often located at terrain discontinuities, such as foot- paths that traverse the slope or large boulders.
Finally, the whole snowpack can suddenly re- lease on the ground to form a full-depth glide- snow avalanche.
Often glide cracks and stauchwalls are ob- served each winter at the same locations. If a fixed obstacle such as a building is located in the gliding snow cover snow pressure loads oc- Fig. 1: Hazard matrix for determining the hazard
level of snow avalanches and snow pressure (VKF, 2005 and BFF/SLF, 1984).
Intensity (kPa)
30
3 0
Return period 30y 100y 300y
Legend:
Red: elevated danger Prohibited area Blue: medium danger Conditional use area Yellow: low danger Awareness zone
______________________
Corresponding author address: Stefan Margreth, WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, CH-7260 Davos Dorf, Switzerland, tel: +41 81 417 0254;
e-mail: margreth(at)slf.ch
Experience shows that different factors must be met before snow gliding starts. The most rel- evant factors are as follows (In der Gand et al, 1966):
The roughness of the ground must be low.
High glide rates and consequently high snow pressure loads occur on smooth slopes with long-bladed grass or on smooth outcropping rock surfaces with stratification planes parallel to the slope. Wet or swampy ground also facilitates glide movements.
At the snow-soil interface the snow tempera- ture must be 0°C so that liquid water is pre-
Snow gliding increases with increasing slope angle and starts at an inclination of about 15°. If the terrain is steeper than 25° strong snow gliding can start. Slopes of gentle in- cline within steep terrain can retard snow gliding.
Finally a deep snowpack increases snow gliding because the weight increases the shear stress.
Fig. 3: The left photo shows a slope with promi- nent cow trails, typical for a glide factor N=1.8.
The right photo shows a smooth slope with a compact grass cover, typical for a glide factor N=3.2.
3 DEFINITION OF AREAS PRONE TO SNOW GLIDING
No well-defined rules exist when and how snow gliding should be considered in a hazard Fig. 2: A glide crack opened and the whole
snowpack is moving slowly downslope. At the stauchwall the snowpack is folded and uplifted.
Tab. 1: Factors relevant for the assessment of snow gliding on a hillside
Factor Criteria Score
1 Ground roughness (Margreth, 2007b) Glide factor N=3.2 3
Glide factor N=2.5 2
Glide factor N=1.8 1
2 Aspect >1000 m a.s.l.
<1000 m a.s.l.
ENE-S-WNW 2
WNW-N-ENE 1
all aspects 2
3 Vertical snow height HA > 2.0 m 3
1.0 – 2.0 m 2
< 1.0 m 1
4 Slope angle > 35° 3
25° - 35° 2
< 25° 1
5 Length of slope L > 30 m 3
15 – 30 m 2
< 15 m 1
6 Slope type Even, smooth 3
Bowl-shaped, concave 2
Hill-shaped, ridge, convex 1
7 Soil humidity Swampy, small brook 2
Dry 1
map. We elaborated a procedure based on sev- en factors how the hazard on a hillside prone to snow gliding can be assessed (Tab. 1; Marg- reth, 2012).
Important factors to be considered are the ground roughness (Fig. 3), the aspect of the slope, the extreme snow height, the slope angle, the length of the slope, the curvature of the slope and the soil moisture. We rate the different factors with one to three points. The higher the overall score the more likely intense snow glid- ing must be expected. We suggest that it is ad- visable to consider a slope in a hazard map as prone to snow gliding if the overall score is more than 10 points. If the overall score is more than 16 points slopes prone to snow gliding should always be considered in a hazard map. This applies also to hillsides where past snow glide events are documented. A hillside where the hazard of snow gliding should be considered in a hazard map has typically a small ground roughness, a slope angle of more than 35°, a snow height of more than 2.0 m, a southern as- pect and an even topography.
4 ASSESSING HAZARD LEVEL AND RETURN PERIOD OF SNOW GLIDING Theoretically the same hazard levels devel- oped for avalanches are also valid for snow glid- ing (Fig. 1). However the hazard due to snow gliding is perceived typically in such a way that a permanent use of an endangered area is not completely out of scope. The snow glide hazard on hillsides can often rather easily be reduced constructive measures. Therefore a hillside with snow glide hazard is in Switzerland typically as- signed to a blue or yellow zone with an intensity of less than 30 kPa (Fig. 4). The hazard matrix (Fig. 1) is interpreted similar to landslides. In the
fields of the hazard matrix with two colours for snow gliding the lower hazard level is applied.
For example, field number 4 of the hazard ma- trix is interpreted for snow gliding as yellow and not as blue. It is rare to assign a red hazard zone because of snow gliding.
Missing data and the complex snow pressure process make it very difficult to allocate a return period and intensity to a potential snow glide hazard. Snow pressure depends not only on the snow height but also on the snow density, the glide factor which is related to the ground roughness and the geometry and position of a building. In practice, if on a hillside snow gliding was observed in the past a return period of 30 up to 100 years is assumed. If on a hillside snow gliding is theoretically possible, but has never been observed in the past a return period of 100 up to 300 years seems to be appropriate.
5 DELIMITATION OF AREAS WITH SNOW GLIDING AND AVALANCHES
It is not possible to clearly define when snow glide hazards and when avalanche hazards have to be considered in a hazard map. Accord- ing to experience, an avalanche hazard is speci- fied in a hazard map, if the dynamic pressure of an avalanche is greater than the static snow pressure of the gliding snowpack. In the Swiss Alps, this is typically the case if the elevation of the area is higher than 700 m a.s.l., the slope is steeper than 28°, the elevation difference H is larger than 30 m and the length of the track L is more than 50 m (Fig. 5). If the slope angle is less than 28° snow gliding is typically the deci- sive hazard. In Switzerland, snow gliding and avalanches are combined in the same hazard map. There are no specific hazard maps for snow gliding.
Fig. 4: Example for a snow glide area and the corresponding hazard map in the commune of Wild- haus - Alt St. Johann. The lower end of the blue zone is derived from the 100 year-scenario and the yellow zone from the 300 year scenario both with intensities of less than 30 kPa (Photo: P. Diener;
Hazard map: Naturgefahrenkommission des Kantons St. Gallen, 2011).
1
1
2 2
3 3
4
4
Fig. 5: The whole snowpack released as a glide snow avalanche and hit the building. This situa- tion is a border-line case whether a snow glide or avalanche hazard should be shown in the hazard map. In the official hazard map at this location a snow gliding area was established for this hillside.
6 PREVENTION OF SNOW GLIDING
In contrast to avalanches, the snow glide hazard on a hillside can be mitigated by relative- ly simple structural measures that prevent snow gliding.
6.1 Terrain modifications
A very effective method to prevent snow glid- ing is to terrace a slope. Snow gliding can be permanently prevented on a hillside if a residen- tial area consists of stepped buildings where horizontal areas such as access roads or green areas alternate with vertical retaining walls. If a single house is built on a hillside prone to snow gliding a horizontal berm with a width of at least 2 m should be planned upslope of the back wall to reduce the effects of snow pressure (Fig. 6).
Fig. 6: Residential house situated on a hillside with snow glide hazard. Two terraces upslope of the building reduce the effect of snow pressure.
The back wall of such an exposed building should be made of concrete. Further the wall should have no openings up to a height of 2 m.
are common (Leuenberger, 2003):
Pilings: The snowpack is anchored to the ground with wooden piles (Fig. 7). A narrow spacing of the single piles is more important than a large pile height. The minimum pile height above ground is 30–50 cm. According to experience the ratio of depth of burial to the pil- ing height above ground has to be equal to 2:1.
For a circular cross section a diameter of 10 cm is appropriate. The slope parallel distance be- tween the piles depends on the slope angle. For a 30° slope the slope parallel distance is 2 m and an arrangement in a triangular grid is pre- ferred.
Fig. 7: Pilings anchor the snowpack to the ground. It is important that the topmost piles cover the top of the release zone or snow gliding area.
Tripod structures: Tripod structures made of wood prevent the snowpack from gliding and also creeping (Fig. 8). Typical structure heights are 1.5 m and structure width is 2 m. The struc- tures are anchored to the ground with wire ropes or steel pilings. Tripod structures are best ar- ranged in a triangular grid where the intermedi- ate distance varies between 1.5 m and 2.5 m.
One tripod structure can secure a surface area of 10-15 m2 and costs around 250 Euros. For the protection of 1 ha approximately 1000 tripod structures are necessary.
L
Fig. 8: Tripod structures increase the ground roughness and prevent snow gliding.
7 CONCLUSIONS
So far no established procedure existed how the risk of small avalanches or snow gliding should be considered in a hazard map. In prac- tice, the assessment was mostly based on ex- pert evaluation. We developed a procedure how the risk of snow gliding on a hillside can be as- sessed. The procedure is based on the evalua- tion of seven factors such as ground roughness, aspect, snow height, slope angle, length of slope, type of slope and soil humidity which are rated for every hillside. The higher the overall score for a hillside the higher is the risk for snow gliding. The procedure allows reaching compre- hensive decisions. Because large parts of Swit- zerland are steeper than 15° and therefore are potential areas of snow gliding the approach had to be as practical as possible. The procedure and the proposed threshold values for the over- all score are currently tested in practice. If nec- essary the procedure will be adapted in future.
8 REFERENCES
Bartelt, P.; Pielmeier, C.; Margreth, S.; Harvey, S.;
Stucki, T., 2012. The underestimated role of the stauchwall in full-depth avalanche release.
Proceedings International Snow Science Workshop ISSW 2012, Anchorage AK, U.S.A., 16-21 September 2012, pp. 127-133.
BFF/SLF, 1984. Richtlinien zur Berücksichtigung der Lawinengefahr bei raumwirksamen Tätigkeiten.
Bundesamt für Forstwesen, Bern. Eidg. Institut für Schnee- und Lawinenforschung, Davos. 34 pp.
In der Gand, H.R. and Zupančič, M., 1966. Snow gliding and avalanches, Symposium at Davos 1965 - Scientific Aspects of Snow and Ice Avalanches, IAHS Publication, 69. IAHS, Wallingford, U.K., pp. 230-242
Leuenberger, F., 2003. Bauanleitung Gleitschnee- schutz und temporärer Stützverbau. Eidg. Institut für Schnee- und Lawinenforschung, Davos.
Margreth, S., 2012. Merkblatt Beurteilung Schneegleiten und Schneedruck (Version 1 vom 19. November 2012). WSL-Institut für Schnee- und Lawinenforschung SLF, Davos. 8 pp.
Margreth, S., 2007a. Snow pressure on cableway masts: Analysis of damages and design approach. Cold Reg. Sci. Technol. 47: 4-15.
Margreth, S., 2007b. Defense structures in avalanche starting zones. Technical guideline as an aid to enforcement. Environment in Practice no. 0704.
Federal Office for the Environment, Bern; WSL Institute for Snow and Avalanche Research SLF, Davos. 134 pp.
Mitterer, C. and Schweizer, J., 2012. Towards a bet- ter understanding of glide-snow avalanche for- mation, Proceedings International Snow Science Workshop ISSW 2012, Anchorage AK, U.S.A., 16-21 September 2012, pp. 610-616.
VKF, 2005. Wegleitung Objektschutz gegen gravitative Naturgefahren.
