When should a hazard map show the risk of small avalanches or snow gliding?

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1 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 Switzerland red, blue and yellow zones are used which are based on scenarios with return periods varying between 0 and 00 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 area or whether the hazard can even be neglected at all are not well defined. Winter 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 0 and 00 years (BFF/SLF, 1984). This three-colour system is used in Switzerland for all types of natural hazards. The main criterion for the definition of the danger level is the impact pressure for a certain return period (Fig. 1). If the impact pressure of a 00 year avalanche is for example larger than 0 kpa a red zone is assigned. Intensity (kpa) 0 0 0y 100y 00y Fig. 1: Hazard matrix for determining the hazard level of snow avalanches and snow pressure (VKF, 2005 and BFF/SLF, 1984). Legend: Red: elevated danger Prohibited area Blue: medium danger Conditional use area Yellow: low danger Awareness zone Return period Corresponding author address: Stefan Margreth, WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, CH-7260 Davos Dorf, Switzerland, tel: ; margreth(at)slf.ch In addition, hazard maps should show areas prone to snow gliding. The methodology for assessing 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 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 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 deformation 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 stauchwall 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 footpaths that traverse the slope or large boulders. Finally, the whole snowpack can suddenly release on the ground to form a full-depth glidesnow avalanche. Often glide cracks and stauchwalls are observed 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- 679

2 cur (Margreth, 2007). Therefore snow gliding can be a threat to buildings and has to be considered in a hazard map. 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 temperature must be 0 C so that liquid water is present. With a dry boundary layer (temperature below 0 C), the snow cover does not glide even on a grass surface. Especially favourable for snow gliding is rainfall prior to the first snowfall or an abundant first snowfall in late fall or early winter. 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 incline within steep terrain can retard snow gliding. Finally a deep snowpack increases snow gliding because the weight increases the shear stress. Fig. 2: A glide crack opened and the whole snowpack is moving slowly downslope. At the stauchwall the snowpack is folded and uplifted. Experience shows that different factors must be met before snow gliding starts. The most relevant factors are as follows (In der Gand et al, 1966): Fig. : The left photo shows a slope with prominent 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=.2. DEFINITION OF AREAS PRONE TO SNOW GLIDING No well-defined rules exist when and how snow gliding should be considered in a hazard Tab. 1: Factors relevant for the assessment of snow gliding on a hillside Factor Criteria Score 1 Ground roughness (Margreth, 2007b) Glide factor N=.2 Glide factor N=2.5 2 Glide factor N= Aspect >1000 m a.s.l. ENE-S-WNW 2 WNW-N-ENE 1 all aspects 2 <1000 m a.s.l. Vertical snow height H A > 2.0 m m 2 < 1.0 m 1 4 Slope angle > < Length of slope L > 0 m 15 0 m 2 < 15 m 1 6 Slope type Even, smooth Bowl-shaped, concave 2 Hill-shaped, ridge, convex 1 7 Soil humidity Swampy, small brook 2 Dry 1 680

3 Fig. 4: Example for a snow glide area and the corresponding hazard map in the commune of Wildhaus - Alt St. Johann. The lower end of the blue zone is derived from the 100 year-scenario and the yellow zone from the 00 year scenario both with intensities of less than 0 kpa (Photo: P. Diener; Hazard map: Naturgefahrenkommission des Kantons St. Gallen, 2011). map. We elaborated a procedure based on seven factors how the hazard on a hillside prone to snow gliding can be assessed (Tab. 1; Margreth, 2012). Important factors to be considered are the ground roughness (Fig. ), 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 gliding must be expected. We suggest that it is advisable 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 5, a snow height of more than 2.0 m, a southern aspect and an even topography. 4 ASSESSING HAZARD LEVEL AND RETURN PERIOD OF SNOW GLIDING Theoretically the same hazard levels developed for avalanches are also valid for snow gliding (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 assigned to a blue or yellow zone with an intensity of less than 0 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 matrix 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 0 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 00 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. According to experience, an avalanche hazard is specified 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 0 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 decisive hazard. In Switzerland, snow gliding and avalanches are combined in the same hazard map. There are no specific hazard maps for snow gliding. 681

4 L H 6.2 Structural measures Snow gliding can be prevented by structural measures often combined with afforestation. The main goal of these measures is to increase the ground roughness. The following methods are common (Leuenberger, 200): Fig. 5: The whole snowpack released as a glide snow avalanche and hit the building. This situation 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 relatively simple structural measures that prevent snow gliding. 6.1 Terrain modifications A very effective method to prevent snow gliding is to terrace a slope. Snow gliding can be permanently prevented on a hillside if a residential 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). 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 0 50 cm. According to experience the ratio of depth of burial to the piling 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 between the piles depends on the slope angle. For a 0 slope the slope parallel distance is 2 m and an arrangement in a triangular grid is preferred. 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 structures are anchored to the ground with wire ropes or steel pilings. Tripod structures are best arranged in a triangular grid where the intermediate distance varies between 1.5 m and 2.5 m. One tripod structure can secure a surface area of m 2 and costs around 250 Euros. For the protection of 1 ha approximately 1000 tripod structures are necessary. 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. 682

5 8 REFERENCES 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 practice, the assessment was mostly based on expert evaluation. We developed a procedure how the risk of snow gliding on a hillside can be assessed. The procedure is based on the evaluation 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 comprehensive decisions. Because large parts of Switzerland 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 overall score are currently tested in practice. If necessary the procedure will be adapted in future. Bartelt, P.; Pielmeier, C.; Margreth, S.; Harvey, S.; Stucki, T., The underestimated role of the stauchwall in full-depth avalanche release. Proceedings International Snow Science Workshop ISSW 2012, Anchorage AK, U.S.A., September 2012, pp BFF/SLF, Richtlinien zur Berücksichtigung der Lawinengefahr bei raumwirksamen Tätigkeiten. Bundesamt für Forstwesen, Bern. Eidg. Institut für Schnee- und Lawinenforschung, Davos. 4 pp. In der Gand, H.R. and Zupančič, M., Snow gliding and avalanches, Symposium at Davos Scientific Aspects of Snow and Ice Avalanches, IAHS Publication, 69. IAHS, Wallingford, U.K., pp Leuenberger, F., 200. Bauanleitung Gleitschneeschutz und temporärer Stützverbau. Eidg. Institut für Schnee- und Lawinenforschung, Davos. Margreth, S., Merkblatt Beurteilung Schneegleiten und Schneedruck (Version 1 vom 19. November 2012). WSL-Institut für Schneeund Lawinenforschung SLF, Davos. 8 pp. Margreth, S., 2007a. Snow pressure on cableway masts: Analysis of damages and design approach. Cold Reg. Sci. Technol. 47: Margreth, S., 2007b. Defense structures in avalanche starting zones. Technical guideline as an aid to enforcement. Environment in Practice no Federal Office for the Environment, Bern; WSL Institute for Snow and Avalanche Research SLF, Davos. 14 pp. Mitterer, C. and Schweizer, J., Towards a better understanding of glide-snow avalanche formation, Proceedings International Snow Science Workshop ISSW 2012, Anchorage AK, U.S.A., September 2012, pp VKF, Wegleitung Objektschutz gegen gravitative Naturgefahren. 68