SNOW STABILITY AND ITS VARIATION IN COURSE OF TIME
* Peter Höller, Federal Forest Research Centre, Institute for Avalanche Research, Hofburg - Rennwg 1, A-6020 Innsbruck,
tel: +43 512 573933, fax: +43 512 572820
ABSTRACT: One of the most important questions in avalanche warning is snowpack stability. A main factor when elaborating avalanche bulletins is the settlement of new snow and the stabilization of the snowpack (e.g. how long does it take that sufficient bonds will exist between a weak layer and the recent snowpack (new snow)).
This paper shows some preliminary investigations on snow stability in March 1998. The measurements were done after a period of snowfall and included shear frame and Rutschblock tests as well as meteorological and nivological investigations.These data were used to determine snow stability in course of time and to compare those values with temperature and other relevant parameters. Results indicate that snow temperature might influence snow stability in a different way. If snow temperatures were relatively high (-2° to -3° C) the shear strength and the stability index increased considerably, in the case of lower temperatures those values did not change significantly, but the shear strength and the stability index decreased when the snow temperatures rised up (especially when the temperatures reached values near the freezing point).
KEYWORDS: Avalanches, Snow Cover, Snow Stability,
One of the most important questions in avalanche warning is snowpack stability. But avalanche bulletins regularly are based on meteorological observations which means that snowpack stability must be estimated by the forecaster (e.g. by taking into account meteorological data or his own experience).However, a more accurate prediction of avalanche formation has to include not only meteorological measurements but rather snowpack and stability factors (e.g. shear strength measurements, Rutschblocktests...). Following McClung and Schaerer (1993) the greatest priority should have factors that give direct evidence of snow stability (Class I), followed by snowpack factors (Class II) and meteorological observations (Class III). Jamieson (1995) pointed out that the stability index for skier triggering, SK38 (in this case SK is calculated for y =38°) is an effective predictor of skier-triggered persistent slabs in surrounding terrain (better than meteorological observations).
To fit those requirements scientists in the past concentrated their investigations on stability parameters for avalanche forecasting (Jamieson and Johnston 1993; Jamieson 1995, Schweizer et al. 1998).Investigations concerning the influence of temperature on snow stability were not given priority treatment.However, in 1996 McClung and Schweizer presented their investigations on the effect of snow temperature on skier triggering of dry slab avalanches.They found that warming may lead to both decreasing stability (warming decreases stiffness significantly) as well as increasing stability (warming increases bond formation rate).In addition this paper wants to describe the changes of snow stability in course of time and to compare those values with temperature and other relevant parameters.
2. EXPERIMENTAL SITE AND METHODS OF MEASUREMENTS
The study field was in the Kühtai about 35 km from Innsbruck, located in a small north east facing slope (approximately 32°) in about 2000 m a. s. l.The investigations included snow profiles (according to Colbeck et al.1990) and shear strength measurements as well as Rutschblocktests (Föhn, 1987a). The shear strength measurements were done with a 0.05 m² shear frame.The shear strength was calculated by taking into account the measured force, the shear frame area as well as a correction factor which depends on the size of the frame; according to Föhn (1987b) I used a correction factor of 0.7. To calculate the stability indices I used the methods as proposed by Föhn (1987) and Jamieson (1995). Additional data (air temperature, humidity, radiation, wind speed ....) were taken from the meteorological observation site in the Längental which is very close to the actual study field.All measurements were done in the second half of March 1998.
3.1. The weather conditions in March 1998 (see fig. 1)
The first days in March were characterized by dry conditions and alternating temperatures. The temperatures in 2000 m were near to -5° C on March 2 and rised up to about +10°C on March 4. The situation changed significantly on March 5 when a front from northwest crossed the Alps and brought new snow of about 10 to 20 cm (the temperatures fell down to -8°C in 2000 m). Another front on March 8 brought about 20 to 30 cm of new snow (between March 8 and March 9). The temperatures decreased rapidly on March 9 and reached -15° C in 2000 m (March 10). High pressure in the next two days leaded to sunny weather with low temperatures (- 10 °C in 2000 m). Between March 14 and March 16 the northern Alps received plenty of snow. The amount of new snow in the study area was about 40 to 50 cm.The temperatures in 2000 m were near to -5 to -7 °C and were slightly increasing up to March 19. Humid air from northwest (March 19 to March 21) brought again a considerable amount of new snow (approximately 20 to 30 cm in the study area). The temperatures fell to -15° C in 2000 m.
Fig. 1.Snow depth and air temperature in March 1998
After a short spell of changeable weather (March 23) and only a few centimeters of new snow a high pressure area up to the end of March leaded to sunny weather. The temperatures did not exceed -5° C up to March 25. But from March 26 it was getting warmer and the temperatures increased continously (-2 to -4°C on March 26, approximately 0°C on March 28 and already more than 0°C in the last days of March).
New snow of about 50 cm between March 14 and March 17 caused increasing avalanche danger. Detailed measurements on March 19 showed that Rutschblocks could already be released by weighting (Rutschblock degree 3), the fracture line (interface 1 in fig. 2) was between a meltfreeze crust (hardness was high to very high) and a layer of clustered (rounded) particles and faceted crystals (hardness was low to medium). The shear strength was about 0.65 kPa. As shown in fig. 3 the shear strength increased from 0.65 kPa (March 19) to approximately 1.9 kPa (March 27). Subsequently the shear strength decreased sligthly and reached
Fig. 2. Simplified snow profile (March 19)1.7 kPa on March 28.
The stability indices S (for natural body force) and S' (for skier and natural body force) were calculated according to Föhn (1987b). As shown in fig. 3 S was about 2.1 at the beginning of the observation period (March 19) and decreased to about 1.8 on March 21. Subsequently S increased to 3.1 (March 27). But already on March 28 the stability index decreased to 2.6.The stability index S' was 1.2 on March 19 (fig. 3) and similar values were calculated for March 21 and March 22. On March 25 S' rised up to 2.1 and reached a maximum on March 27 with approximately 2.5. On March 28 S' was calculated with about 2.1. As shown in fig. 3 a second interface (between small rounded particles and the meltfreeze crust) existed in the snowpack of March 1998 (10 to 15 cm above interface 1).Although this interface seemed to be not so critical I made additional shear frame measurements. I found that the shear strength was approximately 0.85 kPa on March 19 and showed similar values on March 21 and March 25. Thus it can be assumed that the
Fig.3. Shear strength, stability index S and S' (interface 1), snow temperature and snow depth in the second half of March 1998
shear strength did not increase during that period; only between March 25 and March 26 the shear strength increased insignificantly (1.1 kPa). But already on the next day the shear strength decreased to approximately 0.9 kPa.A similar tendency was found for S and S'. The stability index S was calculated with 2.4 for March 19 and decreased to about 2.0 on March 21. On March 26 S increased again and reached approximately 2.5.S' was calculated with 1.6 (for March 19, March 21 and March 25, rised up on March 26 to about 1.9 and decreased to 1.5 on the next day.
Fig. 3 shows that the shear strength and the stability index in the lower interface increased continuously from March 19 to March 26 (a similar tendency was found for the stability index). In contrast, the shear strength in the upper interface only increased insignificanty. However, the snow temperatures in 80 cm were about -2° C to -3° C, while the corresponding temperatures in 100 cm showed significant lower values (approximately -4 to -5 °C).It can be asssumed that the increase of shear strength as well as of the stability index in the lower interface is connected with in the higher snow temperatures in 80 cm, while the insignificant increase of shear strenght in the upper interface might be explained by the lower snow temperatures (see Fig. 3) in 100 cm. The results indicate that the increase of stability takes the longer the lower the snow temperatures are. Between March 26 and March 27 the shear strength and the stability index in the upper interface decreased; next day (between March 27 and March 28) a similar tendency was found in the lower interface. The snow temperatures (Fig. 3) near the upper layer (in 100 cm) rised up significantly from -5.5 to -1.5°C (already between March 26 and March 27) while the snow temperatures in the lower part of the snowpack (80 cm) increased not until the next day (between March 27 and March 28).It can be assumed that the shear strength (as well as the stability index) decreases when the snow temperatures increase (especially when the snow temperatures reach values near to the freezing point).In any case the measurements show that warming may lead to a reduced stability. By the way McClung and Schweizer (1996) pointed out that warmer temperatures will reduce stiffness, failure toughness and strength.The influence of precipitation (new snow) on stability index can be seen between March 19 and March 21.The decrease of the stability index S both in the lower interface as well as in the upper interface corresponds with the increase of normal load (new snow).On the other hand skier stability S'neither increases nor decreases which can be explained by an increasing depth of the weak layer. Similar results were also shown by Schweizer et al. (1998).
The goal of these preliminary investigations was to describe the changes of snow stability in course of time and to compare those values with temperature and other relevant parameters.The measurements of March 1998can be summarized as follows: 1.The shear strength (as well as the stability index ) increased considerably when the snow temperatures were relatively high (-2° to -3°C). 2.In the case of lower temperatures (approximately -5° C) the shear strength (as well as the stability index) did not change significantly.3. Reduced shear strength (as well as stability index) was found when the snow temperatures increased (especially when the temperatures reached values near to the freezing point).
I am grateful to Anja Klebelsberg (TIWAG) for her assistance.; she supported this paper by supplying the meteorological and nivological data from the observation site in the Kühtai.
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