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Extrapolating the skier stability of buried surface hoar layers from study plot measurements
Cold Regions Science and Technology 33 Ž2001. 163–177 www.elsevier.comrlocatercoldregions
Extrapolating the skier stability of buried surface hoar layers from study plot measurements Thomas S. Chalmers a , Bruce Jamieson a,b,) a
Department of CiÕil Engineering, UniÕersity of Calgary, 2500 UniÕersity DriÕe NW, Calgary, Alberta, Canada T2N 1N4 b Department of Geology and Geophysics, UniÕersity of Calgary, Calgary, Alberta, Canada Received 30 September 2000; accepted 18 June 2001
Abstract Buried layers of surface hoar pose a challenge to avalanche forecasters in many areas, partly because some layers stabilise quickly and others remain unstable for a month or more. This paper relates the measurements of two surface hoar layers in a study plot, one buried 30 December 1999 and the other buried 21 February 2000, to skier-triggered slab avalanches within 100 km of the study plot in the Columbia Mountains of western Canada. The two surface hoar layers were monitored at a tree-line study slope at Rogers Pass every 4 to 8 days until the end of March 2000. Physical properties of the slab Žload, thickness, hardness profile., and weak layer Žshear strength, temperature, temperature gradient, crystal size, crystal form. were observed. Approximately once every 2 weeks, the weak layers were photographed in the pit wall to document their texture. On the same days, disaggregated crystals from the weak layers were photographed on a crystal screen. The February 21 layer, which initially consisted of 4–6-mm crystals, was loaded more slowly by snowfall, gained strength and stability more slowly, yielded initially lower stability indices and released many more skier-triggered avalanches than the December 30 layer, which initially consisted of larger, 10–20-mm crystals. Critical study plot values of load, shear strength, and stability are compared with critical values measured adjacent to over 50 skier-triggered slab avalanches. The shear strength of the weak layer, calculated skier stability index Sk 38 , layer thickness, and load on the weak layer show potential predictive value for the stabilisation of buried surface hoar layers. While the time series of photographs of separated crystals shows distinct changes, the time series of photographs of the buried surface hoar layers in situ reveals little useful information on textural changes other than thinning of the layers. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Avalanches; Surface hoar; Snow stability; Skier triggering; Stability indices; Avalanche forecasting
1. Introduction Many skier-triggered slab avalanches fail on layers of buried surface hoar ŽJamieson and Johnston, )
Corresponding author. Department of Civil Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. Tel.: q1-403-220-7479; fax: q1-403-282-7026. E-mail address: [email protected] ŽB. Jamieson..
1992; Schweizer and Jamieson, 2001.. The susceptibility of such layers to skier triggering is often difficult to assess, making them capable of surprising even professional decision-makers ŽJamieson and Geldsetzer, 1999.. Over the winter of 1999–2000 in the Columbia Mountains of western Canada, two particular layers of buried surface hoar were important to many professional and amateur decisionmakers. The evolution of each of these layers was
0165-232Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 2 X Ž 0 1 . 0 0 0 4 3 - X
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monitored at a study site in Rogers Pass, British Columbia, Canada through snow profiles, strength measurements, snowpack tests, and photography over the course of the winter. The following discussion concentrates on the potential value of certain snowpack factors to predict the reduction of widespread skier-triggered avalanche activity on a layer of buried surface hoar over a large forecast area Žup to 100 km from the study site.. Isolated events following such periods of widespread activity, although important to skiers and avalanche forecasters, are more difficult to predict. Furthermore, extrapolating potential predictors of avalanche activity over such a large area will not indicate the specific stability in areas where terrain and weather couple to create snowpack conditions atypical of the forecast area.
2. Literature review Surface hoar or AhoarfrostB are crystal forms that grow on the snow surface by the deposition of water vapour Že.g. Colbeck, 1987; Lang et al., 1984.. Once buried by snowfall, such layers are characteristically weak. Skiers may induce failure in a buried surface hoar layer, thereby causing a slab avalanche of the overlying snow. Most skier-triggered avalanches happen in the first few days following the burial of a surface hoar layer and are relatively easy to predict, but a layer may remain reactive as a potential failure plane for slab avalanches for several weeks or more Že.g. Fohn, 1992; Jamieson, 1995, pp. 141–155.. ¨ Accidental skier-triggered avalanches that occur later in this period are less frequent and often larger, and more difficult to predict. The transition of buried surface hoar layers from unstable to stable conditions is difficult for avalanche professionals to forecast and is the focus of this study. A skier induces stresses on the underlying snowpack that can penetrate to a buried weak layer and produce sufficient total stress to cause the weak layer to fail and release a slab avalanche. Skier-induced stresses on the snowpack decrease with increasing slab thickness, and skiers are not usually effective triggers on slabs thicker than 1 m Že.g. Fohn, ¨ 1987; Schweizer and Jamieson, 2001.. The stability—susceptibility to slab avalanching—of buried weak lay-
ers in the snowpack may be assessed by direct indicators such as observations of other slab avalanches or snowpack stability ŽClass 1 factors., by less direct indicators such as snowpack factors ŽClass 2 factors., or weather observations ŽClass 3 factors. ŽMcClung and Schaerer, 1993, pp. 125–126.. Stability ratios or indices based on shear frame measurements are such Class 1 factors. Schleiss and Schleiss Ž1970. introduced a snow Astability factorB or Stability Ratio ŽCanadian Avalanche Association, 1995., which is the shear strength of a weak layer divided by the weight per unit area above the weak layer. This has been used since ca. 1960 at the Mount Fidelity study plot to extrapolate snowpack stability for natural avalanches in the Rogers Pass highway corridor, but not for skier-triggered avalanche activity. Roch Ž1966. introduced a stability index S, the ratio of shear strength to shear stress on the weak layer, and Fohn ¨ Ž1987. produced a stability index incorporating skier loading, SX . Both Roch Ž1966. and Fohn ¨ Ž1987. made their shear strength measurements on or near avalanche slopes. These indices were corrected for the effects of normal load and shear frame size. Fohn ¨ and Camponovo Ž1996. showed that the shear strength of weak layers strongly correlated with the skier stability index. To apply the stability index S to slopes of differing angles over a geographical region Žup to 30 km., Jamieson and Johnston Ž1993. obtained S 35 , based on a typical inclination of 358 for slopes in slab avalanche start zones. They found a band of transitional stability Ž1.6–1.8. between stable and unstable snowpack conditions, probably due to the increased variability in the snowpack when applying this natural stability index over a large area. Jamieson and Johnston Ž1993. found that SF and S 35 were effective regional predictors of unstable and marginal natural avalanche stability on 75–87% of days, but this study did not include extrapolated regional stability for skier-triggered avalanches. Jamieson and Johnston Ž1994. refined the skier triggering stability index SX , as Sk, to include the effects of ski penetration. In order to use this index for slopes within 15 km, it was applied to a slope angle of 358 as Sk 35 . Jamieson Ž1995, pp. 148–156., applying a slope angle of 388, excluded normal load effects for persistent weak layers and presented daily Sk 38 values for nine buried surface hoar layers in the Columbia mountains. Most layers
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in that study showed a slowing of skier-triggered slab avalanche activity when Sk 38 exceeded approximately 0.5 and a cessation of observed skier-triggered slab avalanche activity when Sk 38 was 1–1.5. Sk was further refined in Jamieson and Johnston Ž1998. for tests done on or adjacent to avalanche slopes. However, to date, no studies have attempted to apply study plot measurements, including stability indices, to extrapolate the regional-scale stability of a buried weak layer in regards to skier-triggered avalanche activity. The stability index Sk assumes that a buried weak layer of surface hoar fails in shear and not in compression. Although a mixed mode of failure cannot be ruled out, buried surface hoar layers are generally thin Ž- 10 mm thickness., and shear failure is assumed ŽJamieson and Schweizer, 2000.. Several studies have measured strength changes of buried surface hoar layers, with stability trends of the layers and associated avalanche activity ŽJamieson and Johnston, 1994; Jamieson, 1995; Schweizer et al., 1998.. These studies considered a study area of approximately 10–15 km around the snow study plot. Jamieson and Johnston Ž1999. focused on the snowpack factors that influence observed strength changes. Snowpack depth, maximum crystal size in the layer, thickness of the overlying slab, load on the layer, and layer thickness were shown to be important to strength changes of buried surface hoar layers; specific threshold values associated with the stabilisation of these layers were not addressed. Some studies have addressed observable changes in buried surface hoar layer properties ŽDavis et al., 1996; Jamieson and Schweizer, 2000., though field-measured layer thickness has not been addressed as a forecasting factor. Changes in texture over time from photography and analysis of section planes of buried surface hoar layers Že.g. Geldsetzer et al., 1997; Davis et al., 1998. have not yet been applied to regional stability forecasting. 3. Methods On 30 December 1999 and 21 February 2000, two layers of surface hoar were buried in many areas of the Columbia Mountains of British Columbia. Subsequent to their burial and until the end of March 2000, measurements of these layers were taken every 4–8
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days at a study site on Mount Fidelity in Rogers Pass, British Columbia, Canada. The study site was located at tree line, 1905 m above sea level, on a predominantly east-facing slope that varied between approximately 158 and 308 inclination. On each measurement day, the buried surface hoar layerŽs. were identified in a snow pit wall with a profile of snowpack layers, compression test, rutschblock test, or shovel test ŽCanadian Avalanche Association, 1995.. A snow profile was observed, including properties of the surface hoar layers Žmaximum and minimum grain size, layer thickness, temperature, temperature gradient, shear strength. and of the overlying slab Žload, slab thickness, density profile, hand hardness profile.. The shear strength of each layer was the average obtained from a set of approximately 12 shear frame tests. The overlying snow was removed, leaving 40–45 mm of undisturbed snow above the weak layer. A 250-cm2 stainless steel shear frame was carefully inserted into the snow such that the frame bottom was 2–5 mm above the weak layer ŽPerla and Beck, 1983.. Details of the shear frame test can be found in Jamieson and Johnston Žin press.. Shear strength is the maximum reading on the force gauge divided by the area of the frame, and adjusted for size effects ŽSommerfeld, 1980; Fohn, ¨ 1987.. The maximum and minimum extent of the characteristic surface hoar crystals in a buried layer were observed. This was done according to industry guidelines ŽCanadian Avalanche Association, 1995., manually separating the crystals from the snowpack and observing them with a low magnification Ž8 = . hand lens on a crystal screen with 1-, 2-, 3-, and 10-mm grids. The thickness of a buried surface hoar layer was measured by placing a millimetre scale against the vertical snow pit wall and recording the layer thickness to the nearest millimetre. The load due to the snow slab overlying a buried surface hoar layer was measured in two ways. The first method involves taking a vertical core sample of the slab by inserting a tube vertically through the snowpack layers above the surface hoar layer. Several such cores were often taken to reduce measurement error. In this first method, Load s mgrA
Ž 1.
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where m is the average mass of the core samples, g is the acceleration due to gravity, and A is the cross-sectional area of the sample tube Ž28 cm2 .. The second method is to calculate the load from the thickness and measured densities of the snowpack layers above the buried surface hoar layer, Load s g Ž r 1 h1 q r 2 h 2 q . . . . ,
Ž 2.
where r i and h i are the density and thickness of the ith layer. For layers, which were too thin Žless than 40 mm. to measure with the small Ž100 cm3 . density sampling tube, the density was estimated from hand hardness and grain type as described by Geldsetzer and Jamieson Ž2000.. Two methods of load measurement are used to minimise the sampling error from each method ŽJamieson and Johnston, 1999., and values from both methods were averaged to obtain the load. Manual measurements of snowpack temperatures and temperature gradient across the buried surface hoar layers were taken on each observation day with
digital display thermometers with a resolution of 0.1 8C. The temperature gradient across a buried surface hoar layer was calculated from the temperatures 50 mm above and below the layer. The temperature and temperature gradient across the weak layers were also monitored continuously for part of the measurement period of each layer. Thermisters, which output to a CR-10 datalogger, were placed 50 mm above and below each layer and their values were recorded hourly, as described in Jamieson and Johnston Ž1999.. Approximately every 14 days, the layers were photographed by two different methods: in situ to reveal their texture, and as crystals disaggregated from each layer Že.g. Davis et al., 1996; Jamieson and Schweizer, 2000.. The in situ photos were made by inserting a black screen approximately 10–20 mm behind the weak layer, parallel to the pit wall to provide background contrast. Disturbed or fractured crystals in front of the screen were carefully removed, leaving only surface hoar crystals in their
Fig. 1. Map of study area showing Glacier National Park ŽGNP. and four helicopter ski areas ŽHS. that reported avalanches.
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natural buried state to be photographed with a macro lens and ring flash. Some crystals were then carefully disaggregated from the pit wall, without breaking them into smaller pieces, and placed on a 10-mm grid to be photographed with a macro lens and ring flash. Meteorological conditions near the Mount Fidelity study slope were monitored continuously by an automated weather station. This provided information about prevailing conditions over the time during which the surface hoar layers were forming. University of Calgary research staff based in Rogers Pass noted skier-triggered avalanche activity within approximately 15 km of the Fidelity study slope in Glacier National Park ŽGNP. over the winter 1999–2000. This avalanche occurrence data was then compiled with similar data from surrounding helicopter skiing ŽHS. operations within approximately 100 km of the study site ŽFig. 1. in order to relate the observed changes in the weak layers to skier-triggered avalanche activity on each layer. Over the entire study area, there were approximately 210 skiers a day during the study period. Considering the area, many slopes lay untouched by skiers over the course of this study.
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Skier-triggered slab avalanches on the surface hoar layer buried 30 December 1999 in the Columbia Mountains occurred frequently during the period 4–7 January 2000 ŽFig. 2., when the layer was 6–8 days old Žage of layer s number of days since burial.. There was some skier triggering up to 15 days after the layer was buried, and an isolated event 35 days after the layer was buried. For the 30 December layer, the period of frequent skier-triggered avalanche activity is defined as up to age 8 days, with occasional activity up to age 15 days. The time between the frequent and occasional thresholds is the stabilisation period ŽFig. 4.. The shear strength of the 30 December layer ŽTable 1. increased from 0.17 kPa Ž4 days old. to a maximum of 5.96 kPa Ž66 days old.. During the period in which skier-triggered slab avalanche activity on the layer slowed, the shear strength of the layer rose from 0.54 Ž8 days old. to 1.25 kPa Ž12 days old.. The significant decreases in measurements
4. Observations 4.1. Surface hoar layer buried 30 December 1999 The surface hoar layer buried 30 December 1999 was formed during the period 21–29 December. During this time, at the weather station adjacent to the study plot, there was no new snowfall, and the relative humidity was 60–100%. Winds over this period were light, between 1 and 6 km hy1 , except for higher winds of 11 km hy1 late 29 December. Night temperatures were between y1 and y6 8C, except for the evenings of 25 and 26 December, when temperatures were at or slightly above freezing. Daytime high temperatures for the formation period were below freezing, except for 25, 26, and 27 December, when temperatures reached up to q5 8C. Conditions such as these are suitable for promoting the growth of surface hoar Že.g. Lang et al., 1984..
Fig. 2. Stability trend and study plot measurements for 30 December 1999 surface hoar layer.
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Table 1 Time series measurements of surface hoar layer buried 30 December 1999 Age Ždays.
Slab thickness Žm.
Shear strength ŽkPa.
Coefficient of variation
Stability index Sk 38
Minimum grain size Žmm.
Maximum grain size Žmm.
Thickness of layer Žmm.
Load ŽkPa.
Layer temperature Ž8C.
Temperature gradient Ž8Crm.
4 8 12 20 27 33 38 42 45 52 59 66 73 79 87
0.33 0.91 1.07 1.34 1.22 1.34 1.18 1.43 1.38 1.30 1.36 1.67 1.70 2.04 2.13
0.17 0.54 1.25 1.35 2.57 3.30 3.70 2.95 4.46 4.72 4.33 5.96 4.51 5.25 3.84
0.15 0.19 0.15 0.16 0.19 0.23 0.18 0.11 0.14 0.15 0.21 0.13 0.22 0.26 0.22
– 0.65 1.34 1.89 2.08 2.88 2.19 1.83 2.60 2.86 2.86 3.19 2.25 2.27 1.50
10 10 9 6 10 8 5 10 9 6 8 4 6 6 8
20 15 12 11 12 12 13 13 14 12 12 8 12 6 12
22 15 15 14 15 15 12 12 11 12 14 10 10 8 10
0.24 0.94 1.37 1.60 2.25 2.33 2.95 3.10 3.37 3.28 3.32 4.02 4.28 4.86 5.54
y8.3 y6.6 y5.4 y5.2 y4.8 y4.5 y4.0 y4.0 y3.8 y3.9 y3.6 y3.1 y2.7 y2.3 y2.4
y9 y4 y6 y7 y6 3 0 0 y3 y2 y3 y3 y2 y1 y1
at ages 42 and 73 days are probably due to the natural spatial variations in layer strength within the study slope. This is assumed to be the case because the layer was over 1.4 m deep in the snowpack on both of these measurement days; its shear strength was not likely affected by the external Žmeteorological. forcing conditions present during the periods when shear strength decreases were observed. This internal study plot variability illustrates a limitation of extrapolating study plot data for avalanche forecasting. Other sources of information and forecasting techniques are required for accurate forecasting Že.g. McClung and Schaerer, 1993, pp. 125–147.. During the critical period when skier-triggered slab avalanche activity on the 30 December layer slowed, the skier stability index Sk 38 calculated from the shear frame tests Že.g. Jamieson, 1995; Jamieson and Johnston, 1998. rose from 0.65 Ž8 days old. to 1.34 Ž12 days old. ŽFig. 2, Table 1.. Sk 38 continued to increase following this period to the maximum score of 3.19, 66 days following burial, at which point observed skier-triggered avalanches on the layer had not been reported for 51 days. Although measurements of this layer were made at age 4 days, Sk 38 is undefined at this point due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. Four days following burial, the 30 December layer of buried surface hoar was observed to be
22-mm thick ŽTable 1.. Over a period of 8 days, the layer compressed by approximately 32%, to a thickness of 15 mm. This was coincident with the time frame in which skier-triggered slab avalanches on the layer became less frequent Ž8 to 10 days old.. After this time, the layer continued to compress more slowly until the end of the observation period Ž87 days after burial., when it was observed to be 10-mm thick. At the time when skier-triggered avalanche activity on the 30 December layer slowed down Ž8 to 10 days after the layer was buried., the load Žweight per unit area. of the overlying slab was approximately 1 kPa ŽFig. 2, Table 1.. The load on the layer rose with new snowfall during the observation period; there was 5.54-kPa load on the layer by the time it reached 87 days of age. After 4 days of burial, the 30 December surface hoar layer was buried under a 0.33-m slab in the snowpack ŽTable 1.. During the period in which skier-triggered avalanche activity on the layer slowed, the overlying slab grew in thickness from 0.91 Ž8 days old. to 1.07 m Ž12 days old.. First observations of the layer buried 30 December showed the disaggregated constituent grains to be 10–20 mm in size Ž4 days old. ŽTable 1.. During the course of the observation period, the recorded size range of the disaggregated grains varied, but was commonly 8–12 mm. As can be seen in Table 1,
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the maximum grain size may be larger than the thickness of the layer itself. This is assumed to be due to crystal orientations deviating from vertical, a phenomenon which increases over time, due to creep on snow slopes. On days when manual snowpack measurements were made, the temperature of the 30 December layer rose from y8.3 8C Ž4 days old. to y5.4 8C Ž12 days old. to y2.4 8C Ž87 days old. ŽTable 1.. The weak layer temperature, continuously monitored after 27 days, shows the slow steady rise also indicated by the manual measurements. The magnitude of the temperature gradient across the 30 December layer was less than 10 8C my1 on all manual measurement days ŽTable 1.. From 27 days onwards, the temperature gradient was recorded continuously; it did not exceed a magnitude of 5 8C my1 , which is in agreement with the manual measurements.
4.2. Surface hoar layer buried 21 February 2000 The surface hoar layer buried 21 February 2000 was formed during the period 16–20 February. During this time, there was no new snowfall, and the relative humidity was 65–95%, except for 20 February, when it was 55–75%. Winds over this period were slight, between 2 and 6 km hy1 . Night temperatures were between y10 and y14 8C, except for the evening of 20 February, when temperatures were around y5 8C. Daytime high temperatures for the formation period were between y1 and y10 8C. Such conditions are conducive to the growth of surface hoar Že.g. Lang et al., 1984.. The surface hoar layer buried 21 February 2000 was steadily triggered by skiers until it had been buried for 9 days ŽFig. 3.. This was followed by a period of less steady activity, with increased activity when 12 days old, and only one skier-triggered avalanche subsequent to this time, when the layer had been buried for 15 days. For the 21 February layer, the period of frequent skier-triggered avalanche activity is defined as up to age 9 days, with occasional activity up to age 15 days. The time between the frequent and occasional thresholds is the stabilisation period ŽFig. 4..
Fig. 3. Stability trend and study plot measurements for 21 February 2000 surface hoar layer.
The measured shear strength of the 21 February was 0.55 kPa, 6 days after burial ŽTable 2.. During the period in which skier-triggered avalanche activity on the layer slowed, the shear strength was 0.56 kPa Žage 13 days.. The shear strength of the 21 February layer was at a maximum of 5.10 kPa at the end of the observation period Žage 37 days.. The skier stability index Sk 38 of the 21 February layer, calculated from the shear frame measurements, was 0.57 at age 13 days, at which time skier-triggered avalanche activity on the layer had become infrequent ŽTable 2.. Sk 38 reached the maximum value of 3.85, 37 days after the layer was buried. Although measurements of this layer were made at age 6 days, Sk 38 is undefined at this point due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. Initial measurements of the 21 February layer showed it to be 9-mm thick Ž6 days old. ŽTable 2..
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Fig. 4. Stability trend and study plot measurements for first 20 days of burial, 30 December 1999 and 21 February 2000 surface hoar layers. Periods of frequent and occasional skier-triggered avalanche activity are shown.
The layer then compressed rapidly to 5-mm thickness, a 44% change, by age 13 days, at which time the skier-triggered avalanche activity on the layer had become very infrequent. By March 29, the end of the observation period, the layer had thinned to 3-mm thickness.
The load on the 21 February layer due to the overlying slab was 0.30 kPa, 6 days following burial of the layer ŽTable 2.. This load increased to 0.84 kPa by day 13, 1.06 kPa by day 20, and reached 2.44 kPa by the end of the observation period Žday 37.. There was a 0.28-m-thick slab over the 21 February layer 6 days after burial, which increased to 0.65 m in thickness by day 13, and to 0.71 m by day 20 ŽTable 2.. At the end of the observation period, the 21 February layer was 1.20-m deep in the snowpack. Following burial of the 21 February layer, the disaggregated constituent grains were observed to be 4–6 mm in size Ž6 days old. ŽTable 2.. Over the course of the observation period, the size of disaggregated crystals varied somewhat, yet the crystals were observed to be 3–6 mm in size 37 days after the layer was buried, little different from the initial observation of the layer. Like the 30 December layer, the constituent grains may be observed to be larger than the thickness of the layer itself ŽTable 2., again due to non-vertical orientation of the grains. The manually measured temperature of the 21 February layer was y6 8C after 6 days in the snowpack, rising to y3.6 8C after 13 days, and y2.7 8C after 37 days ŽTable 2.. The continuously recorded temperature data Ž13 days onwards. shows the slow steady rise in the temperature of the layer indicated by the manual measurements. The measured temperature gradient of the 21 February layer was less than 5 8C my1 during the entire time it was observed in the snowpack ŽTable 2.. The continuously monitored temperature gradient Ž13 days onwards. did not exceed 1.5 8C my1 in magnitude, which agrees with the manual measurements.
Table 2 Time series measurements of surface hoar layer buried 21 February 2000 Age Ždays.
Slab thickness Žm.
Shear strength ŽkPa.
Coefficient of variation
Stability index Sk 38
Minimum grain size Žmm.
Maximum grain size Žmm.
Layer thickness Žmm.
Load ŽkPa.
Layer temperature Ž8C.
Temperature gradient Ž8Crm.
6 13 20 26 34 37
0.28 0.65 0.71 1.08 1.28 1.20
0.55 0.56 1.89 1.87 2.66 5.10
0.20 0.28 0.51 0.23 0.17 0.38
– 0.57 2.07 1.75 2.13 3.85
4 4 4 4 4 3
6 6 8 4 6 6
9 5 6 4 4 3
0.30 0.84 1.06 1.83 2.44 2.44
y6.0 y3.6 y3.6 y3.1 y3.2 y2.7
y4 2 y1 y1 y1 y2
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5. Discussion The surface hoar layer buried on 30 December 1999, was skier triggered only after several days of burial. Such behaviour has been observed in studies of other buried surface hoar layers Že.g. Jamieson, 1995, pp. 149–152.. In a Columbia Mountains study, between 1989 and 1999–2000, detailed profiles and tests were made adjacent to skier-triggered slab avalanches, and 75% of 53 avalanches involved weak layers that were 5 or more days old ŽFig. 5.. Prior to such a period of reactivity, the snow overlying the weak layer may not be cohesive enough to release as a slab. The 21 February 2000 layer was first skiertriggered on the day it was buried due to a large amount of new snowfall. This formed a cohesive slab reactive to skier triggering, although ski penetration in the study plot was through the layer for at least 6 days after burial. It may be that slopes in the forecast region may have had enough wind effect on the snow overlying the surface hoar layer to form a cohesive slab, while the overlying snow in the sheltered study plot had not. Skier-triggered slab avalanches were observed on the 30 December 1999 layer up to 35 days after burial and on the 21 February 2000 layer up to age 15 days. Fohn ¨ Ž1992. observed that persistent weak layers such as surface hoar are most prone to cause avalanches up to around age 20 days. In the 10-year study with profiles observed adjacent to avalanches ŽFig. 5., only 19% of 53 slabs overlying buried surface hoar layers were skier triggered after 15 days of burial. Both layers in this study showed similar patterns of skier-triggered avalanche activity.
Fig. 5. Relative frequency of skier triggering vs. age of buried surface hoar layers on avalanche slopes.
Fig. 6. Relative frequency of skier triggering vs. strength and load of buried surface hoar layers on avalanche slopes.
During the periods in which each of the buried surface hoar layers in this study began to stabilise, as indicated by the region between frequent and occasional activity in Fig. 4, the measured shear strength of each layer exceeded 0.6 kPa and approached 1 to 1.3 kPa. In the 10-year study of slab avalanches on surface hoar, 71% of 52 were skier-triggered when the shear strength was less than 0.8 kPa ŽFig. 6.. Only 21% occurred when the shear strength exceeded 1.0 kPa. Thus, the stabilisation threshold indicated by extrapolated study plot shear strength measurements agrees well with measurements from avalanche slopes, supporting such study plot measurements as a forecasting tool. Shear strength appears to be a promising predictor of the stabilisation of buried surface hoar layers, once it has reached approximately 1 kPa. The Sk 38 values calculated from study plot observations may be interpolated between measurement days for the two layers in this study ŽJamieson, 1995, pp. 147–148.. Sk 38 cannot be defined for the first few days that each layer was buried due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. The skier-triggered avalanche activity on the 30 December 1999 and 21 February 2000 layers of buried surface hoar became less frequent just after the skier-triggered stability index Sk 38 for each layer increased beyond a value of 0.3 to 0.6 ŽFig. 4.. In addition, skier-triggered events became rare when Sk 38 was 1.1 to 1.6. Jamieson and Johnston Ž1998. found that the percentage of slopes with persistent weak layers Žsuch as surface hoar. that were skier
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By the time at which the buried surface hoar layers of this study began to stabilise in regards to skier-triggered avalanche activity, they had thinned by 32–44% from their initially observed study plot thickness ŽFig. 4.. These results are in consistent with those of Davis et al. Ž1996., in which two different layers of buried surface hoar were observed to undergo a rapid initial thinning of 20–35% in the first 20 to 22 days following burial. Davis et al. Ž1996. and Jamieson and Schweizer Ž2000. concluded that such thinning is an indication of strengthening Žand thereby a trend towards stabilisation. of layers of buried surface hoar. In comparing such thinning with regional skier-triggered avalanche activity, this study shows that the time frame during which a buried surface hoar layer in a study plot is observed to be rapidly thinning may be concurrent with frequent avalanching, followed by regional stabilisation of the layer.
Fig. 7. In situ photographs, surface hoar layer buried 30 December 1999 at Mt. Fidelity Study Slope. Down from top: age 4 days, layer 22-mm thick; age 12 days, layer 15-mm thick; age 27 days, layer 15-mm thick; age 87 days, layer 10-mm thick.
triggered dropped from 77% when the slopes’ skier stability index Sk was less than 1, to 6% when Sk was greater than 1.5. The skier stability index Sk 38 , extrapolated over a regional forecast area of up to 100 km, appears to be a good predictor of general trends in skier-triggered avalanches and exhibits similar threshold values.
Fig. 8. In situ photographs, surface hoar layer buried 21 February 2000 at Mt. Fidelity Study Slope. Down from top: age 6 days, layer 9-mm thick; age 13 days, layer 5-mm thick; age 34 days, layer 4-mm thick.
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Fig. 9. In situ photograph of buried surface hoar layer, 13 March 1996, Vermont Study Plot, Purcell Mountains, British Columbia. Layer 8-mm thick. Photograph by J. Schweizer.
The results of this study indicate that the load due to the slab overlying a weak layer of buried surface hoar is also a potential predictor of a layer’s skier stability. Both the 30 December and the 21 February layers became less reactive to skier triggering of slab avalanches when the load was 0.5 to 1 kPa ŽFig. 4..
Skier-triggered avalanches became rare when the load had reached 0.8 to 1.5 kPa. Furthermore, measurements adjacent to skier-tested avalanche slopes ŽFig. 6. indicates that a load of 1 kPa overlying a buried surface hoar layer is associated with a transition to decreasing frequency of skier-triggered
Fig. 10. In situ photograph of buried surface hoar layer, 13 March 1997, Vowell Valley, Purcell Mountains, British Columbia. Layer 20-mm thick.
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avalanches. Thus, this study shows that an overlying load of approximately 1 kPa appears to be important to the stabilisation of buried surface hoar layers, and a useful tool for regional avalanche forecasting based on study plot measurements. The slab load overlying a weak layer is a function of slab thickness and density; the thickness of the slab on a buried surface hoar layer Žthe depth of the weak layer from the snow surface. is another potential predictor of the skier stability of the layer. In the study plot, the slab over the weak layers was 0.65- to 1.00-m thick when the layers became less reactive to skier triggering on a regional scale. Schweizer and Jamieson Ž2001. found that 75% of the skier-triggered slabs at avalanched slopes were no thicker than 0.63 m. This study shows that the study plot slab thickness may be extrapolated as a potential predictor of the skier stability of buried surface hoar layers. It is important to note that the relatively broad range of threshold values indicated by this study may be due to the natural variation of snowpack thickness across avalanche slopes. Load and slab thickness are but two of the properties of the snowpack surrounding a layer of buried surface hoar; further research may examine the influence of other snowpack properties outside the weak layer on regional stability trends. There is little indication that a change in disaggregated crystal size was associated with the widespread stabilisation of the surface hoar layers in this study. Davis et al. Ž1996. found that, Achange in size of disaggregated crystals is probably a poor indicator of strength Žof the weak layer.B. The results of this study apply this idea to extrapolated avalanche forecasting, and show that size of crystals disaggregated from layers of buried surface hoar in a study plot does not appear to be a good indicator of the regional skier stability of the weak layer. The magnitude of the temperature gradient across both buried surface hoar layers in this study was less than 10 8C my1 during the entire time they were observed; in this range, no effect of kinetic metamorphism on the weak layer is expected ŽColbeck, 1987.. The time series of pit wall photos of both the 30 December 1999 and 21 February 2000 layers ŽFigs. 7 and 8. may be compared with photographs of other surface hoar layers taken in previous winters, using the same techniques ŽFigs. 9 and 10.. The previously
Fig. 11. Photographs of disaggregated crystals on 10-mm grid, surface hoar layer buried 30 December 1999. Down from top: age 4 days, crystals 10–20 mm; age 12 days, crystals 9–12 mm; age 27 days, crystals 10–12 mm; age 79 days, crystals 6 mm.
photographed surface hoar layers show the majority of crystals oriented slope normal. Furthermore, it appears that there was a regular columnar or truss-like arrangement of the crystals in these buried surface hoar layers. Other studies Že.g. Jamieson and Schweizer, 2000. have found a similar arrangement of the constituent crystals in buried surface hoar
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visual changes in the layers’ texture concurrent with stabilisation of skier-triggered avalanches on the layers. With no clear, regular initial structure to either layer, it is problematic to identify changes in texture associated with skier-triggered avalanche activity. It would appear that this technique of observing surface hoar layer texture may be of limited use for forecasting applications. Further research may find a better technique for quickly observing the texture of buried surface hoar layers. The time series of photos of disaggregated crystals from both layers ŽFigs. 11 and 12. show some rounding of crystal edges and clustering of grains over time. This is a commonly observed phenomenon Že.g. Colbeck, 1991., which has been proposed as a sign of strengthening of a buried surface hoar layer ŽGeldsetzer et al., 1997; Jamieson and Schweizer, 2000..
6. Conclusions
Fig. 12. Photographs of disaggregated crystals on 10-mm grid, surface hoar layer buried 21 February 2000. Down from top: age 6 days, crystals 4–6 mm; age 13 days, crystals 4–6 mm; age 34 days, crystals 4–6 mm.
layers. In contrast, the in situ pit wall photographs of both the 30 December 1999 and 21 February 2000 layers show no clear structure or consistent orientation of the crystals in either layer. Therefore, not all buried surface hoar layers will have a regular crystal arrangement. The photographic time series of each layer in situ was made with the intent of identifying
The surface hoar layer buried 30 December 1999 in the Columbia Mountains of western Canada was initially 22-mm thick, composed of 10–20-mm surface hoar crystals. The slab overlying this layer was often skier-triggered until age 8 days, with occasional skier-triggered avalanche activity observed until 35 days following its burial. The surface hoar layer buried 21 February 2000 was initially 9-mm thick, composed of 4–6-mm crystals. This layer showed frequent skier-triggered avalanche activity until age 9 days, with some skier-triggered avalanches up to 15 days following its burial. This first attempt at extrapolating study plot measurements to the regional skier stability of buried surface hoar layers shows promise. Comparisons of the stability trends for these two layers of buried surface hoar indicate the potential for several measured snowpack factors to assist in forecasting the regional stability of buried surface hoar layers. A shear strength of approximately 1 kPa appears critical to the stabilisation of buried surface hoar layers. Skier-triggered avalanches on buried surface hoar appear to decrease in frequency when the skier stability index Sk 38 is approximately 0.5, with skier-triggered avalanche activity sharply reduced when Sk 38 is in the critical range 1.0–1.5. The initial rapid
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thinning of buried surface hoar layers by approximately 20–45% indicates strengthening and stabilisation of the layers. Skier-triggered avalanche activity on buried surface hoar layers is not commonly observed after the load on the layer reaches approximately 1 kPa. Finally, layers of buried surface hoar become less reactive to skier triggering when the slab overlying the layer is 0.65–1.00-m thick. Extrapolation from study plot measurements to stability in surrounding terrain can only indicate general trends. This approach must necessarily be supplemented by site-specific observations, other observations of weather, snowpack, and avalanches, and interpreted by experienced decision makers. A time series of photographs of the two surface hoar layers, both in situ and of disaggregated crystals from each layer, was constructed over the course of this study. No clear arrangement of the crystals in the layers was discernible. Comparing these photographs with the stability trends of each layer showed little visual indication of stabilisation in the in situ pit wall photographs and only general indications of strengthening in the time series of photographs of disaggregated crystals. Suggested further research for extrapolating study plot measurements of buried surface hoar layers includes an examination of the properties of the over- and underlying layers, improving techniques for observing changes in layer texture, and gathering data for more layers concurrent with skier-triggered avalanche activity.
Acknowledgements For their careful field work the authors are grateful to Jill Hughes, Michelle Gagnon, Torsten Geldsetzer, Phil Hein, Ben Johnson, Greg Johnson, Alan Jones, Erica Kotler, Kalle Kronholm, and Paul Langevin. Our thanks to Canadian Mountain Holidays for providing the avalanche occurrence reports from their ski guides. For their assistance with field studies, we thank the BC Ministry of Transportation and Highways and the avalanche control section of Glacier National Park including Dave Skjonsberg and Bruce McMa¨ hon.
This study was funded by the Natural Sciences and Engineering Research Council of Canada, Canada West Ski Areas Association, the Canadian Avalanche Association and the BC Helicopter and Snowcat Skiing Operators Association ŽBCHSSOA..
References Canadian Avalanche Association, 1995. Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches. Canadian Avalanche Association, Revelstoke, BC, Canada. Colbeck, S.C., 1987. A review of the metamorphism and classification of the seasonal snow cover. In: Salm, B., Gubler, H. ŽEds.., Avalanche Formation, Movement and Effects. International Association of Hydrological Sciences, Wallingford, Oxfordshire, UK, pp. 3–33, Publication No. 162. Colbeck, S.C., 1991. The layered character of snow covers. Reviews of Geophysics 29 Ž1., 81–86. Davis, R.E., Jamieson, J.B., Hughes, J., Johnston, C.D., 1996. Observations on buried surface hoar—persistent failure planes for slab avalanches in British Columbia, Canada. Proceedings of the International Snow Science Workshop, 1996. Canadian Avalanche Association, Revelstoke, British Columbia, pp. 81– 85. Davis, R.E., Jamieson, J.B., Johnston, C.D., 1998. Observations on buried surface hoar layers in British Columbia, Canada: section analysis of layer evolution. Proceedings of the International Snow Science Workshop 1998. Sunriver, Oregon, pp. 86–92. Fohn, ¨ P., 1987. The stability index and various triggering mechanisms. In: Salm, B., Gubler, H. ŽEds.., Avalanche Formation, Movement and Effects. International Association of Hydrological Sciences, Wallingford, Oxfordshire, UK, pp. 195–207, Publication No. 162. Fohn, ¨ P., 1992. Characteristics of weak snow layers or interfaces. Proceedings of the International Snow Science Workshop 1992. Colorado Avalanche Information Centre, Denver, CO, pp. 160–170. Fohn, P., Camponovo, C., 1996. Improvements by measuring ¨ shear strength of weak layers. Proceedings of the International Snow Science Workshop 1996. Canadian Avalanche Association, Revelstoke, British Columbia, pp. 158–162. Geldsetzer, T., Jamieson, B., 2000. Estimating, dry snow density from grain form and hand hardness. Proceedings of the International Snow Science Workshop 2000. American Avalanche Association, PO Box 1032, Bozeman, MT 59771, USA, pp. 121–127. Geldsetzer, T., Jamieson, J.B., Johnston, C.D., 1997. Visible changes in buried surface hoar over time. Avalanche News, 52, 15–17, Canadian Avalanche Association, Revelstoke, BC, Canada. Jamieson, J.B., 1995. Avalanche prediction for persistent snow slabs. PhD Thesis, Dept. of Civil Engineering, University of Calgary, Calgary, Alberta, Canada.
T.S. Chalmers, B. Jamiesonr Cold Regions Science and Technology 33 (2001) 163–177 Jamieson, J.B., Geldsetzer, T., 1999. Patterns in unexpected skier-triggered avalanches. Avalanche News, Canadian Avalanche Association, Revelstoke, BC, Canada, 58, 7–17. Jamieson, J.B., Johnston, C.D., 1992. Snowpack characteristics associated with avalanche accidents. Canadian Geotechnical Journal 29, 862–866. Jamieson, J.B., Johnston, C.D., 1993. Shear frame stability parameters for large-scale avalanche forecasting. Annals of Glaciology 18, 268–273. Jamieson, J.B., Johnston, C.D., 1994. Monitoring a shear frame stability index and skier-triggered slab avalanches involving persistent snowpack weaknesses. Proceedings of the International Snow Science Workshop 1994. International Snow Science Workshop Committee, PO Box 49, Snowbird, UT 84092, pp. 14–21. Jamieson, J.B., Johnston, C.D., 1998. Refinements to the stability index for skier-triggered dry-slab avalanches. Annals of Glaciology 26, 296–302. Jamieson, J.B., Johnston, C.D., 1999. Snowpack factors associated with strength changes of buried surface hoar layers. Cold Regions Science and Technology 30, 19–34. Jamieson, J.B. Johnston, C.D., in press. Evaluation of the shear frame test for weak snowpack layers. Annals of Glaciology 32. Jamieson, J.B., Schweizer, J., 2000. Texture and strength changes of buried surface-hoar layers with implications for dry snowslab avalanche release. Journal of Glaciology 46 Ž152., 151– 160.
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Lang, R.M., Leo, B.R., Brown, R.L., 1984. Observations on the growth process and strength characteristics for surface hoar. Proceedings of the International Snow Science Workshop 1984. Mountain Rescue-Aspen, Aspen, CO, pp. 188–195. McClung, D.M., Schaerer, P., 1993. The Avalanche Handbook. The Mountaineers, Seattle, WA. Perla, R.I., Beck, T.M.H., 1983. Experience with shear frames. Journal of Glaciology 29 Ž103., 485–491. Roch, A., 1966. Les variations de la resistance de la neige. Proceedings of the International Symposium on Scientific Aspects of Snow and Ice Avalanches. IAHS Publication, Gentbrugge, Belgium, pp. 182–195. Schleiss, V.G., Schleiss, W.E., 1970. Avalanche hazard evaluation and forecast Rogers Pass, Glacier National Park. National Research Council of Canada, Technical Memorandum 98, pp. 115–121. Schweizer, J., Jamieson, J.B., 2001. Snow cover properties for skier-triggering of avalanches. Cold Regions Science and Technology 33, 207–221. Schweizer, J., Jamieson, J.B., Skjonsberg, D., 1998. Avalanche ¨ forecasting for transportation corridor and backcountry in Glacier National Park ŽBC, Canada.. In: Hestnes, E. ŽEd.., Proceedings of 25 Years of Snow and Avalanche Research. Norwegian Geotechnical Institute, Oslo, Norway, pp. 239–244, Publication No. 203. Sommerfeld, R.A., 1980. Statistical models of snow strength. Journal of Glaciology 26 Ž94., 217–223.
Extrapolating the skier stability of buried surface hoar layers from study plot measurements Thomas S. Chalmers a , Bruce Jamieson a,b,) a
Department of CiÕil Engineering, UniÕersity of Calgary, 2500 UniÕersity DriÕe NW, Calgary, Alberta, Canada T2N 1N4 b Department of Geology and Geophysics, UniÕersity of Calgary, Calgary, Alberta, Canada Received 30 September 2000; accepted 18 June 2001
Abstract Buried layers of surface hoar pose a challenge to avalanche forecasters in many areas, partly because some layers stabilise quickly and others remain unstable for a month or more. This paper relates the measurements of two surface hoar layers in a study plot, one buried 30 December 1999 and the other buried 21 February 2000, to skier-triggered slab avalanches within 100 km of the study plot in the Columbia Mountains of western Canada. The two surface hoar layers were monitored at a tree-line study slope at Rogers Pass every 4 to 8 days until the end of March 2000. Physical properties of the slab Žload, thickness, hardness profile., and weak layer Žshear strength, temperature, temperature gradient, crystal size, crystal form. were observed. Approximately once every 2 weeks, the weak layers were photographed in the pit wall to document their texture. On the same days, disaggregated crystals from the weak layers were photographed on a crystal screen. The February 21 layer, which initially consisted of 4–6-mm crystals, was loaded more slowly by snowfall, gained strength and stability more slowly, yielded initially lower stability indices and released many more skier-triggered avalanches than the December 30 layer, which initially consisted of larger, 10–20-mm crystals. Critical study plot values of load, shear strength, and stability are compared with critical values measured adjacent to over 50 skier-triggered slab avalanches. The shear strength of the weak layer, calculated skier stability index Sk 38 , layer thickness, and load on the weak layer show potential predictive value for the stabilisation of buried surface hoar layers. While the time series of photographs of separated crystals shows distinct changes, the time series of photographs of the buried surface hoar layers in situ reveals little useful information on textural changes other than thinning of the layers. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Avalanches; Surface hoar; Snow stability; Skier triggering; Stability indices; Avalanche forecasting
1. Introduction Many skier-triggered slab avalanches fail on layers of buried surface hoar ŽJamieson and Johnston, )
Corresponding author. Department of Civil Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. Tel.: q1-403-220-7479; fax: q1-403-282-7026. E-mail address: [email protected] ŽB. Jamieson..
1992; Schweizer and Jamieson, 2001.. The susceptibility of such layers to skier triggering is often difficult to assess, making them capable of surprising even professional decision-makers ŽJamieson and Geldsetzer, 1999.. Over the winter of 1999–2000 in the Columbia Mountains of western Canada, two particular layers of buried surface hoar were important to many professional and amateur decisionmakers. The evolution of each of these layers was
0165-232Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 2 X Ž 0 1 . 0 0 0 4 3 - X
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monitored at a study site in Rogers Pass, British Columbia, Canada through snow profiles, strength measurements, snowpack tests, and photography over the course of the winter. The following discussion concentrates on the potential value of certain snowpack factors to predict the reduction of widespread skier-triggered avalanche activity on a layer of buried surface hoar over a large forecast area Žup to 100 km from the study site.. Isolated events following such periods of widespread activity, although important to skiers and avalanche forecasters, are more difficult to predict. Furthermore, extrapolating potential predictors of avalanche activity over such a large area will not indicate the specific stability in areas where terrain and weather couple to create snowpack conditions atypical of the forecast area.
2. Literature review Surface hoar or AhoarfrostB are crystal forms that grow on the snow surface by the deposition of water vapour Že.g. Colbeck, 1987; Lang et al., 1984.. Once buried by snowfall, such layers are characteristically weak. Skiers may induce failure in a buried surface hoar layer, thereby causing a slab avalanche of the overlying snow. Most skier-triggered avalanches happen in the first few days following the burial of a surface hoar layer and are relatively easy to predict, but a layer may remain reactive as a potential failure plane for slab avalanches for several weeks or more Že.g. Fohn, 1992; Jamieson, 1995, pp. 141–155.. ¨ Accidental skier-triggered avalanches that occur later in this period are less frequent and often larger, and more difficult to predict. The transition of buried surface hoar layers from unstable to stable conditions is difficult for avalanche professionals to forecast and is the focus of this study. A skier induces stresses on the underlying snowpack that can penetrate to a buried weak layer and produce sufficient total stress to cause the weak layer to fail and release a slab avalanche. Skier-induced stresses on the snowpack decrease with increasing slab thickness, and skiers are not usually effective triggers on slabs thicker than 1 m Že.g. Fohn, ¨ 1987; Schweizer and Jamieson, 2001.. The stability—susceptibility to slab avalanching—of buried weak lay-
ers in the snowpack may be assessed by direct indicators such as observations of other slab avalanches or snowpack stability ŽClass 1 factors., by less direct indicators such as snowpack factors ŽClass 2 factors., or weather observations ŽClass 3 factors. ŽMcClung and Schaerer, 1993, pp. 125–126.. Stability ratios or indices based on shear frame measurements are such Class 1 factors. Schleiss and Schleiss Ž1970. introduced a snow Astability factorB or Stability Ratio ŽCanadian Avalanche Association, 1995., which is the shear strength of a weak layer divided by the weight per unit area above the weak layer. This has been used since ca. 1960 at the Mount Fidelity study plot to extrapolate snowpack stability for natural avalanches in the Rogers Pass highway corridor, but not for skier-triggered avalanche activity. Roch Ž1966. introduced a stability index S, the ratio of shear strength to shear stress on the weak layer, and Fohn ¨ Ž1987. produced a stability index incorporating skier loading, SX . Both Roch Ž1966. and Fohn ¨ Ž1987. made their shear strength measurements on or near avalanche slopes. These indices were corrected for the effects of normal load and shear frame size. Fohn ¨ and Camponovo Ž1996. showed that the shear strength of weak layers strongly correlated with the skier stability index. To apply the stability index S to slopes of differing angles over a geographical region Žup to 30 km., Jamieson and Johnston Ž1993. obtained S 35 , based on a typical inclination of 358 for slopes in slab avalanche start zones. They found a band of transitional stability Ž1.6–1.8. between stable and unstable snowpack conditions, probably due to the increased variability in the snowpack when applying this natural stability index over a large area. Jamieson and Johnston Ž1993. found that SF and S 35 were effective regional predictors of unstable and marginal natural avalanche stability on 75–87% of days, but this study did not include extrapolated regional stability for skier-triggered avalanches. Jamieson and Johnston Ž1994. refined the skier triggering stability index SX , as Sk, to include the effects of ski penetration. In order to use this index for slopes within 15 km, it was applied to a slope angle of 358 as Sk 35 . Jamieson Ž1995, pp. 148–156., applying a slope angle of 388, excluded normal load effects for persistent weak layers and presented daily Sk 38 values for nine buried surface hoar layers in the Columbia mountains. Most layers
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in that study showed a slowing of skier-triggered slab avalanche activity when Sk 38 exceeded approximately 0.5 and a cessation of observed skier-triggered slab avalanche activity when Sk 38 was 1–1.5. Sk was further refined in Jamieson and Johnston Ž1998. for tests done on or adjacent to avalanche slopes. However, to date, no studies have attempted to apply study plot measurements, including stability indices, to extrapolate the regional-scale stability of a buried weak layer in regards to skier-triggered avalanche activity. The stability index Sk assumes that a buried weak layer of surface hoar fails in shear and not in compression. Although a mixed mode of failure cannot be ruled out, buried surface hoar layers are generally thin Ž- 10 mm thickness., and shear failure is assumed ŽJamieson and Schweizer, 2000.. Several studies have measured strength changes of buried surface hoar layers, with stability trends of the layers and associated avalanche activity ŽJamieson and Johnston, 1994; Jamieson, 1995; Schweizer et al., 1998.. These studies considered a study area of approximately 10–15 km around the snow study plot. Jamieson and Johnston Ž1999. focused on the snowpack factors that influence observed strength changes. Snowpack depth, maximum crystal size in the layer, thickness of the overlying slab, load on the layer, and layer thickness were shown to be important to strength changes of buried surface hoar layers; specific threshold values associated with the stabilisation of these layers were not addressed. Some studies have addressed observable changes in buried surface hoar layer properties ŽDavis et al., 1996; Jamieson and Schweizer, 2000., though field-measured layer thickness has not been addressed as a forecasting factor. Changes in texture over time from photography and analysis of section planes of buried surface hoar layers Že.g. Geldsetzer et al., 1997; Davis et al., 1998. have not yet been applied to regional stability forecasting. 3. Methods On 30 December 1999 and 21 February 2000, two layers of surface hoar were buried in many areas of the Columbia Mountains of British Columbia. Subsequent to their burial and until the end of March 2000, measurements of these layers were taken every 4–8
165
days at a study site on Mount Fidelity in Rogers Pass, British Columbia, Canada. The study site was located at tree line, 1905 m above sea level, on a predominantly east-facing slope that varied between approximately 158 and 308 inclination. On each measurement day, the buried surface hoar layerŽs. were identified in a snow pit wall with a profile of snowpack layers, compression test, rutschblock test, or shovel test ŽCanadian Avalanche Association, 1995.. A snow profile was observed, including properties of the surface hoar layers Žmaximum and minimum grain size, layer thickness, temperature, temperature gradient, shear strength. and of the overlying slab Žload, slab thickness, density profile, hand hardness profile.. The shear strength of each layer was the average obtained from a set of approximately 12 shear frame tests. The overlying snow was removed, leaving 40–45 mm of undisturbed snow above the weak layer. A 250-cm2 stainless steel shear frame was carefully inserted into the snow such that the frame bottom was 2–5 mm above the weak layer ŽPerla and Beck, 1983.. Details of the shear frame test can be found in Jamieson and Johnston Žin press.. Shear strength is the maximum reading on the force gauge divided by the area of the frame, and adjusted for size effects ŽSommerfeld, 1980; Fohn, ¨ 1987.. The maximum and minimum extent of the characteristic surface hoar crystals in a buried layer were observed. This was done according to industry guidelines ŽCanadian Avalanche Association, 1995., manually separating the crystals from the snowpack and observing them with a low magnification Ž8 = . hand lens on a crystal screen with 1-, 2-, 3-, and 10-mm grids. The thickness of a buried surface hoar layer was measured by placing a millimetre scale against the vertical snow pit wall and recording the layer thickness to the nearest millimetre. The load due to the snow slab overlying a buried surface hoar layer was measured in two ways. The first method involves taking a vertical core sample of the slab by inserting a tube vertically through the snowpack layers above the surface hoar layer. Several such cores were often taken to reduce measurement error. In this first method, Load s mgrA
Ž 1.
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where m is the average mass of the core samples, g is the acceleration due to gravity, and A is the cross-sectional area of the sample tube Ž28 cm2 .. The second method is to calculate the load from the thickness and measured densities of the snowpack layers above the buried surface hoar layer, Load s g Ž r 1 h1 q r 2 h 2 q . . . . ,
Ž 2.
where r i and h i are the density and thickness of the ith layer. For layers, which were too thin Žless than 40 mm. to measure with the small Ž100 cm3 . density sampling tube, the density was estimated from hand hardness and grain type as described by Geldsetzer and Jamieson Ž2000.. Two methods of load measurement are used to minimise the sampling error from each method ŽJamieson and Johnston, 1999., and values from both methods were averaged to obtain the load. Manual measurements of snowpack temperatures and temperature gradient across the buried surface hoar layers were taken on each observation day with
digital display thermometers with a resolution of 0.1 8C. The temperature gradient across a buried surface hoar layer was calculated from the temperatures 50 mm above and below the layer. The temperature and temperature gradient across the weak layers were also monitored continuously for part of the measurement period of each layer. Thermisters, which output to a CR-10 datalogger, were placed 50 mm above and below each layer and their values were recorded hourly, as described in Jamieson and Johnston Ž1999.. Approximately every 14 days, the layers were photographed by two different methods: in situ to reveal their texture, and as crystals disaggregated from each layer Že.g. Davis et al., 1996; Jamieson and Schweizer, 2000.. The in situ photos were made by inserting a black screen approximately 10–20 mm behind the weak layer, parallel to the pit wall to provide background contrast. Disturbed or fractured crystals in front of the screen were carefully removed, leaving only surface hoar crystals in their
Fig. 1. Map of study area showing Glacier National Park ŽGNP. and four helicopter ski areas ŽHS. that reported avalanches.
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natural buried state to be photographed with a macro lens and ring flash. Some crystals were then carefully disaggregated from the pit wall, without breaking them into smaller pieces, and placed on a 10-mm grid to be photographed with a macro lens and ring flash. Meteorological conditions near the Mount Fidelity study slope were monitored continuously by an automated weather station. This provided information about prevailing conditions over the time during which the surface hoar layers were forming. University of Calgary research staff based in Rogers Pass noted skier-triggered avalanche activity within approximately 15 km of the Fidelity study slope in Glacier National Park ŽGNP. over the winter 1999–2000. This avalanche occurrence data was then compiled with similar data from surrounding helicopter skiing ŽHS. operations within approximately 100 km of the study site ŽFig. 1. in order to relate the observed changes in the weak layers to skier-triggered avalanche activity on each layer. Over the entire study area, there were approximately 210 skiers a day during the study period. Considering the area, many slopes lay untouched by skiers over the course of this study.
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Skier-triggered slab avalanches on the surface hoar layer buried 30 December 1999 in the Columbia Mountains occurred frequently during the period 4–7 January 2000 ŽFig. 2., when the layer was 6–8 days old Žage of layer s number of days since burial.. There was some skier triggering up to 15 days after the layer was buried, and an isolated event 35 days after the layer was buried. For the 30 December layer, the period of frequent skier-triggered avalanche activity is defined as up to age 8 days, with occasional activity up to age 15 days. The time between the frequent and occasional thresholds is the stabilisation period ŽFig. 4.. The shear strength of the 30 December layer ŽTable 1. increased from 0.17 kPa Ž4 days old. to a maximum of 5.96 kPa Ž66 days old.. During the period in which skier-triggered slab avalanche activity on the layer slowed, the shear strength of the layer rose from 0.54 Ž8 days old. to 1.25 kPa Ž12 days old.. The significant decreases in measurements
4. Observations 4.1. Surface hoar layer buried 30 December 1999 The surface hoar layer buried 30 December 1999 was formed during the period 21–29 December. During this time, at the weather station adjacent to the study plot, there was no new snowfall, and the relative humidity was 60–100%. Winds over this period were light, between 1 and 6 km hy1 , except for higher winds of 11 km hy1 late 29 December. Night temperatures were between y1 and y6 8C, except for the evenings of 25 and 26 December, when temperatures were at or slightly above freezing. Daytime high temperatures for the formation period were below freezing, except for 25, 26, and 27 December, when temperatures reached up to q5 8C. Conditions such as these are suitable for promoting the growth of surface hoar Že.g. Lang et al., 1984..
Fig. 2. Stability trend and study plot measurements for 30 December 1999 surface hoar layer.
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Table 1 Time series measurements of surface hoar layer buried 30 December 1999 Age Ždays.
Slab thickness Žm.
Shear strength ŽkPa.
Coefficient of variation
Stability index Sk 38
Minimum grain size Žmm.
Maximum grain size Žmm.
Thickness of layer Žmm.
Load ŽkPa.
Layer temperature Ž8C.
Temperature gradient Ž8Crm.
4 8 12 20 27 33 38 42 45 52 59 66 73 79 87
0.33 0.91 1.07 1.34 1.22 1.34 1.18 1.43 1.38 1.30 1.36 1.67 1.70 2.04 2.13
0.17 0.54 1.25 1.35 2.57 3.30 3.70 2.95 4.46 4.72 4.33 5.96 4.51 5.25 3.84
0.15 0.19 0.15 0.16 0.19 0.23 0.18 0.11 0.14 0.15 0.21 0.13 0.22 0.26 0.22
– 0.65 1.34 1.89 2.08 2.88 2.19 1.83 2.60 2.86 2.86 3.19 2.25 2.27 1.50
10 10 9 6 10 8 5 10 9 6 8 4 6 6 8
20 15 12 11 12 12 13 13 14 12 12 8 12 6 12
22 15 15 14 15 15 12 12 11 12 14 10 10 8 10
0.24 0.94 1.37 1.60 2.25 2.33 2.95 3.10 3.37 3.28 3.32 4.02 4.28 4.86 5.54
y8.3 y6.6 y5.4 y5.2 y4.8 y4.5 y4.0 y4.0 y3.8 y3.9 y3.6 y3.1 y2.7 y2.3 y2.4
y9 y4 y6 y7 y6 3 0 0 y3 y2 y3 y3 y2 y1 y1
at ages 42 and 73 days are probably due to the natural spatial variations in layer strength within the study slope. This is assumed to be the case because the layer was over 1.4 m deep in the snowpack on both of these measurement days; its shear strength was not likely affected by the external Žmeteorological. forcing conditions present during the periods when shear strength decreases were observed. This internal study plot variability illustrates a limitation of extrapolating study plot data for avalanche forecasting. Other sources of information and forecasting techniques are required for accurate forecasting Že.g. McClung and Schaerer, 1993, pp. 125–147.. During the critical period when skier-triggered slab avalanche activity on the 30 December layer slowed, the skier stability index Sk 38 calculated from the shear frame tests Že.g. Jamieson, 1995; Jamieson and Johnston, 1998. rose from 0.65 Ž8 days old. to 1.34 Ž12 days old. ŽFig. 2, Table 1.. Sk 38 continued to increase following this period to the maximum score of 3.19, 66 days following burial, at which point observed skier-triggered avalanches on the layer had not been reported for 51 days. Although measurements of this layer were made at age 4 days, Sk 38 is undefined at this point due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. Four days following burial, the 30 December layer of buried surface hoar was observed to be
22-mm thick ŽTable 1.. Over a period of 8 days, the layer compressed by approximately 32%, to a thickness of 15 mm. This was coincident with the time frame in which skier-triggered slab avalanches on the layer became less frequent Ž8 to 10 days old.. After this time, the layer continued to compress more slowly until the end of the observation period Ž87 days after burial., when it was observed to be 10-mm thick. At the time when skier-triggered avalanche activity on the 30 December layer slowed down Ž8 to 10 days after the layer was buried., the load Žweight per unit area. of the overlying slab was approximately 1 kPa ŽFig. 2, Table 1.. The load on the layer rose with new snowfall during the observation period; there was 5.54-kPa load on the layer by the time it reached 87 days of age. After 4 days of burial, the 30 December surface hoar layer was buried under a 0.33-m slab in the snowpack ŽTable 1.. During the period in which skier-triggered avalanche activity on the layer slowed, the overlying slab grew in thickness from 0.91 Ž8 days old. to 1.07 m Ž12 days old.. First observations of the layer buried 30 December showed the disaggregated constituent grains to be 10–20 mm in size Ž4 days old. ŽTable 1.. During the course of the observation period, the recorded size range of the disaggregated grains varied, but was commonly 8–12 mm. As can be seen in Table 1,
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the maximum grain size may be larger than the thickness of the layer itself. This is assumed to be due to crystal orientations deviating from vertical, a phenomenon which increases over time, due to creep on snow slopes. On days when manual snowpack measurements were made, the temperature of the 30 December layer rose from y8.3 8C Ž4 days old. to y5.4 8C Ž12 days old. to y2.4 8C Ž87 days old. ŽTable 1.. The weak layer temperature, continuously monitored after 27 days, shows the slow steady rise also indicated by the manual measurements. The magnitude of the temperature gradient across the 30 December layer was less than 10 8C my1 on all manual measurement days ŽTable 1.. From 27 days onwards, the temperature gradient was recorded continuously; it did not exceed a magnitude of 5 8C my1 , which is in agreement with the manual measurements.
4.2. Surface hoar layer buried 21 February 2000 The surface hoar layer buried 21 February 2000 was formed during the period 16–20 February. During this time, there was no new snowfall, and the relative humidity was 65–95%, except for 20 February, when it was 55–75%. Winds over this period were slight, between 2 and 6 km hy1 . Night temperatures were between y10 and y14 8C, except for the evening of 20 February, when temperatures were around y5 8C. Daytime high temperatures for the formation period were between y1 and y10 8C. Such conditions are conducive to the growth of surface hoar Že.g. Lang et al., 1984.. The surface hoar layer buried 21 February 2000 was steadily triggered by skiers until it had been buried for 9 days ŽFig. 3.. This was followed by a period of less steady activity, with increased activity when 12 days old, and only one skier-triggered avalanche subsequent to this time, when the layer had been buried for 15 days. For the 21 February layer, the period of frequent skier-triggered avalanche activity is defined as up to age 9 days, with occasional activity up to age 15 days. The time between the frequent and occasional thresholds is the stabilisation period ŽFig. 4..
Fig. 3. Stability trend and study plot measurements for 21 February 2000 surface hoar layer.
The measured shear strength of the 21 February was 0.55 kPa, 6 days after burial ŽTable 2.. During the period in which skier-triggered avalanche activity on the layer slowed, the shear strength was 0.56 kPa Žage 13 days.. The shear strength of the 21 February layer was at a maximum of 5.10 kPa at the end of the observation period Žage 37 days.. The skier stability index Sk 38 of the 21 February layer, calculated from the shear frame measurements, was 0.57 at age 13 days, at which time skier-triggered avalanche activity on the layer had become infrequent ŽTable 2.. Sk 38 reached the maximum value of 3.85, 37 days after the layer was buried. Although measurements of this layer were made at age 6 days, Sk 38 is undefined at this point due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. Initial measurements of the 21 February layer showed it to be 9-mm thick Ž6 days old. ŽTable 2..
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Fig. 4. Stability trend and study plot measurements for first 20 days of burial, 30 December 1999 and 21 February 2000 surface hoar layers. Periods of frequent and occasional skier-triggered avalanche activity are shown.
The layer then compressed rapidly to 5-mm thickness, a 44% change, by age 13 days, at which time the skier-triggered avalanche activity on the layer had become very infrequent. By March 29, the end of the observation period, the layer had thinned to 3-mm thickness.
The load on the 21 February layer due to the overlying slab was 0.30 kPa, 6 days following burial of the layer ŽTable 2.. This load increased to 0.84 kPa by day 13, 1.06 kPa by day 20, and reached 2.44 kPa by the end of the observation period Žday 37.. There was a 0.28-m-thick slab over the 21 February layer 6 days after burial, which increased to 0.65 m in thickness by day 13, and to 0.71 m by day 20 ŽTable 2.. At the end of the observation period, the 21 February layer was 1.20-m deep in the snowpack. Following burial of the 21 February layer, the disaggregated constituent grains were observed to be 4–6 mm in size Ž6 days old. ŽTable 2.. Over the course of the observation period, the size of disaggregated crystals varied somewhat, yet the crystals were observed to be 3–6 mm in size 37 days after the layer was buried, little different from the initial observation of the layer. Like the 30 December layer, the constituent grains may be observed to be larger than the thickness of the layer itself ŽTable 2., again due to non-vertical orientation of the grains. The manually measured temperature of the 21 February layer was y6 8C after 6 days in the snowpack, rising to y3.6 8C after 13 days, and y2.7 8C after 37 days ŽTable 2.. The continuously recorded temperature data Ž13 days onwards. shows the slow steady rise in the temperature of the layer indicated by the manual measurements. The measured temperature gradient of the 21 February layer was less than 5 8C my1 during the entire time it was observed in the snowpack ŽTable 2.. The continuously monitored temperature gradient Ž13 days onwards. did not exceed 1.5 8C my1 in magnitude, which agrees with the manual measurements.
Table 2 Time series measurements of surface hoar layer buried 21 February 2000 Age Ždays.
Slab thickness Žm.
Shear strength ŽkPa.
Coefficient of variation
Stability index Sk 38
Minimum grain size Žmm.
Maximum grain size Žmm.
Layer thickness Žmm.
Load ŽkPa.
Layer temperature Ž8C.
Temperature gradient Ž8Crm.
6 13 20 26 34 37
0.28 0.65 0.71 1.08 1.28 1.20
0.55 0.56 1.89 1.87 2.66 5.10
0.20 0.28 0.51 0.23 0.17 0.38
– 0.57 2.07 1.75 2.13 3.85
4 4 4 4 4 3
6 6 8 4 6 6
9 5 6 4 4 3
0.30 0.84 1.06 1.83 2.44 2.44
y6.0 y3.6 y3.6 y3.1 y3.2 y2.7
y4 2 y1 y1 y1 y2
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5. Discussion The surface hoar layer buried on 30 December 1999, was skier triggered only after several days of burial. Such behaviour has been observed in studies of other buried surface hoar layers Že.g. Jamieson, 1995, pp. 149–152.. In a Columbia Mountains study, between 1989 and 1999–2000, detailed profiles and tests were made adjacent to skier-triggered slab avalanches, and 75% of 53 avalanches involved weak layers that were 5 or more days old ŽFig. 5.. Prior to such a period of reactivity, the snow overlying the weak layer may not be cohesive enough to release as a slab. The 21 February 2000 layer was first skiertriggered on the day it was buried due to a large amount of new snowfall. This formed a cohesive slab reactive to skier triggering, although ski penetration in the study plot was through the layer for at least 6 days after burial. It may be that slopes in the forecast region may have had enough wind effect on the snow overlying the surface hoar layer to form a cohesive slab, while the overlying snow in the sheltered study plot had not. Skier-triggered slab avalanches were observed on the 30 December 1999 layer up to 35 days after burial and on the 21 February 2000 layer up to age 15 days. Fohn ¨ Ž1992. observed that persistent weak layers such as surface hoar are most prone to cause avalanches up to around age 20 days. In the 10-year study with profiles observed adjacent to avalanches ŽFig. 5., only 19% of 53 slabs overlying buried surface hoar layers were skier triggered after 15 days of burial. Both layers in this study showed similar patterns of skier-triggered avalanche activity.
Fig. 5. Relative frequency of skier triggering vs. age of buried surface hoar layers on avalanche slopes.
Fig. 6. Relative frequency of skier triggering vs. strength and load of buried surface hoar layers on avalanche slopes.
During the periods in which each of the buried surface hoar layers in this study began to stabilise, as indicated by the region between frequent and occasional activity in Fig. 4, the measured shear strength of each layer exceeded 0.6 kPa and approached 1 to 1.3 kPa. In the 10-year study of slab avalanches on surface hoar, 71% of 52 were skier-triggered when the shear strength was less than 0.8 kPa ŽFig. 6.. Only 21% occurred when the shear strength exceeded 1.0 kPa. Thus, the stabilisation threshold indicated by extrapolated study plot shear strength measurements agrees well with measurements from avalanche slopes, supporting such study plot measurements as a forecasting tool. Shear strength appears to be a promising predictor of the stabilisation of buried surface hoar layers, once it has reached approximately 1 kPa. The Sk 38 values calculated from study plot observations may be interpolated between measurement days for the two layers in this study ŽJamieson, 1995, pp. 147–148.. Sk 38 cannot be defined for the first few days that each layer was buried due to excessive calculated ski penetration ŽJamieson and Johnston, 1998.. The skier-triggered avalanche activity on the 30 December 1999 and 21 February 2000 layers of buried surface hoar became less frequent just after the skier-triggered stability index Sk 38 for each layer increased beyond a value of 0.3 to 0.6 ŽFig. 4.. In addition, skier-triggered events became rare when Sk 38 was 1.1 to 1.6. Jamieson and Johnston Ž1998. found that the percentage of slopes with persistent weak layers Žsuch as surface hoar. that were skier
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By the time at which the buried surface hoar layers of this study began to stabilise in regards to skier-triggered avalanche activity, they had thinned by 32–44% from their initially observed study plot thickness ŽFig. 4.. These results are in consistent with those of Davis et al. Ž1996., in which two different layers of buried surface hoar were observed to undergo a rapid initial thinning of 20–35% in the first 20 to 22 days following burial. Davis et al. Ž1996. and Jamieson and Schweizer Ž2000. concluded that such thinning is an indication of strengthening Žand thereby a trend towards stabilisation. of layers of buried surface hoar. In comparing such thinning with regional skier-triggered avalanche activity, this study shows that the time frame during which a buried surface hoar layer in a study plot is observed to be rapidly thinning may be concurrent with frequent avalanching, followed by regional stabilisation of the layer.
Fig. 7. In situ photographs, surface hoar layer buried 30 December 1999 at Mt. Fidelity Study Slope. Down from top: age 4 days, layer 22-mm thick; age 12 days, layer 15-mm thick; age 27 days, layer 15-mm thick; age 87 days, layer 10-mm thick.
triggered dropped from 77% when the slopes’ skier stability index Sk was less than 1, to 6% when Sk was greater than 1.5. The skier stability index Sk 38 , extrapolated over a regional forecast area of up to 100 km, appears to be a good predictor of general trends in skier-triggered avalanches and exhibits similar threshold values.
Fig. 8. In situ photographs, surface hoar layer buried 21 February 2000 at Mt. Fidelity Study Slope. Down from top: age 6 days, layer 9-mm thick; age 13 days, layer 5-mm thick; age 34 days, layer 4-mm thick.
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Fig. 9. In situ photograph of buried surface hoar layer, 13 March 1996, Vermont Study Plot, Purcell Mountains, British Columbia. Layer 8-mm thick. Photograph by J. Schweizer.
The results of this study indicate that the load due to the slab overlying a weak layer of buried surface hoar is also a potential predictor of a layer’s skier stability. Both the 30 December and the 21 February layers became less reactive to skier triggering of slab avalanches when the load was 0.5 to 1 kPa ŽFig. 4..
Skier-triggered avalanches became rare when the load had reached 0.8 to 1.5 kPa. Furthermore, measurements adjacent to skier-tested avalanche slopes ŽFig. 6. indicates that a load of 1 kPa overlying a buried surface hoar layer is associated with a transition to decreasing frequency of skier-triggered
Fig. 10. In situ photograph of buried surface hoar layer, 13 March 1997, Vowell Valley, Purcell Mountains, British Columbia. Layer 20-mm thick.
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avalanches. Thus, this study shows that an overlying load of approximately 1 kPa appears to be important to the stabilisation of buried surface hoar layers, and a useful tool for regional avalanche forecasting based on study plot measurements. The slab load overlying a weak layer is a function of slab thickness and density; the thickness of the slab on a buried surface hoar layer Žthe depth of the weak layer from the snow surface. is another potential predictor of the skier stability of the layer. In the study plot, the slab over the weak layers was 0.65- to 1.00-m thick when the layers became less reactive to skier triggering on a regional scale. Schweizer and Jamieson Ž2001. found that 75% of the skier-triggered slabs at avalanched slopes were no thicker than 0.63 m. This study shows that the study plot slab thickness may be extrapolated as a potential predictor of the skier stability of buried surface hoar layers. It is important to note that the relatively broad range of threshold values indicated by this study may be due to the natural variation of snowpack thickness across avalanche slopes. Load and slab thickness are but two of the properties of the snowpack surrounding a layer of buried surface hoar; further research may examine the influence of other snowpack properties outside the weak layer on regional stability trends. There is little indication that a change in disaggregated crystal size was associated with the widespread stabilisation of the surface hoar layers in this study. Davis et al. Ž1996. found that, Achange in size of disaggregated crystals is probably a poor indicator of strength Žof the weak layer.B. The results of this study apply this idea to extrapolated avalanche forecasting, and show that size of crystals disaggregated from layers of buried surface hoar in a study plot does not appear to be a good indicator of the regional skier stability of the weak layer. The magnitude of the temperature gradient across both buried surface hoar layers in this study was less than 10 8C my1 during the entire time they were observed; in this range, no effect of kinetic metamorphism on the weak layer is expected ŽColbeck, 1987.. The time series of pit wall photos of both the 30 December 1999 and 21 February 2000 layers ŽFigs. 7 and 8. may be compared with photographs of other surface hoar layers taken in previous winters, using the same techniques ŽFigs. 9 and 10.. The previously
Fig. 11. Photographs of disaggregated crystals on 10-mm grid, surface hoar layer buried 30 December 1999. Down from top: age 4 days, crystals 10–20 mm; age 12 days, crystals 9–12 mm; age 27 days, crystals 10–12 mm; age 79 days, crystals 6 mm.
photographed surface hoar layers show the majority of crystals oriented slope normal. Furthermore, it appears that there was a regular columnar or truss-like arrangement of the crystals in these buried surface hoar layers. Other studies Že.g. Jamieson and Schweizer, 2000. have found a similar arrangement of the constituent crystals in buried surface hoar
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visual changes in the layers’ texture concurrent with stabilisation of skier-triggered avalanches on the layers. With no clear, regular initial structure to either layer, it is problematic to identify changes in texture associated with skier-triggered avalanche activity. It would appear that this technique of observing surface hoar layer texture may be of limited use for forecasting applications. Further research may find a better technique for quickly observing the texture of buried surface hoar layers. The time series of photos of disaggregated crystals from both layers ŽFigs. 11 and 12. show some rounding of crystal edges and clustering of grains over time. This is a commonly observed phenomenon Že.g. Colbeck, 1991., which has been proposed as a sign of strengthening of a buried surface hoar layer ŽGeldsetzer et al., 1997; Jamieson and Schweizer, 2000..
6. Conclusions
Fig. 12. Photographs of disaggregated crystals on 10-mm grid, surface hoar layer buried 21 February 2000. Down from top: age 6 days, crystals 4–6 mm; age 13 days, crystals 4–6 mm; age 34 days, crystals 4–6 mm.
layers. In contrast, the in situ pit wall photographs of both the 30 December 1999 and 21 February 2000 layers show no clear structure or consistent orientation of the crystals in either layer. Therefore, not all buried surface hoar layers will have a regular crystal arrangement. The photographic time series of each layer in situ was made with the intent of identifying
The surface hoar layer buried 30 December 1999 in the Columbia Mountains of western Canada was initially 22-mm thick, composed of 10–20-mm surface hoar crystals. The slab overlying this layer was often skier-triggered until age 8 days, with occasional skier-triggered avalanche activity observed until 35 days following its burial. The surface hoar layer buried 21 February 2000 was initially 9-mm thick, composed of 4–6-mm crystals. This layer showed frequent skier-triggered avalanche activity until age 9 days, with some skier-triggered avalanches up to 15 days following its burial. This first attempt at extrapolating study plot measurements to the regional skier stability of buried surface hoar layers shows promise. Comparisons of the stability trends for these two layers of buried surface hoar indicate the potential for several measured snowpack factors to assist in forecasting the regional stability of buried surface hoar layers. A shear strength of approximately 1 kPa appears critical to the stabilisation of buried surface hoar layers. Skier-triggered avalanches on buried surface hoar appear to decrease in frequency when the skier stability index Sk 38 is approximately 0.5, with skier-triggered avalanche activity sharply reduced when Sk 38 is in the critical range 1.0–1.5. The initial rapid
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thinning of buried surface hoar layers by approximately 20–45% indicates strengthening and stabilisation of the layers. Skier-triggered avalanche activity on buried surface hoar layers is not commonly observed after the load on the layer reaches approximately 1 kPa. Finally, layers of buried surface hoar become less reactive to skier triggering when the slab overlying the layer is 0.65–1.00-m thick. Extrapolation from study plot measurements to stability in surrounding terrain can only indicate general trends. This approach must necessarily be supplemented by site-specific observations, other observations of weather, snowpack, and avalanches, and interpreted by experienced decision makers. A time series of photographs of the two surface hoar layers, both in situ and of disaggregated crystals from each layer, was constructed over the course of this study. No clear arrangement of the crystals in the layers was discernible. Comparing these photographs with the stability trends of each layer showed little visual indication of stabilisation in the in situ pit wall photographs and only general indications of strengthening in the time series of photographs of disaggregated crystals. Suggested further research for extrapolating study plot measurements of buried surface hoar layers includes an examination of the properties of the over- and underlying layers, improving techniques for observing changes in layer texture, and gathering data for more layers concurrent with skier-triggered avalanche activity.
Acknowledgements For their careful field work the authors are grateful to Jill Hughes, Michelle Gagnon, Torsten Geldsetzer, Phil Hein, Ben Johnson, Greg Johnson, Alan Jones, Erica Kotler, Kalle Kronholm, and Paul Langevin. Our thanks to Canadian Mountain Holidays for providing the avalanche occurrence reports from their ski guides. For their assistance with field studies, we thank the BC Ministry of Transportation and Highways and the avalanche control section of Glacier National Park including Dave Skjonsberg and Bruce McMa¨ hon.
This study was funded by the Natural Sciences and Engineering Research Council of Canada, Canada West Ski Areas Association, the Canadian Avalanche Association and the BC Helicopter and Snowcat Skiing Operators Association ŽBCHSSOA..
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