Evaluating the Stuffblock and Tilt Board Snowpack Stability Tests as Snow Avalanche Forecasting Tools

Snow avalanche forecasting relies on, among other factors, an assessment of snowpack stability derived from careful observation of snow cover stratigraphy. Snowpack profiles and stability tests provide quantifiable information about the location and strength of weak layers in the snowpack. This study found: 1) good comparative results between the stuffblock and compression tests and, 2) a relationship between tilt board test results in level study sites and skier-triggered avalanches. INTRODUCTION The best way to get direct information about snowpack stability is by observing avalanches and by making snow stability measurements in the field. Collecting consistent snow stability measurements to assess snowpack stability is difficult due to avalanche hazard, rapidly changing conditions, spatial variability, methodology problems, and the challenges of performing laboratory experiments in adverse weather. Avalanche forecasters use a variety of tests to assess stability. This study, conducted at Eaglecrest ski area over the winter of 2003-2004, assessed the operational utility of the stuffblock and tilt board tests that were evaluated against the compression test, the shear frame test and triggered avalanches. Ski patrollers mitigating hazard triggered the avalanches observed in this study. The shear frame test, developed by the Swiss Andre Roch, has been used extensively to index the shear strength of weak snowpack layers (Fohn, 1987). The test uses a frame placed just above a weak layer and pulled with a gauge that records the maximum force. The shear strength is calculated by dividing the maximum force by the area of the frame. The tilt board test is a simple method for avalanche forecasters to observe the stability of the surface 40cm of the snowpack. This test is outlined in the Canadian Avalanche Association (CAA) Observation Guidelines and Recording Standards (OGRS) as part of the process to identify weak layers to be tested using a shear frame. The test puts a block of snow extracted from the snowpack on an angle (and tapping it if necessary) to identify the shears within it. The compression test was developed by Canadian park wardens in the 1970s. This test identifies weak layers within a meter of the snow surface using increasing force applied to a shovel blade resting on an isolated test column of snow (Jamieson, 1999). The stuffblock test, developed in Montana in 1993 (Birkeland, 1996, 1999), is also performed on an isolated test column of snow using a nylon stuff sack filled with snow and dropped on the column until a shear failure occurs. SITE DESCRIPTION AND METHODS Study Site The 300 ha ski area is located five kilometers west of Juneau, Alaska, and is on Douglas Island at the headwaters of Fish Creek, a northwest facing drainage. Eaglecrest rises in elevation from 400 to 820 meters above Paper presented Western Snow Conference 2004 1 Environmental Science Program, University of Alaska Southeast, Juneau, Alaska 2 Eaglecrest Ski Patrol, Juneau, AK sea level. Snowpack observations made at treeline snow study plots located at 720 m and 790 m were in accordance with OGRS as required by Eaglecrest’s subscription to the CAA Infoex data exchange. Snowpack Observations Thirty-four snowpack profiles were observed between November 22 and March 28. The profiles recorded weather, snowpack stratigraphy, temperature, density, and snow water equivalent (SWE). Profiles also include the stability tests outlined below, as well as other observations not part of this study. Shear Frame Test Shear strength of weak layers is best measured with the shear frame (Schweizer, 2003, figure 1). The baseline data for this study came from 40 shear frame tests conducted in the level snow study plots. Shear frame tests were also conducted in test profiles in avalanche start zones and along avalanche fracture lines. Figure 1. Shear frame test. Conducting shear frame tests requires discipline, particularly in adverse weather conditions. While providing quantifiable shear strength data, the test is time consuming and difficult (Perla and Beck, 1982). Each shear frame test was conducted at least five times to ensure shear results were accurately reproducible. The equipment used for the shear frame test included a 100 cm2 shear frame, 2 kg and 5 kg Imada pull gauges, digital and mechanical weigh scales, two sampling tubes and a large putty knife. Shear frame test results are used to calculate a dimensionless stability ratio and used to formulate stability indices for the triggering of avalanches (naturally or artificially). Snow stability is a ratio of strength to stress on a weak layer or interface. The shear frame measures the strength of a snow layer, while snowpack weight determines the stress on the layer. The stability ratio used in this study was calculated as shear strength divided by the weight of snow per unit area (Canadian Avalanche Association, 2002), thus an increase in the stability ratio is indicative of an increase in snowpack strength. Stuffblock Test The stuffblock test is a variation of the compression test where a 4.5 kg weight is progressively dropped higher in 10 cm increments onto a 30 cm by 30 cm isolated column. The tests were conducted in profiles on 35o to 40o slopes with generally north aspects representing avalanche start zones (Figure 2). Figure 2. Conducting a stuffblock test at Eaglecrest. Tilt Board Test The tilt board test was conducted at the 790 m tree line weather plot. Tests were also conducted at the level 720 m study site when severe weather conditions affected the results obtained at the higher study site. The test isolates a 30 cm by 30 cm column, tilting the extracted column to 15 degrees angle and gently tapping until a shear is identified. Very easy shears are defined as failure on tilt, easy shears have failure with one gentle tap, moderate shears have failure after the second gentle tap, and hard shears have failure after three or more taps. The equipment used for the tilt board tests included tilt boards located at both treeline study plots, each equipped with a 30 cm by 30 cm metal cutting plate and a crosscut saw, used for extracting the test snow sample from the snowpack. Irrespective of the depth of new or storm snow, the maximum test depth was 40 cm from the surface of the snowpack. When the snow tested exceeded 200 kg/m the test depth was less than 40 cm. RESULTS AND DISCUSSION From December 11 2003 to April 3 2004, 658.1 cm of snowfall (with 113.2 cm SWE) was recorded at the 790 m snow study site (Carter, 2004). During the winter there were eight major avalanche cycles with each cycle producing numerous natural avalanches outside the ski area. On forty days, 416 avalanches were triggered within the ski area ranging from size 0.5 to 2.0 (Canadian Avalanche Association, 2002). Of those avalanches, 227 were triggered using explosives and 189 were skier triggered. The triggered avalanche activity reflected new snow or surface instabilities. One size 2.5 natural avalanche occurred within the ski area. There was one avalanche involvement outside the ski area on March 5 involving a snowboarder who was carried 200 m without injury. Shear Frame Test – Stability Ratio A comparison of skier triggered avalanches with the stability ratio calculated from the shear frame measurements shows that high stability ratios tend to be associated with lower numbers of triggered avalanches 3 Note that this tilt board test is very different from Monty Atwater’s 1970’s test of the same name. Atwater’s test, still used in Utah and elsewhere, collects a snow sample from snow falling on a board suspended above the snowpack. Criticisms of the test are that wind, air temperature and humidity affect all surfaces of the suspended test sample, that it cannot test the interface between the new and old snow and the associated vapor transfer, and that it is repeatable only with more than one suspended collection board. 4 Bruce Tremper outlines a rudimentary version of the test using a shovel (Tremper, 2001). (Figure 3). This study supports previous research showing that stability indices measured in level study plots are effective predictors of snow stability on proximate slopes (Jamieson, 1995). 0 5 10 15 20 25 30 35 40 0 2 4 6 8 10 12 Stability Ratio Sk ie r T rig ge re d A vy s

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