STAB ASSESSMENT:
WIND SLAB HAZARD REPORT
A quick few notes on the reporting standard we are using for this stability hazard.
Assumptions:
You know how to locate and access terrain safely, conduct this assessment, and understand the basic terms and assessment techniques (CT / ECT - Snowform Identification) consistent with your minimum AST2 (Or equivalent) training.
Given the prevailing conditions we are looking for signs of WIND SLAB instability. The two things that we are looking for are initiation ( a weak layer) and propagation (enough cohesion to create a fracture). For this our observation is looking for clues, any events such as whoomping, shooting cracks from ski trails, remote triggered slides, and observed natural slides. If you get these kinds of warnings then we probably already have enough rough intel (Red Flags) about the hazard to raise a RED ALERT.
If conditions are safe enough to permit and / or 'Red flags' have been noted and responded to appropriately, in the process of running this patrol then conduct some pit tests in a suitable (<22˚ incline) aspect. Ideally at an altitude of 1700m or more, although this is not essential.
STEP 1:
Snow Profile & Compression test (CT)
Starting with a simple column test, you will go through the process of isolating any weak layers. If you get quick, easy initiation results at any depth... don't nerd out, jump to Step 2. If not make routine records in your book (CT Scores and Depths), recording the shear quality ( fast/ resistant / stubborn), noting the interface of the weak layer(s) (Crystal forms).
STEP 2:
Extended Column Test (ECT).
Here in Australia we have a variable and temperate climate, the rate of metamorphosis is relatively high. Our snowpack readily stabilises, establishing its ability to propagate is very important. In the life cycle of a windslab there will be that transitional period in which the slab will be cohesive enough to propagate, and not yet well bonded to the bed surface.
The pit we need for this is very specific for consistent reporting. The task is to isolate a column that is 90cm wide by 30cm from the front to back, and isolated to the depth of the weakness (if on crust), or ideally deeper (on rounds / facets etc).
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TECHNIQUE
Having isolated the block as shown in FIG 1, using the same tapping / loading steps as your Compression Test, tap the loading area and observe closely any propagation across the column. If the column propagation extends the width of the column we report this as ECT:P, or if not report ECT:X.
You can record the loading step as (10), for example ECT:P(10)
STEP 3: Propagation Saw Test (PST)
If you got an ECT:P, or ECT:X but easy fast CT results then it will be worth running this quick additional study. Using the side of the pit you have dug, isolate a column that is 30cm wide by 100cm
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TECHNIQUE
Having isolated the block as shown in FIG 2
push the saw along the weakness in an uphill direction. Again we are waiting to see if the weakness propagates. We want to record three things here. The distance of the cut (X) along the total column (Y) when propagation occurred, and what pulled out (Z) (FIG 3) and the depth of the weak layer (A). This is recorded as PST:X/Y(Z)@Acm
Example:
PST:35/100(END)@35cm
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