22 Jan 2012

How low is low, I guess for the last storm this was the basement.
Yesterday I posted some observations , only 2 people (who commented) caught the flaw in the shovel shear test. In the observation guidelines and recording standards for the CAA (Canadian Avalanche Association), the back cut of the column should be no deeper than 70 cm. Here is the standard:

Shovel Shear Test

The shovel shear test provides information about the location where the snow could fail in shear. It is best applied for identification of buried weak layers and does not usually produce useful results in layers close to the snow surface. Soft snow near the surface is better tested with the tilt board and the shear frame or an improvised version of this tilt test.
A pit is dug in undisturbed snow to expose a smooth snow wall on a safe slope representative of the slopes of interest. The pit is dug to ground or until well below any possible significant weak layers (often as much as 2 metres deep). A column of snow 25cm wide (across the slope) and 35 to 40 cm up slope is created as in the diagram at left. The back of the column is not separated from the rest of the snow pack initially. A snow saw can assist in creating the column and making the subsequent back cut described below.
A cut in the back of the column is now made. This cut should be no more than 70 cm deep and should end in medium or hard snow. This is best done with a snow saw and the saw is left in place to identify the depth of the cut. A snow shovel is now inserted in this back cut as shown and force is slowly applied parallel to the top of the slope.
When the column fails in a smooth plane above the low end of the back cut, this level is marked and the depth of the shear failure and force required to cause failure are recorded. The size and type of crystals at the failure plane (often from the underside of the block) are also recorded. If the column does not shear cleanly, the block is then tilted and tapped with increasing force to see if other failures planes can be found.
Another back cut is now made an additional 70cm below the first and the process is repeated to the bottom of the pit. The test is often repeated to verify the results or a Compression Test may be done alongside the first test location.
The amount of effort required to cause the failure is recorded as follows:
  • Very Easy (SV) - fails during cutting or insertion of shovel
  • Easy (SE) - fails with minimum pressure
  • Moderated (SM) - fails with moderate pressure
  • Hard - (SH) - fails with firm sustained pressure
  • Collapse (SC) - block settles when cut
NOTE: Observers are cautioned that identification of the weak layers is the primary objective of the shovel shear test. The shovel shear test is not a stability test. While the rating of effort needed to break the snow may assist with a decision concerning snow failure, it is an inaccurate measurement of snow strength. The ratings of effort are subjective and depend on the strength and stiffness of the slab, on the size, shape, length of the shovel and the length of the shovel handle.

A size 1 ski cut
 Yesterday avalanche control produced size1 to size 2 avalanches with ski cuts and explosives. The new snow was very reactive.

At 2280 meters the temperature was -12 at 06:00. the winds are 35-50 KPH from the SSE. At 1550 meters the temperature is -8 with 95 % humidity and the barometer is dropping. There is 1 cm of new snow. In the valley it is -2. Snow will ramp up during the day and by tomorrow morning we should see amounts near 30 centimetres. The skiing will be exceptional today.

For the up to date Avalanche Advisory click here: Avalanche Advisory

It will be a while before we get this view. Models indicate that there should be no real sunny breaks until Thursday.