Saturday, January 7, 2012

It Depends Where You Dig

Digging snowpits to decide whether or not to ski a slope is a somewhat controversial topic. Most avalanche professionals would say that the decision to ski or not ski a slope should never be made on one single observation like a snowpit. Yet, all too frequently, among recreationalists, this seems to be the case.

Digging pits to make decisions can provide worthwhile information, but you really need to know what you are looking for before you dig. That is, what layer exactly is likely to be the failure plane for an avalanche, where (spatially) is it likely to be preserved, and where does the possible failure layer have enough of a slab over the top to cause a problem.

In December, we had a three week period with virtually no precipitation. During that time, a surface hoar grew large on sheltered north and east aspects, and near surface facetting was found all other aspects. Finally, in late December significant precipitation buried these snow crystals and a persistent weak layer (PWL) was formed. Depending on exact location, this PWL (referred to as the mid-December PWL) is buried between 20 cm (Rossland Range) and 80 cm (Nelson Range).

On Tuesday, we went out for a tour and decided to see how the mid-December PWL was reacting. We chose a sheltered location on an NNE aspect at 1900 metres in an open glade of trees with a 38 degree slope to dig a test pit. This location was specifically chosen because it is a likely area for the surface hoar to be preserved with a good slab of snow overlying it, and, the slope angle was steep enough to give some meaningful results. We got a couple of good "pops" on this layer.

A couple of days later, we were over the east side of Five Mile Creek on a W to NW facing slope with burnt timber (not sufficient to anchor the snowpack) and a 38 degree slope angle, and a desire to ski the slope. We dug another test pit on the slope we were thinking of skiing. A quick probe indicated we should find the mid-December PWL down about 60 cm, if we would find it at all. The location we were thinking of skiing, faces west (afternoon sun) and gets down-slope winds from the valley to the south (both sun and wind frequently destroy surface hoar before it has a chance to be buried). Digging down about a metre, we found only a trace of the mid-December PWL. Looking carefully with a loupe, we could see a few facetted crystals down 60 cm. We got no results on this layer with testing. We skied the slope, but continued to use precautions - putting our up-track in a safe location, skiing steeper sections one at a time, and regrouping in safe locations.

Did we make our decision based on one single observation point? Not really. We knew the slope aspect and exposure to wind and sun was likely to have destroyed the surface hoar before it was buried, and we had no concerns about other layers in the snowpack. Digging down indicated that our hunch was correct, the mid-December PWL is virtually absent in that specific location. Skiing the slope seemed a reasonable choice. Other locations we would encounter during the tour, such as the climb back out of the valley on a sheltered NE aspect still warranted caution, as there we would expect to find the mid-December PWL preserved and with enough snow over the top to cause an avalanche.

The morale of this long and likely tedious blog post, is that test pits can provide useful information, but you must understand the avalanche problem and its distribution well enough to dig your snowpit in a location that is likely to give you the worst result possible. After all, if you are going to trigger an avalanche, it is where that combination of factors (layer depth, slab stiffness, terrain features, etc.) come together to cause the most unstable snow. Of course, the caveat to all this is that you should be digging in a safe location. No point finding the most unstable snow because it avalanches on you when you start to dig
Surface hoar as big as the palm of your hand

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