AdvisorySAAC04

04-10-05 Eaglecrest

We went to Eaglecrest for the last day celebration today, but a series of natural, patrol-triggered, and skier-triggered slabs drew our attention. Yesterday we cautioned that people should watch the bond to any new snow. It snowed hard above 600 m and blew out of the southeast last night, heavily loading lee slopes with snow that turned out to be very sensitive and poorly bonded.

Slabs released on lee slopes on the west side of East Peak, in the Fruit Bowl, and along Pittman's Ridge. We did not have our full snow study kit along today, but we profiled a slab that was skier triggered shortly after midday outside the ski area on a traverse from the Ridge across the west side of East Peak. The skiers were fortunate that it was triggered from a thinner area along the edge and no one was caught.

It was a hard slab, 0.2 - 1.0 m deep, about 100 m wide and 100 m long, dropping from 870 to 810 m. We classified it as HS-AuS-R2D2-I, a hard slab unintentionally skier triggered, relative and destructive size 2, on the interface between old and new (overnight) snow. The slope angle at the crown face ranged from 40° - 42°.

It fractured on a bed surface of old, hard dry snow with an almost imperceptibly thin but slippery melt freeze crust on top from the recent warm weather. The weak layer was 1 cm of soft (fist hard) moist unrimed stellar crystals beginning to change into rounded grains, and the slab was moderately hard (one to four finger) moist windloaded snow that fell and drifted rapidly onto the slope overnight.

A second weak layer 10 cm above the lower one was a 2 cm thick soft (fist hard) moist layer of thawed fine melt freeze grains and thin semifrozen rime crusts. In some areas, the fracture stepped down from an initial break on the upper weak layer.

The graupel layer that has been fracturing in our recent tests was indistinct here. The hard snow layer below the bed surface had graupel grains scattered throughout its lower portion, but no well-defined graupel layers. Neither the graupel nor the bond to the frozen March melt freeze layers was involved in this slide.

As we shoveled out the crown face for profiling, the weak layer at the bed surface sheared out with high energy, fast and clean (Quality 1), and the weak layer above it sheared out a little less often and with average energy (Quality 2). The surface windslab also sheared off easily at average (Quality 2) energy.

These informal shovel shear tests while digging with the top layers removed proved to be the most accurate test for picking out the principal weakness today. Both a tap compression test and an AK Block erroneously indicated more weakness in layers nearer the surface, scoring too-strong values on the deeper principal weakness.

It may be that the thick slab in our test area caused layers nearer to the surface to be triggered more easily than would have been the case if we had tested a thinner flank. Deeper layers are generally harder to trigger, both in block tests and in slab avalanche releases. Spatial variability and strengthening after the slide released probably played a role too.

This weak layer is not likely to be persistent, but may remain sensitive for a few days or more. Avoid steep windloaded slopes and watch for new loading or rapid warming. Watch the bond to any new snow, and for the first rain at elevation.

This is the skier-triggered hard slab we profiled on the west side of East Peak, just outside the Eaglecrest ski area. No one was caught. This traverse is fairly popular, yet it cuts low across many steep slopes that are often heavily loaded. It is the type of terrain best avoided when instability is likely.

View down from the crown face across the bed surface and debris pile, looking toward the East Bowl Chutes.

The crown face, or fracture line. The variable thickness and windloaded nature of the slab are quite evident here.

Closeup of the weak layers at the crown face. Semifrozen thin rime crusts in the upper weak layer 10 cm above the bed surface make it stand out in the profile, though its weakness is evident from the broken-out chunk and the indentation where the softer thawed melt freeze grains brushed easily away. The soft weak layer of rounding unrimed stellar grains is the prominent 1 cm thick indentation just above the bed surface, and the paper thin melt freeze crust atop the bed is barely visible as a slight prominence in the brushed pit wall.

Though most riders avoided slopes of similar angle, aspect, elevation, and loading to the ones that slid, reasoning correctly that fresh slides are the best indicator of instability, not everyone did. The tracks left of and the single set above the slab are on lines with a high probability of releasing similar slabs.

The fact that slopes do not release with the first or second set of tracks is commonly misinterpreted as evidence that they are stable. A better model is to think of potentially unstable slopes as a minefield. The mines are the weak zones or tender spots. Fracture initiates there and spreads out into the stronger areas.

If you make it through a minefield without triggering a mine, you may be tempted to assume there were no mines there at all. Similarly, a rider who happens to miss the tender spots may falsely assume the slope is "safe".

After a track or two is laid down without incident, most riders begin to relax. But then another track may hit the tender spot and the entire slope releases.

Since it is possible to make many tracks without hitting a tender spot, the impression taken away is often misleading. The snowpack is usually quite forgiving of error. It may take years of riding a particular unstable condition before a slab is triggered and the rider learns that a "safe" condition is actually not safe at all.