AdvisorySAAC04

04-12-05 Mt Stewart

Today we went on the upper slopes of Mt Stewart and tested lee slopes to see how the instability that produced a number of slabs two days ago might have changed with space and time. Yesterday was warm, with rain showers to at least 700 m. The mild weather should have given the snowpack a chance to adjust, settle, and bond without much additional load.

As is often the case, the layers on Mt Stewart today did not correspond directly with those we found two days ago near Eaglecrest. But the principal weakness in our tests was similar - a soft thin layer of unrimed stellar crystals turned to rounded grains, sandwiched between hard windslab layers. Our test values were stronger, in the 5 to 6 range. Informal shovel shear tests as we dug the blocks indicated a stronger bond, too.

The graupel layer we have been tracking recently was not present at this site. Graupel often bounces and rolls downslope and collects in pockets, hollows, and benches. So it is a classic weak layer with high spatial variability. Had we dug at the base of the slope instead of the top, we might have found it.

In our block tests, the Rutschblock scored an RB6 Q2 (multiple hard jumps, average shear quality) on the principal weakness on 40°. The Cutback and regular AK Blocks sized for our 75 Kg tester, and an AK Block sized for an 85 Kg tester, all scored at 5 Q2 (second, hard jump, average shear quality). Agreement among the varieties of AK Blocks was perfect. Tester weight and presence or absence of the cutback made no difference. The Rutschblock did not fracture to the cutback, but the Cutback AK Block did.

Snow quality above 600 m was good, 5 cm of dense moist fresh snow over a firm but edgeable base. Below 600 m, the snowpack was wet, sticky, soft thawed melt freeze. The snowpack below 500 m, where the December rains had their maximum effect, is disappearing rapidly.

The weak layers that caused slabs to release two days ago appear to be strengthening, but are still present. Watch them with rapid loading, thaw, or rain, and watch the bond to any new snow.

Snow showers today dropped a mixture of fine 1 - 1.5 mm graupel and big moist snowflake clusters.

Gerry Landry shades the thermometers for our snowpack profile during a sunny period. The blocks for today's test are, front to back, Rutschblock (RB), Cutback AK Block (CAK), AK Block (AK) sized for a 75 Kg tester, and an AK Block sized for an 85 Kg tester. The line in the snow along the front of the Rutschblock is the layout line we trim it to once we are done with our snow profile, so our poking and sampling does not damage the test block.

Gerry Landry shears the Cutback AK Block on the second, hard jump on a 40° slope, CAK5 Q2, on the principal weak layer at 98 cm, 47 cm below the surface. This is one of our rare cutback tests that fractured on the back cut. Our Rutschblock fractured at RB6, on multiple jumps, and did not break at the cutback. All the AK Blocks, cutback and non-cutback for a 75 Kg skier and non cutback for an 85 Kg skier, fractured at a consistent 5 (second, hard jump) and Q2. All the blocks were cut to 1 m depth.