Shear Quality
The National Avalanche Center has been working
with Ron Johnson of the Gallatin National Forest Avalanche Center
on shear quality. Measurements of shear quality are nothing
more than a formalization of what avalanche field workers have
been doing for at least the past several decades. In the past,
many avalanche workers have noted whether their stability tests
resulted in "clean", "fast" shears or "rough",
"dirty" shears. Following their lead, we define three
types of shear quality, Q1, Q2, and Q3. Complete definitions
are in this paper,
but you can think of Q1 shears being the ones that you think
"Oh wow!", Q2 are "Ho hum" and Q3 are "Was
that even really a shear?"
Folks in Canada are using fracture character to
augment stability tests, which is similar to shear quality.
For a comparison of the two, check out this
article.
The biggest danger in stability tests is when
observers place too much emphasis on a single test. Sometimes
an observer does a test, gets a strong result and then goes
onto a slope only to trigger an avalanche. Such occurrences
happen in part due to the spatial variability
of the snowpack. We would like to reduce our chances of making
such a critical error, and our initial results suggest that
carefully noting shear quality in addition to the score of
our stability test might help us better interpret the slope
stability.
Though based on a limited sample, the figure below
graphically demonstrates the usefulness of shear quality. This
graph is only for rutschblocks with a score of 5 or 6, which
typically indicate at least moderately stable conditions, and
the signs of instability include avalanches, cracking or collapsing
observed on adjacent or similar slopes to the one being tested.
For these data, about two thirds of the cases where the test
was associated with a Q1 shear were associated with signs of
instability, whereas none of the tests that had a Q3 shear were
associated with those same signs of instability. Thus, a person
who supplemented their stability test score with shear quality
could have reduced their chances of making a "go"
decision in a "no go" situation.
Finally, we emphasize that assessing shear quality
is just one way to enhance the interpretation of stability test
results, which are themselves only one component of a comprehensive
assessment of the avalanche danger for a particular slope or
area. The data we've collected thus far clearly shows that even
stability tests with high scores and a Q3 shear can be associated
with readily observed signs of instability. As noted in a paper
by Paul Föhn, a researcher from the Swiss Federal Institute
of Snow and Avalanche Research, interpreting stability tests
requires experienced observers, and such tests must be supplemented
with data such as snow profile evaluations, analyses of meteorological
data, knowledge of recent avalanche activity, and knowledge
of the terrain for a comprehensive, and holistic, evaluation
of the avalanche danger.
For more information about shear quality, check
out the following articles:
Birkeland, K.W. and R.F. Johnson. 1999. The stuffblock
snow stability test: Comparability with the rutschblock, usefulness
in different snow climates, and repeatability between observers.
Cold Regions Science and Technology 30, 115-123. [Abstract]
[Article]
Johnson, R.F. and K.W. Birkeland. 2002. Integrating
shear quality into stability test results. Proceedings of
the 2002 International Snow Science Workshop, Penticton,
BC, Canada 508-513. [Article]
Our thanks go to Doug Chabot, Scott Schmidt, Ian
McCammon, Don Sharaf, Allen O'Bannon, and Ethan Greene for collecting
the data that made this work possible.