The issue of relevance

The issue of relevance

The task of gathering and interpreting snowpack information should include a process to determine how relevant the information from observations and tests is to snow stability.

Ensuring Relevancy and Verification of Snowpack Information

Assessing relevance:

Concept of red flags; weigh certain pieces of information and certain values more than others.

Next – ask,

Where did the information come from, and is it representative of the terrain and snowpack in the start zones?


Standardized techniques are important!

• Each snowpack test REQUIRES AN OBJECTIVE prior to commencement.• Practice and experience is required for quality site selection of field test sites.• Tests are skillfully conducted using standardized, practiced techniques. Consistent techniques reduce uncertainty.


Checklists to evaluate snow profile

Challenges to the Observer • Choose sites that are “near start zone” in character, but also safe and accessible. • Ensuring conclusions regarding snowpack instability aren’t “premature.” Conclusions should be confirmed by peer review.• Ensuring observations are repeatable and verified by other relevant tests. • Recognizing the possibility of “false stable indicators.”• Understanding the nature and scope of snowpack variation across the slope. • Operational pressures or group dynamics may compromise available manpower or time and not provide the access required to complete tests properly.

Snow profileObs and test structural profiles of spx

Disadvantage: takes time

Yellow flags: points to layer and layer interface with 70-75% accuracy


Compression test In combination w/frx character useful for identifying thin, persistent weak layers, and obs the likelihood of frx. initiation. Focus on layer ID, initiation (27-70cm deep) Focus on normal stress Initiation need to record frx character

Fracture character• Sudden planar (SP) V , w/mfc• Sudden collapse (SC) V > 1cm



Shovel shear testFocus on shear deformationCan use Frx character with only SP, RPGood only for midpack, 30-70 cmObserving layer changes over time in a study plot as block size and (given the same observer) pull force don’t change as the snowpack depth or “load’ increases. More for snow researcher

Deep tap testNot for upper spx layersGood for stiff layer on weak snowBlock always the same sizeAlways use use Frx character


Limitations (small column tests)When obs frx character: Progressive compression (Q2) type fracture character is impractical to observe using a shovel shear test.

Sudden collapse (Q1-“drops”) are harder to accurately observe using the ST.

Progressive compression fractures are often associated with new snow or storm snow layers.



Extended column test Lg column tests offer information about initiation and propensity for further propagation. A 1 m column gives an idea of whether propagation will continue once the fracture has initiated Observing weak layers under “soft slabs” greater than 30 cm thick but =/< 70 cm thick. Similar depth to CT, 27-70 cm False stable with stiff slab (1F) 90 cm long x 30 cm wide Cut to back wall to 100 cm

Propagation saw test Need to indentify layer Useful for observing fracture propensity for propagation. Useful for correlating information gathered from CT & DT tests. END: frx continues uninterrupted to the end of column ARR, propagation arrests within the weak layer before reaching the end of the columnSF,propagation ends at a fracture through the overlying slab



Layer ID Initiation Propensity further propagation

Propensity propagation - failure

Probing

Shovel tilt

Profiles

Compression

Shovel shear

Deep tap test

ECT

PST

RB

Explosives

Consider all available information prior to making a statement on snowpack instability

• Avalanches: current and past• Snowpack structure: consider history and variability • Weather conditions: season trends, current condition, variation across the range



Fracture characteristicsSmall column tests + fracture characteristics

w/o frx characteristics, 37% w/ fr characteristics, >97% in >4,000 tests


Fracture initiation + fracture propagation = slab releaseIn recent years it has become apparent that snowpack stability tests are better indicators of whether a skier is likely to initiate a fracture in a weak layer than whether ─ once initiated ─ the fracture will propagate.

Learning OutcomesKnow Shear QualityKnow Frx characterKnow the Yellow Flags (e.g., Jamieson and Schweizer, in press)

Unusually clean, planar, smooth and fast shear surface: weak layer may collapse during fracture. The slab typically slides easily into the snow pit after weak layer fracture on slopes steeper than 35° and sometimes on slopes as gentle as 25°. Tests with thick, collapsible weak layers may exhibit a rougher shear surface due to erosion of basal layers as the upper block slides off, but the initial fracture was still fast and mostly planar.

Q1

Shear Quality

Average” shear: shear surface appears mostly smooth, but slab does not slide as readily as Q1. Shear surface may have some small irregularities, but not as irregular as Q3. Shear fracture occurs throughout the whole slab/weak layer interface being tested. The entire slab typically does not slide into the snowpit.

Q2

Shear Quality

Shear surface is non-planar, uneven, irregular and rough. Shear fracture typically does not occur through the whole slab/weak layer interface being tested. After the weak layer fractures, the slab moves a little, or may not move at all, even on slopes steeper than 35°.

Q3

Shear Quality

Fracture character, CAA OGRS, 2007



Yellow Flags:

Identifying potential failure layers

There are three layer properties and three interface properties to check.



Documents

The issue of relevance