Unit 2 Exam
Dylan Craaybeek
Fall 2018

What is alpha angle when measuring avalanche runout? How do you measure it? Does a bigger alpha angle mean a longer or shorter runout relative to a smaller alpha angle? (3pts):

Alpha angle when measuring avalanche runout is the angle from the top of the crown to the toe of the debris where the avalanche came to rest. You can measure alpha angle with an inclinometer, though it is trickier than measuring the slope angle because you are measuring the angle of an imaginary line, so you have to use a line of sight measurement with your inclinometer. The larger the alpha angle the shorter the horizontal runout distance will be relative to a smaller alpha angle.

Describe the difference between trigger points and anchors on a slope (i.e. when is a slope full of trigger points vs well anchored? (2 pts)

Trigger points are typically hidden points of weakness in the snowpack that will often trigger an avalanche when loaded whereas anchors are often clearly visible such as trees, large rocks, bushes. If there are only a few trees on a slope then these could be acting like trigger points more than anchors but the more dense the trees the more effectively they will act as anchors, and they will anchor a hard slab more effectively than a soft slab. Especially in a continental climate, rocks often act as trigger points because a skier is more likely to penetrate through the slab to the weak layers below. An ideal well anchored slope would be a slope dense trees, especially spruce or fir trees with many branches, and a hard slab. An ideal slope full of trigger points would be just a couple trees on the slope with shallow rocks on the slope as well.

Name three terrain characteristics to consider when evaluating the potential for a start zone to produce an avalanche (3pts)

Slope Angle, Slope aspect (with respect to sun, wind, weather, etc...), Terrain traps (trigger points, convexities, gullies, etc...)

List the three main types of blowing snow movement regimes along with associated wind speed ranges. Which one is responsible for the most amount of loading in start zones? (4 pts)

1.) Rolling, or creeping, occurs with winds typically less than 5m/s 2.) Saltation which typically occurs at wind speed 5-10m/s 3.) turbulent suspension which will typically occur at wind speeds exceeding 15m/s.

Saltation is responsible for the most snow transport with respect to loading start zones.

How does the fetch influence blowing snow? Name 3 characteristics. (3pts)

Fetch can influence blowing snow by reducing the total amount of snow transported by reducing the amount of wind across the terrain. Three characteristics that can affect fetch would be vegetation (trees), colouirs/gullies, rock outcroppings/cliffs

How does snow hardness influence blowing snow? (1pt)

The harder/denser the snow the more the wind speed threshold will increase so the less likely snow transportation will occur.

What are the three principal stresses/strengths of an inclined snowpack (3pts)

Tension, Shear, Compression

What is snow density and how do we measure it (include the correct units) (2pts)

Snow density is a measure of how compact the snowpack is in the measured area (mass/volume). According to SWAG snow density is measured in (kg/m^3)

What is the approximate range for average slab density? (1pt)

An approximate range for slab density is 50-450 (kg/m^3) with most dry slab avalanches occurring around 200-250 (kg/m^3)

Define fracture in a snowpack (1pt).

Fracture is the process of crack propagation. It is a catastrophic failure in a substance, in a snowpack fracture occurs when the shear stress approaches the shear strength and the rate of deformation in the weak layer is fast enough to provoke a fracture.

Define stress and strain (2pts).

Stress is the force per unit of area and strain is the rate of stress(deformation/time).

What is fracture toughness with respect to a weak layer in a snowpack? (1pt)

Fracture toughness with respect to a weak layer is the ability of the layer to resist fracture once a crack is present.

Describe the difference between persistent and non-persistent weak layers. Give one example of each (2pts).

The biggest difference between persistent and non-persistent weak layer forms would be the timeline in which they last but there is a number of differences.

Persistent Weak Layer: lasts weeks to months to the entire season, has low fracture toughness, anisotropic, weaken with metamorphism, and resistant to bonding.

Non-persistent weak layers: typically last hours to days, fracture toughness and bonding increase relatively rapidly, strengthen quickly in their metamorphic process.

See my final presentation assignment for more detail*heehee

List three factors that promote persistence in persistent weak layers. (3pts)

Cold snow temperatures, shallow snowpacks, strong temperature gradients

Define plastic and elastic deformation (2pts).

Elastic deformation is a recoverable deformation, in the snowpack, this is often a slow rate of deformity, and the snowpack can recover and strengthen, whereas plastic, or viscous, deformation is a permanent deformation, often caused by a rapid rate of deformation.

Define settlement in the snowpack (1pt).

Settlement in the snowpack is the deformation of the snowpack in the vertical direction(from the surface of the snowpack towards the ground).

Define creep with respect to an inclined snowpack (1pt).

Creep with respect to an inclined snowpack would be the added vector quantities of settlement and shear which causes the snowpack to slowly flow down the slope, occurring more in the upper layers of the snowpack.

Define glide with respect to an inclined snowpack (1pt).

Glide is the slipping of the entire snowpack typically at the ground surface.

Describe the difference in how soft slabs respond relative to hard slabs under the same applied load. (1pt).

A soft slab will be easier to initiate an avalanche but will often be a smaller avalanche than a hard slab if the same load were applied to both because a hard slab will distribute the load to a larger area, often called bridging. This is also why if you do trigger a hard slab it will likely be a larger avalanche than a soft slab.

List the chain of events that leads to dry slab avalanche release (5 pts).

Initiation, the onset of crack propagation, dynamic crack propagation or fracture, tensile failure, sliding of the slab.

What snowpack strength of the slab itself fails when the slab breaks at the crown/fracture line? (1pt)

When the slab breaks from the crown it is a failure in the tensile strength of the snowpack.

What snowpack strength DOES NOT fail if a slab collapses and produces a whump, but does not slide? (1pt)

If you hear a "whumph" then you hear the snowpack fail in compression, not in shear. It does not fail in shear.

In your own words, what does “mixed-mode anticrack” mean? (1pt)

Mixed-mode anticrack is the failure of a slab in compression, shear, and tension due to the collapse of the slab following the fracture.

What are the primary roles of the weak layer and the slab when considering initiation and propagation? (2pts).

Weak layer properties determine crack formation/ initiation propensity. Slab properties determine fracture/ propagation propensity and extent of failure, if any.

To really reduce the chance a slope will avalanche, do we need to break-up/disrupt the slab, the weak layer, or both. Explain. (2pts)

To reduce the chance of a slope avalanching as much as possible you we need to break/disrupt both the weak layer and the slab. If you just broke the slab up there is always a chance a broken section of that slab will still avalanche or you could trigger the weak layer to propagate to another slab and that slab could avalanche. If you just somehow disrupted the weak layer without disrupting the slab the slab always has a chance of sliding on a different weak layer or all the way at the ground surface.

Which is generally easier to trigger: soft slabs or hard slabs? (1pt).

Soft Slabs are generally easier to trigger than hard slabs.

What is avalanche type as defined by SWAG and how is it used? (2pts).

An avalanche type as defined by SWAG is Loose-snow, Wet loose-snow, Soft slab, Hard slab, Wet slab, Icefall, Slush flow, Cornice fall, Roof, or unknown. This system is used to describe or classify an avalanche that has already occurred that an observer is recording.

What are avalanche problems, and what is the utility in using them to describe avalanche hazard? (2pts).

Avalanche problems are a way to characterize the hazard, and thus help determine mitigation strategies.