Avalanches

Avalanches

Avalanches were first imagined as giant snowballs which increased in size from accretion of underlying snow

What are avalanches?

❚ They are flows which move under the influence of gravity

❚ They can be channelized or unconfined

❚ In this sense, they are similar to pyroclastic flows, debris flows, etc.

Avalanches have severe consequences

❚ Direct effects:❙ impact❙ burial

❚ Indirect effects:❙ tsunamis generated if an avalanche

enters a lake

Avalanche zones

❚ a) Starting zone: where an avalanche is initiated

❚ b) Avalanche track: where it goes

❚ c) Runout area: where it dissipates

a) Starting zones of avalanches

❚ Fs = Safety Factor

❚ Fs = (shear strength)/(shear stress)

❚ shear strength: internal resistance to movement

❚ shear stress: force causing movement parallel to slope; increases with slope angle

❚ If Fs is less than 1, then the slope is unstable

and prone to failure

Shear stress

On a slope, gravity has twocomponents, one at right anglesto the slope (gp), and the otherparallel to the slope (gs)

As the slope angle increases, gp

decreases while gs increases

At a certain point, gs exceedsthe shear strength, and failureof the mass occurs

Layering of snow

Two weak layers within a slab

Initial failure - two types

Surface or near-surface

Failure at depth

More dangerous

Loose snow failure

Slab failure

b) Internal structure of the flow

❚ 2 types of snow avalanche (a spectrum exists):

❘ flow avalanches❘ airborne powder snow avalanches

Density and solids concentration gradient

Avalanche flow structure

❚ Note the head, body, and tail of the flow

❚ Vertical and lateral gradients in solids (i.e., snow) concentration and in density:

❘ a lower dense portion which is highly hazardous and destructive

❘ an overlying more dilute portion…also can be hazardous and destructive, since it is turbulent

Flow avalanches

❚ Velocities up to 216 km/hr (60 m/s)❚ Flow heights 5-10 meters❚ Collisions of particles - granular flow

❚ Initially tends to slide as a rigid body (similar to a landslide)…

❚ …but rapidly breaks up into smaller particles and becomes a granular flow

Interior of the flow

-There is a high-density core near the base of the flow

-In this zone, particles collide, resulting in friction and producing heat

-When the avalanche flow stops, freezing can occur, making the deposit very hard

-sets like concrete

High-density core

Mixed flow and powder avalanche

Airborne powder snow avalanches

❚ Velocities can exceed 360 km/hr (100 m/s)

❚ Flow thicknesses may exceed 100 meters

❚ Essentially a highly dilute density current flowing down an incline:

❘ partial entrainment of underlying snow by turbulent, erosive flow

❘ dense core small or absent❘ powder avalanches may develop from flow

avalanches, but the mechanisms are not well understood

Velocity

Newly-fallen snow depth

Powder avalanche: note frontal zone of higher density, low-density cloud behind front

Fully-developed powder avalanche due to cascading down near-vertical cliffs

c) Runout area

❚ Powder snow avalanches flow around obstacles, while flow avalanches do not

❚ When powder snow avalanches hit a barrier, the lower dense portion of the flow is stopped, while the more dilute cloud behaves like a fluid which can flow around or over the obstacle

Some Canadian statistics

❚ Activity 1959-74 74-89

❚ Fatalities: recreational 33 97

❚ activities

❚ Fatalities: buildings, 53 6

❚ roads, worksites

Some interesting statistics from the Canadian Avalanche Association

Avalanche causes

Causes of avalanches Trigger mechanisms

Types of snow and slopes prone to failure

Lee-side avalanche with cornice above

Survival

Some U.S. statistics

Fatalities Property damage (thousands of dollars)

Mitigation

❚ Avoid steep slopes, gullies

❚ Close high-hazard areas to reduce risk and vulnerability

❚ Set off explosive charges to artificially induce avalanches and remove the source material (unstable snow)

HAZARD MAPS, Alta, Utah: note the very fine line between zones of high hazard, potential hazard, and no hazard

Note lack of vegetation, which could help dissipate avalanches

Engineering works

❚ Reforestation:❘ to stabilize slopes and snow

❚ Highways:❘ locate to avoid avalanche tracks❘ use of defense structures: deflectors,

mounds, benches with dams

Avalanche avoidance

Use of defense structures

Starting zone defenses

❚ To help reduce avalanches from forming:

❚ use of terraces

❚ use of supporting structures

Terracing in avalanche starting zones

Supporting structures

Details of supporting structures

Some specific examples of mitigation attempts

❚ Deflectors:

❚ must be gradual, otherwise the avalanche will overflow the deflector

Arresters

❚ Arresters are used to slow or stop avalanches

❚ need adequate height; if too low, flow can accelerate above barrier, increasing damage

Splitters

❚ These are placed directly in front of a single object

❚ They redirect and divert the avalanche flow around the structure

Use of splitters on ski slopes

Mounds

❚ These are used to retard flowing snow at the end of the runout zone

Detail of mounds

Snow sheds

❚ These sheds allow the avalanche to pass over the structure

Snow sheds

Note long dikes to prevent spreading

Current El Niño conditions

Global winter impacts from El Niño

Temperature anomalies from El Niño, Winter 1998

Precipitation anomalies from El Niño, Winter 1998

Avalanches - readings

❚ Committee on Ground Failure Hazards Mitigation Research, 1990. Snow avalanches and mitigation in the United States. Washington, National Academy Press.

❚ Fredston, J., 2005. Snowstruck: In the Grip of Avalanches. New York, Harcourt.

❚ Fredston, J., and D. Fesler, 1994. Snow sense: a guide to evaluating snow avalanche hazard. Anchorage, Alaska Mountain Safety Center, Inc.

❚ International Commission on Snow and Ice of the International Association of Hydrological Sciences. 1981. Avalanche atlas : illustrated international avalanche classification. Paris, Unesco.

❚ McClung, D., and P. Schaerer, 1993. The avalanche handbook. Seattle, The Mountaineers.

Avalanches - web

❚ Canada: http://www.avalanche.ca/

❚ USA: http://www.americanavalancheassociation.org/

❚ North America: http://www.avalanche.org/