Avalanches

Avalanches

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

What are avalanches?

They are rapid downslope movements of snow, ice, rock, or soil.

They can be channelized or unconfined may travel as coherent block or

disaggregate into small particules

What Drives Avalanches?

-The driving force is Gravity-Gravity is a force that attracts objects

toward the EarthFg=mgFg=Force of gravitym=massg=gravitational constante

Avalanche SizeSize Runout Potential Damage Physical Size

1 - Sluff

Small snow slide that cannot bury a person, though there is a danger of falling.

Unlikely, but possible risk of injury or death to people.

length <50 mvolume <100 m³

2 - Small Stops within the slope.Could bury, injure or kill a person.

length <100 mvolume <1,000 m³

3 - MediumRuns to the bottom of the slope.

Could bury and destroy a car, damage a truck, destroy small buildings or break trees.

length <1,000 mvolume <10,000 m³

4 - Large

Runs over flat areas (significantly less than 30°) of at least 50 m in length, may reach the valley bottom.

Could bury and destroy large trucks and trains, large buildings and forested areas.

length >1,000 mvolume >10,000 m

Avalanches have severe consequences

Direct effects: impact Burial

Most fatalities were people killed while building railways (canadian Pacific Railway, in 1886)

The 8 km Connaught channel was constructed beneath the mountains to by pass some of the most dangerous avalanche paths.

Avalanches have severe consequences

-Traffic delays and economic losses (For 100 hours each winter, the Trans

Canadian Highway in Rogers Pass, British Columbia is closed because of avalanches).

-Property damage

-Forests damage: uprooting, breaking trees

Avalanches have severe consequences

Indirect effects: tsunamis generated if an avalanche

enters a lake

Avalanche Triggering

Naturally: after snow storms or normal daytime heating upper part of the snowpack.

The person’s weight increases the shearing force in the weak layer triggering failure

Intentionally with explosives, as part of avalanche-control programs

Avalanche causes

Causes of avalanches Trigger mechanisms

What is a cornice?

A cornice of snow about to fall. Cracks in the snow are visible in area (1). Area (3) fell soon after this picture was taken, leaving area (2) as the new edge.

Wikipidia

A cornice is an overhanging edge of snow on a ridge or the crest of a mountain

Avalanche zones

a) Starting zone: where the snow pack fails

b) Avalanche track: along which the avalanche accelerates and achieves its highest velocity

c) Runout area: where the avalanche decelerates and snow is deposited

Avalanche zones

The three parts of an avalanche path: starting zone, track, and runout zone.

(Photograph courtesy of Betsy Armstrong; Source: NSIDC)  

-At a certain point, gs(go) exceeds the shear strength(stay), and failure of the mass occurs

a.Starting zone

Gravity and slope gradient*shear strength (stay force)internal resistance to movement or force of cohesion and friction

*shear stress (Go force)force causing movement parallel to slopeincreases with slope angle

Angle of Repose

-When the go force is equal the stay force the balance is reached and the angle is 45 degrees.

-The angle of repose is at 40 degrees and is the angle beyond which material will start to move down a slope.

When will a slope fail? Fs = Safety Factor

Fs = (shear strength)/(shear stress)=Stay force/Go force

shear strength (stay force)internal resistance to movement

shear stress (Go force)force causing movement parallel to slopeincreases with slope angle

If Fs is less than 1, then the slope is unstable and prone to failure

Slope Angle

-Most avalanches (slabs) are released from slopes between 30 and 45 degrees

-Slopes less than 25 degrees and steeper than 60 degrees have a very low avalanche risk.

-Wet snow slides, can happen on slopes less than 25 degrees. They contain liquid water between the grains of snow.

Avalanche Initiation

1-Point release avalanches (loose snow) : initial failure of a small amount of snow. More snow is incorporated into the avalanche as it moves downslope.

2-Slab avalanche begins with fracturing of the snowpack along a weak layer at depth. Gravity causes the snowpack to move downslope with the top moving faster than the bottom.

Two weak layers within a slab

Initial failure - two types

Surface or near-surface

Failure at depth

More dangerous

Coherent slab comprising fractured blocks of snow.

Loose snow failure

Angle of repose more than 45 degrees

Slab failure

Coherent slab

b) Internal structure of the flow

2 types of snow avalanche (a spectrum exists):flow avalanchesairborne powder snow avalanches

Density and solids concentration gradient

The powder cloud is less dense than the flowing snow

Avalanche flow structure

Note the head, body, and tail of the flow

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 metersCollisions 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

At velocities above 35km/h flow avalanches generate a cloud of powdered snow. The powder cloud is much less dense than the flowing mass.

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 absentpowder avalanches may develop from flow avalanches,

but the mechanisms are not well understood

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 97activities

Fatalities: buildings, 53 6roads, worksites

Some interesting statistics from the Canadian Avalanche Association

Types of snow and slopes prone to failure

Lee-side avalanche with cornice above

Survival

Burried victims die of suffocation, hypothermia, injuries.

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: Avalanche Risk can be estimated by determining the distribution, frequency, and sizes of avalanches in given area.

Note lack of vegetation, which could help dissipate avalanches

Avalanche frequency is described in terms of a recurrence interval

Engineering works

Reforestation:to stabilize slopes and snow

Highways:locate to avoid avalanche tracksuse of defense structures: deflectors,

mounds, benches with dams

Avalanche avoidance

Use of defense structures

Starting zones defenses to support the snowpack and prevent large avalanges (famous in Europe)

Starting zone defenses

To help reduce avalanches from forming:

use of terraces

use of supporting structures

Terracing in avalanche starting zones

Supporting structures

Prevent large avalanges from starting, they are very expensive and used only where people and property are at risk.

Details of supporting structures

Only practical for protecting inhabited structures, busy roads and critical infrastructure (not commonly used in North America)

Some specific examples of mitigation attempts

Deflectors:

Used in the track and Run-out zones

Deflect avalanches from building or parallel to roads.

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

Slow avalanches and reduce their run-out.

Detail of mounds

Become buried early in the winter, avalanches travel over them

Snow sheds

These sheds allow the avalanche to pass over the structure

Run over the roads and railway

Snow sheds

Avalanche Safety

1-Is the slope to be crossed prone to avalanches?

.using terrain factors as slope angle

2-Is the snowpack unstable?

Using public bulletins, recent avalanches in the area, weather observations.

3-What are the consequences to be caught in an avalanche in this terrain? If an avalanche occurs:

Large or small? A slab or point release? Wet or dry snow?

Could I be swept over a cliff or into trees or boulders?

How to avoid an avalanche?

1-check the surface of the snow. Avalanches are common on slopes with smooth surfaces, such as grass or smooth rock than on treed or rough slopes.

2-Climbing under cornices should be avoided especially during snowstorms. In general convex slopes are more dangerous than uniform or concave slopes.

How to avoid an avalanche

3- check for snow an avalanche reports. If there are any warnings, make sure to avoid the areas. Never ski off marked slopes. If snow build-up has been heavy, avoid going out.

4-If you're caught in an avalanche, try to ski toward the side of the moving mass of snow, keep your mouth closed to avoid choking. To stay on top of the avalanche, make swimming motions. When the avalanche starts to slow down, thrust your hand above the surface.

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/