Avalanche safety

Slab avalanche, Mt Hotham Aug 2017
Avalanche debris, Champex, Switzerland

Avalanches can and do occur in Australia - some have resulted in fatalities. This article provides information about avalanches and safety advice for avoiding them.

Skiers are responsible for their own safety. Understanding what causes avalanches and how to avoid them is very important when skiing in the backcountry.

Many skiers caught in an avalanche were responsible for triggering it.

Always remember: an avalanche doesn’t know that you are an expert.

Contents

Avalanche Types

There are four main types of avalanches: slab, cornice, wet snow and powder. Cornice and slab avalanches have killed people in Australia.

1. Slab avalanche

A cohesive “slab layer” of snow resting on a softer or weaker layer of snow fractures and the whole slab then proceeds down the slope. The top of the slab releases from a distinctive fracture point that usually remains in place after the slab has released. Large volumes of fast moving snow quickly trap skiers, boarders or climbers on the slope.

Worldwide, slab avalanches kill the most people and are often triggered by people. These sometimes occur in Australia and have killed backcountry skiers and snowboarders.

Slabs can remain dormant in the snow pack for some time, particularly during sustained low temperatures.

Causal factors are:

  • Sustained cold temperatures

  • Recent heavy snow fall

  • Wind loaded slopes

  • A weak layer in the snow pack

Weak layers (instability) in the snowpack can be caused by:

  • Fresh snow falling on old snow that has an icy slippery surface formed by previous melting and re-freezing after rain or high temperatures (significant in Australia).

  • Depth hoar (large snow crystals with facets) forming when water vapour deposits onto existing snow crystals

  • Surface hoarfrost that is buried by subsequent snow falls.

2. Cornice avalanche

Cornices form when wind blows snow over a ridge to form a very steep or overhanging snow structure on the leeward side of the ridge.

Cornices are often unstable – walking or skiing too close to the edge, warm weather and sun can cause a cornice to fracture, sending large amounts of snow down the slope that can trap a person and also trigger another avalanche.

Note that the fracture point for a cornice can be well back from its edge.

Large dangerous cornices form on steeper high mountains in Australia including Mt Feathertop, Mt Bogong, Mt Howitt and the Cross Cut Saw, the Main Range and the Bogong High Plains.

Causal factors are:

  • Recent heavy snowfall can result in a fresh/dry cornice

  • Strong winds can increase the size of cornices and load snow on leeward slopes

  • Rising temperatures, rain and melting can weaken cornice structures, particularly in Spring

  • Skiing or walking too close to cornice edge (don’t go there to “have a look”)

3. Wet snow avalanche

Sun on steep snow slopes combined with warm temperatures can soften and melt snow to some depth. As water saturates the snowpack the bonds that hold it together weaken until the snow releases creating a “slip layer”.

Wet snow avalanches are very dense. Skiers caught in even a small slow-moving wet snow avalanche can be swept into rocks or over a cliff or bluff.

These occur in Australia in spring and during or after heavy rainfall.

Causal factors are:

  • Steep slopes

  • Warm temperature

  • Hot sun

  • Sodden wet snow pack

  • Rain permeating snow can create a “slip layer” on grass (particularly in Australia)

4. Powder avalanche

A power avalanche is loose powder snow rapidly descending a mountainside. These can be very large scale involving massive amounts of snow. They usually occur on major mountains in cold conditions. They do not usually occur in Australia.

Smaller scale powder avalanches are called “sloughs” or “surface slides”,

Causal factors are:

  • Cold temperatures

  • Accumulated heavy snow fall

  • Steep terrain

  • Large scale mountains (e.g. New Zealand Alps, European Alps, Himalayas, Rocky Mountains)

Avalanche warning signs

Before the trip, check reports of local avalanche conditions. These are common overseas and are now issued in Australia by the Mountain Sports Collective.

During the trip, look out for any of these warning signs:

  • Snow going “whump” and settling under your skis (slab avalanches)

  • Cracks in the snow radiating from skis when in transit

  • Slab fracture lines observed (slab avalanches)

  • Avalanche debris observed down slopes and in gullies

  • A “slip layer” and slab observed when a snow pit is dug.

  • High temperatures (wet snow avalanches)

  • Absence of trees in areas where there are trees may indicate previous avalanche activity

  • Unusual natural snowballs, some may be quite large

More than one sign indicates high avalanche hazard.

Terrain

  • Most avalanches occur on a slope angle of 30 degrees or steeper. They occur most frequently on slopes 35 to 50 degrees.

  • Lee slopes (relative to the prevailing wind direction) are the most dangerous as they get heavily laden with wind deposited snow.

  • Consider the terrain above your route and look for steeper slopes that may avalanche. Beware when travelling below or across gullies and obvious avalanche chutes and don’t loiter in high risk zones.

  • Slopes that funnel into a gully (a “terrain trap”) are much more dangerous than those that fan out as the avalanche is concentrated and funnelled into the gully (particularly in Australia).

  • Convex rollovers on slopes create tension in the snowpack where the snow is more prone to fracture.

  • Avalanches can travel considerable distances onto terrain that would otherwise be considered safe – including forested snow slopes and gentle slopes in valleys. This is not commonly observed in Australia.

Avoiding avalanches

Avoiding avalanches is the surest way of keeping out of trouble.

  • Check weather conditions prior to a trip including duration of cold weather, wind and snowfalls.

  • If high avalanche factors or conditions are indicated or forecast consider aborting your trip or choosing a location with a lower risk of avalanche.

  • In Australia, the Mountain Sports Collective provide backcountry travel advisory information.

  • Ask locals or other experienced people about conditions and what they have experienced.

  • Watch for avalanche warning signs

  • Monitor terrain for avalanche hazards

Learn about snow stability tests and use them, in order of reliability:

  • Snow pit test – dig a pit and examine snow layers

  • Rutschblock test – dig out a ski length pit and approach it from above on skis

  • Shovel shear test – use a snow shovel to assess the strength of a snow column

  • Ski pole test – probe the snow with your pole to assess weak layers

Note:

  • Digging one snow pit does not guarantee safety - the snowpack and conditions can vary for slopes with different aspects and altitudes

  • These skills are best learnt by instruction on an avalanche or mountain safety course.

Route finding

Choose routes in alpine terrain to mimimise avalanche risk:

  • Stick to ridges, staying well away from cornices

  • Ski on windward slopes that have been stripped of loose snow

Safe practices for traverses and descents

  • Assess avalanche risks using snow stability tests before skiing or traversing a slope. If in doubt, don’t ski the slope

  • Everyone in the party should carry and know how to use an avalanche transceiver, avalanche probe and snow shovel

  • Don’t ski slopes of a similar aspect to those where avalanches have already occurred

  • Undo ski safety straps and pack waist belt and take your hands out of ski pole straps

  • Ski low angle slopes first to test the condition of the snowpack

  • Ski a suspect slope one at a time with others in the party waiting and watching from safe locations

  • Don’t stop below avalanche chutes or paths

  • Don’t descend directly above other group members

If caught in an avalanche

  • Discard equipment to avoid it dragging you down.

  • Try to move to the side of the avalanche path. Head for zones of safety such as rocks or trees.

  • Try to stay on top of the sliding surface by using your arms in a swimming motion

  • When the avalanche is slowing and coming to a stop try to stick an arm or leg out of the surface to make yourself visible.

  • Move your other arm across your face to create an air bubble around your head and mouth.

  • When you come to a stop calm yourself and wait for rescue.

Only about 50 percent of victims who get completely buried beneath an avalanche survive.

Try your best to get out of the avalanche, and if you cannot, try to stay above the snow.

Training

Avalanche Skills Training (AST) courses developed in Canada are now available in Australia.

See also

External resources

Article source: Avalanche safety – Bush Search and Rescue Victoria, reproduced under CC BY-NC-SA 4.0 licence.