Avalanche safety

 

This article provides information about avalanches and safety advice for avoiding them.  Avalanches can and do occur in Australia. Always remember: the avalanche doesn’t know that you are an expert.

Ski patrollers in resorts assess avalanche risks and conduct control measures when necessary, but groups travelling in the backcountry are responsible for their own safety.

Mt Hotham avalanche Aug 2017

Mt Hotham backcountry avalance, source

Avalanche debris Champex valley Switzerland
Avalanche debris visible in Champex valley, Switzerland on the Haute Route. Note ski tracks to the right.

Avalanche Types

There are four main types of avalanches: slab, cornice, wet snow and powder.

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.  Slabs can remain “dormant” in the snow pack for some time, particularly in colder climates.  Causal factors are:
  • Cold temperatures
  • Recent heavy snow fall (although slabs can remain dormant for some time)
  • Wind loaded slopes
  • A weak layer in the snow pack
  • Rain can create a wet layer which re-freezes and forms a slip layer for subsequent snow falls (signficant in Australia)
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 can cause it to fracture, sending large amounts of snow down the slope that can trap a person on also trigger another avalanche.  Note that the fracture point for a cornice can be well back from its edge.  Large dangerous cornices are not uncommon on steeper high mountains in Australia including Mt Feathertop, Mt Bogong, the Main Range and even Mt Nelse.  Cornice avalanches have killed people in Australia. Causal factors are:
  • Recent heavy snow fall can result in a fresh/dry cornice
  • Strong winds increase size of cornice and load snow on leeward slopes
  • Rising temperatures can weaken cornice structures
  • 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 snow pack the bonds that hold it together weaken until the snow releases creating a “slip layer”.  Wet snow avalanches move more slowly but the snow is much more dense.  Skiers caught in one can be swept into rocks or over a cliff or bluff.  These occur in Australia in spring or after during 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 – Loose powder snow rapidly descending a mountainside. These can be very large scale involving massive amounts of snow and usually occur on major mountains in cold conditions.   Smaller scale powder avalanches are also called “sloughs” or “surface slides”.  These do not usually occur in Australia.  Causal factors are:
  • Cold temperatures
  • Recent heavy snow fall
  • Steep terrain
  • Large scale mountains (e.g. New Zealand Alps, European Alps, Himalayas, Rocky Mountains)
Avalanche warning signs
Look for these warning signs.  More than one sign indicates high avalanche hazard.
  • Reports of local avalanche conditions (not normally issued in Australia but are common overseas)
  • 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)

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.
  • Avalanches can travel considerable distances and onto terrain that would otherwise be considered safe – including forested snow slopes and gentle slopes in valleys.  This is not commonly observed in Australia.
  • 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 an avalanche is concentrated and funneled into the gully (particularly in Australia).
  • Convex rollovers on slopes create tension in the snowpack where the snow is more prone to fracture.

Assessing conditions

Avoiding avalanches is the surest way of keeping out of trouble.  If high avalanche factors or conditions are indicated or forecast consider aborting your trip.
Ask locals or other experienced people about conditions and what they have experienced.
Learn about snow stability tests and use them, in order of reliability:
  • Snow pit tests – dig a pit and examine snow layers
  • Rutschblock test – dig out a 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 that these skills are best learnt by instruction on an avalanche or mountain safety course.

Route finding in alpine terrain to mimimise avalanche risk

  • Stick to ridges, staying well away from cornices
  • Ski windward slopes that have been stripped of loose snow

Safe practices for descents

  • Everyone in the party should have an avalanche transceiver, avalanche probe and snow shovel
  • Assess avalanche risks using snow stability tests before skiing the slope.  If in doubt, don’t ski the slope.
  • Don’t ski slopes of a similar aspect to those where avalanches have already occured.
  • Ski low angle slopes first
  • Ski a suspect slope one at a time with others in the party watching from safe locations
  • Don’t stop below avalanche chutes or paths
  • Don’t descend directly above a partner or other group

See also

External resources