An avalanche can fall only where snow has collected on an inclined surface, usually a mountainside. This layer of snow is normally called the Snow Cover.
Avalanches normally recur from time to time in the same mountain locality, called the Avalanche Path (eg. Bealey Valley slide or Avalanche Creek basin). The path consists of three (3) parts...
- Release Zone
- is the area at the top of the path where the avalanche starts; it usually involves the breakaway zone for slab avalanches (see below) and is characterised by accelerating motion.
- The Track
- is the middle part of the path, where steady-state velocity usually prevails unless modified by local terrain variations (eg. cliff)
- Runout Zone
- (sometimes called the deposition zone); usually less steep than the release zone and track, this is the zone in which the avalanche decelerates and deposition of the snow takes place.
These distinctions of these path parts are of more than academic interest...
A prime rule for safe travel in avalanche terrain is STAY OUT OF THE RELEASE ZONES !!
Basically there are two types of avalanches, however the scale at which they occur can dramatically change "danger ratings" of such events...
- Loose Snow
- This type of avalanche originates at a point and propagates downhill by dislodging successively larger amounts of poorly cohering snow grains, typically gaining in width as it falls.
- Slab Avalanche
- occurs when cohesion among snow grains increases, large areas of the release zone (the crown) may break away at once in the form of a slab. This distinct, cohesive layer of snow cover then slides on a clearly defined gliding surface, often facilitated by the presence of a weak or cohesionless snow layer called the lubricating layer.
Avalanches are further classified as wet or dry according to whether or not liquid water is present in the snow at the point of origin.
A wet snow avalanche is easily recognised by the way and form snow is deposited in the runout zone. It may appear as ice blocks, or its density is so high as to require snow or ice cutting tools to modify it. A wet snow avalanche accounts for about 80% of all avalanches in Arthur's Pass National Park, it can be very destructive, burial in wet snow deminishes your survival chances to about 20 minutes, if it hasn't already literally ripped you limb from limb!
A dry snow avalanche is recognised by the way the snow particles interact with the air during its fall through it's path. In most cases a "dust cloud" of snow particles is conveyed into the air. If this dust cloud predominates, such avalanches are called powder avalanches. If sufficiently dense aerosol is formed by the snow particles entrained in the air, the dust cloud may behave as a atmospheric turbidity current (gale force wind), rushing ahead of the sliding snow at high velocity (100 - 300 kmph), and occassionally inflicting heavy damage through wind blast (an example of this is to be found at the treeline in the Bealey Valley).
Areas of extreme hazard in the Park...
- Otira Valley
- Wet, Powder and Slabs in upper valley above foot-bridge.
- Bealey Valley
- Huge wet snow avalanches prevail in areas 400 metres before, and all areas above the treeline. It is these avalanches which give rise to the Bealey Glacier.
- Avalanche Peak
- Large Slab, and Point Release sluffs come down from the ridge amplitheatre surrounding the catchment of Avalanche Creek.
- Rough Creek
- Large Wet Snow avalanches are capable of reaching the creek-bed in many places, just look for the existing damage to vegetation.
- White River
- Anywhere above the Taipoiti confluence, Powder and Wet Snow avalanches should be expected.
- Taipoiti River
- In its upper gorge, Airborne avalanches from above should be expected.
- Campbell Creek
- Anywhere from and above the junction of Campbell Creek and the upper Waimakariri River large Wet Snow avalanches can persist winter and spring. From Waimakariri Col down the Rolleston River, danger also exists.
- Goat Pass
- Large Wet Snow avalanches fall down the slopes between Mingha Biv and Goat Pass.
- Cass Saddle
- Large snow slides can occur off the slopes either side of the saddle.
ACCESS CONDITIONS VERY WISELY - IF IN DOUBT - DON'T !!