Blog Archives

Terrain Traps

Sunday, 1 November 2015  | 

Decisions about where and when to ski off-piste take into account risk-benefit analysis- how much risk am I prepared to take to ski on the slope beneath me?  Sometimes the benefit clearly doesn't justify any off-piste skiing.  I can certainly remember a few days in the last few season where is just wasn't worth it- maybe bullet ice or badly wind-affected snow just made it no fun, so the decision was easy.  However, at other times the decision requires more thought.  Perhaps the snow is good, so the benefit is clearly there. Under these circumstance we need to have clear idea of the other side of the equation- what's the risk?  This takes into account various factors as described, for example, in previous blogs on recognising avalanche terrain, the critical effect of slope steepness on avalanche risk and the presence of avalanche risk red flags.  However, an important factor to take into account is how severe the consequences are if you are taken by an avalanche, and a really important factor in this consideration is the presence or absence of terrain traps.

Terrain traps can be defined as any terrain feature that potentially increase the severity of consequences to you if you are taken by a slide.  They can include the following:

1. Physical obstacles including rocks and trees.  Avalanches move at high speed, sometimes in excess of 60mph, and obviously being slammed into tree trunks or rocks at this speed will present a substantial risk of major traumatic injury.  It's well recognised that the fatality rate for people caught in a avalanche in north America is higher than in Europe, and this is largely due to the higher tree-line in America and Canada, which results in an increase in traumatic injury.

2. Large drops, cliffs and crevaces.  Evan a small avalanche is likely to be fatal if it results in you being swept over a cliff!

3. Gullies, hollows, river beds, ravines, cuttings.  These types of terrain can be dangerous as they can lead to deeper burials.  Gullies and hollows at the end of an avalanche run out tend to trap the snow as it slows.  Even transitions to a flatter slope at the end of the run-out zone can have the same effect. River beds and ravines can act to funnel snow into smaller areas, again resulting in greater depth of snow and deeper burial.   As avalanche survival is closely inversely correlated with depth of burial (very deep burials are rarely survivable), all of these are bad consequences for staying alive.

So to summarise, the presence or absence of terrain traps is a key element to take into account in weighing up the risks in skiing any slope off piste.  

As usual, these blogs are no substitute for proper avalanche education.  However, we hope that they help give a few pointers as to issues to take into account skiing off-piste.  Remember, avalanche fatalities are rare, don't forget to have fun out there!

Gravity Protection Ltd