Avalanche Considerations in Couloirs
Friend of Backdrop, Mike Austin of Avalanche Geeks, shares with us a fascinating article he wrote about things to bear in mind when preparing to ride steep couloirs.
‘Mechanical sintering’ and ‘the angle of repose of aggregates at rest’… geekery terms for avalanche-specific considerations for riding steep couloir lines. This advanced terrain is becoming increasingly popular within the backcountry skiing scene. They are also traditionally the domain of Scottish winter climbers, who may also find these considerations of value.
By their nature all couloirs will fall within the context of ‘Complex’ ATES-rated (that's Avalanche Terrain Exposure Scale) terrain. That is to say, they provide a very high degree of exposure both in ascent and in descent, with high consequence outcomes to being caught by even small Size 1 avalanches. In simple terms, small and generally harmless avalanches can kill you in couloirs.
The terrain issue is compounded by the limited value of avalanche forecasts for couloir skiing. Avalanche problems highlighted by the avalanche forecast can be very different (or non-existent) in couloirs, which by their nature are subject to different loading and temperatures. It’s important to remember that avalanche forecasts are at a regional scale and not slope scale.
Loose snow avalanches, both dry and wet, generally require slope angles greater than 40 degree’s which are common in couloirs. As skiers and riders, we often trigger dry loose avalanches as sluff in steeper terrain which we can potentially manage with experience and skill, but we should be mindful that this avalanche problem-type readily triggers naturally from point-releases off rocks both within the couloir and, perhaps more importantly, from the surrounding side-walls.
Your line may indeed be in deep cold shade, but are the surrounding cliffs that feed into your line, perhaps hundreds of metres above, catching sun or wind?
Another avalanche problem-type prevalent in couloirs is wind slab, particular at entrance rims and gully aprons where the snowpack lays on loose aggregates (talus/ scree) whose natural angle of repose is 37 degrees: the bullseye slab-avalanche angle. By their nature both of these features are exposed to wind slab formation. In maritime climates such as coastal Norway and Scotland there is often a difference in wind speed at the base of the couloir compared to the top. Are the exit slopes and particularly the couloir entrance slopes potentially actively loading? A top-down approach (to accessing the couloir) gives information on that, and the ability to check and manage the problem with roped ski cuts. A bottom-up approach to the couloir allows for wind slab to be identified as we slowly move into the affected zone, and allows turnaround without being committed. Wind slab is usually my primary avalanche concern when couloir skiing.
Cornices are another avalanche problem type that we typically have to contend with when couloir skiing. Whilst cornice-failure exposure time can be reasonable in descent, it can be completely unreasonable in a bottom-up ascent. Are the cornices fresh and delicate or old and stubborn? Are they actively loading/ forming? Is the afternoon sun on their back?
Due to the action of constant natural sluffing, it can be argued that distinct layers that we would find at the same aspect and elevation on more open slopes are less evident in very steep couloirs. Continuously sluffing transported snow physically breaks and batters down into smaller, rounded grains. For this reason, formal stability tests such as Compression Tests (CT) and Extended Column Tests (ECT) are potentially less reliable as a slope scale tool in couloirs and may also lack fidelity on their aprons. It has been evidenced that relative elevation within a couloir plays a key role in snowpack structure.
A person digging a snow pit at the bottom of a couloir during an approach should suspect different conditions higher in the couloir. Likewise, the same concept applies when assessing snow stability from just the top. – Guy/ Birkland: Relating complex terrain to potential avalanche trigger locations.
Given the shear size of many couloirs in the Alps and Norway, this shouldn’t come as a surprise. Non formal tests such as hand shears and ski pole probing can be useful tools to address the snowpack structure in couloirs in ascent.
Strategies for managing hazard within couloirs are best left to others in different forums, but it it’s worth noting that the use of intuition and reliance of a strong skiing skillset (fractionization) in such environments is a poor strategy (see our article: Using Intuition in the Backcountry). Mentorship from those who have a strong and extended background in operating specifically in this subsection of a subsection of skiing is recommended.
Avalanche Geeks run a series of excellent safety courses, both in Scotland and the Alps, from Avalanche Fundamentals to Pro Level Rescue Techniques and Accident Investigations.
This article was written by Mike Austin, he’s a Pro member of A3 holding Level 3 avalanche certification and has worked for avalanche forecasting agencies in Europe & North America and Search & Rescue programs for government agencies in Antarctica, Scotland and The United States.