Part 2. Dealing with wet snow
Compared to recreational adventurers, rescuers loose flexibility on when and where they must go. While avalanches are always formidable foes for rescuers, wet avalanches – as I wrote a couple of weeks ago – are like an angry swarm of African honey bees. Both angry bees and wet avalanches just keep attacking. I suspect that many folks including rescuers and avalanche professionals misjudge wet avalanches. Rescuers armed with misconstrued confidence could be walking into a dangerous trap.
(Sorry for the delay in getting this posted. Extended travel and then a conked-out computer kept me off line for a couple of weeks.)
To most folks from avalanche educators to backcountry adventurers wet snow avalanches are an enigma. Perplexities lead to oversimplification of concepts and actions, or worse, to dismissal or disregard. Avalanche scientists know relatively little about wet snow. The failure and fracture qualities of wet snow are even less understood than in dry snow. For the practitioner wet avalanches are harder to predict and more difficult to trigger. When we know less about something – or face greater uncertainty – we should apply a broader margin of safety to our actions, but sometimes we don’t. Perhaps this is because wet avalanches tend to be smaller (and more numerous) than their dry-snow cousins, so we don’t think of wet avalanches as very dangerous. It’s just a thought.
Rescuers and Flexibility
Recreational backcountry travelers rely on flexibility on when and where they travel. Rescuers loose this flexibility because they have to respond to a specific locale and often right away. Certainly, for a buried or injured avalanche victim their problem is acute and getting worse, and the potential benefit of their rescue will be significant. These circumstances tend to pull rescuers in, which sometimes gets us into trouble. When wet avalanches release naturally they frequently run in clusters and often with little or no apparent change in weather conditions.
With dry avalanches – both naturals and triggered – it’s easy to spot the factors that directly contribute to slides. High intensity snowfall or heavy wind loading are usual culprits. With wet avalanches the contributory factors are often subtle and harder to recognize. An exception is rain on fresh snow, which is usually obvious and easy to spot, but when wet snow is caused by thaw conditions the changes in contributory factors are often indistinct. And may not be even recognizable. A subtle increase in temperature, perhaps by just a degree or two can be too much. A short-timed freeze might be just enough to allow passage or not enough. The old adage “Timing is everything,” describes the situation but fails to provide answers.
Wet Snow Hazards for Rescuers
Difficult travel Certainly avalanches can ruin one’s day but so too can just trying to get through wet snow. Heavy packs and weak, wet, unsupportive snow can slow rescuers to a crawl or even bring them to a standstill. Or worse, wreak havoc to one’s knees, especially for skiers. Extra caution is necessary just moving in wet snow Be ready for post holing and submarining skis when off packed trails. In regions with depth hoar be ready for collapses that that can swallow skiers and snowmobiles.
Natural avalanches When fresh snow warms quickly small rollers and loose releases will occur especially from steep and rocky areas. Once an avalanche runs more are certain to follow. Think African honey bees. Just because there is debris at the bottom of a gully does not mean the gully or slope is “safe,” especially if only a small amount of debris is in the runout zone. Also roller balls or small slides may trigger larger and more destructive slab avalanches. Expect the avalanches to follow the sun starting on the easterly aspects in the morning then going to southerly and westerly aspects by afternoon. North aspects, the last to run may release late int the afternoon or evening.
Gullies, couloirs, and rock faces Complex, rugged terrain often turns into a shooting gallery when wet avalanches start running. These locales are not the place to be when the snow starts moving.
Small, wet avalanches are dangerous
Most wet avalanches are not very impressive looking, especially when seen from across the valley. But once you get up close you may change your mind. No matter the size, wet avalanches are dangerous. An accident researcher may expound on the differences of being struck by a Mini Cooper, a large Buick (or BMW), or a Mac truck, but if you’re the pedestrian struck by a vehicle traveling at 50-70 kph, you are going to be seriously hurt or killed. The same applies to avalanches.
Corn slab avalanches While most wet avalanches run naturally, “corn slab” avalanches are triggered. Though relatively rare corn slabs form after several days of significant warmth followed by a light freeze. The snowpack is often composed of large, faceted grains (depth hoar). On the surface the refrozen snow is strong, solid enough to ski on but perhaps not strong enough to walk on. On the outside (pun intended) the snow cover seems reliable, but the freeze was not long or cold enough to refreeze the snowpack. Instead, the shallow, hard crust becomes the slab. My experience has been that large faceted grains or depth hoar are a key ingredient for these avalanches. A similar melt-freeze crust over finer grained snow or even recent snow usually poses little or no avalanche problem.
Rain on winter snow The combination of fresh snow and rain leads quickly – within minutes to hours – to avalanching. This is a big problem in coastal mountain areas. Prolonged rain can also lead to avalanches, but it seems that rain on older winter snow, say more than 5 days old usually posses little threat as the rain tends to drain through the pack. Sometimes, but not always, even torrential and prodigious rainfall fails to produce avalanches.
Glide snow avalanches But sometimes prodigious rainfall does result in avalanches and these are often “glide” avalanches where the entire snowpack slides. Tensile cracks form as the entire snow cover – to the ground – pulls apart and slips down slope. The ground may be smooth rock slab or covered by grass, or even bamboo, but it is the amount of free (liquid) water at the ground-snow interface that determines the glide rate and the acceleration. Just because large tensile cracks form does not mean the snow is unstable. Hundreds of kilos of explosives have been detonated in these cracks, which is usually unsuccessful to trigger avalanches. Regarding rain-on-snow events, Washington state-based avalanche forecaster John Stimberis and professor Charley Rubin presented an excellent paper at the 2009 ISSW in Davos. An interesting observation they noticed from one particular path was “extensive buckling and bulging prior to glide avalanche release.” Rain is a big offender but rapid and prolonged thaw conditions can also result in glide avalanches. In Utah’s Wasatch Mountains glide avalanches off smooth rocky slabs are sometimes a serious springtime problem. Even in Colorado glide avalanches have been known to occur from summer snowfields late in July or early August. In these very rare events monsoon rains may pour 10+ cm of rain in an afternoon or night. The rain water drains through the snow until it reaches the hard ice of old firn or ancient glacial ice. There it melts the bonds between the past winter’s snow and firn resulting in significant avalanches.
Slush flow avalanches In the high Arctic shallow and weak snow prevails over a frozen ground, but the onset of spring with long days and warm temperatures – intense warm-up or significant rain – can unleash a special beast: slush flows. The rules of slush avalanches are counter to just about everything we know about avalanches. Slush flows occur on very shallow slopes, sometimes as slight as 5 degrees and rarely steeper than 25 degrees. There is no such thing as safe travel when slush flows are possible. The slurry-like mixture of snow, ice, water, soil and rock flows just about anywhere and everywhere and are just as destructive as the biggest and most powerful dry-snow avalanches. All travelers and rescuers can do is wait until temperatures turn cold, refreezing the snow-water concoction. For rescuers this makes the recovery of buried victims extremely difficult because the debris turns to ice. To learn more about slush flows check out Krister Kristensen’s (Norway) presentation at last fall’s International Commission for Alpine Rescue. Four people died in Norway last May (2010) in slush flows.
What Can Rescuers Do
The golden rule of search and rescue is: “Don’t make the incident worse.” So not to escalate an incident we must carefully manage risks to keep ourselves and our teammates out of trouble.
Know the weather Stay abreast of forecasts, especially for temperature trends. Prolonged warming, even seasonal temperatures, can lead to delayed avalanching. Intense warm-ups caused by record and near-record highs are a serious warning.
Dig quick pits As Mike Wiegele is fond of saying, “If you don’t dig, you don’t know.” In your pits look for weak layers, crusts and snow wetness; also look how the wetness spreads or changes. Most people focus on air temperatures and sunshine, but snowpack structure is very important too. Any time you encounter a crust, you should look beneath it.
Extra caution with gullies and couloirs On hot, sunny (or rainy) days having rescuers in a confined gully or couloir is akin to “shooting fish in a barrel.” I am not saying don’t go, just be very cautious or wait until temperatures cool. Even better is to wait until the snow refreezes.
Wet soft snow If you’re sinking to your knees into wet snow, you probably shouldn’t be on that steep slope. Wet snow means you can squeeze water out of the snow while making a snowball. If you’re sinking to your boot-tops, ask yourself what the next several hours will bring. Warmer conditions and wetter snow means you should be thinking to get off the slope soon. If it’s shadows and cooler temperatures, you’ll probably be ok.
Use explosives If you can use explosives to trigger additional avalanches to safe-guard an area, do so. If, however, the snow does not avalanche, do not assume the snow is stable. You may be off in your timing.
Stay away from cornices When the snow is wet and liquid water is dripping off the cornice’s face (and probably your brow too) it’s time to get out from underneath the overhanging monster. Cornices are unpredictable, but the warmer the temperatures the less time you should be exposed to possible cornice fall.
Use a guard Wet avalanches move much slower than dry avalanches, and a guard can likely give fair warning to rescuers when other small natural avalanches release. But don’t be over confident to think that everyone’s can out run even a slow moving avalanche. (When dealing with dry snow avalanches, guards should be used to keep other triggers (i.e. people) off adjacent slopes or to warn rescuers when those triggers – I mean people – approach. It’s very unlikely that anyone will outrun a dry-snow avalanche.)
Have rescuers to rescue the rescuers When natural avalanches are running the situation is high hazard and high risk, but a rescue effort may still be justified and attempted to save a life. (Just remember the worst possible event would be to kill a rescuer.) In higher risks situations hold back rescuers and equipment just in case something happens to the rescuers.
Wait Sometimes the best course of action is to simply wait until the snow cools. It’s even better if the snow refreezes. Frozen snow is strong snow. In some cases when there is little or no benefit to the victim – usually a body recovery – the better course of action might be to suspend the operation until the snow melts.
Wet avalanches pose special problems to mountain travelers and rescuers alike. Both groups should think about conditions a bit differently than we might during wintertime conditions. It has been a big winter across much of western North American and though the calendar says May, conditions in many mountain areas are just now turning spring like. If you pager goes off with the call to an accident, I hope you too will think differently about the wet snow.
My ideas and lists are likely incomplete, so please add your comments and experiences. The more we learn about wet snow, the better off we all will be.
Thanks for reading.