Backcountry Avalanche Safety: A Guide to Managing Avalanche Risk - 4th Edition

Contents

Foreword

Acknowledgements

Introduction

The Anatomy of an Avalanche Accident

Mountain Weather

Snow

Avalanches

Avalanche Terrain

Trip Planning

Avalanche Gear

Travel in Avalanche Terrain

Riding Steep Slopes

Companion Rescue

Bibliography

Glossary

Index

We should condemn men for crossing snow slopes in a condition favourable to avalanches, as we should condemn them for indulging in a cruise in an unseaworthy ship.

Leslie Stephen, 1865

Foreword

In this book I stress the avoidance of avalanche hazard by good routefinding, by recognition and avoidance of hazardous slopes and by staying out of avalanche terrain during periods when avalanche hazard is high.

However, I recognize that days when the snow is stable usually outnumber days when it is unstable, and that there are skiers and snowboarders who want some guidance on riding safely in avalanche terrain. I therefore try to address the difficult problem of evaluating snow stability for backcountry powder seekers and make some recommendations on techniques to use when skiing or boarding steep backcountry slopes. But the book can only point the way; you must go out into the mountains and practise what you learn here. André Roch, in a 1979 address to some of the world’s leaders in avalanche research, put it very succinctly when he said, Remember this, my friends, the avalanche does not know that you are an expert!

Acknowledgements

A work of this nature relies heavily on the knowledge, research and writing of others. This book is an attempt to organize and condense a vast amount of information normally available only to the snow science community or to professional mountain guides, and to interpret that information in light of my experience as a backcountry skier and climber. I have resisted the suggestion that every quote or source should be cited in referenced footnotes. There is a bibliography on page 205 for those of you who wish to read further on the subject.

However, I would like to thank several people who have made a major contribution to backcountry avalanche safety and whose work has had considerable influence on the presentation of material in this book.

Special thanks are due to Doug Fesler of Anchorage, Alaska, for permission to reproduce a major portion of his paper on choosing safe routes and making good decisions, which appears in the Travel in Avalanche Terrain chapter, and to the National Research Council of Canada, under the leadership of Peter Schaerer, who, in conjunction with the British Columbia Institute of Technology, developed the material on the Shovel Shear test and the rationale for hazard evaluation used in the chapter Riding Steep Slopes.

I would also like to acknowledge the contribution to backcountry avalanche safety made by Bruce Tremper and Brad Meiklejohn as a result of their addressing the issue of stability evaluation for the backcountry skier and for promoting safe skiing.

In the previous edition I neglected to mention Dale Atkins from RECCO and Bruce Edgerly from Backcountry Access, along with Manuel Genswein of Switzerland for their independent development of strategic shovelling, now considered a critical stage of backcountry avalanche rescue.

In particular I would like to recognize Grant Statham of Parks Canada for his work on the Avalanche Terrain Exposure Scale, and for his ongoing leadership in risk-based avalanche forecasting, which is the basis for Canadian avalanche forecasts.

Thanks also to Dale Gallagher for his help and encouragement, to Mrs. Martha Atwater for permission to reproduce the extract on page 158 from Monty Atwater’s book The Avalanche Hunters, to Hodder & Stoughton for the quotation from Chris Bonington’s Everest: South West Face, to Pete Martinelli for permission to use material from the Avalanche Handbook, and to Gord and Debbie Ritchie for their input on looking after an avalanche victim.

There were many people who assisted me in various ways. I hope the list is complete and apologize if I have missed anyone. Thanks to Dr. Eizi Akitaya, Tim Auger, Don Beers, Steve Couche, Greg Crawford, Kevin Cronin, Tom Davidson, Jim Davies, Roland Emetaz, Bruno Engler, George Field, Peter Fuhrmann, Dr. F. Furukawa, Lloyd Gallagher, Ethan Greene, Frank Grover, Kyle Hale, Clair Israelson, Bruce Jamieson, Seiiti Kinosita, Leon Kubbernus, Nick Logan, Rudolf Ludwig, Hamish MacInnes, Ian McCammon, Brad Meiklejohn, Kris Newman, Andy Nicol, Ron Perla, André Roche, Steve Rothfels, Tony Salway, Bob Sandford, Al Schaffer, Alf Skrastins, Peter Spear, Chris Stethem, Lars Suneby, Gery Unterasinger, Rod Ward and Knox Williams.

For this edition I co-opted guide and backcountry skier Kevin Hjertaas to cast a critical eye on the practical chapters, in particular Travel in Avalanche Terrain and Riding Steep Slopes. Thank you for your valuable input, Kevin.

Introduction

Snow avalanches are the greatest source of danger for mountain travellers in winter. They catch and very often kill the unwary who literally trigger their own destiny when they venture onto unsafe snow slopes in a moment of inattention or ignorance.

Historically, avalanche victims came from among those people who lived and worked in the shadow of the great mountains; whose houses, even whole villages, were destroyed every generation or two by catastrophic slides considered to be acts of God and so to be suffered with fortitude. Today, the most common victim is the climber, ski tourer, snowmobiler and the backcountry powder-hound.

Avalanches are complex natural phenomena, and in spite of modern technology and years of research no one can predict with certainty when or whether an avalanche will run. Avalanches may occur on any steep snow-covered slope. How steep and how much snow is required to initiate a slide are but two of the many factors to be considered when evaluating hazard. Another complication is that snow conditions vary in different geographical locations and at different times of the year. There is a vast difference between the fluffy powder snow of Colorado and the heavy, wet snow of Washington’s Olympic Range.

Some winter seasons produce an inordinate number of avalanche accidents. British mountaineer and author Frank Smythe, writing in the 1929 Alpine Journal, theorized that years of low early snowfall and high winds in the Alps were accountable for the worst recreational avalanche casualties on record. Conversely, he noted that in seasons of heavy early snowfall and little wind there were few accidents. There is solid fact behind his theorizing: wind and a shallow snowpack have a great deal to do with avalanche accidents, as you will learn.

For instance, in the Christmas/New Year period of 2008, the snowpack all across Western Canada was thin and a period of unusually cold weather had substantially weakened the base. Then a series of winter storms accompanied by high winds swept across the area. Cornices formed and the slopes were loaded with fresh snow. Eight snowmobilers died in southern British Columbia and a skier and a snowboarder were killed out on the Coast. In the first two weeks of 2009 there were more storms—three more snowmobilers died—then a significant warming trend with spring-like conditions on a winter snowpack. Cornices weakened and fell and avalanches on wind-loaded slopes were easily triggered on a layer of weak facets formed during the earlier cold spell. Avalanche danger remained High in many areas for most of January. An unusual start to the winter season.

What are your chances of being caught in an avalanche and what might be the consequences? If you are a backcountry skier travelling on light touring skis along a marked trail, chances are that you’d never have a problem. Because of equipment limitations you’ll rarely venture onto the steeper slopes and so would unconsciously avoid avalanche hazard by sticking to flatter terrain. It is ski tourers travelling off valley trails and over high alpine passes, skiers and boarders out to make turns, and ski mountaineers and climbers who are most at risk.

In a large number of avalanche accidents the recognition, evaluation and qualified acceptance of risk are absent; victims are often totally unaware of the danger. The majority of victims trigger the slide themselves. The chance of being caught by a naturally triggered slide is remote unless you are travelling during or immediately after a heavy snowfall or are climbing in the higher ranges of the world. Gerald Seligman, in his book Snow Structure and Ski Fields, published in 1936, quotes an old Swiss guide as saying, I never fear that any avalanche will catch me unless I have myself brought it down. Statistically, the more time you spend high in the mountains, the more chance you have of being involved in an avalanche accident, either to your own party or to another group.

Spring wet snow slides.

Photo Alf Skrastins.

The answer to that second question—What might be the consequences?—is difficult to put numbers to. There are several sources of statistics compiled by various organizations in North America and Europe. Unfortunately they all depend on accident reports and don’t count many unreported successful companion rescues.

It is estimated that in North America your chance of survival is about 85% if you get caught in an avalanche. Approximately 50% of those who survive either free themselves or are found because some part of their body or equipment was protruding above the surface of the snow. Of the people completely buried, only a dismal 35 to 40% live to tell of the ordeal, despite recent advances in transceiver technology and digging techniques.

Statistics compiled by the Colorado Avalanche Information Center on accidents in the USA from 1997 to 2007 indicate that if you are buried under 30 cm of snow you have a 75% chance of survival, which quickly drops to a 25–30% chance if you are buried a metre deep. All the statisticians seem to agree that if a person is buried under more than 2 metres of snow the chances of survival are almost nil. The weight of snow above the victim and the time it takes to dig down to them make survival unlikely in the majority of cases.

The most disturbing statistic, and the most important one for you to bear in mind, is the time factor. Again figures vary, but not by much. If your companions can dig you out within 15 minutes of the snow coming to rest, you have a 92% chance of survival. A few more minutes and your chance of survival is significantly reduced.

Obviously your first priority is to avoid being caught by an avalanche in the first place, but should the unthinkable happen, your greatest chance of survival lies in your companions locating you and digging you out in the fastest possible time.

The Colorado Avalanche Information Center data shows that since 1997 snowmobilers accounted for about 50% of backcountry avalanche fatalities, skiers and boarders 30%, snowshoers 8% and climbers and hikers 12%.

Some years ago Knox Williams, in his portrait of a typical avalanche victim, said: The victim is a male, 27 years old, has had several years of skiing or mountaineering experience, and didn’t know an avalanche from a snowball. Although the 20–30 age group still leads the way, the Colorado statistics show other age groups are catching up.

If you study published case histories of avalanche accidents you’ll become aware of one important constant in backcountry accidents. A large percentage of accidents occurred when avalanche danger was known to be high. In some cases the victims chose to ignore warnings and proceeded with their trip; in others the victims were completely unaware of the possibility of avalanches. Many victims didn’t know how to pick a safe route through avalanche terrain or, conversely, were unable to recognize potentially dangerous slopes. Few of those involved had any experience in evaluating avalanche hazard, even victims considered experienced leaders.

The biggest advance in avalanche safety in recent years has been in the art and science of Avalanche Forecasting. In the vast majority of avalanche accidents in the past 10 years, the forecaster was right on in predicting the Danger Level, and in most cases the written part of the forecast had warned about the hazard that resulted in the accident.

The bottom line is that your chance of getting caught in an avalanche is greatly reduced if you pay careful attention to the current Avalanche Forecast. Be patient, and wait until conditions are right!

The Anatomy of an Avalanche Accident

Three experienced skiers left the majority of their group skiing on the flats around the Lodge and headed out for a tour to a popular high mountain lake. Conditions were picture perfect, with cloudless skies and mild temperatures. We won’t need our transceivers, we’re only going to the lake.

Alf and his three friends …wanted a carefree, easygoing kind of trip and a fairly short day of skiing. After discussing potential avalanche hazard on the drive to the mountain they decided to head for the same high mountain lake. Even so, we carried avalanche transceivers, snow shovels, avalanche probe poles, a first-aid kit and spare clothing with us as a matter of course.

The local avalanche hazard forecast indicated: Stability decreasing. Soft slab with buried surface hoar—difficult to detect. It went on to add: Safe routefinding imperative due to a weakening snowpack and warm temperatures. Slopes that have not recently avalanched should be considered as suspect.

An overall view of the area described in The Anatomy of an Avalanche Accident. The avalanche ran down the gully below the col in the centre of the picture. Alf’s group were at the top of the treed ridge on the left of the gully. The slide started on the slope above the top of the trees to the right of the gully.

Photo Alf Skrastins.

The ski in to the lake was fast and easy; wax was working and the trail was in good shape. There were tracks from other groups leading to a sunny, gladed plateau above the lake, a fine spot for lunch.

Beyond the plateau an attractive-looking snow-filled basin led up to a col between the main peak and its much lower outlier. The experienced group …found some fresh ski tracks leading up the ridge between the lake and this basin and followed them. After a short distance one of the party decided to turn back and wait for the other two at the lake.

They followed the tracks up through tightly packed trees and decided to carry on to find a clear run out rather than ski down through the trees. We followed the existing ski tracks around the southwest ridge and came out at treeline halfway up from the valley below.

Meanwhile, Alf’s party lunched at the open gladed plateau above the lake, …discussed the option of following the tracks into the basin and decided it might be worth a look. Because it appeared that the other group would either be forced to lose the elevation they were gaining or would have to continue in a mid-slope traverse across steep terrain, Alf decided to choose an easier route through the forest to the bottom of the basin.

The lower section of the basin is divided by a treed ridge splitting the basin into two gullies. The entire ridgeline along the western and northern perimeter of the basin was crowned by very well-developed cornices, indicating heavy wind loading of the slopes in the basin, while the southwest-facing slopes on the east side of the basin showed evidence of cross-loading.

On emerging from the trees at the bottom of the most easterly gully Alf’s group decided that this gully was a potential terrain trap. We double-checked that all of our transceivers were transmitting and then probed the snow with our ski poles. We could feel approximately 25 cm of firm snow with a weak layer underneath.

Proceeding a few metres farther to get a better view, one of the Alf’s party, a qualified heli-ski guide, …did not like the hollow, drum-like sound of the snow as we crossed onto the southwest-facing aspect. Given this information we decided not to ski this half of the basin at all.

Instead they followed the treed ground in the middle of the basin in order to get high enough for a good view of the area and to check out the other half of the basin. The heli-ski guide did a quick shovel shear test in the top 70 cm of the snowpack on the southeast aspect. The column of snow sheared very easily at about 40 cm below the surface while she was still in the process of isolating the column. At that point we decided to avoid the open slopes altogether and to stick to the treed areas.

The photograph mentioned in the text, taken a few minutes before the avalanche released.

Photo Alf Skrastins.

A view of the avalanche path. The slope angle where the skiers triggered the slide is 35°. The average slope angle in the top portion of the gully is 36°.

Photo George Field.

About 30 m above them, and across the gully on the southwest aspect, the two skiers had stopped to decide whether to go farther up or turn around. From here the existing tracks divided, one set going up the valley floor to the col, the other traversing the southwest-facing slope toward the mountain.

We briefly discussed avalanche hazards, noting that a large cornice was an apparent threat to both the valley floor and to the southeast aspect of the valley. We noted that the southwest aspect of the valley did not offer any apparent threat (there was no indication of avalanche activity and there were clusters of trees extending well above us).

One of them probed with his ski pole and felt that the snow was fairly consolidated. We decided to continue to a group of trees 40 m above; we had no intention of carrying on to open slopes beyond.

At the top of the trees, the slope ascended by Alf’s party was wind-scoured and they were in a safe position. While stopped to take a photo Alf noticed two skiers following tracks made by an earlier party. They had just made the switchback at the bottom of the convex roll in the upper portion of the gully. As they approached the switchback at the top of the roll I took a photograph of them and then put my camera away. I was about 200 m distant and 20 to 30 m higher than the skiers.

One of the skiers fell while making the switchback at the top of the roll. When he got up they appeared to be having a discussion. As they continued toward a clump of trees a short distance away, a crack appeared in the snow between the skiers and the trees.

The initial size of the avalanche was small, a slab about 15 m square, and released with no warning. I believe we were standing right on, or next to, the trigger point. We were immediately thrown on our sides and had no chance to ski off the slope.

The skier on the right landed with his head slightly uphill and immediately pulled his skis underneath himself and used his arms to keep his upper body off the snow. The other skier fell with his head downhill and appeared to just hold his position on the snow and to look up at the slope behind him.

The slab broke up and the slab I was on missed the trees and carried me 50 m down the slope and began to slow down. At that point I thought the avalanche was over. However, this small slide triggered the rest of the slope.

Alf watched in horror. The crack ran up the slope from the trees for about 60 m, then cut across the slope toward the ridgeline and then ran just below the ridge crest and below the cornice at the col. It widened very quickly as it ran. It was as if cloth were being ripped or a zipper were being opened. This was accompanied by a hissing sound, like air being let out of a big bag and a blanket being dragged across sand … all at the same time.

I looked over my shoulder to see the entire wall of the valley above begin to move. The slab I was on picked up speed again and disintegrated. I was lying on my back and using my arms to try to swim upright.

The skiers were moving slowly down-slope and the snow they were on was starting to break up as they moved. However, the snow on the steeper slope above them was moving faster than the snow they were on, and began to ride up over the top of the slab they were on.

The upright skier frantically pulled himself up on top of the slabs as they descended onto him. The other skier seemed unable to prevent the first of these faster slabs from covering him.

Successive waves of snow from the higher slopes overran the snow in the middle of the main slide, which ran down the main southwest-facing slope and into the gully. At the same time, a much thicker layer of snow from the slope directly below the col was also sliding in an easterly direction, running over the rest of the debris. This phase of the slide was accompanied by the low rumbling sound usually associated with avalanches.

After another 100 m the avalanche stopped.

My head remained above the snow during the entire slide. When the snow stopped my legs were buried to above my knees. I had lost one pole, my hat and glasses. My skis were still attached but were twisted around at an awkward angle. I was unhurt. When the slide stopped, Alf’s group could no longer see the skiers. I skied down the tree-protected ridge, looking down into the gully when possible. After a few turns I could see one skier on top of the snow at the very bottom of the slide.

When he reached the bottom (I descended within the trees a bit farther to avoid crossing a short, steep slope), the skier on the surface was sitting on the debris, facing downhill, digging out his lower legs. There was no sign of the other skier. The guide was running back and forth down the debris looking for some sign of the missing skier.

After completing the initial search, and with only two hours of daylight left, Alf, whose skis were closest to the bottom, went for help while the others probed likely locations.

We suspected that since I had ridden the avalanche unhurt and on the surface all the way to the toe of the slide, something drastically different must have happened to my companion. Our initial probing was therefore directed at the trees below his entry point, and at a large, deep pile of debris lower down where the larger avalanche had intersected the initial slide.

He was found two hours later by a rescue team, face down under 70–100 cm of snow, still wearing his pack but missing both skis. He did not appear to have struggled at all.

I should note that my friend and I had over 30 years of combined backcountry skiing experience, but despite this we failed to recognize the danger of the slope we were on.

Because of our approach line through the trees on the southwest ridge, which obscured our overall view of the slope and potential avalanche patterns, the danger of the slope was far less obvious to us than to parties which approach the slope from the valley floor. Combined with the results of our pole probe, the fresh ski tracks already on the slope, and our decision to remain in the trees, these circumstances misled us to believe the slope had low risk of avalanche.

An engraving from David Herrliberger’s Topographie der Eydgenossenschaft, 1773, illustrating the common belief that avalanches were giant snowballs crashing down the mountain, gathering animals, trees and even buildings before annihilating the village below.

Mountain Weather

Because changes in the snowpack leading to avalanche hazard are greatly influenced by weather, a knowledge of the effects of temperature change, sun, wind or rain on the snowpack is absolutely essential. It’s possible for the backcountry traveller to decide whether avalanche hazard is likely to increase or decrease by making an intelligent interpretation of present meteorological conditions coupled with a knowledge of past weather.

Winter backcountry enthusiasts are primarily concerned with changes in weather conditions and with the effect of these changes on the old snowpack and on any new snowfall. It is important to remember that any significant change in the weather results in a change in the stability of the snowpack. The most important factors to consider are:

• The amount of snow and the intensity of the snowfall.

• Wind speed and direction of both prevailing and local winds.

• Temperature changes , particularly a sudden rise in temperature.

The Atmosphere

The earth’s atmosphere consists of a mixture of gases, with oxygen and nitrogen making up about 90% of the volume. Other important constituents, as far as weather is concerned, are water vapour, ozone and carbon dioxide, all of which influence the radiation balance; and solid and liquid particles—dust, volcanic ash, tiny droplets of sulfuric or nitric acid, and salt from the sea—called aerosols. Aerosols, which may exist in the mountain atmosphere in concentrations as high as 150,000 particles per cubic centimetre, provide condensation nuclei for precipitation. The atmosphere is bound to the earth by the force of gravity; it has no upper surface, but blends gradually into interplanetary space. The pressure created by the atmosphere is greatest at the surface of the earth, and decreases with altitude. The usual measure of atmospheric pressure is the millibar (mb), which is approximately one 1000th of