Ud 6-81-8 (e) Instruction In Winter Service-snow Awareness

UD 6-81-8 E INSTRUCTION IN WINTER SERVICE SNOW AWARENESS

English edition

UD 6-81-8E Instruction in Winter Service - Snow Awareness

ABOUT UD 6-81-8 E Metadata SHORT TITLE:

UD 6-81-8 E

SECURITY GRADE:

UNCLASSIFIED

VALIDITY:

2011-01-02

LEGAL AUTHORITY:

Letter from the Inspector General of the Army - Delegation of authority.

RESPONSIBLE PROFESIONAL AUTHORITY:

Commander of the Norwegian School of Winter Warfare

VALID FOR:

Royal Norwegian Armed Forces

PREVIOUS VERSION:

English ed. based on the current norwegian ed.

BACKGROUND UD 6-81-8E is produced i accordance with FS 7610-1900. Both the electonic version and the paper edition is produced from a single information source, based on XML

The web edition UD 6-81-8E is stored on the Armed forces database "FOBID". FOBID will hold at all times the current edition of UD6-81-8E as well as prev. editions

The paper edition The paper edition can ordered from FLO Base Østerdalen, Grafisk (LMS), Rena Military camp. The CD edition contains both the web edition and the pdf edition to be used if a web browser is not availiable. The CD edition can ordered, at the cost of the ordering unit, from: FLO Base Østerdalen, Grafisk (LMS), Rena leir PB 24 2451 RENA

Contributer in production Photo cover: Forsvarets mediesenter (FMS) edited by M&K DOK AS Production: Norwegian Army Land Warfare Centre and M&K Dokumentasjon AS. Illustration and photos: FMS/M&K Dokumentasjon AS Printing and binding of the paper edition: AIT Otta

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

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Stipulated by the Norwegian School of Winter Warfare UD 6-81-8(: Instruction in Winter Service – Snow awareness for the benefit of the Norwegian Armed Forces Bardufoss, 2011-01-02

Per Sverre Opedal Major General Inspector General of the Norwegian Army

Harald Østbye Lieutenant Colonel Commander of the Norwegian School of Winter Warfare

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

Sections ToC

Table of Content From pt 1

Chap-1

Chapter 2: BACKGROUND

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Chap-2

Chapter 3: AVALANCHE TERRAIN

3

Chap-3

Chapter 4: SNOW

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Chap-4

Chapter 5: AVALANCHE FORMATION

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Chap-5

Chapter 6: STABILITY TESTS

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Chap-6

Chapter 7: SAFETY FOR PERSONNEL WORKING WITH AVALACHE RISK ASSESSMENT

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Chap-7

8

Chap-8

Chapter 1: INTRODUCTION

Chapter 8: ASSESSMENT OF AV ALANCHE RISK

APPENDICES

Appendix

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Table of Content Chapter 1. INTRODUCTION Preface 1.1

ToC

Application 1.2

Chapter 2. BACKGROUND Introduction 2.1 History 2.2

Chapter 3. AVALANCHE TERRAIN The avalanche area 3.1 The release area......................................................................................... Avalanche path and runout area.............................................................

3.1.1 3.1.2

Exposure 3.2 Terrain formations 3.3 Avalanche frequency in diverse terrain formations...............................

3.3.1

The range of an avalanche 3.4 A simple method to determine the range of an avalanche in the field............................................................................................................. The Norwegian Geotechnical Institute’s topographic model................

3.4.1 3.4.2

Chapter 4. SNOW Snow in the atmosphere 4.1 The snowpack 4.2 Grain types, dimensions and characteristics.......................................... Mechanical decomposition ...................................................................... Transformation of snow – Destructive transformation (destructive metamorphosis)......................................................................................... Snow transformation – Sintering............................................................. Transformation of snow – Constructive transformation (constructive metamorphosis).................................................................. Other types of grains and crystals........................................................... Wet snow, melting transformation (melting metamorphosis).............. Subsidence in the snowpack, the snowpack in steep terrain.................

4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8

Weather that increases avalanche risk 4.3 Snowfall and wind..................................................................................... Rain............................................................................................................. Temperature of the snowpack.................................................................. Solar heating..............................................................................................

4.3.1 4.3.2 4.3.3 4.3.4

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

Meteogram 4.4

Chapter 5. AVALANCHE FORMATION Avalanche types 5.1 Loose snow avalanche............................................................................... Slab avalanche........................................................................................... Slush avalanche.........................................................................................

5.1.1 5.1.2 5.1.3

Forces that influence an avalanche 5.2 An avalanche in motion 5.3

Chapter 6. STABILITY TESTS Snow profile 6.1 Weak and unstable layers in the snowpack............................................ Snow profile form...................................................................................... Classification of snow types...................................................................... Grain types................................................................................................. Grain transformation schematic.............................................................. Fracture surface quality...........................................................................

6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 6.1.6

Shovel test 6.2 Compression test 6.3 Extended compression test 6.4 Rutsch block 6.5 Validity of stability tests 6.6 Use of explosives 6.7 Fracture line inspection 6.8 Free moisture content in the snowpack 6.9 Temperature in snow layers/temperature gradient 6.10

Chapter 7. SAFETY FOR PERSONNEL WORKING WITH AVALANCHE RISK ASSESSMENT Competence level and equipment 7.1 Route planning in connection with field surveys 7.2 Terrain traps..............................................................................................

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7.1.1

UD 6-81-8E Instruction in Winter Service - Snow Awareness

Chapter 8. ASSEMENT OF AVALANCHE RISK Factors to be assessed 8.1 Methodology 8.2

ToC

The military avalanche risk scale 8.3 Avalanche forecast 8.4 Forecasting errors.....................................................................................

8.4.1

Avalanche maps 8.5 Division of the avalanche map into zones................................................ Limitations of the avalanche map............................................................ Using the avalanche map..........................................................................

8.5.1 8.5.2 8.5.3

Establishing an avalanche group 8.6 Organisation of an avalanche group........................................................ Duties and responsibilities of an avalanche group................................. Avalanche group equipment and materiel..............................................

8.6.1 8.6.2 8.6.3

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1

INTRODUCTION

1.1

Preface These regulations have been prepared by the Norwegian School of Winter Warfare in collaboration with the Norwegian Geotechnical Institute and the emergency service/mountain rescue service of the Norwegian Red Cross. Chap-1 The object of this publication is to increase an officer’s understanding and knowledge of snow, the snowpack and avalanche risk in order to determine, to the best of his/her ability, the avalanche hazard and also select a safe marching route. This publication should also support an officer’s theoretical knowledge of snow and avalanches. It should, however, be emphasised that a comprehensive understanding of the respective subjects may only be achieved through regular practice and training. This publication is especially directed at officers who have gained a practical level of experience. This particularly applies to instructors in individual units who are responsible for winter training, staff officers with planning responsibility for winter training, as well as participants in avalanche groups.

1.2

Application These instructions apply to the entire Norwegian Armed Forces.

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BACKGROUND

2.1

Introduction The Norwegian Armed Forces should be capable of engaging in combat in every type of condition in Norwegian topography. This requires our divisions to be able to operate in demanding terrain under the most challenging weather conditions to be found in Norway. Nonetheless, safety and the selection of a safe marching route remain important factors during both war and peacetime. Proper routines must be established through knowledge and training so that the selection of a safe marching route becomes second nature to the individual officer and soldier. Chap-2 In everyday life we readily accept a whole range of safety measures such as seat belts, bicycle helmets and speed limits on roads, for example. In terms of recreation, it would appear that, in an avalanche context, neither research, forecasts or the closing of trails, undertaken by professionals, have made any impact at all. In this respect the Norwegian Armed Forces has a vital role to play in ensuring that its soldiers become familiar with winter conditions and gain insight into the selection of safe marching routes. Each year an average of 5 people lose their lives in avalanches in Norway. In recent years the number of skiers killed in avalanches has increased. If a person is completely buried in snow, the likelihood of survival diminishes rapidly with time and statistics would suggest that only 1 in 10 avalanche victims would still be alive after 3 hours.

Every year an average of 5 Norwegians loose their lives in avalanches. In order to prevent future accidents from occurring it is vital to increase the individual’s knowledge regarding the significant factors relating to avalanche risk. These three factors are: Snow -

Weather

-

Terrain

The first part of this publication will examine these three factors individually. Following this, the interaction and significance of the three factors will be described in greater detail. The final and possibly most important factor in the triangle are people, and our perception of the situation (figure 2.1). The human factor concerns our knowledge, experience, perception, subjective interpretation, evaluation, peer pressure, etc. The human factor will be discussed in several places as it touches upon many of the relevant subjects in this publication.

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Figure: 2.1 The four avalanche factors

2.2

History Every winter major or minor accidents occur as a consequence of avalanches. On average a ‘winter of avalanches’ takes place every 13 years in Norway with a total of 10-20 fatalities both civilian and military. Since 1986 the Norwegian Armed Forces has lost 18 soldiers in avalanche accidents. The most significant accident in a military context occurred on Wednesday 5th March 1986 in Vassdalen during the Anchor Express Exercise. A few minutes after 13.00 hours an avalanche swept down the Storebalak mountain into Vassdalen. The avalanche struck 31 men from the North Norway Brigade while they were in the process of ascending the mountain in snowmobiles along a stream valley on the north side of Storebalak. All personnel were struck by the avalanche and buried to a greater or lesser extent. 15 men survived while a further 16 men perished. The large amount of snow and wind in the week prior to the accident led to circumstances that were exceptionally unfavourable in terms of avalanche risk. The avalanche took the division completely by surprise. Nobody had noticed either subsidence in the snow, local slippages or other signs of movement before they were struck to the ground. Yet there were many old avalanche channels along the trail and there had recently been several avalanches in the area. On Thursday 6th February 1992 at around 06.05 hours an avalanche occurred on a small mountain slope in the Bjørnevatn-Hovden area of Setesdalen. Two cadets from Gimlemoen Army War College were struck by the avalanche during a ski march in darkness and poor visibility.

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Both cadets were buried in the avalanche although one of them was only partially buried and managed to break free from the snow. The terrain at the accident site had been assessed as an avalanche risk on the previous day and it was determined that personnel should not enter the area. However, in darkness and poor visibility the cadets made an error and entered the very area they had previously assessed as posing an avalanche risk. It had also been extremely windy and had snowed in the days prior to the accident. The avalanche victim was found after around 45 minutes in a primary search field in which assisted rescue with ski poles was carried out. The ski pole was barely long enough to reach the avalanche victim. CPR was performed at once as soon as the victim had been dug out and this continued until a doctor arrived at the scene of the accident at 08.15 hours and declared the patient dead, 2 Chap-2 hours and 10 minutes after the avalanche had occurred. In March 1994, 17 Home Guard soldiers were completely buried when an avalanche struck their bivouac area in Tussagjelet near Kvamskogen in the county of Hordaland. The bivouac was located deep inside a gorge and during the course of the night a large amount of snow accumulated due to both strong winds and precipitation, which eventually turned into rain. The avalanche was presumably triggered naturally and resulted in the soldiers being buried in their tents. Fortunately, some personnel had not been buried and they were able to dig out those who had been struck by the avalanche. One soldier perished. The sentry post at the base had been struck by an avalanche 3 hours prior to the main avalanche occurring. However, nobody had fully understood the danger. Following the accident parts of the division were hit by a subsequent avalanche on their way out of the area along a marching route that had been assessed as safe. According to the rescue parties and the police who were present at the scene it was a miracle that further lives had not been lost that evening/night. These are 3 examples from recent times of avalanche accidents that have resulted in fatalities in the Norwegian Armed Forces. In addition to these accidents there have been several near misses. Following the Vassdal accident major emphasis has been placed on avalanche training and choice of safe marching routes. This publication will also contribute to ensuring that the Norwegian Armed Forces will maintain its positive statistics following the Kvamskogen accident, i.e. no avalanche accidents resulting in any loss of life.

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3

AVALANCHE TERRAIN The majority of avalanches that could pose a threat to military units are those that occur in small terrain formations in which the height difference is less than 50 metres. The threat of major avalanches such as we are used to seeing in films and on television is regarded as minor as far as military units are concerned because military units do not operate in this type of terrain. However, large formations can represent a potential threat to roads, terrain axes such as Vassdalen, bivouac areas and military installations situated in valley bottoms. There are particular grounds to warn against stream gorges and smaller leeward slopes that units may seek out in order to gain shelter from inclement weather and wind, or to conceal themselves.

3.1

The avalanche area The avalanche area is divided into: The release area -

The avalanche path

-

The runout area

Chap-3

Figure: 3.1 The avalanche area The figure shows a schematic representation of an avalanche area with the release area at the very top and the avalanche path and runout area at the bottom towards the more level areas where the avalanche comes to a halt. With small avalanches the release area and the runout area often overlap, while in

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major avalanches that cover most of a mountainside there can be a long section (avalanche path) that the avalanche passes through between the release area and runout area.

Smaller terrain formations with height differences from 5 metres to 50 metres are regarded as the greatest threat to military units. 3.1.1

The release area The release area is where the avalanche is triggered. The upper edge of the release area is limited by the avalanche fracture line and the lower limit is the lower part of the slab that slides out. Laterally, the release area is demarcated by the remaining untouched snowpack. All inclines on a mountainside with a steepness of between 30° and 60° may be release areas for larger slab avalanches, providing there is sufficient snow present and no dense forestation. Slab avalanches, which are regarded as the most dangerous type of avalanche, are very seldom triggered when the slope of the slip plane is less than 30°. If it is extremely steep the snow will not attach itself but will gradually slide away as it accumulates. Thus, it is estimated that terrain steeper than 60° would not be a source of major avalanches. When the slope reduces to 30°, a significant amount of snow must be present before an avalanche is triggered because the anchoring forces are generally sufficient to keep the snow in place. If the slope reaches a steepness of 60° then less snow is required to trigger an avalanche. Such avalanches therefore occur more often, but are smaller. Because snow accumulates unevenly on the slope it may accumulate in such a way that the snow surface is steeper than it appears in the terrain information on a map. Studies that have taken place in Norway of around 500 major avalanches have indicated that the majority of avalanches are triggered on a terrain gradient of between 35° and 50°. The studies apply to naturally released avalanches, i.e. avalanches triggered as a consequence of weather conditions or due to changes in the compactness of the snowpack, and which have not been triggered by any other external factors. Surveys of a number of avalanches triggered by skiers indicate that the lower limit for the triggering of a slab avalanche is around 30°.

!Military units should not move in terrain that is steeper than 30 degrees.

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Chap-3

Figure: 3.2 Avalanche frequency in relation to the terrain gradient for major, naturally released avalanches

3.1.2

Avalanche path and runout area Avalanches usually start in hollows on the mountainside due to the fact that more snow accumulates here. When the avalanche is in motion it will seek the lowest point, or the path of least resistance. To a great extent a descending avalanche moves like a river. Prominent ridges and knolls tend to steer the avalanche and will thus determine the avalanche path. If the snow is dry and the avalanche is moving rapidly the snow mass can still pass over such ridges, even if they are 20-30 metres high. The greater the speed and volume of the avalanche, the greater its capacity to move in a straight line, i.e. in the direction of the general path of the mountainside or slope. Usually a dry avalanche will begin to slow down when the slope of the terrain reduces to 15-20°. Major, dry avalanches can extend far across level ground and cross valleys that are several hundred metres wide while smaller avalanches will stop on or close to the slope where they were triggered. This also applies when the snow is damp or wet, mainly because a smaller snow mass is being carried down the avalanche path. An exception to this is a so-called slush avalanche, which contains such a high volume of water that the avalanche virtually flows like a river.

It should be noted that the angle of the gradient in the snow may be steeper than specified on the map due to snow accumulation.

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Figure: 3.3 The slope of the snow surface (blue colour) in relation to the terrain (light colour) and in relation to contour lines on the map (red circles and dotted line)

3.2

Exposure The extent to which the slope is exposed to wind and thus to the accumulation of wind-transported snow must be taken into account when terrain is being assessed for avalanche risk. Slopes and mountainsides that are sheltered from the normal precipitation-leading wind directions, i.e. wind that causes the accumulation of drifting snow, will be most vulnerable to avalanches. Even though precipitation in North Norway is brought on by wind from the west or northwest, it is the high pressure conditions that most often result in powerful offshore winds and the transport of drifting snow onto west and north facing mountainsides. This applies to the counties of Nordland, Troms and Finnmark but it is worth noting that in the coastal areas powerful winds, in conjunction with low pressure from the west and northwest, will result in snow accumulation on east and southeast facing mountainsides and offshore winds will be responsible for less snow transport. Western Norway is the most complicated part of the country in respect of prevailing winds. Local conditions, valleys and fjords are determining factors here. Most snow showers are brought on by southwest to north westerly strong winds, which will result in northeast to southeast facing leeward sides. However, Western Norway can also have prolonged cold winds from the east that result in snow accumulation on west facing leeward sides! East of the watershed in Southern Norway, a significant amount of precipitation and wind is brought on by low pressure systems coming in from the southeast, although wind direction is often more from the southwest. This means that in areas with less snow, from the Swedish border to the Dovrefjell Mountains, most snow will usually

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be present on western and north-western mountainsides. For the most part snow transport will not extend beyond the low pressure period. In Jotunheimen and more central regions of Eastern Norway, the strong, cold winds that come from the west and northwest will be conducive to snow transport, which results in leeward sides being exposed towards the south and southeast. Nonetheless, in respect of wind directions and snowdrifts it is worth noting local variations in the area in which the unit is undertaking exercises.

3.3

Terrain formations In addition to the terrain gradient, the terrain’s actual shape is of great significance to how an avalanche is triggered from a mountainside. The greatest risk of an avalanche occurring is at locations on the leeward side of the wind. This is where most snow accumulates and the possibility of an avalanche occurring increases with the accumulation of snow. Thus, the risk of triggering an avalanche is greater in hollows and bowl-shaped areas than in knolls and on mountain ridges where snow is usually Chap-3 blown away. The most common sheltered areas on a mountainside are stream valleys, bowl-shaped hollows of varying size, ravines and prominent passes. Glacial cirques, i.e. places where there are, or where there have previously been glaciers, are also typical release areas for major avalanches. A significant amount of snow may also accumulate where there is a marked transition from a precipice to a less steep area below, e.g. scree. Bare rock-faces and level grass-grown surfaces that are steep enough are often the source of avalanches, especially during the post-winter period when sunshine or rain make the snow wet throughout so that it is more easily deformed (becomes more pliable) resulting in melt water forming along the ground, reducing friction. Where the terrain becomes steeper down the slope (convex areas) tensile stress is created in the snow. This equally applies just below locations where the snowpack is well anchored in larger rocks or knolls. In such tensile zones it is much easier for an avalanche to be triggered.

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Figure: 3.4 Leeward side with snow accumulation areas On a mountainside the depth of the snow may vary from snow that has been completely blown away to snow that is several metres deep. The uppermost part of a fracture line is often in the upper part of hollows in which the depth of the snow is less and the snowpack is weaker. In the sections of a mountainside in which there is a significant difference in snow depth, e.g. as a result of large blocks of rock, the weakest snow will often be where the snowpack is thinnest (index 4.2.5). The weaker snow and shorter distance to weak layers poses the greatest risk of triggering slab avalanches in such areas.

3.3.1

Avalanche frequency in diverse terrain formations According to a survey of around 250 avalanche zones in Western Norway, types of terrain in avalanche release areas are divided according to avalanche frequency as follows: Shape of terrain

Distribution of avalanche frequency in %

Open hollows, stream valleys

29

Deep passes, gorges

27

Convex areas

12

Bare rock-faces

12

Glacial cirques

10

Beneath anchoring zones

10

Table 3.1 Avalanche frequency divided by types of terrain

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The table shows the distribution of avalanches in diverse terrain formations. As shown in the table, around 60% of avalanches occur in glacial cirques, passes, gorges and bowl-shaped areas. It is in such areas that wind blows the snow and little or no snow is transported away. It is often such terrain formations that are the starting points for large and very large avalanches. However, attention should be paid to protruding ridges and knolls if they are covered in snow as avalanches can also be triggered here, particularly during snowfall with little wind. It is precisely in the transition from a thin snowpack or areas without snow, to hollows that have a significant presence of snow, that there is great tension and weak bonding that may easily result in a fracture. When there is a plateau on top of a mountain, a significant amount of snow from the plateau may be driven onto the leeward side of the mountain. The risk of an avalanche on such a mountainside is greater than when the peak of the mountain is shaped like a sharp ridge. Chap-3 The height of a steep slope need not be significant before avalanches become hazardous. Skiers have lost their lives on slopes where the height difference has been between 5 and 10 metres. The majority of fatal accidents caused by avalanches have occurred on slopes that are 20-40 metres high. As a rule of thumb, military personnel should not dig themselves in or seek shelter on a leeward slope that is higher than 5 metres. If the leeward slope is higher than 5 metres it also poses a threat to units on skis or on foot. It is easy to seek shelter in a stream valley during demanding weather conditions but it is also in such areas that slab avalanches first occur.

Figure: 3.5 Avalanches in stream valleys

!Military personnel should not dig themselves in or seek shelter on leeward slopes that are more than 5 metres high.

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3.4

The range of an avalanche The table below indicates how the range of an avalanche specified in degrees (angle A) is distributed in percentages in relation to the frequency of avalanches. Angle A represents the angle of the outermost accumulation of the avalanche runout to the avalanche fracture line. The distance an avalanche will travel is determined by several factors. A clear indication that an avalanche will travel a long way is a significant presence of snow in the release area (large starting volume) and in the avalanche path (large snow mass movement), in addition to the snowpack being dry, not wet. Wet avalanches (with the exception of slush avalanches) will have a shorter path and range than dry avalanches. Angle (A) – range

Avalanche frequency

20 degrees or less

2%

21-25 degrees

12 %

26-30 degrees

24 %

31-35 degrees

27 %

36-40 degrees

22 %

Greater than 41 degrees

13 %

Table 3.2 The range and frequency of avalanches The frequency of avalanches in relation to the runout angle.

Usually the angle (A) is a little over 30°. In less steep avalanche paths with an even transition to the valley bottom, this angle may decrease to around 20° in optimal avalanche conditions. 3.4.1

A simple method to determine the range of an avalanche in the field One method is based upon the horizontal distance from the avalanche release area. This method is referred to as the ‘20 degree rule’, i.e. when the line of sight from a position at the bottom of a valley to an imagined avalanche crown on the mountainside is 20 degrees, the avalanche will very rarely extend this far. For safety’s sake the line of sight should form an angle of 20 degrees or less, at a ratio of 1:3. Thus, a rough estimate of the range of the avalanche (L) is the vertical drop (H) multiplied by a factor of 3.

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Chap-3

Figure: 3.6 3 x H rule, or 20 degree rule Field rule: The maximum height of fall extending three times out from the mountainside gives the maximum path of a major avalanche (in effect, 18.3 degrees).

3.4.2

The Norwegian Geotechnical Institute’s topographic model For many years the Norwegian Geotechnical Institute (NGI) has mapped and registered several hundred major avalanches in Norway. Through studying these avalanches the NGI has developed a topographic model that forms the basis of the Norwegian Armed Force’s avalanche map production. It has been shown that the maximum range of an avalanche is determined by the steepness and shape of the mountainside. The steepness is defined by a line of sight that links the point in the slope where the terrain gradient is l0° with the top of the release area. The angle of the terrain along this line is described as ‘B’ (refer to figure below). It is possible to determine this 10 degree point according to the distance between the contour lines on a map. On the M711 map on which the scale is 1:50000 and the equidistance is 20 metres, a 10° terrain gradient equates to a distance of 2.3 mm between the contour lines. ((20m/tan10)/50m=2.2685 mm) The 10 degree point is placed on the uppermost of the contour lines between which the 2.3 mm distance has been measured. A simple relationship has been identified between the maximum range of an avalanche (angle A) and the steepness of the avalanche path (angle B): A=0.96 x B – (1.4°+SD), SD=2.3° and the correlation coefficient, R=0.92. With this equation it is possible to determine the estimated range of a major avalanche (angle A). The map is consulted, the 10° point is found and angle B is calculated. The angle’s degree number is then entered into the equation and angle A is calculated. Finally, the height difference H between the release area and the valley bottom is found and the range L of the avalanche is estimated (fig. 3.7). L=H/tgA

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Figure: 3.7 The Norwegian Geotechnical Institute’s topographic model Another method that may be applied to the map (M711) is to identify the 10 degree point. The height of fall from the highest possible fracture line to the 10 degree point is deduced from the map, then half of the height of fall is added to the 10 degree point, horizontally. The point obtained will be the maximum an avalanche is able to move. This is how an avalanche map may be made on an ordinary M711 map if an avalanche map is not available. This method is stricter than the 20 degree rule and will be well within the NGI’s topographic model. L=B+1/2H

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4

SNOW

4.1

Snow in the atmosphere Precipitation is usually caused by air being forced to rise and cool so that water vapour condenses. Warm air can contain a relatively high amount of water vapour but when the air cools its capacity to retain water vapour diminishes. When air cools, the relative humidity increases until the air is saturated with water vapour. With continuous cooling, air becomes supersaturated and water vapour condenses into small droplets of water and ice particles. The temperature that air must cool down to before condensation occurs is called the ‘dew point’. Air temperature and moisture content determine the shape of the snow crystals. When the crystals have grown so large that ascending air currents are unable to support them, they will begin to descend. The crystals will change shape as they descend, depending on the temperature and moisture content of the passing air strata. If the temperature is less than 0 degrees all the way to the ground, precipitation will manifest as snow; if the temperature is close to 0 degrees, several Chap-4 snow crystals will often join together. However, if it is cold the snow crystals may take on a needle shape. Hail is formed when snow crystals melt on their way towards the ground and strong ascending air currents lift them up again so that ice freezes on them when they reach a sufficient altitude. They will then descend towards the ground as hail, if they are large enough and do not melt before hitting the ground. A special type of snow crystal is graupel. These are snow crystals that have collided with supercooled water droplets on their descent. In turn, the water droplets have frosted upon impact with the snow crystal. Such crystals are whitish in colour and are fused together into a rounded shape. Snow crystals have various shapes and may be divided into 9 different primary classifications. Each primary classification has its own subgroups – a total of 37 different subclasses or snow types. The letter codes for the respective primary classifications and subclasses use the initials of gradings for respective snow crystal types. For example, ‘Depth Hoar’ has the letter code DH. The most common is the perfect 6-armed stellar crystal, familiar from drawings and symbols, which has a 60 degree angle between the arms. No snow crystals are identical; all of them have their own distinctive characteristics! In calm, cold weather snow may be extremely light and airy with a density as low as around 10 kg/m³. However, the average density of snow is 100 kg/m³, i.e. 10% of the density of water (water has a density of 1,000 kg/m³). Thus, we may deduce that 1 mm of precipitation produces 1 cm of snow. When the snow has landed it will either subside or settle. This process will happen more quickly if the temperature is higher. Continuous cold weather will result in light and airy snow over a prolonged period.

4.2

The snowpack New snow crystals may be shattered by the wind even on their descent to the ground, or because the wind blows the crystals along the snow surface. New snow that has

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fallen in calm and cold weather, often characterised as loose snow, is light and airy. This type of snow has little grip and bonding and, with such snow, loose snow avalanches may occur providing the terrain is steep enough, i.e. steeper than around 45 degrees and up to around 60 degrees. If the terrain is steeper than 60 degrees, the new snow will usually descend gently in many small loose snow avalanches.

Figure: 4.1 Loose snow avalanches in steep terrain Both the intensity of the snowfall, the temperature during and after the snowfall, as well as the wind strength and direction, will determine how the snowpack accumulates. These conditions will vary and the relationships between the different types of snowfall will cause the snowpack to constantly change. The snowpack will thus comprise many different layers with various types of snow crystals and degrees of hardness. Snow crust layers will also often be present as a result of mild weather or rain: layers of snow covered surface hoar or graupel, which will remain as constantly weak and potential sliding layers. It is the layered composition of snow that is critical in respect of avalanche danger. If there are lasting weak layers in the uppermost 1.5-2 metres of the snowpack then these are the right kind of conditions for the release of a slab avalanche. Certain grain types will form lasting weak layers that will remain in the snowpack for a long time, while other grain types will alter rapidly so that the weak layer will only remain in the snowpack for hours or days after its formation. It should be particularly noted that the layers will, to a large extent, be present everywhere in an area, while their thickness and depth and how much force that will be required to initiate a fracture will vary greatly, also within short distances.

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4.2.1

Grain types, dimensions and characteristics The individual grain types/primary shapes are shown below. Appendix 1 contains all primary classification subclasses.

Chap-4

Figure: 4.2

Figure: 4.3

Precipitation Particles

Decomposing and Fragmented precipitation particles

Primary classification PP, dimension 2-5 mm

Primary classification DF, dimension 1-3 mm

Figure: 4.4

Figure: 4.5

Rounded Grains

Faceted Crystal snow, even surfaces, stripes, lustrous crystals

Primary classification RG Dimension 0.1-0.8 mm

Primary classification FC, dimension 1.5-4 mm

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Figure: 4.6

Figure: 4.7

Depth Hoar

Melt Forms

Primary classification DH Dimension 1-10 mm

Primary classification MF clustered single crystals

Figure: 4.8 Surface Hoar, feather-shaped Primary classification SH Dimension 2-100 mm

4.2.2

Mechanical decomposition When the wind transports a new snow crystal it will rapidly erode, changing from a PP (Precipitation Particles) crystal to a DF (Decomposing and Fragmented) crystal (partially transformed, fig. 4.3). It will erode quite rapidly into a rounded grain crystal, an RG (Rounded Grains) crystal, or drifting snow. In the case of drifting snow the grains will be relatively small, often 0.2-0.3 mm.

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Figure: 4.9 Mechanical decomposition of snow crystals

4.2.3

Transformation of snow – Destructive transformation (destructive metamorphosis) Where a new snow crystal is not transported by the wind but remains static, a destructive transformation will take place. This is because the new snow crystal is no longer in balance with its surroundings; the supersaturation of water vapour in the atmosphere is not present on the ground. Thus, the snow crystal’s shape will start to Chap-4 transform. On the ground, the large new snow crystal will attempt to take on a balanced shape that combines the greatest possible volume with the smallest possible surface area. Thus, the new snow crystal will transform into a spherical shape. The protruding convex parts of the crystal will evaporate and the concave parts will sublimate (into ice). This means that the crystal arms will disappear leaving a small compact crystal, rounded grain snow, type RG. The dimension of the RG crystal is usually from 0.2 to 0.8 mm. It is important to note that this process takes place without any melting occurring. i.e. the temperature is below 0 ºC. Destructive transformation will occur more rapidly the closer the temperature is to 0 ºC. In such cases the process will only take a few hours but at a temperature of -5 ºC it will take 1-2 weeks. The risk of an avalanche will thus remain for a longer period after heavy snowfall in cold weather than when the temperature is closer to 0 ºC. Destructive transformation occurs when the temperature gradient in the snowpack is less than around 10 °C per metre.

Figure: 4.10 Destructive transformation The change in shape causes the snowpack to subside, the snow settles, the crystals fuse together and the avalanche risk diminishes. The snowpack will also compress

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due to the weight of the crystals above.

Destructive transformation stabilises the snowpack diminishing the avalanche risk. 4.2.4

Snow transformation – Sintering When snow crystals come into close contact with each other they also fuse together through the presence of small ice bridge connections between them at the contact points. The process is called sintering and is due to water vapour moving towards an area containing the least vapour. This is in the contact points between snow grains where the vapour will condense, forming ice bridges. A transfer of molecules also occurs on the surface of snow grains towards the contact points between the crystals. Both of these processes occur faster the closer the temperature is to 0 ºC. The smaller the crystals, the more ice connections per unit of space. This means that snow with a large degree of sintering will tolerate relatively major tension, compression and shear loads and is joined together in slabs.

Figure: 4.11 Sintering Sintering is of great significance to the snowpack; before sintering commences the snowpack has minimal strength, particularly in calm weather. As sintering develops ice bridges, the snow’s strength increases, i.e. it requires a greater force to displace the crystals in relation to each other. This has two consequences in relation to avalanche risk: The snow becomes more compact and will tolerate greater loads

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before an avalanche occurs. However, when the snow is more compact coherent slabs are more easily formed, which can transfer forces and stresses from one location to another. In such cases, release points may be around prominent rocks, ridges or bushes where a collapse as a result of lasting weak layers can easily occur when the force or load of a skier is propagated through the snowpack and exerts pressure on a so-called ‘super weak point’. Layers of RG type crystals and well developed sintering in coastal areas can reach a density of 500 kg/m³, but will usually be around 300 kg/m³. Such layers will be white in colour.

Sintering stabilises the snowpack and the avalanche risk diminishes. 4.2.5

Transformation of snow – Constructive transformation (constructive metamorphosis) The constructive transformation creates lasting weak layers of faceted crystal snow and depth hoar. These snow layers are responsible for around 35% of all avalanche Chap-4 accidents. The temperature difference between the snow surface and the ground means that excess moisture on the ground moves upwards into the snowpack. When this moisture reaches snow layers with lower temperatures it condenses on the snow crystals’ surfaces causing the snow crystals to grow and change shape. Water vapour molecules also move on a small scale from one snow crystal to another and condense. The conditions for constructive transformation are more favourable when the snow is looser and the temperature change in the snow is greater. The temperature difference in the snow is described as the ‘temperature gradient’. This must be greater than 1 ºC per 10 cm or 10 ºC per metre for constructive transformation to occur.

Figure: 4.12 Constructive transformation

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!A thin snowpack and prolonged cold weather result in rapid destructive transformation, increasing the avalanche risk. NB! Snowfall and wind following such a period will lead to an acute avalanche risk. The crystals change from a rounded shape (rounded grains and melt forms) or surface hoar into larger crystals with even surfaces and sharp edges. The snow is grainy and lustrous and the crystals are faceted with a dimension of 2-3 mm. The avalanche risk will increase if the contact surface between the crystals diminishes and ice bridges vanish. This shape is described as faceted crystal snow and has the primary classification FC (Faceted Crystals). It should be noted that RG crystals and MF crystals, or melt forms, will constantly move towards faceted crystal snow if the temperature gradient is great enough (1° per 10 cm snow). However, when studying crystals the observer must determine how advanced the process is and which snow type is in the individual layer. When conditions permit (high pore volume and high temperature gradient) the faceted crystal will develop further, the surfaces will gain stripes, the snow grain will become hollow and the walls will form 120° angles towards each other. When the snow grain has become hollow it is described as depth hoar and given the primary classification DH (Depth Hoar). It may grow up to 10 mm. Depth hoar crystals are stacked on top of each other with an opening facing downwards, as in a house of cards. Following prolonged cold periods the whole snowpack can be transformed into depth hoar and become extremely unstable. Depth hoar layers are often held in place by other snow or snow crust layers but the moment it is subject to any load that is not directly from above, e.g. a shear load caused by a skier on a slope, the layer will collapse in an impacted fracture that can easily spread out into the snowpack. Faceted crystal snow and depth hoar may also develop as thin layers of just a few centimetres. This often occurs where there is a snow crust layer buried in the snowpack. The snow crust layer will act as a vapour barrier when the temperature gradient is sufficient enough and constructive transformation will rapidly occur beneath the snow crust. As winter progresses, the process will eat away at the snow crust layer and the layer of faceted crystal snow will move further up if conditions are right. It is also worth noting that a vapour deficiency will occur above a snow crust layer causing the bonding and grains to break down here. This layer is often only 2-3 mm and is therefore difficult to detect. However, several avalanches occur in this very layer above a snow crust layer.

!Danger signs: Rumbling in the snow is caused by layers of faceted crystal snow or depth hoar collapsing. If it is steep enough, above 30°, an avalanche may be triggered.

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4.2.6

Other types of grains and crystals After one or more nights of cold, clear weather, surface hoar will form – Primary classification SH (Surface Hoar). Surface hoar resembles feathers growing on top of the snowpack. Its dimension is usually from 1-50 mm but in favourable conditions hoar crystals up to 130 mm may be identified. Surface hoar forms because during clear nights the snow surface will be colder than air as there are no clouds to reflect radiation back to earth. This causes the surface to lose energy and cool down. The snow surface may be 5 °C to 10 °C colder than the air temperature. So when warmer air comes into contact with the cold surface of the snow, it condenses, forming surface hoar. This surface hoar is extremely slippery but poses no threat until it is buried by other snow. The surface hoar layer may also be destroyed by wind, melted by the sun or higher temperatures. If a surface hoar layer is buried without being destroyed, the crystals will often tip, turning into a thin layer. The layer’s thickness can be from 0.5-1 mm and represents a hazardous and lasting sliding layer. In a small number of cases, as a result of new snow not being compacted or because the surface hoar crystals are not blown over, the surface hoar layer may remain as air strata. In low temperatures, the surface hoar layer is capable of lasting for several weeks. Pockets of surface hoar caused by wind transport will transform into faceted Chap-4 crystal snow more quickly than the surrounding snow when the temperature gradient results in moisture transport in the snow. This is because a surface hoar crystal is actually a faceted crystal but has been given a separate classification as its process of formation is special, and because of its significance in respect of avalanche danger. Thus, a hoar layer will remain constantly weak until the snowpack stabilises through a melt-freeze process. During the winter, especially in coastal climates, graupel layers may be found. Upon closer examination its colour is whiter than traditional lustrous ‘summer hail’. The small pellets are also porous and live up to the name graupel. Graupel is a new form of snow with the primary classification type PP, but subclass gp, i.e. graupel is described as PPgp. Graupel can vary in size from around 1-12 mm. The layers will usually be relatively thin but, in the case of graupel, can also reach up to 50 cm if the wind has resulted in the grains accumulating in smaller hollows in the terrain. Graupel is capable of behaving like pellet layers creating very localised (due to large local variations) unstable conditions where either a minor or even no extra load is required to trigger an avalanche.

4.2.7

Wet snow, melting transformation (melting metamorphosis) When snow has warmed up to 0 °C it begins to melt. This is characterised by the snow becoming sticky. The snow hangs together due to a thin water layer. Further melting will result in the snow pores gradually filling with water. When the moisture content exceeds 8% the water may be forced out of the snow and when it reaches 15% the water will drain out of the snow without any external influence. When snow particles melt, the snow’s strength also diminishes because bonding between snow grains is reduced and eventually disappears. In spring this is described as rotten snow. When the water eventually drains away the snow will resettle and become more compact. The avalanche risk will increase due to increasing snow creep and dwindling surface

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friction when the snow becomes damp and wet. After a period of time the avalanche risk will diminish as the snow settles. When the snow becomes soaked or ‘rotten’, there will be a risk of a slush avalanche occurring, particularly if there has been much rain and significant snow melt. If it becomes cold and the damp snow freezes, the avalanche risk will diminish and the snowpack will stabilise. A snowpack that has been isothermal (0 °C) and which freezes again will lead to particularly stable conditions.

4.2.8

Subsidence in the snowpack, the snowpack in steep terrain On level ground, snow will subside as it settles through destructive transformation. Its density will also increase. Because the pore volume is highest in the uppermost layers of snow, snow movement in steep terrain will be most significant in the uppermost layers, and most of all in new snow – up to 10 cm per day. Snow grains will adhere to the force of gravity in steep terrain. Subsidence will also result in snow moving downwards towards the ground. This type of movement is referred to as ‘snow creep’ and will result in stress and pressure in the snow with shear stresses between snow layers. The stress will be most significant in the uppermost parts of the snowpack. During the subsidence process, the density and strength of the snow will increase from 100 kg/m³ for new snow to 300-400 kg/m³ for older snow. The steeper the terrain, the greater the tension, compression and shear stresses, which will increase the likelihood of an avalanche occurring. The speed of the snow creep will increase as the snow temperature approaches 0 degrees, and the lower the density of the snow. It is new snow that has the lowest density. The bonding between snow grains will also diminish as the snow becomes sticky, causing the avalanche risk to increase as the snow’s temperature rises towards 0 degrees.

!When the snow temperature rises from -2 degrees up to 0 degrees, the avalanche risk will increase. Together with precipitation (new snow) and/or wind, this may pose an acute avalanche risk.

4.3

Weather that increases avalanche risk It is generally acknowledged that avalanche situations most easily manifest in snowy weather and wind. It is the intensity of snow accumulation, i.e. how quickly snow accumulates on a mountainside that is the single most critical factor in respect of the probability of an avalanche being triggered. Experience would also indicate that temperature plays a significant role in the degree of risk. In summary, the three most important weather factors that determine the degree of avalanche risk are: Precipitation and intensity

32

-

Wind

-

Temperature rise

UD 6-81-8E Instruction in Winter Service - Snow Awareness

4.3.1

Snowfall and wind In Norway, the greatest amount of precipitation usually occurs when major migrating low pressure systems move across the country. These are often followed by showery weather that may cause a substantial amount of precipitation to fall on the coast and in the mountains to the west of the watershed. The moisture content of the air is temperature dependent insofar as cold air contains less moisture than warm air.

Chap-4

Figure: 4.13 Low pressure meets a coastal mountain range; snow in higher areas Low pressure that meets coastal mountains is forced upwards and compelled to produce increasing precipitation due to decreasing temperature. Above a certain height precipitation will often fall as snow due to the temperature. The temperature generally decreases at a rate of 0.6 °C per 100 m of elevation, when the air is virtually saturated with moisture. With dry air, the temperature decreases up to 1 °C per 100 m of elevation. When humid air from the sea flows in towards land, air masses are forced upwards and the temperature decreases. The air is then unable to retain as much water vapour, resulting in the formation of clouds and precipitation. This leads to most precipitation falling in mountainous regions several miles inland from the coast. After the mountains have been passed, the air will sink again. Precipitation decreases and temperature increases because the shower/precipitation area descends from the mountains and is able to retain more

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moisture.

Figure: 4.14 Rain shadow The windward side receives precipitation, the leeward side minimal precipitation. As a rule, precipitation increases with height above sea level. It is difficult to provide general guidelines in respect of the amount of new snow and wind speeds necessary to trigger an avalanche because this is determined by the strength of weak layers and the bonding between individual layers. In addition to precipitation and wind, the terrain gradient is of great significance. Avalanches may be triggered even when as little as 20-30 cm of new snow with little grip is on the surface. Following larger snowfalls, e.g. 50 cm or more during the course of a 24 hour period, conditions may become unstable and in steep enough terrain an avalanche will probably occur. If snowy weather remains over a longer period, the snow will eventually settle and more snow will be required for the avalanche risk to be as significant as it would be with a similar size snowfall over a shorter period. Thus, the intensity of the snowfall is of significance in respect of avalanche risk. The relationship between the depth of new snow and the likelihood of a naturally triggered avalanche is shown schematically in the table below.

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Increase in snow over 3 days

Probability of naturally triggered avalanches, and avalanche type

Up to 10 cm

Rare, very localised snow movement (primarily loose snow avalanches)

10-30 cm

Some local slab avalanches. Frequent loose snow avalanches

30-50 cm

Frequent local slab avalanches, primarily on steep mountainsides

50-80 cm

Widespread slab avalanches, also in less steep terrain. General risk above timber line. Some larger avalanches into the valley bottom

80-120 cm

Frequent major avalanches into the valley bottom. Also occasionally outside of known avalanche paths

over 120 cm

Possibility of rare and, up to now, unknown avalanches, both in new locations and also beyond old avalanche paths and risk maps (Zone 2)

Chap-4

Table 4.1 Increase in snow and the probability of naturally triggered avalanches The table applies to wind speeds up to 5 m/s. With stronger and more persistent winds the risk will increase by one or two levels. The wind transports the snow, collects it and distributes it on steep slopes. At temperatures below 0 °C with a relatively soft snow surface, snow transport will commence with winds of 5 m/s, and even at 10 m/s there will be a strong ground storm. Generally, when the wind speed doubles, its potential to transport snow increases to the third power. This means that the accumulated amount of snow per hour on the mountainside must be multiplied by 8 (or 2³) when the wind speed doubles. With wind speeds from ca. gale to strong gale (20-25 m/s) the wind is able to transport large amounts of snow and several metres may accumulate on leeward sides in the course of a few hours. The amount of loose snow that is accessible to wind transport will also determine how much snow is transported. However, it should be noted that with a wind speed of 14-16 m/s, strong snow crust layers will be torn up and hard-packed snow will begin to move.

!The rule of thumb is that with a wind strength higher than gale over several hours, the avalanche risk may increase to a high avalanche risk. Weather forecasts issued by meteorologists in the media and which are available on yr.no specify average lowland winds. Experience would suggest that for exposed

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areas above the tree line the forecasted wind speed may often be doubled. It is also challenging that, in Norway, measurements from mountain-based weather stations are not available, unlike the weather stations that exist in Alpine countries and other countries that face challenges in respect of avalanches. This means that evaluations of measured and forecasted temperature, precipitation and wind speed in mountainous regions are uncertain. Precipitation is usually measured in mm and specifies how much water is produced when snow has melted. 1 mm rain is around 1 cm of snow (density: 1m³ = 100 kg). Accumulated snow is most easily measured by placing a board measuring 1x1 metre on the ground at a location not affected by wind, where snow observations may be made every day and the increase in the amount of snow measured on top of the board. Avalanche 0-8 m/s risk

8-15 m/s

1. Low

>15 cm

15-20 m/s

20-25 m/s 1

0

0

0

2. Modera- >20 cm te

>15 cm

>5 cm

0

3. Considerable

>30 cm

>20 cm

>15 cm

>5 cm

4. High

>50 cm

>30 cm

>20 cm

>10 cm

5. Very high

>80 cm

>50 cm

>30 cm

>20 cm

Table 4.2 Relationship between avalanche risk, snowfall and wind An example of the relationship between avalanche risk, new snow and wind in an extremely wind-exposed area (wind is specified as average wind per 10 mins.). The likelihood of an avalanche being triggered varies from one snowfall to another. In certain cases, for example, 20 cm of new snow is necessary while on other occasions the new snow must exceed 100 cm before an avalanche occurs. Following snowfall and wind the risk of a self-triggered avalanche will diminish quite rapidly although the risk of an avalanche being triggered as a result of human activity will remain high.

!Following snowfall and wind, the risk of a self-triggered avalanche will diminish quite rapidly although the risk of an avalanche being triggered as a result of human activity will remain high. 4.3.2

Rain It is not only snowy weather that is the source of avalanches. Rain on top of snow ¹ Avalanche risk may be high or extremely high even without new snow

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leads to a weakening in the bonding between snow crystals. Rain will also cause an increase in weight and greater strain on the snowpack. During rainy weather the temperature will rise somewhat, which generally increases the speed of the snow creep and weakens the bonds. In order for rain, as a single factor, to significantly heighten the avalanche risk, more than 5-10 mm per 24 hour period needs to fall. Continuous rainfall over several days will soak the snowpack and act as a restabilising factor. The consistency of snow is of significance insofar as rain on new snow more easily results in an avalanche than when it has rained on an old snow surface. Moreover, slush avalanches may be considered in situations with major precipitation in the form of rain.

4.3.3

Temperature of the snowpack The temperature is of significance to the stability of the snowpack. With low temperatures throughout the whole snowpack, all snow crystal transformation and ice bridge formation between snow grains will happen slowly and the snowpack may be unstable for several days following a snowfall. The temperature over longer periods (several weeks) is of significance to the constructive transformation of the Chap-4 snow crystals. Low snow surface temperature combined with a proportionally thin snowpack (large temperature gradient) will cause the lower layers to transform into faceted crystal snow, meaning that during the course of a cold period the snowpack is more unstable. A prolonged cold period with a thin snow layer followed by new snow and wind transport will often result in extremely unstable conditions that may persist for several weeks.

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Figure: 4.15 Constructive transformation during periods of cold weather and a thin snowpack Water vapour rises through the snow and condenses. When the temperature rises the sintering process will increase but when the temperature of the snow approaches 0 degrees it becomes sticky and damp and the ice bridges between the snow grains will diminish in strength and eventually

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disappear. Snow creep will also increase during a rise in temperature due to the snow subsiding more quickly. This weakens the stability of the snowpack. When the whole snowpack has been soaked it will become more stable although it will not regain complete stability before it has frozen all the way down to the ground and ice bridges have formed.

Experience indicates that the avalanche risk increases and is greatest when a warm front passes, i.e. when the air temperature exceeds 0 degrees. The avalanche risk is greatest in the first hours after the warm front has passed. 4.3.4

Solar heating As spring progresses the sun will increasingly gain more of a hold and radiation will heat the snowpack. The amount of solar energy that accumulates on the snow surface depends on the snow’s condition. New snow that has not been contaminated will reflect up to 90% of incoming solar radiation. Older snow will reflect significantly less radiation so that it heats up more quickly and becomes moist. The moisture will Chap-4 be forced further down into the snowpack and stable bonds will dissolve. This may occur even if the air temperature is below 0 °C. Thus, strong solar radiation during the spring heightens the avalanche risk. If it becomes so cold during the night that it freezes down through the snowpack, the snowpack will stabilise. In January and February, solar heating will usually have little effect on the snowpack.

4.4

Meteogram The Norwegian Armed Forces has its own weather database supplied by met.no. Information may be downloaded over the Internet at kilden.met.no, user name forsvaret. The password will vary from year to year. Under ‘Javameteogrammer’ are local forecasts for the specific area in which you are searching. The meteogram should be used in conjunction with the local weather forecast for the area in which you are located. In addition to the Norwegian Armed Force’s meteograms, local weather forecasts are also available at yr.no. These are similar to the meteograms at kilden.met.no. It should be noted that neither the location forecasts at yr.no or the meteograms at kilden.met.no are processed by meteorologists in the same way as text forecasts. Location forecasts at yr.no or meteograms are machine-processed data, using a model that the duty meteorologist regards as the most probable. Thus, it may deviate from the local forecast prepared by meteorologists.

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5

AVALANCHE FORMATION

5.1

Avalanche types Avalanches are divided into three primary groups: Loose snow avalanche

5.1.1

-

Slab avalanche

-

Slush avalanche

Loose snow avalanche

Chap-5

Figure: 5.1 Loose snow avalanche This type of avalanche generally occurs on the surface of the snowpack when bonding between the snow crystals is poor. This typically happens directly after a snowfall with light snow in calm weather or during spring when the sun and rain melt the bonds between snow grains. Loose snow avalanches are most often triggered at one point and spread out in a pear-shape. Loose snow avalanches usually occur in terrain that has a steepness of 45° to 60°. A loose snow avalanche will normally trigger itself when friction on the surface and to the side, as well as the snow’s bonds, succumb to gravity. Through destructive transformation the natural bonding between the new snow grains will be reduced and the avalanche risk will heighten. However, the trigger factor will usually be the weight of snow, in terms of the intensity of snowfall or in terms of snowdrifts, which cause the weight of the snow to be greater than the frictional forces and bonding between snow grains (cohesion). A loose snow avalanche is rarely triggered by

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personnel. In the event that this does happen, the individual/s responsible for triggering a loose snow avalanche will usually be located above the avalanche.

Loose snow avalanches pose little threat to military personnel. 5.1.2

Slab avalanche This type of avalanche is characterised by a more permanent snow layer sliding onto a looser layer further down the snowpack or on the ground. This creates a sharp fracture line along the upper boundary. Slab avalanches may comprise a substantial amount of snow insofar as a slope several hundred metres wide can slide out simultaneously. The fracture line may be several metres high. The slab will break into smaller blocks further down the avalanche path and will end up as a flowing loose mass that may also be airborne along the avalanche path. When the slab avalanche stops the snow immediately hardens. This is due to both compression and heating through friction, after which the snow cools again. The table shows the dimensions of a slab avalanche. Numeric Code

Grading

Volume

1

Slippage

Up to 100 m3

2

Small

Up to 1.000 m3

3

Medium

Up to 10.000 m3

4

Large

Up to 100.000 m3

5

Very large

Larger than 100.000 m3

Table 5.1 Avalanche classification in relation to volume This classification has been issued by the NGI (Norwegian Geotechnical Institute) as a proposal for classification. Even if the snow is light and loose in the bottom of the valley there could still be a risk of a slab avalanche being triggered higher up on the mountainside where the terrain is more exposed to the wind, enabling the snowpack to form slabs. A slab avalanche may be wet or dry. This means that it will behave differently. It is therefore important to understand the differences between wet and dry avalanches, what triggers them and how they behave. The table below shows the differences between wet and dry slab avalanches.

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Dry slab avalanches

Wet slab avalanches

What triggers the avalanche?

The avalanche is triggered because the weight of the snow, or persons, exceeds the forces keeping the snow in place

The avalanche is triggered because the strength of the snowpack/bonding between the snow grains is weakened

To what extent are people involved?

In 90% of cases the av- Very rarely triggered by alanche is triggered by people, usually triggered the victim or by someone naturally accompanying the victim

What type of weather cau- Wind transport of snow Usually triggered by rain, ses this type of avalanche? or intense snowy weather constant melting by the sun or high temperatures How does the avalanche move?

Fast (100-200 km/h), usually in a cloud of snow (airborne)

Slowly (35-100 km/h), like running cement, without a snow cloud

Table 5.2: Schematic representation of the differences between wet and dry avalanches

Chap-5

Figure: 5.2 Slab avalanche

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Dry slab avalanches pose the greatest threat to military personnel in terrain. Wet slab avalanches pose the greatest threat to military bivouacs and vehicles on road axes. 5.1.3

Slush avalanche This type of avalanche comprises a fluid mixture of snow and water in which water is predominant. Slush avalanches may occur following a major snow melt or strong rains on the snowpack. In such conditions it is advisable not to assemble in creek beds or where large streams flow into the bottom of the valley. Slush avalanches may be triggered with a gradient of less than 30 degrees (as low as 0 degrees) and may have a range that is far beyond the 20 degree rule. This type of avalanche is most common in coastal areas during the early part of winter where snow covered ground receives large amounts of rain. Slush avalanches may also occur during spring in the interior and in high mountains, although this is also linked to large amounts of rain, combined with melting snow.

Figure: 5.3 Slush avalanche Precipitation and melting snow that dam up may form the basis of a slush avalanche when the dam gives way.

Neither the Norwegian Armed Force’s avalanche maps nor the avalanche risk scale take into account slush avalanches.

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5.2

Forces that influence an avalanche In order to understand how a slab avalanche is triggered it is necessary to study in more detail the forces at work in the snowpack. Depending on time, temperature and air humidity, a transformation of snow grains in the various layers of snow will occur. We regard the processes as subsidences down through the snowpack. On a slope, snow grains will partially move themselves along the plumb line because of subsidence. However, gravity results in movement (creep) parallel to the terrain gradient. The largest snow creep occurs on steep mountainsides. The snowpack may also slide along the ground if it is slippery, such as, for example, bare rock-face or sward. The consequences of such creep and slippage can be seen from trees on steep, nival slopes, which characteristically bend from the roots and a little way up the trunk.

Chap-5

Figure: 5.4 Stress, compression and shear forces Because the terrain usually alternates in terms of shape, coarseness and gradient, the velocity of snow grains will vary from one location to the next. This causes tension in the snowpack. Gravity affects a snow slab on an incline. In order for the slab to remain in place this force must be absorbed along the sides and towards the surface. It is necessary to distinguish here between shear, stress and compressive forces. The slab’s ability to tolerate stress is determined by the snow’s shear, stress and compressive strength. If the stress exceeds the anchoring forces, an avalanche will be triggered. Of particular significance to avalanche risk are the shear forces that affect weak layers and bonding between layers down in the snowpack. Such layers or layer transitions have very little capacity to tolerate stress and during unstable periods the weight of a skier may be sufficient to initiate a fracture in the loose layer. Moreover, a rapid accumulation of snow, either through precipitation or drifting snow, will cause a shear fracture in a weak layer. The fracture often sounds like a weak rumble or a boom. This will result in the slab losing its anchoring and an avalanche may be triggered if the terrain is steep enough.

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Figure: 5.5 Forces that affect a snowpack on a slope Uppermost are stress forces, at the base are compressive forces, at the sides and towards the surface are shear forces. The vast majority of accidents involving skiers occur because the skiers themselves trigger the avalanche, as opposed to avalanches that are triggered for natural reasons. In unstable conditions a fracture in the snowpack at the foot of a slope can quickly spread up the slope and trigger a slab avalanche higher up the mountainside. In other words, the person who triggers the avalanche may be on level ground such as, for example, a stream gorge in which the terrain above or to the side is 30 degrees or steeper.

!Never approach the foot of steep inclines or enter them when the snowpack is rumbling

5.3

An avalanche in motion When an avalanche is moving down a mountainside its speed quickly increases. Initially, it has a sliding motion insofar as the snow slab breaks into smaller blocks of snow that slide like bricks on a smooth surface. Smaller avalanches with a drop of 10 to 20 metres are characterised by a sliding snow slab that breaks into large blocks. In larger avalanches, the blocks break up as velocity increases and the avalanche changes to a mixture of snow that whirls up into the air (snow cloud) and snowballs that roll, hop or slide towards the surface. (Respectively, saltation and flow layers.)

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Chap-5

Figure: 5.6 Example of a fracture in a layered snowpack At the moment of fracture there is usually a layer of snow at the base that forms a floor for the avalanche. Above this is a sliding layer that is often a thin layer of surface hoar crystals or a layer of faceted crystals/depth hoar. Above this is the avalanche mass itself which, in a slab avalanche, will often comprise fine-grained hard-packed snow (drifting snow). Velocity of a dry slab avalanche:

30-60 m/s (ca 100-200 km/h)

Velocity of a wet avalanche:

10-30 m/s (ca 35-100 km/h)

The velocity will vary according to the terrain gradient and the surface. A wet snow mass will have greater friction towards the surface and will thus not attain the high velocity of a dry avalanche. As an example, an avalanche with average friction on a 30 degree slope will have a velocity of 8 m/s after just 10 metres and, with a density of 200 kg/m³, this would be sufficient to crush the wall of a timber building. The avalanche mass will harden once the avalanche has stopped and the snow will become compact and hard. Such snow is very heavy to dig into and, in the case of rescue, it is necessary to use steel shovels. The avalanche mass may be more than 10-15 metres thick if the avalanche stops in a narrow gorge. On level terrain the accumulation is significantly thinner. If the snow is moisty, avalanche debris has a tendency to end up as deposed tongues with irregular breakthroughs.

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6

STABILITY TESTS

6.1

Snow profile A snow profile comprises a vertical wall around 2 metres wide and down to the ground (or 2 meters down) with a completely smooth wall. The purpose of a snow profile is to study the layering in snow between the surface and the ground to gain an understanding of the properties of the respective layers, as well as to document this appropriately. The purpose of the following stability tests is to calculate how much additional stress the snowpack can tolerate before an avalanche is triggered and which layers this occurs in, seen against the snow profile. Stability tests should be carried out in areas that are representative of the areas in which we wish to assess the avalanche risk. This requires identical exposure and a slope that is more than 30 degrees steep should be located. A snow profile should be carried out above the tree line as the snow here is particularly affected by wind transport. However, if the activity is to be undertaken below the tree line, or because weak layers have been identified below the tree line, then the profile should be carried out here. A snow profile with stability tests has clear limitations. It indicates how much additional load the snow is able to tolerate at that specific location. By moving 10 metres to the side the test may provide completely different answers. It is therefore vital to carry out several stability tests in similar exposures. Ideally two stability tests should be carried out side-by-side in the same snow profile shaft for verification purposes. The choice of location should always be based on the assumption that it is safe for Chap-6 the individual/s carrying out the tests. This includes an evaluation of the size of the slope, the consequences of an avalanche being triggered, terrain traps, and that personnel are available in the event that a rescue needs to be carried out. The slopes being tested should have a height of 5 to 10 metres and should be representative for the area. In the event of any uncertainty about the prevailing conditions, or if it is difficult to locate a safe, small slope of more than 30 degrees, the profiling and tests should be carried out in less steep terrain. In such cases, the individual/s carrying out the stability tests should be aware that signs and results will be less clear than in steeper terrain. The snow profile is made by digging a pit around 2 metres wide and primarily down to the ground to gain a complete overview regarding depth hoar layers closer to the ground. If several profiles are taken and it is more than 2 metres to the ground it will be sufficient to dig to a depth of 2 metres (or to the depth of potentially weak layers based on previous profiles). The wall of the snow profile facing the slope must be vertical, as smooth as possible, and perpendicular to the slope in accordance with figure 6.5. The snow stratification is marked and the hardness of the respective layers is measured using the hand test method. The hand test method should commence with a clenched fist that is pressed with moderate force (ca. 5 kg or 50 N of thrust), followed by four fingers, one finger, a pencil with the blunt end inwards, and a knife. If pure ice is present this should be marked. The purpose of the test is to identify potential layers that could form a slab in an avalanche. In the event that such layers are found on top of loose layers an

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alarm should be raised immediately!

Figure: 6.1 Absolute values of the hand test method

!The location where a snow profile is to be carried out should always be safe for personnel intending to study the snowpack. If hard layers are identified that could form slabs on top of loose layers, the alarm should be raised immediately! 6.1.1

Weak and unstable layers in the snowpack The purpose of a snow profile and stability tests is to identify weak layers and sliding layers in the snow. The object is to locate layers of surface hoar that could create a slip plane, graupel layers that may act as sliding layers (pellet layers) and, naturally, layers of faceted crystal snow or depth hoar that may constitute layers that easily collapse and have poor bonds. In 80% of avalanches triggered by people the fracture (cause) will occur in weak and unstable layers, 47% in layers of buried surface hoar, 26% in layers of faceted crystal snow and 8% in layers of depth hoar.

6.1.2

Snow profile form All snow profile observations must be recorded in a field journal so that this may be maintained as a log on a snow profile form, or on an electronic snow profile form

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such as Snowpilot, (available on the Internet). The snow profile form is like a combat journal for the avalanche forecaster. Here it will be possible to review the condition of the snowpack several days before so that its development may be studied. It will also function as a journal in the event of an accident occurring. The snow profile form is enclosed as Appendix 2.

6.1.3

Classification of snow types The various types of snow grains are divided into primary types in which each type has its own subgroups. Some of these will be of interest as they specify the stage in the development of a snow grain that may provide other properties than the primary group. (All subgroups are specified in Appendix 1.) This is an international classification (ICSI 20120) also used by the Norwegian Armed Forces. Grain types in the individual layers should be entered on the snow profile form.

6.1.4

Grain types Refer to figures 4.2 to 4.8 Primary shape

Primary classification

Symbol

New snow; the crystal is PP (Preciptation Particthe same as or similar to its les) original shape, as well as hail Figure: Machine-made snow

MM (Machine Made snow)

Chap-6

Figure: Decomposing and frag- DF Decomposing and mented; irregular rounded Fragmented preciptation shapes with branches, first particles stage of destructive transformation, partially decomposing Rounded Grains; rounded RG (Rounded grains) individual grains, final stage of destructive transformation, mechanically decomposing crystal shapes

Figure:

Figure:

Faceted crystal snow; FC (Faceted Crystals) grains with even surfaces and stripes, first stage of constructive transformation Figure:

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Depth hoar; hollow crys- DH (Depth Hoar) tals, even surfaces with stripes, final stage of constructive transformation

Figure:

Melt forms; wet crystals, MF (Melt Forms) may be frozen together, polycrystals, slush Figure: surface hoar; surface hoar SH (Surface Hoar) or cavity hoar, feather-like crystals Figure: Ice; pure ice layer in which IF (Ice Formations) the crystals are no longer visible Figure: Table 6.1 Primary shapes of snow crystals with numeric codes and symbols

6.1.5

Grain transformation schematic Snow grains will constantly be in the process of transforming into a new shape until they dissolve. Figure 6.2 shows how snow grains may be transformed through constructive transformation, destructive transformation, stagnation or melt processes.

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Chap-6

Figure: 6.2 Transformation cycle of snow crystals All crystals can transform through melting into melt forms (MF). Through a process of destructive transformation the new snow crystal (PP) will transform into a decomposing and fragmented crystal (DF). Through destructive transformation it will transform into a fine-grained crystal. With minor density and a large temperature gradient on the snow surface, PP and DF crystals can transform directly into small faceted (FC) crystals (fine cold weather over a prolonged period with a thin snowpack). Crystal forms will never revert to new snow crystals or decomposing, fragmented crystals. A small fine-grained crystal has reached the final stage in the process of destructive transformation. The fine-grained crystal transforms into a faceted crystal through constructive transformation and the faceted crystal transforms into depth hoar, concluding the process of constructive transformation.

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6.1.6

Fracture surface quality Recent research indicates that the quality of the fracture surface (Q) is critical to avalanche risk. The quality of the fracture surface is an indication of how slippery the surface is where a fracture/slippage occurs, as well as how easily the fracture occurs. On some occasions the block of snow emerges as if it was spring-loaded, almost shooting out. There is a clear connection between the block shooting out and a major avalanche risk. The table below specifies the fracture surface quality. The fracture surface quality should be used to specify the quality of all fracture surfaces in the stability tests described above.

Figure: 6.3 Q1 – the block shoots out as if it was spring-loaded Fracture surface quality

Description of slip plane Comments

Q1

The block shoots out as if NB! Indication of avalanit was spring-loaded che risk

Q2

Slip plane is smooth, or slightly rough

Q3

Slip plane is uneven

Good grip between layers, or fracture in a homogeneous layer

Table 6.2 Codification of fracture surface quality

!Q1 is a clear indication of a high avalanche hazard.

6.2

Shovel test The shovel test is not a stability test (as many would believe) but a method to identify layers of surface hoar and other potentially critical layers that may be difficult to locate. A buried layer of surface hoar do not need to be more than 1 mm thick. It will be extremely difficult to locate in the profile but, through a shovel test, and if the friction is low enough, the layer will slide. The procedure involves sawing a trapezoidal column of snow in the same way that a cake is cut, to the same width of the shovel and slightly smaller along the rear edge.

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The rear edge should not be cut any longer than the length of the saw. With the shovel inserted vertically behind the column, one layer after another should be loosened in the search for potentially critical layers. The shovel test is allocated numeric codes in order to illustrate friction between layers. However, these must not be used as an index of avalanche risk. ST is the international abbreviation for Shovel Test. Of special interest is a fracture from a shovel test when the fracture surface quality is a Q1 (Shear Quality, index 6.1.6). In such cases, this stratification should be particularly noted in respect of the stability tests.

Figure: 6.4 Shovel test

Chap-6

NB! The shovel must not be used to break free the snow block Numeric code

Grading

Description

ST 1

Very easy

Slides during sawing or insertion of shovel

ST 2

Easy

Slides with little extra load, little pressure on the shovel

ST 3

Medium

Slides with medium extra load, slight increase of force compared to ST 2

ST 4

Hard

Slides with much extra load, significant pressure on the shovel

Table 6.3 Codification of shovel test

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6.3

Compression test This is the first of three different stability tests used by the Norwegian Armed Forces. The test is useful in locating weak layers in the snowpack and for measuring the vertical load required before the snowpack collapses. The test is also relatively quick to perform. The load is applied vertically to the top of an isolated snow column. Snow should be removed from the top so that the shovel pushes vertically down and not at an angle to the top of the column. Fractures may be observed at the front of the column (it would be advantageous if a fellow worker was making the observations). The shovel is placed on the top with the front side of the blade facing downwards towards the column. Pressure is applied through strikes of the hand, the forearm, followed by the whole arm. The procedure involves isolating a snow column measuring 30x30 cm (measured parallel to the layers) that runs straight down passed the weak layers, although not as far as the depth hoar layers on the ground. The sides of the column must be completely smooth and even if the fractures are to be clearly visible. The column should not be so high that it starts to topple, i.e. maximum height of around 1 metre. The height of the column should be recorded. If a layer fractures during isolation of the block or during application of the shovel, this should be recorded as CTV (Compression Test Very easy). Following this, up to 10 strikes or ‘drops’ are applied with the wrist. The load should not exceed more than what is applied by the weight of the hand on the shovel from a 90 degree drop. If a layer fractures after, for example, 7 wrist drops, this should be recorded as CT 7. Following this, the snow should be removed down to the fractured layer, followed by wrist drops 8, 9, 10, etc. From 11 to 20 drops the whole forearm should be used and from 21 to 30 drops the whole arm with fist clenched (but without striking). The test is concluded at 30 and CTN (No fracture) should be recorded in the event that no fracture or collapse occurs. If at any time the uppermost part of the block beneath the shovel is crushed and the force of the shovel is thus no longer transferred to the rest of the block, the loose snow should be removed. The test is not valid on weak layers that are more than 100 cm deep. In such cases the snow must be removed on top and the column being tested. In this type of situation, only information regarding potentially weak layers may be used, while the weight application at the point of fracture (e.g. CT 7) may not be used directly in assessing stability.

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Figure: 6.5 Compression test Numeric code

Grading

Description

CTV

Very easy

Fracture when column is isolated (by sawing)

CT 1-10

Easy

Fracture with little extra load

CT 11-20

Medium

Fracture with medium extra load

CT 21-30

Hard

Fracture with much extra load

CTN

No fracture

No fracture

Chap-6

Table 6.4 Codification of compression test

6.4

Extended compression test The extended compression test is a comparatively new test that experience has so far shown to produce extremely reliable results in relation to the snowpack’s ability to propagate a fracture. How easily a fracture propagates in the snowpack is of great significance to how easily an avalanche can be triggered. It would be natural to perform this test if a CT 0-10 was recorded during the compression test. If the compression test indicates a relatively stable snowpack there would be little reason to assume that an ECT (Extended Column Test) would produce any results. So far, the ECT has shown itself as the most reliable individual test for assessing whether the snowpack is stable or unstable.

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Instead of isolating a snow column of 30x30 cm, in this case a 90x30 cm column is isolated, the 30 cm section being upwards on the slope. The shovel is applied to one corner of the column and the loading of the column is undertaken in the same way as in the compression test. Initially, 10 strikes/drops with the wrist are applied, followed by 10 strikes/drops with the forearm and, finally, 10 strikes/drops with the whole arm. When a fracture/crack appears in the snow column the number of strikes should be recorded, as well as whether the fracture spread across the whole surface. If this does not happen, apply one more strike and if the fracture then spreads throughout the whole column the snowpack may be deemed unstable. If the fracture does not propagate immediately, or on the first strike following the initial fracture, the snowpack may be regarded as stable. Fracture surface studies very often indicate that if the first sign of a fracture spreads itself throughout the whole column, or in a subsequent strike, the block will emerge with a Q1 (index 6.1.6). An extended compression test should be recorded as, for example, ECT 6/6, ECT 6/7 or ECTNP (No Propagation). The first figure specifies the number of strikes until the initial fracture occurred. The second figure specifies the number of strikes until the fracture propagates throughout the whole column. The test is not valid for weak layers deeper than 100 cm. If the weak layer is lower then the snow on top must be removed. The same proviso for the interpretation of test results in the compression test also applies here.

Figure: 6.6 Extended compression test An extended compression test should be undertaken following the shovel test and the compression test. The respective tests should then be compared.

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!If a layer of slippery surface hoar or faceted crystal snow has been found where slippage/collapse is measured at CT 0-10, as well as fracture propagation in an ECT, an alarm must be raised.

6.5

Rutsch block The rutsch block is a direct test of stability in the snowpack using an increasing vertical load. A rutsch block takes around 25-40 minutes to excavate. This is labour-intensive in relation to the amount of information obtained. The test area is 3 m². When the snow is loose it may be difficult to test the uppermost layers (20-30cm). There is also uncertainty as to whether this method provides a true picture of weak layers deeper than 1 metre beneath the surface. A vertical wall is dug in the snow to a length of 2.5-3 m and a depth of 1.5-2 m. In order to isolate the 3 m² block, 1. 5 m long trenches are dug on either side. The upper or rear long side is cut with an avalanche cord or snow saw, if it is long enough. Specially long saws and cords are available for cutting the sides, rather than excavating. If a saw is being used on the sides, the block must be cut trapezoidally in order to prevent it from jamming. Once the block has been isolated it should be loaded in 7 stages with increasing loads until it eventually slides out in a weak layer, in accordance with the scale in table 6.5.

Chap-6

Figure: 6.7 Rutsch block

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Stage

Loads that cause the block to slide out

RB 1

Excision without additional load

RB 2

A skier cautiously moves onto the block (on the upper half of the block, 30-40 cm below the upper edge)

RB 3

The skier performs a rocking motion without raising the heels or the skis

RB 4

The skier jumps and lands in the same place

RB 5

The skier jumps again in the same place

RB 6

Jump without skis in the same place. If the snow is loose, descend to the middle of the block wearing skis, rock once and then jump three times

RB 7

None of the preceding stages result in a clean fracture Table 6.5 Codification of rutsch block

The American rutsch block classification is specified above. The Swiss classification differs slightly.

6.6

Validity of stability tests In the case of compression tests, values of 13 or less indicate avalanche risk, while values of 14 or higher indicate stability, although these are not absolute. Experience from field tests would indicate that it is possible to have a CT 5 while avalanche risk 2 is still present, as well as a CT 19 and avalanche risk 3. It is therefore important that not only one test method is used but as many as possible in different exposures. In addition, two CTs should be undertaken side-by-side and the results should be approximately the same. The rutsch block provides a tolerably quantifiable measurement of snowpack stability. Nevertheless, it should be noted that on a relatively homogeneous mountainside there may be significant variations in the snowpack. Thus, a rutsch block on its own can give a completely false impression. However, the rutsch block shows minor variations in test results compared with other tests. The ECT indicates a positive relationship between the results and the actual avalanche risk. However, it may appear as being too conservative when there is a permanent layer of, for example, drifted snow above the fracturing weak layer. None of the stability tests take into account the stabilising effect of the layers above the weak layer. An example of this could be compact drifted snow above a layer of faceted snow where a CTV is attained, but in which it is possible to jump on the snowpack without anything happening, in spite of snow being released underneath and to the sides – without any cuts to the rear edge. Stress forces in the snowpack

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keep the snow in place, with a person acting as the additional load. This is a weakness that is present in all of the specified tests as the columns (CT, ECT) or surface in the rutsch block are totally released from the edges, i.e. only the strength and friction of the weak layer are tested. The Norwegian School of Winter Warfare, together with the NGI (The Norwegian Geotechnical Institute) are currently working on a new test (SLAB) that also takes into account the effects of the forces that hold the snowpack in place. In this test, the column is not released at the rear edge. Tests are performed in less steep terrain (with the exception of the rutsch block). This may be a correct assessment when snow conditions are so unstable that it would be regarded as too risky to expose personnel to smaller formations above 30 degrees. Thus, personnel carrying out such tests should be aware that the results will differ, or will be more difficult to interpret than previous tests, as well as requiring more experience.

6.7

Use of explosives As a rule, the Norwegian Armed Forces does not use explosives either to assess an avalanche risk or to trigger an avalanche. Explosives should only be used in special circumstances and such activity should be undertaken by avalanche group personnel. If, for safety reasons, it is necessary to blast mountainsides or snow banks this should be carried out as shown in figure 6.8. Charges should be buried as deep as possible at the rear edge of the snow bank or assumed fracture line. It may also be advantageous to lower a larger charge onto the surface of the snow, a little below the assumed fracture line. This should be carried out with a rope. The charges should be Chap-6 simultaneously ignited or a delay unit should be utilised in which the surface charge is ignited first. The size and number of charges will vary according to the amount of snow, terrain formation and extent of the avalanche risk area. However, as an example, the charges at the rear edge may range from 5 kg to 25 kg and the surface charge from 5 kg to 50 kg. Regardless of the size of the charges, the snowpack must be unstable. In a stable snowpack it will not be possible to trigger an avalanche (neither is this necessary). For stability testing of the snowpack with explosives, small charges from 0.5-1 kg should be used. The most efficient impact on the snowpack in respect of pressure is a charge that is ignited one metre above the assumed fracture line. In such cases, the charges are mounted on snow poles or furring strips. A helicopter will be required for the emplacement of these items.

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Figure: 6.8 Emplacement of explosives during avalanche blasting

6.8

Fracture line inspection A fracture line inspection where a recent avalanche has occurred can provide valuable information. The purpose of the inspection is to identify the cause of the avalanche, as well as the layers that fractured. Before ascending or entering out into an avalanche path, the risk of new snow sliding out of the fracture line – or other avalanche paths that could affect the area – must first be assessed. Normally, an avalanche path along which an avalanche has recently occurred is a safe area as it is unlikely that the remaining snow would be capable of loosening. This is because the usual former stress load has been eliminated and the anchoring forces are greater than the forces that could pull the rest of the snow downwards. The fracture line inspection is carried out by excavating a profile and performing stability tests. The profile should be excavated 1 metre above the fracture line.

6.9

Free moisture content in the snowpack Free moisture content describes the content of water in the snow in volume percentage. Free moisture is only present in snow that has a temperature equal to 0 degrees. Dry gloves or mittens are required to test the dampness of the snow. Before starting to test for free moisture content, the surface layer in the profile wall must be removed in order to reach layers that have not been affected by air temperature or solar radiation. The moisture content is registered by squeezing the snow with a glove or mitten to ascertain the degree to which it becomes sticky, or whether water can be squeezed out of the snow. The table below specifies the qualitative and quantitative gradings.

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Numeric code

Grading

Properties

1

Dry

Snow temperature (T) is 0 below 0 °C. The snow is difficult to shape

(blank)

2

Moist

T = 0 °C. Free moisture is < 3 not discernible but the snow is sticky and may be shaped

|

3

Wet

T = 0 °C. Free moisture 3-8 between some snow grains can be observed through a magnifying glass but it is not possible to squeeze the water out with moderate pressure

||

4

Extremely T = 0 °C. Water can be 8-15 wet squeezed out with moderate pressure but the snow pores still contain an amount of air

|||

5

Slush

||||

T = 0 °C. The snow is soaked with water and contains little air

Corresponding moisture content in%

>15

Graphic symbol

Chap-6

Table 6.6 Codification of free moisture in snow The moisture content of the snow is a factor that must be assessed in an analysis of the snowpack. A high moisture content will weaken the bonding between the snow crystals while an increase in moisture content following rainy weather will contribute to increasing the weight of snow layers that have a high moisture content. However, extremely wet snow or slush will contribute to stabilising previously smooth surface hoar or graupel layers.

6.10

Temperature in snow layers/temperature gradient In addition to the air temperature being measured at the profile 1.5 metres above ground, the temperature of the various snow layers should also be measured. The intervals are dependent upon the conditions although an interval of 20 cm is normal, with the exception of the surface. The first measurement should be at the surface, then at 10 cm, 30 cm, 50 cm, 70 cm, etc. Temperature measurements should be taken in the shade. If the snowpack is isothermal (0° C) throughout the intervals should not be measured. In the case of potentially weak layers in the snowpack, the temperature may be measured on both the upper side and underside of the layer in order to establish the occurrence/non occurrence of processes that could further weaken the

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layer. It is recommended that a digital thermometer is used to measure the temperature of the snow. The thermometer must be calibrated. It should be possible to insert the thermometer’s sensor 15 cm into the snow. The purpose of studying the differences in the snow’s temperature is to determine the snow’s temperature gradient. If this is more than 10 °C per metre, or 1 °C per 10 cm, a constructive transformation will occur, which in turn means that weak layers will develop and become even weaker. Where it is warmer in the upper layers as the result of solar radiation or a change in weather, this may weaken the bonds and increase snow creep. However, when it freezes again this will lead to greater stability. When such a thawing/freezing cycle takes place over many days on the snow surface, faceted crystal snow can form. This may create a lasting weak layer once it has been buried in the snowpack. Please note that all snow thermometers should be calibrated by regulating them in slush ice (0 °C).

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7

SAFETY FOR PERSONNEL WORKING WITH AVALANCHE RISK ASSESSMENT Many former avalanche experts and guides are no longer with us because avalanches do not distinguish between personnel with or without avalanche training and expertise – this must also be taken into consideration.

7.1

Competence level and equipment Personnel intending to work with avalanche risk assessment in the field should always work in pairs. This is a mandatory requirement. Both parties should be trained in the use of avalanche transceivers, probes and shovels. This equipment must be used. The working pair should always be in possession of some form of communication, preferably a mobile phone with coverage or a satellite telephone. Personnel intending to work with avalanche risk assessment for military units should have completed the avalanche assessment course at the Norwegian School of Winter Warfare. Norwegian safety regulations also stipulates the same requirements for personnel carrying out avalanche forecasting during exercises.

7.2

Route planning in connection with field surveys Prior to a working pair travelling out to carry out a snow profile and avalanche risk assessment, it is vital that a plan detailing their movements is prepared. This plan Chap-7 should also be available to personnel based at a camp. The weather will often be demanding, which may hamper navigation. Thus, it is important that preparations are made to ensure the safest possible navigation. When one member of the working pair is required to enter a slope in order to carry out a snow profile it is important that his/her partner always remains on safe ground until the party entering the slope gives the go-ahead and is confident of the stability of the location where the profile is to be excavated. Slopes higher than 5 metres may contain sufficient snow to bury a person so it is vital to assess the consequences of an avalanche being triggered. In the event that you trigger an avalanche or slippage it would be advantageous to be as high up in the avalanche as possible, i.e. you should always enter a slope from above and never from the base. If you have a shovel at hand, you may use it as an anchor to prevent yourself being carried downwards with the avalanche. If, however, you are standing at the base of the mountainside or slope, the avalanche will run over you. You should also be aware that you may trigger an avalanche and that the avalanche can spread out into terrain that is less than 30 degrees steep, if the terrain is steeper to one of the sides, or above you. Thus, you can expose yourself to avalanche risk in terrain that is 25 degrees steep if the slope of the terrain somewhere is above 30 degrees on the same mountainside or slope. You should also be aware that the snow slope may be steeper than 30 degrees, even

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though the map may indicate that it is less than this. (ref. figure 3.3.)

!Always enter a slope as high up as possible. 7.1.1

Terrain traps Terrain traps are areas in which the terrain will worsen the situation if an avalanche occurs. Examples of this are where the avalanche path enters a forest, runs over cliffs, as well as avalanche channels with visible rock formations and stream gorges. Avalanche channels containing terrain traps should be avoided in respect of avalanche risk assessment work.

66

Figure: 7.1

Figure: 7.2

Terrain trap in a river gorge

Terrain trap down into flat terrain

Figure: 7.3

Figure: 7.4

Terrain trap over cliff/ into water/frozen water

Terrain trap into a forest

UD 6-81-8E Instruction in Winter Service - Snow Awareness

8

ASSEMENT OF AVALANCHE RISK

8.1

Factors to be assessed Avalanche risk assessment is like a large jigsaw puzzle with many pieces missing. Assessing the avalanche risk where you are located is relatively easy. However, very often a forecast or a future assessment needs to be made that also covers areas where testing and profiles have not been undertaken, but which are assumed to have similar conditions to where a snow profile has previously been carried out. Field work also provides other vital observations that cannot necessarily be quantified but which represent important supplementary information. There may have been recent avalanche activity in the area, rumblings in the snowpack, fracturing and/or slippage in the snowpack. Factors that must be assessed: Weather conditions in the preceding 3-4 days. -

Condition of the snow in the relevant areas; variations in the snowpack; how has snow accumulation and the weather been throughout the winter?

-

Lasting weak layers, as well as thickness and depth of weak layers.

-

What signals (values) the stability tests produce.

-

What other observations were made during field work?

-

What type of weather is anticipated over the next 24-48 hours and how will it affect the snowpack?

!Remember that you are not familiar with all factors that may influence the avalanche risk.

8.2

Methodology When a group is to provide an avalanche forecast for an area, the whole group must be familiar with recent weather conditions, as well as the history of the weather. Lee and windward sides are important. Following this, each individual working pair that has carried out a snow profile must present their observations. These must also include everything that has been witnessed regarding avalanche related observations during the day’s field work. A weather forecast for the period you are forecasting is vital. In this respect it is wind, rising temperature and precipitation that are important. It should be noted that weather forecasts from meteorologists, yr.no or meteograms apply to lowland regions, whereas military units often move in the high mountains where the wind, in particular, is stronger than in valleys – often twice as strong as lowland winds. Also, temperature differences mean that precipitation that has fallen as rain in lowland regions will have fallen as snow in the mountains. In addition, at a given height snow will change from sticky to dry – something which increases the wind’s ability to transport snow. Natural questions to pose are:

Chap-8

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

-

How will the forecasted weather affect the snowpack – will it make it more or less stable?

-

Will mild weather affect weak layers and, if so, to what depth?

-

What is the significance of precipitation in the form of rain?

-

How will strong winds affect the snowpack?

Major naturally triggered avalanches usually start high up and descend all the way down into the valley bottom Following this, the involved parties must be in agreement on the avalanche forecast. Group consensus is important. Each member of the group should be heard and all opinions are vital in order to shed as much light as possible on the prevailing conditions. Nevertheless, experience is important and the views and opinions of the most experienced members of the group should count the most when a forecast is being made.

8.3

The military avalanche risk scale The military avalanche risk scale is new as of 1st November 2008. The scale is identical to the international avalanche risk scale but instead of making assessments in relation to infrastructure the military scale includes supplementary provisions that provide instructions relating to the Norwegian Armed Force’s avalanche map.

Figure: Table 8.1: The Norwegian Armed Force’s Avalanche Risk Scale

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

8.4

Avalanche forecast During all exercises in which an avalanche may occur, the officer in charge of the exercise should ensure that there is a qualified person or group that issues daily avalanche forecasts. According to Norwegian safety regulations this person or persons must be qualified through an avalanche hazard warning course at the Norwegian School of Winter Warfare. During exercises at brigade level or higher an avalanche group should be established that issues forecasts every evening that are valid for 24 hours (ref. Norwegian safety regulations). The Norwegian Armed Force’s avalanche forecast scale should be used. The avalanche forecast should, as a minimum, include the degree of risk, as well as the military supplementary provisions. These should be stated verbatim (word for word). The forecast should have a duration of 24 hours. In the event of significant changes to weather conditions it must be possible to increase the number of avalanche forecasts before expiry of the most recent forecast period. The avalanche forecast must reach all participating units, right down to individual teams and patrols.

An avalanche risk is not equal everywhere. Even if a forecast has been issued, the field commander must undertake an independent assessment 8.4.1

Forecasting errors One of the challenges in forecasting avalanches is the objective assessment of observations, tests and weather forecasts. To a great extent, people have a tendency to adapt observations and elements in a way that corresponds with the picture they have already formed, or wish to form. If a unit is to undertake a march in challenging mountainous terrain, many preparations will have been made and expectations formed. In addition, there will be a desire to carry out the mission, as well as group pressure. The avalanche forecaster or avalanche group must keep a distance from such issues Chap-8 in order to be afforded the best possible basis upon which to provide as objective a forecast as possible. If this does not happen then our knowledge and proficiency regarding snow and avalanches will be of little use. The avalanche forecaster must therefore have a definitive task quite apart from the division's operative plans, objectives and desires. Avalanche forecasting work must be transparent and must include a quality control system. This could be, for example, a work log with conclusions and valid forecasts on a day-to-day basis. All avalanche related observations must be registered in the log. This could include slippages, rumblings in the snow, amount of loose snow, accumulated precipitation, etc. Avalanche forecasts must not be based on one snow profile and one set of tests. The avalanche risk must be determined by several profiles in all exposures, as well as field observations and weather forecasts.

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

Figure: 8.1 The human factor and our assessment It is vital to avoid or reduce the subjective elements of avalanche risk assessment and that the forecaster considers the facts that are available. It is important to distinguish between facts and an assessment.

8.5

Avalanche maps The map series M711 to the scale of 1:50000 and with an equidistance of 20 m now covers virtually the whole of Norway. Avalanches may be triggered down to a gradient of 30°. This corresponds to a distance of 0.7mm between the 20m contour lines and a distance og 3.5mm between the 100m contour lines. If the distance between height contours is less, or the same, the terrain is steep enough for an avalanche to be triggered. This is how potential release areas are delineated on the map. It should be noted that steep slopes lower than 30-40 metres may not be apparent on the map. Calculations can therefore only be based on the height difference between two contour lines, and may be inaccurate for some areas. Thus, maps of this scale are not accurate enough to indicate the steepness of smaller slopes. The avalanche map is based on an ordinary M711 map. However, by utilising the NGI’s (The Norwegian Geotechnical Institute) topographic model the map also shows areas in which avalanches may occur. In addition, individual avalanche paths are surveyed and assessed by avalanche experts. The map has been prepared for the Norwegian Armed Forces for the planning and implementation of exercises and should not necessarily be used for any other purpose. A total of 87 avalanche maps have been produced, covering the areas in which the Norwegian Armed Forces undertakes the majority of its exercises. The avalanche maps are hiking maps and are based on the premise that an avalanche will not extend beyond Zone 2 within a 100 year period. Infrastructure maps are significantly stricter and take into account an

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

avalanche every 1,000 years. The Norwegian Armed Force’s avalanche map is also available for civilian use on the Internet, skrednett.no, and at snoskred.no. An extract of an avalanche map is shown in figure 2.43 (from 1532 IV KIRKESDALEN).

Chap-8

Figure: 8.2 Avalanche map Dark red denotes terrain steeper than 30 degrees. Light red denotes the maximum run out area. The arrow denotes a typical trail at the valley bottom that would be closed in the event of an avalanche risk 4 being issued – major avalanche risk.

8.5.1

Division of the avalanche map into zones The map is divided into the following zones: Zone I: Avalanche release area

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

-

Zone II: Avalanche run out area (based on 100 years of avalanches)

Zone I Zone 1 is indicated by the contour lines on the map and is marked where the distance between 20 m contour lines is 0.7 mm or less, i.e. 30° or steeper and not in dense forest. The terrain is steep enough for an avalanche to be triggered. Some steep areas have been omitted because the forest is so dense that the likelihood of an avalanche is considered minor (according to surveys). Zone II Terrain that lies beneath release areas and which can be reached by an avalanche. Zone II is indicated as the result of a method of calculation of how far an avalanche may extend, based on experience of a large number of avalanche paths (NGI’s topographic model).

8.5.2

Limitations of the avalanche map The map only shows release areas that can be identified on the basis of contour lines. Thus, avalanches may occur on smaller slopes that are not marked on the map. In stream valleys the height difference is often less than 40 metres and will generally not appear on the map. However, in such valleys there are often snow conditions that pose an increased avalanche risk. Neither does the map provide information on avalanche frequency. In some areas avalanches occur every year while in other areas many years may elapse between avalanches.

8.5.3

Using the avalanche map Avalanche maps are well suited to the planning of exercises and route selection. When an exercise is being planned, consideration must be given to the map's limitations and how it was formulated. Due to the prevailing wind direction, there may be an absence of snow on certain mountainsides. It may therefore be safe to walk on such mountainsides even though the chosen route falls within Zone 1 or Zone 2 on the avalanche map. It should be noted that even though the mountainside may appear free of snow, there could still be snow in gorges and stream valleys on the mountainside. If there is any doubt as to whether such snow may loosen, personnel should select an alternative route. Local experts may also provide useful information about where avalanches normally occur. It is therefore sensible to mark avalanche channels where local experts have stated avalanches have previously occurred. Thus, declarations about ‘safe mountainsides’ should be regarded with scepticism if the shape of the terrain and the gradient indicate the possibility of an avalanche occurring.

8.6

Establishing an avalanche group An assessment of possible avalanche risk should commence when the planning of the field training exercise has progressed to a point where the training ground has been determined. Initially, the avalanche risk assessment should be based on the avalanche map. The assessment should be followed up with a reconnaissance of bare ground. The responsibility for carrying out the avalanche assessment will be assigned to the officer who plans and is in charge of the exercise. The technical responsibility should be assigned to an individual who has received special

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

avalanche training. The avalanche risk assessment is given to the officer in charge of the exercise and constitutes a recommendation. The forecast should not be prepared or transcribed by any other staff member. Even if an avalanche group is established, it is important to emphasis that other personnel on the ground are not exempt from making continuous local avalanche risk assessments in the area in which they are moving. Usually, units carrying out exercises should not moderate the stipulated forecast. However, if the local commander assesses the avalanche risk to be higher, he/she may escalate the forecast issued by the avalanche group.

8.6.1

Organisation of an avalanche group With larger field service exercises at brigade level or higher, a separate avalanche group should be established. The manning requirements of the avalanche group are dependant on the training ground’s geographic area and the number of participating units, and are stipulated for each individual exercise. The avalanche group should comprise a minimum of 2 patrols in addition to the group commander. During larger exercises, civilian avalanche experts should reinforce the avalanche group so that it comprises 3 to 4 patrols. The commander of the avalanche group should report directly to the officer in charge of the exercise. The avalanche group is linked administratively to the officer in charge of the exercise.

8.6.2

Duties and responsibilities of an avalanche group Below are listed typical duties and responsibilities of an avalanche group: Procurement of information concerning avalanche risk -

Adaptation of information

-

Preparation of avalanche forecasts 1 to 2 times per day. Ensuring that the initial avalanche forecast reaches the respective units before they arrive at the training ground

-

Providing avalanche training to foreign units upon request

-

Rendering assistance in an avalanche accident in the form of advice regarding the potential risk of further avalanches occurring at the accident site, and also in the selection of a safe route in and out of the area Chap-8 Participating in the planning of the exercise by the officer in charge, if required

-

8.6.3

-

The avalanche group commander must/should attend a daily briefing for management and observers/controllers

-

The avalanche forecast must be given high priority. One avalanche forecast should be issued for the entire training ground (may be waived in the event that the training ground covers several distinct climatic areas)

Avalanche group equipment and materiel The avalanche group should possess the following equipment: Means of communication, satellite telephones -

Avalanche probes

-

Shovels

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UD 6-81-8E Instruction in Winter Service - Snow Awareness

74

-

Transcievers

-

4x4 vehicles with trailers

-

2 Snowmobiles for each patrol

-

Use of a helicopter

-

Temperature gauge

-

Raster plates

-

Snow profile forms

-

PC with Internet access

-

Tent/Jerven bag

-

Cooking stove

-

Alpine skis, boots and poles

-

Snow saw

-

Rutch block saw

APPENDIX A: GRAIN SHAPE CLASSIFICATION ** A.1 Main and subclasses of grain shapes Morphological classification Basic classification

Subclass

Code

Place of formation

Physical process

Prismatic crystal, solid or hollow

PPco

Cloud; temperature inversion layer (clear sky)

Growth from water vapour at –3 to –8°C and below–30°C

Needle-like, approximately cylindrical

PPnd

Cloud

Growth from water vapour at high super-saturation at –3 to –5°C and below –60°C

Plate-like, mostly hexagonal

PPpl

Cloud; temperature inversion layer (clear sky)

Growth from water vapour at 0 to –3°C and –8 to –70°C

Six-fold star-like, planar or spatial

PPsd

Cloud; temperature inversion layer (clear sky)

Growth from water vapour at high supersaturation at 0 to –3°C and at –12 to –16°C

Clusters of very small crystals

PPir

Cloud

Polycrystals growing in varying environmental conditions

Heavily rimed particles, spherical, conical, hexagonal or irregular in shape

PPgp

Cloud

Heavy riming of particles by accretion of supercooled water droplets Size: 5 mm

Laminar internal structure, translucent or milky glazed surface

PPhl

Cloud

Growth by accretion of supercooled water Size: >5 mm

Transparent, mostly small spheroids

PPip

Cloud

Irregular deposits or longer cones and needles pointing into the wind

PPrm

Onto surface as well as on freely exposed objects

Freezing of raindrops or refreezing of largely melted snow crystals or snowflakes (sleet) Graupel or snow pellets encased in thin ice layer (small hail) Size: both 5 mm Accretion of small, supercooled fog droplets frozen in place. Thin breakable crust forms on snow surface if process continues long enough

Precipitation Particles

a

Columns

j Needles

k Plates

Additional information on physical processes and strength

Shape

PP

l Stellars, Dendrites

13

m Irregular crystals

n Graupel

o Hail

p Ice pellets

q Rime

r

Dependence on most important parameters

Increase with fog density and exposure to wind

Notes: Diamond dust is a further type of precipitation often observed in polar regions (see Appendix E). Hard rime is more compact and amorphous than soft rime and may build out as glazed cones or ice feathers (AMS, 2000). The above subclasses do not cover all types of particles and crystals one may observe in the atmosphere. See the references below for a more comprehensive coverage. References: Magono & Lee, 1966; Bailey & Hallett, 2004; Dovgaluk & Pershina. 2005; Libbrecht, 2005

* * IHP-VII Technical Documents in Hydrology N° 83 IACS Contribution N° 1 UNESCO, Paris, 2009

Common effect on strength

Morphological classification Basic classification

Subclass

Shape

Machine Made snow

b

Additional information xon physical processes and strength Code

Place of formation

Physical process

Dependence on most important parameters

Common effect on strength

MM

Round polycrystalline particles

s Crushed ice particles

Small spherical particles, often showing protrusions, a result of the freezing process; may be partially hollow

MMrp

Atmosphere, near surface

Machined snow, i.e., freezing of very small water droplets from the surface inward

Liquid water content depends In dry conditions, quick sintermainly on air temperature ing results in rapid strength and humidity but also on increase snow density and grain size

Ice plates, shard-like

MMci

Ice generators

Machined ice, i.e., production of flake ice, subsequent crushing, and pneumatic distribution

All weather safe

t References: Fauve et al., 2002

14 Decomposing and Fragmented precipitation particles

c

DF

Partly decomposed precipitation particles

Characteristic shapes of precipitation particles still recognizable; often partly rounded.

DFdc

Within the snowpack; recently deposited snow near the surface, usually dry

Decrease of surface area to reduce surface free energy; also fragmentation due to light winds lead to initial break up

Speed of decomposition decreases with decreasing snow temperatures and decreasing temperature gradients

Regains cohesion by sintering after initial strength decreased due to decomposition process

Shards or fragments of precipitation particles

DFbk

Surface layer, mostly recently deposited snow

Saltation particles are fragmented and packed by wind, often closely; fragmentation often followed by rounding

Fragmentation and packing increase with wind speed

Quick sintering results in rapid strength increase

u Wind-broken precipitation particles

v

Morphological classification Basic classification

Subclass

Shape

Rounded Grains

d

Additional information xon physical processes and strength Code

Place of formation

Physical process

Dependence on most important parameters

Common effect on strength

RG Small rounded particles

w Large rounded particles

x Wind packed

y

RGsr

Within the snowpack; dry snow

Decrease of specific surface area by slow decrease of number of grains and increase of mean grain diameter. Small equilibrium growth form

Growth rate increases with increasing temperature; growth slower in high density snow with smaller pores

Strength due to sintering of the snow grains [1]. Strength increases with time, settlement and decreasing grain size

Rounded, usually elongated particles of size  0.25 mm; well sintered

RGlr

Within the snowpack; dry snow

Grain-to-grain vapour diffusion due to low temperature gradients, i.e., mean excess vapour density remains below critical value for kinetic growth. Large equilibrium growth form

Same as above

Same as above

Small, broken or abraded, closely-packed particles; well sintered

RGwp

Surface layer; dry snow

Packing and fragmentation of wind transported snow particles that round off by interaction with each other in the saltation layer. Evolves into either a hard but usually breakable wind crust or a thicker wind slab. (see notes)

Hardness increases with wind High number of contact points speed, decreasing particle size and small size causes rapid and moderate temperature strength increase through sintering

RGxf

Within the snowpack; dry snow

Growth regime changes if mean excess vapour density is larger than critical value for kinetic growth. Accordingly, this transitional form develops facets as temperature gradient increases

Grains are changing in response to an increasing temperature gradient

15

Rounded, usually elongated particles of size < 0.25 mm; highly sintered

Faceted Rounded, usually elongated rounded particles particles with developing facets

z

Reduction in number of bonds may decrease strength

Notes: Both wind crusts and wind slabs are layers of small, broken or abraded, closely packed and well-sintered particles. The former are thin irregular layers whereas the latter are thicker, often dense layers, usually found on lee slopes. Both types of layers can be represented either as sub-class RGwp or as RGsr along with proper grain size, hardness and/or density. If the grains are smaller than about 1 mm, an observer will need to consider the process at work to differentiate RGxf from FCxr. References: [1] Colbeck, 1997

Morphological classification Basic classification

Subclass

Shape

Faceted Crystals

Additional information xon physical processes and strength Code

Place of formation

e FCso

Within the snowpack; dry snow

Solid kinetic growth form, i.e., a solid crystal with sharp edges and corners as well as glassy, smooth faces

Growth rate increases with temperature, increasing temperature gradient, and decreasing density; may not grow to larger grains in high density snow because of small pores

Strength decreases with increasing growth rate and grain size

Faceted crystals in surface layer

FCsf

Within the snowpack but right beneath the surface; dry snow

May develop directly from Precipitation Particles (PP) or Decomposing and Fragmented particles (DFdc) due to large, near-surface temperature gradients [1] Solid kinetic growth form (see FCso above) at early stage of development

Temperature gradient may periodically change sign but remains at a high absolute value

Low strength snow

FCxr

Within the snowpack; dry snow

Trend to a transitional form reducing its specific surface area; corners and edges of the crystals are rounding off

Grains are rounding off in response to a decreasing temperature gradient

16

B Rounding faceted Faceted crystals with rounding particles facets and corners

C

Common effect on strength

Solid faceted crystals; usually hexagonal prisms

A Near surface faceted particles

Dependence on most important parameters

Grain-to-grain vapour diffusion driven by large enough temperature gradient, i.e., excess vapour density is above critical value for kinetic growth

FC

Solid faceted particles

Physical process

Notes: Once buried, FCsf are hard to distinguish from FCso unless the observer is familiar with the evolution of the snowpack FCxr can usually be clearly identified for crystals larger than about 1 mm. In case of smaller grains, however, an observer will need to consider the process at work to differentiate FCxr from RGxf. References: [1] Birkeland, 1998

Morphological classification Basic classification

Subclass

Shape

Depth Hoar

Additional information xon physical processes and strength Code

Place of formation

f D Hollow prisms

E Chains of depth hoar

Within the snowpack; dry snow

Formation of hollow or partly solid cup-shaped kinetic growth crystals [1]

See FCso.

Usually fragile but strength increases with density

Prismatic, hollow skeleton type crystals with glassy faces but few striations

DHpr

Within the snowpack; dry snow

Snow has completely recrystallized; high temperature gradient in low density snow, most often prolonged [2]

High recrystallization rate for long period and low density snow facilitates formation

May be very poorly bonded

Hollow skeleton type crystals arranged in chains

DHch

Within the snowpack; dry snow

Snow has completely recrystallized; intergranular arrangement in chains; most of the lateral bonds between columns have disappeared during crystal growth

High recrystallization rate for long period and low density snow facilitates formation

Very fragile snow

Large, heavily striated crystals; either solid or skeleton type

DHla

Within the snowpack; dry snow

Evolves from earlier stages described above; some bonding occurs as new crystals are initiated [2]

Longer time required than for any other snow crystal; long periods of large temperature gradient in low density snow are needed

Regains strength

Hollow skeleton type crystals with rounding of sharp edges, corners, and striations

DHxr

Within the snowpack; dry snow

Trend to a form reducing its specific surface area; corners and edges of the crystals are rounding off; faces may lose their relief, i.e., striations and steps disappear slowly. This process affects all subclasses of depth hoar

Grains are rounding off in response to a decreasing temperature gradient

May regain strength

crystals; usually cup-shaped

17

G Rounding depth hoar

H

Common effect on strength

DHcp

Striated, hollow skeleton type

F Large striated crystals

Dependence on most important parameters

Grain-to-grain vapour diffusion driven by large temperature gradient, i.e., excess vapour density is well above critical value for kinetic growth.

DH

Hollow cups

Physical process

Notes: DH and FC crystals may also grow in snow with density larger than about 300 kg m –3 such as found in polar snowpacks or wind slabs. These may then be termed 'hard’ or 'indurated’ depth hoar [3]. References: [1] Akitaya, 1974; Marbouty, 1980; Fukuzawa & Akitaya, 1993; Baunach et al., 2001; Sokratov, 2001; [2] Sturm & Benson, 1997; [3] Akitaya, 1974; Benson & Sturm, 1993

Morphological classification Basic classification

Subclass

Code

Striated, usually flat crystals; sometimes needle-like

Striated, planar or hollow skeleton type crystals grown in cavities; orientation often random

Surface Hoar

g

Additional information xon physical processes and strength

Shape

Place of formation

Physical process

Dependence on most important parameters

Common effect on strength

SHsu

Usually on cold snow surface relative to air temperature; sometimes on freely exposed objects above the surface (see notes)

Rapid kinetic growth of crystals at the snow surface by rapid transfer of water vapour from the atmosphere toward the snow surface; snow surface cooled to below ambient temperature by radiative cooling

Both increased cooling of the snow surface below air temperature as well as increasing relative humidity of the air cause growth rate to increase.In high water vapour gradient fields, e.g., near creeks, large feathery crystals may develop

Fragile, extremely low shear strength; strength may remain low for extended periods when buried in cold dry snow

SHcv

Cavity hoar is found in large voids in the snow, e.g., in the vicinity of tree trunks, buried bushes [1] Crevasse hoar is found in any large cooled space such as crevasses, cold storage rooms, boreholes, etc.

kinetic growth of crystals forming anywhere where a cavity, i.e., a large cooled space, is formed or present in which water vapour can be deposited under calm, still conditions [2]

Within the snowpack; dry snow

Trend to a form reducing its specific surface area; corners and edges of the crystals are rounding off; faces may lose their relief, i.e., striations and steps disappear slowly

Grains are rounding off in response to a decreasing temperature gradient

May regain strength

SH Surface hoar crystals

I

Cavity or crevasse hoar

J 18 Rounding surface hoar

K

Surface hoar crystal with rounding SHxr of sharp edges, corners and striations

Notes: It may be of interest to note more precisely the shape of hoar crystals, namely plates, cups, scrolls, needles and columns, dendrites, or composite forms [3]. Multi-day growth may also be specified. Surface hoar may form by advection of nearly saturated air on both freely exposed objects and the snow surface at subfreezing temperatures. This type of hoarfrost deposit makes up a substantial part of accumulation in the inland of Antarctica. It has been termed 'air hoar’ (see [2] and [4]). Crevasse hoar crystals are very similar to depth hoar. References: [1] Akitaya, 1974; [2] Seligman, 1936; [3] Jamieson & Schweizer, 2000; [4] AMS, 2000

Morphological classification Basic classification

Subclass

Code

Place of formation

Physical process

Dependence on most important parameters

Common effect on strength

Clustered rounded crystals held by large ice-to-ice bonds; water in internal veins among three crystals or two grain boundaries

MFcl

At the surface or within the snowpack; wet snow

Wet snow at low water content (pendular regime), i.e., holding free liquid water; clusters form to minimize surface free energy

Meltwater can drain; too much water leads to MFsl; first freezing leads to MFpc

Ice-to-ice bonds give strength

Individual crystals are frozen into a solid polycrystalline particle, either wet or refrozen

MFpc

At the surface or within the snowpack

Melt-freeze cycles form polycrystals when water in veins freezes; either wet at low water content (pendular regime) or refrozen

Particle size increases with number of melt-freeze cycles; radiation penetration may restore MFcl; excess water leads to MFsl

High strength in the frozen state; lower strength in the wet state; strength increases with number of melt-freeze cycles

Separated rounded particles

MFsl

Water-saturated, soaked snow; found within the snowpack, on land or ice surfaces, but also as a viscous floating mass in water after heavy snowfall.

Wet snow at high liquid water content (funicular regime); poorly bonded, fully rounded single crystals – and polycrystals – form as ice and water are in thermodynamic equilibrium

Water drainage blocked by capillary barrier, impermeable layer or ground; high energy input to the snowpack by solar radiation, high air temperature or water input (rain)

Little strength due to decaying bonds

MFcr

At the surface

Crust of melt-freeze polycrystals from a surface layer of wet snow that refroze after having been wetted by melt or rainfall; found either wet or refrozen

Particle size and density increases with number of melt-freeze cycles

Strength increases with number of melt-freeze cycles

Melt Forms

h

Additional information xon physical processes and strength

Shape

MF Clustered rounded grains

L Rounded polycrystals

M Slush

N

completely immersed in water

19 Melt-freeze crust

Oh

Crust of recognizable melt-freeze polycrystals

Notes: Melt-freeze crusts MFcr form at the surface as layers at most a few centimetres thick, usually on top of a subfreezing snowpack. Rounded polycrystals MFpc will rather form within the snowpack. MFcr usually contain more refrozen water than MFpc and will not return to MFcl. Both MFcr and MFpc may contain a recognizable minority of other shapes, particularly large kinetic growth form FC and DH. See the guidelines (Appendix C) for examples on the use of the MFcr symbol.

Morphological classification Basic classification

Subclass

Shape

Ice Formations

i

Additional information xon physical processes and strength Code

Place of formation

Physical process

Dependence on most important parameters

Common effect on strength

Ice layers are strong but strength decays once snow is completely wetted

IF Ice layer

Horizontal ice layer

IFil

Within the snowpack

Rain or meltwater from the surface percolates into cold snow where it refreezes along layer-parallel capillary barriers by heat conduction into surrounding subfreezing snow, i.e., snow at T < 0°C; ice layers usually retain some degree of permeability

Depends on timing of percolating water and cycles of melting and refreezing; more likely to occur if a stratification of fine over coarse-grained layers exists

Vertical ice body

IFic

Within snowpack layers

Draining water within flow fingers freezes by heat conduction into surrounding subfreezing snow, i.e., snow at T < 0°C

Flow fingers more likely to occur if snow is highly stratified; freezing enhanced if snow is very cold

Basal ice layer

IFbi

Base of snowpack

Melt water ponds above substrate and freezes by heat conduction into cold substrate

Formation enhanced if substrate is impermeable and very cold, e.g., permafrost

Weak slush layer may form on top

Thin, transparent glaze or clear film of ice on the surface

IFrc

At the surface

Results from freezing rain on snow; forms a thin surface glaze

Droplets have to be supercooled but coalesce before freezing

Thin breakable crust

Thin, transparent and shiny

IFsc

At the surface

Melt water from a surface snow layer refreezes at the surface due to radiative cooling; decreasing shortwave absorption in the forming glaze enhances greenhouse effect in the underlying snow; additional water vapour may condense below the glaze [1]

Builds during clear weather, air temperatures below freezing and strong solar radiation; not to be confused with melt-freeze crust MFcr

Thin breakable crust

P

Ice column

Q Basal ice

20

R Rain crust

S Sun crust, Firnspiegel

T

glaze or clear film of ice on the surface

Notes: In ice formations, pores usually do not connect and no individual grains or particles are recognizable, contrary to highly porous snow. Nevertheless, some permeability remains, in particular when wetted, but to much a lesser degree than for porous melt forms. Most often, rain and solar radiation cause the formation of melt-freeze crusts MFcr. Discontinuous ice bodies such as ice lenses or refrozen flow fingers can be identified by appropriate remarks (see Appendix C.2). References: [1] Ozeki & Akitaya, 1998

RNoA

SNOWPROFILE

Observation site(Name,Chart No., UTM coord.)

Date

Air temp

Terrain slope at chute

◦

Weather notations

C

Avalanche risk assessment

Time

Ca

Snowprof. no Elevation in meters Slope direction

◦

Special observations (for ex. Avalanches in the area)

Observer Sign explanation: Ts:Snow temp K:Hardness Z:Distance form ground D:Grain size O:Density

T ◦C 20 18 16 14 12 8 6 4 2 O ◄—————————————————————————————Z D K6 5 4 3 2 1 cm mill.mtr kg/m3 (lce) (Knife) (Pensil) (1 finger) (4 fingers) (Fist)

Notes: (Ex.Sliding layers and slope)

Bl 0289B (Utg 12-2010) ENG