AGRICULTURE HANDBOOK 489

U.S. DEPARTMENT OF AGRICULTURE

FOREST SERVICE

U.S. DEPARTMENT OF AGRICULTURE

FOREST SERVICE

July 1976

USDA, National Agricultural Library

NALBIdg ^

10301 Baltimore Bivd

Beltsville, MD 20705-2351

Avalanche

Handbook

RONALD I. PERLA and

M. MARTINELLI, Jr.

Alpine Snow and Avalanche

Research Project

Rocky Mountain Forest and

Range Experiment Station

USDA Forest Service

Fort Collins, Colorado

(Dr. Perla is now with

the Glacioiogy Division,

Environment Canada,

Calgary, Alberta)

Agriculture Handbook 489
Acknowledgments

We wish to thank André Roch and Hans Frutiger

of Switzerland and many persons in the United

States who furnished photographs. Special mention

is due the instructors at the 1972 and 1973 National

Avalanche Schools, who used and improved early

versions of much of the material presented here, and

Alexis Keiner, who did the art and graphic work.

Substantial help was also received from numerous

reviewers in the Forest Service and from:

WILLIAM HOTCHKISS

National Ski Patrol and

U.S. Geological Survey

Helena, Montana

DR. E. R. LACHAPELLE

University of Washington

Seattle, Washington

DR. JOHN MONTAGNE

Montana State University

Bozeman, Montana

PETER SCHAERER

National Research Council of Canada

Vancouver, British Columbia.

Cover photos: Wingle (front), Keiner (back),

Standley (inside).

Perla, Ronald I., and M. Martinelli, Jr.

1975. Avalanche Handbook. U.S. Dep. Agrie, Agrie. Handb. 489, 238 p.

Avalanches seldom touch man or his works, but when they do they can be disastrous. This illustrated handbook sets forth procedures for avoiding such disasters in

ski areas, near roads and settlements, and in the back country.

New snowfall and old snow redeposited by winds are the major causes of avalanches. Melting and freezing or the presence or absence of a temperature gradient

in the snow are difficult to observe directly but can make the snowpack either more

or less susceptible to avalanching. Such variations often can be identified by digging

pits in the snow.

When a snow slope fails, the impact of the avalanche depends on such variables as

the length, shape, and roughness of the avalanche path. If disaster strikes, buried victims must be rescued quickly, since the chance of survival decreases sharply with time.

Methods of avalanche control include artificial release by explosives, defense structures, public warnings, and land-use legislation.

Oxford: 111.0+111.784+384.1

Keywords: avalanche, avalanche classification, avalanche control, avalanche reporting, avalanche rescue, meteorology, snow, snow-cover stability, snow mechanics,

snowfall, snowpack, weather, weather observations, zoning.

Library of Congress Catalog Card Number: 75-600061

For sale by the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C. 20402—Price $3.95.

Stock No. 001-000-03466-9/Catalog No. A 1.76:489.
Preface

This book is for ski patrolers, snow rangers,

mountaineers, rescue teams, and others who

encounter avalanches in work or recreation. It is

suitable also for persons generally interested in

mountain weather, snow, avalanches, and avalanche

control. Although avalanches are treated mainly

as a problem of physical science, this is not an

engineering text. Those interested in engineering

design will find references to more technical material

in the "Further Reading" lists at the end of each

chapter and in the "Literature Cited" section at

the end of the book.

In the few places where equations or numerical

values are given, the international system of metric

units is used, in view of the trend toward the metric

system. Moreover, quantities such as snow density,

atmospheric pressure, snowpack stress, and avalanche

impact pressures are easily expressed in metric

units. Tables and arithmetic methods for converting

to English units are in Appendix A.

For brevity, avalanche is used to mean a mass

of snow that sometimes contains rocks, soil, and ice

moving rapidly downslope.

This book is meant to supplement, not substitute

for, field experience. Hopefully, the reader will

gather his field experience painlessly under

competent leadership.

ni
Abbreviated units of measure

The commonly accepted abbreviations for units

of measure used in this book are hsted below, along

with the words for which they stand. Appendix A

discusses the metric system at some length and

gives methods and tables of equivalents for converting

from metric to English units and vice versa.

b bar

°C degree Celsius

cm centimeter

cm- square centimeter

cm^ cubic centimeter

cm/h centimeters per hour

ft foot

g gram

g/cms grams per cubic centimeter

h hour

Hz hertz (cycles per second)

in inch

kg kilogram

kg/m3 kilograms per cubic meter

km kilometer

m meter

m2 square meter

m3 cubic meter

m/s meters per second

mb millibar

mi mile

mi/h miles per hour

mm millimeter

mm^ square millimeter

mm^ cubic millimeter

mm/h millimeters per hour

s second

t metric ton

t/m2 metric tons per square mete

The use of trade, firm, or corporation names in this

publication is for the information and convenience

of the reader. It does not constitute an official

endorsement or approval by the U.S. Department of

Agriculture of any product or service to the exclusion

of others that may be suitable.

IV
Contents

Chapter ^^Se

1. Effects of avalanches 1

Disasters of the past 3

Further reading 5

2. Avalanche meteorology 7

Flow of large airmasses 9

Precipitation 13

Snow crystals 18

Windflow over mountain terrain 23

Wind redistribution of snow 25

Wind analysis 29

Heat exchange at the snow surface 31

Storm analysis 35

Further reading 37

3. The mountain snowpack 39

Structure of the snowpack 40

Types of metamorphism 42

Equitemperature metamorphism 43

Temperature-gradient metamorphism ... 45

Melt-freeze metamorphism 49

Fracture mechanics of snow 52

The inclined snowpack 55

Snowpack analysis 57

Further reading 63

4. Avalanche phenomena 65

Failure of snow slopes 66

Slab analysis 69

Mechanics of slab failure 74

The avalanche path 76

Identification of avalanche paths 81

Avalanche movement 85

Further reading 90

5. Stability evaluation 93

The general problem 94

Snow-cover distribution and avalanche

activity 96

Snowpack structure 98

Local meteorological data 100

National Weather Service data 103

Ski and explosive tests 104

Further reading 109

Chapter Page

6. Protection of ski areas Ill

Control by restrictions 113

Control by compaction 115

Control by explosives 117

Choice of explosives 119

Use of explosives 121

Assembly of explosives 123

Explosive safety 127

Cornice control 130

Use of artillery 136

Artillery techniques 139

Planning the control program 143

Further reading 147

7. Protection of highways

and villages 149

Defense structures 150

Reforestation 157

Control by explosives 158

Planning the protection of highways .... 160

Protection of the Trans-Canada Highway 163

Land-use regulation 167

Avalanche zoning 171

Public information and warnings 173

Further reading 175

8. Safety and rescue 177

Avalanche accidents 178

Decisionmaking and route selection .... 181

Precautions for back-country travel .... 186

Probing for the victim 190

Search by electronic transceiver 194

Avalanche dogs 196

Revival and evacuation of the victim . . . 200

Organized rescue in ski areas 203

Special rescue problems 206

Further reading 209

Appendix

A. Converting units of measure 211

B. Snowpit data 217

C. International avalanche classification . . . 223

D. Avalanche reporting in the United States 225

E. Literature cited 231

Index 235

Effects of

avalanches

Avalanches are common on steep, snow-covered

mountains everywhere. Although most are remote

from man and his improvements and damage

little except timber, some do considerable damage

to man and his works. This chapter describes

briefly a few of the most devastating North and

South American avalanches of recent times.

Figure 1.—Bingham Canyon, Utah, February 17, 1926: searching

for victims amidst wreckage and avalanche debris (death toll 40).

Figure 2.—Recovering victims at Ophir, Utah, February 26, 1939.
Figures.—The mining lownof Alt;i, Utah, was nearly destroyed by an avahmche on Iclniiai) 13. 1885. Photo July 3, 1885.

Figure 4.—Avalanche damage to a ski lift terminal at Sun Valley. Idaho, February 11, 1959.

EFFECTS OF AVALANCHES
Disasters of the past

Snow avalanches are powerful forces of nature. A

large one may transport not only ice and snow but also

rock, soil, and vegetation. Avalanches thus play a

significant role in carving and weathering the world's

most spectacular peaks. Most of this action occurs in

places remote from civilization, but when man and

avalanche interact the results can be terrifying, as

this description by Benjamin Morales (1966) makes

clear;

In the Santa Valley, Peru, on January 10th, 1962,

a great ice avalanche occurred, the first one known

in the country, which fell from one of its highest

and most beautiful peaks. The avalanche was

caused by the breaking oflF of the west front of the

hanging glacier on the summit of North Huascaran

at the approximate altitude of 6,300 m. The quantity of ice involved is estimated at 2.5 to 3 million

m-', dragging along a great volume of granodiorite

blocks from the cliff.

This avalanche traveled 16 km, descended 4,000 m

in elevation, and destroyed and demolished everything in its path. The average speed of the avalanche was 60 km/h. More than 4,000 human lives

were lost and 9 small towns were destroyed. Cultivated fields were devastated, thousands of animals

were killed, and great destruction was caused in an

area which had been famous for its fertility and

beauty.

The 1962 avalanche spared the nearby city of

Yungay, which was protected by a small hill. But on

May 31, 1970, a large earthquake shook Peru and

released another monstrous avalanche from Mt.

Huascaran. This time, the avalanche spilled over the

hill, buried Yungay, and took 20,000 lives.

Worldwide, the settlements most vulnerable to

very large avalanches are those located where earthquakes, landslides, and avalanches could combine

effects, as happened in Peru. Avalanche destruction

on this scale, however, is not likely in North America

because the higher, steeper mountains (for example,

McKinley and Logan) are remote from civilization.

On the North American Continent, most avalanches

occur on land set aside for recreation, logging, grazing, and watersheds, rather than on areas of dense

residential use. Even small avalanche disasters that

affect only tens or hundreds of people are bound to be

rare in North America because the degree of snow

slope instability (except when triggered by earthquakes) can be evaluated with moderate precision

Figure 5.—Bent steel beams on bridge near Teton Pass, Wyo.,

testify to avalanche force, February 27, 1970. (Photo by

Martinelli)

and controlled by modern technology. Other natural

disasters such as earthquakes, floods, volcanic eruptions, and tornadoes are much harder to anticipate

and often impossible to control.

Small, isolated avalanche disasters have occurred

since the early settlement of the West. Reports of

avalanche deaths before the Civil War appear in

Mormon church records at Salt Lake City. Vivid

accounts of later avalanche disasters in the nearby

mining communities are described in the Salt Lake

Daily Tribune:

Alta, Dec. 26, 1872, 8:30 pm (telegraph) Number

killed not ascertained, seven sleighs found, four

more missing. Three teamsters got out alive. Eight

teamsters and some passengers reported lost.

Occurred one mile below. Storm unabated. Aha,

Dec. 30, 1872 . . . 200 men probing with 12 foot

rods of iron, one sleigh carried across canyon and

200 feet up mountain.

On March 7, 1884, at least 12 people were killed

in Alta, and on February 13, 1885, the town was

nearly destroyed by an avalanche that claimed 16

lives. The survivors took shelter in mine shafts.

On February 12, 1899, two avalanches ran simultaneously near the town of Silver Plume, Colo.,

killing ten people and injuring three others. On February 28, 1902, near Telluride, Colo., an avalanche

hit the shaft house of the Liberty Bell Mine and killed

seven miners; a rescue party was summoned, and two

of its members perished in a second avalanche. Three

other avalanches on the same day killed ten others.

Disasters of the past
Ä

-TI

Figure 6.—Avalanche damage to a ski lift, Alyeska, Alaska,

April 14, 1973. (Photo by O'Leary)

Although avalanche accidents were rather frequent

in the mining camps, the scattered arrangement of the

mines and homes usually limited the number of people

involved in any one accident.

The worst avalanche disaster in the United States

occurred on March 1, 1910, in Washington State;

96 people died when two snowbound passenger trains

near the town of Wellington were swept off the track

and into a steep-walled canyon. The ordeal is described in the book Northwest Disaster by Ruby El

Huit (1960). By coincidence, Canada suffered her

worst avalanche disaster in the same month and

year, when 62 workmen perished at Rogers Pass,

British Columbia, while attempting to rescue a train

stalled by an earlier avalanche (Anderson 1968).

On March 22, 1915, in another tragedy in British

Columbia, 57 men, women, and children were killed

by a rock and snow slide at the "Jane Camp" of the

Brittania Mine.

On February 17, 1926, 40 men, women, and

children were killed by an avalanche in the mining

community of Bingham Canyon, Utah. Twenty-five

families were left without homes, and 200 people were

seriously affected by the disaster. As human nature

often dictates, the community resettled in the same

Figure 7.—Damage to the reinforced concrete base of a lift tower, Alyeska, Alaska, April 14, 1973. (Photo by 0"Leary)

EFFECTS OF AVALANCHES
place, only to suffer another disaster on February 8,

1939 (four killed and four injured).

More recently, avalanche disaster has struck the

mountain community of Twin Lakes, Colo. (January

21, 1962-7 dead) ; the Grand Due Mine, British Columbia (February 18, 1965-26 dead); and a small

community near Terrace, British Columbia (January

22, 1974-7 dead). Gallagher (1967) and Williams

(1975) give detailed accounts of avalanche accidents

in the United States since 1910. European avalanche

accidents are reported annually in winter reports of

the Swiss Federal Institute for Snow and Avalanche

Research at Davos.

Avalanches in the United States are reported to

the U.S. Forest Service's Rocky Mountain Forest

and Range Experiment Station (240 W. Prospect

Street, Fort Collins, Colo. ) by a network of observing

stations in the western United States. Statistics from

this network for the 5 years preceding the winter of

1973-74 show about 10,000 avalanches per year.

About 1 percent of these harm man or his property.

These account, however, for an average of seven lives

lost and $300,000 in property damage each winter.

Although mountain communities will continue to

suffer occasional avalanche disasters, the greatest

potential for loss of life is in ski areas. On a typical

day at the height of the season, several thousand

skiers are dispersed on and around avalanche paths

in North American ski areas. In some areas, more

than 100 skiers may be under major avalanche paths

at one time. It is a tribute to avalanche technology

that only a few lives are lost to avalanches each season. This fine record is due to the conscientious efforts

of those responsible for recognizing and controlling

the avalanche hazard.

Further reading

Atwater, Montgomery M.

1968. The avalanche hunters. 235 p. Philadelphia: MacRae

Smith. One man's account of avalanche control, research,

anil rescue work in the western U.S. ami South America.

Fraser, Colin.

1966. The avalanche enigma. 301 p. London: John Murray.

Good account of avalanches, avalanche research and

control techniques, and rescue operations, mostly in

Switzerland.

Wechsberg, Joseph.

1958. Avalanche. 254 p. New York: Alfred A. Knopf. The

true report on the people and events associated with a

devaslatina avalanche that hit the high alpine village of

Blons, Austria, on January 11, 1954.

Figure 8.-A reminder of mining days: an avalanche slams into the ski town of Alta, Utah, January 1. 1974 (3 lodges damaged,

2 people injured, 35 cars damaged or destroyed).

Disasters of the past

Avalanche

meteorology

Changes in the weather provide the best dues to

when and where avalanches are likely to release.

With today's technology it is not possible to observe

directly subtle changes in the snowpack that

signal the onset of its failure and release from the

mountainside. However, it is possible to relate

changes in weather to changing tendencies of the

snowpack toward either failure or increased

stability.

This chapter begins with a brief discussion of

large-scale meteorology. Avalanche workers are

not expected to be weather forecasters, but they

must be able to communicate with meteorologists at

the well-equipped centers of the National Weather

Service. Avalanche workers are supplied with a

forecast and in return supply the meteorologists

with data in order to improve forecasts. This

exchange works most smoothly when avalanche

workers and meteorologists are familiar with one

another's problems.

Figure 9. About 90 percent of the wind-transported snow

travels in the 50 cm just above the snow surface. (Photo by Keiner)
160" 150° 140° 130° 120° 110° 100° 90° 80° 70° 60° 50'

Figure 10.—Map of 500-mb height contours over North America. Contours represent the height in ineters of the 500-mb level.

Note the wavelike regularity; a ridge (R) alternates with a trough (T). Broadly speaking, winter weather at high mountain

stations is determined by the upper-air conditions. A pressure ridge over a mountain area signifies fair weather, and a trough

signiñes storm conditions.

I HCMMtf I

Figure II.—Left, microbarograph and right, microbarograph chart

from Berthoud Pass, Colo., for the week beginning December 13,

1973. Midrange position and trough and ridge periods are identified on the chart.

AVALANCHE METEOROLOGY
Flow of large airmasses

Most destructive avalanche cycles are caused by

sustained heavy snowfall; other meteorological factors

are secondary. Heavy and sustained snowfall in a

mountain range occurs when the large-scale flow of

the atmosphere assumes certain patterns with respect

to the mountain range. To understand these patterns,

it is helpful to understand a few basic ideas about an

important feature of the atmosphere-barometric

pressure and its variation.

Barometric pressure decreases generally with increase in elevation. In metric units, the barometric

pressure at sea level is about 1 bar (b), or 1,000 millibars (mb). Pressure decreases with elevation at the

rate of about 0.1 mb per meter or, in metric shorthand,

0.1 mb/m.

Meteorologists find it convenient to talk about pressure levels in the atmosphere. The 1,000-mb level is

about sea level; the 700-mb level is about 3,000 m

above sea level; and the 500-mb level is about 5,500 m

above sea level. However, the exact elevation of any

pressure level fluctuates with the movement of large

airmasses.

A large number of weather stations around the

worid simultaneously send up balloons to gather data

on pressure, wind, temperature, and humidity. From

these upper-air data and from surface observations,

weather maps are prepared for the earth's surface and

for selected pressure levels, usually for the 850-mb,

700-mb, and 500-mb levels and for several higher

(lower pressure) levels.

Except over the higher mountain ranges, airflow at

the 500-mb level is not much influenced by terrain.

Typically, the 500-mb pressure patterns have a

smooth, wavelike appearance, with alternate ridges

ELEVATION

(Meters, m)

9000m

APPROXIMATE

PRESSURE

LEVELS

(Millibars, mb)

300mb| A

V

Mt. Everest

6000m 500m b

Mt. McKinley

Sea level 1013mb

Figure 12.—Atmospheric pressure is measured in millibars

(mb). The mean pressure at sea level is 1,013 mb. Pressure

decreases with increase in elevation.

Flow of large airmasses
Low pressure

High pressure

Low pressure

— 1000

"-1005

—-!0)0

'OlSmb

Igh pressure

Figure

SOOmb SURFACE AT EARTH'S SURFACE

13.—At the 500-mb level (left), wind flows nearly parallel to the height contours. At the surface (right),

flow across contours from high-pressure to low-pressure areas.

wind tends to

VHIGHy""----,. ~~-.

LOW

CYCLONIC LIFTING ANTICYCLONIC SINKING

Figure 14.—Cyclonic lifting, or convergence (left), and anticyclonic sinking, or divergence (right). The diameter of the circular

flow is between 100 and 1,000 km.

and troughs. These pressure waves are widespread;

in fact, four or five waves may link to encircle the

globe. The waves tend to move eastward across north

temperate regions, but occasionally they retrogress toward the west.

The waves decay and build up while moving, so that

pressure patterns change constantly. Superimposed on

the large waves are smaller, faster moving waves. The

large waves (long waves) determine the general

weather over sustained periods of several days to a

week; the smaller waves (short waves) are responsible

for hourly or daily changes. Prediction of these combined changes in pressure is done by the National

Weather Service on high-speed computers.

10 AVALANCHE METEOROLOGY
Figure 15.—Lifting of airmasses during the passage of fronts.

Warm fronts (top left) on the average advance at about 5

to 10 m/s; cold fronts (top right) advance at about twice

the speed of warm fronts. Mountain ranges intensify the

lifting effect of warm fronts (bottom right).

The wavelike regularity of pressure patterns, apparent at the 500-mb level, is obscured at the surface,

where frictional effects of terrain cause the pressure

patterns to assume more complex shapes. The 700-mb

level is a transitional level between 500 mb and the

surface. The 700-mb pressure patterns have the general regularity of the 500-mb patterns, but they also

are influenced by the surface. Since most hazardous

avalanche paths are in the pressure band between

1,000 mb and 500 mb, 700-mb balloon measurements

of wind, temperature, and humidity are especially useful for evaluating avalanche conditions.

Barometric pressure is easily and accurately measured; it is probably the easiest meteorological variable

to monitor unambiguously. Many instruments are

available; the microbarograph has proven satisfactory

at avalanche stations. With rare exceptions a barometer falling steadily below midrange is an omen of bad

weather at high-altitude stations. The barometer rising

steadily above midrange usually heralds a clearing

trend.

Large airmasses flow from regions of high pressure

into regions of low pressure. As shown in figure 13, at

the 500-mb level in the free atmosphere, the pressure

and the rotational force of the earth are approximately

balanced, and the resultant force alines the wind direction parallel to the pressure contours. Between sea

EFFECT OF MOUNTAINS

Warm air

level and 1,000 m above, the friction of the surface

upsets the balance between pressure and rotational

force, causing the wind to deflect across the contours

toward lower pressure areas. These deflections may be

as great as 45° over very rough surfaces.

This two-dimensional picture is modified by the

rising and sinking of airmasses. Vertical speeds are

much lower than horizontal windspeeds, but are important in forming and dissipating clouds and precipitation.

Frontal lifting is the result of either warm air flowing up over a cold airmass or a cold airmass wedging

beneath a warm mass. Airmasses do not readily mix,

Flow of large airmasses 11
Figure 16.—Maximiim oiographic lifting occurs when the wind is perpendicular to the mountain barrier (left). Orogiaphic

lifting is reduced when the wind strikes the barrier at angles other than perpendicular (right).

and the boundary between two colliding airmasses is

a front. Vertical air velocities induced depend on the

slope of the front; the steeper the slope, the greater

the lifting power. A warm front advances as a gently

sloped wedge of warm air with relatively low vertical

speeds at the frontal surface. A cold front advances

as a steep wedge with rapid lifting of the warm airmass. Mountain ranges trap cold airmasses on their

windward sides and intensify the lifting effect of warm

fronts.

Orographie lifting, or the forcing of air up and over

terrain barriers, is by far the most powerful lifting

mechanism. It induces vertical speeds on the order of

1/10 of the horizontal windspeed (compared to

l/IOO for cyclonic and frontal eflfects). Orographie

vertical speeds depend on windspeed, the angle at

which the wind strikes the mountain barrier, and the

slope of the barrier. For example, if the wind strikes

perpendicular to a mountain with a slope of 10°, the

vertical speed is about 17 percent of the horizontal

windspeed. Vertical speeds are reduced if the wind

strikes the mountain barrier at an angle other than

perpendicular and, in principle, are reduced to zero

when the wind is parallel to the barrier. Considering

the whole earth, orographie eflfects are small in comparison to convective, frontal, and cyclonic lifting.

However, in mountain regions they are clearly the

most important mechanism for lifting airmasses.

Under certain conditions, airmasses sink rapidly

on the lee sides of mountain ranges. Very strong,

warm and dry winds, called chinooks or foehns, are

caused by airmasses sinking into low-pressure areas

to the lee of ranges. These winds are a consequence

of the airmass losing moisture while rising on the

windward side of the range and then being heated by

compression on the descent down the lee side.

Chinooks rapidly warm and evaporate snow cover.

Vertical motions in the atmosphere depend on the

vertical distribution of temperature, pressure, and

water vapor. If a parcel of air is lifted, it expands and

cools because of lower pressure at the higher elevation. If it moves downward, it contracts and gets

warmer because of the higher pressure. This decrease

of temperature with elevation is the lapse rate. For

dry air the lapse rate is about 1 ° C/100 m. For saturated air it is less (0.6° C/100 m is a useful approximation). When a lifted parcel is colder and heavier

than the surrounding air, it tends to sink back to its

former elevation. However, when a lifted parcel is

warmer and lighter than the surrounding air, it tends

to keep rising. Once it reaches a temperature called

the dew point, the water vapor condenses into droplets and clouds form. Further lifting, cooling, and

condensation result in precipitation.

During warm seasons, local heating of the ground

produces significant lifting. If enough moisture is

available, convective clouds may form and showers

are possible. In winter, convection plays a small role

in vertical motion compared to three other important

eflfects: cyclonic circulation, frontal lifting, and orographie lifting.

12 AVALANCHE METEOROLOGY
High

pressure

Low

pressure

Dry

Warmed

by

compression

Figure 17.—Mechanism of Chinook winds. Although precipitation is not required, it greatly accentuates the Chinook

effect.

In cyclonic circulation, aiiflow is counterclockwise

around a low-pressure area and is directed toward the

low-pressure center of the cyclone. Hence, at the center, air must rise up from the surface to upper levels.

This is called convergence. In anticyclonic circulation,

the situation is reversed; air sinks downward and out

from the high-pressure center of an anticyclone. This

is called divergence.

Precipitation

The amount of water vapor an air parcel can hold

depends on the parcel's temperature; the warmer the

air, the greater its holding capacity. Rising air parcels

expand as they encounter the lower pressures of

higher elevations. The expansion results in cooling

and, therefore, a loss of capacity to support water

vapor. Droplets or ice crystals form and enlarge in

moist rising parcels, eventually dropping out as precipitation. Thus, the two essential elements for precipitation are moist airmasses and vertical lifting.

As discussed in the preceding section, vertical motions are produced by convective, cyclonic, frontal,

and orographie mechanisms. Since orographie lifting

produces the highest sustained vertical speeds, mountain regions usually receive far heavier snowfalls than

flatlands. When the wind is parallel to a mountain

range, orographie lifting is minimized, and adjacent

PACIFIC

CYCLONE

BELT

Berthoud

Pass

510cm

Squaw

Valley

555cm

Figure 18.—Centimeters of new snow, December through

March, at Mt. Rainier, Wash., Squaw Valley, Calif., Alta,

Utah, and Berthoud Pass, Colo.

mountain and flatland areas receive comparable

amounts of precipitation. Orographie lifting is reinforced by cyclonic and frontal lifting and nullified by

the sinking motions of anticyclonic flows; large

amounts of precipitation rarely occur in the mountains

when the airflow is anticyclonic.

Distribution of precipitation in the major mountain

ranges of the United States clearly shows the roles of

orographie lifting and available moisture. The primary

moisture supply for the western United States is the

Pacific Ocean. The greatest snowfall is in the Cascade

Range of Washington and Oregon, where moistureladen air carried from the Gulf of Alaska by the Pacific

cyclone belt is lifted by a north-south chain of mountains. Substantial precipitation also falls in the Sierras,

but there the moisture supply depends on storms that

stray south of the main cyclone belt.

Farther inland, precipitation in the mountains falls

off rapidly. The north-south running Teton and Wasatch Ranges are precipitation anomalies in an otherwise dry intermountain region. Both ranges are steep

Precipitation 13
Crest

(El. 3300m)

W E

Prevailing

winds

(800cm)

ALTA

(El. 2700m)

(600cm)

BRIGHTON

(El. 2700m)

(400cm)

PARK CITY

(El.2100m)

(150cnn)

El. 1300m

**^ALT LAKE CITY

(El. 1300m)

Figure 19.—Seasonal snowfall in the Wasatch Range of Utah.

orographie barriers that rise 2,000 ni above relatively

flat terrain. Abundant precipitation occurs in the San

Juan Range, Colorado's first major barrier to storms

tracking from the west and southwest. Storms from the

northwest usually deliver heavy snows to the Park

Range near Steamboat Springs. Because the mountains of western Colorado pull down much of the

precipitation, the Front Range just west of Denver

receives less snowfall from Pacific storms. Occasionally, moist air is carried into Colorado from the Gulf

of Mexico by storms from the southeast, in which case

large amounts of snow fall on the eastern side of the

Front Range.

Precipitation increases with elevation on the windward side of mountain ranges, usually reaching a

maximum slightly windward of the crest; the lee side

of the range is generally the drier side. Figure 19 illustrates this distribution for the Wasatch Range. It must

be emphasized that high precipitation on the windward side and scarcity on the lee side is a large-scale

characteristic pertaining to entire mountain ranges.

The actual distribution for small-scale terrain features.

Figure 20.—Recording and nonrecording precipitation gages mounted on a tower. A low railing around the platform is

recommended for the observer's safety.

14 AVALANCHE METEOROLOGY
such as ridges and gullies subject to wind transport and

deposition of snow, follows the exact opposite rule:

erosion of snow on the windward side and accumulation on the lee side.

Precipitation measurements are conveniently given

in centimeters (cm) of snowfall or in millimeters

(mm) of water equivalent. Some typical measurements of precipitation are:

• Snowfall—typical value, 10 cm

• Water equivalent—typical value, 10 mm

• Snowfall rate—typical value, 1 cm/h

• Precipitation intensity (water equivalent rate)—

typical value, 1 mm/h.

The latter measurement represents the rate at which

load is added to the snowpack.

Snow density is the fundamental property relating

snow and its water equivalent. Density is mass per

unit volume; in scientific units, kilograms per cubic

meter (kg/m-'). The density of water, 1,000 kg/m',

should be remembered as a convenient reference.

Alpine snow commonly has a density between 30

kg/m-' for light, newly fallen snow and 500 kg/m-' for

old, highly compressed layers. When measured in

areas sheltered from the wind, newly fallen snow is

found to have a density that averages between 70 and

120 kg/m'', depending on the temperature. In areas

exposed to the wind, the average densities are about

250 kg/m'. As a general rule, high densities correlate

with warm air or high winds, and low densities correlate with cold air or low winds.

Precipitation is measured either manually from

stakes or snowboards or by gages. When windspeeds

are low, a gage captures a representative sample of

the precipitation. However, high winds cause substantial amounts of precipitation to miss the gage. Recording gages offer one big advantage : they operate around

the clock, telling at least when precipitation fell, if not

the precise quantities.

Manual measurements are the most accurate way

of sampling the quantity of newly fallen snow, but it

is impractical to make them hourly around the clock.

Recording and manual measurements should be used

together so that recorded data tell when the precipitation fell, and manual measurements tell how much.

For avalanche stability evaluation, manual measurements should include:

• Total depth of the snowpack

• 24-hour new snow and water equivalent

Figure 21.—Total snowstake (left), fixed to the ground, and

portable snowboard (right) for measuring 24-hour, storm,

or interval amounts of precipitation. Various designs are

possible, but it is important that the board be made of light,

nonmetallic material and that the vertical scale be sturdy.

• Total snow and water equivalent of storm

• Snow and water equivalent for short intervals

during the storm.

The total depth of the snowpack is read from a

master stake, fixed rigidly to the ground. Three portable, lightweight snowboards, each with a vertical

scale, are used to measure the 24-hour, storm, and

interval snowfalls (see fig. 21). The 24-hour snowboard is reset daily, the storm snowboard is reset at

Precipitation 15
ié«4 "^xr

i

16

Figure 22.—Gathering and weighing a core sample of precipitation from a portable snowboard.

AVALANCHE METEOROLOGY
Amount of precipitation.

(Does not vary with slope.)

H= Heigtit measured vertically.

(Does not vary with slope.)

D= Thickness measured

perpendicular to slope.

(Does vary with slope.)

A= Unit area

O 0|

— ,_ 1 » II 1 X Ql

m ^^11

Figure 23.—In the absence of wind, snow height (H), measured vertically, is independent of slope angle. Snow thickness (D),

measured perpendicular to the slope, varies as the cosine of the slope angle.

the end of the storm, and the interval snowboard is

read and reset periodically throughout the storm.

Depending on manpower, the interval snowboard

may be read every hour, every 3 hours, or every 6

hours during the storm. A large aluminum can is used

to obtain a core sample from the snowboards, and the

sample is weighed in a sheltered enclosure (fig. 22).

When precipitation occurs without wind, a rare

case in mountain storms, precipitation amounts and

vertically measured snow heights are the same for all

slopes. Snow-cover thickness (measured prependicular to the slope ) varies as the cosine of the slope angle,

as shown in figure 23. Under the influence of wind,

snow is transported and deposited in uneven layers

that do not follow the simple cosine rule. Accumulations in adjacent areas of similar slope may then vary

by a factor of more than 10.

Since it is feasible neither to install a host of recording gages nor habitually to take manual measurements

in a large number of areas, the most practical scheme

is to measure precipitation at one study plot, in a

sheltered area where deposition does not depend

greatly on windspeed and direction. The study plot

should be level and located in a sheltered clearing, the

diameter of which is about one tree height. If this condition can be met, recording and manual measurements will be in fair agreement. The plot should be

easily and safely accessible during storm periods and

avalanche cycles.

Interpreting study-plot data to show what is actually being deposited on wind-exposed slopes is a central problem in avalanche technology. At best,

observers synthesize study-plot data and wind information and make rough estimates of deposition on the

slopes in question. Maintaining a high-altitude study

plot not far below the avalanche slopes is a considerable advantage, but the final choice of location will no

doubt be a compromise between accessibility, shelter

efficiency, and ability to provide meaningful data for

avalanche slopes.

Precipitation 17
Figure 24.—When the number of water molecules condensing

onto a droplet equals the number leaving the droplet, the

air is said to be saturated with respect to the droplet. If the

air and water-droplet temperatures are lowered, there will

be a net flow of water molecules toward the droplet until

a new balance is reached.

Snow crystals

To understand how snow crystals form and change

shape, it is necessary to discuss briefly certain molecular processes. First, imagine a drop of water suspended in the atmosphere. Water molecules are exchanged constantly at the surface of the drop. The air

is said to be saturated with respect to the drop's surface when the number of molecules leaving the drop

is equal to the number falling onto the drop. Now

suppose the air temperature is lowered. The result is

an increase in the number of molecules condensing

onto the drop because, as pointed out in the preceding

section, the water vapor capacity of air decreases with

decreasing temperature. The transfer of molecules

continues until a new balance is reached; that is, until

saturation is reached for the new air temperature. The

amount of water vapor that the air can support over

the drop is called the saturation vapor pressure with

respect to the surface of the drop.

Next, consider a rising, moist air parcel that is

cooled to saturation with respect to water. In this state,

water molecules tend to condense onto water droplets,

// droplets exist. But where do the droplets come from

to begin with? Although droplet formation is complex

and not fully understood, it is known that initial condensation occurs on microscopic dust, salt, or soil

particles that are lifted from the earth by wind. These

so-called condensation nuclei have dimensions on the

order of 10-" m, or 1 /im. Each cubic centimeter of

air contains 10 to 10,000 such nuclei.

Even if the air temperature is well below 0° C, a

droplet will not automatically freeze unless it contains

another type of impurity called a freezing nucleus.

These are much rarer than condensation nuclei; a

cubic centimeter of the atmosphere may contain only

10 freezing nuclei that are active at —10° C and

above. However, the colder the air, the greater the

probability of finding active freezing nuclei and hence

frozen droplets. At —10° C, only about one in a

million droplets freeze; at —30° C, one in a thousand;

and at —40° C, all droplets freeze spontaneously. A

rising parcel brought to saturation at air temperatures

between 0° and —40° C, therefore, consists of a mixture of ice crystals and supercooled water droplets,

Finally, consider an ice particle suspended in the

atmosphere. Again, the transfer of molecules between

the ice surface and the air is controlled by temperature; the wanner the temperature, the higher the

saturation vapor pressure over the ice surface. A

fundamental law of snow physics is that for a given

temperature below freezing less vapor can be retained

over an ice surface than over water. In other words,

the saturation vapor pressure is less with respect to

ice than with respect to liquid water.

Ice crystals grow at the expense of the water droplets in two ways. First, water vapor deposits directly

onto the crystal. As already noted, the vapor pressure

over an ice crystal is less than over a water droplet;

hence, there is a net transfer of water molecules from

droplets to the air and thence to the ice crystals. By

this vapor process, ice crystals grow into a variety of

hexagonal forms. In the second mode of growth, ice

crystals collide with the supercooled droplets, and the

droplets freeze onto the crystals. The crystals become

coated with a layer of frozen droplets, called rime.

Vapor deposition and riming may occur simultaneously, although one process usually dominates in

1Í AVALANCHE METEOROLOGY
Droplet

Droplet

O

Droplet

O

ô

o

VAPOR RIMING

Figure 25.—Two modes of growth of ice crystals at the expense of water droplets. In the vapor deposition mode (left), there

is a net transfer of water molecules from the droplet to the air and then from the air to the crystal. In riming (right), water

droplets collide with and freeze to the crystal.

each atmospheric layer. In general, riming tends to

obscure the hexagonal form of the parent crystal.

The wide variety of solid precipitation forms

observed in nature does not lend itself to a simple

classification scheme. However, a system proposed

in 1949 by the International Commission on Snow

and Ice uses a letter-number code to describe the basic

forms, their major modifications, and their average

sizes. In "The International Classification of Solid

Precipitation" (Mason 1957, UNESCO/IASH/

WMO 1970), 10 basic forms are distinguished and

coded Fl, F2,..., FO (fig. 26). Four modifications or

additional characteristics of the basic forms are shown

by the following letters: broken (p), rimed (r),

cluster or flake (f), and wet (w). Average size is

designated D and is best given in millimeters and

fractions, but if size categories are preferred, symbols

are given for five categories as follows:

Dd

De

Symbol

Da

Db

Dc

Size

Very small

Small

Medium

0-0.49 mm

0.50-0.99 mm

1.00-1.99 mm

Large

Very large

2.00-3.99 mm

4.00 mm and larger

A natural snowfall should be described on the basis

of the most frequent basic form, not the relatively

few most easily recognized forms. Size refers to the

greatest extension of a particle or the average of the

greatest extensions when many particles are considered. For a cluster of crystals, it refers to the average

size of the crystals composing the cluster. In coded

form FlwDO.3 means wet plates 0.3 mm in diameter,

and F2frDc means clusters of rimed stellar crystals

between 1 and 2 mm in diameter.

In avalanche technology, it is felt that modifications

are of equal or greater importance than the basic

forms. Broken crystals are usually the result of deposition in a strong wind. The crystals collide with one

another, are dragged along the ground, or bounce off

terrain obstacles. Delicate ones, especially form F2,

cannot remain intact under such treatment. The important modification, rime (r), described above as

growth by accretion of supercooled droplets on the

Snow crystals 19
i 4^m

F1

Plates O

F2

Stellar

crystals ^

F3

Columns r 1

F4

Needles «

F5

Spatial

dendrites ®

F6

Capped

columns t=l

FT

Irregular

particles ?r>

F8

Graupel A

F9

Sleet A

FO

Hail A

Figure 26.—The international classification of the basic forms

of solid precipitation.

parent crystal, is most common when snow crystals

are transferred turbulently through moist atmospheric

layers. Some riming is almost always present, especially in orographie storms. The heaviest riming occurs

during the passage of a cold front, when turbulent

transfer is strongest. Deposition of the solidly rimed

form F8, graupel, usually signals cold-front passage.

The term snowflake technically means a coagulation

Figure 27.—Modifications of basic forms—above, rimed

crystals and below, broken, partly metamorphosed, and

clustered crystals. (Photos by LaChapelle)

of several snow crystals. Whereas large crystals have

dimensions up to 0.5 cm, snowflakes may have a

spread of 5 cm or larger. Large snowflakes are predominantly composed of interlocked stellar crystals,

form F2.

The international classification scheme may suffice

for most avalanche work, but its apparent simplicity

leads to much misclassification. Because many particles do not fit into its relatively few categories, there

is a tendency to classify samples on the basis of the few

that do fit or to overuse the "'irregular particle" category. The more detailed system of Magono and Lee

( 1966) offers a much better chance of proper classification. Its 80 categories, based on extensive field and

laboratory work, may appear overwhelming, but with

good optical equipment and a field guide, it is easy to

use.

20 AVALANCHE METEOROLOGY
Crystal with

sectorlike branches

P1C

Crystal with

broad branches .^

Pid

Stellar crystal

Pie

Ordinary

dendritic crystal

Pif

Fernlike crystal

P2a

Stellar crystal

with plates at ends

P2b

Q'j^'O Stellar crystal with

sectorlike ends

P2c

Dendritic crystal

with plates at ei

P2cl '^

Dendritic crystal

with sectorlike ends

P2e

Plate with

simple extensions

sectorlike extensions

P2g

Plate with

dendritic extensions

Two-branched crystal

P3b

Three-branched crystal

P3c

Four-branched crystal

P4a

Broad branch crystal

with 12 branches

P4b

Dendritic crystal

with 12 branches

%

Mallormed crystal

P6a

Plate with spatial

Plate with

spatial dendrites

il plat^ 1^^

P6c

'^ Stellar crystal

with spatial plates

P6d

Stellar crystal

witti spatial dendrites

P7a

Radiating

assemblage of plates

P7b

Radiating assemblage

ol dendrites

CPla

Column with plates

CPlb

Column with dendrites

CPIc

f^ultiple capped column

CP2a

Bullet with plates

CP2b

Bullet with dendrites

CP3a

Stellar crystal .^^^

with needles i^^H

. ^ CP3b •

»|]j5>-4ff' Stellar crystal

with columns ^^^

CP3c ^^

"W'-W Stellar crystal

with scrolls at ends

CP3d

•^r<* Plate

with scrolls at ends

SI

Side planes ^^^_

Scalelike side planes

S3

Combination ot side

planes, bullets, columns j

Ria

Rimed needle crystal

Rib

Rimed columnar crystal

Ric

Rimed plate or sector

Rid

if

O

R2a

Densely rimed

plate or sector

R2b

Densely rimed

stellar crystal

I2c

Itellar crystal with

rimed spatial branches

R3a

Graupel-like snow

ot hexagonal type

R3b

Graupel-like snow

of lump type

R3c

Graupel-like snow \

nonrimed extensions

R4a

Hexagonal graupel

R4b

Lump graupel

R4c

Conelike graupel

^ II

Ice particle

12

Rimed particle

fl3a

Broken branch

I3b

tJifo

Jf »Í

<^

^

Rimed broken branch

14

Miscellaneous

G1

Inmute column

G2

Germ ol skeletal form

G3

Minute hexagonal plate

G4

Minute stellar crystal

G5

Minute assemblage

of plates

G6

Rimed stellar crystal Irregular germ

Figure 28.—The classification of snow crystals accoiding to Magono and Lee ( 1966).

J

Snow crystals 21
Basal

plane a-axes

Crystallographic axes Example of a-axes growth Example of c-axis growth

Figure 29.—Crystallographic axes of snow and ice crystals. If the growth rate along the «-axes exceeds that along the c-axis,

the crystals tend toward a platelike structure. If c-axis growth dominates, the crystals assume a columnlike appearance. The

mechanisms that cause differences in growth rates are not fully understood; air temperature plays an important role.

1

1

~ Hotlow pfisms

1 i

t

Seclor piales \

,/"'

1

a 5

I /

- ^/

.■■'' .-;

y

-

X

^••....- '''

W..■■■■"" {.-■■ SOJKJ prisms

-,v3 / \

-10 -20 -30 -40

So4>c)

prisms TEMPERATURE, (°C)

"igure 30.—Snow crystal type as a function of temperature

and supersaturation, based on the laboratory observations

of Mason ( 1957) and others. Note that as the temperature

changes from 0" C to —25° C, the crystal shape of midsupersaturation changes from plates (a-axis growth) to

needles (c-axis) to hollow prisms (c-axis) to sector plates

(a-axes) to dendrites (a-axes), and finally to hollow prisms

(c-axis). At —15° C, as the supersaturation increases from

0 to 15 percent, the crystal structure increases in complexity from very thick plates, to thick plates, to sector plates,

and finally, to dendrites. Dashed lines indicate uncertainty

due to insufficient data.

The crystal structure of snow is described by four

intrinsic axes, three a-axes and a c-axis. The a-axes

lie in the basal plane of the crystal; the c-axis is perpendicular to the basal plane.

Depending on meteorological conditions, crystal

growth occurs either in the basal plane or perpendicular to the basal plane. Growth in the basal plane

results in flat, platelike crystals like forms Fl and F2,

while growth along the c-axis results in columnlike

structures like forms F3, F4, and F6. Many field and

laboratory observations have demonstrated that relative growth along the a- and c-axes is controlled mostly

by air temperature and somewhat by the abundance of

water vapor in excess of saturation. The latter condition is called supersaturation; it is thought to control

the complexity of the crystal. High supersaturations

favor the development of intricate arms and branches,

or dendritic growth, as in forms F2 and F5. The most

peculiar feature of snow crystal formation is the

extreme dependence of crystal form on small changes

in temperature within a narrow range. As air temperature changes from 0° C to —25° C, the patterns

change from a-axis growth to c-axis growth and back

to a-axis growth.

In the field, large crystals such as graupel and stellars are easily identified by the naked eye. Identification of medium and smaller specimens and their

modifications requires 10- to 25-power magnification.

Pocket magnifiers with good depth of field are most

useful in day-to-day work.

In making observations, one should beware of classifying newly fallen snow solely on the basis of one or

two very prominent symmetrical specimens. The life

history of a snow crystal is complicated by a journey

through different temperature, supersaturation, and

wind layers, and the final product is rarely in the form

of the textbook illustrations.

22 AVALANCHE METEOROLOGY
Windflow over mountain terrain

Windspeed and direction are quite variable over a

mountainside from ridgecrest to canyon floor. About

10 m above the ridgecrest, windspeed and direction

are approximately that of the free air as measured by

balloon at a nearby upstream station. For example,

over a 3,000-m isolated ridgecrest. the windspeed and

direction have approximately the 700-mb values. The

wind below the ridgecrest depends on the orientation

of the terrain. At a canyon floor, winds usually are

deflected to aline with the canyon's direction. The

amount of deflection depends on the steepness and

length of the canyon. Often, flow at the canyon floor

is deflected 90° or more from the free airflow over the

ridgecrests.

Terrain obstacles exert drag forces that disturb the

typically smooth flow of free air. Hence, flow over

CANYON

FLOW

Figure 31.—Flow above the ridgecrests is approximated by

the free airflow. Canyons tend to deflect the wind to aline

with the canyon direction. Flow over the mountainside is

generally irregular and unsteady.

Figure 32 (right).—Airflow over a wedge-shaped ridge.

mountain terrain is relatively irregular. As the terrain

exerts drag on the wind, the wind in return exerts a

shear stress on the terrain. This shear stress loosens

the snow from the terrain. The eroded snow is carried

by the wind and redeposited.

Since deposition of snow is controlled by wind, it is

important to understand how air flows over and around

terrain obstacles. Picture, first, the flow over a wedgeshaped ridge, as shown in figure 32. The flow lines

represent the trajectories of air parcels. At levels high

above the ridge (region B), the flow lines show almost

none of the ridge's influence. However, a parcel that

flows close to the ridge is accelerated in region A, the

windward side of the ridge, reaches a maximum velocity above the crest (region C), and decelerates in

region D, the lee side of the ridge. It is easy to see that

the acceleration and deceleration effect will depend

strongly on the "sharpness" of the ridge, as measured

by apex angle 6. Accelerations and decelerations ocWindflow over mountain terrain 23
Figure 33.—Flow across a slope. Acceleration regions are

designated A. deceleration regions, D.

Figure 34. '^\ o\ei a surface. The shearing force of

the wind depends on the gradient of the windspeed at the

surface. The gradient at the surface is represented by the

slope of line OL.

cur over a variety of terrain geometries. Of particular

importance is flow across a slope where accelerations

and decelerations are caused by shoulders and gullies.

Figure 35.—Flow over a blunt ridge. The windspeed gradient

at B (BL) exceeds the gradient at A (AL). Hence, the

shear at B is greater than the shear at A. Gradients reduce

on the decelerating or lee side of the ridge.

Next, imagine the thin windflow layer just above the

surface of a terrain obstruction. The wind velocity is

zero at the surface and varies with height above the

surface. This variation is the windspeed gradient. As

already noted, the wind exerts a shear stress against

the surface. The amount of stress depends on the

gradient of the windspeed at the surface.

Terrain obstacles cause variations in gradients and

thus variations in shear stress. Consider flow over a

blunt ridge, as shown in figure 35. In the accelerating

region, the gradient increases with increasing proximity to the ridgecrest. Since the gradient is larger at

B than at A, there is a greater shear stress at B than

at A. In the decelerating region, the gradient decreases, and therefore shear stress decreases. The

amount of shear stress at some point in the decelerating region, say point C, depends on the terrain,

particularly the sharpness of the ridge and the steepness of the lee slope.

The decelerating effect intensifies as the lee side

steepens or the ridge becomes sharper. Over steep lee

slopes or sharp ridges, the wind profile may have the

24 AVALANCHE METEOROLOGY
Figure 36.—Over part of a steep lee slope, the windspeed

gradient at the surface may vanish. In this case, the shear

at the surface also reduces to zero.

Figure 37.—Reverse flow and shear on steep lee slopes. The

reverse flow joins the main flow in a circular pattern called

a vortex.

shape shown in figure 36. The shear stress vanishes on

a surface beneath such a profile. On even steeper lee

slopes, typical of many avalanche paths, the deceleration is sometimes strong enough to reverse airflow and

so reverse shear, as shown in figure 37. The reverse

flows join the main stream to form circular swirls

called vortexes or eddies. Each terrain feature induces

a unique vortex pattern strongly dependent on the

speed and direction of the main flow. Vortexes are

generated in all sizes, from large "rolls," with dimensions on the order of the ridge height, to pea-sized

eddies. In general, vortexes are stationary in neither

position nor size, but are shed by the terrain and

carried downstream by the main flow. Vortexes build

up and decay continuously, and new ones take the

places of dissipated ones. The passage of a vortex is

observed as a gust or a sudden change in wind direction and by "snow devils."

The foregoing discussion is an idealized picture of

flow over terrain obstacles. Actual small-scale flow is

much more complicated. Each small protrusion of

terrain has an effect, producing accelerations, decelerations, and vortexes on its own scale.

Wind redistribution of snow

In the absence of wind, snow crystals fall at rates

that vary between about 0.3 m/s for flat, platelike

crystals and 2 m/s for graupel. Very light winds deflect the snow crystals in the horizontal direction,

plastering snow against the windward sides of obstacles, especially when the snow is wet. However, when

windspeed exceeds a few meters per second, as it usually does during mountain storms, the shearing forces

exerted by airflow against the snow surface erode the

snow from regions of high wind stress. Eroded snow

is redeposited in regions of low wind stress. Looked

at in terms of windspeed, the snow is picked up where

windspeed is increasing and deposited where windspeed is decreasing. When snow is transferred, it tends

to accumulate in localized zones instead of spreading

out evenly. These zones are usually terrain indentations bounded by rock outcroppings, gully walls, tree

groups, or other irregularities. The deepest accumulations are in gullies and bowls, where the snow may be

several meters deep, but large accumulations can also

form on flat or even convex slopes.

Wind redistribution of snow 25
^r.&V. ACCELERATiriM O't-'

.w 4 ^á.

d^

^ ,.._^g^ffi

jüsSptf SíaHnzÍ7iraT t

^^^^M1

<? ^^^^^M^^ ^8M14

^i^^^^B ^sliSÉ

H^^^^^^^g ^^W ^H|i^^

3ffl|H|ffl^riillr!flY|^^ Sl^öi feffgWy ^^^M^i^^^^ ^^^Hi

B

Figure 38.—Snow is eroded from acceleration regions and redeposited in deceleration regions. Four examples of transport are

shown: A, transport from the windward to the lee side of a ridge: ß, transport to a region of increasing slope steepness; C,

transport into gullies from side and top; and D, transport through notches in a ridgecrest.

Figure 39.—Examples of natural accumulation zones in a

mountain basin. The accumulation zones, shown by arrows,

depend on the prevailing wind direction.

The amount of snow eroded by high winds depends

on the "looseness" of the surface. The largest amounts

are transferred during or immediately after storm periods, since newly fallen snow is especially susceptible

to erosion. Wet snow is not as easily sheared loose.

However, some snow can always be moved during

winter if the wind is strong.

It is difficult to give threshold values of windspeed

that shear loose all types of snow from all types of terrain, but it is observed that the amounts transported

depend strongly on the windspeed. According to some

studies, the volume of snow transported depends on

the third power of the speed; that is, doubling the

windspeed increases the horizontal transport by a factor of eight.

26 AVALANCHE METEOROLOGY
TURBULENT SUSPENSION

Snow is transported in three ways: rolling, saltation, and turbulent suspension. Rolling is limited to a

thin layer, no higher than 5 mm thick, on the surface

of the pack. This is not thought to be important in

loading avalanche slopes. Saltation involves bouncing

particles. Each particle is supposedly kicked up into

the airstream by the impact of another particle. Although saltation may lift particles as high as 100 cm,

it is most effective in the first few centimeters above

the surface. Turbulent suspension refers to particles

suspended in the wind by aerodynamic forces. Heavy

particles are suspended near the surface, light particles

as high as 100 m above; however, the greatest mass

transport by this process occurs in the first 2 m above

the surface, and it is thought that 90 percent of the

transfer is limited to the first 50 cm above the surface.

There is some disagreement as to whether saltation

or turbulent suspension is the dominant mode for

loading avalanche slopes, but working together the

two modes rapidly transfer snow to regions of low

wind stress. In a test at Berthoud Pass, Colo., snow

was deposited in a lee-exposed gully at a rate of

45 cm/h.

Due to pulverization, the average size of the blowing snow particles may be only Vjo the size of those

that fall undisturbed. Because of small particle size,

wind-deposited snow is two to four times denser than

snow that falls in a sheltered study plot. Also, due to

Figure 40.-Three modes of wind transport (modified from

Mellor 1965).

Figure 41.—Snow deposition, especially with wet snow, is

heavier on the windward side of a terrain obstacle during

very light winds of only a few meters per second.

Wind redistribution of snow 27
«awdw."

Roof

,0:i:i(iíim,i¡ü¿.•*><:

Figure 42.—Basic structure of a snow cornice.

k

Figure 43. —A large cornice at Rotlalsattel, near Jungfraujoch, Switzerland. (Photo by Roch)

the small particle size and many particle contacts,

wind-transported snow quickly takes on a firm, slablike structure.

Large amounts of snow accumulate in the form of

cornices at sharp terrain bends. The most common

cornice locations are at ridgecrests, sides of guUies,

and other local steepenings. The basic structure of a

cornice consists of a root, roof, face, and scarp.

Cornices grow as successive layers that are added

during each period of snow transport. In the Bridger

Range of Montana, cornices were observed forming

with winds as light as 7 m/s, whereas winds of 27 m/s

scoured the cornice surface and abruptly reduced its

height (Montagne et al. 1968). As each layer accretes onto the roof, it tends to extend out over the

cornice face, supported as a cantilever sheet. The

mechanisms that favor this growth are not fully understood. After attachment to the roof, the layers are

deformed slowly by gravity and bend toward the cornice face as a curved tongue that often encloses an

airspace.

The far end of the top layer with its poor attachment to the old cornice surface and the cavities caused

by the deformed layers are major sources of structural

weakness. Also, throughout its life, the entire cornice

deforms steadily outward over the slope, usually

reaching a position of precarious balance. Overhanging cornices can be extremely massive. Cornices often

extend as much as 15 m upward and outward from

28 AVALANCHE METEOROLOGY
the ridgecrest, with size and shape a function of ridge

shape and lee-slope steepness. The density of cornice

snow can exceed 400 kg/m'. Cornices are usually

more stable over slopes of less than 50° than over

steeper slopes where support is lacking.

Wind analysis

Through a combination of human observations and

wind instrumentation, it is possible to judge qualitatively the effects of wind on the snowpack. In evaluating the avalanche hazard, there is no substitute for

an experienced observer being at the area in question

and seeing where snow is being deposited. The observer first watches the visual evidence of transport

(snow plumes over ridgecrests, vortexes, etc.). Based

on these clues, he makes as thorough a field inspection

as possible to verify his suspicions about where snow

is being deposited. This approach is generally limited

by lack of manpower, severe weather, and the size or

inaccessibility of important accumulation zones. It

is therefore necessary to rely heavily on wind instruments.

Many types of wind systems are available, featuring

simple and complex, inexpensive and expensive components. It has not been demonstrated that any one

system solves all the problems of measuring mountain winds. The most popular sensors from an ecoFigure 44.—Rolating-CLip anemometer and wind vane. These

instruments should be located on a high, isolated ridgecrest

and set on a tower at least 3 m above the ridge.

nomic and operational standpoint are the rotating-cup

anemometer for measuring windspeed and the wind

vane for measuring wind direction. The sensors can

be wired to recorders, or, in the more deluxe systems,

the signals can be transmitted to receivers that are

wired to recorders.

Figure 45. The formation of rime on wind sen.sors (left). One solution is to heat the sensore with infrared lamps (right).

(Photos by Judson)

Wind analysis 29
Wind run Wind

past direction

station

S W N E

Isw INWINEI SE

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Figifte 46.—Digital wind-direction and speed record. Each

small notch in the left trace indicates 1 mile of wind travel.

Each of the large notches indicates 10 miles of travel. Wind

direction is determined from the other traces, by the relative number of marks from each direction. For example,

at 4 p.m. the wind was from the west-northwest.

Since it is generally not practical to install a large

number of sensors, one good location must be chosen.

The sensors should be placed where they can monitor

the windflow that feeds the greatest number of the

most important lee slopes. A high, isolated ridgecrest

is a natural choice if it is reasonably accessible in winter. Long experience teaches that even the most reliable wind system needs periodic attention to help it

stand up to the rigors of winter at high elevations. The

sensors should be erected on a tower so as to always

clear the snow surface by at least 3 m. The windspeed

at the 3-m level is roughly 90 percent of the speed at

the 10-m level.

Wind sensors on high ridgecrests are exposed to

rime and lightning. Rime can completely cover the

wind sensors, locking them and often causing damage.

One solution is to heat the sensors with infrared

lamps. Lightning also presents a serious problem. The

instruments must be removed from the tower when

threatened by lightning. Circuitry should be connected through lightning arrestors. When not in use,

circuitry should be removed from the tower and electrically grounded.

Wind measurements can be based on either digital

or analog signals. In the simplest digital system, an

impulse is recorded after a predetermined amount of

wind passes the sensor. From the digital data, it is

simple to compute the average windspeed, perhaps on

an hourly basis.

Analog systems give a continuous reading of windspeed and direction. This enables one to judge the

gustiness of the wind, or the difference between the

average windspeed and the peak values. Since the

amount of transported snow depends on some exponential power of the windspeed (perhaps the third

power), large amounts of snow are transported by

gusts. Gust information can be obtained in digital

form, but this requires expensive and complex equipment.

Many gusts originate as vortexes shed by terrain

obstacles; therefore, gustiness, as measured at a sensor, depends somewhat on the position of the sensor

with respect to upstream terrain features that shed

the gusts. For a given wind direction, the flow may be

gusty over one ridge but smooth over a neighboring

ridge that has a slightly different orientation. If the

wind shifts, the situation could be just the opposite.

Hence, for broad answers about wind transport over

a wide variety of terrain, the average windspeed is

probably sufficient.

Because wind direction fluctuates considerably, it

is often difficult to compute average wind direction

from digital charts, unless again one wants to resort

to complex and expensive components. Comparing

the wind-direction data in ñgures 46 and 47 shows

that average wind direction is more easily computed

from analog traces than from digital traces.

30 AVALANCHE METEOROLOGY
Direction Wind speed

10 00

09 00

08 00

07 00

06 00

05 00

04 00

S§gg§K§§ggí5S8 ° 8 s § § g 8

Figure 47.—Analog windspeed and direction record. Between

0800 and 0900, the average windspeed is about 35 mi/h

(16 m/s). Peaks are in the range of 45 to 48 mi/h (20-21

m/s). One advantage of the analog system is the clarity

with which the information is displayed.

When local wind sensors are not operating, avalanche workers may contact the National Weather

Service for information on windspeed and direction

at ridgecrest level. Weather Service information is

based on balloon soundings of the upper air. The

usefulness of this information depends, first, on how

far downstream the avalanche area is from the nearest

balloon-release station; second, on how dense the

network of balloon-release points that surrounds the

area is; and third, on the complexity of the terrain.

Many avalanche-threatened areas in the United States

are well located to use Weather Service wind data.

BRW

SYA SNP

ALASKA

GTF

GGW

LND

;w 7

ü

,VBG

SLC DEN]

UCC GJT *

INW

'TUS

ELP

Figure 48.—National Weather Service upper-air network in

the western United States. Instrumented balloons released

from these stations twice a day send back information on

windspeed, wind direction, temperature, humidity, and

pressure. Additional upper-air soundings are sometimes

taken at local airports and by agencies involved in cloud

seeding and related projects.

Heat exchange at the snow surface

Heat energy is exchanged constantly between the

snowpack and the atmosphere. Generally speaking,

heat energy can enter and leave the snowpack by

molecular or radiative processes. Both processes

occur simultaneously, and together they determine

how the snowpack heats up and cools down near the

surface.

The basic law for molecular transfer of heat energy

between the snow and atmosphere is simple. If the

snow is warmer than the air, the snow loses heat to

the atmosphere; conversely, if the atmosphere is

Heat exchange at the snow surface 31
Atmosphere

Distance

above

snow Air

surface temperature

60cm ^ o

Heat transfer

40cm 5 o

20cm _g„ I

0cm .- Snow surface

1 Snow «

Atmosphere

Distance

above

snow Air

surface temperature

60cm

Heat transfer

40cm _^^„

20cm _^5„ I

0cm ^Q, Snow surface

Snow

Figure 50.—Standard instrument shelter for housing thermometers and other meteorological instruments. The metal

frame and pulley allow the shelter to be raised or lowered

to keep it the proper height above the snow.

Figure 49.—Examples of air-temperature gradients above the

snow surface in calm air. For temperature profile A, the

transfer of heat is into the snowpack; for profile B, the heat

transfer is out of the snowpack.

warmer than the snow surface, heat flows from the

atmosphere to the snow surface. In the absence of

wind, molecular transfer is comparatively slow. With

increasing wind, there is increasing opportunity for

molecular contact due to mixing, and the energy

exchange occurs as much as 10,000 times faster. Like

the snow-transport mechanism discussed in the previous sections, the rate of removal or addition of heat

depends on wind shear against the snow surface. The

greater the shear, the greater the exchange of heat.

The rate of molecular heat exchange also depends

on how much water vapor is being transferred between

atmosphere and snowpack. If the air layer above the

snow is oversaturated with respect to the snow surface,

energy transfer into the snowpack is increased by

vapor deposition of molecules onto the snow surface.

Conversely, if warm, dry air is blown over the snow

surface, water vapor molecules may sublimate from

the snow surface and thus return some heat energy

back to the atmosphere. As snow grains fall on the

snowpack, they either bring heat energy into the pack

or absorb heat, depending on the relative temperatures of the precipitation particles and the snowpack.

Rain can only add heat.

In the above discussion, the concept of air temperature is used rather imprecisely. Air temperature

32 AVALANCHE METEOROLOGY
rTT^wïîS^îiTJ^^rTi^iSTîS^

SOLAR COMPONENTS TERRESTRIAL COMPONENTS Radiation of clouds i,

into space ;

Terrestrial

radiation

Into

atmosphere

Back

radiation

of

atmosphere

Terrestrial

radiation

into

clouds

SNOW SNOW

Absorbed

solar

radiation

Figure 51.—Some of the important components of the radiation balance; incoming solar radiation, absorbed solar radiation,

reflected solar radiation, terrestrial radiation into the atmosphere, radiation of atmosphere toward space, back-radiation of

atmosphere toward earth, terrestrial radiation into cloud, back-radiation of cloud toward earth, and radiation of cloud

toward space.

varies with height above the snow surface; this variation is called the air-temperature gradient. Heat

energy flows so as to minimize the air-temperature

gradient above the snow.

Because temperature varies with height, it is standard practice to measure air temperature about 1.2 m

above the surface. At this level the gradient is reduced.

Air temperature should be measured in a well-ventilated shelter, facing north and constructed according

to National Weather Service specifications (fig. 50).

Determining the temperature gradient just above

the snow requires knowledge of the snow surface

temperature, which is not easily measured. The main

difficulty in determining the surface temperature is due

to radiation—the other important process by which

heat is exchanged at the surface. Radiation is emitted

from the sun (solar radiation) and reradiated from

the earth (terrestrial radiation).

The intensity of the solar radiation that arrives at

the surface depends on the time of day, calendar date,

cloud cover, and aspect of the slope. Much of this

radiation is reflected back into space. Some is absorbed by the snow, adding heat energy. New, dry

snow reflects more than 90 percent of the solar radiation, making less than 10 percent available as heat

energy. (This is still a considerable amount of energy,

especially at midday in spring.) Wet snow can absorb

almost 50 percent of the incoming solar radiation. Absorption takes place almost exclusively in the top

meter of the pack and mostly in the top 20 cm. The

ratio of the amount of solar radiation reflected by a

surface to the amount incident upon it is called the

albedo of that surface. It is usually expressed as a percentage.

The entire surface of the earth reradiates energy

into space. This so-called terrestrial or longwave radiation is not as penetrating as solar radiation, and it is

absorbed by the atmosphere, which in turn radiates

energy back toward the earth as well as into space.

Thus, the net flow of terrestrial radiation is extremely

complex, depending critically on such factors as snow

surface temperature, cloud temperatures, and the humidity and temperature structure of the atmosphere.

If the atmosphere is clear and dry, relatively large

amounts of terrestrial radiation can escape from the

snow surface into space. On the other hand, if a warm

cloud passes over, the back-radiation from the cloud

can counteract the surface emission.

Considering this complex state of afi^airs, it is possible to make only broad generalizations about the combined effects of solar and terrestrial radiation:

Heat exchange at the snow surface 33
'"'''ayga^r-

• During the middle of the day in winter, December through March, there is a small gain of radiant

energy on south-facing slopes due to the excess of

incoming solar radiation over outgoing terrestrial

radiation. On steep, north-facing slopes, there is a

continued loss of terrestrial radiation only partly

offset by small amounts of incoming solar radiation.

• During the middle of the day in spring, there is a

large gain of radiation on south-facing slopes and a

small gain on north-facing slopes.

• During evenings, all slopes radiate terrestrial

energy into space. The amount that escapes depends mostly on humidity and cloud cover.

Using the air temperature and some qualitative information about radiation, it is possible to guess the

trends in the energy balance at the snow surface and

be right more times than wrong. For example, one

may assume the following:

(1) If the air temperature is rising, the snow temperature near the surface is also rising, at least up to

0° C. If the snow is wet and the air temperature rises

above 0° C, then the snow will get wetter. This correspondence between snow- and air-temperature

change is especially true when there is wind.

(2) Similarly, falling air temperatures imply falling

snow surface temperatures.

(3) The important exception to (1) and (2) occurs

in the evening, under clear skies, when terrestrial

radiation loss from the surface is so strong that snow

temperatures fall regardless of air-temperature trends.

The terrestrial loss is even strong enough to compete

with warm winds.

Cold

surfaces

Figure 52.— lemperaUire invei">ion in a müiinlum canyon.

Inversions are the result of nocturnal cooling of the snow

surface by radiation loss.

nings, when the temperature of the snow surface drops

due to terrestrial radiation loss, cold layers of air form

over the snow surface. These cold layers, being

denser and heavier than the warm air, drain down to

the canyon floor. The warmer air that still retains

some of its daytime heat energy is suspended in the

canyon. On the upper slopes, the warm air mixes in

with the cold surface layers, tending to produce

warmer air and snow surface temperatures. This stratification of warm air over cold air is referred to as an

inversion. During early morning hours, when the inversion is fully developed, air temperatures at the

canyon floor may be 15° C colder than at midslope.

(4) During the daylight hours of winter, there may

be some exceptions to (1) and (2), especially when

winds are light. The air temperature may be observed

to rise, but the snow could lose terrestrial radiation

through clear skies, in which case, the snow temperature would be maintained at fairly constant levels.

This exception is more likely on north-facing slopes.

Some interesting examples can be given to illustrate the combined effects of exchange of energy at a

snow surface. Consider a canyon bounded by two

ridges. The terrain is snow covered. On clear eveThe growth of surface hoar affords another interesting example of surface energy exchanges. During

the day, the relatively warm air layer above the snow

may contain an appreciable amount of water vapor.

Although the air is unsaturated at the daytime temperature, it may become saturated with respect to the

snow surface at the colder evening temperature. Thus,

at night the vapor condenses on the surface of the pack

in the form of surface hoar, the solid equivalent of

dew. Surface hoar layers are a few millimeters to

several centimeters thick. They are extremely weak

and cohesionless. Once buried in the snowpack, they

represent serious structural weakness.

34 AVALANCHE METEOROLOGY
Figure 53.—Surface hoar crystals. (Photo by LaChapelle)

Storm analysis

It is an observed fact that the vast majority of avalanches release during or shortly after storm periods.

In this text, the term storm period is used in the broadest sense to include periods of snowfall, snow transport, or both. Each storm is a unique combination of

meteorological variables; the combinations are almost

infinite. As already pointed out, meteorological data

cannot be gathered at every avalanche path of interest.

For practical reasons, wind, precipitation, temperature, etc., are measured at one or two locations. Thus,

storm analysis consists of interpreting available data

in terms of what is happening on the slopes of interest.

Storm data can be recorded and analyzed in many

ways. Each avalanche worker sooner or later develops

a system that best fits his own day-by-day operation.

As a minimum, some analysis must be made of wind

and precipitation measurements. More refined systems also include measurements of temperature, radiation, snow-crystal type, etc. To visualize the interaction of storm variables, it is helpful to plot the data

in graph form. This is called a Jiorw p/o/ (see fig. 54).

From a storm plot, one tries to deduce which slopes

are being loaded with new deposition, the amount of

loading, and the rate of loading. The analysis is

weighted heavily by the observer's experience and

feel for the terrain. Beyond this, only the following

general indications can be mentioned:

Indications of which slopes are being loaded. Windspeed and direction determine which slopes are being

loaded :

WATER EQUIVALENT

OF PRECIPITATION

(mm)

360° r

180°

WIND DIRECTION

(Degrees)

J L. I [II I I I I"

40 WINDSPEED

(Meters/sec)

0700 1300

_24 Jan..

1972

1900 0100 0700

-25 Jan.

1972

1300

Figure 54. Analysis of storm data provides clues as to which

avalanche paths are being loaded, the amount of loading,

and the rate of loading. Further application of storm analysis to stability evaluation is explained in chapter 5.

• In the absence of ridgecrest winds, or for very

light winds, windward and lee slopes have an equal

probability of being loaded.

• For moderate ridgecrest winds, the loading is

almost all on lee slopes or the lee sides of gullies.

• For very strong winds, loading is on the lee side,

but snow tends to accumulate in relatively isolated,

deep pockets rather than being spread over the entire lee slope. In general, the higher the wind, the

more difficult it is to locate these pockets. A field

inspection is essential.

• Glaze, an ice layer resulting from freezing of

supercooled water, tends to be deposited on windward exposures.

• Rain afïects all slopes.

Storm analysis 35
Indications of the amount of loading. The amount

of loading is inferred from ridgecrest winds and precipitation observed at a study plot.

• Heavy loading generally correlates with high

precipitation intensity, as observed at a study plot.

• Regardless of the study-plot precipitation values,

heavy loading correlates with sustained winds if the

snow surface is dry and loose.

Indications of the rate of loading. The rate of loading is inferred from the precipitation intensity and

ridgecrest wind data.

As an illustration, consider the storm plot in figure

54. Based on precipitation, this storm consists of three

phases: an initial period from 0700 to 1600, January

24, during which precipitation intensity (represented

by the slope of the curve of water equivalent) is comparatively low; an intermediate period, from 1600,

January 24, to 0400, January 25, characterized by

relatively high precipitation intensity; and a final

tapering-off period, from 0400 to 1300, January 25,

with a relatively low precipitation intensity. The precipitation data can be summarized as:

Water Precipitation

equivalent intensity

Phase I (9 h) 10 mm 1.1 mm/h (moderate)

Phase II (12 h) 30 mm 2.5 mm/h (intense)

Phase III (9 h) 5 mm 0.6 mm/h (light)

Total 45 mm

During phase I, the wind is light to moderate and

has a definite southerly component. The windspeeds

are probably high enough to transport snow from

windward to lee slopes; hence, loading is presumably

on north-facing slopes. From the precipitation and

wind data, it is reasonable to conclude that moderate

loads are developing, and at moderate rates.

During phase II, the wind direction changes from

south-southwest to northwest. Since windspeeds are

clearly high enough to transport snow, accumulations

are most likely forming on east-facing slopes. Precipitation data suggest that total loads and loading rates

are significantly large.

During phase III, wind and precipitation data indicate that only small loads are developing on southfacing slopes. Also, the north winds of phase III do not

appear strong enough to significantly erode the accumulations from phase I. At the conclusion of this

storm, large depositions should be expected on eastfacing avalanche paths and moderate depositions on

north-facing paths.

Storms also may be analyzed on the basis of temperature trends. There are two broad categories:

standard trends and inverted trends. In the standard

trend, initial storm temperatures are relatively high,

and the temperature falls as the storm progresses. In

the inverted trend, the initial temperatures are relatively cold, and temperatures rise as the storm progresses. The storm plot shown in figure 54 is an

example of the standard trend. For the mountain

ranges of the western United States, the standard trend

usually begins with southwesterly airflows that carry

in warm, moist airmasses. Eventually, the winds shift

to the west and finally to the northwest. In the inverted

trend, the initial surge of moisture is carried in on a

northwesterly airflow. This is followed by a wind shift

to the southwest and consequently warmer and usually

more intense precipitation.

36

AVALANCHE METEOROLOGY
Further reading

Bell, Corydon.

1957. The wonder of snow. 269 p. New York: Hill and

Wong. A nontechnical account of many features of snow

and snow-covered terrain, including avalanches.

Bentley, W. A., and W. J. Humphreys.

1962. Snow crystals. 225 p. New York: Dover Publications,

Inc. Unabridged republication of a classic work first published in 1931. Hundreds of superb photographs of new

snow crystals.

Cantzlaar, G. L.

1964. Your guide to the weather. 242 p. New York: Barnes

and Noble, Inc. A nontechnical introduction to meteorology, emphasizing observations and measurements.

Flohn, Hermann.

1969. Climate and weather. 253 p. New York: World University Library (McGraw-Hill). A well-illustrated introductory treatment of meteorology for the university

student or the general public.

Geiger, R.

1965. The climate near the ground. 611 p. Cambridge,

Mass.: Harvard University Press. A technical but readable discussion of small-scale meteorology, with special

reference to snow cover and mountain terrain.

Judson, A.

1965. The weather and climate of a high mountain pass in

the Colorado Rockies. USDA For. Serv., Res. Pap. RM-^

16, 28 p., illus. Rocky Mt. For. and Range Exp. Stn.,

Fort Collins, Colo. Mountain meteorology of the

Berthoud Pass, Colo., area, with application to avalanche planning. Comparison of upper-air and surface

weather conditions.

Knight, Charles, and Nancy Knight.

1973. Snow crystals. Sei. Am. 288(1) : 100-107. A concise

summary of snow-crystal growth in the atmosphere.

LaChapelle, E. R.

1969. Field guide to snow crystals. 101 p. Seattle, Wash.:

University of Washington Press. A discussion of the

formation, structure, and metamorphism of solid precipitation. Includes the International Classification System

and the more detailed system of Magono and Lee. Many

photographs.

Lehr, P. E., R. W. Burnett, and H. S. Zim.

1957. Weather. 160 p. New York: Golden Press. One of

the Golden Nature Guides. Written in an elementary

form, but contains a wealth of general weather information.

Magono, Choji, and Chung Woo Lee.

1966. Meteorological classification of natural snow crystals. J. Fac. Sei., Hokkaido Univ. (Sapporo, Japan).

Ser. VII, 11(4) :321-335. A detailed, well-illustrated presentation of the various types of new snow crystals.

Mellor, Malcolm.

1965. Blowing snow. Cold Reg. Sei. and Eng., Part III-A3c,

79 p. U. S. Army Corps Eng., Cold Reg. Res. and Eng.

Lab., Hanover, N.H. A technical discussion of blowing

snow, with application to snow drifting around structures and on highways. Explanation of the mechanisms

of transport, with emphasis on turbulent suspension.

Middleton, W. E. K., and A. F. Spilhaus.

1953. Meteorological instruments. 286 p. Toronto, Canada:

University of Toronto Press. A summary of standard

devices used for measuring wind, temperature, precipitation, humidity, and radiation.

Nakaya, Ukichiro.

1954. Snow crystals: natural and artificial. 504 p. Cambridge, Mass.: Harvard University Press. A well-written

account of exhaustive laboratory and field studies of new

snow crystal types, carried out in Japan. The basis for

a detailed classification of new snow crystals. Many outstanding photographs of natural and artificial new snow

crystals.

Peterssen, Sverre.

1969. Introduction to meteorology. 333 p. New York:

McGraw-Hill Book Co., Inc. An up-to-date and comprehensive introduction to large-scale meteorology. Well illustrated. Mostly descriptive text with some mathematics.

U.S. Department of Agriculture, Forest Service.

1969. Fire weather. U.S. Dep. Agrie, Agrie. Handb. 360,

229 p. Washington, D.C.: U.S. Gov. Printing Office. An

introduction to large- and small-scale meteorology, with

application to forest-fire control.

U.S. Department of Agriculture, Forest Service.

1973. Instrumentation for snow, weather, and avalanche

observations. 80 p. revised. Rocky Mt. For. and Range

Exp. Stn., Fort Collins, Colo. A complete guide to the

instruments recommended for avalanche observers in

the Forest Service mountain weather and avalanche observing network.

Further reading 37
ri^

.^
Chapter 3

The mountain

snowpack

As soon as a snow crystal falls onto the snowpack

it fastens to its neighbors and becomes part of

an interconnected skeleton of ice grains. The ice

grains are quite free to move into available voids

or pores; therefore, the ice skeleton constantly

deforms in response to its own weight. In addition,

the shape of each ice grain changes continually

until eventually the grain loses all resemblance to

the original snow crystal. This chapter discusses

these processes and how they influence the

snowpack. The emphasis is on broad characteristics

that are fundamental to stability evaluation and

control of avalanche slopes.

The intricate changes that take place within the snowpack,

depicted by the three snow crystal photos, are often not obvious at the

snow surface. (Photos by LaChapelle and Keiner)
TEMPERATURE, C

0° -5° -10°

Figure 55.—Example of the temperature variation in a mountain snowpacic. The topmost layer is characterized by temperatures

that change throughout the day in response to the energy exchange at the snow surface. During most of the winter, temperature gradients exist throughout the snowpack. A typical strong temperature gradient is 0.2° C/cm to 0.3° C/cm.

Structure of the snowpack

The snowpack is bounded on top by the atmosphere

and underneath by the ground. The top boundary is

called simply the snow surface; the bottom boundary,

the ground surface. The snow surface is a moving

boundary, fluctuating up and down in response to

deposition, settlement, and loss of mass due to

melting, evaporation, erosion, and sublimation. As

explained in chapter 2, the heat exchange across the

snow surface may be directed into or out of the pack,

depending on conditions. At the ground surface, heat

generally flows in one direction—into the snowpack

from the ground. The heat is supplied mostly from

the energy stored in the surface soil layers during

summer. In temperate zones, the ground surface is

maintained throughout the winter very close to the

melting point, 0° C. In northern zones and at high

alpine locations, the ground surface may be colder

than 0° C. Heat can also be supplied to the snowpack

from tree trunks, rocks, and other objects extending

up through the snow.

Every snowpack, therefore, contains some temperature variation. The variation in temperature with

distance, measured up or down, is the snow-temperature gradient. Assuming the ground temperature to

be fixed at 0° C, the average temperature gradient

through the snowpack is determined by the thickness

of the pack and the mean snow surface temperature.

If the snowpack is thin and the snow surface temperature is well below 0° C, then the temperature

gradient is relatively large. Such conditions could

exist, for example, early in winter on shaded slopes.

In the spring, when the snowpack finally warms up

to the meltpoint, temperature gradients vanish, and

the snowpack is isothermal (the same temperature

throughout).

A snowpack consists of many separate layers, some

relatively thick, others microscopically thin. The difference between layers can range from well defined

to practically indistinguishable. Thick layers are

deposited by steady and consistent snowfalls, by wind

drift, or by prolonged periods of similar weather that

wipe out the original differences in layers near the

surface. Typically, thin layers consist of snow crystals

deposited during special storm conditions, such as a

thin graupel layer laid down during the passage of a

front. Distinct thin layers also form at the snow sur40 THE MOUNTAIN SNOWPACK
face between storms; for example, ice crusts formed

by melting and refreezing, surface hoar, and wind

crusts.

The structural details of a snow layer are rather

difficult to examine in the field. However, it is possible

to prepare thin sections of snow layers in a laboratory

and to examine them under a microscope. When this

is done, it is found that the texture of the layer consists

of a complex, randomly arranged skeleton of ice and

much vacant space. The vacant space is called pore

space.

Pores contain a mixture of air and water vapor. If

the snow layer is warmed, liquid water may also be

present. The water vapor in the pore space is maintained very close to the saturation vapor pressure over

a flat ice surface. In most seasonal snow layers, the

pore spaces are interconnected and are described as

communicating with one another. For densities typical

of glacier ice ( 800 kg/m ' and higher), the pore spaces

are isolated, or noncommunicating. In communicating

pore spaces the mixture of air and vapor is free to circulate and hence to readily transfer water molecules

from one part of the ice skeleton to another.

Figure 57 shows details of snow texture in a

medium-aged layer. The bulkier units of the ice skeleton are called grains.^ Grains are interconnected by

relatively narrow links called necks. The thicker the

necks, the stronger the ice skeleton, all other properties being the same. The ice skeleton consists of convexities (regions of positive curvature) and concavities (regions of negative curvature). It will be seen

that these differences in curvature determine, in part,

the future evolution of the snow layer.

Density is one of the most fundamental and easily

measured properties of a snow layer. Some thin layers,

such as ice crusts, may have relatively high densities

(above 500 kg/m-*), but most of the thicker layers in

a seasonal pack have average densities in the range of

100 to 500 kg/m-\ depending on the age of the layer

and its depth below the surface.

Compared to other natural and artificial solids,

snow layers have the unique ability to sustain large, irreversible density changes. A layer deposited with an

original density of 100 kg/m'' may densify to 400

kg/m'' in the course of the winter. Higher densities are

the result of metamorphism and the weight of the overiWhen referring to particles of the snowpack, the term

i;rain is used in preference to the term cryslat, which has a

more specific meaning relating to the arrangement of atoms

and molecules of the material. As used here, the term i;rain

refers to the general appearance of the ice particle when observed with the naked eye or with a simple hand lens.

Figure 56.—Microscopic photographs of thin sections of snow

and ice layers: Top, newly fallen snow a few hours old;

middle, old, dense snow; and bottom, thin ice crust. Note

the random arrangement of the ice skeleton and the general decrease in pore space with age.

Structure of the snowpack 41
Figure 57.—Illustration of the microscopic detail of the ice skeleton-pore structure. Typical regions of positive curvature,

convexities, are designated +; regions of negative curvature, or concavities, are shown as —.

lying snowpack. Large density changes can occur in a

relatively small time; a 100-kg/m^ layer may be compressed to 150 kg/m^ in a day by the weight of that

day's snowfall. However, as the density of the layer increases, so does its resistance to further densification.

Evidence of densification is observed in a gradual

settling of the snow surface. Settlement occurs at rates

on the order of 1 cm/day and depends on snow cover

depth and crystal types. Higher rates are observed

during or immediately after heavy snowfalls. It is not

uncommon for settlement to be on the order of

10 cm/day when new snow is being deposited at the

rate of about 30 cm/day. Then, most of the settlement

takes place in the top layers. During light snowfalls,

the settlement rate of the older snow may exceed the

new snowfall rate, in which case the total depth of the

pack decreases, in spite of the incoming snow.

The high compressibility of low-density snow is

largely due to the flexibility of the ice skeleton. When

load is applied, relatively high forces concentrate at

the necks and other narrow regions of the skeleton.

The necks are easily distorted by the concentrated

forces, and the grains are free to move into new positions, filling up vacant pore space. The necks then

re-form. This type of deformation is irreversible; once

the ice skeleton has been rearranged with new necks, it

cannot spring back to its original shape.

Types of metamorphism

Unlike most materials encountered in nature, snow

exists quite close to its melting point. When any solid

is near its melting point, its molecules have a great

amount of mobility and it changes quickly in response

to changes in external conditions. Newly fallen snow

is one of the most unstable natural substances on

earth.

The instability of snow is revealed by the drastic

changes or metamorphism of crystal form that begin

as soon as the snow is deposited. Metamorphism is a

term borrowed from the geological sciences. In snow

hydrology it refers to changes in snow texture caused

by pressure and temperature conditions. The temperature of the layer determines the rate of metamorphism,

and the temperature gradient across the layer largely

determines the type of metamorphism. Within a short

time after deposition, a few days at the most, it is

normally impossible to identify the original form of

the deposition. Within about 2 weeks a deposited

crystal may shrink to a small ice grain of only onefourth its original size or, under a different set of

conditions, grow to a grain of four times the original

size.

In a dry, seasonal snowpack, it is convenient to

distinguish two types of metamorphism. If the metamorphism is in response to a strong temperature gra42 THE MOUNTAIN SNOWPACK
^^^^^^^^p^^^p^P®^?^5i¿g^ppp|^^^

¿ä:^.

Weight of above layers |

Figure 58.—Because of the mobility of the grains, snow layers can undergo large compressive deformations. The driving force

for deformation is the weight of overlying layers.

dient, the process is called TG metamorphism {temperature-gradient metamorphism). If the process is

not driven by the temperature gradient, but instead

results from the tendency of the snow to "simplify" its

form and knit into a tighter, stronger texture, the

process is called ET metamorphism (equitemperatiire

metamorphism ).

A third type of metamorphism takes place when

snowpack temperatures reach 0° C and surface snow

layers undergo frequent melt-freeze cycles. Under

these conditions, the smaller ice grains in the surface

layers melt during the day, and melt water refreezes

at night. Repeated freeze-thaw cycles produce large,

rounded clusters of ice grains by a process called MF

metamorphism {melt-freeze metamorphism).

Equitemperature metamorphism

To understand ET metamorphism, consider again

saturation vapor pressure over an ice grain. When this

subject was introduced in chapter 2 under "Snow

Crystals," the effects of surface curvature were not

explained. The curvature of the ice surface influences

^

Ice grain

\ [convex

Concave / High vapor

Low vapor .^^^^^ pressure

pressure ■■•'''''^'^^^r

\ /^'.^—-***^

i ^ H2O transfer

Ice grain |

1

/

/

\

1

J^

Figure 59.—Vapor pressure is higher over a convex ice surface than over a concave ice surface. Hence, water molecules tend to be transferred from convexities to concavities.

This mechanism has the effect of removing sharp curvatures in the ice skeleton and causes individual grains to assume simpler, more rounded configurations.

Equitemperature metamorphism 43
^^99^^^^^^^^^!^

'éU:'^ - V life ^

i;2è< .^ V

-wfê'lê^^^i'iao^v^

Figure 60.—Examples of grains in succeeding states of ET metamorphism. Grains are evolving toward rounded forms.

(Photos by LaChapelle)

Ice grain

\

Figure 61.—The weight of the snowpack presses grains into contact. Sharp concave regions are thus formed between grains.

Water molecules migrate into these concavities from convex regions. This process, which joins individual grains together

in an ice skeleton, is called sintering.

the amount of water vapor that can be supported in

the air above the surface. More vapor can be supported over a convex surface than over a flat surface,

and more can be supported over a flat surface than

over a concave one. Thus, there is a vapor-pressure

difference between convex and concave surfaces and

consequently a net transfer of water molecules from

convexities, such as crystal corners and dendritic

branches, to concavities.

ET metamorphism, the result of this preferential

transfer of water molecules, causes two important

changes in snow texture: ( 1 ) a general rounding out

of grains, and (2) strengthening of the ice skeleton by

the formation of necks between grains. The rate of

these changes increases with increasing temperature,

being very rapid near 0° C and almost nonexistent

at -40° C.

44 THE MOUNTAIN SNOWPACK
Consider first the general rounding out of grains.

In the beginning stages of ET metamorphism, each

original grain loses its fine details. The basic shape of

the grain can still be recognized, but sharp corners and

fine branches disappear. The apparent size of the grain

begins to decrease. Beginning stages require from a

few hours at warm temperatures to a few days at cold

temperatures. In the intermediate stage of ET metamorphism, the original shape can no longer be identified. The grains continue to become more rounded,

and large grains tend to grow at the expense of smaller

grains. In the final stages of ET metamorphism, there

is a strong tendency toward uniform, rounded, wellbonded grains. Almost all crystal types are strongly

affected by ET metamorphism. An exception is graupel. This snow grain type, which is rounded to begin

with, resists metamorphism so much that its original

shape is easily identified several weeks after deposition.

The second important texture change caused by ET

metamorphism is strengthening of the ice skeleton.

When two grains make contact, a highly concave

region is formed between the grains. There is a strong

tendency for water molecules to migrate to this region.

The inñux of mass fills up the concavity, forming a

neck between the grains. This joining by forming necks

is sintering. Since the neck material migrates from

convex regions, sintering and rounding of grains occur

together.

It is thought that the migration of molecules to the

contact point and their later deposition in these concavities is mostly by sublimation. This is a process

whereby a substance goes from the solid state (ice)

to the vapor state (water vapor), or vice versa, without going through the liquid state (water). Some additional migration occurs within the ice skeleton and on

the surface of the ice skeleton by poorly understood

processes. In any case, ET metamorphism takes

place in the absence of an appreciable temperature

gradient.

Temperature-gradient

metamorphism

It was mentioned in chapter 2 that the vapor pressure over an ice surface depends on the temperature;

the higher the temperature, the higher the vapor

pressure. This principle, which explains snow crystal

growth in the atmosphere, also explains how ice grains

grow in the snowpack. The fact that vapor pressure

depends on temperature means that more vapor can

be supported in a warm pore space than in a cold

Low

temperature

HEAT

FLUX

High

temperature

Figure 62.—Water vapor flows in the ice skeleton from regions of high temperature to regions of low temperature.

Temperature-gradient metamorphism 45
pore space. This implies that if there is a temperature

gradient in the snowpack, vapor flows from high- to

low-temperature regions of the snowpack. As the

vapor circulates it carries heat energy from warm

pores to colder pores.

Vapor now is not fully understood. It is thought that

vapor diffuses from the warmer grains across the pore

spaces to neighboring colder grains. Another idea is

that "channels" of vapor flow are established. Once

these channels are primed, flow proceeds rapidly.

Only small quantities of vapor are required to transport heat in this manner because of the high energy of

sublimation and vapor deposition (about 600 calories are released for every gram of water vapor that is

converted to ice). In any case, because the vapor flow

is driven by the temperature gradient, the resulting

change in the ice skeleton is called TG metamorphism.

For heat energy to be transferred efficiently, vapor

is deposited on the grains instead of on the necks

between grains. Hence, in TG metamorphism the

individual grains enlarge while the neck thickness

remains essentially constant. This is in direct contrast

with ET metamorphism, in which neck regions grow

at the expense of grain convexities.

The enlarged grains that result from TG metamorphism are called TG grains. They are characterized by flat, sometimes steplike or striated faces. The

faces intersect, forming sharp, angular corners that

give the grains a coarse texture as compared with the

rounded, smooth grains of ET metamorphism. In the

advanced stages of TG metamorphism, TG grains

enlarge to about 8 mm in diameter and have very

distinct faces and corners. These large TG grains are

called depth hoar.

The larger TG grains are easily identified with the

naked eye. However, it is instructive to view these

grains with 10- to 25-power magnification and note

how much they differ from ET grains. Under magnification, depth hoar grains appear as assorted prisms,

cups, and pyramids.

TG metamorphism and ET metamorphism occur in

all snow layers, but one process usually dominates at

a given time in a given layer. Because grains of newly

fallen snow contain sharply curved convex and concave regions, there is an initial tendency to simplify

the shape of the newly fallen snow by mass transfer

from convexity to concavity. At first, therefore, metamorphism of a layer is ET-controlled. After the sharp

COLD

HEAT

FLUX

HjO vapor

HOT

HjO vapor

H2O vapor

Figure 63.—In temperature gradient (TG) metamorphism water vapor deposits on the grains instead of on the neck between

the grains. This leads to an increase in grain size and an overall decrease in the strength of the ice skeleton.

46 THE MOUNTAIN SNOWPACK
V«

J

Figure 64.—Top left, ET grains, top right, intermediate stage

of TG grains, and bottom right, advanced stage of TG

grains.

branches and corners have been removed by ET metamorphism and the crystal has turned into a smaller

and more rounded grain, ET and TG processes compete for control. If the temperature gradient across the

layer is strong enough, the TG process dominates. As

winter progresses and the snowpack becomes thicker,

temperature gradients become minimal, and again ET

processes tend to dominate. At the end of the season,

when the snowpack becomes isothermal and temperature gradients vanish completely, TG metamorphism is no longer possible, and ET metamorphism

competes with melt-freeze processes for control.

Clearly, the properties of a snow layer depend

strongly on whether the ice skeleton has ET or TG

texture. If the ice skeleton is predominantly ETmetamorphosed, the grains are relatively small and

well joined; if TG metamorphism dominates, the

grains are large and not well joined and have many

fewer bonds. It is easy to see that the ET structure is

stronger than the TG structure if other variables

(such as density and temperature) are equivalent.

The strength of snow and its relation to metamorphism are discussed more fully later in this chapter

under "Fracture Mechanics of Snow."

The temperature difference across the layer is

fundamental to determining whether ET or TG metamorphism dominates. However, another important

factor is snow density. In low-density snow, vapor

flow is much easier, and for the same temperature

conditions and time the TG process can go faster

and produce larger grains. As density increases, the

process slows.

Depending on the duration and intensity of the

temperature gradient, the advanced types of TG grains

Temperature-gradient metamorphism 47
Figure 65.—TG layers are most commonly found just above the ground surface. Occasionally, thin TG layers form above, or

less commonly below, ice crusts. TG grains also are likely to be found near terrain irregularities, such as rocks and vegetation.

appear about a week to a month after the TG process

begins. Depth hoar normally requires at least 2 weeks

to form from newly fallen crystals or ET grains, but

there are reports of depth hoar layers that formed

within a week.

The strongest, most sustained temperature gradients are on north-facing slopes and in deep, shaded

gullies. In such places, the snow surface is kept at

relatively low temperatures by continuous radiation

loss. TG layers occasionally form in areas that are not

protected from solar radiation, and sometimes even

on south-facing slopes, but the more protected the

slope from the sun's rays, the greater the chance of

finding a TG layer.

The probability of forming a TG layer is highest

early in the winter when the snowpack is thin and

unconsolidated. The climatological combination that

normally leads to TG layers is an early storm, say in

October or November, followed by a month or so of

cold, clear weather. Such conditions occur frequently

in the inland mountain ranges of the United States,

such as the Rockies and the Wasatch and Teton

Ranges, but less frequently in the Pacific Coastal

Range, which usually has an almost continuous storm

cycle once winter begins. Advanced TG metamorphism is rare in maritime climates.

In continental climates, TG layers generally are

found in the first 30 cm above the ground, but layers

more than 1 m thick have been observed in the

Colorado Rockies. Advanced TG metamorphism

often occurs next to trees and between rocks, presumably because surface irregularities support the

snow, preventing densification and providing sites for

vapor circulation. Thin TG layers sometimes form in

the upper and middle layers of the pack. These layers

are often difficult to locate, but they often form immediately above or below ice or sun crusts, especially

when such layers are at 0° C.

TG layers are more prevalent at higher elevations

for many reasons. First, the snow surface is colder at

higher elevations; hence temperature gradients are

stronger. Second, strong winds at high elevations tend

to erode the snow and thin out the snowpack in exposed places. Finally, avalanches remove snow from

high elevations, and temperature gradients can arise

in the remaining snow. Considering these complications, one cannot be assured of conditions in the higher

elevation snowpack if observations are made only at

a low-elevation study plot. A field inspection of the

high elevations is essential. The necessary techniques

for such field inspections are discussed later in this

chapter under "Snowpack Analysis" and in chapter 5.

48 THE MOUNTAIN SNOWPACK
Melt-freeze metamorphism

Liquid water present in a snow layer is called free

water. Free water is usually measured in percent by

mass. Very wet snow may contain 15 percent or more

free water; for example, 150 g of water in 1 kg of

snow.

At present, devices that measure free water are

relatively complex and are not used routinely in

avalanche control and forecasting. It generally suffices

for avalanche observers to note qualitatively the "wetness" of the snow according to the nomenclature of the

International Association of Scientific Hydrology:

dry, moist, wet, very wet, and slush (see table 1 ).

The two sources of free water are rainfall and melting. If the temperature of the top layer of the pack is

below 0° C, then light rainfall is likely to freeze onto

the snow surface as a thin ice layer. If the top layer is

raised to 0° C by sustained heavy rain or by warm

weather, rainfall is absorbed by the pack as free water.

It should be emphasized that rain is important as a

source of free water, but not as a melting agent. In

winter, rain is rarely warm enough to cause significant

melting, since Ig of water contains 1 calorie for each

degree above 0° C, and it takes 80 calories to melt I g

of snow at 0° C.

The main cause of melting is heat input at the snow

surface. The convective and radiative components of

the heat exchange both contribute to melting, evaporation, and sublimation, but radiation is usually the

most important cause of melting and free-water production.

Radiation produces free water at an accelerating

pace. When snow is dry and clean, it absorbs little

solar radiation. When wet, it absorbs much more radiation, and melting quickens. The process accelerates

to a peak by midafternoon, usually about 1300 to

1600 hours. The largest amounts of free water are

TABLE 1.—Free water contení

Term Remarks

Dry Usually T is below 0° C, but dry snow can occur at

any temperature up to and including 0° C. When

its texture is broken down by crushing and the

loose grains are lightly pressed together as in making a snowball, the grains have little tendency to

cling to each other.

Moist T = 0° C. The water is not visible even with the

aid of a magnifying glass. When lightly crushed, the

snow has a distinct tendency to stick together.

Wet T = 0° C. The water can be recognized by its meniscus between adjacent snow grains, but water cannot be pressed out by moderately squeezing the

snow in the hands.

Very T = 0° C. The water can be pressed out by moderwet ately squeezing the snow in the hands, but there still

is an appreciable amount of air confined in the snow.

Slush T = 0° C. Snow flooded with water; contains a relatively small amount of air.

Z

O

o

<

ce

Oui ^^

^5

O^

LU

>

LU

ir

Incoming

solar radiation

Free water

production

0600 1200 1800

TIME

Figure 66.—Comparison of typical daily variation of solar radiation and free water production. The largest amount of free water

is produced between noon and late afternoon.

Melt-freeze metamorphism 49
Figure 67.—In one mode of flow, free water percolates

through the pore space, under the influence of capillarity

and gravity.

produced next to rocks and other terrain features that

absorb radiation efficiently and can transfer heat to

the snow. Large amounts of free water concentrate in

gullies and form streams under the snowpack.

Because of surface tension, small amounts of free

water cling to melting grains. As melting accelerates,

free water begins to flow into the pack. The rate of flow

depends on the texture of the snow, the temperature

of the ice skeleton, and the amount of water available.

Although some water flows by capillary motion

through the pore space, it is thought that a large

amount flows in special channels melted by heat stored

in the free water. In any case, free water flows down

until it either freezes on contact with a cold layer or is

blocked by a hard layer. The water tends to spread out

over a hard layer until a channel is eroded or melted.

Free water flows very slowly, and the flow is referred

to as percolation. When free water percolates into a

.j^

r^<^

Figure 68.—Comparison of ET grains (top) and MF grains

(bottom). Note that the MF grains are large clusters of

smaller, angular grains.

50 THE MOUNTAIN SNOWPACK
Water channels

Hard layer

i

Hard layer iwafi,. n iim-mimmimifmrr^~':^'-' imiJ •

Figure 69.—Free water penetrating the snowpack tends to spread out on top of hard layers and melts or erodes the attachment

between the hard layer and the layer above. This effect is known as "lubrication."

cold region of the pack, the water often freezes to form

an ice lens.

In spring, the entire snowpack achieves a temperature very close to 0° C. The snow temperature at the

surface may vary widely between day and night extremes, however. These fluctuations originate at the

surface, but penetrate deeper into the pack as a wavelike disturbance of the average temperature. Where

temperature fluctuations cause melting and rcfreezing,

a special type of metamorphism occurs. This meltjreeze (MF) metamorphism builds clusters of large

grains in the following way. During melting of the ice

skeleton, the smaller bonds and grains have a very

slightly lower melting temperature than the larger

bonds and grains. The difference in melting temperature is exceedingly minute, perhaps on the order of

10""'° C. Nevertheless, this tiny difference causes

small grains to melt first, before larger grains begin to

melt. Some melt water is retained between neighboring

grains; it tends to bond the remaining grains together

by surface tension. When the temperature fluctuates

back to freezing, the bond freezes and locks the surviving large grains into polygranular units. It appears that

there is also vapor transport from the smaller grains

to the larger grains, similar to TG metamorphism in

dry snow; this produces many large grains.

After several melt-freeze cycles, it is possible for

each polygranular unit to consist of 50 or more large,

coarse grains. Depending on the number of cycles, the

time of day, the depth of the layer, etc., the polygranular units may form into any of a variety of wellknown forms of spring snow, such as "corn snow" or

"rotten snow." The large polygranular forms that result from repeated melt-freeze cycles (MF metamorphism) are called MF grains.

The strength of a layer of MF grains varies widely,

depending on whether the layer is in the melt or the

freeze part of the cycle. In the melt part, the grains

are essentially separate and are held together only by

surface tension. The structure is then extremely weak

and ductile. When the layer refreezes, it can assume

enormous strength. A case of interest occurs when a

cold snow layer of early winter is drenched by rain and

then freezes. In such a layer, enlargement of grains

by MF metamorphism is at a minimum, and the layer

freezes into a very strong structure.

The great changes in strength that result from melting and freezing have important implications in avalanche stability evaluation (see chapter 5). A prime

cause of spring avalanches is lubrication of the sliding

surface. This is the result of free water percolating

Melt-freeze metamorphism 51
Lubricating flow

Ice crust —

Figure 70.—Intensive lubrication of an ice layer in an inclined snowpack. Lubrication is also commonly observed above the

ground surface.

down to a hard suiface such as an ice layer or the

ground. If the water cannot penetrate the surface, it

tends to spread out and melt or erode the bond between this surface and the snow above.

Fracture mechanics of snow

The properties of steel, wood, ceramics, and other

common materials have been worked out in the laboratory and in theory to an accuracy that permits confident engineering design. Snow has also been studied

as a material. As would be expected, the higher its

density, the more it behaves like other materials. In

fact, many studies have been made of the suitability

of high-density snow (more than 400 kg/m') as a

construction material in polar regions.

Some studies have also been made of the lower

density snow typical of alpine, seasonal snowpacks.

It is fair to conclude from these tests that low-density

snow is a complex and unique material. Unfortunately,

its properties are so complex that it is hardly possible

to predict confidently how it will behave on the mountainside. On the other hand, although many properties

of low-density snow cannot be identified accurately,

laboratory tests provide at least a broad picture of its

behavior.

In the laboratory, when a compressive force is

applied to a snow sample, the sample deforms considerably. The amount of deformation depends on the

intensity of the force and the rate at which the force

is applied. When the force is removed suddenly, the

sample springs back slightly, but a large amount of

permanent deformation remains in the sample. The

important point is that in the deformation of snow under load there are both a springy or elastic component

and a permanent deformation component. The elastic

component represents energy that is stored and recovered, and the permanent deformation component

represents energy that is unrecoverable or has been

dissipated.

The recoverable component of the deformation is a

consequence of the springiness or elastic nature of the

grains and necks in the ice skeleton. The unrecoverable energy is lost in the sliding of grains and necks.

Some energy is also lost through viscouslike flow of

the skeleton. Many other substances, which are classified as viscoelastic, have recoverable and unrecoverable components. Almost all solids have a viscoelastic

response when deformed at temperatures near their

melting point.

As mentioned earlier, the ice skeleton is highly

compressible and can sustain large amounts of per52 THE MOUNTAIN SNOWPACK
FORCE

Figure 71.—A snow sample is compressed. When the force is suddenly removed, most of the deformation appears to be

permanent or unrecoverable (U). However, there is a slight tendency for the sample to spring back or recover (R). This

indicates that elastic energy is stored in the sample and available to propagate fractures.

TENSION COMPRESSION SHEAR

Figure 72.—Snow in three modes of brittle failure.

manent deformation. If the load is applied slowly

enough that deformation is slow, then, in principle,

the sample can be compressed to a mass of solid ice.

If the applied force is strong enough and pushes fast

enough on the sample, then the snow cannot respond

by gradual collapse, and brittle fracture occurs.

Fracture mechanics of snow 53
I ,0

5 m"

O 200 400 600 800

DENSITY, Kg/m'

Figure 73.—Shear strength envelopes of small snow samples

as a function of snow density (Mellor 1974).

What causes brittle fracture in snow? The answer,

which is of deep significance in avalanche stability

evaluation, is that any snowmass, depending on its

temperature and texture, has two limitations: (1) it

can store only a limited amount of elastic strain energy, and (2) it can dissipate the extra energy at a

limited rate.

During brittle fracture propagation, the energy of

the system is quickly redistributed into fracture surfaces, kinetic motion, and heat. Fractures initiate at

flaws or localized regions of stress concentration.

Because of the random nature of the ice skeleton, any

snowmass is bound to have a weakest site where fractures are most likely to originate. In the mountain

snowpack, trees, rocks, ski tracks, etc. are regions of

stress concentrations and likely spots for fracture

initiation.

The ability of a material to withstand catastrophic

failure is called its fracture toughness. This is difficult

to measure and not necessarily related to the apparent

strength or hardness of snow. Hard, strong snow

Relatively

short

levers

TG ET

Figure 74.—Due to lever action, applied force is redistributed as high stress concentration at the neck between grains. The

larger the grains compared to the neck cross section, the greater this effect.

54 THE MOUNTAIN SNOWPACK
layers are as apt to propagate fractures as weak, soft

snow.

From laboratory and field tests, some qualitative

information has been assembled concerning the fracture of snow. Like all materials, snow can fracture

when loaded in tension, compression, or shear. Furthermore, like most materials, snow has substantial

resistance to fracture in compression but is easily fractured in tension. The usual explanation for the different behavior in compression and tension is that in

compression, the potential fracture surfaces are

pushed together so that the material gains strength

from friction. Also, in the case of snow, forcing the

grains together by compression allows the material

to gain strength by sintering.

The fracture toughness of snow is a strong function

of density; polar snow with a density of 800 kg/m^

may be more than 1,000 times as strong as alpine

snow with density of 200 kg/m'' (see figure 73). At

the lower densities typical of the mountain snowpack,

snow strength varies greatly depending on grain texture. For example, fine-grained ET layers may have

10 times the shear strength of large-grained TG layers

of the same density. As a general rule, the larger the

grains, the weaker the snow, all other factors remaining the same. One explanation for this dependence on

grain size is shown in figure 74.

How does temperature affect the fracture toughness of snow? Intuitively, one suspects that the colder

the snow, the more brittle its behavior. On the other

hand, laboratory tests indicate that snow gains

strength as its temperature decreases. At present, the

overall effects of temperature on the fracture toughness of snow are largely unknown. Certainly, snow

exhibits brittle fractures at all temperatures up to

0° C, and sometimes even when an appreciable

amount of free water is present.

The inclined snowpack

To understand failure of inclined snowpacks, it is

helpful to study first the simpler case of horizontal

snowpacks. Consider the idealized case of a long, horizontal snowpack, as shown in figure 75. For this

simple case, it is evident that the deformation of the

snowpack is downward and that each element of the

snowpack is subject to a compressive stress component in the vertical direction, caused by the weight of

Direction of gravity force

and deformation

(settlement)

Horizontal snowpack Inclined snowpack

Figure 75.—Left, in a horizontal snowpaclc, the gravity force and deformation (settlement) are both directed perpendicular to

the ground. Right, in an inclined snowpack, the shear component of the gravity force produces a downslope component of

deformation.

The inclined snowpack 55
Compression

CO',0^ e->^^:\M

Tension

B

Anchor

region

Figure 76.—Possible sources of tensile stress are: A, a weak substratum; B, slope curvature; C, an inhomogeneity, such as a

snow cushion; or D. anchoring at the top boundary.

the snow above. The compressive stress at any level

is easily calculated as the weight or force per unit

area exerted by the column of snow above that level.

It is also evident that there is no component of shear

stress acting in the horizontal snowpack.

What happens when the horizontal pack is tilted at

angles typical of avalanche paths (30° to 50°)? To

analyze this situation, it is convenient to resolve the

gravity force into compressive and shear components

as shown in figure 75. It is then possible to compute

the approximate compressive and shear stresses (in

N/m-) along any surface parallel to the snow surface as:

Compressive stress = p D g cos 6

Shear stress = p D g sin 9

where 9 is the slope angle, D is the depth in meters

beneath the snow surface (measured perpendicular

to the snow surface), p is the average density of the

snow (in kg/m'*) between the snow surface and depth

D, and g is the acceleration due to gravity.^

In the simplest model, failure occurs because the

shear stress exceeds the shear strength at a certain

depth. However, another important factor to consider

is tensile stress. Mathematical analysis shows that significant tensile stress can develop in an inclined snowpack through:

Substratum weakness. If an inclined snowpack rests

on a locally weak substratum that cannot support its

weight, then, in order for the snowpack to be in equilibrium, a tensile stress must develop in the snowpack

upslope from the weak substratum, and a compressive

stress must develop downslope. The redistribution of

stress due to the weak substratum is shown in part A

-Taking g as 9.8 m/s-, stresses are in newtons per square

meter (N/m-).

56 THE MOUNTAIN SNOWPACK
^7>^

X

* J/A

^ i««»

Figure 77.—The complex motion of an inclined snowpuck is

the total of creep and glide components. Glide is the slip

of the snowpack with respect to the ground. Creep is the

internal deformation of the snowpack.

of figure 76. The weak substratum may be a layer of

TG grains, graupel, surface hoar, or any weak, cohesionless snow.

Localized curvature. Regions of tension and compression can develop wherever the inclined snowpack

is curved. Tension regions develop over convex terrain features, and compression regions develop over

concave terrain features (fig. 76, part B).

5«ow cushions. Another source of localized curvature is the nonuniform thickness of a snowpack deposited by strong winds. The pack may be shaped like

a pillow or cushion, and hence is called a snow

cushion. As shown in part C of figure 76, a snow cushion has several regions of comparatively high stress.

Anchoring or clamping effects. The ends of snowpacks are often wedge shaped, as shown in part D of

figure 76. The top of the wedge tends to be anchored

to the terrain. The snowpack pulls on the anchoring

point, and a region of tension develops below the

anchor. Trees, rocks, and other protrusions may also

function as anchoring points and induce high local

stresses.

Figure 78.—Dramatic evidence of glide on a grass-covered

mountain slope. (Photo by Frutiger)

The deformation of an inclined snowpack is rather

complicated. In response to compressive stress, the

inclined layer settles and densifies, and hence strengthens with time. The rate of settlement depends on the

amount of compressive stress and on the snow temperature; the higher the temperature, the faster the

settlement. In response to the shear stress, the snowpack glides and creeps downslope as shown in figure

77. Glide is slippage of the snow layer with respect to

the ground. Creep is often defined as the internal

deformation of the snowpack (determined by subtracting glide from total motion ). The creep and glide pressures of the snow must be taken into account in designing ski lift towers and other structures on slopes.

Snowpack analysis

The previous sections of chapter 3 have explained

some of the fundamental qualities of mountain snowpacks. Definite guidelines for stress and failure analysis could not be given. The question, therefore, arises:

What field observations are useful in evaluating slope

stability, in spite of the complexities of the snowpack?

It is generally agreed that some measure or index of

strength is of primary importance. Other helpful observations are snow depth, density, relative hardness

or strength of individual layers, snow temperature,

grain type, and degree of bonding within as well as

between layers. This section emphasizes only those

very basic techniques of snowpack analysis that are

generally agreed to be important. Interpreting observations in order to make stability evaluations of avalanche slopes is discussed in chapter 5.

Snowpack analysis 57
Figure 79.—Cross section of a snowpit on a steep slope.

Snowpits should be excavated as close as possible to the

avalanche starting zones. Special safety precautions, including rope belays, may need to be taken.

H = Weight of hammer

T = Weight of tube

Figure 81.—The ram number for a snow layer is computed

from the amount of ram penetration p for a hammer fall

height /; n is the number of hammer blows.

Figure 80.—The ram penetrometer is a convenient tool for

snowpack observations from the surface. Using the ram,

an observer can quickly determine the location of weak

layers. Snowpits are required to confirm ram results.

Snowpack observations are made essentially by digging snowpits as close as possible to avalanche release

points. Snowpits are relatively easy to dig on steep

slopes because disposing of the snow is not hard. A pit

much deeper than 2 m is usually not worthwhile because of the large variation in the thickness of the

mountain snowpack and the small return of information for the labor involved.

Thus, after a representative slope has been chosen

for analysis and all safety precautions have been taken

for travel and work on the slope, the first step is to

determine the depth of the pack. If the pack is deeper

than a ski-pole length, the depth can be determined

conveniently with a collapsible probe. The type of

probe used in avalanche rescue work (see chapter 8)

is perfectly suitable. Although it is common practice

to measure the amount of snow on the ground by

probing and to refer to it as snow depth, snow profiles

(appendix B) are plotted in terms of height above the

ground, because the constantly changing snow surface

is a poor reference plane.

After depth has been established by probing, there

is some advantage to taking a second, more refined

58 THE MOUNTAIN SNOWPACK
sounding with a special instrument called a ram penetrometer. This instrument is driven slowly into the

snowpack by blows from a hammer that is dropped

down a guiderod from known heights. After one or

more blows, the penetration of the ram is noted. The

amount of penetration per blow increases substantially when the ram passes through a weak layer such

as a layer of TG grains. The ram gives the observer

quick and convenient information about the relative

strength of the snowpack in many locations without

the work of excavating a pit in each place.

From ram penetration data, it is possible to compute a relative strength index called the ram number.

The most popular formula for computing the ram

number is:

nfH

P

RAM NUMBER

R = T + H +'

where

R is the ram number

T is the mass of the tube (usually 1 kg per section)

H is the mass of the hammer (usually 1 kg)

/! is the number of blows of the hammer

/ is the fall height of the hammer

p is the penetration after /; blows.

This formula is based on the assumption that all the

energy of the hammer blow is transferred to the tip of

the ram. Although this is not strictly correct, there is

no harm in using the formula for computing a relative

strength index. T and H are measured in kilograms,

and / and p are measured in centimeters. The ram

number usually ranges between 1, for newly fallen

snow, and 1,000, for ice crusts. The ram number 1

represents the lowest resolution of the ram, that is, the

penetration due to the weight of the ram tube. A plot

of the variation of ram number with depth is shown

in figure 82.

Besides the ram number, several quantitative tests

for snow strength have been devised. Most of these are

time consuming, and none has been demonstrated to

give more insight into stability than qualitative observation.

Preliminary probing and the more refined rampenetrometer test give the observer a feel for the snowpack depth and structure. After several probes and

ram tests, the observer should excavate a pit. Except

in the very deep snowpacks of maritime climates, the

pit should be large enough to permit unobstructed

observation of the bottom layer, which in many cases.

Figure 82.— \ .niatiüii üf lam number uilh depth. Flagstaff

avalanche path. Alta, Utah. Feb. 8, 1970, 1345 h.

especially in continental climates, contains the only

obvious weakness in the snowpack.

Several tests can be performed on the walls of the

snowpit. However, if time is limited, the observer

should concentrate on a qualitative investigation of

the texture of the pitwall. The pitwall should be

smoothed with the tip of the shovel, and then gently

brushed with horizontal strokes of a soft brush, to

bring out variations in hardness of the layers.

The first thing to look for is major weak layers. In

the deep snowpacks of maritime climates, these may

be buried layers of cold, dry snow or thick layers of

hard ice that may later be lubricated by melt water

and act as avalanche bed surfaces. In more continental

climates they are most likely layers of TG grains,

usually just above the ground surface. The stage of

TG metamorphism should be noted: beginning, intermediate, or advanced (depth hoar). The strength of

the TG layer should be noted qualitatively as weak,

medium, or strong. No guidelines can be stated for

these strength categories, except that the observer

learns quickly the great difference in strength between

firm, well-sintered ET layers at one extreme and weak,

cohesioniess layers of depth hoar at the other.

Having searched for major weak layers, the observer should look next for more subtle weaknesses.

Snowpack analysis 59
í

Figure 83.—Snowpit stratigraphy observation. Top left, smoothing wall with shovel; top right, brushing wall to accentuate

stratigraphy; bottom left, running straightedge down wall; and bottom right, hand lens observation of snow texture.

The existence of any of the following may be worth

noting: thin TG layers; weak layers above or below

ice crusts; graupel layers; buried layers of surface

hoar; or wet, cohesionless grains.

To help locate crusts and thin layers, the tip of a

small straightedge can be run down the pitwall. It is

also instructive to view the pitwall texture through

a hand lens.

60 THE MOUNTAIN SNOWPACK
150

E

u

Z

(3

UJ 100 I

5

O

z

CO

50

+

+

+

+

Snow surface

+

+

+ New snow

+

+

V Old surface hoar

A. Early stage

^ ET

X if Old graupel

Ice

layer

Melt-freeze

Advanced

ET

D a

o a Early TG

D O

A A

A A

A A

A A

Advanced

TG

(depth hoar)

Snow Type RAM NUMBER, kg TEMPERATURE, C DENSITY, Kg/m

300 400 500

3

Figure 84.—Typical plot of snowpit data, showing symbols for various types of deposited snow.

Occasionally it is worthwhile to note the temperature variation in the snowpack. Temperature data give

the observer a feel for whether TG metamorphism can

be expected to intensify in the near future. They also

can indicate how close the snowpack is to thaw (0° C

throughout). The temperatures of the weak layers

can indicate how rapidly they will gain strength

through ET metamorphism and sintering. Temperature measurements should be taken in a north-facing

pitwall as soon as the pit is excavated. This is especially critical in spring, when pitwalls are likely to

heat up rapidly. Because of their ruggedness, dial-stem

thermometers are more suitable for pit work than

breakable glass thermometers. (However, the 0° C

point of each dial-stem thermometer should be

checked by putting the stem in a well-stirred slush of

clean snow and water.) The procedure for measuring

pitwall temperature is first to set the thermometer

stem into the layer and allow a few minutes for the

stem to come into equilibrium with the snow, then

shift the stem horizontally to a fresh spot for the final

reading. Working with several thermometers simultaneously saves time.

There is, of course, no limit to the amount of data

that can be measured in a snowpit; each avalanche

worker forms his own opinions on what data are most

useful for daily operations. The wise observer strikes

a compromise between thorough examination of one

snowpit and hasty examination of several snowpits

on many different exposures. A snowpit examination

plot that includes temperature, density, ram number,

and grain texture is shown in figure 84. A complete

set of field notes and the resulting snow profile are

given in appendix B.

A suitable kit for carrying on skis might consist of

the following items:

Snowpack analysis 61
-^I*»»'-

Figure 85.—Above, required and, below, optional equipment

for snowpack observations. The shovel pictured here may

be too light for some snow conditions.

Required

Collapsible probe 3 in long and marked

in centimeters

Aluminum shovel (coal and street

cleaners model or grain scoop)

Brush

Folding rule (3 m)

Notebook and pencils

Rucksack

Optional

Ram penetrometer

Hand lens (and millimeter grid)

Two thermometers (dial-stem preferred)

Portable density kit

Any of a variety of strength-measuring devices,

such as a shear frame or Canadian hardness

gage.

62 THE MOUNTAIN SNOWPACK
Further reading

Akitaya, Eizi.

1974. Studies on depth hoar. Contrib. Inst. Low Temp. Sei.,

Hokkaido University (Sapporo, Japan). Ser. A, No. 26,

67 p. Results of detailed laboratory and field studies.

Growth of depth hoar is related to snow temperature,

magnitude of negative temperature gradient, and size of

airspace in snow.

Keeler, Charles M.

1969. Some physical properties of alpine snow. U.S. Army

Corps Eng., Cold Reg. Res. and Eng. Lab., Res. Rep.

271, 70 p. Hanover, N.H. Technical comprehensive

study of the mountain snowpack.

Keeler, Charles M., and W. F. Weeks.

1967. Some mechanical properties of alpine snow. Montana 1964-66. U.S. Army Corps Eng., Cold Reg. Res.

and Eng. Lab., Res. Rep. 227, 43 p. Hanover, N.H.

Technical study of the snowpack in the Beartooth and

Bridger Ranges of Montana. Comparison of strengthtesting techniques.

Kingery, W. D., ed.

1963. Ice and snow: Properties, processes, and applications.

684 p. Cambridge, Mass.: The M.LT. Press. Collection

of technical papers, some of which deal with the metamorphism and properties of snow and the mechanics of

the inclined snowpack.

Martinelli, M., Jr.

1971. Physical properties of alpine snow as related to

weather and avalanche conditions. USDA For. Serv.,

Res. Pap. RM-64, 35 p. Rocky Mt. For. and Range Exp.

Stn., Fort Collins, Colo. Technical study of the mountain snowpack at Berthoud Pass, Colo. Emphasis on the

strength of snow.

Mellor, Malcolm.

1964. Properties of snow. Cold Reg. Sei. and Eng., Part

III-Al, 295 p., edited by F. J. Sänger. U.S. Army Corps

Eng., Cold Reg. Res. and Eng. Lab., Hanover, N.H. A

technical discussion of the properties of snow, with

emphasis on high-density snow.

Mellor, Malcolm.

1974. A review of basic snow mechanics. U.S. Army Corps

Eng., Cold Reg. Res. and Eng. Lab., Intern. Rep. 352.

42 p., Hanover, N.H. A detailed and technical paper

that summarizes the state of the art in snow mechanics.

Also in the proceedings of the International Symposium

on Snow Mechanics held at Grindelwald, Switzerland

1-5 April 1974.

Oura, H., ed.

1967. Physics of snow and ice. Proc. Int. Conf. Low Temp.

Sei. (Sapporo, Japan, 1966) 1(2): 713-1414. A collection of technical papers on the properties of high- and

low-density snow. Several papers deal directly with

avalanches.

Sommerfeld, R. A.

1969. Classification outline for snow on the ground. USDA

For. Serv., Res. Pap. RM-48, 24 p. Rocky Mt. For. and

Range Exp. Stn., Fort Collins, Colo. A classification of

the types of snow found on the ground. The classification

is based on metamorphic processes. Semitechnical.

Sommerfeld, R. A., and E. LaChapelle.

1970. The classification of snow metamorphism. J. Glaciol.

9(55):3-17. A slightly more technical coverage of the

process-based classification scheme for snow metamorphism than that given in the above publication.

Yen, Yin-chao.

1969. Recent studies on snow properties. In Advances in

hydroscience, vol. 5, Ven Te Chow, ed. New York:

Academic Press. Technical summary of recent work on

metamorphism, thermal snow properties, and the radiation properties of snow.

Yosida, Zyungo, et al.

1955. Physical studies on deposited snow. I-V. Contrib.

Inst. Low Temp. Sei., Hokkaido University (Sapporo,

Japan). (Original out of print; photocopies available

from University Microfilms, 300 N. Zeeb Road, Ann

Arbor, Mich. 48106, Class No. OP 12926.) Reports a

series of detailed laboratory studies on thermal, mechanical, and dielectric properties of deposited snow.

Further reading 63

Chapter 4

Avalanche

phenomena

The first half of this chapter explains some current

ideas about avalanche release mechanisms. The

intent is to supply the reader with the foundations

needed for stability evaluation and avalanche

control, practical subjects treated in later chapters

of this text. It will be seen that slope failure

occurs in many different ways, depending on

meteorological conditions, snowpack structure, and

the way the slope is "triggered."

The second half of the chapter deals with avalanche

paths and avalanche motion. Once a snow slope

fails, the energy and impact forces of the avalanche

are a function of path length, shape, vegetation,

and other path variables. Approximate methods

are given for estimating impact forces.

Figure 86.—Slab avalanche triggered by skier. (Photos by Ludwig)
/ 7*

y /Â

^ /Ä

- _ /^^

Loose snow avalanche

/ /''^

/ ■■ ,

/-'Ml

r ~—-—_^ //:Ä w

IH^^^^^^-^~~~~^V j^^

Slab avalanche

Figure 87.-TWO modes of snow slope failure: (left), loose-snow avalanche and (right), slab avalanche: slab avalanches

are usually more dangerous.

Figure 88.—L>tiiii('. íIL-\\ ^ii^>\^ ii,i^ a Lufiipuialively high angle

of repose, as evidenced by snow clinging to steep slopes

and trees; Denny Mountain, Wash. (Photo by Emetaz)

Failure of snow slopes

Avalanche failure may occur by either of two distinct processes. In one process, failure begins near the

snow surface. A small amount of cohesionless snow,

usually less than 1 m-*, slips out of place and starts

down the slope. The initial mass may set an increasing

amount of snow into motion if the snow in its path is

also fairly cohesionless. When this process is observed

from a distance, the avalanche seems to start at a

point, and the sliding snow spreads down from the

point, leaving an inverted V-shaped scar. Avalanches

that form this way are called loose-mow avalanches.

The other failure process begins with brittle, catastrophic fracturing of cohesive snow that frees a slablike region of the slope. The slab quickly breaks into

smaller cohesive blocks, whose size varies with their

cohesiveness and the roughness of the avalanche track.

Avalanches that begin in this manner are called slab

avalanches. The initial slab may vary from about 100

to 10,000 m- in area and from about 0.1 to 10 m in

thickness. Clearly, movement of the larger slabs releases enormous amounts of energy. In contrast to

loose-snow avalanches, slab avalanches depend on the

propagation of fractures by stored energy in the relatively cohesive layers.

66 AVALANCHE PHENOMENA
Figure 89.—Depending on its lenipeiatuie, welness, and texture, each snow type has a critical angle of repose, above which it

will not cling to the snowpack. Loose-snow avalanches result when this critical angle is exceeded.

Much of this chapter is devoted to studying the slab

avalanche, which is by far the more dangerous type.

Loose-snow avalanches occur mostly because the

steepness of the slope exceeds the angle of repose of

certain types of weak snow. The exact critical angle of

repose depends on the temperature, wetness, and

shape of the snow grains. For example, wet, slushy

snow has very little strength for its weight and can

avalanche off slopes as gradual as 15°. Newly fallen

snow has relatively low cohesion but generally has

enough strength to cling to 40° to 50° slopes. However, if the new snowfall is cold enough, avalanches of

cold, dry snow may occur on slopes of 30° to 40°

because sintering cannot proceed fast enough at the

cold temperatures to anchor the snow. Rounded

graupel crystals generally will not cling to slopes

steeper than 40° and often roll down steep slopes like

ball bearings. In the Andes, conditions exist that permit thick slabs to cling to inclinations as steep as 50°

or 60°.

The sequence of failure in a loose-snow layer that

is resting at a steeper-than-critical angle of repose is

as follows (fig. 90) :

(1) The layer is disturbed by any of several natural

or artificial processes: overloading, from the added

weight of newly fallen snow or a skier; vibration, from

Initial failure

confined to small mass

at snow surface.

Local loss

of cohesion

Figure 90.—Start of a loose-snow avalanche. Initial failure is

confined to a small mass at the snow surface.

an earth tremor or explosive force; or, most important,

internal changes such as the warming of the layer to a

state of drastic loss of cohesion.

Failure of snow slopes 67
í'^*^'v>'

V V

r''^:¡a-^-^'^-

-»^-j

>í*> '#*'1

/

/

jf '' ^

Figure 91.—Examples of dry loose-snow avalanches. Most loose-snow avalanches are small, harmless releases called sluffs,

although occasionally they reach hazardous sizes. (Photos by Roch and O'Leary)

(2) A small piece of the layer slips out; the piece

can be as small as a single grain but is typically the size

of a large snowball.

{3) The loose piece either comes to rest at a new

angle of repose or imparts enough energy to the snow

in its track to cause an avalanche.

Loose-snow avalanches occur frequently throughout the snow season, from early fall to late spring. It

is customary to distinguish between dry and wet loosesnow avalanches, although there is no sharp dividing

line between these categories, since avalanches may

have any of the wetness values outlined in table 1.

Loose-snow avalanches occur in a variety of sizes.

Most fall as small, innocuous masses called slu^s.

Dangerous wet loose-snow avalanches are sometimes

observed, especially in the Pacific Coastal ranges,

where they have destroyed life and property. The

hazard from large, dry loose-snow avalanches is not

well documented. Many observers feel that dry loosesnow avalanches pose a threat only when they trigger

slab avalanches. In any case, the effects of loose-snow

avalanches depend on the amount of snow set in motion. It is worthwhile to summarize the more important

effects :

Hazard to mountaineers and skiers. Loose-snow

avalanches are generally small, but they are large

enough to knock a mountaineer or skier from a safe

stance. Small loose-snow avalanches have sent skiers

on leg-breaking rides and have taken mountaineers

over cliffs and into crevasses.

Hazard to facilities. Occasionally, loose-snow avalanches are large enough to threaten moving or parked

cars, fixed facilities, etc. Most of the notorious cases

occurred either in spring or after continuous rain,

when the snowpack became soaked and cohesionless

to depths of a meter or more.

Stabilization of liigli-angle slopes. Loose-snow avalanches remove snow from steep slopes. During

storms, sluffing occurs almost continuously where the

slope angle is steeper than about 50°. Because of

sluffing, dangerously thick layers rarely build on such

steep slopes. The exceptions to the 50° rule occur

where snow is plastered against perpetual icefields

and snowfields, in narrow couloirs, or where snow

conditions result in unusually steep angles of repose.

68 AVALANCHE PHENOMENA
Stabilization of lower slopes. In terrain where highangle slopes empty down onto lower slopes, sluffing

from the higher slopes may force the lower slopes

gradually to shed small avalanches and become stable.

Loading of lower slopes. Sluffing may transfer snow

to the lower slopes, building up large deposits that

later may release as slab avalanches.

Triggering of slab avalanches. Loose-snow avalanches that would be harmless by themselves may

spill onto lower slopes and trigger dangerous slab

avalanches.

Before discussing slab avalanches in further detail,

the customary distinction between natural and artificial avalanches (loose-snow and slab types) should

be made. Natural avalanches are not triggered directly

by man or his equipment. A falling cornice, sluffing

snow, stress change due to metamorphism, stress

change due to weight of new snow, earth tremors,

snow falling from trees, etc., can all trigger avalanches.

Artificial avalanches are triggered by man or his

equipment. A ski pass, a mountaineer's weight, an

explosive blast, a sonic boom, etc., commonly precipitate artificial avalanches. There seems to be no difference in appearance between natural and artificial

avalanches. The important fact is that artificial triggering leads to a far greater frequency of avalanches on a

given path than if the path were left to avalanche

naturally.

Slab analysis

Slab avalanches begin with the fracturing of snow

slopes. Cracks usually propagate quickly and follow

unique and complex paths that depend on slope geometry and slab anchorage. Obviously, for a slab to

detach completely from the slope, fractures must

proceed around the entire slab boundary. The following nomenclature (see fig. 92) is used to describe the

boundary fracture surfaces:

Figure 92.—Top, nomenclature for the fracture surfaces of a snow slab. Bottom, a classic example of a soft slab avalanche.

Slab analysis 69
Crown surface. The top fracture surface of the slab,

usually a smooth, clean cut, generally perpendicular

to the slope. The snow that remains on the slope above

the crown surface is called the crown.

Flank surface. The side boundary of the slab, often

sawtoothed.

Crown

Bed surface

Stauchwall

Bed surface. The main sliding surface of the slab,

generally smoothed and compacted by the sliding

blocks.

Stauchwall. The downslope fracture surface of the

slab, often difficult to identify since it is usually overridden and obliterated by the sliding blocks.

Viewing the fracture pattern in ideal cross section,

the bed surface and the crown surface typically intersect at approximately 90°; the slab has a wedgelike

shape at the stauchwall (see fig. 93).

Because the bed surface is hidden entirely during

the fracture process, the sequence of fracturing is not

known. Although the first observed fractures are

usually spectacular crown fractures, the crown may

in fact be the last link to break.

Crown fractures may extend considerable distances, jumping gullies and bowls and linking together

several smaller slabs. Crown lengths of more than a

kilometer have been recorded. A characteristic feature

of the fracture boundary is that crown length usually

exceeds flank length. In some cases, the crown surface

blends directly into the stauchwall, forming an oval,

flankless slab.

Regardless of their geometric peculiarities, slabs

often contain large masses of snow and great amounts

of potential energy. It is not uncommon for a slab to

have an area the size of a football field and an average

Figure 93.—Cross section of a typical snow slab.

Figure 94.—Crown fractures may propagate over long distances, connecting a series of smaller slabs, (Photo by Wilson)

'^ AVALANCHE PHENOMENA
thickness of 1 m. Moreover, when a slab breaks loose

and slides down the mountainside, it may bring down

100 times the initially released amount of snow.

Slab avalanches originate on a wide variety of

terrain. The main requirement is steepness. One

hundred observations of bed-surface inclinations for

dangerous-sized slabs are summarized in figure 95.

Dangerous slabs are most likely to start on slopes in

the 30°-to-45° range. Fortunately, good recreational

skiing, even under deep powder conditions, can be

enjoyed on slopes that are not this steep. Explanations

for the distribution shown in figure 95 are:

• For slopes of less than about 30°, shear stress on

the bed surface is not large enough to cause shear

failure. It is interesting that collapse and fracturing

are often observed on slopes with inclinations of

less than 30°, and even on horizontal surfaces, but

in those cases vertical collapse is not followed by

avalanching.

• The upper limit of about 45° probably indicates

the tendency of snow to slufï gradually off steep

slopes (45° to 60°).

It must be emphasized that many isolated examples

of slab avalanches are outside the 30° to 45° range.

Certainly, mountaineers should not venture confidently onto 50° snow slopes. In cases where snow

accumulates on steep terrain (45° to 60°), slab avalanches are a definite possibility. Also, slab fracture

can propagate from high-angle slopes to slopes of

less than 30°.

As shown in figure 76, slope curvature can influence the stress distribution in a slab. However, the

bed surfaces of almost all slabs are quite planar.

Moreover, crown fractures usually form just below

a terrain bend rather than directly on the bend. Thus,

it appears that bends do not play the fundamental

role of increasing stress, but instead contribute to

avalanching by influencing the way snow is deposited

and anchored.

A few studies have been made of the properties of

slab avalanches. These studies provide the following

limited information:

Crown thickness. This dimension can vary widely,

from a few centimeters in the case of sluffs to several

meters. Typically, slab avalanches become hazardous

to skiers when the crown thickness exceeds 15 cm,

the minimum thickness for extensive fracture

propagation.

Slab density. Normally slab avalanches consist of

snow with densities between 100 and 400 kg/m''. In

Slab analysis

almost all slabs, there is substantial variation in

density between snow surface and bed surface. The

density just above the bed surface is usually about

twice the density at the snow surface. In the vast

majority of larger slabs (crown thickness more than

1 m), average snow density lies within the narrow

range of 200 to 300 kg/m'. This range is typical of

wind-deposited new snow or sintered snow.

Slab hardness. It is customary to distinguish two

hardness categories, hard slab and soft slab. This distinction is based on subjective observations. For example, extreme hard slab snow cannot be penetrated

by ski tracks; ski edges leave only a faint line. In contrast, excellent powder skiing is often possible through

soft slab snow. Another subjective basis for distinction

is that on slopes of similar roughness, hard slab blocks

can survive lengthy trips down the slope without disaggregating, while soft slab blocks break up very

quickly into smaller lumps. There is, of course, no

fundamental distinction between hard and soft slab;

slabs come in a wide range of hardnesses. From a practical viewpoint, the harder slabs pose special control

problems in ski areas. Generally, hard slabs consist of

high-density snow (300 kg/m'' or higher), either de50 -

w

LU

I

Ü

z

<-JI

m

<

O

oc

LUm

z

20 25 30 35 40 45 50 55

BED SURFACE INCLINATION, Degrees

Figure 95.—Bed-surface inclinations of 100 large slabs

observed in the United States, Switzerland, and Japan.

(Average slab thickness is about 1 m.)

71
Figure 96.—Examples of slab avalanche fracture patterns. (Photos by Gabriel, Judson, Keiner, and Roch)

72 AVALANCHE PHENOMENA
-/ 19

o 200

DENSITY, Kg/m

400 o -2 -4 -6

5 TEMP.. °C

0 1.000 2,000 3.000

SHEAR FRAME INDEX, N/m

Figure 97.—Snow stratigraphy of a 1-m-thick slab that released at Aha, Utah. As indicated by the shear-frame measure of

strength, either the graupel or the TG layer was a possible bed surface; in fact, a nearby slide ran on graupel the same day.

posited by high winds or the result of aging and compaction.

Slab temperature and wetness. Slab temperatures

reflect the air temperatures of the mountain range.

Avalanches have been observed in the cold environments of the Himalayas and on the slopes of Mount

McKinley. At the other extreme, slab avalanches have

been observed when the snow was thoroughly

drenched with free water (see table 1 ). It is customary

to distinguish dry and wet slabs as follows: Dry slab

snow when squeezed in a gloved hand will not make

a snowball, but liquid water can be squeezed from

wet slab snow. This test should be made with snow

from near the starting zone, not with avalanche debris.

Slab stratii-raphy. The slab layers above the bed

surface consist of a wide assortment of snow types.

The most frequently observed layers are:

• Newly deposited snow up to a few days old. Most

of the basic forms and modifications (see fig. 26)

have been identified. Firm, wind-deposited layers

are very common.

• Metamorphosed snow. Mostly ET grains and

grains in the early stages of TG metamorphism.

Quite often thin TG layers, crusts, graupel layers.

and other special layers are sandwiched between

thick ET layers.

• Wet layers (table 1).

Bed surface. Because the bed surface is altered by

the sliding slab, definite information on its structure is

rarely available. A limited number of studies on the

extension of the bed surface into the crown reveal the

following:

• In some cases it is possible to identify a distinct

discontinuity, such as a transition from ET grains

to TG grains, a graupel layer, the ground surface,

an ice crust, or thin layers of surface hoar.

• In many cases it is not possible to identify any

special discontinuity of snow stratigraphy at the bed

surface extension, although snow at the bed surface

is typically about 50 percent weaker than snow immediately above the bed surface.

Slab substratum. In most cases investigated in continental climates, the slab substratum beneath the bed

surface consisted of TG grains. In maritime climates,

TG substrata are rare, and the limited data indicate

that most of the substrata consist of new snow, partly

metamorphosed snow, or wet snow.

Slab analysis 73
Mechanics of slab failure

In one possible sequence leading to slab avalanche

release, failure begins when shear stress exceeds shear

strength at the bed surface. This may be the result of

either an increase in the bed surface shear stress or a

decrease in the bed surface shear strength, or a combination of the two. Such changes may be caused by

the following situations:

• High-intensity snowfall or wind redistribution of

older snow

• Rapid application of shock load, explosive blast,

cornice fall, etc.

• Load of one or more skiers traversing the slab

• Weakening of the bed surface by TG metamorphism

• Weakening of the bed surface due to slow straining

• Weakening of the bed surface by melting.

The initial bed surface failure is always hidden from

sight, so it is not obvious how rapidly failure progresses. There may be rather widespread shear fractures, or possibly slow, progressive straining. In any

case, after shear failure begins, tensile stress begins to

increase in the slab. Finally the slab fractures in tension, and the entire slab releases very rapidly as the

shear support along the flanks and the compressive

support at the stauchwall are overcome. This sequence

could result in crown fractures propagating above a

skier, trapping the skier as shown in figure 98.

A second possible and related sequence of slab failure begins with the sudden collapse of a thick, weak

layer of TG grains, or low-density snow. The collapse

can be due to new snow load, weakening by metamorphism, or sudden shock. As shown in figure 99,

the sequence of events would be first, collapse of the

substratum; second, high bending stress upslope from

the collapsed area; third, tension fracture in the region

of high bending stress; and fourth, shear fracture at

the bed surface. It is known that substratum collapse

is an important mechanism, since it explains the intense instability associated with thick TG layers. However, in many cases slabs have failed on hard substrata, where collapse was not possible.

A third possible mechanism of slab failure begins

with tension fracture, which in turn activates fracture

at the bed surface (fig. 100). This requires a previous

internal failure or straining in the snowpack to build

up the necessary tensile stresses. Tension fracture

could result from:

Figure 98.—In one possible sequence, initial slab failure occurs in shear along the bed surface. High tensile stress

develops upslope, ahead of the propagating shear failure.

Finally, the slab layer fractures in tension.

• Sudden ski traverse across crown, explosive

blast, etc.

• Decrease in tensile strength due to warming of

slab

• Increase in tensile stress due to thermal contraction brought on by protracted cooling

• Increase in tensile stress due to creep or glide.

In all three proposed mechanisms, the necessary

condition for failure is that a relatively stiff layer (the

slab layer) rests on a relatively weak layer that contains the bed surface. Regardless of the mechanism,

tensile stress eventually develops in the slab layer,

which stores elastic strain energy. When the slab layer

fractures, a sudden jolt is thrown onto the bed surface.

This jolt reinforces any bed surface failure that began

earlier. Finally, the shear fractures at the bed surface

and the tension fractures at the crown reinforce one

another, so that the slab rapidly breaks free on all

boundaries.

It is not difficult to see that heavy snowfall, rain,

thaw, and dynamic loads are important triggers of

slab avalanches. However, can abrupt temperature

changes cause instability in dry snow deeper in the

74 AVALANCHE PHENOMENA
Figure 99. Initial failure may occur as collapse of a weak substratum. This would induce high bending stress, tension fracture,

and finally shear fracture at the bed surface.

snowpack than about 0.5 m? The question comes up

because avalanche slopes in ski areas often seem to

be stable when tested with explosives in the early

morning but release a deep slab in the afternoon. It

is sometimes said that such postcontrol avalanches

are caused by an abrupt rise or fall of snow

temperature.

This so-called "temperature-release" mechanism is

controversial. Rapid changes in snow temperature

are certainly confined to the surface layers. The

thermal conductivity of snow is very low—about

1/10,000 that of copper—and it is difficult to see how

a dry slab any thicker than about 0.5 m could be

affected by rapid fluctuations in the surface energy

balance. Moreover, solar radiation is the most important variable in the short-time energy balance,

yet most avalanches attributed to temperature release have been on north-facing slopes, where

hourly radiation changes are minimal. Finally, in

mountain ranges where sudden temperature changes

are most extreme (for example in the Canadian

Rockies, where the temperature may go up or down

25° or 30° C in a few hours), there are few complaints about temperature release.

Figure 100.—Initial failure may occur as a tension crack.

The opening of the tension crack would induce shear fracture at the bed surface.

Mechanics ofslab failure 75
Figure 101.—Three sections of an avalanche path: starting

zone, track, and runout zone.

What alternative explanation, then, can be given

for latent instability of tested slopes? If a slope is

structurally unstable, early-morning explosive tests

might jar the slab loose from its substratum. Tensile

stress would then begin to build up in the slab as the

day progressed. The slab may reach a hair-trigger

state later in the day; the fact that the snow temperature is also changing is of secondary importance and

most likely a coincidence. It is interesting that most

cases of "temperature release" have occurred where

noticeably weak substrata could be identified.

To summarize current ideas about temperature

releases and postcontrol avalanches:

• Temperature changes at the snow surface may

cause failure and sliding in the surface layers. This,

in turn, could trigger deeper slab releases.

• Sudden temperature changes at the surface very

likely do not cause deep slab instability in dry

snowpacks.

• There is growing concern that explosives alone

do not give an infallible test of snowpack stability

and in some rare cases may weaken the slope.

The avalanche path

Once set in motion, avalanches can move great

distances. The larger avalanche paths in North America are about 3,000 m long and have a vertical drop

of about 1,800 m. Some avalanche paths in the Alps,

the Andes, and the Asiatic mountains are much

longer. The average inclination for most avalanche

paths that extend for a long distance is between 20°

and 35°.

The term avalanche path is used to describe terrain

boundaries of known avalanches or suspected avalanches. It is customary to divide an avalanche path

into three sections: starting zone, track, and runout

zone. The typical dimensions of each section of a

given path are best determined by systematic records

of field observations. However, the maximum dimensions can only be guessed from historical evidence

and assumptions about "what could happen" under

unusual conditions. An avalanche may overrun its

presumed boundaries so that a former runout zone

becomes a track for a new runout zone. The vast

majority of avalanches that run on a given path will

not be observed to reach their maximum boundaries

in a short-time study. There are examples from the

Alps of buildings destroyed by avalanches after

standing for 100 years or more (Fraser 1966).

The essential characteristics of the starting zone

have been described. It must be steeper than about 30°

and must receive large amounts of snow. The latter

characteristic is satisfied for certain wind directions

if the slope is in the lee of a source of blowing snow

or if the lee slope is an efficient collector of snow

(see chapter 2). Gullies and bowls are especially

efficient collectors and make up a large proportion

of the most active starting zones. Buttresses and ñat,

exposed faces are less efficient collectors and therefore less likely to contain active starting zones. However, when large amounts of snow are deposited with

little or no wind, normally inactive and exposed

starting zones may become quite active. The most

active starting zones are gullies bounded at the top by

horseshoe ridges or cliffs. Most starting zones are bare

of trees, some have sparse timber, and a few contain

stands of fairly dense timber. In the last case, slab

fracture may spread into the timber from the clearings.

The term track refers to the part of the path between the starting zone at the top and the runout zone

at the bottom. As a very general rule of thumb, avalanche tracks have an inclination of at least 15°; more

76 AVALANCHE PHENOMENA
^s.

^».

t, >

#

\

X"

if

Figure 102.—Active starting zones .i¡c iwnna .a a,^ iops of gullies and bowls and in the lee of ridges. Many starling zones are

bounded by cliffs or rock outcrops. (Photos by Keiner and Roch)

The avalanche path 77
Figure 103.—The terrain loolvs safe, bul a skier was killed in this avalanche path. Open timber does not guarantee safety. Floral

Park Avalanche. Berthoud Pass, Colo.

commonly, 20° to 25°. Avalanche tracks can be subdivided into two categories: channeled and unconfined. Channeled tracks are gullies, couloirs, gulches,

etc., with or without summer streams. Unconfined

tracks are on plane, open slopes. Most of the longer

avalanche tracks in the United States are channeled,

because the confining action of the channel tends to

concentrate the flow and propel the moving snow

efficiently.

An avalanche track may have several branches. A

common situation is a main channeled track fed by

several small tracks, each beginning at a separate

starting zone. It is important to keep in mind that

multibranched tracks may run several times in quick

succession, as each branch empties into the main

track. A number of fatal accidents has occurred

when workers clearing the debris of a first avalanche

were struck by a second that ran down the same track

within hours of the first.

Dry avalanches tend to follow straight lines. Where

a gully makes an abrupt turn, the fast-moving "snow

dust cloud" of a dry avalanche may jump the gully

walls and continue on its straight-line path. Wet avalanches flow more slowly and are more easily channeled to follow curved gullies. An interesting case that

illustrates the different paths taken by dry and wet

avalanches occurred recently at a popular United

States ski area. Based on the recollection of the older

Figure 104.—An example of main tracks fed by several

branches. Cement Creek, near Silverton, Colo.

78 AVALANCHE PHENOMENA
.^.?'^"

Bffü"''"

'M-W

1.» r'SS', .!.

Figure 105.—Comparison ol a channeled track and an unconfined track: lop, N.U. KCU walanche, near hmpire. Colo.;

bottom, Ironton Park Avalanche, near Ouray, Colo.

The avalanche path 79
ELEVATION ABOVE SEA LEVEL, Meters

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Runout zone

Figure 106.—Longitudinal profiles of four avalanche paths in Colorado.

residents of the area, a building site was planned in

a supposedly avalanche-free gully. There was an avalanche above, but everyone remembered that it customarily failed to make an abrupt turn halfway down

the gully and jumped the gully walls, finishing on a

straight course. Just before construction a wet spring

avalanche did not jump the gully and continued into

the middle of the proposed site.

Although starting zones must have the required

steepness, avalanche tracks may include gentle slopes

or even level terrain. The runout zone is usually the

valley floor, but it may in extreme cases extend uphill.

As an example, an avalanche in the Alps, with a starting zone at elevation 2,500 m, ran down its track to

elevation 1,740 m and then continued across the valley and uphill to elevation 1,960 m. In an incident

from United States history (1872), a convoy of horsedrawn sleds was struck by an avalanche from Mt.

Superior near Alta, Utah. Several bodies were found

in debris on the opposite side of the valley, 100 m

above the road.

The runout zone is the bottom boundary of the

path. For several reasons, it is difficult to be certain of

this boundary. One problem, of course, is the great

variation in weather from one winter to the next. In a

severe winter, starting zones and tracks may be loaded

with abnormally large amounts of snow. In those

winters, avalanches may overrun historical boundaries. There is also the possibility that two large avalanches down the same path in fairly rapid succession

will result in the second avalanche exceeding the

known boundaries because it flows down a track and

across a runout zone smoothed by the first avalanche.

Finally, a series of avalanches may gradually force a

80 AVALANCHE PHENOMENA
Figure 107.—Avalanche debris across two-lane highway.

Little Cottonwood Road, Utah.

path through a forest stand that before stopped the

avalanche. Once most of the timber is destroyed, a

major avalanche can break through and continue into

a formerly protected area. As a general guideline, runout zones on slopes of 5° to 10° can extend 300 to

500 m.

In determining the limits of the runout zone, it is

necessary to account for the airblast caused by the

moving snow (described later in this chapter under

"Avalanche Movement"). It appears that the airblast

zone may extend about 100 m beyond the boundaries

of a major avalanche path.

The snow deposited in the runout zone is about two

or three times denser than the starting-zone snow, and

it is much harder. While moving down the track, some

of the snow is pulverized into fine particles. When the

mass comes to rest, the fine particles and the larger

pieces sinter rapidly to form a very firm aggregate.

Wet avalanche debris is remarkably hard and icelike,

probably due to regelation—thc refreezing of pressureinduced melt water. To dig into the debris of a large

wet snow avalanche requires pickax and shovel (fig.

107).

Identification of avalanche paths

One method of studying avalanche paths is direct

observation for as long as possible by a trained, reliable observer. The second method is to rely on a

synthesis of indirect evidence: topography, climate,

avalanche damage to vegetation, and information

from the local population. Since long-term avalanche

records are usually unavailable, most planning of

facilities in avalanche terrain relies heavily on the

second method, with due consideration for any existing observations.

If a ski area, highway, village, etc., is to be built in

suspected avalanche country, 3 to 5 years of continual

observation are desirable before the area is opened to

the public. The observation should be recorded in a

systematic way. In 3 to 5 years it should be possible

to observe at least the avalanches that run on the

average of once each season and to acquire a feel for

the potential of inactive paths. The difficulty is, of

course, that some avalanches run on the average of

once per season, others once every 5 to 15 years, and

others less frequently. Another complication is that

many avalanche paths are not active every winter but

run several times in winters when they are active.

A good way to begin is to gather as much information as possible from the local population or from

people who have lived and worked in the area for long

periods. Highway crews, electric power companies,

telephone companies, sheriffs' crews, etc., may be able

to contribute information. The first avalanche maps of

the areas can be based on their recollections. Information collected from local people will probably be incomplete, and it probably will include only the most

spectacular avalanches of recent years.

Path identification should be conducted throughout

the year. In winter the obvious features are fracture lines and avalanche debris. Winter observations

should be made immediately after storms. Some fracture lines and debris will be plainly visible, and some

will be obscured by new snow and wind drifts. Avalanches that run early in the storm cycle may be obscured and quite diflficult to identify. In spring also,

avalanche paths are most active after storms, especially those that are followed by warming trends.

Major avalanches may occur whenever the snowpack

is thawed. Late in spring, and even in summer and fall,

avalanche debris may be seen in gullies. This lumpy

debris, which is dense and hard and often contains

rocks, branches, mud, small animals, etc., persists in

gullies long after the snow melts from the neighboring

Identification of avalanche paths 81
• irí-S"^-

Figure 108.—At a distance, avalanche paths often appear as treeless strips. Seven Sisters Avalanche Paths, near Loveland Pass,

Colo. (Photo by Frutiger)

Figure 109.—Destruction of trees is an indication of avalanche activity. Broken trees are alined in the direction of the moving

snow. Trees are disaster, or pioneer, species, which normally do not dominate. Superior Avalanche Path, Alta, Utah.

82 AVALANCHE PHENOMENA
terrain. In summer, detailed inspection can be made

of the vegetation and topography of avalanche paths.

Many avalanche paths are not obvious. This is

especially true of small paths that would be hazardous

in a ski area. A good way to investigate and map these

small avalanches before construction of a proposed

ski area is to carefully and thoroughly ski tour the area

and record observations on high-quality aerial oblique

photographs.

In alpine and subalpine ski areas it can be assumed,

as a start, that all treeless slopes, gullies, and bowls

steeper than about 30° are possible avalanche paths.

There is hardly any lower limit to the size of hazardous

paths in ski areas. People have been buried and killed

by avalanches on paths 10 m long.

Identification of large avalanche paths presents

several problems. For one thing, it is necessary to view

the entire path. For very long paths, this may be impossible without using air reconnaissance or ascending the ridge across the valley from the suspected path.

A view up from the valley floor often gives a distorted

impression and may miss completely the starting zone.

In some cases, large starting zones are hidden by cliff

bands or other terrain features. A view up from a proposed development site may make the suspected

avalanche path seem deceptively short. The view

down the path from the starting zone gives the best

perspective of potential damage. A proposed site

might diminish to a small point, well within the possible boundaries of the path.

Large avalanches have enough power to destroy

trees, and landscape scars provide important clues to

avalanche activity. From far away, an active avalanche path below timberline normally appears as a

treeless strip, often following a gully. Less active paths

appear as strips of smaller trees, or strips of trees of

a different species from those outside the path. From

the air or from an opposing ridge, the runout zone

may be outlined clearly by changes in vegetation.

It is often impossible to enter large starting zones

in the winter because of risk or inaccessibility. In those

cases, the starting zone should be entered in summer

to collect information not available from maps. Of

particular interest are the slope angle and aspect. It

may be necessary to study the ground surface of the

starting zone in order to plan defense structures or

reforestation.

Close inspection of vegetation in the runout zone is

important. Vegetation gives important clues to boundaries. The things to look for are:

'^

Figure 110.—Branch damage on the uphill .de of a tree may

be evidence of avalanche activity. Bethel Avalanche Path,

near Silver Plume, Colo.

Obvious destruction of trees and branches. The

most convincing evidence of past avalanche activity

is a patch of fallen trees, alined in the same direction

and sheared at about the same height above the

ground. The common shear height can be assumed to

be about the height of the snowpack at the time of

the avalanche. At the boundary of the runout zone

branches of upright trees may be damaged. Some of

this damage may be caused by airblast or low-density

snow dust. In many cases, branches are removed from

the uphill sides of trees.

Change in species. As noted above, another indication of past avalanche activity is a marked change in

vegetation in the neighborhood of a suspected runout

zone. A typical example might be a patch of slide

alder, yellow cedar, or aspen surrounded by a closed

forest of spruce and fir or other climax species. The

explanation for these patches is that certain species of

shrubs and fast-growing, light-tolerant trees invade

disturbed sites more readily than climax plants. Trees

of the climax species usually become established in

the shade of the pioneer species and eventually crowd

them out if given enough time without further disturbance.

Identification of avalanche paths 83
c »••.

W«»'.'^'^^-*^

Figure 1 I 1.—Stout limber pine growing in the middle of Flagstaff Avalanche Path, Alta, Utah.

Differences in height of trees. Trees survive avalanches up to a height that varies with the strength

and flexibility of the species and the thickness of the

pack. Since avalanches ordinarily do not penetrate

deeply under the pack in the runout zone, short trees

may escape destruction. Thin, flexible trees up to

about 3 m high may be bent by the moving snow and

then recover. Certain rugged subalpine trees, such as

thick limber pines, may be resistant to avalanches,

and an occasional high tree is found in the middle of

a path. In the great majority of cases, though, trees in

the runout zone are shorter than trees outside the zone.

Vegetation can be used for making a rough estimate

of avalanche frequency. Table 2 gives some idea of

how frequency can be correlated with vegetative

clues. Increment cores from selected trees give their

relative ages (Chapman and Meyer 1949, and Cuno

1938 ). It should be kept in mind that there is an indeterminate time lag between avalanche activity and

establishment of trees. The lag may be 1 to 10 years

or more. Thus, detailed age analysis by increment

coring is justified only for activity less frequent than

once in 25 years. The ages of mature trees that are

definitely outside the runout zone provide a reference

for increment core analysis. Trees in a suspected

runout zone should be older the closer they are to the

reference. It helps to summarize information about

tree species, age, and height on a vegetation map.

Identified paths should be marked on as detailed a

topographical map as is available. To begin with, avalanche areas can be located on standard U.S. Geological Survey maps (for example, 7- to 15-minute quadrangles). However, for zoning purposes (see chapter

7), it is necessary to use maps as detailed as possible,

with contour intervals of 5 m or less. Planning defense

structures (see chapter 7) may require even more detailed surveys.

84 AVALANCHE PHENOMENA
TABLE 2.—Venelalion as a rouf>h indicator

of avalanche frequency

Frequency: at least

one avalanche in an

interval of—

1-2 years

Vegetation clue.i

Bare patches, willows and shrubs, no

trees higher than about 1 to 2 m. Broken

timber.

2-10 years Few trees higher than I to 2 m. Immature trees of disaster or pioneer species. Broken timber.

10-25 years Predominantly pioneer species, young

trees of the local climax species (increment core data).

25-100 years Mature trees of pioneer species, young

trees of the local climax species (increment core data).

Over 100 years Increment core data.

Avalanche movement'

Avalanche motion may be described as flowing, airborne powder, or mixed. Flowing motion is the turbulent, tumbling action of snow moving mostly along the

ground. In airborne-powder (usually called simply

powder) motion, most of the snow is swirling through

the air. Pure powder motion seldom occurs except

when the snow goes over a cliff (fig. 112). Almost all

avalanches move with mixed motion; large blocks and

particles tumble and bounce along the ground, and

smaller particles are airborne.

A slab avalanche usually passes quickly through

several types of motion as it attains its maximum

velocity. After breaking loose from the starting zone,

a snow slab accelerates down the track and very

quickly splits into smaller blocks. Within a short distance, the blocks begin tumbling and colliding. If the

track is extended and the motion continues, the blocks

disaggregate into chunks and particles. The smallest

particles are tossed into the air, forming a snow dust

cloud. This results in mixed avalanche motion, with

part of the snow flowing along the ground and part

moving in a snow dust cloud.

It is thought that in large, mixed-motion avalanches

the great majority of debris is transported close to the

*In this and other sections that deal with engineering

aspects of avalanches, pressures are given in the more conventional units of metric tons per square meter (t/m-) rather

than newtons per square meter. See appendix A for conversion factors.

Figure 112.—An airborne-powder avalanche shooting over a

cliff in central Colorado. (Photos by Standley)

Avalanche movement 85
Figure 113.—The mixed type of avalanche motion is characterized by a prominent snow dust cloud. In this sequence, the snow

dust cloud is about 40 m high and 150 m wide. One of the Jones Brothers Avalanche Paths, near Empire, Colo. (Photos by

Cleveland)

snow surface. Debris drops out continuously. The

avalanche continues undiminished as long as the

snow in the track feeds the moving mass and replaces

the debris that drops out. This condition is fulfilled

when the track snow is relatively unconsolidated or of

soft slab texture. Thus, in a sense, a moving avalanche

is the result of successive failures connected by a common track. Under certain conditions, avalanches can

run long distances over level terrain or up adverse

grades (figs. 114 and 115). Extremely powerful avalanches like the Huascaran avalanches described in

chapter 1 can run long distances even over bare

ground. It must be remembered, however, they were

largely avalanches of glacial ice, not seasonal snow.

If the snow density and the avalanche velocity are

known, it is possible to compute approximate impact

pressures, in kilograms per square meter, from the

following simple relationships:

Maximum impact pressure = pV'-/g

Mean impact pressure = pV'-/2g

where

P is the snow density ( kg/m'' )

gis the acceleration due to gravity (9.8 m/s-)

K is the speed of the avalanche ( m/s).

It is not known how the density varies in a mature avalanche. One possibility is that close to the snow surface, the density is about that of the undisturbed track

snow, perhaps 100 to 200 kg/m-' or higher. It is

thought that density falls off rapidly with height above

the snow surface. At the upper boundary of the visible

snow dust cloud, the density may be 10 kg/m-' or less.

86 AVALANCHE PHENOMENA
j^H^PPH^^'^

^^^^^BK?'. -t'^'^- ''^^^^^^^^^K^^

^^^^--.^^^

t^' '^0-

Figure I 14.—This mixed-motion avalanche ran down the

slope, over the roof of an

avalanche shed, across a lake

150 m wide, and up the opposite slope near Oberalp Pass,

Switzerland. (Photos by Frutiger)

ä^^^^r.w

>

^^^^^^'H

¿^^^^^^^^i|

,y ' ,'K

^mL

A valanche movement 87
li'à.-1

IV. Vi,

vî^ -'H!»»^.?^

L-

..i>-

L .

>

Figure 115.—Large, wet, midwinter avalanches that ran for 450 m across the level Plains of Abraham on the east side of Mt.

St. Helens, Wash. (Photo by Emetaz)

Avalanche velocities are also a matter of rpeculation; however, many have been clocked, and typical

speeds seem to be in the range of 25 to 75 m/s. It is

reported that avalanches have reached speeds up to

125 m/s. Observed speeds are higher than would be

expected on the basis of a balance between the pull

of gravity and frictional drag resistance of the snow

surface and the air. It has been suggested that large

avalanches adjust their shapes to minimize drag and

that friction is minimized by an air-cushion effect.

Probably other mechanisms also contribute to the high

observed speeds.

Speeds in excess of 30 m/s are observed only for

large, dry avalanches with well-developed snow dust

clouds. If an avalanche is very wet, it moves along the

snow surface as a slurry with little or no snow dust

cloud. The speed of wet-snow avalanches is typically

in the range of 5 to 30 m/s; the higher limit is approached only by very large, wet avalanches.

Using reasonable values for density and speed, it

is possible to make simple calculations that demonstrate the enormous impact pressures of a moderatesized avalanche. Consider an avalanche moving at

50 m/s with a density distribution that varies from

100 kg/m' at the snow surface to 10 kg/m-' at the

top of the snow dust cloud. For this hypothetical avalanche, the maximum impact pressure at the snow

surface is about 25 t/m-; impact pressure at the top

of the snow dust cloud is about 2.5 t/m-. From more

detailed theoretical calculations, it appears that maximum impact pressures should be in the range of 5 to

50 t/m-'. These values have been confirmed by measurements on large avalanches and by inspection of

damage to homes, railroad cars, concrete foundations,

and other property. In one test on a very large and

fast-moving avalanche, the maximum impact pressure

exceeded 100 t/m- (Salm 1965). Table 3 gives a

rough correlation between avalanche pressure and

potential damage.

From the viewpoint of engineering design, avalanche impact pressures represent an extremely high

external load; even concrete structures require substantial reinforcement.

Avalanches cause damage in several ways. First is

the direct thrust of the avalanche, with impact pressures as given above. In addition, avalanches may

88 AVALANCHE PHENOMENA
Figure 116.—Possible speed and density variations in the

profile of a large mixed-motion avalanche.

TABLE 3.—Correlation between impact pressure

and potential damaf>e

Impact

pressure

(t/m^) Potential damage

0.1 Break windows

0.5 Push in doors

3 Destroy wood-frame

structures

10 Uproot mature spruce

100 Move reinforced-concrete

structures

exert upward and downward forces; they have been

known to lift up and move large locomotives, road

graders, and buildings. From field measurements and

observations of damage, it appears that the upward

and downward forces are about one-fourth to onehalf the direct thrust. Thus, a large avalanche overriding a structure could exert a vertical pressure of 25

t/m^. Finally, the airblast of a fast-moving avalanche

(speed in excess of 30 m/s) may exert pressures up to

about 0.5 t/m-. Although small compared to the

main thrust of the moving snow, such pressures are

capable of destroying doors, windows, and poorly

designed roofs. There are also reports that the airstart of motion

Flowing motion.

à1 ^B

Mixad motion.

Figure 117.—A possible model for the buildup of a dry snow

avalanche. The stress wave of the avalanche advances in

the snow slightly ahead of the avalanche front. As the

avalanche passes over, new mass is fed into the main body

An airblast sometimes precedes the avalanches.

blast has induced lung injuries. The strongest airblast

effects are confined to within 100 m of the observed

boundary of the snow dust cloud. As a very rough

rule, the extension of the airblast is about equal to the

height of the snow dust cloud. Slow-moving wet avalanches that cling to the snow surface have no airblast.

Intuitively, it would seem that the impact pressure

should be higher in channeled paths than in unconfined paths. Some data support this notion. PresumA valanche movement 89
Air blast /// '

(0.5ton/m^) ,/i

Figure 118.—The horizontal thrust of a large avalanche may cause impact pressures in the range of 5 to 50 t/m-. Vertical impact

components are typically one-fourth to one-half the magnitude of the horizontal components. Airblast pressures are much

smaller, perhaps no higher than 0.5 t/m-.

ably, the confining walls of the channel increase the

speed and density of the flowing snow.

Further reading

Bader, Henri.

1939. Der Schnee und seine Metamorphose, Beitr. zur

Geol. der Schweiz, Geotechnische Ser.. Hydrol., Lieferung 3. Bern. (Snow and its metamorphism. 1954. U.S.

Army Corps Eng., Snow, Ice and Permafrost Res. Estab.,

Transi. 14, 313 p.) Tliis classic ¡realise on snow and

avalanches contains chapters on snow and avalanche

mechanics, field invcstiftalions, and meteorological

effects on the snowpack.

Frutiger, Hans.

1964. Snow avalanches along Colorado mountain highways. USDA For. Serv., Res. Pap. RM-7, 85 p. Rocky

Mt. For. and Range Exp. Stn., Fort Collins, Colo. Describes and illustrates characteristics of 80 avalanche

patlis in the Colorado inoiiiitains and discusses possibilities for stnictiiral control. Pictures and maps are given

for each avalanche path.

Hotchkiss, W. R.

1972. Avalanche awareness and safety for snow scientists

in the field. Proc. West. Snow Conf. 40: 75-80. A nontechnical coverage of avalanche features and liazard

evaluation.

International Association of Scientific Hydrology. Commission of Snow and Ice.

1966. International symposium on scientific aspects of

snow and ice avalanches. Publ. No. 69, 424 p. Gentbrugge. Belgium. A collection of more than 45 papers on

all aspects of avalanche phenomena. Most papers technical or scmilcchnical. English and French.

Martinelli, M., Jr.

1974. Snow avalanche sites—their identification and evaluation. U.S. Dep. Agrie, Agrie. Inf. Bull. 360, 26 p.

Many illustrations of avalanche patlis in winter and

summer conditions. Nontechnical.

Mellor, Malcolm.

1968. Avalanches. Cold Reg. Sei. and Eng.. Part III-A3d,

295 p. U.S. Army Corps Eng., Cold Reg. Res. and Eng.

Lab.. Hanover, N.H. Written for the engineer, this technical monograph contains an excellent section on avalanche movement and forces.

90 AVALANCHE PHENOMENA
Sommerhaider, E.

1965. Lawinenkraefte und objectschutz. Winterbericht des

Eidg. Institut fuer Schnee- und Lawinenforschung Nr.

29 1964/65 (In English as: Avalanche forces and the

protection of objects. Alta Avalanche Study Center,

Transi. 6, 14 p. Available from USDA For. Serv.,

Rocky Mt. For. and Range Exp. Stn., 240 W. Prospect,

Fort Collins, Colo.) Engineering criteria for calculating

the size and strength of avalanche sheds, deflecting walls,

guide walls, and wedges.

Voellmy, A.

1955. Über die Zerstörunskraft von Lawinen. Schweiz.

Bauzeitung, Jahrg. 73, S. 159-165, 212-217, 246-249,

280-285. (In English as: On the destructive force of

avalanches. 1964. Alta Avalanche Study Center, Transi.

2, 63 p. Wasatch Nati. For., USDA For. Serv., Salt Lake

City, Utah. Loan copy of English translation available

from Rocky Mt. For. and Range Exp. Stn., 240 W.

Prospect, Fort Collins, Colo.) Estimates of avalanche

forces based on the analysis of damage to buildings and

other structures. Presents a theoretical model of avalanche movement and forces.

Further reading 91
>

\

..ífí

Jt
Chapter 5

stability

evaluation

The previous three chapters concentrated on snow

and avalanche fundamentals. In this chapter, an

important application of fundamentals is discussed

—the forecasting of likely avalanche activity. This

evaluation of slope stability, with no reference to

hazards to humans and their facilities, is strictly a

geophysical problem and therefore is given the

technical name "stability evaluation."

Stability evaluations are based on six kinds of

''inputs," or categories of information. These

include quantitative meteorological measurements,

such as study-plot precipitation and ridgecrest

winds, as well as such qualitative observations as

test skiing and visual observations of avalanche

path loading.

Figure 119.—Test skiing may be performed "belayed" or "free." Free

test skiing should be restricted to miniature or lightly loaded

slopes that involve minimum risks. If this slope had been more heavily

loaded, the skiers would have roped up or used explosives, or both.
HAZARD

Human usage of tfie

avalancfie patfi

Consequences of an

avalanche

STABILITY

EVALUATION

Present and future

slope conditions

ADMINISTRATIVE

CONTINGENCIES

Available equipment

and manpower

Safety and Economics

Public needs

1 ^ 1

1—1

PROTECTION AND

CONTROL DECISIONS

Hazard announcements

Restrictions

Artificial release

_l

4r

ACTIONS

Figiiie 120.—Day-by-day decisions concerning avalanche protection and control are influenced by many complex factor

Stability evaluations play an important role.

The general problem

An estimate of the likelihood that avalanches will

release in a given region and within a given time interval is called a stability evaluation. The region may be

a single avalanche path or a group of paths. The time

interval is measured from the present into the future.

Usually, stability evaluations are made for the immediate future; that is, within several hours of the present. Occasionally, evaluations for longer times are

required.

Stability evaluations influence decisions on whether

or not to control the avalanches in the region and on

what type of control or protection is appropriate. They

also determine when avalanche warnings should be

issued. To understand the role of stability evaluation

in decisionmaking, it helps to consider briefly two very

different avalanche problems; the first concerns ski

areas, and the second has to do with highways.

In developed ski areas, small as well as large paths

contribute to the avalanche hazard. Because of the

large number of skiers on the slopes, any instability

is very likely to be triggered. It is also highly probable

that an avalanche triggered by one skier will sweep

into another. Finally, there is a high likelihood that

the encounter of a skier with even a small avalanche

will result in a serious accident.

A simple mathematical notion connects all these

probabilities; if an event depends on several independent preliminary actions, then the probability of

that event is equal to the products of the probabilities

of the preliminary actions. Consider a case in which

stability evaluation indicates that there is one chance

in 100 (or a probability of 1/100) that a particular

slope is unstable enough to be released by a skier.

Next suppose that it is certain (or 1/1) that a skier

will cross the slope during the day. Moreover, there is

one chance in two (or 1/2) that a skier can be found

in this potential avalanche path at any time during the

day. For this situation the probability of a serious accident is (1/100) X (1/1) X (1/2), or (1/200).

Although this is a small probability, it is significant

and cannot be ignored. In practice, one does not indulge in probability calculations, but the point is clear:

Any suspeeled instability in a ski area entails a risk

that should motivate some sort of hazard control or

protective action.

94 STABILITY EVALUATION
Snow

cover

distribution

Current and

past

avaianchies

SnoWpack

structure

Local

meteorological

measurements

Weather

Service

information

Ski and

explosive

tests

II

Hazard

A J

Stability

evaluation

Protection

and control

decisions

Administrative

contingencies

Actions

Figure 121.—Stability evaluations are based on six kinds of inputs. Current avalanche events greatly influence the evaluations.

Highway problems may allow a little more leeway

in accepting risk. Consider the hypothetical evaluation

that gives a 1/100 chance that a given slope will be

triggered naturally and avalanche across a highway

in the next 8 hours. In that 8 hours, suppose traffic

will be under the path in question 1/100 of the time.

For this situation, the probability of a serious accident

is 1/10,000. Here it may be quite reasonable to

chance the accident, leave the highway open to traffic

(or at least to supervised convoys), and wait for an

opportune time to take action. (Chapters 6 and 7

describe avalanche control.)

The above figures are fictitious, but they demonstrate that decisions about protective and control

actions must be based on stability evaluation of the

slopes plus analysis of human problems. Although the

stability evaluation is not the sole determiner of the

protective decision, the more precise the stability

evaluation, the sounder the decision. This chapter

discusses methods for evaluating slope stability. The

more complex problem of making protective decisions

for ski areas and highways is discussed in later

chapters.

Broadly speaking, stability evaluations are based

on six kinds of input:

• Snow-cover distribution

• Current and past avalanches

• Snowpack structure

• Local meteorological measurements

• National Weather Service information

• Ski and explosive tests.

Several of these inputs require a close liaison between the control team and the person who makes the

evaluation. The most precise stability evaluation for

a slope must be based on knowledge of recent avalanche activity on nearby slopes. For this reason, the

stability evaluation and the control work are generally

performed either by the same team or by teams working in close cooperation. Often stability evaluation

and control work are so interlinked that the boundary

is not clear. This interlinking is a unique feature of

avalanche technology, in contrast to the forecasting

and control of wildfires, earthquakes, and other natural hazards.

The general problem 95
How are the above six kinds of inputs converted

into a stability evaluation? Generally, working systems in mountain ranges around the world follow

similar lines. A team of experienced workers studies

the inputs, issues broad directives about the likelihood

of instability, and in most instances supervises control

work. These systems rely almost entirely on human

judgment, and they obviously depend critically on the

competence of the team. Many other decision systems

follow impersonal "cookbook" formulas, but replacement of even small parts of the customary subjectivity

in stability evaluation remains a research topic. Moreover, teams are structured like trade guilds; a "master" hands down his skills to "apprentices," often

chosen more for mountaineering and administrative

talents than for formal scientific training.

In the next sections, it will be shown how each input

contributes to the overall picture. Although the inputs

will be discussed separately, the reader should keep in

mind that in practice the separate inputs are not considered separately but instead are integrated and

weighted subjectively to form a single judgment.

Snow-cover distribution

and avalanche activity

This section concerns the first two inputs; namely,

( 1 ) snow-cover distribution and (2) current and past

avalanches. For the most part, these are simple, visual

observations, but they form the basis of the most reliable stability evaluations.

Snow-cover distribution is determined by direct

visual observation of whether an avalanche path is

filled with snow and primed for release. To begin

with, significant avalanches cannot occur on a path

until, first, terrain irregularities such as boulders and

brush are covered with snow, and then additional

snow is deposited on this smoothed foundation. For

most paths, this requires about 1 m of snow in the

starting zone and track. The exceptions are paths on

permanent snowfields, smooth rock, dirt, grass, etc.,

where avalanches may run with perhaps 15 cm of

coverage in the starting zone.

Normally, the beginning of the avalanche season

on a particular slope can be anticipated from visual

observations of snow-cover amount and distribution

and conditions in the starting zone and track. If visual

observations cannot be made, it may be helpful to

correlate statistically the earliest significant avalanche

on a given path with snow depth at an accessible study

plot. An example of such a correlation is given in

table 4. Many years of historical data are required

for the correlation, which still must be used with a

large safety factor.

After the beginning of the avalanche season, direct

observation continues to provide the most reliable information on snow buildup in the starting zones. For

ski areas, direct observations include watching the

wind-deposition patterns and field checking the starting zones. During operating hours, it is essential to

keep track of new snow buildup in or near the avalanche paths in the area. Generally, 15 to 30 cm of

new snow in the starting zone can be considered significant and worthy of further investigation. Observation of snow loading in large avalanche paths above

highways or villages may not be possible during critical periods. Here, direct observation may be limited

to occasional inspections from a distance during temporary clearing of the weather.

TABLE 4.—Study-plot snow depth (cm) at time of earliest avalanche onto Little Cottonwood Road, Utah

Season Depth

1950-51 244

1951-52 158

1952-53 145

1953-54 132

1954-55 256

1955-56 216

1956-57 259

1957-58 256

1958-59 137

1959-60 226

Season Depth

1960-61 198

1961-62 183

1962-63 203

1963-64 208

1964 65 206

1965-66 191

1966-67 132

1967-68 132

1968-69 218

1969-70 165

Note: There has never been an avalanche to the road with less than 132 cm of snow in the study plot.

96 STABILITY EVALUATION
Figure 122.—Avalanche release on slopes of common aspect. All areas between avalanches are suspect.

After avalanches empty out a starting zone, direct

observations provide information on when the zone is

reprimed with new snow. The old crown surface is an

excellent natural snow stake.

In ski areas, where some avalanche paths may also

be ski runs, the amount of ski compaction on the path

in question is important. Lightly skied slopes are far

more unstable than heavily skied slopes. Moguled

slopes are virtually immune to avalanches, except during very wet or slushy conditions, and rarely as a dry

slab on depth hoar.

The second input, "observations of current and past

avalanches," includes the following:

Observation of slopes with common aspect. Because loading is sensitive to wind direction, avalanches

tend to occur on slopes that have a common aspect.

For example, after a storm, it may be noticed that avalanche activity was most intense on southeast-facing

slopes. All southeast-facing slopes that did not avalanche during the storm should then be suspected of

instability.

Observation of slopes with common elevation. Similarly, in an area that has a wide range of starting-zone

elevations, it may be observed that avalanche activity

is confined to particular elevation zones. For example,

the higher zones may be relatively inactive, but serious instability may exist at lower elevations. This phenomenon is usually caused by wind or temperature

patterns.

Frequent and infrequent paths. This observation is

most applicable to large avalanche paths that threaten

highways and villages. Based on historical data, avalanche paths can be classified according to frequency.

It is accepted practice to defer evaluation of infrequent

paths until the stability conditions of the more frequent paths are known. If frequent paths are moderately unstable, then instability can be expected on the

infrequent paths. Conversely, if the frequent paths are

stable, it is usually possible to evaluate as stable the

infrequent paths.

Thaw warnings. Small, wet avalanches of either the

loose-snow or slab type often give ample warning that

free water is accumulating in the thawed snowpack.

Wet avalanches of increasing size can be expected.

Repeaters. Statistical evidence shows that once an

avalanche path is activated, there is a good chance it

will be reactivated later in the season. Many avalanche

paths that are not active every avalanche season tend

to run more than once in seasons when they are active.

There is some evidence that the second avalanche on

a path is usually larger than the first.

Stuffing. The remarks made on sluffing in chapter 4

under "Failure of Snow Slopes" should be recalled.

Visual observation may indicate that conditions were

highly unstable during a storm and that small avalanches released to remove the instability. These avalanches may be partly obscured by additional snowfall.

Snow-cover distribution and avalanche activity 97
Figure 123.—Sluflfing activity covered up by fresh snow. Parts of these slopes self-stabilized during the storm. (Photo by

LaChapelle)

Snowpack structure

During storms, observers are occupied with keeping

track of storm parameters, conditions of the newly

fallen snow, and the increased avalanche activity that

comes with heavy precipitation. Between storms it is

possible for them to investigate the condition of the

deeper snowpack.

Diagnosis of snowpack structure involves searching

for weak layers. Some kinds of weak layers that are

definitely correlated with instability are those that contain TG grains; loose, cold snow; surface hoar;

graupel; radiation-recrystallized grains; or wet snow.

Although a hard crust cannot be considered a weakness, there is much evidence that instability is greatly

intensified when any of these weak layers is immediately above or beneath a firm crust (fig. 124).

Methods of analyzing pits for weak layers are described in chapter 3 under "Snowpack Analysis." It is

worth reemphasizing that digging pits near the avalanche release points is the only reliable and efficient

way to locate significant weak layers. The farther the

pits are from the avalanche release points, the less

reliable the pit information.

TG-metamorphosed grains. An explanation of TG

metamorphism was given in chapter 3. Weak TG

grains are considered the most obvious clue that the

snowpack is potentially unstable. Thick TG layers

stand out from ET layers in the pitwalls and are

easily identified. The importance of TG metamorphism in stability evaluation cannot be overemphasized. Instability is most apt to exist where

metamorphism has advanced toward weak, large,

depth-hoar grains.

When weak TG layers have been identified, the

following should be kept in mind:

• The probability of deep slab instability is many

times greater on slopes that have depth hoar than

on those that have an ET structure throughout.

• Although instability is most likely to be triggered

during or immediately after storms, slopes with

weak TG layers remain unstable for long periods

between storms. Instability can be triggered at almost any time.

• The first storm that deposits snow on a TG layer

may not produce avalanches. This should not give

98 STABILITY EVALUATION
,-^

^-f^

Crust or

hard surface

/ ^■■""

o

o o

Figure 124.—Investigation of snowpack structure near the starting zones is an important input. Generally, one looks for both

thick and thin weak layers. A thick, weak TG layer is the most obvious clue of potential structural instability. A thin, weak

layer sandwiched between a stiff slab and a crust is also highly unstable, but this combination is difficult to detect in practice.

observers a false sense of security. Two or more

storms may be required to build a slab on the TG

substratum.

• Slopes with weak TG layers do not always

respond immediately to explosive control (see

chapter 6, "Use of Explosives"). Instability is

sometimes triggered several hours after avalanche

control.

• If a slab is removed down to the TG substratum,

TG metamorphism normally continues in the

thinned snowpack, and instability reappears and

sometimes intensifies.

• Instability remains until the TG grains are sintered under the pressure of a thick snowpack or

until MF metamorphism dominates. Normally, TG

instability is most acute during December and January and begins to diminish in February. Strengthening of the TG layers usually occurs by early

March.

• After TG layers are strengthened, instability

may reappear in spring because of melting of the

bonds (thaw instability). Full-depth avalanches

have run on layers of wet TG grains.

These seven points make it clear that the evaluation

must show potential instability when TG layers are

prevalent. The situation becomes especially critical

in a ski area or along a ski-tour route where localized

patches of TG grains produce small but dangerous

slab avalanches. Stability varies remarkably from season to season, depending on the weakness and extent

of TG layers.

Loose, cold sno-w. If the snow surface is maintained

at a low temperature for a long period after a storm

and the surface snow is relatively loose and cohesionless, there is a good chance that any new snow may

not bond well to the loose, cold surface. Typically,

potential instability is acute after air temperatures

have remained below about —15° C for several days

and the new storm brings in at least moderate amounts

of precipitation before the old snow surface has a

chance to warm up significantly. Extreme instability

can be expected when the old surface is a cold crust

and the first few centimeters of new snow consist oí

very cold, cohesionless grains.

Siirjace hoar. Surface hoar growth was described

in chapter 2 under "Heat Exchange at the Snow Surface." Uncompressed layers of surface hoar are extremely weak. It is therefore not surprising that widespread coverage of a slope by surface hoar leads to

instability. Surface hoar layers must be detected while

they are still on the surface, before they are buried

and compressed. Once buried, they are almost impossible to identify. It appears that in many cases, surface

hoar instability is relatively short lived, lasting for

only one or two storms.

Graupel. (See "Snow Crystals" in chapter 2.)

Graupel is easily observed in snowpits. This type of

Snowpack structure 99
grain is known to cause instability; however, for some

unclear reasons, graupel does not cause the intense

instability that would be expected on the basis of its

weak structure in snowpit walls. Graupel seems

mainly to induce slab avalanches of one storm thickness. It appears that few deep slab avalanches have

released on graupel bed surfaces.

Radiation-recrystallized grains. These grains are

caused by a rather peculiar radiation phenomenon

that occurs only rarely during the avalanche season

and then only at high altitudes on south-facing slopes

in late winter. On clear, dry days, an intense temperature gradient may be established between the snow

surface, cooled by terrestrial radiation loss, and the

sublayer, warmed by penetrating solar radiation. This

produces a weak layer of recrystallized grains resembling depth hoar and, generally, an underlying ice

crust.

Wet snow. In practice, wet snow instability is evaluated more from meteorological data than from snowpits. Snowpits are useful in determining when the

temperature of a large part of the snowpack reaches

0° C and therefore approximately when the snowpack

is susceptible to thaw. Snowpits are also useful in

evaluating the amount of free water remaining in the

pack after a rainstorm.

Ice crusts are potential glide surfaces. The closer

ice crusts are to the snow surface, where free water is

produced, the greater the possibility that the free

water can dissolve the bonds between ice and slab.

Deeply buried ice crusts are usually not a problem.

Figure 125.—A glide crack between two wet snow avalanches.

(Photo by Frutiger)

Slowly opening tensile cracks may sometimes be

observed when a wet slab is gliding along a wet, lubricated surface. Most glide cracks that open early in

the season (November through January) do not develop into dangerous slabs. Glide cracks later in the

season are more likely to become unstable, but the

process is slow enough to provide ample warning.

Local meteorological data

Field observations of snow buildup on or near avalanche release areas are often not feasible. When conditions are especially critical it is rarely possible to

tour to the release zones of high-elevation paths that

threaten highways and villages. Even in ski areas, field

observations of avalanche paths are limited to daylight

hours. It is therefore necessary to rely heavily on

meteorological measurements.

As discussed in chapter 2 under "Storm Analysis,"

indirect evidence of avalanche path loading can be

inferred from study-plot precipitation measurements

and ridgetop wind measurements. From chapter 3, recall that the requirement for catastrophic snow failure

is rapid loading to some critical value. Thus, the basic

problem in making stability evaluations from meteorological measurements is to infer from the study-plot

and ridgetop data whether the rate of loading and total

load are approaching critical values in the avalanche

paths in question. The simplest way to use the numerical data furnished by instruments is to identify, for

each path or group of paths, the following critical

conditions:

(1) Wind directions that load the paths in question

(for example, winds from southwest to northwest)

(2) Windspeed required for lee-slope loading (usually above 5 m/s)

(3) Critical precipitation intensity (for example, 2

mm/h)

(4) Critical total water equivalent (for example, 20

mm).

Each storm is then monitored and analyzed. The

slopes in question are evaluated as unstable when the

four critical conditions just mentioned are satisfied

simultaneously. An example of this kind of analysis

for the storm plot of figure 54 is shown in the example

below. The critical values of the four conditions for

each path or group of paths are derived from at least

3 to 5 years of records of meteorological measurements and avalanche conditions. New data are used

100 STABILITY EVALUATION
continually to improve the choice of critical conditions. Thus, the success of the above method depends

on keeping careful records of meteorological and avalanche observations. For avalanche problems in general the importance of good records cannot be

overemphasized.

For example, suppose historical data indicate that

a group of east-facing slopes that affect a highway

becomes unstable when the following conditions are

satisfied simultaneously: wind direction 270° ±60°,

windspeed above 5 m/s, precipitation intensity 2

mm/h, and total water equivalent 20 mm.

The storm plotted in figure 54 would then be analyzed as follows in terms of water equivalent of

precipitation, wind direction, windspeed, and air

temperature:

From 0700 to 1600, January 24, Winds are approximately critical, but the precipitation intensity of 1.1

mm/h is below critical. Instability is only gradually

increasing (explosive control may be premature).

From 1600, Jamiary 24, to 0400, January 25.

Winds are critical, precipitation intensity is above

critical at 2.5 mm/h, and the total water equivalent

deposited at the critical rate exceeds 20 mm. A high

level of instability is reached (explosive control

should be effective during the early morning of January 25).

From 0400, January 25, to end of storm. All factors, except accumulated water equivalent, are less

than critical. Instability is beginning to decrease.

Critical conditions (3) and (4) can be modified to

account for snowpack structure. If the snowpack is

known to have a weak structure, the critical values of

precipitation intensity and total water equivalent can

be reduced. Similarly, if the snowpack has a relatively

strong structure, it is possible to use increased critical

values. However, even with these corrections, there

will be a great amount of uncertainty in the evaluation, and the analysis must be interpreted cautiously

in making control decisions. For ski areas, where a

stability evaluation error could be disastrous, a large

safety factor is required, and the above analysis may

be of less practical use. For highways, where slight

error is less dangerous, the above method, or even a

more complex one, may be useful. Also, as a general

rule, the larger the starting zones of the paths in question, the more applicable a numerical analysis of the

simultaneous effects of precipitation and wind. A

numerical technique for issuing "extreme hazard"

warnings for large areas is given in chapter 7.

For small paths, normally a threat in ski areas and

on back-country tour routes, critical loading due to

wind transport is always possible regardless of studyplot precipitation measurements. Thus, it is worthwhile to monitor ridgetop windspeed and direction

continually. Wind redistribution is greatest for a few

days after a new snowfall, but transport may occur as

long as the surface snow remains uncrusted.

Meteorological measurements are useful for evaluating wet snow instability. The important variables to

monitor, at least qualitatively, are rainfall, radiation,

and air temperature. Heavy rain causes wet snow

instability by adding weight, decreasing cohesion in

the surface layers, and lubricating a potential bed

surface. The amount of rain required for instability

depends on the temperature of the top layers of the

snowpack. If these layers are near 0° C before the

rain, then relatively little rain can cause avalanches.

Cold snowpacks have a high capacity for absorbing

rain.

Wet snow instability is most likely during the first

warmup (thaw) after a heavy snowfall. Wet snow

instability should be expected after late spring and

summer snowstorms. The delay between the end of

the storm and the thaw can be estimated from the air

temperature. If the air remained cold during the storm

and then climbed slowly afterwards, instability may

be delayed for several days. Air temperatures at high ,

elevations can be estimated most of the time by subtracting 6° C per 1,000 m of elevation above the elevation at which measurements are taken. Wet snow

instability is most intense from midafternoon, shortly

after solar radiation reaches its peak. Melting is at

first delayed, since the heat of fusion (80 calories per

gram of ice) must be supplied. Once melting begins,

the process accelerates because wet snow absorbs far

more solar radiation than dry snow, and the onset of

wet snow instability may seem to come rather rapidly.

Similarly, wet snow instability may persist into late

afternoon, until the melt water releases its heat of

fusion (again, 80 calories per gram).

Usually, wet snow instability is confined to the most

recently deposited layers, to a depth of about 1 m.

Prolonged thawing occasionally triggers deeper or

even full-depth slabs. These deep, wet slabs may release almost any time during a prolonged thaw.

Several other meteorological features may influence

stability evaluation. Humidity, air-temperature trends

during a storm, and the form of the new snow crystals

have been proposed.

Local meteorological data 101
^^^í*i

t^M^eP«^*'^'"''

SCAB

ted bV »'9*^^-

>Nlnds)

Figure 126.—New snow instability is usually a result of a

heavy, stiff layer deposited on

a light, weak layer. This pattern may be caused by changes

in temperature, snow-crystal

form, and wind.

Humidity could be important because of two different effects. First, it controls how far a windblown

snow grain can travel before sublimating. When relative humidity is high, wind transport of snow is more

effective. When the air is dry, large amounts of blowing snow sublimate and never reach the lee slopes.

High humidity is also thought to promote a slablike

texture in deposited snow and therefore a more

fracture-sensitive structure. This second possibility

has not been substantiated.

Air-temperature trends are known to correlate with

slab stability. Rising air temperature during a storm is

considered an unstable trend, presumably because

heavy snow is being deposited on lighter snow. Falling

air temperature is considered a stable trend. In the

north temperate zone, temperatures normally fall at

the end of a storm, but when one storm follows on the

tail of another, it is possible that some precipitation

could fall at cold temperatures, and then the bulk of

the precipitation could follow during the warmer temperatures of the next storm.

Many avalanche workers believe that stability is

influenced by the form of the new snow crystals. In

particular, it has been suggested that instability increases with riming. Of course, heavily rimed crystals

are deposited as layers that have a high water equivalent, so it is not clear how much of the observed instability is due to high water equivalent and how much

to snow-crystal form.

It is possible to generalize the effects that humidity,

temperature, and crystal form have on the stability

of new snow. The unstable pattern seems to be a

relatively heavy or strong layer over a relatively light

or weak layer. This pattern might be generated by:

• Rising temperatures during a storm

• Lightly rimed or rime-free crystals falling early

in the storm, followed by a thick layer of heavily

rimed crystals

• Initial deposition during light winds, followed by

rising winds toward the end of the storm.

102 STABILITY EVALUATION
Finally, it must be emphasized that humidity, temperature, wind, and the form of snow crystals are at

best secondary compared to precipitation and its

transport. The glaring fact is that 90 percent of all

avalanche activity occurs during or shortly after

storms. Regardless of secondary influences, the probability of instability on a given path rises rapidly as

the path is loaded with snow.

National Weather Service data

The National Weather Service may assist avalanche

workers by providing data on winds, temperature, and

humidity at upper levels; short-term forecasts to guide

immediate control decisions; and extended forecasts

for operation planning.

The National Weather Service releases balloons

from major weather stations twice a day. These balloons are released simultaneously all over the world at

Greenwich Time 0000 Z and 1200 Z. The corresponding Rocky Mountain and Pacific Standard times of

release can be found from table 5. About 2 hours after

balloon release, weather stations may be contacted for

upper-air data. Normally, avalanche workers in the

Rockies and Sierras should be interested in data taken

between the 700-mb and 500-mb levels (see fig. 10).

For the Cascades, the band of interest would be mainly

between 850 mb and 700 mb. The balloon data of

most use in avalanche work are windspeed, wind

direction, temperature, and dewpoint.

Balloon wind data supplement local measurements

for estimating the avalanche path loading in a large

region. When local wind instruments are rendered

inoperative by rime, lightning, or some other cause,

balloon wind measurements must be used as backup.

Balloon temperature data for the same elevation as

the starting zones give a reasonable estimate for air

temperatures near the ground in the starting zones.

Some avalanche-threatened areas in the Cascades

and Wasatch Mountains are immediately downstream

from balloon release points. Balloons occasionally fly

directly over these areas and transmit accurate ridgeTABLE 5.—Release times of weather service balloons

Greenwich Greenwich

Time 1200 Z Time 0000 Z

Mountain Standard 5 a.m. 5 p.m.

Pacific Standard 4 a.m. 4 p.m.

Note: 0000 Z is 1 day ahead of continental U.S.

time. Thus, 0000 Z on December 12 is equivalent to 5 p.m. December 11 Mountain

Standard Time.

top information. Where avalanche stations are not

favorably located with respect to release points, it may

be necessary to interpolate data from surrounding release points. For example, upper winds near Aspen,

Colo., can be inferred by interpolating Grand Junction

and Denver measurements.

It is a relatively easy task for Weather Service personnel to give the raw balloon data to avalanche

workers. However, interpreting the data and issuing

forecasts for specific avalanche areas require some

effort and time on the part of the Weather Service and

a reciprocal effort by avalanche workers.

It is easy to understand that interpretations and

forecasts of small-scale or local weather are imprecise.

Forecasting mountain weather is especially complex.

Nevertheless, with existing physical models, reporting

networks, and experience, weather forecasters usually

do a creditable job of predicting air temperatures,

windspeeds and directions, and the hkelihood of precipitation. Forecasters still have trouble, however,

determining the amount of precipitation for mountainous areas. Good communications between the

field and the forecast center, especially during storm

periods, help forecasters to adjust precipitation forecasts and, in the long run, to improve their forecasting

skills.

How is this communication established? First, working through appropriate channels, avalanche workers

must find out which weather center is in a position to

issue special forecasts and to exchange information on

a personal basis. A preseason meeting is then set up

between avalanche workers and the meteorologist-incharge of the chosen station. A plan for exchanging

information is developed; it will no doubt have to be

renewed and improved each year. Routine communication can be by telephone, radio, or teletype. A direct

telephone link into the forecast center poses the fewest

operational problems. Unlisted telephone numbers,

which ensure prompt service, can be obtained if

avalanche workers explain to the meteorologist-incharge the need for this special service. It may help

for Weather Service personnel to visit the avalanche

workers' area, and avalanche workers may benefit

from an inspection of the weather center.

Generally, the communication plan should provide

for a morning exchange of routine information and

supplementary exchanges during emergencies.

Morning exchange. Each morning, at a set time

after balloon data are available, an avalanche worker

should phone the Weather Service. He should furnish the Weather Service with local weather informaNational Weather Service data 103
tion, following a format set up in the preseason meeting. The following kinds of local information may be

useful to the forecaster:

• Present state of the weather

• Present precipitation rate

• Cloud cover

• Present temperature

• Dewpoint

• Local windspeed and direction

• Maximum and minimum temperatures during

the previous 24 hours

• Precipitation during the past 24 hours

• Observed freezing level

• Evidence of front passage or other special

weather phenomena

• Other information that could be communicated

by the Weather Service to the public; for example, highway conditions, avalanche warnings, etc.

In return, the Weather Service may supply some or

all of the following information to the avalanche

worker:

• 1200 Z and 0000 Z upper-air data

• Maximum and minimum forecast temperatures

for the next 24 hours

• Expected amount of precipitation in the next 24

hours and expected times of precipitation surges

• Trends in ridgetop windspeeds and directions

• Freezing level

• Cloud cover

• Extended forecast of general weather for the

next 2 days, 3 days, and 7 days.

Emergency supplements. Arrangements should be

made for additional communication during critical

periods. There are times during the day when the

Weather Service staff is loaded down preparing and

issuing general public bulletins. Requests for supplementary information should be made at other times.

The avalanche worker should first give his local measurements. The forecaster can then give a revised forecast. How well the forecaster can pinpoint precipitation surges or lulls may depend on his experience and

knowledge of local effects. Weather stations often

have personnel with special interests in mountain

weather, avalanches, skiing, etc. Avalanche workers

should learn who these people are and work with them

during critical periods.

Ski and explosive tests

The stability of new snow layers is most conveniently determined by test skiing relatively steep slopes

and simply observing the behavior of the snow under

and around the skis. Test skiing is conceptually simple

and plays a very important role in stabiHty evaluation.

Skis have two advantages over other mechanical testing gadgets. First, skis test a large sample of snow

quickly and efficiently; this is important since the

snowpack varies greatly from point to point. Second,

they submit the snow to a realistic test of fracture

toughness. However, certain limitations of ski tests

must be noted:

• Ski tests involve some risk and require strict

safety precautions. They cannot be used where the

skier may be exposed to the severe forces of large

moving slabs.

• Although ski tests are the most reliable way to

measure the stability of surface layers, they do not

give a reliable measure of hard-slab conditions nor

of deep instability.

• Ski tests (or, for that matter, and mechanical

test) cannot be used on remote slopes.

Performing and interpreting tests are skills that are

developed by experience; there are no quantitative

guidelines. Ski tests can be made "belayed" or "free."

In belayed skiing, the tester is tied to a rope that is

anchored to a secure point, such as a tree or lift tower.

In free test skiing, the tester moves across the slope

without the protection of a rope. The following safety

precautions apply:

(1) All test skiing on avalanche paths should be

observed by a second person.

(2) Test skiing parties should carry electronic

transceivers and collapsible probes (see chapter 8 for

an explanation of rescue devices).

(3) Free test skiing should be strictly limited to

those slopes where an avalanche ride could not have

serious consequences.

(4) Test skiers should never enter a dangerous

slope unbelayed on the assumption that they can ski

across the slope fast enough to avoid being trapped.

(5) Before entering the slope, the tester should

have his transceiver in the transmit position. It is

recommended that the transceiver be turned on when

the tester gathers his equipment in the morning and

left on all day.

(6) For belayed test skiing, 11-mm mountaineering

rope should be used. The tester should be tied in

104 STABILITY EVALUATION
Figure 127.—In belayed skiing, one end of an 11-mm rope is tied securely to the tester by a bowline on the coil, and the other

end is anchored to a secure point, such as a tree or life tower (Manning 1960). (Photos by Martinelli and Keiner)

Ski and explosive tests 105
Figure 128.—The test skier should move diagonally across

the crown from one safe position to the next. His progress

should be observed by a companion. Test skiers should

never rest in the middle of the slope.

around the waist with two or more loops (or a harness) knotted by a "bowline on the coil"; the rope

must be anchored to a strong fixed point, such as a

tree or lift tower. There should be as little slack as

possible in the rope.

(7) Standard belays that are acceptable in rock

climbing may not hold against the steady forces of

snow pounding on a body. In any case, test skiing

should not be performed where there is danger of such

large forces against the tester's body.

(8) Test skiers should move as quickly as possible

between safe points, and they should never stop in the

middle of the slope.

(9) Test skiing should not be performed if there is

any possible hazard to people in the track or runout

zone.

Test skiing may be practiced on miniature avalanche slopes and on the major avalanche paths of a

ski area if the above safety precautions are taken. The

larger slopes are test skied as a check on meteorological observations that indicate small amounts of new

precipitation have been added to the slopes in question. Regardless of precipitation amounts, test skiing

should not be performed on large paths that have not

been thoroughly ski packed (see chapter 6, "Control

by Compaction") before the new snow loading. If the

tester finds larger amounts of new snow than indicated by meteorological observations, he must perform belayed skiing or resort to explosive tests (or

retreat if he is unprepared to do the job safely).

If the slope can be free tested, the standard procedure is for the tester to enter at one corner, under

the watchful supervision of a second tester. The first

tester skis diagonally down and across the crown

region, bounding and thrusting his skis to add extra

stress to the slope. The line of travel begins in a safe

spot outside the starting zone, crosses the starting

zone, and ends in a safe place. After the first tester

arrives at the end of the diagonal, roles switch, and the

first tester watches as the second tester comes across.

The second tester traverses a slightly lower line. Testing may then continue diagonally down and back

across the slope. A slope should not be judged absolutely stable on the basis of one or two ski traverses.

A good habit to develop in skiing all avalanche

paths, regardless of their apparent stability, is to end

ski runs at the side of the path. Stopping to rest in the

middle of the path should be avoided. In many cases

skiers have been trapped because the crown fractured

above them while they stood resting in the middle of

the slope.

During test skiing, one looks for the following signs

of structural instability in the new snow:

Piopasicition of fractures. This is the most direct evidence of new snow instability. The deeper and more

extensive the fracturing, the more unstable the new

snow. Fracture propagation on the order of one ski

length or more generally indicates structural instability.

Collapse noise. In some cases, although sudden collapsing sounds in the snowcover are clearly audible,

106 STABILITY EVALUATION
Figure 129.—Fiactuie propagation ¡s diiect evidence of instability. Fractures may propagate from the tester's skis or. in cases

of extreme instability, around the periphery of the slab. (Photo by LaChapelle)

fracture may not be observed. This effect can be observed on horizontal as well as inclined slopes. It is an

indication of a weak structure and extreme instability.

The exception is collapse noises from the fracture of

very thin surface crusts in the spring. These are usually

harmless.

Hard surfaces. It is necessary to distinguish between

wind-scoured slopes and deep, hard deposits in

lee pockets. Wind-scoured surfaces are identified by

erosional features and are generally not an avalanche

problem. However, hard deposits in lee pockets may

be quite unstable. When skis cannot penetrate a lee

deposition, the tester should suspect hard slab instability, and should further investigate the structure by

digging a pit to search for the combination of a hard

slab over a weak substratum or by placing an explosive charge.

Very unstable thin slabs. Occasionally, test skiing

shows that the new snow is extremely unstable but

that the instability is confined to a thin surface slab

less than 15 cm thick. These thin slabs are comparatively harmless. However, avalanche activity can be

expected when new deposits build on this weakness.

Obviously, test skiing is an efficient and reliable

tool when the test slopes are located very near to

the problem slopes or, better yet, when the test slopes

are the problem slopes. A slightly different application of the technique is to test miniature slopes that

behave similarly to more remote and larger problem

slopes. This approach is normally used to evaluate

stability on paths that threaten highways and villages.

Reliability decreases with increasing separation and

elevation difference between test and problem slopes.

The limitations of test skiing can be resummarized:

hazard to testers on large slopes, unreliability for

evaluating deep slab instability, and unreliability for

evaluating conditions on remote slopes. The alternative and more expensive test is to bomb the slopes

with hand-placed or artillery-launched explosives.

This tests the stability of the slopes and at the same

Ski and explosive tests 107
Figure 130.—Effects of wind. Left, erosional features, such as the sastrugi here, indicate that the slope is wind scoured and probably not an avalanche problem; right, note that the tester's skis are not penetrating the smooth, hard surface of the lee-exposed

pocket. This indicates potential hard slab instability. (Photos by Keiner)

time accomplishes control objectives (see chapter 6

under "Control by Explosives"). In some areas, avalanche workers are criticized for using unnecessary

and uneconomical explosive testing instead of a more

"analytical" stability evaluation. It should be understood that explosive testing is the avalanche worker's

main tool. Improved analysis can replace some of the

current explosive testing, but it cannot be expected to

replace very much. Many ski areas are operable only

because of exact and thorough use of explosives.

Here are a few guidelines for explosive testing:

• In general, extensive explosive testing is justified for stability evaluation in ski areas or in any

situation where uncertainties cannot be tolerated.

Extensive explosive testing is seldom required over

highways and villages.

• Explosive testing should be used in connection

with other inputs. For example, after precipitation

driven by sustained south winds, north-facing

slopes should be tested thoroughly and south-facing slopes less thoroughly.

• Testing should be concentrated on those elevations and aspects that respond positively to explosive tests.

• Testers should look for patterns in the way

slopes respond to tests. Which elevations and aspects seem to be unstable? If all but one of neighboring slopes of similar aspect and elevation test

unstable, retest the anomaly, perhaps in a slightly

different target area.

The use of explosives for control, as well as testing,

is described in more detail in the next chapter.

108 STABILITY EVALUATION
Further reading

Bois, Ph., Ch. Obled, and W. Good. Judson, Arthur, and Bernard J. Erickson.

1974. Multivariate data analysis as a tool for day by day 1973. Predicting avalanche intensity from western data: A

avalanche forecast. Proc. Int. Symp. on Snow Mech. statistical analysis. USDA For. Serv., Res. Pap. RM-112,

(Grindelwald, Switzerland, April 1974.) (Unpublished.) 12 p. Rocky Mt. For. and Range Exp. Stn. Fort Collins,

Gardner, Noel C, and Arthur Judson. °'

1970. Artillery control of avalanches along mountain high- Perla, R. I.

ways. USDA For. Serv., Res. Pap. RM-61, 26 p. Rocky 1970. On the contributory factors in avalanche hazard evalMt. For. and Range Exp. Stn., Fort Collins, Colo. uation. Can. Geotech. J. 7(4) :414-419.

Further reading 109

Chapter 6

Protection

of ski areas

A valanche control in ski areas uses several

methods, including barring the public from

hazardous runs, stabilizing slopes by compaction,

and artificially releasing avalanches by explosives

or artillery. This chapter explores these methods

in detail. Judging by the excellent safety record

of avalanche workers in today's ski areas, it is

fair to say that avalanche control is a successful

technology. However, avalanche control cannot

work miracles if a ski area is poorly planned to

begin with. Though the need for planning is

universally recognized by the ski industry,

sometimes avalanche planning is dismissed quickly

with the thought, ''the problems will somehow

be controlled." The result is future wrestling

with an avalanche hazard and sacrificing much of

the operating budget. In this chapter, planning

is discussed last, but it should be stressed that

avalanche control properly begins in the planning

stages, when a ski area is first conceived.

Figure 13 I.—Recoilless rifle, 105 mm, fired from a fixed position

on a cement block tower. (Photo by Bob and Ira Spring)
Ski

with

Caution

Avalanche

Area

Avalanche

Area

Figure 132.—Ski area avalanche control sign in two positions. In its normal position, this center-hinged sign closes the slope.

When folded upward and latched at the top, it opens the slope but reads "Ski with Caution." This emphasizes that the best control is not completely effective and that the skier shoulders some responsibility for his own safety.

Figure 133.—Avalanche sign lines, used to close off large areas.

112 PROTECTION OF SKI AREAS
^.

:j;w<t'.ffHe

ÍKI RUN CON

|LOWER PERUVIAN filDGE

'UPPER PERUVIAN RIDGE

COLLINS FACE

.BEAR PAW

WILDCAT FACE

HIDEAWAY

WESTWAR^-HGSCHUSS GULLY

,m^ CURVE

^'CORKSCREW

'LOWER RUSTLER

-JORTh RUSTLER

¿ROCK GULLY

FOR VOUR PROTECTKM

KNOW

THESE

SIGNS

AVALANCHE ;|

^ DANGER ^

' _^—J — . >. '--T- ,-. 1

(

^1

Figure 134.—Lift-terminal displays that give backup information on avalanche and other warning signs in the area.

Control by restrictions

It is essential that ski area operators develop effective ways to restrict public access to avalanche areas.

An effective system includes:

• Making the skiing public aware of what parts of

the area are hazardous and therefore closed to

skiing, either permanently or from time to time as

conditions warrant

• Closing ski runs only as long as the avalanche

hazard exists

• Enforcing restrictions consistently and fairly.

If a ski area is lax on any of the above points, the

public will lose respect for the area's avalanchecontrol program.

Well-placed signs help make the public aware of

hazardous areas. Avalanche signs should be about 75

by 75 cm and should be painted international orange

with clear black letters. For simplicity, signs may be

made square or rectangular, but the international

octagonal shape is preferred. A hinged avalanche closure sign is shown in figure 132.

Ultimately, avalanche safety is best achieved by

"educating" the public. It is, therefore, in the public's

Control by restrictions 113
>^ X

r

V ^

Figure 135.—Pcinuinciit closuies should be used to keep skiers out of terrain traps such as gulhes fed by large starting zones. A

skier was trapped in this gully and buried under 5 m of snow.

interest that the sign actually say "AVALANCHE,"

so that the public is informed of the reason for the

closure. Similarly, when part of a ski area is closed

because of ice, rocks, insufficient snow, etc., signs

should not give the impression that closure is due to

the avalanche hazard. One excellent system is to use

the international sign for all closures, but to attach

removable plaques that identify the main hazard. This

achieves safety and educational goals and at the same

time is consistent with international practice.

In placing avalanche signs, every advantage should

be taken of natural terrain features. It is good to place

avalanche signs at fixed locations so that at least the

"return customers" know where to look. Standard

traverse paths are natural choices. Occasionally, it is

necessary to set several signs in a neat line to convincingly block off an extensive avalanche area.

Whenever possible, the signs in such a line should be

connected with colored or flagged rope.

It is good practice to supplement closure signs with

master bulletin boards or chalkboards that tell which

areas are closed. This avoids temptations that arise

when a skier goes out a long way to the closure sign

and asks himself, "Did they really intend to close

this area?" It is also good practice to post at lift terminals additional displays asking the public to be on

guard for the various warnings in the area. Much

confusion is eliminated if all posted signs are in agreement as to whether an area is open or closed. This is

critically important. To keep signs in agreement requires good communications and organization. In the

long run, such efforts are rewarded by the skiers' respect for warnings.

Closures are most effective when used as temporary

measures. It is best to remove the closure as soon as

the slope is tested and the hazard removed. Permanent closures should be kept to a minimum and

strictly enforced. However, permanent closures are

justified where the hazard cannot be cleared confidently, especially where the terrain forms deadly

traps. As an example, it is good practice to keep skiers

permanently out of deep gullies that are fed from

above by large catchment basins. Although an area

is permanently closed, it still should be monitored

and possibly controlled by ski release or explosives.

Several skiers have been killed when they accidentally

or purposely entered permanently closed areas.

A ski area strongly compromises its stabilization

program when it does not lift restrictions shortly after

slopes are evaluated as stable. For one thing, the ski114 PROTECTION OF SKI AREAS
ing public may begin to question the closure, especially when all neighboring slopes are skied out. More

important, continual heavy skiing is by far the best

way to solidify and thus stabilize weak substrata.

Many slopes that once produced major avalanches

are now relatively inactive because of heavy skiing.

Violations of restrictions can be quite serious. They

may affect violators as well as innocent skiers below,

and they add to public confusion. The normal enforcement procedure is to deny violators lift privileges. While many violators are sincerely naive and

victims of unclear sign programs, many other violations are premeditated. For habitual violators, it is

worthwhile to have a fairly strong county ordinance

that allows prompt arrest and fining.

When a violation could have serious consequences,

it is sometimes necessary to post guards temporarily

at the closure sign. Such precautions should be used

to guarantee that skiers do not enter an area during

blasting operations that are conducted during normal

lift operating hours.

Control by compaction

Trampling the snow by ski or boot is one effective

technique for densifying and strengthening the snow

in avalanche starting zones. It is an excellent way to

reduce instability due to TG metamorphism. Such

compaction does not stop TG metamorphism, but it

does remove much of the attendant hollowness and

tendency to collapse. Because ski and boot packing

takes considerable effort, they are used normally on

fairly small starting zones of readily accessible paths

and typically on paths with runout zones that cross

lifts, base facilities, or areas where people congregate.

Systematic compaction of such areas should begin

as soon as possible after an early-season snowstorm.

To make the task easier, it helps to start by sidestepping the slope on skis. For maneuverability, "shorty"

skis with climbing skins are an asset. After ski compaction, skis can be removed, and compaction can

be finished by repeated foot traverses.

It is not necessary to compact the entire starting

zone; the job can be cut in half by compacting alternate bands from the top down. This tedious process

is best performed by a squad of several packers, but

one or two strong avalanche workers can stabilize

small starting zones in reasonable time. Systematic

ski or boot packing may be repeated several times

each season on the more critical starting zones.

GOI.NG BETWEEN

OR ABOVE THESE

SIGNS IS A

MISDEMEANOR

PUNISHABLE BY

A FINE OF S299.00

AND OR 180 DAYS

IN JAIL

CLOSED AREAS

MVALAÄCHEIJAfjlR^

Figure 136.—County ordinance in support of avalanche control signs.

Compaction needed

Figure 137.—An example of avalanche paths that should be

thoroughly compacted beginning with the first snowstorm

of the season. These paths threaten base facilities.

Control by compaction 115
.-^/

•slopes '~

Figure 138.—Compacted and uncompacted slopes, side by side. Despite its steepness, the compacted slope (center and right)

is a negligible avalanche problem. The uncompacted slopes (left) avalanche frequently.

4. i

Figure 139.—Some avalanche paths remain uncompacted throughout the season. If ihc paths threaten developed areas they

should be stabilized.

116 PROTECTION OF SKI AREAS
Once the recreational season is underway, compaction consists of two phases:

Control or protective skiing. Preliminary compaction of potentially dangerous slopes is done by experienced avalanche workers. During this phase, the

slopes are closed to the public.

Public skiing. The slopes are evaluated stable, and

they are opened to the public for normal use.

To minimize risk, ski compaction should be preceded by appropriate tests, either test skiing or explosive control or a combination of both.

Test skiing is discussed in chapter 5 in connection

with stability evaluation. The precautions used in

test skiing also apply during control skiing. Often

control skiing is just a foUowup phase to test skiing,

and in most cases there is no fine dividing line between

the end of the test phase and the beginning of the

control phase.

In control skiing the team skis cautiously, one at a

time, down through the starting zone, deliberately

making wide and deep turns which chop up as much

of the starting zone as possible. The compaction

should continue down the track. Wiggling down the

fall line is ineffective; it also makes public relations

more difficult, since it gives the impression that the

control skier is hogging the good powder. Control

skiing (and sometimes boot packing) cannot be used

on hard slabs. Normally, hard slabs must be broken

up by explosives.

The intensity of control skiing is an important

variable. The team chooses an intensity that depends

on slope conditions. Fairly intense control skiing is

needed on slopes that have not had continual public

skiing (and thus have comparatively weak substrata)

and have received large amounts of new precipitation.

Control-skiing intensity can be reduced on slopes that

are skied steadily. Intense control skiing involves

chppping up the starting zone with ski tracks 1 to 5 m

apart. Of course, even such intense control skiing is

not a safeguard against deep instabilities caused by

failure to maintain compaction from the beginning of

the season. Intensity may be reduced to a few passes

where small amounts of new snow have been added

to a well-compacted substratum. Whenever possible,

control skiing should be performed continuously during sustained precipitation periods or periods of drifting snow. This keeps ahead of the problem.

After control skiing and as soon as a ski path is

judged stable, it should be opened to the public. It is

important that avalanche workers monitor the amount

of public skiing on critical paths. If for some reason

the public is avoiding a path and stability of the path

becomes suspect because of new precipitation, wind

drift, etc., avalanche workers should reclose the path

and prepare for appropriate test and control at the first

opportunity.

There is increasing evidence that even the best

planned and executed control programs are not 100

percent effective. Public skiing of avalanche paths involves a small but not negligible risk. The local ski

population should be informed of this in the schools,

in ski clubs, and through radio and television programs. Instruction can be given to nonlocal skiers at

lodges and in warming huts. The important points to

get across to the public are the following:

• Because a slope is open, don't assume it cannot

avalanche.

• Enter the slope cautiously.

• Don't stop to rest in the middle of the slope. End

all turns on the side of the slope, preferably in the

shelter of trees.

• Make a practice of skiing avalanche slopes with

a companion. Keep watch on each other by skiing

one at a time.

• If the snow acts suspiciously (sudden settling or

fracturing), turn back and notify ski area personnel.

It must be emphasized that these precautions are

for developed ski areas. Many additional precautions

must be followed when skiing in the back country on

uncompacted paths. These precautions are outlined in

chapter 8.

Clearly, the small risk that the public assumes when

skiing avalanche paths is more than compensated for

by the compaction benefits. However, the risk can increase considerably if the area, for economic reasons,

uses the public to perform the control skiing phase. To

maintain a proper balance between control and public

phases, the ski area must provide a competent, welltrained control team of a size to match the area hazard. The economics of supporting this team must be

worked out in the initial planning of the area.

Control by explosives

Explosives are important tools in avalanche work;

they are used regularly to test and control slopes in

ski areas. As pointed out in chapter 5, often the test

and control objectives are inseparable and are

achieved simultaneously. In that chapter the test objectives of explosive blasting were explained. In this

Control by explosives 117
Figure 140.—Explosive blasts cause cracks to shoot out from the crater. Some of the blast energy is converted into stress waves

that travel through the air, snow, and ground. These stress waves reinforce fracture propagation.

section emphasis is on the control objectives. Some

of the more important control objectives are:

Release of avalanches from wicoinpacted paths.

Many avalanche paths that cross developed areas are

not regularly skied and therefore remain uncompacted throughout the avalanche season. These paths

must be stabilized. Usually explosive techniques are

most effective, although structures sometimes offer a

better solution (chapter 7). The more frequent the

blasting of unskied paths, the less likely they are to

shed large, unexpected avalanches. Although frequent

blasting greatly increases the number of avalanches,

the average size of the avalanches is greatly reduced.

Release of medium to large avalanches on skiable

paths. In many cases, setting up safe belays for test

and control skiing is too time consuming. It is generally more economical to control larger paths initially

with explosives. If a path does not avalanche when

controlled with explosives, it probably will not avalanche when entered by a ski control team. Because

there is not a 100-percent guarantee of safety, the

team may still wish to enter on belay and take other

precautions as outlined in chapter 5 under "Ski and

Explosive Tests."

Release of deep-slab instability. Ski control is normally ineffective against deep-slab instability and hard

slabs. Explosive blasting is needed to control deepslab instability and to break up hard slabs. The observed difficulties in controlling deep-slab instability

reemphasize the need for a thorough compaction program.

118 PROTECTION OF SKI AREAS
Explosive blasting may release avalanches in several ways. It is speculated that the following are

important:

Mechanical fracturing around the crater. The explosive blast causes cracks to shoot out from the

crater. Fractures continue to spread, branching into

weak regions and eventually cutting loose the entire

slab. This is considered the most important effect of

blasting; the observed fact that explosives are not very

effective against wet slabs confirms the central role of

brittle fracture. Fracturing is reinforced by thrust and

three types of stress waves.

Thrust. The explosive blast also thrusts or heaves

against the slab. When applying explosives to rocks

and related materials, optimum thrust is normally

achieved when the detonation speed of the explosive

(see the next section) is approximately equal to the

speed of sound in the rock. Although convincing experiments are lacking, there is evidence that speed

matching to optimize thrust is not important in snow

slab blasting. Instead, field preference generally is for

explosives with speeds about 10 times as great as the

speed of sound in snow. For blasting snow cornices,

however, field preference is for reducing the explosive

speed and thus gaining thrust effects.

Snow stress waves. Compared to rock, metals, and

other solids, snow is a poor carrier of stress waves.

Just how poor depends somewhat on the frequency

of the waves. It is thought that only a very small

amount of the blasting energy is transmitted across

the slab via stress waves in the snow, and that any

wave disturbance that propagates a long distance from

the blast is transmitted via the ground or air. Nevertheless, snow stress waves work locally to generate

and extend cracks.

Ground stress waves. The ground is much more

effective than snow for transmitting stress waves. The

waves of largest amplitude travel at the ground surface and are known as Rayleigh waves. Occasionally

a subsurface explosive blast in a slab triggers instability in another slab separated from the first by a continuous rock ridge. This indicates that ground stress

waves carry enough energy to induce avalanching. It

is also known that artillery shells fired into rock bands

often cause avalanches on adjacent slopes. The relative importance of ground waves in comparison to air

stress waves is unknown.

Air stress waves. Compared to snow, air is an effective medium for spreading shock waves. Since most

avalanche charges are exploded on the surface, air

stress waves may play an important role. Several experiments have demonstrated that avalanches can be

released by explosive bursts above the snow. As a

special case of air shock, sonic booms from jet aircraft sometimes release avalanches on highly unstable

slopes.

Choice of explosives

Several types of explosives can be used for avalanche blasting. Each has something to offer in terms

of performance, safety, storage, handling, and cost.

Two fundamental properties of an explosive largely

determine its performance: detonation speed and density. Detonation speed is the rate of propagation of

the explosive reaction through the explosive material

(measured in meters per second). Density is the massto-volume ratio of the explosive (measured in grams

per cubic centimeter; the density of water is 1 g/cm^ ).

From detonation speed and density it is possible to

compute a performance index known as the detonation pressure of the explosive. Detonation pressure

varies, approximately, as the square of the detonation

speed and directly as the density. Thus, an explosive

that is twice as fast as another has roughly four times

its detonation pressure. The higher the detonation

pressure, the greater the shattering effect of the explosive. Table 6 gives speed, density, and detonation

pressure of explosives used in avalanche work.

Avalanche workers generally prefer explosive mixtures with high detonation pressures, the denser and

faster explosives. It is claimed that higher detonation

pressures are more effective in activating instability,

all other factors being equal. There is no obvious

explanation for this claim, since low-pressure explosives are known to be quite effective for blasting soft

rock (see comments on thrust in previous section).

The difference may be that avalanche charges are

normally detonated as unconfined surface blasts (external charge), while rock blasting frequently is done

in confined boreholes. More studies are needed to

confirm and explain the advantage of a high detonation pressure for avalanche blasting.

In choosing an explosive for avalanche work, the

following requirements should also be considered:

• The explosive and its detonation system should

be safe, simple, and usable under severe winter

conditions.

• The explosive should not be adversely affected

by moisture, cold, frost, or other elements of the

winter environment.

Choice of explosives 119
TABLE 6.—Characteristics of explosives used in avalanche work

Explosive

Cast TNT

PETN

Cast pentolite

Military composition

A3, B, C3, C4

Military tetrytol

Amatol

Straight" gelatin

(80% wt.)

Straight gelatin

(60% wt.)

Straight gelatin

(40% wt.)

Straight gelatin

(20% wt.)

Two-component

explosives

"Ammonia gelatin

what less detonation

aient percent weight

Content

TNT

PETN

TNT and PETN 1.65

RDX, TNT, and 1.65

others

Tetrytol 1.60

TNT and ammonium 1.50

nitrate

65% nitroglycerin 1.35

50% nitroglycerin 1.40

32% nitroglycerin 1.50

20% nitroglycerin 1.70

Ammonium nitrate and .95

nitromethane

and semigelatin explosives have somepressure than straight gelatin for equivratings. "High-velocity" gelatins are deApproximate Detonation

Approximate confined pressure

density detonation relative to Typical

(g/cm^) speed (m/s) cast TNT application

1.56 6,700 1.0 HE ammunition, primer,

booster, demolition

1.60 7,600 1.4 Detonating cord, caps (unconfined velocity—6,400 m/s)

7,400 1.3 Primer, booster

7,600 1.3 Demolition, shaped charges,

(63% RDX) HEP ammunition

7,000 1.2 Demolition

6,200 .9 Primer, booster, HE ammunition

(60% TNT)

6,700 1.0 Hard rock blasting, primer,

deep wells

6,100 .9 Hard rock blasting, primer,

deep wells

5,000 .6 Hard rock blasting, primer,

deep wells

3,400 .3 Softer rock

4,300 .4 Miscellaneous storage advantages

signed to detonate at rated speeds regardless of confinement

and are preferred for avalanche work.

• Misfires (or duds) should be infrequent. If a

misfire is lost on a slope and exposed to the elements, it should not become shock-sensitive.

• The explosive should be packaged in a nonshrapnel container.

• The explosive should be reasonably nontoxic

under normal outdoor handling.

• The explosive should have a high density so that

field loads are not bulky.

Many commercial explosives have been tested and

found satisfactory for avalanche blasting. The preferred avalanche explosives are known commercially

as primers or boosters, a class of explosives used

mainly to detonate insensitive blasting agents such

as ammonium nitrate. Primers are divided into two

categories: cast primers and gelatin primers. Their

composition varies according to manufacturer.

Cast primers are usually high-density, pressed or

cast cylinders of TNT and other ingredients. TNT

is a fast, powerful explosive that is not extremely

sensitive to accidental detonation by shock. It was

developed by the military to withstand the rigors of

the battlefield. Its fumes do not produce headaches,

as do those of some other explosives; they are toxic,

but this is not a problem in normal outdoor use. TNT

has the peculiarity of reacting with atmospheric oxygen. This adds some energy to the blast, especially

when the charge is detonated on the surface. One disadvantage of TNT is that it leaves a messy black

crater; another disadvantage is high cost. TNT is

not reliably detonated by No. 6 or No. 8 blasting

caps, so that cast primers of TNT include a more

sensitive explosive, such as PETN. The PETN content of cast primers for avalanche work should not

exceed 50 percent.

Gelatin primers are cheaper and do not leave a

black crater. They are as fast as TNT but slightly

more bulky. They have a high percentage of nitroglycerin and share the disadvantages of all nitroglycerin mixtures; they produce headaches, deteriorate,

and are more shock-sensitive than primers that consist, for example, of TNT.

120 PROTECTION OF SKI AREAS
Gelatin and cast primers are classified as high explosives and must be stored and handled according

to strict codes (see chapter 6 under "Explosive

Safety"). Because of regulations dealing with explosives security, storage is expensive. Where there is

a limited need for explosives, avalanche workers may

wish to avoid the more expensive storage requirements by using a "two-component system." Stored

separately, the components are not high explosives.

They are classified as high explosives only when

mixed. The storage advantage is offset by higher cost

of materials, lower detonation speeds, bulkier charges,

the inconvenience of mixing the explosive in the field,

and the requirement of a mixing time of about V2

hour to bring the mixture to full strength. Mixing

should be done at temperatures of 0° C or above,

but once mixed, the explosives will detonate at

— 50° C or lower.

Use of explosives

Ideally, explosive efficiency could be achieved in

several ways: by selecting the minimum amount of

explosive, by detonating the charge in the most critical

region of the slab, or by properly timing the control

action. All these refinements have their limitations,

but timing is the most limited. Control work in ski

areas generally is performed at set times while the

public is off the slopes, and there is little chance to

adjust the timing. (The situation is quite different for

highway control, where timing can be adjusted.)

It is conventional to blast each target with 1 kg

of TNT or its equivalent. The 1-kg-TNT equivalent

charge is called the "standard charge." Depending on

the circumstances, it may sometimes be necessary to

use more than the standard charge. The following

guidehnes have evolved from field experience:

• A charge of 2 kg often can be used to trigger

marginally stable deep slabs.

• Charges larger than 2 kg no doubt "manufacture" instability in some instances when a 2-kg

charge is insufficient, but it is highly probable that

such avalanches could not be ski-released from the

vicinity of the target and would certainly not be

released by internal natural triggers.

• For explosives other than TNT, amounts should

be scaled to the detonation pressure relative to TNT

(table 6). As an example, 2 kg is the standard

charge of an explosive with half the detonation

pressure of TNT.

• The effectiveness of a surface-placed charge is

determined by the detonation pressure. Scaling

exclusively to detonation pressure probably overpenalizes low-detonation-pressure explosives, but it

does provide an estimate on the safe side. There is

no reason to penalize for low detonation pressure if

the charge is buried in a borehole. Further comments on buried charges will be made shortly.

To interpret the above guidelines, it is necessary to

make some assumptions about the minimum target

area tested for instability by a given amount of charge.

This area may be thought of as the radius of influence

of the charge. Within this radius, an explosive ought

to release any avalanche that could be released by a

skier. From field experience, it may be assumed as a

safe estimate that the radius of influence of the standard 1-kg charge is about 10 m. In many cases the disturbance of the charge propagates over a much larger

area, but 10 m is taken as the minimum radius. By this

guideline, a series of 1-kg charges should be spaced no

closer than 20 m when blasting a wide, continuous

slab. Considering the uncertain basis of avalanche

blasting theory, it is fortunate that the target areas of

most slabs are well defined by terrain barriers such as

gully walls, rock outcrops, timber stands, etc., so it

is rarely necessary to estimate the number of charges

needed to cover a given slab area.

For field purposes, it may be assumed that doubling

the amount of surface-placed charge increases the

radius of influence by a factor of J2, or a little over

40 percent. Thus, for blasting wide slabs, two or more

1-kg charges spread out over the width are likely to

be more eflicient than fewer 2-kg charges. However,

if the slab width (distance between flanks) and slab

length (distance between crown and stauchwall) are

about equal, a 2-kg charge with its 14-m radius of

influence may be more efficient than two 1-kg charges.

It is tempting to blast with larger and larger charges,

especially if pit studies indicate that a slope has deep

slab instability and the above-recommended amounts

fail to activate the instability. A little sober reflection

shows that no matter how large the charge, it may only

loosen the slab to the brink of failure. When a 1- or 2-

kg charge fails to trigger instability and the control

team is stifl uncertain about the slope, there are several alternatives, depending on conditions. For small

slopes and shallow snow cover, the workers may enter

the slope to perform systematic compaction by boot

packing or intense control skiing, provided proper

belays and other safety precautions are taken. For

larger slopes with deeper snow cover, where boot

Use of explosives 111
•"'4 ":^ \ ^

Figure 141.—Target areas along control routes at a Utah ski area. Explosives should normally be placed within the slab

boundaries about halfway between the slab center and crown. If possible, the radius of influence should sweep into shaded

pockets where structural weakness is likely to be most pronounced.

packing would be ineffective, the area may have to be

closed for a half or full day and then retested with

explosives.

In connection with deep-slab instability, one must

keep in mind that avalanches may release, although

fortunately infrequently, several minutes to several

hours after explosive control failed to trigger instability. There is no reason to believe that these postcontrol releases can be avoided by "forcing instability" with larger and larger charges. "Forcing

instability" is questionable in terms of economics and

environmental damage, and it may even be questioned

in terms of safety objectives.

There is still some dispute as to where explosives

should be placed for maximum efficiency. Field opinion is that charges normally should be placed within

the boundaries of the slab as determined by the observed crown, stauchwall, and flanks. As discussed at

length in the earlier chapters, slab boundaries are

determined by terrain and wind-deposition patterns.

Hence, boundaries shift with conditions, and speciñc

target points must be adjusted accordingly.

There is some difference of opinion as to how far

the explosive should be placed from the slab center.

For most conditions, the favored position is about

halfway between crown and slab center, but arguments can be given for placing charges at or above

the crown, at the slab center, or even at the stauchwall

region. The case for keeping the charge high is that

tension fractures are easier to activate than shear or

compression fractures. Occasionally, charges can be

placed in narrow, steep gullies, cliff bands, and other

deposition zones above the crown. Here, the idea is

to release small slabs or loose-snow avalanches that

could trigger the main slab.

As an added refinement, it may be possible to improve efficiency by carefully choosing the target so

that the radius of influence sweeps into rock outcrops

or trees. This exploits stress-concentration effects and

helps generate tensile cracks, which tend to shoot out

from rocks and trees. Also, TG metamorphism and

other structural weakness are apt to develop in the

shade of rock outcrops and trees, and slab failure is

most likely to originate in such regions. Pit studies

122 PROTECTION OF SKI AREAS
can confirm the degree of weakness in a given shaded

region. Although blasting next to rocks may produce

stress waves in the ground that aid in releasing avalanches, this practice sometimes produces flying pieces

of rock that are an added hazard.

Refinements in target location should in no way

jeopardize the safety of the control crew. The charges

must always be thrown from a safe stance that offers

a foolproof escape route from the area of explosive

blast. Normally, this entails throwing the charges

down from a ridge, perhaps over a cornice or cliff, and

there may be little opportunity to refine the target

selection.

An important question related to crew safety concerns the feasibility of burying charges. In principle,

placing a charge in a well-prepared borehole increases

the efliciency of the blast. Also, as discussed earlier,

there is reason to suspect that low-cost, low-detonation-pressure explosives such as 40-percent gelatin are

quite effective in borehole blasting. However, any advantages of borehole blasting must be balanced

against safety precautions imposed on the control

team. Borehole blasting must be done on belay, and

foolproof escape routes must be worked out. Because

of these required precautions, the great majority of

explosive control must be done with surface charges.

The main exception is cornice blasting.

In conclusion, it should be understood that many

of these field practices have not been tested objectively. Research may show that acceptable safety results can be obtained with reduced amounts of lowcost explosives, used in new and more efficient

schemes.

Assembly of explosives

This section describes accepted field techniques for

preparing and delivering explosives. No attempt is

made to treat explosive technology thoroughly; avalanche workers are strongly urged to consult Dick

(1968), DuPont de Nemours (1969), Mellor (1965),

and U.S. Department of the Army (1959) for more

complete discussion of explosive techniques.

Avalanche blasting is based on nonelectric detonating systems for the following reasons:

• Casualties have been caused by the static electricity of snowstorms unexpectedly setting off electric detonation systems.

• The electric-field intensity of the atmosphere

in the vicinity of ridgecrests is prohibitively high.

Safety fuse

Copper or aluminum shell -

Flash chargePriming charge -

Base chargeFigure 142.-Typical construction of a blasting cap.

• Electric blasting is not practical under the severe

weather and terrain conditions normally encountered in avalanche blasting.

However, even with nonelectric blasting caps, avalanche blasting should not be done when there is

evidence of a strong static field (cumulonimbus

clouds, electric buzzing). For safety, the nonelectric

system must be as simple and foolproof as possible.

The recommended system consists of an explosive

charge, a blasting cap, a safety fuse, and a safety-fuse

igniter. Some general considerations for charge preparation follow:

Blasting cap. The explosive charge is detonated by

a nonelectric cap that contains explosives far more

heat- and shock-sensitive than the main explosive

charge. Typical construction of a cap is shown in figure 142. Most primers or boosters can be detonated

by a No. 6 cap; however, under severe winter conditions, some primers with a high ammonium nitrate

content require a No. 8 cap. Some compounds (like

TNT) may require the equivalent of a No. 10 cap.

It is preferable to use primers that can be detonated

reliably by a No. 6 cap under all field conditions (providing the charge is made up properly).

Assembly of explosives 123
B

III m

1

/L--j ^~-- No Gap

Cap /

//

/

u

RIGHT WAY

D E F

Figure 143.—Assembly of cap and fuse (DuPont de Nemours 1969), A. Use only the proper tool, a cap crimper. B, Cut a

sufficient length of fuse. Square the tip of the fuse. C, Select one cap, large enough to detonate the charge reliably under winter

conditions. D. Gently insert fuse into cap until fuse butts against cap wall. E, Crimp cap in two places as shown. F. Do not

allow any gaps between fuse and cap wall.

Safety fuse. The highest quality safety fuse should

be used in avalanche work. It should have excellent

water resistance and flexibility. The standard burning

rate of fuse marketed in the United States is 0.5 m in

65 seconds (s) (± 10 percent) at sea level. At elevation 2,500 m, standard fuse burns at a rate of about

0.5 m in 70 seconds (±10 percent). After the fuse

is purchased, a test segment should be ignited and

the burning rate timed. The minimum length of the

safety fuse depends on the time needed for escape

from the blasting location. Under no circumstances

should a fuse be cut to a length that allows less than

90 seconds burning time. The 90-second burning time

allows a trained blaster to make two attempts to ignite the fuse. There are several sad cases of casualties

caused by short safety fuse. Where the escape route

is complex or difficult, it may be necessary to use

burning times longer than 90 seconds.

124 PROTECTION OF SKI AREAS
Figure 144.—Arming cast primer. It is important that the explosive end of the cap be inserted into the off-center hole (the capsensitive hole). Misfires will result if the cap Is inserted in the wrong hole or is not firmly fitted into the proper hole with snug

contact at the firing end.

Safety fuse-cap assembly. Safety fuse should be

stored, uncoiled, and assembled to the cap at room

temperatures. Working at comfortable temperatures

helps ensure correct assembly and minimizes misfires

and duds. The procedure for assembling cap and

fuse is shown in figure 143. Just before inserting the

fuse into the cap, the tip of the fuse must be clipped

square. The fuse is then gently inserted into the cap

until flush against the inside wall of the cap. There

should be no gap between the fuse and cap wall. The

cap is then crimped onto the fuse with, and only with.

a cap crimper. Scrupulous observance of these details

minimizes misfires or duds. On request, several fuse

manufacturers supply the fuse and cap already assembled according to procedures recommended by

the industry.

Fuse, cap, explosive assembly—general considerations. As soon as the cap is inserted into the explosive,

the system is armed. From this instant, the relatively

insensitive explosive contains a sensitive cap and is

vulnerable to accidental detonation. For this reason,

arming should be delayed as long as possible in the

field. Sometimes it is possible to arm the explosive

just before tossing the charge onto the target. In other

cases, wind and temperatures on the control route

are quite severe, and overall safety is served if the

explosives are armed in a shelter before starting out

on the control route.

Three examples of explosive assemblies are presented below:

Arming of cast primer. Figure 144 shows the steps

in the arming of cast primers. Most cast primers are

manufactured with two axial holes, a central one and

an off-center one. The central hole is designed to be

detonated by high-explosive detonating cord. The

ofi'-center hole is usually lined with a primer, which

is sensitive to No. 6 blasting cap. It is essential to

place the cap in the proper hole, or a misfire is likely.

In avalanche work, it is convenient to lace the safety

fuse tightly through the central hole and then into

the off-center hole, snug against the end of the hole.

This requires a high-quality safety fuse that can function reliably despite sharp bends. The assembly is

then taped securely. Because of its simplicity, this

assembly lends itself nicely to "field mass production,"

in which the control team assembles perhaps 100 or

more charges for a morning operation. Because of

the insensitivity to shock of the cast primer and the

fact that the cap is protected in the middle of the

charge from shock and static electricity, this assembly

is the safest armed charge that can be carried on a

severe mountain ridge.

Assembly of explosives 125
Figure 145.—Arming gelatin-primer by lacing. Two diagonal holes are punched; the upper one goes completely through the

cartridge, and the lower one is a little deeper than the length of the cap. The capped fuse is then threaded through the upper

hole and the cap inserted into the bottom of the lower hole. The assembly is then taped securely.

Arming of gelatin primer. The arming of a gelatin

primer is shown in figure 145. Gelatin primers do

not have precast holes; it is necessary to punch two

diagonal holes. First, a diagonal hole is punched

through the charge with the punch end of the crimper. Then the charge is rotated one-fourth turn, and

a second diagonal hole is punched slightly deeper than

the length of the cap. The fused cap is then laced

through the first hole, and the cap is inserted into the

latter hole. The assembly is taped securely.

Arming of detonating cord. Some explosives (for

example, military tetrytol) are detonated by a highexplosive cord known as detonating cord. Such

charges are armed by taping the cap to the detonating cord or joining the cap and cord with special connectors. The explosive end of the cap should point

along the detonating cord, toward the main charge.

Because in both these systems the cap is exposed and

vulnerable to accidental shock, the final connection

of cap and detonating cord should be made only at

the blasting position. Further comments on the use

of detonating cord are made in a later section on

"Cornice Control."

The prepared charge is carried in the patroller's

jacket or pack. Patrollers should not be loaded so

heavily that skiing is clumsy. About 15 kg of explosives is a maximum load. Igniters are carried separately from the explosives. The procedure at the blasting position is as follows:

(J) Make sure all possible runout zones are free of

people and traffic. For areas not visible from the

blasting point, arrange for signals from an observer.

(2) Work with only one charge at a time.

(3) Step into blasting position and make final check

of target and escape route.

(4) Clip end of safety fuse and firmly insert the

fuse into the igniter. The igniter should be activated

immediately. Caution: occasionally the act of inserting the fuse into the igniter may cause ignition (before

the igniter is activated).

(5) Make a quick check ( 1 or 2 seconds at the

most) to see that the fuse has ignited, then immediately place charge.

(6) Get to a safe position and await detonation.

If it appears that the igniter has failed to ignite

the fuse:

(1 ) Quickly remove the igniter.

(2) Make a fast visual check of fuse end.

126 PROTECTION OF SKI AREAS
Figure 146.—Fastening safety fuse to detonating cord.

(3) If there is no sign of ignition, reclip the end,

insert a new igniter, ignite, and immediately toss the

charge.

Be properly organized, so that the above steps for

reignition can be completed within 45 seconds, since

there is a very small, but not negligible, chance that

the powder was ignited on the first attempt, despite

lack of visible signs. In at least one case a blaster

could not see a visible sign and was killed while trying a second ignition.

Explosive safety

All avalanche blasting work, including storage,

transportation, and handling of explosives, must comply with Federal, State, and local laws. The U.S. Code

of Federal Regulations (CFR) treats explosive regulations in detail. In particular, U.S. avalanche workers should be acquainted with the following sections:

Department of Treasury (Alcohol, Tobacco, and

Firearms Bureau), sections 181.181 to 181.198.

These sections pertain to licensing, record keeping,

and the location, construction, and maintenance of

explosive storage magazines of various types.

Department of Labor, Title 29 CFR, section

1910.109 and sections 1926.900 to 1926.909. These

sections broadly define explosives and explosive terminology and review storage and transportation regulations. They discuss blaster's qualifications, safety

procedures for handling and preparing explosive

charges, and the use of safety fuse, caps, detonating

cord, and primers.

Department of Transportation, Title 49 CFR, sections 173.50 to 173.114 and section 177.848. These

sections first define in detail various types of explosives and ammunitions and then give the transportation regulations for each type.

In addition to general Federal, State, and local

regulations, employees should be thoroughly familiar

with:

Employing agency regulations. For example, U.S.

Forest Service Health and Safety Code, sections 6.1

to 6.19 (USDA Forest Service 1972). These pertain

to Forest Service employees and may, by terms of the

special use permit, pertain to permittee's employees.

Manufacturers' literature. Instructions, warnings,

and other literature, published by the manufacturer,

that pertain to the chosen explosive.

Explosive safety m
Figure 147.—Tossing charges from ski lifts entails special

risks. Take special precautions against entangling charges

in any of the lift equipment. All chairs should be vacant

except for the blasting team. Charges should be tossed

downslope and away from chairs that are moving uphill.

Since avalanche blasting is conducted under severe

weather and terrain conditions, it is necessary to supplement the above extensive regulations with additional guidelines for avalanche safety. To a large

extent, the following guidelines briefly summarize

points made in chapter 6.

General:

(1) An avalanche should not be released artificially

until the avalanche path, including the potential

runout zone, is cleared of people.

(2) Avalanche guards should be positioned at the

entrance to the path if there is any chance that people

will enter the path during blasting.

(3) During very unstable conditions, artificial release of one avalanche may trigger sympathetic release over a wide area. Avalanche workers should consider such possibilities and clear the appropriate area.

(4) Explosives should be handled always with

utmost care.

(5) In all mountaineering work, safety is achieved

by simplicity. Avoid any undue complications.

128 PROTECTION OF SKI AREAS
Personnel:

(1) The blasting party should consist of the blaster-in-charge and at least one assistant.

(2) All members of the blasting party should be in

good physical and mental condition and should be

competent ski mountaineers.

(3) All members of the blasting party should be

properly trained and qualified by the employing

agency.

(4) Responsibility for the preparation and placement of the charge should not be divided. The blasterin-charge is responsible for supervising all phases.

Explosives:

(1) Explosives should have a shelf life in normal

storage of at least one operating season.

(2) Explosives should not be packaged in metal

containers.

(3) Explosives should have excellent v^ater and

frost resistance.

(4) Misfires should not be shock-sensitive.

(5) Recommended explosives are industrial primers (or boosters) that consist mainly of TNT or gelatin.

Detonation systems:

(1) Except for exploding-bridge-wire caps, electrical blasting caps should not be used.

(2) The detonation system should be as simple as

possible; the recommended system is: explosive, cap,

safety fuse, and fuse igniter.

(3) Blasting caps should not be larger than No. 8

(No. 6preferred).

(4) Caps should be protected fully from external

sl]ock during control maneuvers.

Safety fuse:

(1) Use only the highest quality safety fuse, v^hich

has excellent water resistance and excellent flexibility.

(2) Safety fuse should burn no faster than 0.5 m

per minute. A section of fuse should be tested after

purchase and before use.

(3) Safety fuse lengths should be selected to allow

the control crew to escape from the blast area under

all reasonable contingencies (falls, release of safety

bindings, etc.). Burning time should be at least 90

seconds.

Preparation of detonating systems:

(1) Blasting caps should be crimped onto the safety

fuse only with special crimper tools.

(2) To prevent misfires, the fuse-cap

should be fastened or taped securely to the

charge.

(3) Charges should be armed with caps as late as

possible in the blasting operation.

(4) The igniter should not be attached to

fuse until the control crew is at the blastin:

and ready to release the charge.

assembly

explosive

the safety

g position

Firing of charge:

(1) The blaster should be at the blasting position

before attaching the igniter.

(2) Ignite only one fuse at a time. (Double fusing is

an unnecessary complication. )

(3) Check quickly that the fuse is ignited, then immediately toss charge.

Tossing charge from control route:

(1) Normally, the charge is tossed down onto the

target from a safe position, preferably a ridge.

(2) The control team then escapes to a safe position

behind a terrain barrier at least 30 m from the target.

(3) In cases where the charge could slide down on

a hard snow surface, it should be belayed with nylon

cord.

Tossing charges from ski lifts and trams:

(1) The number of charges tossed from lifts and

trams should be kept to an absolute minimum.

(2) The lift operation crew should be informed of

the blasting plans.

(3) The lift crew should stand by in full readiness

for emergency procedures (transfer of lift to auxiliary

power, evacuation, etc.).

(4) The lift crew and the blaster-in-charge should

be in radio contact at all times during the blasting

operation.

(5) Only the blasting crew and the essential operating personnel should be on the lift or tram.

(6) The Hft should be in motion when the charge is

tossed, and the blasting crew should be moving upslope. The charge should be tossed downslope and to

the side.

(7) Charges should not exceed 2 kg of TNT equivalent.

Explosive safety 129
(4) Secure fixed belays should be set up, and all

necessary precautions to guarantee a safe entry to the

slope should be taken.

(5) The normal procedure is to retrieve the charge

and escape to an avalanche-free location as soon as

possible, preferably by retracing steps in the upslope

direction.

(6) Deviation from the normal procedure (for example, planting a second charge next to the misfire)

depends on the cause of the misfire and the sensitivity

of the explosive. These procedures should be worked

out in consultation with the manufacturer.

(7) Following recommended procedures for assembling primers, it should be possible to reduce misfire

incidence to less than 0.1 percent. Any organization

that is not maintaining this standard should thoroughly investigate equipment and procedures.

Figure 148.—Small cornices are routinely kicked off on daily

control routes.

(8) The minimum distance from target to closest

point of lift should be 20 m.

(9) Fuses should be cut so that blasting crews have

moved on the lift at least 100 m from the target by the

time of detonation.

(10) All tram and gondola cars should be at least

100 m from the target at time of detonation.

(11) Precautions should be taken to avoid tossing

charge into any of the lift equipment, moving chairs,

cables, lift towers, etc.

Blasting from aircraft:

In addition to the regulations outlined above, avalanche blasting from aircraft is subject to special permission from the Federal Aviation Administration, in

accordance with FAA Regulation Part 103. Hachease

must be considered individually in context with the

alternatives. (Helicopter blasting is treated in more

detail in chapter 7.)

Retrieving misfires (duds):

(1) A conscientious effort should be made to retrieve each misfire.

(2) If conditions make it impossible to retrieve the

misfire, the slope should be closed and a search begun

as soon as possible.

(3) The control team should wait at least 1 hour

before approaching any misfire. Any misfire that is

aflame or emitting smoke should be left alone.

Cornice control

Cornices are described in chapter 2, under "Wind

Redistribution of Snow." Falling cornice blocks may

be quite massive and hard; they can survive a long

ride without pulverizing and then smash into a skier.

They may trigger slab avalanches, sometimes even

where explosive charges failed to trigger instability.

Because of these hazards, it is necessary to watch

closely cornice buildup in ski areas and to take control action before cornices fall naturally.

Whenever possible, cornices should be controlled

routinely, especially after periods of snow transport.

Small cornices can be kicked or skied off by an unbelayed ski mountaineer who takes standard precautions. Larger cornices are kicked off from sturdy

belays; more massive cornices, which consist of several cubic meters of hard, dense snow, should be

released only by explosives. A proper belay affords

enough protection for work on small to medium cornices, but it offers incomplete protection from the

crushing forces of gigantic cornice blocks.

The general safety principles for work on massive

cornices are:

• Post guards to restrict the public from entering

slopes beneath the cornice. Maintain radio contact

as required.

• Never work on a massive cornice without secure

belays using 11-mm mountaineering rope.

• Allow only one person to work on the roof of

the cornice at a time.

• Work on cornices only when weather is favorable.

130 PROTECTION OF SKI AREAS
Figure 149.—Cornice collapse may place enormous forces on the belay system. The belayer should be in a sitting position and

as far down the windward side of the ridge as possible. It is important to keep the belay rope taut.

The belay should be set up as far downslope as

possible on the windward side of the ridge. Full use

should be made of trees and natural anchors. The

belayer should be sitting, and the belay rope should

be kept as tight as possible. Full use should be made

of standard mountaineering equipment, such as ice

axe, snow pickets, slings, carabiners, etc., to help anchor the belay as required. Whenever the belay position is changed, the worker should retreat back from

the cornice roof.

Although the belay is essential, it must be regarded

as a second line of defense, for backup protection

only. Every possible precaution should be taken to

remain on the safe side of the cornice roof. Before

proceeding out on the cornice roof, establish the closest safe working line to the edge. Here, mountaineering experience and skill are the only guidelines, and

such experience teaches that cornices can break a

considerable distance back from the edge. Sometimes,

an observer stationed at a nearby vantage point can

help establish the safe line. It is good practice to flag

the working line and strictly obey the flagging.

Cornices should be blasted only after they achieve

a mature overhanging structure. Pushing vertical cornice walls over the ridge is a waste of explosives.

Knowing when to blast is, again, based on mountaineering judgment.

The simplest and safest procedure for cornice blasting is based on 1-kg surface charges of cast or gelatin

primers, or the equivalent. The charges are placed

at 2-m intervals along the estimated tension line of

the cornice roof, at arm's length from the working

line. The charges are linked to a main line of detonating cord. The recommended details are as follows:

(1) Select the number of charges necessary to

cover adequately the tension line of the cornice roof.

Lace each charge with a 0.5-m length of detonating

cord, referred to as branch line.

(2) Set out a main line of detonating cord along the

safe working line.

(3) Set the first charge into position along the

working line. Tie the branch line of the first charge to

the main line with a girth hitch or clove hitch.

(4) In a similar manner, connect the next charge

2 m from the first, and so on down the working line.

(5) After all branch lines are connected to the

main, carefully push each charge from the working

line to the presumed tension line of the cornice.

(6) Aline each branch line approximately perpendicular to the main line.

(7) After the charge is in place, tape a blasting cap

with safety fuse to one free end of the main line. The

Cornice control 131
Safe

working

^fe,^„

^.

*^-

To

belayer

VA.

Figure 150.—Before proceeding out toward the cornice roof it is important to flag a safe working line.

explosive end of the cap must point down the main

line toward the charges.

(8) In order to allow enough time to escape from

the blast area, the safety fuse length should be at

least 1 m long. Before igniting, check with the posted

guards.

Cast primers can be connected in series without

branch lines; the main line of detonating cord is simply run axially through the appropriate hole in the

primer. As in the above steps, the system is assembled

and pushed into place without stepping over the safe

line. This shortcut should not be used to connect

buried charges.

A more efficient blasting scheme is to bury the

charges in a row of boreholes. In borehole cornice

blasting, one may achieve satisfactory results with

about half the explosive used in surface blasting. It is

also possible to blast effectively with low-cost, lowdetonation-pressure explosives. Although borehole

blasting of cornices increases efficiency, boring holes

along the presumed tensile fracture line exposes the

avalanche worker to considerable danger. Safety in

borehole blasting depends critically on:

• The ability of the control team to judge correctly

the safe working line.

• The feasibility of maintaining a tight, secure

belay.

• The ability of the driller to bore holes along the

tension line while remaining on the safe side of the

working line.

132 PROTECTION OF SKI AREAS
Main line

of detonating cord

Cast primers

with axial holes

Figure 151 —Cornices may be blasted with 1-kg surface charges of cast or gelatin primers, or the equivalent. The charges are

spaced 2 m apart and connected to a main line of detonating cord by branch lines. For some types of cast primers, it is

possible to run the main line directly through the axial hole of the primer, as shown.

The recommended steps in borehole blasting are

as follows:

(1) The driller, belayed securely, steps into position on the safe side of the working line and drills a

row of holes up to 1 m in depth, but no deeper than

half the thickness of the roof. The boreholes should

be as close as possible to the potential tensile fracture

line of the cornice.

(2) The diameter of the holes should be such that

the charges fit as tightly as possible. The holes should

be spaced 2 m apart. Since cornice snow is normally

quite hard, boring will require a soil auger of diameter

appropriate to the charge.

(3) After all holes are bored, the main line of

detonating cord is strung out. To prevent loss of explosives in the event of a sudden cornice collapse, a

free end of the main line should be secured to an

anchor until the system is ready to be detonated.

(4) A charge laced with a 2-m branch line is inserted into the first hole. The branch line is connected

to the main line, and the hole is refilled with snow and

stemmed (tamped) compactly.

(5) After all boreholes are prepared in a similar

manner, a free end of the detonating cord is armed

with cap and fuse. Firing takes place after the usual

check with the posted guard.

Whereas surface blasting of cornices requires 1-kg

charges, buried charges can be limited to about 0.5

kg per hole. All of the recommended explosives for

slope control (see table 6) can be buried. Moreover,

it is not necessary to penalize explosives for low detonation pressure in comparison to TNT; in fact some

Cornice control 133
Hole

tamped compactly

with snow

r

Branch line

To main line

'/2kg charge

Figure 152.—Borehole blasting leqiiiies half the explosive

used in surface blasting (0.5 kg of buried charge compared

to 1 kg of surface charge). Buried charges should fit snugly

in the base of the borehole. The hole is then filled and

stemmed (tamped) compactly. The charges should be buried

no deeper than about 1 m.

field evidence favors low-detonation-pressure explosive for buried-charge cornice blasting.

Because detonating cord plays an important role in

cornice blasting, one should be acquainted with the

basic techniques for working with this high explosive.

The following points should be kept in mind:

• Use reinforced detonating cord. This gives an

extra margin of safety by retaining the explosive

system should the cornice fail during preparation.

• Because of knot-tying problems, do not use detonating cord with an outside plastic sheath.

• Kinks and sharp bends are to be avoided.

• Special care should be taken in cold weather to

avoid breaking or cracking the cord.

• Detonating cord lines should be laid out as

straight as possible but not stretched taut.

• Main lines can be spliced with conventional

square knots; the knots should be pulled tight.

Leave 10 cm of free end. Splices should be kept to

a minimum.

• Branch lines should be connected to the main

line with a girth hitch or clove hitch to achieve a

90° tie. Tighten up securely and leave a 10-cm free

end.

• Wet detonating cord loses sensitivity. Keep the

cord as dry as possible during storage and handling.

While blasting wet-snow cornices, take special precautions to keep the free end, where the cap is attached, as dry as possible.

• Any number of branch lines can be connected

to the main line; however, connections should not

be made at a splice.

It should be clear that blasting massive cornices requires adequate manpower and special equipment. A

typical blasting operation should include a minimum

of four qualified avalanche workers plus the blasterin-charge, who distributes and supervises the various

jobs. The minimum equipment needed in massive

cornice operations is quite impressive. The blaster-incharge should be certain that the following equipment

is taken:

• Adequate clothing for ridgetop conditions (Hypothermia is a serious threat and can dull senses just

when alertness is needed.)

• Adequate footwear (generally double mountaineering boots, rather than competition plastic ski

boots)

• The required explosives, caps, detonating cord,

safety fuse, and igniters

• Flagging to identify the working line

• Auger, if charges are to be buried

• Belay rope (and slings and carabiners as required

for anchor)

• Snow pickets, if required for anchors

• Radios

• Collapsible probes and avalanche transceivers

(see chapter 8).

Blasting a massive cornice presents many technical

problems as well as high costs. Serious thought should

be given to the alternative, controlling the cornice with

preventive structures installed near the ridgecrest. The

general subject of structures for avalanche control is

presented in chapter 7.

Figure 153 (on page ¡35).—A, Sections of main line are

spliced with a conventional square knot. B. Branch lines

are joined to the main with a girth hitch. C, Technique for

connecting branch line to cast primer.

134 PROTECTION OF SKI AREAS
Cornice control

135
Use of artillery

Military weapons may be used where it is unsafe

or impractical to blast with hand-placed explosives.

Many remote avalanche paths that endanger highways

are routinely blasted with artillery (according to techniques described in chapter 7). Artillery is also used

to blast remote avalanche paths that affect ski runs.

There are several problems, however, that planners

must consider before embarking on an artillery control

program in a ski area. For example :

• Obtaining military weapons and ammunition is

becoming increasingly more difficult and expensive.

• The military does not remanufacture weapons no

longer required for national defense, and it is difficult to obtain artillery pieces, replacement parts,

and ammunition for weapon systems no longer in

current production.

• Obsolete weapon systems brought into the ñeld

after long-term storage suffer some loss in reliability

and safety.

• Artillery pieces, replacement parts, and ammunition are relatively expensive.

• Reliability and safety decrease if ammunition is

not stored properly in the ñeld. Ammunition storage

facilities are expensive.

• Reliability and safety decrease if pieces are not

maintained properly. Maintenance involves additional expense and manpower.

• Gun positions must be sturdy, and the artillery

pieces should be secure from theft, which means

more expense.

• Artillery rounds are designed to detonate against

hard objects. Deep snow slabs have a cushioning

effect; the result is an intrinsic dud rate in avalanche

work of about 1 percent (compared to about 0.1

percent for hand-thrown explosives). Duds are

clearly undesirable in any area used by the public.

• Backblast and concussion can shake buildings

and break windows up to 500 m away.

• Shrapnel is a major problem to be considered at

all times. While it is true that most shrapnel is confined to within 100 m of target center, freak pieces

of shrapnel travel considerably farther. In at least

one case an avalanche worker was struck by a piece

of shrapnel that ricocheted 1,000 m from the target.

Shrapnel is also a serious hazard in storage and in

some situations results in prohibitive costs.

The above problems are serious enough that full

consideration must be given to alternatives. In planning new developments, the developer should realize

that the difficulty of procuring obsolete weapon systems, irrespective of cost, is increasing. Existing ski

areas with operations that depend on military artillery

are expected to deplete the present inventory of obsolete ammunition in the near future. Planning for any

new ski developments must demonstrate the feasibility

of avalanche control and snow safety without military

weaponry.

Many of the problems associated with military

artillery are solved when the pieces are fired and

maintained by military units serving winter or mountain-warfare training assignments. Avalanche workers who are familiar with local terrain can function

as civilian advisers to military units. This system

works quite effectively in Switzerland and Canada

(see chapter 7). If civilians are responsible for firing

and maintaining weapons, it is essential that they receive training from military units. No attempt will be

made in this handbook to cover military guidelines

and field-manual material, such as U.S. Department

of the Army (1957, 1958a, 1958b, and 1962), which,

if followed strictly, result in a high standard of safety

and efficiency.^

At present, the military artillery used in ski area

control are the 75-mm recoilless rifle, the 75-mm

howitzer, and the 105-mm recoilless rifle. The 105-mm

howitzer is excellent for highway control, but it requires a five-man military crew and is too cumbersome for ski-area work. It is thought that the 57-mm

military ammunition has too small a charge to be

effective,^ while the 155-mm howitzer is too large and

complex. Unfortunately, existing high-explosive (HE)

ammunition for the 90-mm recoilless rifle has insuflicient range for avalanche work. The 106-mm recoilless rifle should be satisfactory, but it is in use by the

military and has not been released for avalanche

work. Considering accuracy, crew safety, dud problems, and simplicity, most mortars, rockets, and related systems are poor second choices to recoilless

rifles or howitzers.

Characteristics of the 75-mm recoilless rifle (fig.

154), 75-mm howitzer, 105-mm recoilless rifle (fig.

^D.C. DuLac. Firing Manual, 75-mm and 105-mm Recoilless Rifle. Information on file at U.S. Forest Service, Region

6, Portland, Oreg.

•'^However, a 57-mm projectile designed specifically for

avalanche vt'ork could carry a standard charge of 1 kg of TNT.

136 PROTECTION OF SKI AREAS
131 ), and 105-inm howitzer are summarized in table

7. Information on the 106-mm recoilless rifle is also

included for possible future reference. Since all the

listed weapons deliver a large enough payload to release avalanches, the choice of weapons should be

based predominantly on range requirements. As a

general rule, slab targets should be within half the

rated maximum range of the weapon. At half range,

it should be possible to maintain acceptable accuracy

despite high winds and other contingencies. Also, at

half range it should be possible to fire the weapons

accurately with aiming-stake references, as well as

with direct visual sight on target. For targets beyond

half range, accuracy decreases drastically, and the

number of duds becomes unacceptable.

If range is not a problem, the 75-mm recoilless rifle

is probably the best military piece available, considering cost and flexibility. It is light enough to be

moved routinely by truck, snowcat, toboggan, or lift

from a sheltered and secure storage to a temporary or

permanent firing position. One class of ammunition

for the 75-mm weapons has somewhat less payload

than the recommended 1-kg TNT equivalent. However, judging by the excellent safety record for the

75-mm weapons in past years, the payload seems

adequate. (In this connection, postcontrol releases

have occurred after slopes were hit with 105-mm

ammunition; firing larger rounds is no guarantee

against postcontrol release.)

Only two types of recoilless-rifle and howitzer ammunition are considered satisfactory for avalanche

work; namely. High Explosive (HE) and High Explosive Plastic (HEP, HEP-T). The High Explosive

Anti-Tank (HEAT) round is intended for armor

piercing and is not suitable for avalanche work. The

90-mm HEXM is also disqualified because of its low

range.

HE rounds contain a payload of TNT (or the more

powerful composition B) in a relatively thick steel

encasement. The rounds are intended to produce

shrapnel and blast damage. HE rounds may be detonated by several types of fuses. Avalanche workers are

advised to use only the impact type of fuse, specifically

the Type M557 Impact Fuse. This fuse does not arm

the round until the round has traveled a fairly safe

distance from the barrel. Thus, crew safety is improved should the round detonate prematurely because of collision with precipitation particles or some

other obstacle. Other types of fuses, known as time

fuses and time-impact fuses, are designed to detonate

HE rounds in midflight, or at a certain distance from

A IK^

Figure 154.—Recoilless rifle, 75 mm, fired as a portable

weapon from a temporary position. Gunner's ears should

be protected from blast noise.

the target. Rounds equipped with these fuses should

be handled only by military crews.

The impact fuse on HE rounds is a point-detonating

(PD) component in the nose cone of the round. The

PD component can be set to detonate either instantaneously on impact (the so-called "super-quick"

setting) or with a small time delay of 0.05 second after

impact. If the "super-quick" element does not function, the delay component takes over to detonate the

round, giving extra assurance against obtaining a dud.

HE rounds are normally shipped with the "superquick" setting. Avalanche workers are advised to use

only the "super-quick" setting in order to maintain

a low dud rate.

HEP (HEP-T) ammunition carries about 75 percent more payload than HE ammunition. The intended effect of HEP ammunition is to squash against

a tilted armor plate, detonate, and produce "scabbing"; that is, turn the back side of the plate into

shrapnel. Shrapnel from the round itself is minimal,

but still dangerous. Shrapnel weight is replaced by

composition A3, which is more powerful and sensitive

than TNT. Because the round is designed to "squash"

against armor, the explosive is detonated by a fuse

located in the base of the round. The base-detonating

fuse (Type M91 ) is actuated by the deceleration of

the charge on impact. The round is armed almost

immediately after leaving the weapon; however, the

Use of artillery 137
base-detonating fuse is not apt to function prematurely on collision with precipitation particles. After

hitting a solid target, the round detonates without

delay. There is no setting on the fuse.

The increased payload and reduced shrapnel of the

HEP round is, of course, very desirable in avalanche

work. Unfortunately, the base-detonating fuse may

not be activated by a snow target. A rather high proportion of duds can be expected when HEP rounds

are fired against deep slabs. The dud rate is substantially reduced if the HEP round is fired into a shallow

region of the slab, perhaps into rock bands immediately above or to the side of the slab. The increased

explosive payload of the HEP round appears to generate a large enough shock wave in the ground to compensate for deviation from normal target center.

Because of the sensitivity of composition A3, HEP

rounds are more hazardous in storage than HE

rounds. Accidental detonation of an HEP round is

likely to detonate the entire storage supply.

Up to the mid-1970's, about 100,000 rounds were

fired at avalanche slopes with no casualties caused by

malfunction of ammunition or artillery piece. The one

death attributed to artillery occurred when an experienced avalanche worker inexplicably stepped behind

a 75-mm recoilless rifle. The victim had fired the

weapon many times and was well acquainted with the

backblast force. During this time, about 1,000 duds

were destroyed in the field without mishap. Most duds

were not armed, but each was handled with great respect (see the next section, "Artillery Techniques").

TABLE 7.—Military artillery suitable for avalanche control

Weight of Length of

Artillery Maximum complete complete Muzzle Armed

piece range"^ Payload^ round round velocity distance

and weight Round Fuse (m) (kg) (kg) (cm) (m/s) (m)

75-mm howitzer HE

MlAl M48 PD 0.7

Weight: 230 kg (4 bags) M557 8,800 TNT 8 60 380 61-f

75-mm recoilless HE PD 0.7

rifle. M20 M309A1 M557 6,350 TNT 10 73 300 61 + Weight: 80 kg HEP-T BD 1.2

M349 M91A1 6,350 A3 8 67 430 1.5

105-mm howitzer HE PD 2.3

M2A4, M4A1, M49 Ml M557 11,300 TNT 19 79 480 61 + Weight: 2,260 kg HEP-T BD 3.5

M327 M62A1

M91

8,670 A3 15 74 720 1.5

105-mm recoilless HE PD 2.0

rifle. M27A1 M323 M557 8,600 TNT 19 79 480 61 + Weight: 320 kg HEP-T BD 3.4

M326 M62A1

M91

7,550 A3 19 97 380 1.5

106-mm recoilless HEP-T BD 3.5

rifle.'» M40A1 M346A1 M91A2 6,850 A3 19 97 500 1.5 Weight: 210 kg

"For avalanche work, target distance should not exceed

half the maximum range.

^'Composition A3 is a military explosive that has about a

40 percent higher detonation pressure than TNT. It contains

91 percent RDX and 9 percent desensitizing wax. The explosive is more sensitive than TNT and can be detonated with

a No. 6 blasting cap.

•"Distance out from barrel required for fuse to arm round.

*106-mm recoilless rifles have not been released for avalanche work.

138 PROTECTION OF SKI AREAS
Artillery techniques

Before firing an artillery piece, avalanche workers

should be quite certain that no person or facility will

be hit by shrapnel or blast. Except for the gun crew,

no one should be allowed to stand exposed within

1,000 m of the target area. Targets should be no

closer than 300 m to ski lifts or other vulnerable

facilities. Gun-to-target distance should be no less

than 500 m for 75-mm ammunition and no less than

700 m for 105-mm ammunition.

Quality control of ammunition is good, but not perfect. Rounds may land short of target by a wide error.

To prevent damage caused by short rounds, one

should avoid aiming over vulnerable facilities. Also,

the line of fire should diverge away from roads and

other developed areas.

Under no circumstances should a round be fired

when there is chance of overshooting the target and

hitting a developed area. If there is any question, the

flight trajectory should be plotted on a topographic

sheet. Targets near ridges should be hit purposely low,

at least 50 m below the ridgecrest for each 1,000 m

of target distance. The sight assembly should be

mounted correctly and firmly in its housing on the

artillery piece; this should be checked routinely, especially before high ridge shots. It is also important

periodically to check sight alinement by boresighting

the weapon. A sure way to prevent overshooting is

to install an overshoot guard as shown in figure 156.

Firing positions are selected on the basis of several

considerations: safety, backblast and concussion,

target coverage, accessibility, and working conditions.

There can be no compromise on crew safety. All firing

positions should be free from avalanche danger either

originating at the target or as sympathetic releases

from nearby slopes. Backblast and concussion effects

on fixed facilities should be minimum. Dangerous

backblast cones extend about 100 m behind recoilless

weapons. Concussion zones can extend much farther,

depending on the weapon, terrain, amount of snow on

the ground, and atmospheric conditions. Large windows directly behind a 75-mm recoilless rifle and

within about 500 m are apt to be broken. The concussion is not as great to the side, but it can still break

windows perhaps 250 m directly to the side. Windows

may be protected by shutters. Concussion from

75-mm howitzer fire is not as intense.

If crew safety and concussion effects are not problems, the choice of firing positions is made by weighing the benefits of target coverage versus accessibility

Section of discarded cartridge ^

fitted witfi cross fiairs. w '

«

Back section

of discarded cartridge

-small peep tiole

drilled in center.

Figure 155.—Sight alinement should be checked occasionally

by boresighting on a target about 1,000 m from the weapon.

Boresighting equipment can be made from a discarded

cartridge sliced into two sections as shown in the figure.

Artillery techniques 139
Figure 156.—Overshoot guard at Carson Pass, Calif.

Figure 157.—Reinforced concrete enclosure used to protect

a 75-mni howitzer at Snowbird. Utah. The howitzer pivots

on a large steel plate.

and working conditions during inclement weather.

Before operation, at least 1 year of reconnaissance is

needed to select the best sites.

Figure 158 shows two gun towers; one constructed

with tubular steel, the other with corrugated pipe.

Both types of construction are satisfactory. The tower

should be protected from lightning. Figure 157 shows

a reinforced concrete enclosure for a 75-mm howitzer.

The howitzer pivots on a large steel plate to facilitate

rapid and efficient coverage of the target area. When

the howitzer is not being used, the enclosure is sealed

off by sliding wooden doors. The cost of such a sophisticated facility is prohibitive unless a ski area has

very pressing avalanche problems.

There are two methods for sighting artillery. If the

starting zones are visible, artillery may be sighted

directly with a telescopic sight. This is called direct

sighting. The alternative, which is used when the starting zones are not visible, is indirect sighting. Essentially, indirect sighting consists of:

• Putting the artillery piece in a sturdy mount

• Setting the horizontal and vertical adjustments

on an indirect sight, which is firmly mounted on

the piece

• Elevating and traversing the weapon until the

cross hairs of the indirect sight aline with a reference point on an aiming stake about 10 to 30 m

from the weapon.

Indirect sight data must be set up carefully and

periodically checked by firing indirectly when starting

zones are visible. Before firing, it is necessary to

140 PROTECTION OF SKI AREAS
Figure 158.—Examples of gun towers used in avalanche control.

% %

Figure 159.—Direct sighting of 75-mm recoilless rifle during clear visibility.

Artillery techniques 141
Figure 160.—Indirect firing during poor visibility. Gunner alines sight on reference (right).

DANGER

iXPLOSIVES ON THE MOUNTAI

unexploded warheads used m avalaoche comitrol

may be found m target areas

UNEXPLODED WARHEAD

(warhead may be dostorte«

IF YOU FIND A WARHEAD DO THE F0LL0WIH6

1- do not CUDS

2- mark locatoon, DO ft. away, woth rock, bright dlotl

3- ómmedóateDy report location to dístróct forest

ranger at 524-5042

, etc.

142

Figure 161.—Dud warning sign on the USDA Forest Service garage at Alta, Utah.

PROTECTION OF SKI AREAS
Safety fuse

(length as explained below)"

Clove hitch

Detonating cord

kg

gh detonation

pressure explosive

Figure 162.—Duds should be destroyed by a qualified blasting

crew. Two blocks of high-detonation-pressure explosive,

each at least the equivalent of 1 kg of TNT, are placed

along the sides of the dud, close to the dud but not touching.

The explosives are detonated simultaneously by detonating

cord. Leave enough escape time in the fusing to retreat

from the shrapnel zone (1,000 m in the open or behind

good cover).

double check the indirect settings as follows: The

first gunner calls out the target name and the horizontal and vertical settings; the second gunner sets the

indirect sight; and the first gunner checks the settings.

As mentioned earlier, a 1-percent artillery dud rate

can be expected in avalanche work. All duds should

be considered dangerous and armed; the duds should

be disposed of in place, preferably by military demolition crews. The location of each dud should be carefully noted on a photograph of the target area. Duds

will penetrate to the ground and cannot be recovered

until the snow melts in early summer. It is wise to

begin the search as soon as the snow melts, before the

summer growth of vegetation. Searching for duds can

be tedious if performed by one searcher. It is advisable

to flag the area and bring in a rather large crew.

If a military demolition team is not available, duds

may be destroyed by qualified blasters. Since the blasting operation is conducted in the summer, precautions

against fire may be necessary. If the duds are not

armed, they may be surprisingly difficult to detonate.

The recommended procedure is to use two charges of

a military or commercial explosive with a high detonation pressure. The amount should be at least the

equivalent of 1 kg of TNT per charge. The charges are

placed on both sides of the dud, as close as possible

without disturbing the dud. The charges are detonated

simultaneously by detonating cord, using a fuse long

enough to leave time to withdraw from the shrapnel

area, at least 5 minutes. Dud terrain should be posted

with warning signs that advise the public about procedure to be followed should a dud be encountered.

Planning the control program

Many avalanche problems at ski areas could have

been avoided had the developer given more attention

at the outset to studying and planning the avalanche

control program. At the time a new development or

expansion is proposed, the developer should prepare

an initial avalanche study. The initial avalanche study

should include ( 1 ) identification of the hazard and

(2) proposed control, including costs.

Hazard identification is based on techniques described in chapter 4 under "Identification of Avalanche Paths." The starting point is a thorough reconnaissance of the area in winter and summer. On the

basis of the reconnaissance, it is possible to prepare a

map and description of the hazardous paths in the

proposed development. At the same time, meteorological and snowpit data should be collected, to indicate avalanche size and frequency. An example of a

hazard identification is shown in figure 163 and

table 8 (on page 144).

After the hazard is identified, a control plan must be

prepared. Among other things, this shows:

• Method of control for each avalanche path (or

group of paths)

• Routes for ski-testing and control and for blasting

• Facilities to shelter the control teams

• Storage facilities for explosives

• Stability evaluation instruments and equipment

• Operational procedure for various conditions

• Manpower needs and organization of manpower,

including leadership, training, and career ladder

• Costs.

Planning the control program 143
TABLE 8.—Hazard identification and control for Berthoud Pass, Colorado (mean elevation 3,500 m)

Approximate ApproxiNumber ot\[ number of mate Snow

photo map'I

avalanches starting weather

(fig. 163) Name per season zone angle Aspect Description Hazard conditions

1 Timber 6 42 NNE Several small

paths in

timber

This group of

paths is a

hazard to deWind loaded by strong prevailing west to northwest

winds. Deep instability

2 Trough 4 37 ENE Wide gully scending skiers

and to the base

caused by TG metamorphism

is quite common. Slab

3 Lift

gully

16 42 E Deep gully

beneath

cornice

facilities. thickness varies widely;

average about 1 m thick.

4 Cliffs 9 40 E Spills over

cliffs

5 Roll 4 40 ENE Open slope beneath convex

roll

*Avalanche paths 3, 4, and 5 are permanently closed to the

public. Avalanche paths 1 and 2 are ski compacted by a

professional ski patrol and opened to the public. Handthrown

explosives are periodically used on paths 2, 3, and 4. Path 5

is not controlled.

If the expansion or new development is feasible and

is approved, the next step is to work up a detailed

avalanche control plan for the new area. This valuable

document, written during the construction of the new

area, serves as an operating manual for day-by-day

problems. It is an extension of the initial avalanche

study. Whenever possible, it should be written by experienced personnel who are intimately acquainted

with the terrain and meteorological conditions. The

services of experienced avalanche workers should be

enlisted from the beginning. The avalanche control

plan may be part of a more general snow-safety plan;

however, many areas with severe avalanche problems

may prefer to work up the avalanche control plan as a

separate manual.

Undoubtedly, the control plan will evolve and improve with operating experience. However, a developer is responsible for maintaining safety from the

instant the development is opened to the public, and

the first edition of the control plan should be well

along toward a finished product by the time the area

opens. From the first the person responsible for snow

safety, such as a mountain manager or ski patrol leader, must be designated clearly and distinctly.

One important function of the control plan is to

explain how hazardous areas are checked systematically by control routes. Control routes are normally

skied early in the morning before the public arrives.

Each control route is skied by at least two avalanche

workers who should not have to carry any more than

15 kg of equipment per man, including explosives, a

radio, an avalanche transceiver, and other safety

equipment. Control routes usually work down the

mountain, following ridges and threading from one

safe location to another. The routes may crisscross,

and the progress of one crew may depend on the results of another crew. On the more complex routes,

signs should be posted indicating blasting positions.

If control routes are intricate, it is wise to permanently assign one crew to each route. Permanent assignments define responsibilities. Also, a crew can

become quite familiar with snow conditions on its

route and can watch for developing instability throughout the day. It is wise, however, to rotate crew members often enough that others can take over a route

in case of sickness or accidents to the regular crew

members.

Daily control methods depend on the stability evaluation, which, as explained in chapter 5, is adjusted

continually depending on control results. Normally,

the avalanche control leader briefs the control teams

in the morning on stability conditions as they appear

before the control routes are skied. He then advises

the teams on the amount of explosives to be taken on

each route, the slopes to be opened and closed, the

slopes to be intensively control skied, etc. Teams often

run into conditions not anticipated in the morning

stability evaluation. This requires decisions concern144 PROTECTION OF SKI AREAS
Figure 163.—Hazard identification for a small ski area at Berthoud Pass, Colo. The existing lift is AB. The proposed (never

seriously) expansion by a lift BC is not feasible because of severe weather and avalanche problems. If lift BC were built, the

public would need protection from the avalanche hazard in the cross-hatched areas (see table 8).

ing public safety to be made at the control-team level,

and the control teams must have experience and competence to match the responsibility. They should also

maintain radio contact with other teams and the avalanche control leader so that significant differences

from anticipated conditions are quickly passed on.

Many excellent control plans have been prepared.

The interested reader is referred to Aspen Skiing

Corp. (n.d.), Hastings et al. (1969), and Krisjansons

(1971).

To be profitable, few ski areas can rely entirely on

closing runs and allowing the snow to stabilize naturally, even though such techniques require the least

capital outlay. Most ski areas that suffer from recurring avalanche hazards use artificial stabilization to

some extent.

In estimating the cost of reducing hazards, a number of conditions must be considered:

• Frequency of avalanche hazards during the skiing season.

• Amount of available, skiable terrain affected by

the hazard.

• Need to protect lifts and other physical improvements such as buildings, access roads, and parking

lots from avalanche damage.

Figure 164.—On the more complex routes, it is necessary to

mark blasting positions with small signs.

Planning the control program 145
Figure 165.—Intricate system of control routes at the Jackson Hole Ski Aiea, VVyo.

• Kind of clientele served by the area. If the customers are vacationing skiers who have traveled a

considerable distance, they will not be pleased to

find the slopes closed. Day skiers who live nearby

are more willing to accept avalanche closures and

can be more easily informed by radio and television

announcements.

• Relative costs and benefits to the area of various

intensities of control programs. For example, during times of high avalanche hazard the policy may

be to have: ( 1 ) all slopes open daily at 9 a.m., (2)

some slopes open daily at 9 a.m. and others closed

until noon or 1 p.m. to allow for avalanche control

during the morning hours, or (3) no slopes opened

until the snow stabilizes naturally.

• Relative demand for specific kinds of slopes.

Spending large sums to keep certain slopes open

may not be economic if they are used by only a few

exceptionally skillful skiers. Such a procedure generates little additional revenue.

146 PROTECTION OF SKI AREAS
Further reading

Livingston, Clifton W.

1968. Explosions in snow. U.S. Army Corps Eng., Cold

Reg. Res. and Eng. Lab., Tech. Rep. 86, 124 p. Hanover,

N.H. Reports results of more than 140 test blasts under

the snow in Greenland using three types of explosives.

Failure was described as viscous damping, which differs

markedly from shock type in brittle material and from

shear type characteristics of plastic materials.

Mellor, Malcolm.

1965. Explosives and snow. Cold Reg. Sei. and Eng., Part

III-A3a, 34 p. U.S. Army Corps Eng., Cold Reg. Res.

and Eng. Lab., Hanover, N.H. Rather technical coverage

of the effects of explosives on snow and of snow on explosives. Mostly related to polar conditions. Use for avalanche control is given very brief mention.

Mellor, Malcolm.

1968. Avalanches. Cold Reg. Sei. and Eng., Part III-A3d,

215 p. (see especially p. 121-139). U.S. Army Corps

Eng., Cold Reg. Res. and Eng. Lab., Hanover, N.H.

State-of-the-art paper covering all phases of avalanches

and their control.

Mellor, Malcolm.

1973. Controlled release of avalanches by explosives, p.

37-50. In Advances in North American avalanche technology. USDA For. Serv., Gen. Tech. Rep. RM-3, 54 p.

Rocky Mt. For. and Range Exp. Stn., Fort Collins, Colo.

Effect of explosives on snow is discussed from viewpoint

of rock blasting technology. Explores possibility of liquid

and slurried explosives in avalanche control.

Further reading \A1
V ',
Chapter 7

Protection of highways

and villages

This chapter starts with a discussion of structural

control of avalanches. Certain kinds of structures

can be built in the starting zones to keep avalanches

from starting and to influence snow deposition

patterns. Other kinds are placed in the track and

runout zone to guide avalanches away from

objects or to give direct protection. Artillery

can be used to protect highways, but it has

serious limitations when avalanches threaten

towns or dwellings. In the case of highway

protection, shooting is intended to release the

avalanches before they get large enough to reach

the road. This requires a good method of stability

evaluation, backed up by an organized artillery

team ready to fire at any time and under any

weather conditions. The chapter continues with

factors to be considered when planning a

highway avalanche-control program, using the

Trans-Canada Highway as an example. Finally,

avalanche zoning and avalanche warnings are

discussed as ways of avoiding additional avalanche

problems and of informing the public during

high-hazard periods.

Figure 166.—A century of avalanche defense at Davos,

Switzerland. Ancient masonry walls and terraces, with

Arlberg fences added later, are flanked by modern structures.

(Photo by Wengi)
Figure 167.—Rigid supporting structures in the starting zone

can be made from a variety of materials: Top left, aluminum frame and braces with wood crossbeams on the grate.

Single snow bridges are no longer used; a continuous line

of bridges is better. Mattstock Avalanche, near Amden,

Switzerland (Wagner and Hopf 1959). Bottom left, aluminum snow bridges. Mattstock Avalanche, near Amden,

Switzerland (Wagner and Hopf 1959). Top right, prestressed concrete snow bridge. Kiihnihorn, near St. Antönien-Castels, Switzerland (Frutigerand Martinelli 1966).

Defense structures

Many types of structures are used throughout the

world to defend against avalanches. Massive earth,

stone, or concrete walls, terraces, and mounds that

require little or no detailed design have been used for

more than a century in central Europe. Lighter weight,

open structures of wood, aluminum, steel, prestressed

concrete, or some combination of these materials have

evolved in recent years, after enough was learned

about snow pressures to permit proper design.

The wide variety of avalanche defense structures

can be classified in four groups : ( 1 ) Supporting structures in the starting zone, (2) deflecting and retarding

structures in the track and runout zone, (3) directprotection structures in the runout zone, and (4)

snowfences and wind baffles.

Supporting structures in the starting zone. This kind

of structure is built in the upper part of the avalanche

path to prevent avalanches from starting or to catch

any that do start before they gain momentum. The

earliest supporting structures were massive walls and

terraces made of stone and earth. These were intended

150 PROTECTION OF HIGHWAYS AND VILLAGES
to interrupt the continuity of the snowpack and were

moderately successful in some places as long as snow

depths were not great and the structures were kept in

good repair (fig. 166).

During the 1930's and 1940's scientists working

mainly at the Swiss Federal Institute for Snow and

Avalanche Research (SFISAR) in eastern Switzerland learned enough about the forces in snow cover

lying on steep slopes to permit the design and construction of lightweight structures. At first these were

open wood and metal units with the grates almost

horizontal. They looked something like bridges and

functioned much like terraces.

Modern supporting structures in the starting zone

may be rigid, flexible, or a combination. They can be

made of wood, steel, aluminum, prestressed concrete,

or a combination of these materials (fig. 167). Today

the most commonly used material is steel. Some structures have crossbeams that are horizontal, and some

have crossbeams that are roughly perpendicular to the

slope. The former are called snow bridges, the latter

snow rakes. These structures are tilted about 15°

downhill from the perpendicular to the slope to get

better support from the back braces without sacrificing too much effective height. The structures are usually arranged in continuous lines or segments of lines.

In a few cases, the grates or open supporting structures are vertical; these are called fences. Snow

bridges, snow rakes, or fences may be combined with

earth walls or terraces to gain height in areas of deep

snow.

Snow nets are flexible supporting structures (fig.

168). Steel cables are used to form the nets, which

are held up by steel poles guyed with cable at the top.

Although good foundations are needed for all supporting structures, bedrock anchors are required for

nets. Use of this kind of structure is limited to slopes

with snow depths not much greater than 2 m and with

little rockfall.

Supporting structures are expensive to install and

maintain, not only because of the materials involved,

but because of the access problem at most sites. Their

reliability when properly planned, installed, and maintained makes them desirable where the starting zones

of the avalanches are relatively small and the objects

to be protected are numerous and valuable, such as

the homes and buildings of a town. To be effective,

supporting structures must be taller than the snow

cover and must be designed to support the snow above

themselves and to catch and hold small avalanches

that might start in the protected area. Details for planFigure 168.—Snow nets are flexible supporting structures. The

triangular-shaped nets of wire rope are hung on tubular

steel posts. Kneugrat, Braunwald, Switzerland (Frutiger

and Martinelli 1966).

ning and designing avalanche control projects using

this type of structure are given in SFISAR (1961) and

Frutiger and Martinelli (1966).

Deflecting and retarding structures. These are massive structures usually made of earth, rock, or concrete

and located in or near avalanche tracks or runout

zones. They are intended to keep the moving snow of

an avalanche away from valuable objects.

Structures to deflect moving snow should divert it

as little as possible from its normal direction of flow

(fig. 169). Deflections no greater than 15° to 20°

have been most successful. Walls built at sharp angles

to the flowing snow are likely to be overrun by fastmoving masses of dry snow. The contact faces of

deflecting walls and dikes should be as smooth and

steep as possible.

Retarding structures are usually earth mounds or

dams (fig. 170) built on benches or gently sloping

parts of paths where avalanches slow or stop naturally. The additional roughness and cross-currents set

up in the moving snow by these structures usually

Defense structures 151
EARTH-BAMK DEFLECTOR EARTH-BANK DEFLECTOR WITH MASONRY FACING

Figure 169.—Deflecting structures: Top, sketches of earth or

earth and masonry walls (Mellor 1968); bottom, concrete

walls backfilled with earth to keep the avalanche that runs

down the stream channel left of the fence from spreading

to the right (Beratungsstelle für Stahlverwendung 1962).

Stop all but large dry-snow avalanches. Mounds are

inexpensive to install and relatively easy to maintain;

however, they are ineffective on slopes steeper than

20°. A height of 5 to 8 m generally is sufficient. Where

frequent, large, wet-snow avalanches occur, dry

masonry or a concrete slab on the uphill face helps

reduce erosion. Earth dams or concrete walls are built

perpendicular to the flow direction of the avalanche

and are intended to stop slow-moving avalanches. The

trapping capacity of the dam must be adequate for

the size of avalanches expected. A cover of native

vegetation softens the visual impact of such structures

and helps stablize them.

A snow dam bulldozed up from deposited snow can

be used in some places as a substitute for a small earth

dam. Benches with snow dams are adequate protection against small spring-thaw avalanches.

Direci-proiection structures. This type of structure

is built immediately adjacent to the object to be protected; in a few cases it is part of the object itself. The

aim is to render complete protection regardless of

avalanche size, type, or frequency of occurrence. The

avalanche gallery or avalanche shed is a good example ( fig. \1\). Avalanchesheds are merely roofs over

roads or railroads that allow avalanches to cross without interrupting traffic. They are more economical for

railroads or narrow roads than for the multilaned

superhighways now being built in some mountainous

areas, but they offer the most positive protection

against avalanches. Design criteria are given in

Schaerer (1966) and Sommerhaider (1972). The

Figure

^•Jí'í' ^

170.—Retarding stiuclinos used in asaianchc paths: Left, mounds near Innsbruck, Austria. Right, mounds in the runout

zone of Mt. Tupper Avalanche, Rogers Pass, B.C. (Photos by LaChapelle)

152 PROTECTION OF HIGHWAYS AND VILLAGES
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Figure 171.—Direct-piotection structure: Avalanche shed

over U.S. Highway 160, near Wolf Creek Pass. Colo. Two

avalanche paths cross the three-lane highway at this point.

(Photo by Martinelli)

Figure 172.—Direct-protection structures: Top, earth ramps

and wedges to protect buildings (Mellor 1968). Bottom,

every house of the alpine settlement of St. Antönien has a

mound of earth on the uphill side to protect it from avalanches. Gädmen-Matten-Meierhof, St. Antönien, Switzerland. (Photo by Frutiger)

Figure 173.—Direct-protection structures: Left, a wedge-shaped concrete base for a high-voltage transmission tower located in

an avalanche path (Beratungsstelle für Stahlverwendung 1962). Right, solid concrete blocks uphill from high-voltage towers.

(Photo by Frutiger)

Defense structures 153
Figure 174.—Direct-protection structure: Special construction

of the church at Davos-Frauenkirch, Switzerland, to withstand the impact of moving snow. (Photo by LaChapelle)

Figure 175.—Snowfences on a flat ridge to catch drifting

snow before it goes over the steep rim where it might

become an avalanche. Clünas, Flan, Switzerland. (Photo

by Frutiger)

Figure 176.—Direct-protection structure: A sketch of a concrete wedge protecting a high-voltage transmission tower.

(Mellor 1968)

choice between sheds and supporting structures in the

starting zone is discussed in Frutiger and Martinelli

(1966). Sheds are not without problems; there is

evidence that soine sheds cause more accidents because of restricted visibility and ice on the road than

would result from avalanche encounters.

Where isolated buildings must be protected from

avalanches, special precautions are needed. Earth

ramps or wedges on the uphill side often deflect most

of the snow away from the structure (fig. 172). Special roof design and construction allow any snow that

gets past the earth wall to slide over the roof and

down the valley. This type of direct protection is used

for the homes and schools of several mountain communities in Switzerland, even though in at least one

of the cases there are also many supporting structures

in the avalanche starting zones. A building in the path

of slow-moving, small avalanches may be built with a

wall that is perpendicular to the avalanche and strong

enough to resist its impact.

When an object such as a high-voltage transmission

tower or a ski lift tower must be built directly in an

avalanche path, it can be built on top of a massive

concrete base designed to take the impact of an

avalanche. At other times, it is better to build a wedgeshaped wall or mound uphill from the structure to

divert the snow.

154 PROTECTION OF HIGHWAYS AND VILLAGES
The splitter-wedge principle has also been built

directly into some buildings located, for one reason or

another, in avalanche paths. A striking example is the

old church at Davos-Frauenkirch, Switzerland (fig.

174). The uphill wall of this church is a massive

wedge that has split several avalanches in the past

century.

Snowfences and wind baffles. The snow cover in

most avalanche starting zones is influenced by the

winds common to mountain summits and ridgecrests.

In areas where the snow distribution pattern leads to

avalanches, it is reasonable to assume that a change

in this pattern will lead to a change in the size and

frequency of the avalanches. Snowfences and several

types of wind baffles have been used in many areas to

reduce the number and size of avalanches and to

prevent the formation of cornices.

Traditional snowfences are effective where snow

can be trapped on relatively flat ridges and gentle

slopes before it gets to steeper terrain (fig. 175). The

volume of snow caught by a series of such fences can

appreciably reduce the size of cornices or the amount

of snow in avalanche starting zones. The number,

size, and location of fences depends upon the amount

of precipitation, wind direction during drifting, length

of the upwind contributing area, fence density, and

several terrain features.

Three types of wind baffles have been used to prevent or reduce cornices. One type designed to create

severe wind turbulence is a vertical wooden wall.

These "eddy panels" (called Kolktafeln in Austria

and Switzerland) are built at the terrain break where

a cornice might form (fig. 177). They are seldom

more than 4 m long or 3 m tall and are usually solid,

although densities as low as 75 percent have been

used successfully. A bottom gap of 1 to 1.5 m is often

advantageous.

Figure 177.—Wooden eddy panels (Kolklafclit) on a slope (above) and a ridgecrest (below) above supporting structures. Note

the disruption in the snow deposition caused by winds eddying around the baffles. (Photos by Frutiger)

Defense structures 155
h>2.5m

Figure 178.—Two types of sloping wind baffles used to prevent cornices: Left, pulpit used to carry wind and snow beyond the

usual accumulation area. Right, jet roof that deflects wind down the lee slope so a cornice cannot form. (Photo by Frutiger)

The Other two types of wind baffles used for cornice

prevention are sloping. One (fig. 178), called a "pulpit" is shaped to carry wind and snow upward at the

cornice-prone ridgecrest so the snow will be distributed more uniformly and farther down the lee slope.

Pulpits seem to be most successful on narrow ridges

with steep lee slopes.

The second type of sloping wind baffle (fig. 178),

called a "jet roof," is designed to trap wind blowing

over the crest and deflect it down the lee slope, to

prevent snow accumulation in the shelter normally

offered by the ridge. For best results, the roof should

have about the same inclination as the lee slope, and

the gap on the lee side should be 1 to 1.5 m. The

height on the windward edge is governed by the slope

and width of the roof, which is seldom more than 4 m

wide. Air jetting under the roof keeps the upper 10

to 15 m of the lee slope snow-free or nearly so, but

creates a snow deposit farther down the slope.

Role of defense structures. The various types of

structures have been discussed here one at a time as a

matter of convenience. In practice, it is common to

use many types of structures on a single path. For

example, to protect a village and roads from large

avalanches, supporting structures, wind baffles, and

snowfences may be used in or near starting zones to

stabilize the upper parts of avalanche paths. Mounds,

walls, and dams may be used farther down the mountain to catch any avalanches that start below the

supporting structures. Ramps, wedges, and other

direct-protection structures may be needed to protect

isolated objects such as electric power poles, ski-lift

towers, mines, or buildings. In addition, most European avalanche defense systems include reforestation

(see the next section) up to the natural treeline.

Obviously, the most desirable and effective protection against avalanches is to locate buildings,

roads, and other objects away from avalanches. With

plenty of room and a population wise to the ways of

the mountains, this is not hard to do. However, as the

population grows and less desirable sites must be

used, advanced planning and often enforced zoning

appear the best solutions. In some cases, even these

are not able to afford complete freedom from avalanches, and certain risks must be assumed, especially

in the case of roads, powerlines, and railroads. These

risks can be reduced, however, if appropriate structural controls are used.

156 PROTECTION OF HIGHWAYS AND VILLAGES
Figure 179.—The results of reforesting an avalanche path about 2.5 km south of Bergün on the Albula route of the Rhatische

Bahn Grison, Switzerland. Left picture was taken in 1907; middle in 1938; right in 1957. A gallery 117 m long was built over

the railroad at the foot of the right avalanche path. More than 10 km of retaining walls were built above timberline and over

much of the rest of the mountain, then trees were planted. (Photos from Swiss Snow and Avalanche Research Institute with

the permission of the Rhätische Bahn)

Reforestation

Since large, destructive avalanches seldom start on

densely forested slopes, there is a strong incentive to

try reforesting existing avalanche paths. In some

mountainous countries, the need for additional forest

land is another incentive for reforesting avalanche

paths. There are many examples from central Europe

of avalanche paths that have been successfully planted

(fig. 179). The task, however, is neither easy nor

cheap and should be undertaken only after a careful

check of growing conditions and a firm conviction

that reforestation is the proper action for the site.

Most avalanche paths that have adequate soil can

be reforested, provided the time and money are available. On some paths, thin, droughty soils and large

areas of bare rock offer few places for trees to grow.

Reforestation is not feasible on such sites.

Avalanche paths that start above timberline or

those that start on steep, rocky slopes or in rocky

gullies are also poor prospects for reforestation unless

supporting structures are used to stabilize the snow

in the starting zone. Paths that are completely within

the forest and have deep soils and good moisture conditions offer the best chances for reforestation.

An avalanche path, even under the best conditions,

must be considered a harsh planting site that requires

special care, not only in such site-preparation necessities as terraces, fertilizers, and supplemental water,

but also in the selection of species and handling of

planting stock. High-quality planting stock of the

fastest growing native species is preferred. Seedlings

should be from locally collected seeds that are germinated and grown at a nursery in biodegradable pots.

This potted stock suffers considerably less setback

when it is moved from the nursery to the planting site

than the more traditional bare-rooted stock. Lightsensitive species, such as Engelmann spruce, should

be planted in the shade of rocks, stumps, logs, or

other objects.

On some sites, it may be better to plant one of the

successional species rather than the climax species.

For example, in the central Rocky Mountains, consideration should be given to lodgepolc pine on sites

where there is insufficient shade for planting spruce.

The pine is fast growing, has a good survival rate,

prefers sunlight to shade, and provides a good microclimate for the natural establishment and growth of

spruce, the climax species.

Because of the harsh environment, heavy losses

must be expected the first year, and several replantings

are usually needed for complete coverage. In some

European areas, avalanche paths are planted at the

rate of 2,000 seedlings to the acre in an attempt to get

Reforestation 157
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Figure 180.—Young trees on an avalanche slope in Switzerland with creep- and glide-control structures. (Photo by Frutiger)

at least 1,800 trees to the acre. It is not known that

even this density of stocking eliminates all avalanches.

Once planted, the young trees must be protected

from avalanches and from snow creep and glide for

many years (fig. 180). This usually means that supporting structures must be installed and maintained

for at least 25 to 50 years. Where spruce or other slowgrowing species are planted, longer protection is

needed, since they often take 75 to 100 years to reach

diameters of 10 to 15 cm, 1 m above the ground.

Snow distribution may have to be modified on some

sites before planting. Windy locations with shallow

soils are very droughty and hard to reforest. On the

other hand, areas of late-lying snow may have a growing season 30 to 60 days shorter than nearby places.

Trees growing in areas of deep, persistent snow are

subject to damage and possibly death from snowmold

fungi. Snowfences are one way to modify snow distribution before planting.

Reforested areas also must be protected from domestic grazing animals and fire.

Control by explosives

The strategy of artificially releasing avalanches

above highways is intended to produce small avalanches that clear out the track and either stop short

of the road or deposit only small loads on it. Ideally,

the avalanches should stop just before crossing the

highway. Experience demonstrates that it is much

more efficient to plow out many small deposits than

one very large deposit. Also, the release of many small

avalanches in the winter prevents large avalanches

from running in the spring thaw.

Thus, highway control strategy differs from control

strategy in ski areas. For the latter, an important objective is to use explosives to test and prepare the way

for ski compaction, which can proceed cautiously

even if explosives do not cause slab release. By contrast, for almost all highway paths, as well as inaccessible avalanche paths in ski areas, the release of small

avalanches in response to explosives is the desired

effect. The notion that explosives always stabilize a

slab, regardless of whether or not the avalanche releases, is disputable. In some cases, explosives may

158 PROTECTION OF HIGHWAYS AND VILLAGES
release tensions, but such relief is only temporary. Of

course, if the slab is saturated with explosive holes,

stability may be reached by force. However, the environmental damage of saturation blasting is rarely

justified.

What is the strategy for explosive control of paths

that have the potential to run into residential areas or

other developments? The primary objective is to safeguard the lives of people that need to move in and

around the threatened facilities; protection of facilities

is secondary. It should be understood that buildings

in a developed area protected solely by explosive

release are on borrowed time. Sooner or later, a sequence of climatological events will cause avalanches

that cannot be completely controlled by explosives.

Developers must account for this economic risk, and

they have full responsibility to explain to their clients

the vulnerability of the facilities. In addition, developers must either work out a satisfactory procedure for

evacuation during critical periods or provide adequate

avalanche shelters. Moreover, threatened lodges,

buildings, condominiums, etc., should be equipped

with complete rescue caches including probes, shovels,

and medical supplies.

From limited experience with the few developments

that are gambling on artificial release for protection,

it appears that the strategy of releasing small avalanches is helping to extend the life of the development

over what would be expected if the avalanches were

left uncontrolled. Just as in highway control, the release of many small avalanches has a strategic advantage over waiting for one large avalanche. Nevertheless, avalanche workers called upon to blast in such

a risky situation must be released from liability before

control is begun. This may require a civil ordinance

that delegates authority to perform control on specific

slopes for the "general public welfare," in full understanding of risks and uncertainty of results.

Inaccessible avalanche paths over highways, villages, and ski areas are routinely controlled by artillery

or helicopters. Of these two principal techniques, artillery control is apt to be the more expensive, since

artillery ammunition plus purchase and maintenance

of weapons cost many times more than handthrown

charges plus helicopter time. On the other hand, helicopters are limited to favorable weather. Since visibility constraints should not be imposed if control is to

be most effective, helicopters are best used in combination with artillery rather than as the sole technique.

Artillery methods were discussed in detail in chapter 6. Because range is a problem, most highway conFigure 181.—75-mm lecoilless rifle tower on Washington State

highway. Winches are used to lift gun onto tower.

trol is performed with military weapons. At present,

the alternatives to military weapons do not have adequate range and accuracy for most highway control.

Whenever possible, military units stationed at the area

should fire the weapons and maintain the weapons and

ammunition. The weapons should be fired from fixed

mounts that permit accurate indirect aiming during

poor visibility. Firing positions should be chosen to

minimize avalanche hazard to firing crews. Recoilless

rifles and howitzers (75 mm to 105 mm) are suitable; the howitzer should be used where steep gun

elevations might bounce recoilless-rifle backblast into

the gunner.

After firing, it is important to note which paths

failed to release; on the next shoot such stubborn

paths can be hit twice. When large snow accumulations have built up, it may be possible to decrease the

avalanche runout by firing a special target sequence;

for example, firing on a low target before hitting the

Control by explosives 159
high target. In other cases, there is a gain of efficiency

in first hitting the high target and allowing the slide

to clean out the lower slopes. This procedure, however, usually results in a large avalanche.

During firing and hazardous periods, roads should

be closed by strong blockades or by a patrol car with

red lights, or by both. Road closures should be on

wide, level stretches that allow traffic to turn around

(Gardner and Judson 1970). The "waiting-out" area

should be avalanche-free and not exposed to recoilless rifle backblast. Enough space should be maintained between parked vehicles to avoid exhaust gas

poisoning. Whenever possible, a public telephone and

shelter should be provided.

When weather is favorable, helicopter control is a

valuable supplement to artillery. Helicopters may be

used either to transport control teams to ridgetops or

• for direct bombing from the air. This may save time

and money, make possible the control of paths not

accessible to artillery, and add safety to the overall

control program. This last advantage must be qualified

immediately, since helicopter flying over mountainous

terrain is not without risk. Just how much safety a

helicopter can add or subtract from the program

depends on the ability of the pilot and the capability

of the helicopter, as well as on the blasting crew and

their equipment and procedures.

Pilot requirements for avalanche blasting are not

yet standardized nationally; however, blasting from

aircraft requires special permission from the Federal

Aviation Administration (FAA), and permission is

granted only to the most experienced and responsible

pilots who have had considerable mountain flying

experience. Excellent knowledge of local terrain is

essential, and the pilot should be acquainted with

avalanches and mountaineering in general. The FAA

waiver also is based on using a helicopter with suitable power, range, and cabin capacity. In addition,

the FAA must review the blasting procedures and

emergency plan, all of which may be specified in the

waiver. Operating on Government land may require

other approvals and regulations. All crew members

must be experienced in basic helicopter procedures.

Dummy charges can be used for simulated drills on

transportation, charge preparation, and jettison of

explosives during emergencies.

Charge preparation should follow the techniques

oudined in chapter 6. The cap-fuse assembly can be

prepared before flight. It is the pilot's responsibility to

determine the point in the operation when the charges

are armed. In flight, cap-fuse assemblies and explosives should be carried in containers that are securely

fastened to the helicopter floor. Cast-primer explosives consisting mainly of TNT are preferred; however, the castings should be quite solid and properly

encased to resist impact shattering. Explosive loads

should be kept to a minimum; 1-kg units are the

normal charge, but charges in excess of 1 kg can be

used at the blaster's discretion, since it is sometimes

difficult to achieve target accuracy when blasting from

a helicopter. Charge preparation is the responsibility

of one crew member. At the pilot's discretion, a

second crew member can be added for assistance and

training.

It is possible to control remote paths by preplanting

explosives in the starting zone before the avalanche

season. Preplanted charges are detonated by transmitting a VHF-coded signal. Because a preplanted

unit, complete with signaling equipment, is much

more expensive than an artillery round, the technique

is justified only for remote paths that cannot be

reached by artillery, yet must be occasionally controlled during bad visibility when helicopter flight is

restricted. Preplanted charges should be anchored

securely to the terrain. Using a shock-insensitive explosive, it is possible to anchor a large number of units

in the same starting zone (at about 5-m spacings)

and detonate each unit separately. At present, preplanted-charge technology is still in the development

stages. Although preliminary results are promising,

this method is expensive.

Planning the protection of highways

Avalanche planning begins when a highway is first

conceived. The basic location of the road is determined by many constraints that have little to do with

avalanches; for example, grades, soil, curves, climate,

etc. Nevertheless, avalanche problems could affect

some of the fine points in the road layout.

If the avalanche hazard is serious enough, closing

the road during the avalanche season should be

considered. Economics and politics will determine

whether this simple solution is possible. If politics

indicate that the road should be kept open yearround, an experienced consultant should estimate the

approximate costs of protecting the maintenance staff

and the general public. Assuming the estimate does

not disqualify the road for winter operation, a more

detailed study can be made. Avalanche paths that

threaten the highway should be inventoried and

mapped according to techniques discussed in chapter

160 PROTECTION OF HIGHWAYS AND VILLAGES
4. Important data to collect are avalanche frequency,

expected amounts of snow deposition on the highway,

extent of the runout zone, type of slides (dry winter

or wet spring), and weather conditions for activity.

Normally, it takes 3 or more years to collect such

information.

After the paths have been inventoried, control

methods can be established for each path. The choice

is usually from among several methods that can be

used in various combinations: Do nothing; Temporarily close the road; Artificially release avalanches;

Build sheds; Use miscellaneous déjense structures;

Relocate the road; and Control traffic.

Do nothing. It is usually uneconomical to control

avalanches that release naturally and cross the road

less than once in 10 years. Explosive control of such

infrequent avalanches could offset tree growth, increase the frequency of activity on the path, and build

a significant hazard where one did not exist before.

Temporarily close the road. The public can be

protected from infrequent avalanches by temporary

closure during and immediately after severe storms

or thaws. Closure can be relaxed to allow passage of

supervised convoys of groups who are familiar with

the risk, have well-equipped vehicles, and have urgent

business on the other side of the path. Maintenance

crews and the general public should not be allowed to

cross the closure. This procedure is impractical on

heavily traveled routes.

Artificially release avalanches. Temporary closures

can be shortened by controlling avalanches with explosives. The normal technique is to close the road

and release the avalanche by using explosives. Artificial release increases the avalanche frequency significantly. Decisions about road closure and explosive

control are made by experienced avalanche workers

who understand both the stability-evaluation techniques described in chapter 5 and the administrative

problems of highway operations. Provision for a competent decisionmaking team is an important part of

overall planning.

Build sheds. This costly technique may be justified if traffic flow must be maintained and protected

from paths that release large avalanches one or more

times each season. The construction of a shed costs

more in the long run than a combined artillery and

plowing program. In addition, sheds are hazardous because of limited visibility, icing, and obstacles close to

the roadway.

Use miscellaneous déjense structures. Mounds,

dams, bridges over avalanche paths, and supporting

structures in the starting zone can be justified where

traffic flow is critical and where paths have a configuration that permits economical use of such structures. Earth structures such as mounds and dams deFigure 182.—Miscellaneous defense structures for protection of mountain highways.

Planning the protection oj highways 161
Figure 183.—Major relocation can rarely be used to avoid a hazard, although in some cases it may be possible to introduce

bypass loops, angle the road to minimize shed length, or curve the road around the runout zone.

fend best against slow-moving wet avalanches. They

are effective on paths that usually have no more than

two avalanches per year. It is sometimes possible to

bridge the road over a deeply channeled avalanche,

but this must be done cautiously, since it is difficult to

estimate the boundaries of the flowing avalanche. Supporting structures in the starting zone may be economical protection on small, steep slopes that do not

dump large volumes of snow on the road but still pose

a threat to fast-moving traffic (fig. 182). There may

be opportunity to combine the defense structures with

reforestation.

Relocate ilie road. As mentioned above, highway

construction is based on many constraints, and relocation can seldom be counted on to avoid hazard. However, on low-traffic, low-priority roads, it may be possible to construct small bypass loops that can be used

as temporary detours in the winter during periods of

high hazard. Minor relocations may help minimize the

length of protective sheds. For example, it may be

possible to angle across the narrowest section of the

path. Also, it may be economical to extend the road

by curving it around runout zones or causing it to cross

back and forth along a canyon bottom so as to intersect the runout zones of paths that have smaller and

fewer avalanches (fig. 183). The ecological damage

of adding detours must be weighed; the avalanche

hazard is no excuse for scarring the landscape. Except

for road segments protected by sheds, it is best to

facilitate snow removal by locating avalanche-threatened segments on fills rather than cuts.

Control traffic. Temporary closures, artificial release, mounds, and dams are not perfect control methods. Unexpected avalanches sometimes reach roads

even where defense measures have been taken.

Most avalanche accidents on roads occur when traffic has stopped in front of avalanche paths, usually

because an earlier avalanche blocked the road or because cars have stalled due to slippery conditions or

people are resting, sightseeing, or putting chains on

their vehicles. Exposing stationary vehicles to avalanches can be prevented by:

• Placing signs ("Avalanche Area, Do Not Stop"

and "End Avalanche Area") at either side of avalanche paths

• Directing vehicles to safe areas as soon as avalanches have covered the road

• Requiring chains or adequate snow tires during

slippery conditions.

Road-maintenance personnel should be trained in

safety measures and search-and-rescue procedures.

Rescue equipment should be kept in areas that have

frequent avalanches.

162 PROTECTION OF HIGHWAYS AND VILLAGES
Protection of the

Trans-Canada Highway

The Trans-Canada Highway crosses nearly 100

avalanche paths in the 145 km (91 mi) between

Golden and Revelstoke, B.C. Some of the starting

zones that affect the highway are 1,600 m above the

road. The hazard is most intense in the vicinity of

Rogers Pass, where, according to traffic surveys, it is

probable that at least one motor vehicle is under a

major avalanche path at any given time. The combination of the number of avalanche paths, the frequency of avalanches, and the volume of traffic makes

this the most hazardous highway in North America

with respect to avalanches. Despite the hazard and the

constant pressure to keep the road open, the safety

record over the past 10 years of operation has been

commendable. This is a credit to years of planning before the highway was opened (fig. 184).

Snow and avalanche observations were formally

begun in 1953, when the route was proposed. In 1956,

the route was approved for construction. Avalanche

observations were then extended to provide detailed

information for design of highway defenses, including

an artillery program. Based on these early studies,

planners decided to install and improve the defense

system in three phases (Schaerer 1962a), the general

features of which are:

First phase. Initial defenses were to be in service

when the highway was first opened to the public.

With the completion of the first phase, the highway

was expected to be closed about 280 hours in an

average winter. The duration of any one closure

would vary from 4 hours to 6 days, including time

for snow removal.

Second phase. These defenses would be added

later, based on experience with the first phase. The

highway was expected to be closed about 120 hours

each winter.

Third phase. The final goal would be to extend

defenses so that the highway would be closed only

for periods of about 2 hours. Once in 10 years, closure

could extend up to 2 days.

After establishing these general goals, the next step

was to select a specific defense for each path. This

required a rather complex engineering-cost study

(Schaerer 1962a, 1962b, 1962c). With the second

phase in effect, highway closure has averaged 100

hours each season. For example, in the winter 1968-

69, there were 88 closures of 76.5 total hours for

artillery shooting, one closure for 8.5 hours because

of visibility and road conditions, and one closure of

11.5 hours to clear a major avalanche that released

naturally.

The success of the program is based on the cooperative efforts of five teams:

Snow Research Group. This team consists of two

analysts, two observers, and four assistant observers.

They are responsible for:

• Collecting and recording weather, snow-cover,

and avalanche data

• Analyzing all technical data and preparing forecasts of avalanche hazards

• Ordering closures of the highway

• Determining when and where to use artillery

• Conducting research for improving forecasts.

Army Detachment. This group consists of one detachment commander, seven gunners, and one gun

mechanic; it is equipped with two 105-mm howitzers.

Maintenance Group. This group consists of about

28 men with the equipment needed for snow removal,

ice control, and all other maintenance required to

keep the highway open and safe for the traveling

public.

Warden Service. This team consists of five men

trained in avalanche rescue; it is responsible for traffic

control, public information, and general safety.

Equipment Maintenance Team. This group consists of five men responsible for maintaining and repairing all equipment.

The operation center is at the summit of Rogers

Pass (elevation 1,320 m), which is headquarters for

all five teams and includes maintenance sheds, warden

headquarters, dormitories, military facilities, avalanche-forecasting instruments, and a snow study

plot. In addition, the Snow Research Group makes

snow and weather observations at a higher station

(1,900 m) on Mt. Fidelity and measures wind at two

ridgetop sites. A weekly tour is made to a remote highelevation study plot (2,100 m ) on Mt. Abbott to read

charts and make snowpit tests.

The method of stability evaluation used by the

Snow Research Group is of particular interest. Essentially, evaluations are based on all six inputs discussed

in chapter 5, but they are biased strongly toward new

snow instability rather than deep slab or wet snow

instability. The main idea is to determine when the

new snow is unstable and then use artillery to bring

out small slabs that clean out the track. Little snow

Protection of the Trans-Canada Highway 163
To Revelstoke

and Vancouver

To Golden

and Calgary

East boundary

Glacier National Park

LEGEND

TRANS-CANADA HIGHWAY

CANADIAN PACIFIC RAILWAY

MAIN AVALANCHES

Scale in kilometers

Figure 184.—Main avalanche paths near Rogers Pass, B.C.

164 PROTECTION OF HIGHWAYS AND VILLAGES
is left in the starting zones, and the spring thaw hazard

is minimized. Deep slab instability due to TG metamorphism is not usually a serious problem at Rogers

Pass, since long periods of clear, cold weather between storms are uncommon. (Deep slab instability

is a more serious problem in the Canadian Rockies

near the Banff-Jasper Highway, which is influenced

by a continental climate.) This is not meant to imply

that deep slab and wet snow instability are ignored;

snowpit measurements of density, temperature, grain

structure, and hardness are taken routinely.

To distinguish between stable and unstable layers

of newly fallen snow, the Snow Research Group uses

meteorological data and the following test procedure

(Schleiss and Schleiss 1970):

Penetration test. This test is performed at study

plots that represent conditions at midtrack and in the

lower part of the track for most of the avalanche paths

in question. The first section of the ram penetrometer

is allowed to sink into the newly fallen snow. Depth of

penetration is monitored. Normally, about 40 cm of

penetration indicates that enough snow can be entrained in a moving avalanche to cause hazard to the

highway.

Inclined-table test. The next step is to determine

by a rather delicate procedure if a critical shear plane

exists in the newly fallen snow. First, a 40-cm cubical

block is cut out of the topmost layer. The block is set

gently on a table, which is tilted, then given a jarring

blow. If the snow is unstable, a shear fracture propagates, and the block separates as shown in figure

185.

Shear-frame test. The final step is to determine the

stability index of the critical shear plane with a special

snow instrument called a shear frame. The load on

the critical shear plane is computed from the density

and thickness of the snow above the critical shear

plane. If the ratio of the shear strength to the load is

less than 1.5, the critical plane is considered unstable.

These tests are performed routinely at 7:30 a.m.

and 4:30 p.m. and on a concentrated schedule during

intense storms. Results are reported to the analyst

at the Rogers Pass headquarters who considers all six

inputs discussed in chapter 5 and arrives at a stability

evaluation, on the basis of which he issues written

directives. These may call for an "alert" to a developing hazard, or for "action," to close the highway and

begin artillery firing. Examples of typical written

directives:

Figure 185.-Rogers Pass test procedure for evaluating new

snow instability.

Protection of the Trans-Canada Highway 165
ALERT

Date: January 10 1967

Hour of issue: 2200

7. Avalanche warning: High winds and heavy snowfall

will cause avalanche activity which may affect

the Trans-Canada Highway in form of dry dust

action. Extra attention is advised.

2. Stabilizing gunfire may be recommended between:

2300 hours and 2400 hours

3. Gun positions likely to be occupied are those between:

Summit East to Connaught

Mortar to Fidelity

4. Guns likely required:

105mm Howitzer

75mm Pack Howitzer

5. C.P.R. representation may be required at

approximately: 2400 hours for

Observatory to Fidelity gun positions.

Analyst: John Doe, Forecaster

Received by: Maintenance, Warden i.C, Gateways,

C.P.R., Army.

for Park Superintendent

Issued by: Joe Doe of Snow Research Section

ACTION

Date: January 10 1967

Hour of issue: 2200

7. Avalanche warning: High winds and heavy snowfall

are causing avalanche hazard to the

Trans-Canada Highway.

Expect periodic closures not exceeding 2 hours.

Expected action time: 2300 to 0400, January 10, 1967.

2. Gunfire should commence at: 2300 hours.

3. Sequence of gun positions:

Summit East to Connaught

Mortar to Fidelity

to

4. Guns and estimated ammunition required:

105mm Howitzer x and 40 rounds.

75mm Pack Howitzer and rounds.

5. C.P.R. representative required: between 0030

liours and 0200 hours at

Observatory to Fidelity gun positions.

Residents of fan house and station areas to be warned for

0300 hours.

A nalyst: John Doe, Forecaster

Received by: Maintenance, Warden i.C, Gateways

C.P.R., Army.

for Park Superintendent

Issued by: Joe Doe of Snow Research Section

Artillery firing is done by the Canadian Army detachment on assignment for winter training. Firing

may be performed at any hour of the day and under

any conditions, and it is therefore based strictly on

indirect techniques using fixed aiming stakes. The

I05-mm howitzer appears to be the best weapon

because of its range and ability to fire at a high barrel

elevation. The weapon is fixed on special concrete

pads (fig. 186). There are about 144 targets. Most

are at the very tops of the starting zones, often in steep

cliff bands. The idea is to release small sluffs that

trigger the bulk of the starting zone. The amount of

166

ammunition fired varies considerably from year to

year (423 rounds minimum to 1,850 maximum). In

the severe winter of 1971-72, 129 avalanches crossed

the road, and about 480,000 m'^ of snow were removed

by plows, loaders, and graders.

Special emphasis is given to personal safety. All

members of the Rogers Pass operation are instructed

in basic avalanche safety and rescue. The research

PROTECTION OF HIGHWAYS AND VILLAGES
Figure 186.—Firing of 105-mni howitzer from concrete pad, Rogers Puss. B.C. A marker on the highway shed is used as

reference for indirect firing.

Figure 187.—Avalanche warning sign on the Trans-Canada

Highway. Signs are essential for the protection of maintenance staff as well as the general public.

group is given additional training in safe travel over

avalanche terrain. Everyone carries a manual that

includes avalanche charts with reference mileage,

radio communication instructions, and a list of avalanche safety rules.

Motorists who enter the threatened highway section

receive a pamphlet, "Winter Guide to Rogers Pass

Section," which explains the avalanche problem, tells

how avalanches are controlled, and gives general advice. The safety program is backed up by a large number of warning signs (fig. 187).

Land-use regulation

Federal, State, and local governments may regulate

or prevent construction in avalanche paths. Since most

avalanche paths are on Federal land, agencies such as

the USDA Forest Service and the USDl Bureau of

Land Management and National Park Service play a

central role. Their main tool is to withhold approval

of Federal land-use or concession permits until satisfied with the avalanche protection proposed by the

applicant. The Federal Government may also purchase land with the object of reducing the avalanche

hazard. The Federal Government does not have

authority to regulate the use of non-Federal land.

Regulation of non-Federal land in the interest of

general public welfare is delegated by the Constitution

to the State governments. The use of individual property may be controlled if a State government can

demonstrate that the owner's intentions conflict with

general public welfare. This, of course, leads to disputes between individuals and the government. On the

principle that such disputes are best handled at the

local level, the States have passed enabling acts that

delegate their constitutional authority to home rule

municipalities, statutory municipalities, and counties.

However, the trend is beginning to reverse, as more

and more State governments realize that land-use

planning is crucial enough to require State intervention. As an example, the Oregon State Legislature

passed à bill making zoning mandatory in all cities and

Land-use regulation 167
counties and further required that local government

begin work on comprehensive plans. Should the local

government not comply, the State would intervene and

do the job for them. Although the law was tough

sounding, it was supported by the voters when it was

challenged a year later at the polls.

On the basis of delegated authority, local government can regulate land use with combinations of the

following tools:

Zoning ordinances. This important tool will be discussed in more detail in the next section. The fundamental idea is for the local government to prepare

maps that divide the land under their jurisdiction into

several zones, delineated on the basis of the avalanche

hazard. Appropriate restrictions on construction and

habitation are matched to the degree of hazard. The

zoning statute of the Swiss canton of Nidwaldon is a

good example:

STATUTE

CONCERNING THE AVALANCHE ZONING PLANS

OF 26 APRIL 1964

Amendment No. 470

Art. 1 The avalanche zoning plans define areas in

which the erection of buildings is forbidden or placed under

restrictions.

Art. 2 For districts in which ground suitable for construction lies in zones endangered by snow slips or avalanches,

the Agriculture and Forestry Commissions, in cooperation

with the district government, are required to prepare avalanche zoning plans.

In districts for which an avalanche zoning plan still does

not exist, construction may be forbidden or placed under

certain restrictions, for a period not to exceed one year, by

the Agriculture and Forestry Commissions prior to publication of the avalanche zoning plan.

Art. 3 Before being placed in force, the avalanche zoning plans shall be open to public inspection at the district

government offices for a period of 30 days, during which

objections may be lodged.

The Agriculture and Forestry Commissions shall decide

on the merit of any objections.

Art. 4 The avalanche zoning plans prepared by the

Agriculture and Forestry Commissions are public documents.

Special property restrictions involving public rights introduced by these avalanche zoning plans shall be entered in

the land registry.

Art. 5 Decrees of the Agriculture and Forestry Commissions may be appealed within 20 days to the cantonal

court, whose decision shall be final.

Art. 6 The provisions of Art. 3 shall apply to later

alterations or additions to the avalanche zoning plans.

Art. 7 Violations of this Statute shall be punished according to the penalty provisions of the Cantonal Construction Law of 30 April 1961.

Art. 8 The stipulation concerning the registering of

property restrictions (Art. 4, Par. 2) is subject to ratification

by the Federal Legislature (Art. 962 ZGB).

Art. 9 This Statute takes effect upon enactment by the

Landsgemeinde, except the paragraph requiring ratification

by the Federal Legislature.

So concluded before the LandsgemeLnde

Wil an der Aa 26 April 1964

Art. 4, Par. 2, ratified by the Federal Legislature on

15 May 1964.

(Translated by LaChapelle)

Subdivision regulations. This shifts some of the

burden of mapmaking to the subdivider, who must

submit a map of his parcels, showing avalanche paths

and other hazardous areas. A local government board

(with the help of consultants) reviews the map and

requires modification according to its technical analysis of the problem. Subdivision regulations should

supplement, not replace, zoning ordinances.

Building, housing, and miscellaneous codes. These

important supplements to the basic zoning ordinances

allow local governments to examine construction and

maintenance details of proposed and existing developments. In particular, structures weakened by avalanches can be made to conform to the most recent

standards for new structures. Protection can be required for sanitary and water facilities.

Taxation. The justification for taxation is that improper use of avalanche land burdens the community

with special problems; the property owner must contribute accordingly. Likewise, lower taxes can be

assessed for avalanche land not developed by the landowner.

Comprehensive plan. In some instances, the local

government (and State and Federal governments) can

develop roads, parks, ski lifts, and water and sanitation

facilities in a way that discourages the development

of hazardous private land.

Public acquisition. This includes land purchase,

easement purchase, and land exchange. The State can

assist by providing funds and land-exchange possibilities when it can be shown that the State will save

money in the long run.

Despite all these options, the local government

is not in an enviable position. In practice, land-use

planning can be limited by a barrage of legal and social pressures. Legally, each case tends to be unique

and is handled individually through the tiring process

of hearings and appeals. Socially, local government

often faces traditional public sympathy for property

168 PROTECTION OF HIGHWAYS AND VILLAGES
Figure 188.—An example of the false sense of security that may be offered by a small band of timber at the foot of an avalanche

path. The housing development in the scattered timber at the foot of the mountain was invaded by avalanches, which destroyed

two cabins and killed several people. (Photo by Emetaz)

rights and the right of individuals to talce risks. To

malie matters more complicated, delegated authority

implies responsibility, and the local government is

obligated to protect those who place themselves in

danger, knowingly or not. Caught in the middle, local

governments face court action if they block development, but they may face damage suits if they fail to

block development and an avalanche strikes.

To withstand legal and social attack, local governments must carry out a three-pronged attack. First,

technically sound data, maps, and recommendations

must be compiled. This invariably means hiring independent avalanche experts; the courts typically respond to a thorough, professional job. Second, local

governments must comply strictly with procedure and

regulations. Third, and perhaps most important, local

governments must carry out active public-educaton

campaigns to encourage public acceptance.

By contrast, insurance companies and banks can

exert direct influence without the legal and social complications facing government agencies. Since avalanche insurance is not generally available, banks and

loan corporations can refuse financial support for avalanche-threatened property. Insurance companies can

raise their fees for liability protection. At best, financial organizations cooperate closely with local government. Unfortunately, in some cases banks may support a development that seems lucrative despite the

avalanche potential. Thus, the government cannot always depend on cooperation from financial institutions.

Land-use regulation 169
» • »Tk-rfUri

1.

^- ft <^. »fFigure 189a.—Aerial photograph, Alta, Utah.

170 PROTECTION OF HIGHWAYS AND VILLAGES
Avalanche zoning

Techniques for identifying and mapping avalanche

paths are presented in chapter 4. In summary, mapping is based on quaUtative analysis of topography,

climate, vegetation, and past records. For some cases,

the numerical methods of Voellmy (1955) can be

used to estimate boundaries of runout zones. Because

of uncertainties in the analysis, avalanche maps are

never finished; they constantly evolve as new information becomes available.

The Swiss have given much thought to avalanche

maps and avalanche zoning plans (Frutiger 1970).

After long study and debate at both scientific and government levels, they are using a "three-zone" system

of mapping (sometimes referred to as a "three-color"

map). The three zones are: "high hazard" (red),

"potential hazard" (blue), "no hazard" (white). The

criteria for each zone are the estimated avalanche

force and an estimate of the average time between avalanches. The latter is called the return interval of the

avalanche. The three zones are defined as follows:

High-hazard (red) zone. This zone includes terrain

exposed to frequent and powerful avalanches that satisfy either of the following:

• Any avalanche with return interval of 30 years

or less.

• Avalanches with impact pressures of 3 t/m- or

more and with a return interval up to 300 years.

Buildings and winter parking lots generally are not

permitted in high-hazard zones. Special bunkers may

be installed for equipment.

Potential-hazard (blue) zone. This is the transition

zone betwen high hazard and no hazard, and includes

avalanches that are either small or infrequent:

• Avalanches with impact pressures less than 3

t/m- and a return interval from 30 to 300 years.

Private homes may be erected in potential-hazard

zones if care is taken to design for the above forces.

Schools, hospitals, lodges, and other buildings that encourage gatherings of people should not be erected.

The local government is responsible for closure,

evacuation, and rescue during periods of hazard.

Figure 189b.—Detailed avalanche map for the lodge area, Alta. Utah.

A valanche zoning 171
* Mt. Juneau

LEGEND

Potential hazard

hazard

172

Figure 190.—Initial zoning map for Juneau, Alaska (Daniel et al. 1972).

PROTECTION OF HIGHWAYS AND VILLAGES
No-hazard (white) zone. Occasionally, the terrain

may be affected by small airblast pressures, up to 0.1

t/m-. There are no building restrictions with respect

to the avalanche hazard."

Avalanche zones can be plotted first on standard

topographic maps, but if decisions are needed for individual building sites, it is necessary eventually to

generate special maps at a scale no coarser than

1:5,000, with contour intervals of 5 m or less. Figures

189b and 190 show, respectively, a detailed and an

initial avalanche zoning map.

Enforcing restrictions for a high-hazard zone is

usually not an administrative problem, since in most

cases the hazard is obvious. On the other hand, it may

be hard to agree on appropriate restrictions for some

potential-hazard zones. Assuming that human life is

not gambled, the question often boils down to economic risk (in some benefit-cost studies, a money

value is even assigned to human lives). Economic risk

is determined from the probability that a facility will

be hit by an avalanche within the estimated life of the

facility; this probability, which is called the encounter

probability, is a function of the return interval of the

avalanche and the design life of the facility. The latter

depends mainly on construction methods and materials. Table 9 shows approximate encounter probabilities in percent as a function of design life and return interval.

TABLE 9.—Encounter probabilities based on

design life of structures

Design Encounter probability (in percent) based on

life of an avalanche return interval of:

structure 5 10 20 JO 50 100

(years) years years years years years years

1 20 10 5 3 2 I

5 67 41 23 16 10 5

10 89 65 40 29 18 10

25 >99 93 72 57 40 22

50 >99 96 92 82 64 40

Source: LaChapelle (1966).

It is common for a site to be affected by several

geological hazards in addition to avalanches. Many

avalanche gullies are in fact mud and rock slide gullies

in the spring and summer. In some regions, such as

part of Alaska, it is necessary to consider the combined effects of avalanches and earthquakes, since

ground vibration can trigger very large avalanches.

Landslides

High

hazard

Potential

hazard

No

hazard

«

0)

.c

u

c

JO

(0>

Î

c

M

T3

II ^^HDD

S"2

C (0

0)

T3

N DDD O CO

^ to

sz

I^^^HIHl

^H^l

« A four-zone system which takes into account airblast and

highly infrequent avalanches is under study in Switzerland.

Figure 191.—An example of a system for assigning a dualhazard index.

It is possible to compute encounter probabilities

for multiple hazards. Suppose, for example, a site

has an avalanche encounter probability of PA and a

landslide encounter probability of P,,. The combined

encounter probability for either a landslide or an

avalanche is PA + Pi. — PAPI.-

Local planners can use various schemes to classify

land exposed to muUiple hazards. For example, a

dual-hazard index for landslides and avalanches is

shown in figure 191. Such schemes lend support to

regulations against developing sites with multiple

hazards.

For a model study of zoning for multiple hazards,

including administrative and legal procedures, the

reader may refer to the planning work of the City of

Juneau, Alaska (Daniel et al. 1972).

Public information and warnings

An important part of avalanche hazard control is

public education—convincing people that the zoning

ordinance, ski-run closure, or highway restriction is

in their best interest. The avalanche worker has much

on his side. Avalanches are spectacular and can be

portrayed in exciting and convincing presentations via

television, radio, newspapers, ski magazines, films,

lectures, and pamphlets. Besides the general public,

Public information and warnings 173
some groups that may benefit from avalanche information and education include:

Ski clubs

Ski patrols

Public utility crews

State highway crews

Ski area operators

Snowmobile clubs

Rescue organizations

Sheriff departments

National Guard units

State and Federal Agencies

Ski schools

Mountaineering clubs

Scout groups

Local planning commissions

Mining corporations.

The information program should underscore the

"common sense" of avalanche safety.

An effective way to develop awareness of the avalanche hazard is to release periodically a report of

avalanche conditions in the neighboring mountains.

The report can be made daily, on weekends, or intermittently during periods of high and extreme hazard.

Daily report. An estimate of current avalanche

conditions can be included in the early morning snow

and ski report. To minimize distortion by the media,

it is advisable either to prerecord a radio tape or to

prepare a formal written statement that gives exactly

the message that should reach the public. Before the

avalanche season, proper contact should be established with the media to ensure that reports are treated

as routine bulletins rather than sensational news

scoops. Reports may include a variety of information on ski and highway conditions; for avalanche

purposes, they should include:

• 24-hour snowfall

• Present weather, including temperature and

winds

• Forecast weather, including amount of snow,

temperature, and wind

• Recent avalanche activity (natural and artificial)

• Expected avalanche activity in the next 24 hours

• Back-country travel recommendations.

Preparing and releasing the report depends on the

combined efforts of field observers in the mountains.

a coordinator who may or may not be stationed in the

mountains, and interest on the part of the media. In a

system that works well in the Salt Lake City, Utah

area, observers at various ski resorts in the Wasatch

Mountains take 5:30 a.m. weather observations that

are phoned to a coordinator in Salt Lake City, along

with avalanche observations and recommendations

for back-country travel. The coordinator prepares a

tape summarizing the observations. The tape is played

on public radio at about 8:00 a.m. each morning.

Weekend report. For small population centers, it

may not be economically feasible to issue daily reports

of snow and avalanche conditions. As a compromise,

a report of anticipated weekend conditions can be

issued Friday afternoon, Friday evening, or early

Saturday morning. For large metropolitan areas, a

special weekend report that emphasizes touring problems may supplement the daily report.

Intermittent warning. Intermittent warnings are

released via the National Weather Service Network

when conditions are especially critical. Although

intended for a more general audience, intermittent

warnings may serve to supplement any daily or weekend report. Used properly, intermittent warnings are

avalanche workers' most powerful method of establishing public awareness. The rest of this section will

describe this important tool.

In contrast to daily and weekend reports, intermittent warnings are released exclusively through the

National Weather Service, as part of its program

of warning against natural hazards. Intermittent warnings are not released informally to the press, since

they may be misrepresented as a sensational curiosity

or, just as likely, set aside if a better curiosity comes

across the editor's desk. When received at the local

branch of the National Weather Service, the warning

is officially relayed on teletype loops to television,

radio, and newspapers. At the same time, the warning

is broadcast on VHF radio with a coverage of about

100 km (60 mi). The warning may also be fed into

the national teletype circuit for the benefit of meteorologists in neighboring States.

The warning message must be simple and concise

and should assume the listener has no prior knowledge

of the phenomenon. The message should define the

exact area affected and include a termination time. A

statement about protective measures is equally important. As an example, a warning for the mountains west

of Denver, Colorado, was issued:

174 PROTECTION OF HIGHW^AYS AND VILLAGES
AVALANCHE WARNING BULLETIN NO. 1

IMMEDIATE BROADCAST REQUESTED

U.S. FOREST SERVICE FORT COLLINS, COLORADO

ISSUED 11AM MST FRIDAY, DECEMBER 14, 1973

NORTHERN COLORADO MOUNTAINS

AN AVALANCHE WARNING IS IN EFFECT FOR

THE MOUNTAINS OF COLORADO NORTH OF A

LINE FROM COLORADO SPRINGS TO GRAND

JUNCTION. THIS WARNING IS VALID THROUGH

MONDAY, 17 DECEMBER 1973. HEAVY SNOW AND

HIGH WINDS HAVE CREATED DANGEROUS

AVALANCHE CONDITIONS.

BACK-COUNTRY TRAVELERS SHOULD AVOID

KNOWN AVALANCHE PATHS, STEEP SLOPES, AND

GULLIES.

THE NEXT AVALANCHE WARNING BULLETIN

WILL BE ISSUED SATURDAY AT 8 AM OR EARLIER IF CONDITIONS WARRANT.

JUDSON U.S.F.S. FORT COLLINS, COLORADO.

Intermittent warnings can be based on two hazard

levels:

• Moderate hazard. Avalanche hazard will most

likely result from artificial release of starting zones

in the high elevations.

• High hazard. Avalanche hazard is due to larger

avalanches reaching developed areas and roads.

The warning is usually made to include the lower

elevations. Wording may be equivalent to: "Mountain travelers should avoid steep slopes, gullies, and

narrow valleys," or as follows:

AVALANCHE WARNING BULLETIN NO. 2

IMMEDIATE BROADCAST REQUESTED

U.S. FOREST SERVICE FORT COLLINS, COLORADO

ISSUED 8 AM MST SATURDAY, DECEMBER 15,

1973.

HEAVY SNOW HAS INTENSIFIED AVALANCHE

DANGER AT ALL AREAS ABOVE 9000 FEET FROM

LEADVILLE NORTH ALONG CONTINENTAL DIVIDE TO CAMERON PASS. LARGE AVALANCHES

ARE EXPECTED THROUGH 19 DECEMBER. MOUNTAIN TRAVELERS ARE ADVISED TO EXERCISE

EXTREME CAUTION.

For continuity of record, at the end of the instability

period, the analyst should issue a termination bulletin.

The termination bulletin may mention that "isolated

pockets of instability remain in the higher elevations"

or some such statement of the type and degree of

hazard remaining. For example :

AVALANCHE WARNING TERMINATION

LETIN NO. 3

BULIMMEDIATE BROADCAST REQUESTED

U.S. FOREST SERVICE FORT COLLINS, COLORADO

ISSUED 9AM MST THURSDAY, DECEMBER 20, 1973

NORTHERN COLORADO MOUNTAINS

AVALANCHE WARNINGS FOR THE NORTHERN

COLORADO MOUNTAINS ARE NO LONGER IN

EFFECT. ISOLATED POCKETS OF INSTABILITY

REMAIN IN THE HIGH ELEVATIONS.

The criteria for deciding when to issue a warning

were covered in chapter 5. The analyst must rely heavily on his observer's subjective analysis of weather and

avalanche conditions. The main objective criterion

for issuing an avalanche warning is the water equivalent of the new snow accumulated over a 3- to 5-day

period. To provide for systematic improvement in the

warning program, it is possible to use a numerical

technique for totalling water equivalent. One scheme

is to use an index / of the form

where W^, Wi, Wo,... are the respective water equivalents of the snowfall on the present day, 1 day in the

past, 2 days in the past, etc. The «i, ao, ûH, ... are decay coefficients that represent the fading "memory" of

the snowpack for past amounts of precipitation. The

decay coefficients have values less than one, such that

fli is greater than a^, which is greater than a¿, etc. For

example:

I =Wo + 0.6 Wr + 0.3 W. + 0.1 W^,

A warning may be issued when / exceeds some

critical value. The critical / and the optimum values

of the decay coefficients are found from computer

analysis of several years of avalanche and precipitation data for a given area.

Further reading

Judson, Arthur.

1975. Avalanche warnings: Content and dissemination.

USDA For. Serv., Res. Note RM-291, 8 p. Rocky Mt.

For. and Range Exp. Stn., Fort Collins, Colo. Gives

examples of wording and format for effective avalanche

warnings, based on several years' experience in Colorado.

Lynch, Dennis L., and Standish R. Broome.

(n.d.) Mountain land planning. 38 p. Fort Collins, Colo.:

Colo. State For. Serv. General coverage of the need and

aims of land planning in mountainous areas.

USDA Forest Service.

1975. Avalanche protection in Switzerland. USDA For.

Serv., Gen. Tech. Rep. RM-9, 169 p. The English translation of a series of well-illustrated articles on avalanche

control structures and avalanche zoning procedures in

Switzerland. An up-to-date treatment of the subject.

Further reading 175
^f> f.

•»í^ .
Chapter 8

Safety

and rescue

This chapter discusses route finding and decision

making for back-country travel, based on terrain,

snow, and weather conditions. The careful selection

of a route or the wise decision to turn back are

the best ways to avoid accidents. Once a person

is buried, outside help is usually needed for rescue.

A buried victim must be recovered as soon as

possible, because the chances of survival decrease

sharply with increasing time. For back-country

areas, this means members of the party must carry

out the rescue, since there is seldom time to go

for help. For ski areas, it emphasizes the need to

have rescue manpower and equipment at the top

of the mountain. This chapter emphasizes in

considerable detail rescue techniques with and

without special aids such as avalanche dogs or

electronic transceivers. It also outlines revival and

evacuation procedures, with special attention to

resuscitation techniques, since most avalanche

deaths are from suffocation.

Figure 192.—Members of a touring party hurry to the aid of a

comrade caught in a small avalanche. The accident occurred when a

skier crossed in midtrack above the victim. (Photo by Roch)
Avalanche accidents

Although avalanches kill people in many ways, the

great majority of fatalities are due to suffocation. In

a typical avalanche burial, rather little air is trapped

in the space around the victim, and it is only a matter

of time until the victim loses consciousness and dies.

The weight of snow bears down on the victim's throat

and chest and further accelerates respiratory failure.

The snow usually packs so tightly that the victim is

immobilized and must helplessly await his fate.

Some victims are killed outright or severely injured

by the moving avalanche. The victim may be dashed

into a tree or building or hit by flying debris. Head

injuries, abdominal injuries, and broken necks, backs,

and legs are common. There are also reports of lung

injuries caused by avalanche pressure forces. Some

victims die of hypothermia, exhaustion, or shock.

Less than 20 percent of the victims buried with no

trace showing are recovered alive.

Statistics compiled in Switzerland and in the United

States (Vanni Eigenmann International Foundation

1963, WilHams 1975) show that the victim's chance

of survival diminishes rapidly with burial time (fig.

193) and depth of burial. Statistically, after half an

hour of burial, the victim's chance of surviving is

about 50 percent. A victim often cannot survive a

15-minute burial in an unfavorable position (snow

packed tightly around mouth and nose). This is understandable, since after stoppage of breath, a person

loses consciousness in about 45 to 120 seconds. The

first permanent brain damage occurs in about 4 minutes, and after 8 minutes, survival is unlikely even if

it were possible to restore breathing and circulation.

On the other hand, if the victim is buried in a favorable position, without snow packed tightly around

mouth and nose, he may survive for many hours.

Table 10 summarizes data from 61 organized rescues conducted in the United States. A great contrast

appears between rescues conducted in the back country and those conducted in developed areas such as

ski areas, villages, and highways. The victim has

almost 10 times the chance of being found alive if the

burial occurs in the immediate vicinity of organized

rescue groups.

Almost 50 percent of the fatal accidents resulted

from slides that ran less than 100 m. This supports

the mountaineer's guideline that small slides can kill

as easily as large slides. The danger is determined not

only by the length of the path but also by the terrain;

100

90 -

80

LU

I 70

<

Q

Z

O

60 -

LU

ü

ce

LUÛ.

50 -

40

30

-

1

Data from

\ (32 avalanche

\

rescues

in U.S.A.

,„:>s^

1

11 ,jL.. .. J1 1

20

10

0

0 12 3 4 5

BURIAL TIME, hours

Figure 193.—The victim's chances diminish rapidly with

burial time. Based on 82 cases in the United States (taken

from Williams 1975), 50 percent of the buried victims

perish in the first half hour.

a relatively small avalanche can produce a deep burial

in gully terrain.

Avalanche accidents in North America fall into the

following categories :

Back-country accidents. At present, this category

accounts for the largest number of fatalities in North

America. The victims are usually ski tourers, helicopter skiers, or mountaineers. In almost all cases, the

victims triggered their own avalanche while crossing

the starting zone. In a few rare cases, the avalanche

was triggered naturally and swept into the touring

party lower down in the track or runout zone. In a few

cases, the avalanche overran a camp area and buried

victims in their tents. The back-country category of

TABLE 10.—Avcraí^e delay in organized rescues

of buried victims

Hours after accident b

Arrival of

efore:

Sounding rescuers at Discovery

Location of alarm accident of victims'

Developed areas Q.IS 1.25 2.5

Back country 5.25 10 38

All rescues 3.5 6.25 20.5

"In six cases, the rescue was abandoned and the victim was

not found until summer. These cases have been deleted from

this column.

178 SAFETY AND RESCUE
<^rr*

Figure 194.—Rescuers uncover mountaineering victim on Mt. Washington, N.H. (April 6, 1964). Back-country accidents

account for the largest number of fatalities. (Photos by Warren)

accidents can be traced to a combination of inexperience and taking a calculated risk (and losing). Rescue

depends on the actions of the unburied survivors.

Organized rescue from afar usually turns into a bodyrecovery operation rather than a live rescue.

Ski-area accidents. The victims include ski patrollers as well as the general public. Ski-patrol accidents

usually result from carelessness or errors. Typically,

the patroller should have used explosives but decided

to ski the slope instead. In other cases, the explosive

test was negative, but the avalanche released during

skiing (postcontrol release). Generally, too much reliance was placed on the explosive test, and the ski

entrance was not carried out cautiously. For example,

in several accidents, the patroller skied down the fall

line (and in a few cases, stopped to rest in the middle

of the path). Ski patrollers and others who carry electronic transceivers (described later in this chapter)

can be more readily rescued.

A valanche accidents 179
Figure 195.—Make camp well away from the runout zone, since distances in the mountains can sometimes be deceptive. This

campsite on the American Dhaulagiri Expedition is properly set back from the threatening wall in the background. (Photo by

American Dhaulagiri E.xpedition)

Accidents involving the general public usually occur

on uncompacted slopes (postcontrol releases have

also taken their toll on the general public). Several

accidents have been due to skiers who willingly or

unknowingly skied into restricted areas. Rescue depends on fast and efficient mobilization of organized

probe teams.

tiighway accidents. In this category, the largest

number of fatalities are suffered by highway maintenance crews. Several fatalities have resulted when

the victim was plowing out debris from one avalanche

and the avalanche path ran a second time. In one fatal

accident, a plow operator stopped for lunch under a

path. Another accident occurred while a plow operator

was giving assistance to a motorist caught in avalanche debris. Some avalanches have enough force to

toss around the largest highway-maintenance machines. The operator usually has a better chance to

survive if he remains in the vehicle, although snow

can be expected to crash through windows and pack

tightly around the victim. Prevention requires a thorough set of avalanche signs, safety education, and a

good avalanche-control program.

Buildings hit by avalanches. Mining camps were the

traditional targets, but an increasing number of lodges

and condominiums are being constructed in avalanche

paths as good building sites become scarce (see chapter 7 for "Avalanche Zoning"). Rescue in demolished

buildings is very tedious and time consuming, since

the snow and building debris pack together in an almost inseparable mass. However, building debris may

provide breathing space that prolongs the victim's

chances of survival. In one case the avalanche started

a fire, and the victims burned to death while trapped

in the debris.

Railroad accidents. The largest single avalanche

disaster in North America occurred in 1910 near Wellington, Wash. (El Huit 1960). This case provides a

testament to the enormous forces of avalanches. Two

railway cars of people and several locomotives were

swept ofï their tracks. In all, 96 people perished. The

avalanche was not unusually large; its vertical drop

180 SAFETY AND RESCUE
was slightly more than 500 m. Rescue required the

tedious disentangling of train wreckage and avalanche

debris. There were 22 survivors.

Miscellaneous. Tragic accidents have struck snowmobilers, utility servicemen, hunters, and dam construction workers. Families have been buried while

playing in the runout zones of avalanche paths. Two

children were buried fatally while playing in a ravine

in the middle of a major eastern city. For such isolated

cases rescue invariably comes too late; the best hope

is prevention by public education.

Decisionmaking and route selection

Back-country safety is first of all a matter of controlling enthusiasm and exercising sound judgment. In

the spirit of adventure, even the most experienced skimountaineer may cause an accident by taking what

he thinks is a small risk to reach his objective. This

enthusiasm is reinforced by the feeling of group security that comes when several people push forward together; no member of the group likes to admit concern

and turn the party around prematurely.

Another important cause of accidents is the physical

strain imposed by severe weather and terrain. Working

under this strain, one may exhibit a carelessness that

one would condemn while sipping tea in front of the

fireplace.

It may be unrealistic to expect the weekend enthusiast to have a flawless record when he escapes to

the mountains. On the other hand, professional mountain guides must set the highest safety standards and

cannot habitually lead unknowing clients into areas of

extreme avalanche danger. In a famous case, the

courts in Switzerland did in fact accuse a guide of

negligence when he led 14 people into an avalanche

path that took the lives of Olympic skiers Bud Werner

and Barbara Henneberger (Schild 1965).

Figure 196.—Glide cracks on a wet, steeply dipping rock face in the Wasatch Mountains of Utah. Left photo taken in late

February 1975; right photo, early May 1975. Such highly unstable slopes are no place for touring. (Photos from Wasalcli

Tours by Keiner and Hanscom )

Decisionmaking and route selection 181
Figure 197.—The best time to tour is in spring, when avalanche activity is most predictable. (Photo by Keiner)

Figure 198.—A good example of bad route selection and poor

technique. Skiers are in an obvious avalanche path. They

have made at least three traverses and are still a long way

from a safe spot. The slope is one that is loaded by the

wind, as evidenced by the large cornices. Fresh snow has

recently been deposited on the slope. The party is small

and much too close together.

Figure 199.—The most efficient route is often the safest. Here

winter mountaineers thread their way on the windward

side of the ridgecrest. (Photo by Keiner)

Decisionmaking depends mainly on snowpack stability evaluation. The basic principles of stability evaluation are set forth in chapter 5, which explains the

six inputs needed for routine operations, namely:

snow-cover distribution, current and past avalanches,

snowpack structure, local meteorological measurements. Weather Service information, and ski and explosive tests. It is unlikely that a tour leader will have

personal access to all of these inputs, but he can call

on nearby avalanche specialists for help and opinions.

For example, in Switzerland, touring parties may dial

on the telephone prerecorded tapes that summarize

most of the pertinent weather and avalanche conditions. In North America, it is usually necessary to

make contact with the nearest ski patrol, Forest Service office, park warden office, etc., on an individual

basis. If the touring club is large, it is preferable to

assign one club member the task of establishing and

maintaining contact with mountain-based observers

throughout the season. In this way, continuity will be

provided and duplication minimized.

Stability evaluations are adjusted according to the

lateness of the season:

Early season. This usually includes the period from

November to February. The main problem is that the

snowpack is unconsolidated. Touring is most hazard182 SAFETY AND RESCUE
Figure 2ÜÜ.—Tourer enjoying .iig on slopes of 20" to 30°; steeper slopes should be ¿ivoided until spring. (Photo by

Keiner)

ous during and immediately after storms, but instability persists between storms. In general, the most

dangerous slopes are north-facing or shaded. Such

slopes are most likely to have significant TG layers

during the early part of the season. If enough time is

allowed for new snow to stabilize, it may be possible

to tour on south-facing slopes.

Mid-season. This is the transition period, usually

from February through April. The snowpack is consolidating, and deep slab instability tends to relax a

few days after a storm. Touring must still be conducted with caution.

Late season. From April through summer, the

mountain snowpack compacts and becomes isothermal. This is the best time for alpine touring. With

rare exception, slab avalanches are restricted to the

first few days of thaw after a new snowfall. Deep slab

instability is rare; the thickness of the slab will be confined to the layer of new snow. After a few days of

warm temperatures, the slab hazard will be essentially

stabilized, and the main threat to tourers will be loose,

wet avalanches that run "on schedule" in the afternoon. These avalanches can be completely avoided by

making an early morning start across the frozen pack

and completing the tour as the snow surface turns to

'"corn" (when the skiing happens to be best). This

usually means getting off all hazardous terrain before

noon.

Regardless of the lateness of the season, proper attention should be given to warning signs, most of

which are discussed in chapter 5. Sudden collapse of

the snow, fractures propagating out from skis, increasing wet snow instability, wind slab, persistent

heavy snow, heavy rain, etc., are reasons to turn back

or consider a safer route.

It is not possible to give universal rules for route

selection. Each tour has peculiarities that overrule

idealizations; the route is chosen to optimize efficiency, safety, and pleasure. Fortunately, the most

efficient route and the safest route are very often the

same. For example, since one can travel much faster

on flat, windblown slopes than on steep, snow-filled

slopes, route selection generally favors ridges or lowangle windward slopes rather than lee slopes.

Decisionmaking and route selection 183
Figure 201.—When approaching a saddle or ridge, cross the starting zone as high as possible. Avoid suspected areas of wind

deposition and TG metamorphism. Dotted arrow shows recommended route. (Photo by Keiner)

However, when instability is suspected, it is necessary to avoid avalanche paths completely. This may

mean using a roundabout or inefficient route that gains

or loses precious elevation, or it may mean traveling

over difficult but avalanche-free terrain. For example,

one may avoid open slopes and travel instead on forested slopes. As a simple rule, if trees are spaced

within a few meters of one another (just close enough

to make ski travel annoying), the protection they offer

should be adequate.

Generally, one must stay clear of slopes steeper

than 30° during periods of potential instability. This

is not overly restrictive, since downhill skiing can be

enjoyed on slopes in the range from 20° to 30° under

•most snow conditions, including deep powder. The

more spectacular tours on steeper slopes must be

saved for spring.

If it is necessary to cross an avalanche path during

a period of potential instability, it is better to move

quickly across the runout zone rather than across the

starting zone or track. This is because most avalanches

are triggered by the victims. If an emergency makes

it necessary to cross a starting zone, the tourer can improve his chances by the following strategies:

• While climbing to a ridge or saddle, enter the

starting zone as high as possible. If the slab fractures, the chance of ending up on the surface is

better (fig. 201).

• When climbing or descending in a starting zone,

try to keep to the flanks rather than the slab center.

If the slab fractures, the flank snow tends to be

deposited over snow from the slab center. More

important, there is a better chance to escape by

traversing out of the slab area.

• If there is a choice, favor starting zones that feed

into flat, open runout zones, as opposed to gullies.

There is a better chance for shallow burial on flat,

open slopes.

• Avoid starting zones that feed into crevasses,

cliff^s, icefalls, and other terrain hazards.

• When TG instability is a possibility, choose a

route that favors sun-exposed sections over shaded

sections.

• Avoid suspected areas of wind slab.

• Expect crown fractures to propagate between

snowpack anchors such as rocks and trees; try to

contour above the suspected fracture path (see fig.

94).

Besides these route-finding principles, many other

safety practices to be used when crossing avalanche

terrain are discussed in the next section.

184 SAFETY AND RESCUE
Figure 202.—Four ski-tour areas with the safer routes shown by solid lines and the more dangerous ones by dotted lines.

Decisionmaking and route selection 185
Figure 203.—A good example of the incorrect (left) and the correct (right) procedure. Only one person should cross an

avalanche path at a time, while other members of the party watch from safe positions. The selection of a route through such

an extensive avalanche path must be questioned. (Photos by Keiner)

Precautions for back-country travel

If a victim is buried completely, according to

present data, there is statistically one chance in five he

will be found alive. The "one-chance-in-five" statistic

is based on a wide variety of accidents; in many cases,

the victims neither paid attention to route selection

nor took precautions. This section will discuss actions

that may improve a victim's chances:

Mountaineering rope. A rope is often essential for

crossing starting zones, especially where paths feed

into gullies, crevasses, cliffs, etc. Although the rope

may give protection against the forces of small powder

avalanches, the dynamic forces of moderate to large

avalanches far exceed the forces sustained in mountaineering falls. Avalanche forces are also of much

longer duration. To sustain these forces, the belay

should be tied to a fixed anchor, such as a tree, rock.

or rock pitons, rather than hand held. In an emergency, the belay may be supported by ice pitons,

snow picket, or ice axe. A rope is especially useful in

descending slopes where avalanches can be kicked

off to clear the descent route.

Probes and shovels. Collapsible avalanche probes

that fit into rucksacks can be purchased at some

mountaineering shops. As an alternative, some special

ski poles have removable baskets and can be attached

together to make a probe about 3 m long. One or

more strong, lightweight shovels should be taken on

hazardous tours. Digging only with skis and hands,

it takes almost half an hour in typical avalanche debris

to dig a hole 1.5 m deep and wide enough to give

assistance to a buried victim. The time can be cut to

about 10 minutes with a stout mountain shovel. In

a 1974 Colorado tragedy, the victim was located

immediately by the transceiver technique, but the

186 SAFETY AND RESCUE
\

Figure 204.—Rope protection across an avalanche path.

survivor had no shovel and was unable to dig down

to the victim buried under 2 m of soft slab debris. The

survivor was forced to go for help, and the victim was

dead when recovered.

Picking out escape route. Before starting across a

slide path, carefully study it and make a plan for

escape should it fracture. When a slab fractures

around and above a victim, his best chance is to

escape to one side. If on skis, he should try to keep

his balance and ski out the nearest flank exit. This

means keeping track of the escape route and reacting

almost instantly when the slab fractures. The closer

the victim gets to the flank, the better his chance for

a shallow burial. Very often the crown fracture occurs

exactly along the victim's ski tracks, in which case he

should attempt to hold his position above the moving

slab.

Crossing starting zones one at a time. The correct

procedure is for one person to cross at a time; the

other members of the party watch from safe positions.

When the first skier arrives at a safe location, he

signals for the next to cross. Repeat the process until

the entire party is at the safe location. Never assume

that a slope is stable because one, two, three, or more

people have crossed without triggering the slab. The

first to cross may disturb the slab and initiate a

relatively slow bed surface failure. The last to cross

may trigger the final fractures.

Precautions for back-country travel 187
i

ffr-,

Avalanche

cord

Ih

II,

o

Transceiver

Collapsible

probe

Small shovel f

ft

S

First aid kit

Ski pole probe

Figure 205.—Safety equipment for back-country travel: avalanche cord, small shovel, collapsible probe or ski pole

probe, transceiver, and first aid kit.

Electronic transceiver. A transceiver is a radio that

can transmit and receive. It is the most effective safety

device, and one should be carried by every avalanche

worker and sici mountaineer. Before starting out, all

members of the party should switch to transmit. If a

meriiber of the party is buried, the survivors switch to

receive and home in on the signal according to procedures described more fully later in this chapter.

Avalanche cord. Trailing out at least 15 m of

brightly colored nylon cord known as "avalanche

cord," which is commercially available at most mountaineering shops, can be a safeguard. Avalanche cords

are only partly reliable. According to tests conducted

by the Swiss Avalanche Institute, in only 40 percent

of the burials can a completely buried victim expect

to have part of his avalanche cord on the surface. In

/

'S

.\'

Figure 206.—Comrades rush to aid a partially buried victim

(arrow). Note that the victim's avalanche cord is on the

surface. Such cords are a worthwhile safety device even

though there are only a few authenticated cases where the

victim was completely buried and his cord stayed on the surface. (Photo from Wasatch Tours by Keiner and Hanscom)

actual rescues, there is one documented case of an

avalanche cord saving the life of a completely buried

victim. Using the cord may be worthwhile, however,

if it makes the traveler stop and reconsider his route

selection and the possible danger. Effectiveness of the

avalanche cord is improved by attaching a small

helium-filled balloon to the free end.

Removing ski-pole wrist straps. If the victim is

carried down, his survival chances are improved if his

hands are free. For this reason, the wrist straps are

removed before entering an avalanche slope, so that

the ski poles can be discarded when balance on skis

becomes impossible. Whether or not a victim wants to

keep his skis attached will vary from case to case;

thus, no general rule can be given for removing ski

safety straps before entering a slope. First of all, the

victim wants to keep his skis on long enough to get

over to the flank. However, if the victim is carried

down, his skis will flail against his body, compounding

his injuries. There is also a chance that his skis will

drag him under to a deeper burial. On the other hand,

buried victims have been found because part of a ski

was on the surface or because a probe pole hit a

buried ski.

188 SAFETY AND RESCUE
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Figure 207.—This survivor of an avalanche at Mission Ridge, Washington, owes his life to thrusting a hand out of the snow.

(Photo by Whitmire Photography, Yakima, Wash.)

Swimming. If he is caught in an avalanche and

unable to make a flank exit, or if he loses balance,

the victim should discard his ski poles and attempt to

stay above the snow by swimming. The swimming

motion should be as efficient as possible so that the

victim is neither winded at the bottom nor choked

by snow.

Thrusting a hand up. If snow starts to pile up on

the victim after he reaches the bottom, he should

attempt to thrust a hand up as high as possible before

the snow sets up. A few victims owe their lives to

having a hand above the snow. The other arm and

hand should be in front of the chest and face to form

a breathing space.

Precautions for back-country travel 189
Figure 208.—One useful system for ski touring is to carry ski

poles that can be assembled into probe-poles.

Conserving oxygen. In order to conserve oxygen,

the victim should relax and not fight the sensation of

blackout.

Action of the survivors. A buried victim's chance

for survival depends on what the surviving members

of the party do in the next few minutes. Hopefully,

the survivors have followed the first precaution in this

section and are standing in a safe place from which

they can watch the victim's path. The survivors should

search with whatever resources are available. If the

victim is not wearing an electronic transceiver, the

survivors must probe with skis, poles, collapsible avalanche probes, improvised probes made from tree

branches, etc. Only if the touring party is large or if

additional manpower is a few minutes away should

a messenger be spared and sent for help. The history

of avalanche accidents has too many instances of

survivors departing the scene in panic to seek help

when a few minutes' search would have uncovered

the victim.

Probing for the victim

Compared to transceivers or avalanche dogs (see

the later section "Avalanche Dogs") probing is a

slow and tedious method of search. However, it is the

only practical method if the victim is not carrying a

transceiver of if a dog is not available. More complex

detection systems that rely on the victim's body heat,

body odor, sound, electromagnetic properties, gravitational field, etc., are still in the research stages

(Vanni Eigenmann International Foundation 1963).

The first problem in making a speedy rescue may

be the hazard to rescuers. The fact that an avalanche

ran is proof of instability on neighboring slopes. After

precautions are taken to safeguard rescue personnel,

including a lookout in a safe spot to watch for and

warn probers of additional avalanches (see the later

section "Special Rescue Problems"), the search

moves quickly and efficiently through the following

steps:

(1) From witnesses or by examination of clues,

establish where each victim was at the time of the

avalanche.

(2) Determine the area where each victim disappeared—the "last-seen area."

(3) Using this information, establish probable trajectories for each victim.

(4) Determine the regions of highest priority search

(usually a rough estimate).

(5) Make a rapid but systematic search of the avalanche debris surface in the regions of highest priority.

Mark the location of all clues. Searching the surface

is the first part of the probing operation, and it offers

the greatest probability of a live rescue.

Immediately after completing the surface search,

rescuers should be organized into probe lines, using

whatever probe equipment is available. Ideally, probe

poles should consist of rigid steel or aluminum tubing,

3 to 4 m long. Longer poles are difficult to manage;

besides, the chance for making a live rescue of a victim buried deeper than 3 m is extremely small. There

is an advantage to standardizing probe length, since

this may make it more obvious to the probe leader

when one probe is stopped short of full probe depth

by an obstacle.

Probing is simple and requires very little practice;

it is a technique as old as avalanche disasters and is

readily learned by well-meaning volunteers. It is easy

to feel the difference between striking a body and

striking a snow layer; the real problem is discriminat190 SAFETY AND RESCUE
ing between the ground surface and a deeply buried

victim. Before starting to probe the avalanche debris,

the probers should be briefed on their mission and

given directions for evacuating quickly.

Probe lines must be ordered and properly spaced

for probing to be effective. About 20 probers per line

is satisfactory, and 30 is an upper limit. If extra

manpower is available, a person can be placed at each

end of the line to hold a string for alining the probers.

The string should be marked to help properly space

the probers.

The probe line advances steadily upslope. Advancing uphill automatically helps set the proper pace and

keeps order. Even so, proper spacing and discipline

often become a problem after probing has continued

for some time. Downhill probing is more difficult to

control, although, if the rescue team approaches from

above, the initial surface search can proceed downslope. Probing does not come to a halt when a possible strike is made. The probe line continues to advance, and a small crew falls out and shovels down

to investigate the strike.

Coarse probing. As long as there is any hope for a

live rescue, probing is conducted at a vigorous pace

according to a system known as "coarse probing."

The idea behind coarse probing is to sacrifice some

thoroughness to improve speed and thus maximize

the chance of finding the victim alive. Coarse probing

gives about a 70-percent chance of finding the victim; this may be compared with the alternative and

more thorough method of probing known as "fine

probing," which gives nearly a 100-percent chance of

finding the victim. However, fine probing takes four

or five times longer than coarse probing, and it is

best to make several passes using coarse probing

before resigning the operation to slow, thorough fine

probing. Coarse probing does sacrifice some thoroughness because of the grid size, and it must be

carried out precisely to keep the sacrifice to a minimum. Coarse probing proceeds as follows (see fig.

210):

(1) Probers are spaced along a line, 75 cm (center

to center) apart. A distance of 50 cm is straddled,

leaving 25 cm between the toes of adjacent probers.

(2) The probe pole is inserted once at the center of

the straddled span. (An alternative method, used

where terrain is steep or there are only a few probers,

is to stand "fingertip-to-fingertip." The prober then

probes first on one side of his body, then on the other

side. If done carefully, this gives the same probe spacing as the single-insertion system.)

Figure 209.—If two points of the victim's trajectory can be

established, a high probability exists that the victim will be

near the downhill flow line passing through these two

points.

(3) On signal from the probe-line leader, the line

advances one step (about 70 cm) and repeats step 2.

Usually one signal suffices for the complete sequence—insertion of probe, retraction of probe, and

advancement of line. The signals should be at a

rhythm that enforces the maximum reasonable pace.

Strict military discipline and firm, clear commands

are essential for efficient probing. The probers should

work silently. The spacing of the probe holes is 75 by

Probing for the victim 191
FigLiie 210.—Execution of coarse probe. The coarse probe is used whenever live rescue is expected. The above spacing can be

achieved by dressing the line "elbow-to-elbow" in "hands-on-hips" position. One probe insertion is made, and the line advances

one step. An alternative method, useful when the number of probers is limited or terrain is steep, is to have probers stand

fingertip-to-fingertip. Probers then probe once to the left of their body and once to the right before the line advances.

70 cm, which is equivalent to 1.9 probes per m-. The

chances for direct strike of victim are:

Person lying on his stomach or back 95%

Person on his side 75 %

Person in vertical position 20%

Average position 70%

On the average, 20 men can search an area 100 by

100 m in 4 hours. Depth of probing is adjusted to

depth of deposition, but should be no deeper than 3 m.

Fine probing. After repeating the coarse probe several times, it may be obvious that the objective is body

recovery rather than live rescue. In order to preserve

the strength and morale of the volunteers, the pace

and discipline of the operation may relax to the more

thorough and less vigorous fine-probe technique,

which functions as follows (fig. 212):

(1) Volunteers are arranged the same as for the

coarse probe.

192 SAFETY AND RESCUE
Figure 211.—Ideally, a probe line should consist of about 20 rescuers. A large group tends to get out of alinement, with consequent loss of efficiency.

(2) Each volunteer probes in front of his left foot,

then in the center of his straddled position, and finally

in front of his right foot.

(3) On signal, the line advances 30 cm and repeats

the three probes.

The spacing of the probe holes is 25 by 30 cm, or

13 probes per m-. This means that if the victim is in

a favorable position, five direct strikes are likely to be

made. If he is in an unfavorable position, one direct

strike is likely. The overall probability of finding the

victim is therefore 100 percent, providing the burial is

not deeper than about 3 to 4 m. On the average, 20

men can fine probe an area 100 by 100 m in 16 to 20

hours, depending on the depth of probing.

In most cases the victim is carried by the moving

avalanche to the places of greatest snow deposition—

usually the toe of the slide. Occasionally he is snared

by rocks, trees, or benches in the terrain. If the avalanche follows a wandering gully, all bends with snow

deposits are likely burial places; again, the victim is

likely to be where the greatest amount of snow is

deposited. Finally, the victim may be thrown out of the

slide. In a few cases, victims have been found dangling

from trees. There is a sad case of a young boy who

escaped from an avalanche but died of hypothermia

in the woods while his rescuers carefully probed the

avalanche debris.

Since there is usually a shortage of manpower during the initial and critical phase of the probing operation, it is necessary to decide which areas have the

highest priority for search and in what order these

areas will be probed. The decision involves weighing

the approximate probability that a victim is in a given

area versus the relative time required to search that

area. Normally, initial search is made of the main

debris area, since this area can be searched most efficiently with the greatest chance of success. However,

there may be clues that indicate a high probability

that the victim was snared above the main debris or

perhaps that the victim has escaped the slide. In such

cases, manpower should be allocated to follow up the

clues.

Figure 212.—Execution of fine probe. The fine probe is used

when live rescue is not anticipated. The setup is the same

as in coarse probing. Three probe insertions are made, and

the line advances about a half step.

Probing for the victim 193
Benches or

roads i

Figure 213.—Most victims are carried to areas of greatest deposition. However, other burial clues may require followup.

Search by electronic transceiver

People who travel regularly in avalanche terrain

cannot depend on probe lines for backup protection.

They need a device that offers speedy detection regardless of the size of the avalanche. The electronic

transceiver mentioned in several earlier sections enables the victim to be found within 10 minutes after his

signal is first picked up, providing rescuers know the

correct procedure for using their transceivers and participate in practice drills at least once each season.

As an example, the basic procedure for using one

of the transceivers^ is outlined below.

'The "Skadi" system developed by Lawtronics, Inc., was

the first transceiver to be marketed and proven by saving lives.

It is based on a 2275-Hz audio-induction signal. There are

two important reasons for choosing this audio frequency as

opposed to a higher radio frequency: (a) There is no interference from radio broadcasts; and (b) the electronic circuitry is relatively simple, and there is less chance for a malfunction. Unfortunately, several other systems are being marketed at noncompatible frequencies. In a recent accident involving 20 helicopter skiers who were wearing transceivers.

194 SAFETY AND RESCUE
Sufficient searches to

cover avalanche with

30 meters maximum

between searches.

large avalanche.

Two search parties on

long deposition zone.

Figure 214.—Initial search procedure using transceivers with effective range of 30 m (Lawion 1969).

Consider the most common situation, in which the

victim is swept down by the avalanche, and the rest of

the party is above the area where the victim was last

seen. Switch the transceiver to receive, and proceed as

follows (Lawton 1969):

(1) Carefully note and mark the last-seen area. If

there is danger of further avalanches, post an avalanche guard and be prepared to switch back to transmit.

(2) Be sure all transceivers are switched to receive.

The leader must check this, since a single searcher

with his transceiver left on transmit can frustrate the

entire search.

(3) Deploy a line of searchers at the level of the

last-seen area and search downward. The spacing between searchers should not exceed 30 m (fig. 214).

(4) Keep volume control turned all the way up

until a signal is received. You should be able to hear

some background noise (static).

there was added confusion when units that were incompatible

with those of the victims were brought to the scene. It is

recommended that rescue teams purchase only brands that

operate at 2275 Hz. '

(5) Move in unison, keep the line straight, and

keep noise and talking to a minimum. All searchers

stop every 10 paces and slowly rotate their transceivers left and right, then front and back.

(6) When a signal is heard, everyone should be informed, but the line of searchers should not be broken

up. Orient the transceivers for maximum signal

strength. Turn the volume down until the signal can

just be heard; the ear detects changes in signal strength

much more readily at low levels.

(7) Do not change the orientation of the transceiver in space while moving. This is very important

because it causes false changes in signal level.

(8) Halt every few paces to refine the orientation

for maximum signal strength and reduce the volume.

The best orientation will change as you get closer.

19) When the signal starts to get weaker, the victim

has been passed. Scribe a line A in the snow to mark

this point (fig. 215).

(10) Again orient the transceiver for maximum

signal, maintain this orientation, and reduce the volume until the signal can just be heard. Now, back up

until the maximum signal point has been passed and

the signal fades again. Mark this fade point (line A'

Search by electronic transceiver 195
Figure 215.—Terminal phase of search by bracketing (Lawton 1969).

in fig. 215). The two fade points bracket the position

of the victim.

(11) Locate yourself halfway between the fade

points. Orient the transceiver for maximum signal,

then reduce the volume until the signal can just be

heard. Search at right angles to the original search direction. Establish bracket B-B\ Hundreds of practice

sessions have shown the right-angle technique to be

the fastest way of finding a victim.

(12) Continue systematically to improve the estimate of the victim's position until it is pinpointed.

Three or four crisscrosses are usually sufficient.

(13) It pays to pinpoint the position of the victim.

Otherwise, a great deal of snow may have to be moved

and a good bit of time lost before the victim is found.

Experience has shown that it is possible to refine the

fix to about one-fourth of the burial depth.

If more than one person is buried, turn each victim's

transceiver off as soon as possible. In most cases, it is

not very difficult to separate signals from more than

one stationary transmitter; however, the added complication of searching for several transmitters reemphasizes that only one person should cross an avalanche path at a time. It is recommended that groups

who purchase transceivers attend practice field clinics

arranged by the manufacturer.

One problem with transceivers is their cost. They

are a specialty item that must be manufactured and

tested for the highest standards of reliability. Detailed

testing of all components takes time and money—costs

that the consumer pays if he wants a dependable unit.

However, mountaineering shops or ski-touring clubs

can purchase several units at reduced prices and rent

them to individuals on a temporary basis.

Finally, some victims wearing transceivers have

been located quickly but not quickly enough to save

their lives. Although the transceiver is an important

tool for backup protection, the user should not believe

that transceivers (or any rescue system) permit increased risks to be taken on potential avalanche

slopes.

Avalanche dogs

Working mostly by scent, a well-trained avalanche

dog can make a rough search of an area 100 by 100 m

in about half an hour or about eight times as fast as a

20-man team could "coarse probe" the same area. A

dog requires about 1 to 2 hours to make a detailed

search, whereas a "fine probe" would take 16 to 20

hours.

Avalanche dogs are used extensively in the Alps,

where dense settlement makes practical the time-consuming training of dogs to search efficiently for buried

victims. According to Swiss statistics, avalanche dogs

participated in 305 rescues from 1945 to 1972; 45

victims were found alive, 224 victims were found

dead, and in 36 cases the dog failed to find the victims.

The large number of dead recoveries is no reflection

on the ability of the dogs; it is proof of the slim chance

of survival of a buried victim.

All told, there are about 500 certified avalanche

dogs in Europe. The system is most highly organized

in Switzerland, where a call to a central telephone

number brings the nearest avalanche dog to the scene

of the accident (by plane or helicopter, if necessary).

The famous Parsenndienst rescue patrol at Davos

maintains an avalanche-dog kennel near their headquarters at the top of the Weissflujoch ski area. Keeping dogs on call at the top of a ski area is the best

system for live rescue.

Because the avalanche hazard in North America is

dispersed in space and time, a different approach has

evolved—the training of dogs for multipurpose search.

In addition to avalanche victims, dogs are trained to

find flood victims, people lost in the wilderness, planecrash victims, earthquake victims, etc. Thus, the dogs

196 SAFETY AND RESCUE
are used much more often than if they were saved for

avalanche work. This approach has been proven in a

wide assortment of rescues when dog and leader were

quickly airlifted to the disaster scene.

The advantage of housing special avalanche dogs

at ski areas to improve the chances for a live rescue is

often somewhat outweighed by the fact there are seldom enough avalanche rescues to keep dogs and

handlers working at peak efficiency. Special efforts

must be made to keep search skills and physical conditioning high. On the other hand, the versatility of

multipurpose search dogs means they and their handlers usually have enough missions to stay in training.

Unfortunately, it also means the dogs are normally not

housed at ski areas and therefore often are not available soon enough for live rescues.

Avalanche dogs work best to retrieve persons still

alive or victims who have died within a few hours.

They are less successful at finding dead persons

buried deeper than about 2 m, although under favorable conditions a dog has made a live rescue of a

victim buried under 5 m of snow. Cold temperatures,

strong winds, wet snow, and contamination of the

search area are other important limiting factors. The

exhaust and gasoline fumes from hovering or landing

helicopters make it very difficult for dogs to work.

There are several prerequisites for dog and master.

The dog must have :

• A well-developed sense of smell

• A strong constitution and the ability to withstand the rigors of winter travel, wading through

snow, etc.

• A hair coat resistant to cold

• A temperament suited to training

• A healthy physical and mental structure.

German Shepherds and related breeds are proven and

are an excellent choice, but satisfactory results may be

obtained with other breeds, such as Labrador retrievers, golden retrievers, border collies, etc. A trainer will

be operating at a disadvantage if he chooses breeds

that are "game-oriented" or are either too large or too

small for vigorous travel.

Besides an understanding of canine psychology and

a talent for training dogs, avalanche dog leaders need

impressive qualifications:

• Knowledge of mountaineering and avalanche

safety

• A strong constitution and reasonable ability to

hike and ski

Figure 216.—A trained avalanche dog is a welcome addition

to any touring party.

• Training in first aid

• Training in flying and helicopter safety

• Availability during the winter on short notice.

In the Swiss Alps, dogs and masters attend weeklong training clinics, followed by continual local

practice. Dogs are rated according to three classes:

Class A (beginning), Class B (intermediate), and

Class C (operational). A dog is about 3 or 4 years old

before he achieves Class C; this leaves about 4 to 6

years of useful service.

In one system of training, explained more fully in

Austrian Mountain Rescue Service of Tirol (1962),

the dog is given the following four initial exercises:

Exercise 1. A shallow pit large enough for two men

to lie in is excavated on a slope, preferably one covered with avalanche debris. Participants are the dog's

master (M), a trainer (T), and two helpers (Hi and

Hj). M delivers his dog to T and then hurries to the

pit, calls the dog, and disappears into the pit. With

his left hand, T holds the leashed dog by his collar,

stimulates the dog's eagerness, and at the moment M

disappears into the pit, points impressively with his

right hand, commands "search!", and releases the

dog. The dog should rush to M and be received with

praise. M puts the dog on a leash and returns it to T.

Exercise 2. M quickly returns to the pit and is given

a shallow burial ( 10 to 20 cm) by H,. M has a delicacy ready in hand. Hi quickly returns past the dog,

giving the dog a chance to sniff and determine that M

has not returned. T performs as above but follows

A valanche dogs 197
^

Figure 217.—Training the avalanche dog. Exercise 1.

the dog to the pit. If the dog does not start digging,

T points to the pit, commands "search!", and begins

digging and scratching on the snow. M helps by

groaning. After detection, the dog is rewarded with

praise and a delicacy.

Exercise 3. Hj buries Hi and M together, as shown

in figure 219, and then returns past the dog. The

search proceeds as above. Upon detection, H, and M

both praise the dog, and H, hands the dog a delicacy.

Exercise 4. In the last of the initial exercises, M

does not hand the dog over to T but enacts T's role.

Hi is buried alone and receives the dog with a delicacy

and praise.

These exercises are done quickly in sequence. The

dog is then given several hours of rest. When the dog

has mastered completely the above sequence, he

moves on to more difficult exercises:

• Helper buried in the original pit, out of sight of

the dog

• Burial out of sight of the dog, about 10 m beyond

the original pit

• Burial out of sight of the dog, shifting the exercise pits to the right or left

• Burial at random anywhere on the exercise

slope, out of sight of dog

• Deeper burials (taking adequate precautions to

ensure the safety of the helper)

• Enlargement of the search area, adverse conditions, etc.

• Tactical missions, including long approach

marches.

In general, these advanced exercises are practiced in

order of increasing difficulty; however, the dog is

sometimes reinforced with easier problems that he can

solve readily. Should the dog fail on an actual rescue,

he should be provided with a solvable problem (such

as digging up a planted glove) and praised when he

succeeds.

198 SAFETY AND RESCUE
(M is about 20cm

under the snow.)

/

Figure 218.—Training the avalanche dog. Exercise 2.

Figure 219.—Training the avalanche dog. Exercise 3.

A valanche dogs 199
Figure 220.—Tiaiiiing the avalanche dog. Exercise 4.

Revival and evacuation of the victim

Suppose a victim has been located and uncovered

by an efficient and speedy rescue. Will the victim

survive? The answer may be determined by what

takes place in the next few minutes.

To all external appearances, the victim may exhibit

no signs of life, but unless there are unmistakable

signs of death, resuscitation must begin immediately.

At a European ski area, a buried victim was uncovered and pronounced dead by two doctors. Against

the protests of an avalanche dog leader, resuscitation

was halted and the victim was transported to a hospital. On arrival, the victim was revived but died 15

hours later. In another case, a Swiss soldier buried

by an avalanche for 8 hours was found at a depth

of 1.5 m with minimum life functions and was saved.

He had lost consciousness quickly after burial. His

Figure 221.—Suction pump for clearing obstruction from

breathing passages.

body temperature at time of rescue had dropped to

30° C (86° F); his blood pressure was 80/65; his

pulse rate was 44/minute; and his breathing was

extremely superficial and slow. Other victims have

been resuscitated from similar states of greatly

lowered body temperature and metabolism. Although

this reduction of body functions to an absolute minimum helps the victim to conserve oxygen and thus

prolongs his survival time while buried, it poses a

problem to the rescuers, since body heat must be

restored to aid revival.

If the victim is breathing normally, it is important

to restore the body's heat as soon as possible. It can

be assumed that the victim is suffering some degree of

hypothermia. Further chilling must be avoided. The

victim's reaction to the cold consumes large amounts

of energy. Not only must existing heat be conserved,

but external heat must be supplied to counteract the

body's cooling. A warm sleeping bag is essential. This

may be heated by chemical hot pads or hot water

bottles, or by preliminary occupation by a healthy

person. A large sleeping bag is best, to make it possible for one of the rescuers to get in with the victim.

Wet clothing should be removed, his body dried, and

dry clothing (blankets, down gear) provided. If adequate shelter from wind and snow is in question, a

small, lightweight tent that can be quickly erected and

a small portable stove should be brought to the scene.

200 SAFETY AND RESCUE
Figure 222.—A, Breathing-bag apparatus and B, portable oxygen unit for administering artificial respiration.

Because of continuing research into the treatment of

hypothermia, the most recent pubhcations on the

subject should be consulted.

Restoration of heat is secondary if the victim is not

breathing; the first urgent treatment is restoration of

breathing by artificial respiration. As soon as the victim's head is exposed, his nose and mouth should be

cleared of snow, blood, and vomit. After preliminary

cleaning, it may be necessary to apply suction. Organized rescue teams will need to carry a foot-operated

suction pump. If a suction pump is not available, then

one should attempt to suck the liquid from the throat

and pharynx with a catheter or with a small rubber

tube. Meanwhile, the unconscious victim should be

positioned horizontally with the upper body and head

lowered; this helps prevent water, blood, and vomit

from entering the deeper breathing passages. In some

cases, an airway tube or resuscitube will be useful

to keep the passage clear, both during resuscitation

and after normal breathing is restored. Suction attempts are not always required, although if the airways are clogged, artificial respiration is pointless. In

any event, very little time should be lost debating

whether or not the airways are clogged, because during the first attempts to inflate the victim's lungs it

will be clear whether there is a major obstruction.

If respiration equipment is not available, the preferred technique is mouth-to-mouth resuscitation,

which is taught in first-aid clinics. The first puffs of

breath are often decisive; the victim should be respirated at least 10 times in quick succession before

the more normal rhythm of 10 to 12 breaths per

minute is started. If a victim is recovered in a state of

cardiac arrest (no pulse or heartbeat, pupils dilated),

artificial respiration should be augmented by closedchest heart massage. This technique is covered in

advanced first-aid clinics; it requires training and

practice. At high elevations, prolonged application of

cardiopulmonary resuscitation, or even mouth-tomouth resuscitation, is quite demanding, especially if

performed by one rescuer. For this reason, rescue

teams must bring respiration equipment to the scene

of the accident.

Revival and evacuation of the victim 201
The type of resuscitator that uses a self-inflating

bag (AMBU, Hope, etc.) is preferred. The elasticity

of the bag forces a proper rhythm so that danger of

under- or overrespiration is lessened. The breathing

bag and its auxiliary equipment must function properly under severe weather conditions. Atmospheric

air is almost always sufficient for revival. If oxygen is

available it may be administered as an extra measure,

but the resuscitator, not the oxygen bottle, is the

important thing to get to the victim first. It is possible

to obtain sufficient blood-oxygen saturation with

forced breathing of air alone up to an elevation of

5,800 m.

Even if the victim shows signs of life (swallowing,

slight movements, weak breathing), supported respiration with the breathing apparatus must continue.

When the blue discoloration of the lips, tongue, and

fingernails disappears and the normal rosy color

returns, it is a sign that respiration and circulation

have improved.

Below is a list of resuscitation equipment for medical worker and physician:^

Medical worker

Suction pump with suction catheter

Breathing bag with air and oxygen inlet and breathing

valve (resuscitator complete with accessories)

"Airway" or "resuscitube" breathing tube

Tent, stoves

Sleeping bag, blankets

Standard first-aid equipment (splints, bandages, back

splint)

Physician

Drugs

Demerol solution (50 mg/cm^^)—30 cm*'»

Demerol tablets (50 mg)—6

Lidocaine (1% or 2% solution)—50 cm-^

Epinephrine (1:1000 solution)—3 l-cm"» amps

Sodium bicarbonate solution (3.75 gm—50 cm"^)—3

amps

Atropine (1 mg)—3 or 4 vials

Isuprel (1:5000 solution)—2 or 3 1-cm'^ amps

Aramine (1% solution)—10 cm-'»

Levophed (0.2% solution—8 mg)—3 to 4 amps

Dextrose solution (50% )—50 cm-^

Sodium chloride (0.9% solution for injection)—30

cm3

Sterile water for injection—30 cmDextrose and water (5% solution)—1,000 cm'^

Sodium chloride (0.9% solution)—1,000 cm*^

Blood plasma—2 250-cm"' bottles

Tubing, tape, etc., for administering last three items

Hypodermic Needles

#18 X 11/2"—3

#20 x 11/2"—5

#22 X 11/2"—5

#20 X 6" (intracardiac)--2

#20 X 4" (intracardiac)--2

#15 X IVi" (emergency airway)--2

#15 X IVi" (emergency airway)--2

Syringes

21/2 cm-î-3

5 cm3—3

10 cm3—3

30 cm-î—2

50 cm3—1

^Prepared by Burton Janis, M.D., College of Medicine,

University of Utah, Salt Lake City.

Supervision Equipment

Blood-pressure apparatus

Flat stethoscope

Cardboard tags and pencil

The drugs listed above should be kept in a cool, dry

place and they should not be subjected to freezing

temperatures. Most of these drugs should have a 2- to

3-year expiration period, and the person purchasing

them should try to buy compounds that all expire at

about the same time, so that replacements can be

made all at one time. The procurement, storage, and

packing of these items in a suitable rucksack should

be supervised by a physician.

The possibility of mechanical injury is always present. The rescue team should exercise special care to

check for the possibility of neck fracture before

manipulating the patient to clear the air passages and

begin resuscitation. If a neck fracture appears possible, traction is permissible, but flexing of the neck

should be minimized. If breathing has stopped, resuscitation must begin in any case.

As long as the victim remains unconscious, full

attention has to be paid to keeping the breathing

passages open and to the possible need for artificial

respiration. The unconscious victim is positioned on

sled or stretcher, on his back, with neck slightly extended; the head and upper part of the trunk lie flat

or are inclined slightly downward. According to his

condition, the rescued person is transported to the

hospital or to his home in the company of an attendant who observes him continually. Medical supervision must be continued; in one tragic case, a buried

victim was revived and appeared fully recovered, but

died of shock the following day.

Avalanche victims under maintenance of artificial

respiration can withstand air flight to a valley hospital

in the company of emergency clinic personnel. Immediate air transportation to a hospital may, in fact, be

the victim's only hope for survival. (See Washington,

202 SAFETY AND RESCUE
State of, 1972, for discussion of helicopter rescue and

evacuation.) Throughout Europe, ski tourers and

mountaineers can purchase an insurance policy that

provides air-rescue service to a hospital. Telephoning

a central number alerts the nearest air-rescue team,

which is fully prepared with probes, avalanche dogs,

artificial-respiration equipment, etc., and medical personnel as required.

The following is an outline of the procedure to follow in caring for avalanche victims :^

Determine whether victim is breathing or has an

obviously fatal injury.

If the victim is not breathing:

(1) Examine for fractured cervical spine

(2) Clear snow away from chest and examine for

penetrating injuries

(3) Treat any penetrating chest injuries

(4) Determine whether heart is beating by checking for:

(a) Bleeding from open wounds

(b) Peripheral pulses

(c) Audible heartbeat

(5) Institute artificial respiration

(a) Clear mouth and throat of all foreign

material

(b) Extend head

(c) Give mouth-to-mouth artificial respiration

(6) Institute cardiac resuscitation

(a) Sharp blows over chest

(b) Cardiac massage

// the victim is breathing (or after resuscitation attempts have begun):

(1) Examine for major open wounds

(a) Control bleeding with direct pressure

(b) Apply temporary bandage

(2) Examine for and treat fractures

(a) Fractured spine

(b) Fractured pelvis

(c) Fractured arms and legs

(d) Fractured ribs (flail chest)

(3) Treat for shock

(a) Elevate legs and lower body

(b) Control pain

(c) Administer plasma or plasma substitutes

(4) Treat for exposure

(a) Protect from environment

(b) Supply warm clothing

(c) Supply external heat (v/7a//)

(d) Provide nutrients

Trepared by James A. W^ilkerson, M.D., Merced Pathology

Laboratory, Merced, Calif.

Evacuation

If unconscious:

(a) Keep airway open (vital!)

(b) Keep head level with body or slightly

lower

(c) Continue treatment for shock and exposure

Signs of death

(1) Livor mortis (black and blue death color)

(2) Rigor mortis

(3) Dilated pupils unresponsive to light.

Organized rescue in ski areas

It must be reemphasized that many more lives will

be saved by public education, avalanche control, and

caution than by rescue of buried victims, regardless of

how organized, sophisticated, or well equipped the

rescue efforts. Nevertheless, ski areas are responsible

for providing the public with fast and efficient rescue

service in full accordance with modern standards and

equipment.

To give a buried victim a better than 50-percent

statistical chance of surviving, he must be uncovered

in less than half an hour. This is generally impossible

if the rescue team is mobilized in a valley and transported up a major ski lift system. It is generally possible, however, to find the victim within half an hour

if a small, well-equipped rescue team is on the alert

at a high ski terminal. The Parsenndienst can be cited

again as an example of preparedness. This team maintains headquarters at a summit station. In case of

burial they act as follows :

(1) The entire crew at the station is called into the

patrol room by the alarm siren.

(2) The situation is announced on a loudspeaker;

for example, "two persons buried at Meirhofer, one

survivor at the accident site."

(3) The acting rescue leader on duty at the station

issues necessary instructions.

(4) Rescue teams, including avalanche dog and

leader, move out within 4 to 8 minutes after receiving

the accident call.

(5) Announcement is made at the lift terminal. Additional volunteers, doctors, mountain guides, ski instructors, and good skiers are asked to contact rescue

headquarters. Volunteers are equipped and moved out

to the accident scene with qualified leaders.

Since every minute counts, ski developments with

serious avalanche hazards need to follow the ParsennOrganized rescue in ski areas 203
Figiiie 223.—Speedy rescue of a buried victim on many ski

areas requires a professional team to stand alert at an upper

terminal station.

dienst's example. A loss of 10 to 15 minutes may be

incurred by transporting an initial search party up a

lift from a midmountain station. A buried victim cannot be penalized this transportation time. Of course,

if the avalanche hazard is spread out over a great range

of elevation, supplementary manpower and equipment

can be dispatched from a midway terminal; however,

an upper station is still essential.

A speedy organized rescue depends crucially on

developing a plan of action before the accident. This

is called the rescue plan. It is a written plan that is

studied by all employees of the area. It is updated

before each ski season begins. The plan is precise

about how an accident alarm is transmitted and received. It precisely spells out manpower responsibilities and equipment management. Rescue plans drawn

up before 1975 should be reevaluated on the basis of

new evidence indicating a 50-percent chance of survival after half an hour, rather than 1 hour as previously thought. In general, the plan provides for the

following key personnel:

Rescue leader. He is the overall coordinator of the

operation. Typically, he would be head or assistant

head of the ski patrol or a person with equivalent experience in all facets of avalanche rescue, and he

should be thoroughly familiar with the area. There

H n 1

10. • • • .2

». /—3

« .T. 4

7 6 5

TIME

O min

Stage I

10 min

Stage II

20 min

Stage III

J

REPORT OF ACCIDENT

Contact Rescue Headquarters.

Sound alarm.

Freeze personnel.

Appoint Column Leaders for Stage I

Dispatch initial column.

(Hasty rescue or Immediate

rescue group.)

Appoint Accident Site Commander.

(Does not have to be Initial

Column Leader,)

Dispatch additional Stage I columns.

Contact qualified medical personne

Appoint Stage II Column Leader(s)i

Dispatch First Aid-Evacuation Group.

Appoint Support Leader.

Contact support groups.

Dispatch support columns.

60 min

Figure 224.—Time schedule for organized rescue in ski area.

should be one qualified rescue leader on duty at all

times to receive alarms and at least begin dispatching

workers. In addition to coordinating the quick dispatch of manpower, the rescue leader is responsible

for contacting medical support, air support, law enforcement groups, and government officials and for

handling the public-relations problems of the rescue.

Throughout the rescue, he usually remains stationed

at the summit terminal; he appoints an accident site

commander to be in charge in the field.

204 SAFETY AND RESCUE
PERSONNEL

Hospital Support

V /

Law enforcement

\ /

RESCUE LEADER

/ \

Air Support / \

Public relations

(via Supervisor)

ACCIDENT SITE

COMMANDER

Column leaders

(Stage 1)

First aid

and evacuation

leader

(Stage II)

Support leader

(Stage III)

1

Figure 225.—Personnel requirements for organized rescue in ski area.

Accident site commander. He is sent as quickly as

possible to the accident site and relieves whomever is

temporarily in charge. He decides where to search

and arranges probe lines accordingly. His badge is a

power-operated megaphone through which he calls

out instructions to column leaders.

Column leaders. These are experienced patrollers

who lead rescue columns of five to ten professionals

or volunteers to the accident site. Column leaders

screen volunteers, equip them with probes and

shovels, and lead the columns by a safe route to the

accident, where they report to the accident site commander. A column leader may be asked to join an

existing probe line or to direct the formation of and

lead a new probe line. The first column should consist of professionals on standby at the top terminal;

later columns can be formed of patrollers, instructors,

and competent skiers who arrive at the top terminal.

The first column leader is temporarily in charge at

the accident site until relieved by an appointed accident site commander. Each column leader is responsible for checking in the members of his column at

the end of the rescue.

Most rescues in ski areas proceed according to

three stages:

Stage I—Immediate action. This stage consists of

speedy dispatch of columns equipped with probes,

shovels, light first-aid equipment, and personal gear.

Their objective is to locate the victim by a fast surface

search or vigorous coarse probe. They are not

equipped for a long, drawn-out rescue, since they

travel fast and light to the accident site.

Stage II—Revival and evacuation. Although patrollers in Stage I columns have first-aid kits for dealing

with immediate problems, they cannot be slowed

down by transporting the bulkier equipment needed

for resuscitation and evacuation. Thus, a special teain

of three to five patrollers (plus one physician, if possible) is organized to transport toboggans, blankets,

sleeping bags, resuscitation equipment, tent, stove,

physician's kit, and other equipment needed to revive

and evacuate the victim. As soon as the alarm is received, the rescue leader will have appointed an

experienced patroller to organize Stage II and collect

the necessary equipment. In a well-organized rescue,

the Stage II column is only a few minutes behind the

first stage columns.

Stage III—Support. The first two stages are dispatched with a speed which hardly leaves time to prepare for a prolonged rescue. The support stage is

Organized rescue in ski areas 205
organized to transport hot beverages, food, warm

clothing, illumination equipment, and extra manpower. The extent to which the support stage is activated varies widely according to the size of the

accident, weather conditions, and terrain problems.

In some cases, it may hardly be necessary to provide

support. In other cases, usually prolonged searches in

major avalanches aimed more at body recovery than

saving lives, the third stage may be the main component of the operation.

One of the essentials for a speedy rescue is to have

equipment fully organized and ready for distribution

as soon as the alarm is sounded. There are many ways

to organize equipment caches, depending on the area

layout; details must be specified exactly in the rescue

plan. In general, a major cache containing equipment

for all three stages should be maintained at the summit station. Small caches containing equipment for

Stage I should be set up at each lift terminal or in

the vicinity of each potential slide path. The rescue

plan should also include a list of sources for Stage III

equipment and manpower.

In summary, rescue organizations should use practice sessions to reduce lost time and improve efficiency. Four areas where time can be saved are in

reporting the accident, responding to the alarm,

traveling to the site, and carrying out procedures on

the site. Good on-the-hill communications and public

education help reduce reporting time. Response time

is reduced if key rescue personnel are prepared to go

into action immediately, equipment caches are completely stocked and well placed, and the group is well

enough trained to avoid false starts. Travel time can

be shortened if crews are in good physical shape,

know the terrain well, and are experienced enough to

make accurate field decisions about the route quickly

and efficiently. Finally, procedural time can be minimized if the crews know what needs to be done, have

proper equipment, and have enough practice performing the necessary jobs to do them without undue

delay or fatigue. Repeated drills and dry runs are

needed to perfect the individual steps and to blend

them all into an efficient rescue effort.

Following is a list of equipment to be stored in

caches for organized rescues in ski areas:

Stage I caches (at lift terminals near avalanche

paths)

One or more rucksacks, each containing ten collapsible probes and one aluminum shovel (grain-scoop

design)

Single-piece probes about 3 m long. The number of

probes should be chosen to match the hazard (10,

25, or 50, arranged in bundles of 5). Either steel

tubing (about 11 mm o.d.), or aluminum pipe similar

to í4-in, 606-1T6, Schedule 40 (13.7 mm o.d., 9.3

mm i.d.).

Main cache (at summit station)

Several rucksacks as described above

Bundles of one-piece probes as described above

Extra shovels (some for heavy-duty work)

Revival and evacuation equipment (Stage II)

Toboggans (sectional and one-piece)

Blankets

Sleeping bags

Tent

Headlamps for night operation

Floodlights (gas or propane)

Flares

Megaphone (battery-powered)

String line

Flagging and wands

Climbing skins (as needed)

Rope, ice axe, etc. (as needed).

Special rescue problems

Speed is essential, but safety must not be compromised. Rescuers have been buried and killed on

approach marches and at accident sites. In a classic

case near Snow Basin, Utah (March 27, 1964), a

rescue column crossed a slope above another column

and released a slab that killed a volunteer below. In

a worse tragedy, 62 workers perished in an attempted

rescue of a train stalled by an earlier avalanche

(Rogers Pass, B.C., March 5, 1910).

Where a second avalanche is a problem, the rescue

leader or accident site commander often faces a difficult decision: Should artificial release by explosives

be attempted on a slab that overhangs the accident

site? If the answer is yes, the victim's chances are

reduced considerably by the time spent in postponing

the rescue for explosive control, and also by the

deeper burial that may result should the avalanche

run. Each case must be considered individually. If the

avalanche is small and the victim has not been buried

for a long time, then a team of experienced rescuers

may gamble on a fast rescue before resorting to artificial release of the remaining hazard. The rescue

attempt should not persist for long, especially if storm

conditions intensify the hazard. If it is possible to

gamble briefly that a natural release will not occur, it

may be necessary to post guards to ensure that no one

enters the starting zone above the rescue operation.

For some terrain configurations, the guards may have

time to sound an alarm via megaphone or whistle if

206 SAFETY AND RESCUE
Figure 226.—First aid and support facilities are set up in a protected area. An avalanche guard is posted to warn the rescue

teain of further avalanches.

Special rescue problems 207
Figure 227.—-Weber County sheriff's patrol illuminates slope during avalanche rescue near Snow Basin, Utah, March 29, 1964.

the slab releases, whereupon rescuers could break for

cover. All Stage II and support equipment should be

in a safe location to begin with.

The well-being of the rescue columns is the first

consideration on approach marches into the back

country. It is foolhardy to take risks in the interest of

speed, since the object is most likely body recovery.

Approach routes should be chosen carefully, and rescuers should be led by experienced column leaders.

AH personnel should be properly equipped for travel

and prepared to withstand conditions on the approach

march and at the accident site. Volunteers should be

screened for fitness, experience, and personal gear.

Well-meaning volunteers from a ski area should not

be permitted to set out for an extended rescue under

severe winter conditions if they are equipped only

with downhill ski equipment (plastic boots, racing

garments, no heel lifts).

When weather conditions are favorable, helicopter

transport has an overwhelming advantage over a land

approach. The most efficient combination is helicopter transport of avalanche dog and leader. If an

avalanche dog is available, dog and leader should be

transported to the accident ahead of probing columns,

which remain on standby in case the dog fails.

The first flight from a heliport usually requires

daylight conditions; however, landing places and

floodlights may be set up so that flying operations

may continue into the evening. Heliports should be

equipped with strong canvas bags containing rescue

equipment. These may be arranged according to the

three stages of rescue.

Volunteers should receive instructions in helicopter procedures before joining the operation. Even

then, the pilot and accident site commander must be

208 SAFETY AND RESCUE
Gallagher, Dale, ed.

1967. The snowy torrents: avalanche accidents in the

United States 1910-1966. 144 p. USDA For. Serv.,

Wasatch National Forest. (Available from Rocky Mt.

For. and Range Exp. Stn., Fort Collins, Colo.) Accounts of numerous avalanche accidents in the U.S.

for the period stated. A good way to learn from the

experience of others.

Keiner. A., and D. Hanscom.

(n.d.) Wasatch tours. Salt Lake City: Wasatch Trails. (In

press.) A guide to ski lour routes and avalanche safety

in the Wasatch Mountains of Utah.

LaChapelle, E.

1961. The ABC of avalanche safety. Denver, Colo.:

Highlander Publishing Co. A small pamphlet giving

basic safety procedures in avalanche terrain.

Lathrop, T. G., M.D.

1972. Hypothermia: killer of the unprepared. 24 p. revised. Portland, Oreg.: Mazamas. An excellent article

that points out the dangers from what used to be called

exposure, ways to recognize the beginning of hypothermia, and steps to take to protect yottrself and others.

Figure 228.—Evacuation of victim from accident near Snow

Basin, Utah. March 30, 1964.

on the alert for carelessness caused by hypothermia

or fatigue.

Air transport is needed when large-scale disaster

strikes reinóte mountain settlements. Quite often,

road access is either cut off completely or limited to

oversnow vehicles. In preparation for emergency, any

threatened mountain settlement should select and

maintain a safe heliport area complete with floodlights for night operation. The community via its

sheriff should be aware of procedures for contacting

the nearest center for coordinating air rescue, including avalanche dog service. Victims buried amidst

building debris may survive lengthy burials, and rescue

operations should continue until all victims are found.

Further reading

Bowman, W. D., M.D.

1972. Winter first aid manual. 52 p. Denver, Colo.: National Ski Patrol System, Inc. Special emphasis on winter

and snow problems.

Maclnnes, Hamish.

1972. International mountain rescue handbook. 218 p.

London: Constable & Co.. Ltd. Well-illustrated, very

readable coverage of all types of mountain search and

rescue. Good material on avalanche rescue, avalanche

dogs, and helicopter techniques, as well as information

on use of ropes, cables, litters, searchlights, and radios.

Manning, Harvey.

1960. Mountaineering: the freedom of the hills. 430 p.

Seattle, Wash.: The Mountaineers. A general book on

all phases of mountaineering, including sections on alpine rescue, mountain weather, and snow and ice

climbing.

Tejada-Flores, Lito, and Allen Steck.

1972. Wilderness skiing. 310 p. San Francisco; Sierra

Club. A Sierra Club "totehook" easily carried in a pack.

Very readable with a common-sense approach. Covers

roule finding, physical characteristics of snow, orientation, ami weather and avalanche danger and ways lo

avoid it. Also a summary of emergency medical and

evacuation procedures.

Wilkerson, James A.

1967. Medicine for mountaineering. 309 p. Seattle, Wash.:

The Mountaineers. A fine coverage of medical problems

likely to he encountered on mountaineering expeditions

and how to handle them.

Williams, Knox.

1975. Avalanche fatalities in the United States, 1950-75.

USDA For. Serv., Res. Note RM-300. 4 p. Rocky Mt.

For. and Range Exp. Stn., Fort Collins, Colo. Reviews

U.S. fatalities by activities and localities; also gives

selected data from other countries.

Further reading 209

Converting units of measure

Appendix A

General conversion technique

There are many ways to convert from one system of

units to another. One of the simplest is cross-multiplication to cancel out undesired units. Three examples

of increasing difficulty illustrate this technique. The

reader is urged to work the given examples.

Example 1. Convert 24 inches to feet.

Solution:

24>rf-lil-]=2ft. U2>r/

Note that multiphcation is by a fraction equal to

one. The numerator and denominator are chosen to

cancel out the undesired units.

Example 2. Convert 10 miles per hour to feet per second.

Solution:

I ix^ / I I íHf / 13600 s y

Example 3. Convert 100 ounces per square inch to pounds per

square foot.

Solution:

im^] lA^] (}i^\[^^i^\ = 900 ib/ft^>.

I \Mi^ I [ lóprj V ift iV ift j

The metric system

There is a worldwide trend toward expressing all

measurements in one system of units, the metric system. Two basic metric units commonly used in avalanche work are the meter and the kilogram, which are

respectively units of distance and mass. These two

basic units are the building blocks for more complex

units of such quantities as speed, density, and stress.

Distance and speed. A meter (m) is a little more

than a yard; more precisely, 3.28 ft or 39.37 in.

Meters are used for measuring mountain elevations,

thicknesses of slab fractures, runout distances of avalanches, artillery range, etc.

Problem 1. The range of a 105-mm HE round fired from a

howitzer is 11,300 meters. Convert this to miles. (Problem

solutions appear at the end of Appendix A.)

Large distances are measured in kilometers (km),

which are equal to 10^ m, and small distances are

measured in millimeters (mm), which are equal to

10-3 ^

Problem 2. How many inches in diameter is a 75-mm projectile?

It is sometimes convenient to express certain measurements, such as the amount of new snow, in centimeters (cm). A centimeter is equal to 10-^ m, or

about 0.39 in. This means 1 in equals 2.54 cm.

Problem 3. John Doe wants to sell his old 7-foot skis. What

size should he advertise?

We all want to minimize confusion by using the

smallest possible number of different units. It is

argued that all our purposes are served if we stick to

units based exclusively on factors of 1,000; for example, megameters (10*^ m), kilometers (10^ m), meters

(1 m), millimeters (10-^ m), and micrometers

(10-^ m). Hence, the centimeter (10-- m) is not a

preferred unit. Nevertheless, it is a handy unit in avalanche work.

Units of area and volume are based on distance

units raised to the second and third powers, respectively (for example, m- and m^).

Problem 4. A standard density sample tube has a volume of

500 cm^ What is its volume in cubic inches?

Units of speed are based on distance divided by

time. Avalanche speed, for example, is expressed in

meters per second (m/s). A brisk walking pace is

about 2 m/s.

Problem 5. An avalanche is clocked at 50 m/s. What is its

speed in miles per hour?

Problem 6. An avalanche is triggered by a sustained precipitation intensity of 1 mm/h. What is this in inches per hour?

Mass and density. The amount of material, mass, is

based on the fundamental unit, the kilogram, which is

equal to 2.20 lb. Density is mass per unit volume and

is properly expressed in kilograms per cubic meter

(kg/m'^). The density of water is 1,000 kg/m^

Converting units of measure 211
Problem 7. The average density of a slab is measured as 200

kg/m-'. What is its density in pounds per cubic foot?

Snow density is sometimes expressed in grams per

cubic centimeter (g/cm-^). In these units, the density

of water has the convenient value of 1.0 g/cm^.

Problem 8. Show that 1,000 kg/m' equals 1 g/cm'.

Force and stress. The preferred unit of force is the

newton (N). The preferred unit of stress (force per

unit area) is newtons per square meter (N/m-). Unfortunately there is much confusion over force units.

Although it is not possible in this short appendix to

resolve the confusion, the reader should understand

that some of the confusion is due to the fact that forcemeasuring devices such as spring scales are calibrated

in units of mass (kilograms, grams) rather than units

of force (newtons). Thus, it is a common (although

poor) practice to speak of avalanche impact pressures, shear stress, ram strength, etc., in units of mass

per unit area (for example, kg/m- for pressure).

Problem 9. Use the equation on page 56 to calculate the shear

stress of a 2-m slab of density 250 kg/m" on a 38° slope (sin

38^ = 0.616).

Problem 10. Use the equation on page 86 to calculate the

impact pressure of an avalanche that has a speed of 50 m/s

and a flowing density of 100 kg/m^

Although the newton may not be familiar to many

readers, it is handy in force and stress computations.

For example, the impact force equation on page 86

is simply

Impact pressure = p Fwhere the impact pressure is now in N/m-, if density

is in kg/m^ and V (speed) is in m/s. Note that the

newton is equivalent to (kg) (m/s^), a result that is

derived from Newton's second law of motion: force

= (mass) (acceleration).

Problem 11. Use the data of problem 10 to compute impact

pressure in N/m".

The common unit of atmospheric pressure is the

millibar (mb), which is 10- N/m^. One bar (1,000

mb) equals 29.5 inches of mercury.

Problem 12. What is the atmospheric pressure in inches of

mercury at the 750-mb level?

Temperature. Snow and avalanche observations

are based on the Celsius temperature scale. A comfortable room temperature is slightly over +20° C.

Snow melts at 0° C and is cold enough to squeak at

—20° C. The formulas for converting from Celsius to

Fahrenheit and vice versa are:

F = |c + 32

C = |(F-32).

Problem 13. Convert —20*' C to the equivalent Fahrenheit

temperature.

Solutions to problems

Problem 1

Problem 2

Problem 3

Problem 4

Problem 5

(500 (^'[-ïiikV='' '''''

(-T:^) (-Th-J i^-^j 1^:2805?)-'•'"""'•

Problem 6

(^) (lè^) (riís)-»»-*

Problem 7

I 200>r\ /2.2 1b^ i_iZ!l^^-i9^iK/ftS [-^^) \-ïw) lx287r] -i^-^i^/^^'^-

Problem 8

(^^) (^) (KÍ&) = '-

Problem 9

Shear stress = p D ^ sin 6

= (250kg\ (2 m) (9.8 !n) (0.616)

\ m-^ / s3018 N/m-*

Problem 10

Impact pressure = ^—^

= 25,510

m

kg/m2 - 25.5 t/m2.

212 APPENDIX A
Problem 11

Impact pressure — pV-

^ nOOkg\ / 50m\2

Problem 12

(750 mb) i

Problem 13

1 bar

KOOOmb

] (29.5inHg\^^^j.^

7 \ 1 bar /

= 25x 104 N/m2. (-20) +32 = -4^

Conversion tables

TABLE Al.—Distance

iches Feet Yards Miles Centimeters Meters Kilometers

1 0.083333 omins — 2.540005 0.0254 —

12 1 0.33333 0.000189 30.48006 0.304801 0.000305

36 3 1 0.000568 91.44018 0.914402 0.000914

5280 1760 1 — 1609.347 1.609347

1.3937 0.032808 0.010936 — 1 0.01 —

39.37 3.280833 1.09361 0.000621 100 1 0.001

3280.83 1093.61 0.62137 — 1000 1

TABLE A2.—Area

Square

inches

Square

feet Acres

Square

centimeters

Square

meters Hectares

Square

kilometers

1

144

0.006944

1

43560

0.000023

1

6.451626

929.034

0.000645

0.092903

4046.87 0.404687 0.004047

0.155

,549.997

0.001076

10.76387

107,638.7

10,763,867

0.000247

2.471044

247.104

1

10,000

0.0001

1

10,000

1,000,000

0.0001

1

100

0.01

1

TABLE A5.— Volume

TABLE A3.-—Weight

Cubic Cubic Cubic Cubic

Ounces Pounds Grams Kilograms inches feet centimeters- meters

1

16

0.035274

35.27396

0.0625

1

0.002205

2.204622

28.34953

453.592

1

1000

0.453592

0.001

1

1

1728

0.061023

0.000579

1

35.3145

16.3872

28,317

1

1,000,000

0.028317

0.000001

1

TABLE A4.—Weight per unit >area

Pounds per

square foot

Pounds per

square inch

Kilograms per

square meter

Grams per

square

centimeter

1

144

0.204817

2.04817

0.006944

1

0.001422

0.01422

4.88241

703.067

1

10

0.488241

70.3067

0.1

1

TABLE A6.—Speed

Feet

per second

1

1.46667

1.68894

3.28083

Miles

per hour

0.681818

1

1.15155

2.236932

Knots

per hour

0.592086

0.868393

1

1.94253

0.91134 0.621370 0.539593

Meters

per second

0.304801

0.447041

0.514791

1

0.27778

Kilometers

per hour

1.0973

1.60935

1.85325

3.6

1

Converting units of measure 213
TABLE A7.—Grade percent and equivalent degree of slope TABLE A9.—Natural trigonometric functions

Grade Slope

(percent) (dcfirees)

5 2 51.7

10 5 42.6

15 8 31.8

20 11 18.6

25 14 2.2

30 16 42.0

35 19 17.4

40 21 48.1

45 24 13.7

50 26 33.9

55 28 48.6

60 30 57.8

65 33 1.4

70 34 59.5

75 36 52.2

80 38 39.6

85 40 21.9

90 41 59.2

95 43 31.9

100 45 0

Degree Cos Tan

TABLE A8.—Degree of slope and equivalent grade percent

Slope Grade

(degrees) (percent)

1 1.746

2 30 4.366

5 8.749

7 30 13.165

10 17.633

12 30 22.169

15 26.795

17 30 31.530

20 36.397

22 30 41.421

25 46.631

27 30 52.057

30 57.735

32 30 63.707

35 70.021

37 30 76.733

40 83.910

42 30 91.633

45 100.0

1 .0175 .9998 .0175

2 .0349 .9994 .0349

3 .0523 .9986 .0524

4 .0698 .9976 .0699

5 .0872 .9962 .0875

6 .1045 .9945 .1051

7 .1219 .9925 .1228

8 .1392 .9903 .1405

9 .1564 .9877 .1584

10 .1736 .9848 .1763

11 .1908 .9816 .1944

12 .2079 .9781 .2126

13 .2250 .9744 .2309

14 .2419 .9703 .2493

15 .2588 .9659 .2679

16 .2756 .9613 .2867

17 .2924 .9563 .3057

18 .3090 .9511 .3249

19 .3256 .9455 .3443

20 .3420 .9397 .3640

21 .3584 .9336 .3839

22 .3746 .9272 .4040

23 .3907 .9205 .4245

24 .4067 .9135 .4452

25 .4226 .9063 .4663

26 .4384 .8988 .4877

27 .4540 .8910 .5095

28 .4695 .8829 .5317

29 .4848 .8746 .5543

30 .5000 .8660 .5774

31 .5150 .8572 .6009

32 .5299 .8480 .6249

33 .5446 .8387 .6494

34 .5592 .8290 .6745

35 .5736 .8192 .7002

36 .5878 .8090 .7265

37 .6018 .7986 .7536

38 .6157 .7880 .7813

39 .6293 .7771 .8098

40 .6428 .7660 .8391

41 .6561 .7547 .8693

42 .6691 .7431 .9004

43 .6820 .7314 .9325

44 .6947 .7193 .9657

45 .7071 .7071 1.0000

214 APPENDIX A
G)

JZC

0)

CO

0)

G) k.

Ö) 0Q

¿1oi1Un

—-

_—

—

z

200-

—

^

-

z_

190-

-

z

180-

-

=

_-

170-

-

L

160-

—

—-

150-

-

^

1 /in_

—

- 1 'tU" - -

130-

-

^

—

—

120-

5

110-

-

—

-

z_

100-

-

z

90-

—

r_

80-

_=

z

—

70

3

__

—

—

60^

^

c;n"

—

_—

—

—

40—: 3— -

~"

30-3

3

20—

—

—_

--—_

10—

5

0-^

3

——

--

-10-3

L

-20—ï

-

100

90

80

70

60

50

40

30

20

10

COD

i2

(DÜ

CO0

CDk.

Ö)

(DQ

-10

-20

-30

Converting units of measure 215

Snowpit data

Appendix B

As mentioned in chapter 3, much information can

be gathered from a snowpit and a few simple instruments. A spiral-bound pocket notebook with waterproof paper is a convenient way to record the data.

It is good to write column headings and other preliminary information and reminders in the book before going to the field. This reduces the chances of

omitting vital information and speeds up fieldwork.

The data are often converted to a graphic display

called a snow profile, using symbols and a procedure

that are internationally understood and accepted

(UNESCO/IASH/WMO 1970). Notice that the

vertical scale against which the various features are

plotted is height above the ground; the snow surface

is not a suitable long-term reference for features in

the snowpack.

This appendix presents the international symbols

and measurements used to describe snow features, a

typical set of field notes, and the snow profile plotted

from the field notes.

International symbols and measurements

Grain type

Symbol

+ + +

+ + +

\ A> <

-V ^ *.

• • •

DDO

DDO

A A A

A A A

O O O

O O O

1—w~r

n ä i.

Description

Freshly deposited snow. Initial forms can

be easily recognized.

Irregular grains, mostly rounded but often branched. Structure often feltlike.

Early stages of ET metamorphism.

Rounded, often elongated, isometric

grains. End stages of ET metamorphism.

Grains usually less than 2 mm in diameter.

Angular grains with flat sides or faces.

Early stages of TG metamorphism.

Angular grains with stepped faces; at least

some hollow cups. Advanced stages of TG

metamorphism.

Rounded grains formed by MF metamorphism. Grains usually larger than 1 mm

and often strongly bonded.

Graupel. Occasionally appreciable layers

of this form of solid precipitation can be

identified in the snow cover.

Ice layer, lens, or pocket.

Hardness

Ram number

Symbol Description Hand test* (kg)**

Very soft Fist 0-2

Soft Four fingers 3-15

TX Medium hard One finger 16-50

Hard Pencil 51-100

XÏ Very hard Knife Over 100

Ice

* In the hand test the specified object can be pushed into

the snow in the pit wall with a force of about 5 kg. In hard

snow, for example, a pencil can be pushed into the snow, but

with the same pressure, a finger cannot.

** Ram number is from a cone penetrometer with a 60°

apex and 4-cm diameter.

Snowpit data 111
Grain size

(Average maximum grain diameter in millimeters)

Diameter Term

0-0.5 mm Very fine

0.5-1.0 mm Fine

1.0-2.0 mm Average

2.0-4.0 mm Coarse

More than 4.0 mm Very coarse

Free water content

Symbol Term

Dry

Moist

Wet

Very

wet

Slush

Description

Snow usually, but not necessarily, below 0° C. Grains have

little tendency to stick together

in a snowball when lightly

pressed in gloved hand.

Snow at 0° C. No water visible

even with hand lens. Snow

makes good snowball.

Snow at 0° C. Water visible as a

meniscus between grains but

cannot be pressed out by moderate squeezing in the hand.

Snow at 0° C. Water can be

pressed out by moderate squeezing in the hand. There is still an

appreciable amount of air confined within the snow.

Snow at 0°C. Snow flooded with

water and containing relatively

small amounts of air.

Snow surface deposits

Symbol Deposit Description

^ " ^ Surface hoar Sublimation crystals

formed directly on the

snow surface.

Soft rime White, opaque deposit

formed on objects by rapid

freezing of supercooled water droplets.

^ Hard rime Formed the same way as

soft rime but more compact

and amorphous.

^^""^ Glazed frost Coating of ice, generally

(Verglas or clear and smooth but usuglaze) ally including some air

pockets formed on objects

by freezing of films of supercooled water. Denser,

harder, more transparent

than rime or surface hoar.

These symbols are also used in snow profiles if the deposits

are identified inside the snowpack.

Snow density

Snow density samples are usually taken with cylindrical tubes. A tube 57 mm in diameter and 196 mm

long gives a convenient volume of 500 cm^. The tube

may be inserted parallel to the layering or vertical,

depending on whether one is interested in layer properties or integrated snowpack properties. Tubes are

weighed empty and then full of snow, and density

is computed.

Snow temperature

Snow temperatures should be taken in the northfacing wall as the pit is being opened up. Dial thermometers are sturdy and fast to use. When taking

readings near the surface, shade the area above the

thermometer to keep penetrating light from heating

the stem. It is good practice to insert the thermometer

and allow it to reach apparent equilibrium, then withdraw it and insert it in another place before taking a

reading. In crusty or very hard snow, use an ice pick

to punch a guide hole for the thermometer.

Total snow depth or height

The amount of snow on the ground is measured

vertically in centimeters, using the ground as a reference plane. On sloping terrain, slope angle must be

given. When taking snowpit data, measure ram number first. Then leave the ram in place and dig the pit

adjacent to it. This way the ram serves as a height

guide for recording features of the pit wall.

New snow depth or height

The daily new snowfall measured vertically from

the old snow surface.

Total snow thickness

The thickness of the snow cover measured in centimeters perpendicular to the slope, with the ground as

reference.

New snow thickness

The thickness of the daily new snowfall measured

perpendicular to the old snow surface.

218 APPENDIX B
Typical set of field notes

^--rtM + ^^

W+.of A/^,o-Ç Rxi Locafi'úA Pine- n4l f(aiv\ f+eijlit

l^beV- J31oWS He.'jl,!- of 'rujion P l^mW ¿tijov'e

Hi/Hwitr Point C/T] ^Wti/ij CW^ fi«,r^me^ u/t. = 1 K4

TfH in f P R ¿/N^

(9r^/ 3AOV/ su/ ^CLce. (lo

/ 0 Ö /i II 0 1 /7^

.? 0 0 )l 0 /7^ /)oUed( ¿^li/c/e 7^ù4 i- /-¡O-MMer

(o 5" 13 a r,'-/6 n m

J

3 /o âo 7 mi ^ï lió

H- ID 34 /t %.5 s iSk

8 10 ^.? If %-s:\ 7.7 ¡a

Í S ó't? Ä i^3^ H-.^' If-ô

iS 10 é^ /X %-iÂ I'i-.S Ià8

16 lo 7:L /Ù %-l^' n 118

i(> lo so 8 't-ao Si lio

S 10 u X \.fo ^A lùS

.3 2^ o lù8 ñdátd â'^ -^ech'ùA or ruLc

/5 i^ jo JO %'^i^ J2/ 100

¿ do fa 5- %-).'{ 47 1^

y JiO ¡05 8 'W^ /4 87

âO as ¡5 %"^i,^ f-é 7^

\ âo 158 :to %^\ -f 5-^

3o 10 If-'i- & 't'^so 5.3 U

35 ^^ ¡¿>0 Ik %"^^ S8 2o

Û ¿?.tr /7f /^ %-'S,k C.b /é

^ m 0 lé AdcltJíd'^áecÜnt. ofkU

Y \ iO l9o li> '^^.^ ^,lo Û Û^ÔÙOnJ J

Snowpit data 219
SnoW Pr¿LÍn DarcL

Location'. Paf^5 Í<M^ DCLU^ JiSrAar, ^7^-

Mdk\/eVûund

Cm

Ho

I90-/H

ß'^-ns

ó:ze

Í¡nry¡_

mr¿'u£¿i

l-%

Ù.S-I

ns-iu

1%-no

I7ö'l'^3

m'í'H>

m-Hi

Il3( 'lio

/.S-Ä

Q.S-i.ö

LS'^.ù

ûS-l.ô

Ho-91

\/.6ùH

V'óoilFtree

Wdher

mûL-sf

dr

XCf I'MKE.

SöH

Ö0Ç^

iUáL

tûedÀaùi

dù^

ci,

Qra.'i''

Y^L¿De.

ó^ars ^pLhs

W¡r\ci hrûk&n

/yiO^H[/ /)foken

.'Sp^ria-I c/endrii-es

^eit-tre^ze 'àz£

^

é

f

a:Œ Lf\HtR

¡ 2. nûMÀâùL

Jùf

di4-

attains

íe/íeJ old 6r\ûW

£T dkhhJ

ßarieJl óu/^race

hoa-r

M.es

K^noW ^^u.rtace.

ôld suryoOLce. crus/-

órnall rou.r\Jltdarcûr\S

^a^S A.JO

W^il bônJeà ßf-dif^i

vj'ij-k f-la.j- -ÇûLC&S-^

Q-n^ es

9/-S3' /-;? ô^£t M

f

More ClA

^

ks ^ /})/aJ. heá SurioLce @i'3 C/Vi

óíeps on races olûpe cf Ltc^ diir^cLC t ^'i^

Si'lo ^'3 hiLLçL drf ^Uf^amS 77)i^ la/i'.r> óh/Cíí í/\ pLce.

3Û-/7 .3-3^ ^ML ¿

f

W'cil formed

^

ra. t r\S

óomt CùLpS

loose ^pôôrly ¿oncied

ff^JCÙMÂ. J'f "¿jidL ^^y ^a./Ae, ij/pft Q-f

jra.ins QîJU ¿jrc^'nj fa./10^4 o^p'^T'

Uha&r- >tlooser Wa.d

220 APPENDIX B
i//Aie', li-'.dO ûSser. -ynyr^

Den.-5¡-H ¡e/n/Oerä^Tùcre^

Hf.ixAü)/& vil uu^ W. ^ l)ôns'i h Hh ß^hcvc 'Te/npúrouñcl '4^ s/)o\V ö/IDW QrOLLhC^ Ö' c

'(c^) (a>r)s) (^yy\fi) Kùlyy)'^ ' T'en.)

^Tcir^e wtf OC )Occ iuhe. - 3/0gAn3 /^7 0 /^

ÍS^ 3é^ 61 lis /SX '^ 1 /¿Ay»,

J-lo '^ [HXJ

b^ 39ÙÎ. îù% /¿/ ¡¿.ô -^ ^--^.

I^I^D -^

/>^f ^97t S7^Q. /7r lAD '8

lùO '^.

i3^ 4-/S /Dt) Jilo 30 ^fi

> áo -^

ilö^ ^10 1 bo Jlûb H-Ö -3

30 -X

löX i-H I3L ^"IX 15- '1

qrôc(.r\ci ô

7é> •i-ié /öG Ji/Si J

fS 397 87 ¡yiJS -^L'f ¡s.< 3/Ô

30 ^él'lí /óa.í ^ôS

y 4^1c Uo 3-Jio

Snowpit data 221
Snow profile plotted from field notes

^ 09 .CZ)

O CZ) Co

.¿r

1 fin.^

T

>

EIVIPE:R/S

-<3

RE (T

8

),°c

-10 -12

n i

=3

1 c^n. \\ 1 DU h \

140- J

>

1 H-U V^

\

^>^ ^-T

1 90. 1

V

S\

1 ¿LU' L.1

1 nn. \

1 uu

r

J ^

^

RDJ

rf^X

ou f

RH. (

^^-R

DU- j

__ 1

HUAH. r L^

-—Í¿

Of). 7

^^^

/

^

nuu^A

r1 1 0 20 30 40 50 60

>,

RAM INDEX (R), kg

Air temperature in sliade

222 APPENDIX B
Appendix C

International avalanche classification

A system of classification by observable properties of avalanching snow has been proposed by the

International Commission on Snow and Ice (De Quervain et al. 1973). This so-called morphological classification system takes into account three zones, which

correspond to the starting zone, track, and runout

zone:

Zone of origin. In this zone the appearance and

movement of an avalanche are characterized by the

manner of starting. For a slab avalanche the zone of

origin extends down to the stauchwall fracture; for a

loose-snow avalanche there is no sharp lower limit.

The zone of origin is seldom more than 100 m long.

Zone of transition. In this zone flow is independent

of the manner of starting. The speed may be increasing, steady, or decreasing. No particular avalanche

deposit is visible in this zone after movement has

stopped, except for snow retained by rough terrain or

narrow gullies.

Zone of deposit. In this zone a natural deposit is

produced by loss of energy due to friction and compaction. The zone of deposit can exhibit a wide range

of slope angles; it may even extend uphill on the

opposite side of the valley from the main part of the

avalanche path. For powder avalanches the zone of

deposit is the sediment zone of the snow dust cloud.

Letter-number symbols

Within each zone, several criteria are used in classifying avalanches. The list below gives the criteria for

each zone, along with the characteristics important to

each criterion. The letter-number symbols are used for

coding purposes.

Zone of Origin

A Manner of starting

Al Starting from a point (loose-snow avalanche)

A2 Starting from a line (slab avalanche)

A3 Soft slab

A4 Hard slab

B Position of sliding surface

Bl Weithin snow cover (surface-layer avalanche)

B2 New-snow fracture

B3 Old-snow fracture

B4 On the ground (full-depth avalanche)

C Liquid water in snow

Cl Absent (dry-snow avalanche)

C2 Present (wet-snow avalanche)

Zone of Transition

D Form of path

Dl Path on open slope (unconfined avalanche)

D2 Path in gully or channel (channeled avalanche)

E Form of movement

El Snow dust cloud (powder avalanche)

E2 Flow along ground (flow avalanche)

Zone of Deposit

F Surface roughness of deposit

Fl Coarse (coarse deposit)

F2 Angular blocks

F3 Rounded clods

F4 Fine (fine deposit)

G Liquid water in snow debris at time of deposition

Gl Absent (dry avalanche deposit)

G2 Present (wet avalanche deposit)

H Contamination of deposit

HI No apparent contamination (clean avalanche)

H2 Contamination present (contaminated avalanche)

H3 Rock, soil

H4 Branches, trees

Below are explanations of some of the criteria in the

list.

Manner of starting

Loose-snow avalanche. This type of avalanche

starts at a point and may be triggered by a falling

object (stone, ice chunk, etc.) or by a skier. In the

latter case the point fracture mechanism is obscured.

Slab avalanche. This type of avalanche starts at a

line. The origin of the movement may be propagated

as an invisible fracture from a distant point of initiation. (The term slab is often used synonymously with

slab avalanche. This should be avoided unless there is

no doubt about the correct meaning.) Slab fracture

may be observed without subsequent avalanche (often

related to the slow gliding movement of wet snow on

the ground).

Distinction between soft and hard slab may be

based on testing the snow at the fracture site, considering any changes that might have occurred, or, less

reliably, on the appearance of the avalanche.

International avalanche classification 223
• Soft slab. Broken snow layer is very soft, or soft

and of low density. Slab disintegrates into loose

material immediately after the start.

• Hard slab. Broken snow layer is medium hard,

hard, or very hard and of high density. A hard slab

preserves chunks or blocks over longer avalanche

paths depending on their roughness.

Position of the sliding surface

Within snow cover. If the sliding surface is within

the snow cover, it is called either a new snow fracture

or an old snow fracture. A new snow fracture is in a

uniform layer of snow deposited within 1 to 5 days

before the avalanche. An old snow fracture takes

place in the older snow layers. It thus contributes old

snow to the avalanche at the fracture line. A fracture

separating new snow from old snow is a new snow

fracture, even if the surface condition of the old snow

(surface hoar, sun crust, loose surface, etc.) favored

the fracture.

On the ground. If the avalanche slides on the

ground surface, full depth avalanche should be noted

even if some snow patches are left on the ground because of roughness of the ground.

Liquid water

A wet-snow avalanche requires liquid water to be

present throughout the avalanching layer; otherwise

the avalanche would be dry or mixed. Discrimination

may be difficult without considering genetic elements

(development of temperature, rain).

The classical term "ground avalanche," often used

as the opposite of "powder avalanche," is reserved for

heavy, wet spring avalanches that drag along rock or

soil material.

Form of path

Many channeled avalanches start as unconfined

avalanches and are concentrated in a channel only in

the lower part of their course. If the main part of the

path is channeled, an avalanche is characterized as

channeled; otherwise a mixed type is reported, describing the unconfined and channeled sections.

The longitudinal profile of an avalanche path is

often very important (changes in slope angle, intermittent steps). A quantitative description of the profile is better than an elaborate classification of ail possible terrain profiles.

Form of movement

Mixed types are very frequently observed. "Mixed

ñowing-powder avalanche," "powder avalanche with

flowing component," and "flowing avalanche with

powder component" are ways to characterize mixed

avalanches. A movement detached from the groundeither of powdery or flowing type—may be called a

"cascade."

Liquid water in snow debris

Large avalanches that are dry in the zone of origin

may pick up wet snow in lower parts of the track and

change their character. Wet snow in debris causes

hard and solid deposits, practically impermeable to

air, an important fact for rescue work and avalanche

clearing.

224 APPENDIX C
Appendix D

Avalanche reporting in the United States

X

Observers at about 30 locations in the western

United States and Alaska fill out forms to document

avalanche control activity and avalanche occurrences

within their area of interest. The reports may apply to

ski areas, mining areas, sections of major mountain

highways, or short stretches of secondary or private

roads. The U.S. reporting system is described and

illustrated below:

Symbol Type avalanche

HS Hard slab

SS Soft slab

WS Wet slab

L Loose

WL Wet loose

Trigger (activating agent)

N Natural

AS Artificial, ski

AE Artificial, hand charge

AA Artificial, artillery

AL Artificial, avalauncher

AO Artificial, other

To describe an avalanche these symbols are given

in sequence as listed. For example:

HS-AA-2-G Hard slab avalanche released by artillery.

It was of small size and ran to ground in

the starting zone.

SS-AE-4-0-J Soft slab avalanche released by hand

charges. It was large for that path and ran

on an old snow layer. Airblast was observed.

L-N-l-O A loose-snow sluff that released naturally

and ran on an old snow surface.

Avalanche classification is only part of the information routinely reported on avalanches in the United

States. The full report form and instructions for completing it are on the next few pages.

Size (based on volume of snow for the path in

question)

1 Sluff (Any slide running less than

150 feet (50m) slope distance regardless of other

dimensions)

2 Small (Relative to the path)

3 Medium (Relative to the path)

4 Large (Relative to the path)

5 Major or

maximum (Relative to the path)

Running surface

G Avalanche ran to ground in the starting

zone

O Avalanche rar1 on an old snow surface in

the starting zone

Airblast

Airblast was observed with the avalanche.

A valanche reporting in the United States 225
U.S. FOREST SERVICE

AVALANCHE CONTROL AND OCCURRENCE CHART

REVISED 10/71

TIME ¿f?. STANDARD AVALANCHE

CLASSIFICATION (3) z

o

i

u.

O

Ul

o. >

(4

FRACTURE

LINE z

-1

LL

O

o

Kjr

CONTROL

OR

OCCURRENCE

(1)

AVALANCHE PATH

AND/OR TARGET NAME < <

<a:

> m

< m

si 1- ^

o ai "s

(2)

TYPE

HS

SS

WS

L

WL

<

< in

ÙJ

N

o

o

i

i

i

(r

II

<

ONLY) %0F

TOTAL

AVAL.

AREA

THAT

SLID il

(6)

u.

-loi

<oÜZ

>Q

FT.

I

FT.

(0

ce

UJ>- <-J

(5

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Í 0 7 0 0 S r Pt N L e y 4 a S S A A '^ O 3 'Ä a 0 T R 3 O O

¿ 0 7 f S B E- fî. T H 0 u D ^ A L U S if Í S s A A 3 Ô M if 3 O T C 1 0 0

é 9- ^ I 7 D A M S S t^ 3 O 5 Ç D T R. 7 0 0

7 a q- 0 5 f=- u 0 ^ A L P A H K S s (Ni 4 0 4 B 7 ç M 3 O O

1 S 9- ^ 0 5 C u Ci R E- r^ T C R E E K ^4 P W L S s N St 0 a 7 ¿) r C â 0 O

3 c 0 ? 6 0 S T A hf L E y 4 1 K s A A ^ 0 M é 7 5 T L i 5 o O

3 0

3 0 0 li 0 0^ P Ac H 4 3

3 \_ 9.± 1 7 H 1 Cr l-l r R A I L C L i P F S L iv/ / 0 5 r s 0

~

^ ~~

"~

~ ~

~ ~~

~" ~

~ ~

~ ~

"~ ~

"" ~

~ ~

~ "~

"" ~

~

~

~ ~

(1) Use 24-hr clock-0800. 0915, 1530, etc. When (2) Control Measures: Col. 36 - Type of control; 1 = Ski. (3) Examples- HS-AA-2-G = Hard slab

time is unknown, enter 2405 if event is suspected in 2 - H.E.. 3 = 75mmR, 4 = 75mnnH. 5 = 105mmR. small size ran to around- SS-AEuiJ)

A.M.; 2417 if suspected in P.M. 6 - 105mmH, 7 - Avalauncher. 8 = other. ârtfficill hand rLrl («r^f. t^l„:

Col. 37 - No. of shots or ski passes or lbs. of H.E. If l?th » li«? . M í^' ? *' ^ '

greater than 9, enter 9. ^'*^ f J^'f*' L-N-l "0 = Loose snow.

Col. 38 - If snow fractured w/o avalanche, enter 1 ; **'^" ^^° *^- *'°P« distance, ran on ol

otherwise, blank.

irtif

J = Î

jno

nati

dsn

cial art

>oft slab

Id snow

jral, ran

ow sfc.

llery

sfc

less

226 APPENDIX D
AREA BefiTHO^ PAS^i 0.^4^0 STA. NO. > ^ aMONTH JAS/ YR. 7t PAGE < OF ^

Wli-UAMS

OBSERVERS

GC

03

LU

Û

U.

O

U

3

(7)

IF AVALANCHE AVALANCHE ACCIDENTS & DAMAGE (8)

REMARKS

(Please use for clarifying statements about any entries. Note unusual or interesting events,

give details on avalanche accidents, make dollar estimates on avalanche damage, etc. If

necessary, use more than one line.)

REACHED A ROAD PEOPLE VEHICLES 1 DAMAGE

MAX. TO

DEPTH

ON

CENTER

LINE

FT.

LENGTH

OF

CENTER

LINE

COVERED

FT.

1-

I

D

<

Q

UJ

tr

D

m

Q

LU

ÛC

D

i

Q

LU

-1

-J

QC

O

D DC

O m

Q

UJ

<

Q

z

D

-J

D

QQ

\-

u.

_J

^

S

59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

I SMS/I slide c^^fineX to rí¡j4i't s,de of paHn.

B

8 Ñ,i¿ki h^nJ^ Hot.

C 3 3 Q 0 R'di^ s^metX^e beirre é^ A, t^. \ fir^'fure lifi^ '^ ^00 '

foh^j rztin on oUi ice l^^/er ^' eidep uJkic/i ^^<í

•for/Pi-cçL in eah/^ D^c.

B 3 sef>:>r»'te si iJes ö¿ií?K^'^A

B ^ 0 9- 0 O 1 / Hit t/^pper rooJ-^ c^id- not t^e^ch lower roOcL

ba^acfe^a q^r&Jer pdirk^cL on roçiJL ¿it

c-d^ of PiifA. <^rÁc(er u/Bs Un c ccixf i^^.

d rhoiy /iVc^ ^o Section.

S d< sm^ll y/uf-fs,

(4) Type of motion: S = Sliding, F = Flowing or (6) Location of starting point: (8) Enter no. of people caught, partially or wholly buried.

Tumbling, P = Airborne Powder, M = Mixed Air & Col. 53 - T = Top, M = Middle, B = Bottom injured, and/or killed; no. of vehicles partially or

Ground Motion. Col. 54 - L = Left, C = Center, R = Right wholly buried and/or damaged; and no. of buildings,

(5) If fracture line is confined to new (storm) snow layer, (7) Where avalanche stopped: ¡ifts or miscellaneous damaged. Explain misc. in

enter A; if fracture line penetrated and includes old A = Fracture Zone. B = Transition Zone, Remarks column.

layer, enter B; if uncertain, leave blank. C = Foot of Runout Zone

A valanche reporting in the United States 227
U.S. FOREST SERVICE

AVALANCHE CONTROL AND OCCURRENCE CHART

Revised October 1971

14. The original must be received on or before the 10th of the following

month to be included in the monthly summary.

General Instructions

1. The U.S. Forest Service Avalanche Control and Occurrence Chart is

designed to give national uniformity to recordings of avalanche events.

The chart is intended as an aid in snow safety and avalanche control

programs and as a source of data for avalanche research.

2. The 80-column format facilitates data keypunching for computer usage.

The observer needs to fill out columns 8-80 and enter all pertinent

remarks.

3. These forms are printed on self-duplicating paper. Put the rear cover

cardboard flap behind the second sheet to give an original and one "carbon" copy. Test to be sure that the sheets are in the proper order to give a

"carbon" copy on the second sheet.

4. Use a medium-hard pencil. Please do not use a ballpoint pen. This way

mistakes can be erased and rewritten. Entries must be neat and legible.

Enter only one letter or number per column.

5. Use a separate line for each avalanche path controlled or for each avalanche reported. Feel free to use several lines in the Remarks column if

necessary.

6. Use as many sheets per month as needed.

7. Start a new sheet on the first of each month.

8. Fill in only data of which you are reasonably certain; otherwise, leave the

column(s) blank. Higher quality data are obtained if the entries are made

quickly following avalanche control work or observation. A small notepad

or tape recorder is useful for recording observations while outdoors. These

can then be transferred to the avalanche chart indoors.

9. If several columns are available to make a certain numerical entry and the

number of columns exceeds the number of digits to be written, the number should be right-justified with blanks appearing in the leftmost

coiumn(s). For example, if a 2-foot fracture line is observed, enter__2 in

Cols. 47-48, not 2._. Or if an avalanche fell 80 feet, enter 8 0.in Cols.

55-58, not_ 8 0_or_8 0 .

10. If an entry in a column or columns is the same for several consecutive

lines, ditto marks or a continuous arrow pointing downward in all

column(s) after the first is acceptable as long as legibility is maintained.

For example, if 10 avalanche events are recorded (using 10 consecutive

lines) on the 8th day of the month, enter 8 in Cols. 8-9 of the first line

(for the first avalanche) and follow this either with ditto marks in Col. 9

for lines 2 through 10 or with a continuous arrow in Col. 9 extending

from line 2 through 10. Either way will indicate that all 10 events

occurred on day 8. This convenience can be used for any columns which

have identical entries for several consecutive lines.

11. Upper left corner: Fill in names of the area, state (abbreviation), station

number, month, and year. State and federal highways with avalanche

problems are considered to be individual reporting stations, unless the

control work on these roads is done specifically to safeguard travel to a

particular ski area or mine, in which case the road bears the same station

number as the ski area or mine. Mark the first sheet of each month Page 1.

If all avalanche activity for the month can be recorded on one page, this

will read Page 1 of 1. If 2 sheets are needed, the first will read Page 1 of 2

and the second. Page 2 of 2.

12. Upper right corner: Print the name of the observer or observers.

13. At the end of the month, keep the "carbon" copy for your reference and

send the original to

Rocky Mountain Forest and Range Experiment Station

240 West Prospect Street

Fort Collins, Colorado 80521

Attn: Alpine Snow and Avalanche Project

Cols. 8-9: Enter the day of the month when avalanche control or occurrence took place. For days 1-9 make entry in Col. 9.

Cols. 10-13: Use 24-hour clock - 0800, 0915, 1520, etc. When time is

unknown, enter 2405 if event is suspected to have occurred in the

a.m.; 2417, if suspected in the p.m.

Cols. 14-32: Enter the name of the avalanche path. Please print; enter only

one letter per column starting at the left, and abbreviate if necessary. When many paths responded in the same way and a general

term is used here, list the actual slide paths in the Remarks

column.

Cols. 33-35: If the avalanche paths have been assigned numbers, enter the

3-digit number which corresponds to the avalanche name, such as

003, 021, etc.; otherwise, leave blank.

Control measures: columns 36-38. For natural avalanches, leave blank.

Col. 36: Enter the number indicating the type of control: 1 = ski, 2 = hand

charge or explosive, 3 = 75 mm. recoilless, 4 = 75 mm. howitzer,

5 = 105 mm. recoilless, 6 = 105 mm. howitzer, 7 = avalauncher, 8

= other (specify in Remarks).

Col. 37: Enter number of shots or ski passes or lbs. of H.E. If 9 or greater,

enter 9. Note: if one man skis across the path, this is one pass; if

two men cross, this is two passes.

Col. 38: If snow visually or audibly settles or fractures but does not avalanche, enter a "1"; otherwise, leave blank.

Standard avalanche classification: columns 39-45.

Cols. 39-40: Enter type of avalanche: HS = hard slab, SS = soft slab, WS = wœt

slab, L = loose snow (either column), WL = wet loose. Note: Slab

avalanches release from a line; loose snow avalanches, from a

point. A slab avalanche is soft slab if snow disintegrates into

loose material immediately after the start; hard slab, if angular

blocks of snow are preserved over long distances of the avalanche

path depending on the ruggedness. The distinction between hard

and soft slabs is based on the density and hardness of the snow,

with hard slab avalanches consisting of higher density snow

(-.30grams/cm3), It is often not clear whether an avalanche is

hard or soft slab; in such a case, soft slab is probably the more

correct entry. A slab or loose snow avalanche is called wet if free

water can be seen or squeezed from the debris, or if the debris is

obviously refrozen when inspected several hours after the event.

Cols. 41-42: Type of avalanche release or trigger: N = natural (either column);

AS = artificial, ski release; AE = artificial, hand charge; AA =

artificial, artillery; AL = artificial, avalauncher; AO = artificial,

other, such as backblast, snowshoe, snowcat, snowmobile, sonic

boom, etc. (specify in Remarks).

Col. 43: Avalanche size: The size of an avalanche is designated with a

number scale ranging from 1 to 5. A size 1 avalanche, or sluff, is

any snowslide running less than 150 feet slope distance (approx.

75 ft. vertical) regardless of its other dimensions such as width,

fracture line, etc. All other avalanches are classified by a number

2 to 5 that designates their sizes. This size classification is based

on the concept that size should convey an estimate of the volume

of snow that is transported down an avalanche path rather than

an estimate of threat to life or property. In addition, sizes 2 to 5

are reported relative to the slide path. A "small" avalanche is one

that is small (moves a small volume of snow down the path) for

the particular avalanche path; a "large" avalanche is large (moves

a large volume of snow down the path) for that path.

With these specifications in mind, the avalanche size classification

becomes:

1 = a sluff, or snowslide less than 150 ft. slope distance (approx.

75 ft. vertical) regardless of volume of snow

2 = small, relative to the avalanche path

228 APPENDIX D
3 = medium

4 = large

5 = major or maximum

Col. 44: Running surface: enter G if avalanche ran to the ground ¡n the

starting zone, 0 if avalanche ran on an old snow surface in the

starting zone.

Col. 45: Enter J if airblast was observed with avalanche; otherwise, leave

blank. Airblast should not be confused with the dust cloud

observed with many dry soft slabs. True airblast is the potentially

destructive strong wind that may extend well beyond the visible

dust cloud or moving snow front.

Col. 46: Type of motion: Sliding (S) is when the snow breaks loose and

moves downslope without rolling or tumbling. In flowing or

tumbling motion (F), the snow, whether granular or in blocks,

moves along the snow or ground surface in a rolling, turbulent

action. Airborne powder (P) refers to snow that billows up in a

dust cloud; motion is turbulent and very fast. Mixed air and

ground motion (M) is probably the most common with some

snow moving along the ground or snow surface in sliding or

flowing motion, and the rest billowing up as a dust cloud. Leave

this column blank unless the avalanche motion is clearly

observed.

Fracture line data: columns 47-49. Fill in only for slab avalanches.

Cols. 47-48: Estimate thickness or depth of fracture line at right angles to the

slope, to the nearest foot. If less than .5 foot, enter zero; .5 to 1

foot, enter 1. Remember only slab avalanches have fracture lines;

loose snow avalanches start from a point, not a line.

Col. 49: The fracture line of a slab avalanche may penetrate only into

newly deposited snow from the current or most recent storm, or

it may penetrate deeper into an old snow layer or layers. Entries

should be made in this column only if the observer is reasonably

certain of the snow layers penetrated in the starting zone. If only

new snow is involved, enter A. If an old layer is included, enter

B. If uncertain, leave blank.

Cols. 50-52: Estimate the percent of the total avalanche path affected by the

avalanche being reported. If, for example, an avalanche starts at

the top of its path, covers the entire width of the path, and runs

to the runout zone, 100 percent of the path is affected and

should be.recorded as 10 0. If an avalanche starts at the top of

its path, covers only the right half of the path, and stops halfway

down the track, only one-half the width and one-half the height

of the path are affected. This is 25 percent and should be

recorded as_2ê.

Cols. 53-54: This locates the starting area when the avalanche path is viewed

from below. In Col. 53, enter T, M, or B for top, middle, or

bottom, and in Col. 54, enter L, C, or R to indicate a fracture

line left, center, or right of the midline of the path. T, M, or B

with no suffix will indicate a fracture line extending across the

entire width of the starting zone or avalanche track.

Cols. 55-58: Give an estimate in feet of the vertical fall distance of the avalanche, not slope distance. On most avalanche paths, vertical fall

distances are considerably less than slope distances. For example,

on a 30®(58%) slope, vertical fall distance is only half the slope

distance. Topographic maps are helpful in making accurate

estimates.

Col. 59: Location of debris or where avalanche stopped: A = fracture or

starting zone; B = transition or bench partway down track; C =

bottom of track or runout zone.

If avalanche reached a road normally kept open in winter: columns 60-65.

Cols. 60-61: Estimate in feet the maximum depth of snow on the centerline.

Should this depth ever exceed 99 feet, enter 99.

Cols. 62-65: Estimate in feet the length of centerline covered by debris.

Should this length ever exceed 9999 feet, enter 9999.

Avalanche accidents and damage: columns 66-80.

Cols. 66-67: ^ Enter number of people caught by an avalanche. A person is

considered caught if he is in any way involved in the moving

avalanche snow. This can involve ski patrolmen or snow rangers

doing control work, recreational skiers, people in automobiles or

buildings, etc. If this person skis out of the avalanche or is

thrown completely to the snow surface when the snow stops

moving and is uninjured, he would be entered only in the

"caught" category. (See examples below.)

Cols.,68-69: Enter number of people partially or wholly buried by an avalanche. A person is considered partially buried if he is covered by

debris (after it has come to rest) anywhere from his ankles to his

neck. He is also considered buried if trapped inside a vehicle or

house which is buried. Every person entered in the "buried"

category is by necessity entered in the "caught" category also.

(See examples below.)

Cols. 70-71: Enter number of people injured by an avalanche. Every person

entered in the "injured" category is by necessity entered in the

"caught" category also. He may be entered in the "buried"

category too, if he is both buried and injured, which is quite

likely. (See examples below.)

Cols. 72-73: Enter number of people killed by an avalanche. Every person

entered in the "killed" category is by necessity entered in the

. "caught" category also. He may be entered in the "buried"

category too, if he is both buried and killed, which is likely.

However, it is not possible for one person to be both injured and

killed; being killed supersedes being injured.

Examples: Two skiers release a slab avalanche. One is momentarily caught by the moving snow but manages to ski out. The

other is carried downhill partly or wholly submerged, but when

the avalanche stops, he is completely on top of the snow and

uninjured. Enter 2 caught.

Two skiers are carried downhill by an avalanche, both are partly

buried, one is injured. Enter 2 caught, 2 buried, I injured.

A skier is caught by an avalanche, strikes a tree, and is injured

but not buried. Enter I caught, I injured. If he has been killed,

enter I caught, I killed.

A motorist is killed inside his car which is buried by an avalanche. Enter I caught, I buried, I killed, I vehicle buried, and

possibly I vehicle damaged.

Ten people are inside a cabin when it is buried by an avalanche.

Four people are injured and two others are killed. Enter 10

caught, 10 buried, 4 injured, 2 killed, 1 building darr.aged.

Should one of those injured later die from his injuries, reduce the

number injured from 4 to 3 and increase the number killed from

2 to 3.

Cols. 74-75: Enter number of vehicles caught, partially buried, or wholly

buried by an avalanche. This includes automobiles, trucks, snowmobiles, snowcats, snowplows, bulldozers, graders, etc.

Cols. 76-77: Enter number of vehicles damaged by an avalanche.

Col. 78: Enter number of buildings of any kind damaged by an avalanche.

Col. 79: Enter number of ski lifts damaged by an avalanche.

Col. 80: Enter number of other structures damaged by an avalanche. This

miscellaneous category includes bridges, roads, telephone lines

and poles, transmission towers, etc.

Remarks: This part of the chart is less formal than the entries by columns;

it is not keypunched, and therefore, remarks may be made in

longhand.

Be generous with remarks. Use for clarifying statements about

any entries. Note any unusual or interesting events such as extra

large slides, the presence of an ice or depth hoar layer in the

snowpack, etc. Give details on all avalanche avx:idents, and make

dollar estimates on avalanche damage. Use as many lines as

needed, skipping down that many lines before making the next

avalanche entry.

A valanche reporting in the United States 229

Literature cited

Appendix E

Anderson, Frank W.

1968. Frontier guide to the incredible Rogers Pass.

Frontier Book No. 8, 56 p. Calgary, Alberta: Frontier

Publ. Co.

Aspen Skiing Corporation.

n.d. Snowmass ski area safety plan. Aspen, Colo.: USDA

Forest Serv. and Aspen Skiing Corp. White River Nati.

Forest, Glenwood Springs, Colo.

Austrian Mountain Rescue Service of Tirol.

1962. Das ABC des Lawinenhundfuhers (In English as:

A primer for the avalanche dog leader. 74 p. USDA

Forest Serv. Alta Avalanche Study Center, Trans. 4.

Wasatch Nati. Forest, Salt Lake City, Utah.)

Beratungsstelle für Stahlverwendung.

1962. Lawinenverbauung in Stahl und Stahlbeton. (Avalanche control structures of steel and reinforced concrete.) 19 p. Bestell Nr. 316. Düsseldorf: Beratungsstelle

für Stahlverwendung.

Chapman, Herman H., and Walter H. Meyer.

1949. Forest mensuration, 522 p. New York: McGrawHill Book Co., Inc.

Cuno, John B.

1938. Increment borer and core technic. J. For. 36(12):

1234-1236.

Daniel, Mann, Johnson, & Mendenhall.

1972. Geophysical hazards investigation for the city and

borough of Juneau, Alaska: a summary report. 92 p.

(a special report prepared by DMJM consultants of

Portland, Oreg., for the city and borough of Juneau).

De Quervain, M., L. de Crecy, E. R. LaChapelle, K. Losev,

and M. Shoda.

1973. Proposal of the working group on avalanche classification of the International Commission on Snow

and Ice. Hydrol. Sei. Bull. 18(4): 391-402.

Dick, Richard A.

1968. Factors in selecting and applying commercial explosives and blasting agents. U.S. Bur. of Mines, Inf.

Circ. 8504. 30 p. U.S. Dep. of Inter. Bur. of Mines,

Washington, D.C.

DuPont de Nemours and Company.

1969. Blasters handbook—a manual describing explosives

and practical methods of use. 15th ed., 525 p. Wilmington, Del.: E. I. duPont de Nemours & Co., Inc.

El Huit, Ruby.

1960. Northwest disaster: avalanche and fire. 228 p. Portland, Oreg.: Binfords and Mort.

Fraser, Colin.

1966. The avalanche enigma. 301 p. London: John

Murray.

Frutiger, Hans.

1970. The avalanche zoning plan. 24 p. USDA Forest

Serv. Alta Avalanche Study Center, Transi. 11. Wasatch

Nati. Forest. (Copies available from Rocky Mt. For.

and Range Exp. Stn., Fort Collins, Colo.)

Frutiger, H., and M. Martinelli, Jr.

1966. A manual for planning structural control of avalanches. USDA For. Serv., Res. Pap. RM-19, 68 p.

Rocky Mt. For. and Range Exp. Stn., Fort Collins,

Colo.

Gallagher, Dale, ed.

1967. The snowy torrents: avalanche accidents in the

United States 1910-1966. 143 p. USDA For. Serv.,

Wasatch Nati. For. (Available from Rocky Mt. Forest

and Range Exp. Stn., Fort Collins, Colo.)

Gardner, Noel C, and Arthur Judson.

1970. Artillery control of avalanches along mountain

highways. USDA For. Serv., Res. Pap. RM-61, 26 p.

Rocky Mt. For. and Range Exp. Stn., Fort Collins, Colo.

Hastings, E. A., W. Bassett, B. Sandahl, and others.

1969. Alta avalanche control and safety plan. 51 p. USDA

For. Serv., Wasatch Nati. For., Salt Lake City, Utah.

Krisjansons, Juris.

1971. Jackson Hole ski area avalanche control plan. 21

p. USDA For. Serv., Jackson Ranger Dist., BridgerTeton Nati. For., Jackson, Wyo.

LaChapelle, Edward R.

1966. Encounter probabilities for avalanche damage.

USDA For. Serv., Misc. Rep. 10, 10 p. Alta Avalanche

Study Center, Wasatch Nati. For. (Available from

Rocky Mt. For. and Range Exp. Stn., Fort Collins,

Colo.)

Lawton, John G.

1969. Instruction manual: Skadi III avalanche search and

rescue system. 10 p. Available from Lawtronics, 326

Walton Drive, Buffalo, N.Y. 14226.

Magono, Choji, and Chung Woo Lee.

1966. Meteorological classification of natural snow crystals. J. Fac. Sei., Hokkaido University (Sapporo,

Japan). Series VII (Geophys.) 2(4): 321-335. (Also

available in reprint form from Rocky Mt. For. and

Range Exp. Stn., Fort Collins, Colo.)

Manning, Harvey, ed.

1960. Mountaineering: the freedom of the hills. 430 p.

Seattle, Wash.: The Mountaineers.

Mason, B. J.

1957. The physics of clouds. 481 p. London: Oxford

Univ. Press.

Literature cited 231
Mellor, Malcolm.

1965. Explosions and snow. Cold Reg. Sei. and Eng.,

Part III-A3a, 34 p. U.S. Army Corps Eng., Cold Reg.

Res. and Eng. Lab., Hanover, N.H.

Mellor, Malcolm.

1968. Avalanches. Cold Reg. Sei. and Eng., Part III-A3d,

215 p. U.S. Army Corps Eng., Cold Reg. Res. and

Eng. Lab., Hanover, N.H.

Mellor, Malcolm.

1974. A review of basic snow mechanics. U.S. Army

Corps Eng., Intern. Rep. 352, 42 p. Cold Reg. Res. and

Eng. Lab., Hanover, N.H.

Montagne, John, J. T. McPartland, A. B. Super, and H. W.

Townes.

1968. The nature and control of snow cornices on the

Bridger Range, Southwestern Montana. USDA For.

Serv., Misc. Rep. No. 14, 16 p. Alta Avalanche Study

Center, Wasatch Nati. Forest. (Available from Rocky

Mt. For. and Range Exp. Stn., Fort Collins, Colo.)

Morales, Benjamin.

1966. The Huascaran avalanche in the Santa Valley, Peru.

Proc. Int. Symp. Sei. Aspects Snow and Ice Avalanches,

p. 304-315. International Association of Scientific

Hydrology, Commission of Snow and Ice, Publ. No. 69.

Perla, Ronald I.

1968. Modern avalanche rescue. USDA For. Serv., Snow

Safety Guide No. 1, 94 p. Alta Avalanche Study Center,

Wasatch Nati. Forest. (Available from Rocky Mt. For.

and Range Exp. Stn., Fort Collins, Colo.)

Salm, Bruno.

1965. Contribution to avalanche dynamics. Proc. Int.

Symp. Sei. Aspects Snow and Ice Avalanches, p. 199-

214. International Association of Scientific Hydrology,

Publ. No. 69.

Schaerer, P.

1962a. Avalanche defences for the trans-Canada highway

at Rogers Pass. Div. Build. Res., Tech. Pap. 141 (NRC

7020), 35 p. Nati. Res. Counc, Ottawa, Canada.

Schaerer, P.

1962b. The avalanche hazard evaluation and prediction

at Rogers Pass. Div. Build. Res., Tech. Pap. 142 (NRC

7051), 32 p. Nati. Res. Counc, Ottawa, Canada.

Schaerer, P.

1962c. Planning avalanche defence works for the transCanada highway at Rogers Pass, B.C. Eng. J. 45(3):

31-39. (Also: Div. Build. Res., Res. Pap. 152. Nati.

Res. Counc, Ottawa, Canada.)

Schaerer, Peter A.

1966. Snowshed location and design. J. Highw. Div. ASCE

92(HW2): 21-33. (Also: Div. Build. Res., Tech. Pap.

252 (NRC 9676), Nati. Res. Counc, Ottawa, Canada.)

Schild, M.

1965. Durch Lawinen verursachte Unfaelle und Schaeden.

In Schnee und Lawinen in den Schweizeralpen Winter

1963/64, p. 73-122. Winterbericht des Eidg. Institutes

fuer Schnee- und Lawinenforschung Weissfluhjoch/

Davos. (Part of this article in English as: The WernerHenneberger Accident, USDA For. Serv., Alta Avalanche Study Center, Transi. 5, 14 p. 1966. (Available

from Rocky Mt. For. and Range Exp. Stn., Fort Collins,

Colo.)

Schleiss, V. G., and W. E. Schleiss.

1970. Avalanche hazard evaluation and forecast: Rogers

Pass, Glacier National Park. p. 115-121. Ice engineering

& avalanche forecasting and control. Proc. of Conf. at

Univ. Calgary, 23-24 Oct. 1969. (Also: Nat. Res.

Counc, Tech. Memo 98, Nat. Res. Counc, Ottawa,

Canada.)

Sommerhaider, E.

1966. Lawinenkraefte und Objectschutz. Winterbericht

des Eidg. Institut fuer Schnee- und Lawinenforschung,

Nr. 29, 1964/65. (In English as: Avalanche forces and

the protection of objects. 14 p., Alta Avalanche Study

Center, Transi. No. 6, 1967. Available from USDA

For. Serv., Rocky Mt. For. and Range Exp. Stn., Fort

Collins, Colo.)

Sommerhaider, E.

1972. Ahlenkverbau. Lawinenschutz in der Schweiz

Buenderwald, Beiheff Nr. 9, p. 155-169. (In English as:

Deflecting structures, avalanche protection in Switzerland, p. 117-127. USDA For. Serv., Gen. Tech. Rep.

RM-9, 1975.)

Swiss Federal Institute for Snow and Avalanche Research

(SFISAR).

1961. Lawinenverbau im Anbruchgebiet: Richtlinien für

den permanenten Stützverbau. Mitt. 15. Bern: Eidg.

Institutes für Schnee- und Lawinenforschung Herausgegeben durch Eidg. Inspektion für Forstwesen. (In

English as: Avalanche control in the starting zone:

guidelines for the planning and design of permanent

supporting structures. Transi, by Hans Frutiger and

published as Stn. Pap. 71, 60 p., 1962. USDA For. Serv.,

Rocky Mt. For. and Range Exp. Stn., Fort Collins,

Colo.)

UNESCO/IASH/WMO.

1970. Seasonal snow cover. 38 p. Technical papers in

hydrology No. 2, United Nations Educational, Scientific,

and Cultural Organization; Place de Fontenoy, 75,

Paris-7°, France.

U.S. Department of the Army.

1957. Operation and organizational maintenance: 75-

MM rifles M20 and T21E12, tripod mount M1917A2,

and weapon tripod mount M74. U.S. Dep. Army Tech.

Man. 9-3140, 186 p. Headquarters, Dep. Army, Washington, D.C.

U.S. Department of the Army.

1958a. Crew-served weapons and gunnery. U.S. Dep.

Army, ROTC Man. 145-41, 589 p. Headquarters, U.S.

Dep. Army, Washington, D.C.

232 APPENDIX E
U.S. Department of the Army.

1958b. 106-MM rifle M40A1. U.S. Dep. Army, Field

Man. 23-82, 235 p. Headquarters, U.S. Dep. Army,

Washington, D.C.

U.S. Department of the Army.

1959. Explosives and demolitions. U.S. Dep. Army, Field

Man. 5025, 215 p. Headquarters, U.S. Dep. Army,

Washington, D.C.

U.S. Department of the Army.

1962. 75-MM pack howitzer MlAl. U.S. Dep. Army,

Field Man. 6-78, 141 p. Headquarters, U.S. Dep. Army,

Washington, D.C.

U.S. Department of Agriculture, Forest Service.

1972. Forest Service health and safety code. USDA For.

Serv., Handb. 6109.13, Amend. No. 5, p. 185-411.

Vanni Eigenmann International Foundation.

1963. Sauvetage des victimes d'avalanches. Proc. Symp.

held at Weissfluhjoch, Davos, Switzerland, 18-20 Jan.

1963, 63 p. Fondation Internationale Vanni Eigenmann,

Milano, Italy.

Voellmy, A.

1955. Über die Zerstörunskraft von Lawinen. Schweiz.

Bauzeitung, Jahrg. 73, s. 159-165, 212-217, 246-249,

280-285. (In English as: On the destructive forces of

avalanches. 63 p. USDA For. Serv., Alta Avalanche

Study Center, Transi. 2, 1964. Available from Rocky

Mt. For. and Range Exp. Stn., Fort Collins, Colo.)

Wagner, Oswald, and Joseph Hopf.

1959. Nouvelles données scientifiques sur la neige, la

couverture neigeuse, les avalanches et leur correction.

88 p. Organisation des nations unies pour l'alimentation

et l'agriculture. Document d'information No. 8, Rome.

(FAO/EFC/TORR. 23, 15 Juin 1959; FAO/59/7/-

5547)

Washington, State of.

1972. Helirescue manual. 35 p. Dep. Emergency Serv.,

State of Washington.

Williams, Knox.

1975. The snowy torrents: avalanche accidents in the

United States 1967-71. USDA For. Serv., Gen. Tech.

Rep. RM-8, 190 p. Rocky Mt. For. and Range Exp.

Stn., Fort Collins, Colo.

Literature cited 233

Index

Accident site commander, 204, 205, 206,

208

Airblast from an avalanche, 81, 83, 89,

90,171

Air stress waves from explosives {see

Stress waves)

Air temperature gradient (see Temperature gradient)

Albedo,33

Ammunition, artillery, 136-139

Anemometer, 29

Angle of repose, 66, 67, 68

Anticyclonic circulation, 10, 13

Artificial avalanches (see Avalanche)

Artificial respiration, 201, 202

Artillery control, 136-143, 158-160

Artillery weapons, 138

Avalanche

artificial (trigger), 69

classification, 223-224

definitions, iii

natural (trigger), 69

size, 225

types, 66-69, 78, 225

Avalanche accidents, 3-5, 178-181

Alta, Utah, 2, 3

Bingham Canyon, Utah, 1, 4

Grand Due Mine, B.C., 5

Huascaran, Peru, 3, 86

Jane Camp, Brittania Mine, B.C., 4

Ophir, Utah, 1

Rogers Pass, B.C., 4, 206

Silver Plume, Colo., 3

Snow Basin, Utah, 206

Sun Valley, Idaho, 2

Telluride, Colo., 3

Terrace, B.C., 5

Twin Lakes, Colo., 5

Wellington, Wash., 4, 180

Werner-Henneberger, 181

in backcountry, 178, 179

in buildings, 180

on highways, 180

on railroads, 180

on ski areas, 179

Avalanche beacon (see Transceivers,

electronic)

Avalanche control plan, 143, 144, 145

Avalanche control routes, 144, 146

Avalanche control signs (see under Signs)

Avalanche cord, 188

Avalanche debris, 81, 86, 180, 186, 190,

193

Avalanche dog, 190, 196-200, 203, 208,

209

Avalanche frequency, 85, 97

Avalanche guard (lookout), 128, 206,

207

Avalanche motion, 85-90, 223

airborne powder, 85

flowing, 85-89

mixed, 85, 86, 87, 89

Avalanche path, 76

identification, 81-84

profiles, 80

Avalanche reporting, 225-229

Avalanche sheds, 153, 154, 161

Avalanche signs (see under Signs)

Avalanche speed, 88, 89

Avalanche track, 76, 77

channeled, 78, 79

unconfined, 78, 79

Avalanche trigger, 69, 74, 98, 178, 184,

187,225

Avalanche victim, 178, 186, 187, 188,

189, 191, 194, 195, 196, 197,200

burial time, 178,200

care of, 200-203

Avalanche warning, 173-175

Avalanche zoning (see Zoning)

Backblast from recoilless riñe, 136, 139,

159

Back-country accidents, 178, 179

Back-country safe travel, 186-190

Barometric pressure, 9, 11

Bed surface, 70, 71, 73, 74, 75, 186, 223,

225

Belay, 104, 105, 106, 108, 118, 130, 131,

132,186

Blaster-in-charge, 129

Blasting cap, 123, 125, 129,131,132

Blowing snow

average particle size, 27

density of deposited layer, 27

rate of deposition, 27

Boot packing, 115, 117, 121

Borehole blasting, 123,132, 133, 134

Boresighting, 139

Branch line for explosives, 131, 133, 134,

135

Brittle fracture of snow,'53, 54, 55, 66,

119

Burial time in an avalanche (see under

Avalanche victim)

Buried charge, 121, 123

Burning time {see Fuse, burning time)

Cap crimper, 125, 126

Cap-fuse assembly, 124, 125, 160

Cardiac resuscitation, 201, 203

Cast primers

(See under Primers)

Chinook winds, 12

Climax plant species

as related to avalanche paths, 83

as related to revegetation, 157

Closure for avalanche control, 114, 115

Closure signs {see under Signs)

Coarse probing {see under Probing)

Collapse failure or fracture {see under

Fracture)

Collapse noise, 106

Column leader, 205, 208

Compaction for stabilization of slopes,

115-117

Compressive stress equation, 56

Condensation nuclei, 18

Controlskiing, 117, 118, 121

Convergence, 10, 13

Conversion of units of measure, 211-215

Conversion tables

area, 213

degree of slope, 214

distance, 213

grade percent, 214

natural trigonometric functions, 214

speed, 213

temperature, 215

volume, 213

weight, 213

weight per unit area, 213

Core sample from snowboards, 16

Corn snow, 51, 183

Cornice

control, 119, 123, 130, 131, 132, 133,

155,156

growth and location, 28

Creep, 57,74, 158

Crown, 69, 70, 97

Crown surface, 69, 70, 97

Crown thickness, 71

Crystal {see Snow crystal)

Cyclonic circulation, 10

Index 235
Deep slab instability (see under Instability of snowpack)

Deflecting and retarding structures, 150,

151,152

Deformation of snow, 43, 52, 53, 55

Density of explosives, 119

Density of snow (see Snow density)

Depth hoar, 46, 59, 98

Detonating cord, 126, 127, 131, 133, 134

Detonation pressure, 119, 121

Detonation speed of explosives, 119

Detonation systems for explosives

electric, 123, 129

nonelectric, 123, 129

Dewpoint, 12, 104

Direct protection structures, 150, 153,

154

Direct sighting of artillery, 140, 141

Divergence, 10, 13

Drugs for first aid use, 202

Dry-snow avalanches, 89

Dual-hazard index, 173

Duds or misfires, 120, 125, 130, 136, 138,

139,142, 143

Earth mounds or dams, 150, 152, 156,

161

Earth ramp or wedge, 153, 154, 156

Eddy panel, 155

Electronic transceivers (see Transceivers, electronic)

Encounter probability of avalanches,

171,173

Energy, elastic strain, 74

Energy exchange with atmosphere, 33,

34,40,100

Energy required for melting snow, 49

Equations

compressive stress, 56

encounter probability, 173

impact pressure, 86

shear stress, 56

ram number, 59, 219

water equivalent index, 175

Escape route, 187

Exploding-bridge-wire caps, 129

Explosive control, 117-119, 161

Explosive testing for instability, 108

Explosives used in avalanche work, 120

Fade point, 195, 196

Failure of snow layers (see Fracture)

Fences (avalanche), 151

Field notes, 219-222

Fine probing (see under Probing)

Flank surface, 69, 70, 122

Foehn winds, 12

Fracture or failure of a snow layer

collapse of weak layer, 74, 183

in shear, 74, 75

in tension, 74, 75

Fracture propagation, 106, 107

Fracture toughness, 54, 55, 104

Free water, 49, 50, 51, 97, 218

Freezing nucleus, 18

Fronts, weather, 11, 12, 104

Fuse, artillery, 137

Fuse, burning time (or rate), 124, 129

Fuse-cap assembly (see Cap-fuse assembly)

Gelatin primers (see under Primers)

Glaze, 35, 218

Glide, 57, 74, 100, 158

Glide cracks, 100

Glide surface, 100

Grains, snow (see Snow grains)

Graupel, 20, 40, 45, 57, 60, 67, 73, 98,

99, 100,217

Ground stress waves from explosives

(see under Stress waves)

Ground surface, 40

Hardness of snow, 217

Heat exchange (see Energy exchange)

Heat of fusion, 49, 101

Helicopters for avalanche control, 130,

159,160

Howitzers, 136, 137, 139, 159, 166

Hypothermia, 134, 178, 200, 201, 208

Ice lens, 51

Ice skeleton, 41, 42, 43, 44, 45

Igniter (see under Safety fuse)

Impact pressure

equations, 86

estimates, 88, 89, 90

related to damage, 89

related to zoning, 171

Inclined table test for snow stability, 165

Indirect sighting of artillery, 140, 142

Inputs for stability evaluation (see under

Stability evaluation)

Instability of snowpack

deep slab, 76, 98, 100, 104, 118, 121,

122,163,183

new snow, 102, 106, 163, 165

wet snow (thaw), 99, 100, 101, 163,

183

Instrumentation

air temperature, 32, 33

atmospheric pressure, 8

dial-stem thermometers, 61, 62

for snowpit study, 62

ram penetrometer, 58, 59, 62

snow depth (height), 15

wind, 29

Inversion, temperature, 34

Isothermal snowpack, 40, 47, 183

Jet roof, 156

Kolktafel (see Eddy panel)

Lapse rate, 12

Last-seen area, 190

Lifting of air masses

cyclonic, 13

frontal, 11, 12, 13

orographie, 12, 13

Loose-snow avalanche, 66, 67, 68, 183,

223

Lubrication, 51,52, 100

Medical supplies for avalanche accidents,

159,202

Metamorphism, 42-52

ET metamorphism, 43, 44, 46, 47, 61,

73

MF metamorphism, 43, 49, 51, 98, 99

TG metamorphism, 43, 45, 46, 47, 48,

59,61,73,74,98, 115, 122

Microbarograph, 8

Misfires (see Duds)

Natural avalanches (see under Avalanches)

Necks between grains, 44, 45, 46, 54

New snow instability (see under Instability of the snowpack)

Overshoot guard, 139, 140

Penetration test for new snow stability,

165

Percolation, 50, 51

Pioneer plant species as related to avalanches, 83, 157

Point detonating fuse (see Fuse, artillery)

Pore space, 41

Postcontrol avalanches, 75, 76, 122, 137,

179, 180

Precipitation as related to terrain features, 13, 14, 15

Precipitation, solid, 13, 14, 15, 16, 17

classification, 19, 20, 21

distribution, 13, 14

intensity: typical values, 15; critical

values, 100

methods of measuring, 15, 16, 17

236 Index
Preplanted charges, 160

Pressure, barometric, 9,11

Primers

cast, 120, 121, 125, 132, 133, 135, 160

gelatin, 120, 121, 126

Probability estimates, 94, 95

Probes, avalanche, 104, 159, 186, 188,

190, 191,205,206

Probing for avalanche victims, 190-193

coarse, 191, 192, 196,205

fine, 191, 192, 193

Protective skiing {see Control skiing)

Pulpit, 156

Radiation (^^e Energy exchange)

Radiation-recrystallized grains {see under Snov/ grstins)

Radius of influence of an explosive, 122

Ram data, 219

Ramnumber, 59, 61,217

Ram penetrometer, 58, 59, 62

Rayleigh waves, 119

Recoilless rifles, 111, 136, 137, 159

Reforestation of avalanche paths, 156,

157,158

Regelation, 81

Rescue caches, 159, 206

Rescue leader, 204

Rescue plan, 204, 206

Rescue procedures, 203-209

Rescue stages, 204, 205

Stage I immediate action, 205

Stage II revival and evacuation, 205,

208

Stage III support, 205, 206

Restrictions for avalanche control, 113-

115

Resuscitation, 200, 201, 203

equipment, 200, 201, 202

Return interval of avalanches, 171

Rime and riming, 18, 19, 20, 30, 102, 218

Road closures

for artillery shooting, 160

for avalanche control, 161

Road relocation, 161, 162

Rolling type of snow transport, 27

Rope, mountaineering, 186

Rotten snow, 51

Route selection, 181, 182, 184, 185

Running surface {see Bed surface)

Runout zone of an avalanche path, 76,

80,81

Safety fuse, 123, 124, 127, 129, 131

igniter, 123, 126, 129

Saltation type of snow transport, 27

Saturation vapor pressure {see under

Vapor pressure)

Shear failure {see under Fracture)

Shear frame, 165

Shear frame test of strength, 73, 165

Shear strength of snow, 54

Shear stress

equation, 56

of slab against bed surface, 74

of wind on snow, 25, 32

of wind related to terrain, 23, 24

Short rounds, 139

Shrapnel, 136, 138, 143

Signs

avalanche warning, 112, 113, 115, 167

closure, 114

Sintering, 44, 45, 55, 67

Ski testing {see Test skiing)

Slab

density, 71

fracture, 74-76, 184

hardness, 17, 217

stratigraphy, 73

substratum, 70, 73

temperature, 73

wetness, 73

Slab avalanche, 66, 67, 69, 71, 72, 74, 85,

223

Sliding surface {see Bed surface)

Sluffs or slufling, 68, 69, 97, 98

Snow as a material, 52

Snow bridges, 151

Snow cover distribution, 96, 97

Snow crystal

classification, 19, 20, 21

definition, 41

fall rate, 25

formation in atmosphere, 18, 19, 22

size, 19

structure and growth, 18, 19, 20, 21,

22

Snow cushion, 56, 57

Snow density, 218

as related to metamorphism, 47

changes with time, 41, 42

definition, 15

ice crusts, 41

in avalanches, 86

in cornices, 29

thick layers, 41

typical values, 15, 55

windblown snow, 27

Snow depth, 17,218

Snow devils, 25

Snow-dust cloud, 85, 86, 88, 89

Snow grains, 41, 217

ET, 48, 50,73,217

MF, 50, 51,217

radiation-recrystallized, 98, 100

size, 218

TG, 46, 47, 73,98,99, 122,217

type, 61,217

Snow hardness, 217

Snow height (or snow depth), 17,218

Snow nets, 151

Snow profile, 61, 222

Snow rakes, 151

Snow settlement rate, 42

Snow stress waves from explosives {see

///zi/<?r Stress waves)

Snow surface, 40

deposit, 218

Snow surface deposits, 218 {see also Surface hoar, Rime, and Glaze)

Snow temperature, 61, 218

Snow thickness, 17, 218

Snow transport by wind

rolling, 26

saltation, 26, 27

turbulent suspension, 26, 27

Snow wetness {see Free water)

Snowboards, 15, 17

Snow-cover thickness, 17, 218

Snowfall

rate, 15

typical value, 15

Snowfenees, 150, 154, 155

Snowflake, 20

Snowpits, 58, 59, 60, 61, 98, 217-222

data, 219-222

Snowstake

master, 15

portable, 15

Snow-temperature gradient, 40

Solid precipitation {see Precipitation,

solid)

Sonic boom, 119

Stability evaluation (avalanche), 15, 57,

93,94,95, 100, 143, 163,182

critical conditions for, 100, 101

field tests of new snow, 165

inputsfor, 95, 96-108

test skiing, 104, 106, 107, 117,118

Standard avalanche charge, 121

Starting zone, 76, 77, 80, 99, 184, 186,

187

Stauchwall, 69, 70, 122

Storm period, 35

Storm plot, 35, 36, 100, 101

Stratigraphy, 60, 73, 222

Stress

compressive, 56, 57

shear, 56

tensile, 56

Stress waves from explosives, 119, 123

Index 237
Structural control of avalanches, ISOISO, 161

Study plot, 17

Sublimation, 32, 40, 45, 49

Suction pump, 200, 201, 202

Suffocation, 178

Surface hoar, 34, 41, 57, 60, 98, 99, 218

Surface search for avalanche victims or

clues, 190

Supporting structures in starting zone,

148, 150, 151, 154, 156, 161

Sympathetic release, 139

Temperature gradient, 33, 48

in air, 33

in snow, 48, 100

Temperature release of avalanches, 75

Temperature trends, 36

Tensile cracks, 100

Tensile failure (see under Fracture)

Tensile stress (see under Stress)

Test skiing for instability, 104, 106, 107,

117,118

Thermal conductivity of snow, 75

Thrust of explosives, 119

Track (see Avalanche track)

Transceivers, electronic, 104, 179, 186,

188, 194-196

Turbulent suspension type of snow transport, 27

Upper air data as source of wind data,

31, 103

U.S. Code of Federal Regulations for

explosives, 127

Vapor pressure, 18

saturation, 18, 43

supersaturation, 22

Vegetation as related to avalanches, 82-

84

Victim (see Avalanche victim)

Viscoelastic properties of snow, 52

Vortex, 25

Water equivalent

critical value, 100

index, 175

typical value, 15

Wet-snow avalanche, 88, 97, 183

Wet-snow instability (see under Instability of the snowpack)

Wind baffles, 150, 155, 156

Wind data, 29, 30, 31

Wind direction

relative to pressure contours, 10

relative to terrain, 23, 24

Wind shear (see Shear stress)

Wind transport of snow (see Blowing

snow)

Windspeed

gradient, 24, 25

needed to move snow, 26

Zoning of avalanche areas, 168, 170-173

jîr U.S. GOVERNMENT PRINTING OFFICE: 1976 O 587—973

238 Index

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