S N O
W A V A L A N C H E
their
characteristics, forecasting and control
by Edward R.
LaChapelle
Avalanche Hazard Forecaster
Wasatch National Forest,
Utah
U.S. Department of Agriculture
Introduction
Snow avalanches are active geological agents of
erosion and have been a source of natural disasters as long as man has
dwelled in the mountains. A common feature of mountainous terrain
throughout the temperate and arctic regions of the earth, they may fall
wherever snow is deposited on slopes steeper than about 30 degrees. Small
avalanches, or sluffs, run in uncounted numbers each winter, while the
larger avalanches, which may encompass slopes a mile or more wide and
millions of tons of snow, fall infrequently but inflict most of the
destruction. The avalanche hazard arises whenever man and his works are
exposed to sliding snow. Such hazard has been familiar to inhabitants of
the Alps and Scandinavia for many centuries, while it is a more recent
development in other parts of the world.
Avalanches run in the same
paths year after year,the danger zones often being well known in normal
circumstances. Exceptional weather at intervals of many years may produce
exceptional avalanches which overrun their normal paths and even break new
ones where none existed for centuries. Unwise timber removal in alpine
terrain can also create avalanches where none existed before. Given
exceptional snow conditions, even short slopes like the walls of a ravine
can become dangerous. Snow avalanche may fall anywhere that enough snow is
deposited in the right circumstances on an inclined surface.
These
right circumstances sometimes consist of abnormally large snowfalls, but
not always so. Avalanches find their genesis in snow cover structural
weaknesses which are often induced by internal changes. A large overburden
of snow alone may not result in avalanching if it is anchored to a solid
underlayer. On the other hand, even a shallow snow layer can slide from
the mountainside if poorly bonded. The snow avalanche is a complex problem
in mechanical stability which can best be understood in terms of the
physical processes taking place in the changeable winter snow and the
dependence of these on temperature.
Types of Avalanches and Their
Characteristics
The wide variety of snow avalanche origin,, nature of
motion, and size reflects the highly changeable nature of snow. The
fundamental classification of avalanches is based on conditions prevailing
at the point of origin, or the release zone. There are two basic types,
loose snow and slab avalanches. Each is subdivided according to whether
snow involved is dry, damp or wet, whether the slide originates in a
surface layer or involves the whole snow cover (slides to the ground), and
whether the motion is on the ground, in the air,
or
mixed.
Fig.
1 The classification of snow avalanches
Loose
snow avalanches form in snow with little internal cohesion among
individual snow crystals. When such snow lies in a state of unstable
equilibrium on a slope steeper than its natural angle of repose, a slight
disturbance sets progressively more and more snow in downhill motion. If
enough momentum is generated, the sliding snow may run out onto level
ground, or even ascend an opposite valley wall. Such an avalanche
originates at a point, growing wider as it sweeps up more snow in its
descent. The demarcation between sliding and undisturbed snow is diffuse,
especially in dry snow.
Three processes commonly leave snow in a
state of unstable equilibrium on a slope steeper than its natural angle of
repose: (1) Deposition of stellar or dendritic crystals with little or no
wind, (2) reduction of internal cohesion among crystals by metamorphism,
and (3) reduction of internal cohesion by intrusion of liquid water.
Though very numerous, most dry loose snow avalanches are small and few
achieve sufficient size to cause damage. With advent of spring melting,
wet loose snow avalanches also are common. Most of the latter, too, are
small, but they are more likely to develop occasional destructive size,
especially when confined to gulleys.
Slab avalanches originate in
snow with sufficient internal cohesion to enable a snow layer, or layers,
to react mechanically as a single entity. The degree of this required
cohesion may range from very slight in fresh, new snow (soft slab) to very
high in hard, winddrifted snow (hard slab), according to circumstances of
layer attachment to the external environment. A slab avalanche breaks free
along a characteristic fracture line, a sharp division of sliding from
stable snow whose face stands perpendicular to the slope. The entire
surface of unstable snow is set in motion at the same time. A slab release
may take place across an entire mountainside, with the fracture racing
from slope to slope to release adjacent or even distant slide paths. The
mechanical conditions leading to slab avalanche formation are found in a
wide variety of snow types, both new and old, dry and wet. They may be
induced by the nature of snow deposition (wind drifting is the prime agent
of slab formation),, or by internal metamorphism.
Slab avalanches
are often dangerous and unpredictable in behavior. Providing most of the
winter avalanche hazard, they axe the primary object of avalanche defense
and control measures.
Avalanches composed of dry snow usually
generate a dust cloud as part of the sliding snow is whirled into the air.
Such slides, called powder snow avalanches, most frequently originate as
soft slabs. Under favorable circumstances, enough snow crystals are mixed
with the air to form an aerosol which behaves as a sharply bounded body of
dense gas rushing down the slope ahead of the sliding snow. This wind
blast can achieve high velocities to inflict heavy and capricious
destruction well beyond the normal bounds of the avalanche
path.
Wet snow avalanches move more slowly than dry ones and seldom
are accompanied by dust clouds. Their higher snow density can lend them
enormous destructive force in spite of lower velocities. As wet slides
reach their deposition zones, the interaction of sliding and stagnated
snow produces characteristic channeling.
Direct action avalanches
fall as the immediate result of a single snow storm. They usually involve
only the fresh snow. Climax avalanches are caused by a series of snow
storms or a culmination of weather influences. Their fall is not
necessarily associated with a given current storm or weather
situation.
The Mechanism of Avalanche
Release
Most avalanches of dangerous size originate on slope angles
between 30 degrees and 45 degrees. They seldom occur below 30 degrees and
hardly ever below 25 degrees. Above 45 degrees to 50 degrees sluffs and
small avalanches are common, but snow seldom accumulates to sufficient
depths to generate large slides.
Though internal metamorphism or
stress development may sometimes initiate snow rupture, avalanches are
often dislodged by external triggers. An overload of new snow may dislodge
an existing slab. Falling cornices or chunks of snow from trees are common
natural triggers, similar in action to the sunballs or snow wheels which
frequently initiate wet slides. In the absence of external triggers;
unstable snow may revert to stability with passage of time and no
avalanche occurs. Artificial triggers in the form of mechanical
disturbance may be intentionally introduced for control purposes.
Unintentional triggers are a major cause of accidents; most skiers who
fall victim to an avalanche trigger the slide which traps
them.
Fig. 2 The fracture line of a
slab avalanche, showing the sharp boundary between the stable
snow and
that which slid away to the left. Note blocks of the hard slab
resting on the sliding
surface.
Slab avalanches
fall when a well defined snow later breaks free and slides away. The
sliding surface is usually the interface between distinguishable layers of
snow which has been formed by variations in weather or snow deposition. In
some cases the sliding surface may be the ground (entire snow cover
avalanches). There often exists a lubricating layer of low shear strength
which allows the slab to break free from the sliding surface. This
lubricating layer may be generated by deposition of fragile crystal forms
(e.g., surface hoar), internal metamorphism, or the intrusion of melt
water. In some cases the lubricating layer is absent, instability being
provided simply by a poor bond between snow slab and the sliding
surface.
The primary instability develops when the component of
force parallel to the slope due to the weight of the slab exceeds the
shear strength of the bond to the underlayer (sliding surface). The
situation is mechanically complex due to irregular attachment of the slab
to stable snow or other anchorages at the head, toe,, and sides. In
general., slab avalanche release occurs when one of these attachments is
broken by a trigger; redistributed stresses then exceed the strength of
the other bonds and an avalanche falls. Only part of the slab attachments
may be weak, while others are strong. In this case a trigger will initiate
fracturing in the snow but the slab remains in place and no avalanche
falls.
Snow settles as destructive metamorphism proceeds, and on an
inclined surface it also creeps downhill under the influence of gravity by
internal plastic deformation and slip on the ground. Creep velocity varies
with temperature., snow type, snow depth, slope inclination and profile.,
and ground cover. These variations from one zone of the snow cover to
another develop creep stresses. The zones of creep tension are favorable
locations for slab avalanche fracture lines; these commonly occur on
convex profiles or at the head of open slopes where the snow cover first
finds anchorage (trees, ridge top, etc.). A snow slab under tension may
not only break free when triggered but shatter into blocks as well when
the stress is relieved. In hard snow of high tensile strength this release
may achieve almost explosive violence. Creep stresses are in large measure
responsible for the dangerous and unpredictable character of slab
avalanches.
Forecasting Snow Avalanches
Although the general features of snow
instability are known,, many details of avalanche formation are not
clearly understood. Forecasting snow avalanches is therefore largely an
empirical art based on accumulated experience. Known physical and
mechanical principles of snow behavior provide a qualitative understanding
of avalanche origin, but quantitative extension of these principles to
specific situations is difficult,, for nature presents too many variables
to allow exact calculation of snow stress and strength variations with
time. The precise time a given slope will avalanche cannot be predicted,
but the general degrees of instability in a given area can be estimated
with reasonable accuracy.
There are two basic methods of
anticipating avalanche hazard. One is the examination of snow cover
structure for patterns of weakness, particularly those leading to slab
avalanches. This method finds its greatest success in forecasting climax
slab avalanches caused by structural weaknesses which may be evolved over
a period of time and by a variety of weather conditions. The second method
is analysis of meteorological factors affecting snow depositions. The
latter is now successful in forecasting direct action soft slab avalanches
which run in fresh surface snow layers where structure is poorly
differentiated. In practice the two methods overlap and both are used.
Emphasis on one or the other depends on local climate,, snow type, and
avalanche characteristics. Both apply principally to winter avalanches in
dry snow; forecasting wet spring avalanches depends on knowledge of heat
input to the snow surface as well as elements of the foregoing methods.
Fig. 3 Typical snow structure at the
fracture line of a slab avalanche.
Layers of new, partly
metamorphosed and old snow are separated
from an icy crust by a thin
layer of very fragile depth hoar crystals.
The profile of ram
resistance at right indicates low strenght in the
slab layer which
slid away.
Snow cover
structure is investigated directly by digging pits and examining the
exposed stratigraphy. Snow temperature, density, strength properties, and
crystal type are all important for determining stability. Time variations
in these properties are examined by a succession of pits in a given study
area, the result being plotted in a time profile. Indirect evidence on
snow structure can be gathered by instruments probing from the surface.
The most useful of these is the ran penetrometer which measures snow
strength variations with depth by means of a pointed rod driven by a
falling weight. Periodic observations at representative study plots
(usually on level ground) are compared with snow profiles from actual
avalanche fracture lines to determine and anticipate stability
trends.
The basic structure leading to slab avalanche formation is
a cohesive snow layer resting on a weak substratum which offers poor
support or attachment. Actual combinations of slab layer and substratum
strength vary widely. A heavy,, hard slab of great thickness may exert
enough shear stress at its base to rupture a relatively strong supporting
layer which would provide adequate anchorage for a lesser overburden. On
the other hand, even shallow layers of soft, weak snow may break free as a
slab avalanche if the substratum is sufficiently fragile. A common source
of weakness is depth hoar formed in the early winter snow cover. This
provides very poor support for subsequent snowfalls which often slide off
fully developed depth hoar regardless of their individual character. Thin
layers of depth hoar, surface hoar, or graupel can also provide a fragile
bond (good lubricating layer) when sandwiched between stronger layers. The
general process of constructive metamorphism always weakens snow layer
strength and bonds; it may precipitate an avalanche long before
recognizable depth hoar crystals actually appear. Another frequent cause
of slab avalanching is an ice layer or crust which provides a smooth
sliding surface. Crusts formed by refreezing following a rain storm offer
especially poor anchorage to subsequently deposited snow layers. The bond
between slab layer and a crust can be poor at low temperatures., while it
rapidly gains strength if the interface is near the freezing point. Other
patterns of snow stratigraphy also lead to slab avalanche formation in dry
snow though these are the most important.
Soft slab avalanches
usually run during or immediately after a storm. In motion they are
similar to dry loose snow avalanches and sometimes are confused with the
latter when they fall during poor visibility. The characteristic fracture
line and initial motion as a cohesive layer is nevertheless present,
identifying them as true slab avalanches. Observation of contributory
weather factors before and during a snow storm provides the basis for
forecasting this hazard situation. The depth and surface character of the
existing snow base, established by previous storms, must be known. A deep
snow cover favors avalanching by smoothing the terrain, while certain
surface conditions such as a crust (see above) offer a good sliding
surface. The new snow depth, type, and density also offer clues to
stability. New snow layers more Wain 25-30 cm thick most frequently lead
to soft slabs, with graupel end intermediate stages of rimed crystals the
most favorable crystal type. New snow densities above about 0.12
g/cm-3 are a warning sign. (Very low new snow densities, 0.05
g/cm-3 or less., are usually associated with dry loose snow
avalanches.). Settlement in the new snow Is a stabilizing
factor.
Rising temperature during a storm accompanied by rising new
snow density tends to cause avalanching,, while falling temperatures have
the opposite effect. New snow precipitation intensity is a signifi cant
factor, for it represents the rate at which the slopes are being
overloaded. Values above 2.5 mm of water per hour warn of impending
hazard. In practice this factor may not be measured directly; instead, new
snow density and snowfall intensity are observed. The wind is also
critically important, for soft slab avalanches seldom occur unless
sustained average wind velocity exceeds 6 to 7 m/sec -1. The
most reliable indicator of developing avalanche hazard is a sustained
period of coincident high wind and high precipitation
intensity.
Wet snow avalanches are generated by intrusion of
percolating liquid water (rain or snow melt) in the snow cover. The rapid
temperature rise-quickly alters snow behavior, while the water itself
reduces snow strength. Liquid water accumulating at an impervious crust
provides an especially good lubricating layer for slab release. The most
extensive wet snow avalanching occurs during winter rains or the first
prolonged melt period in spring, when liquid water intrudes into
previously subfreezing snow. Snow melt by solar radiation is the commonest
source of wet snow avalanching and this is amenable to quantitative
prediction. It is essential., though., that the total snow surface energy
balance be considered in estimating amount of melt, for longwave
radiation, vapor exchange and sensible heat from the air all play an
important part. A warm., windy,, overcast day may produce more melting
(and avalanche activity) than sunshine and cloudless
skies.
Accuracy of formal forecasting procedures is enhanced by
frequent field checks of snow stability. For this purpose small,
accessible avalanche paths are sometimes chosen as sites for test skiing ,
where snow conditions are checked by actually trying to set the snow in
motion. This technique is particularly useful in detecting incipient soft
slab formation during storms. It is less effective (and more dangerous) on
hard slabs formed by heavy wind drifting. Tests of the latter are usually
restricted to blasting with high explosives.
Avalanche Control
Techniques
Avalanche hazard can be mitigated or eliminated by the
application of operational and engineering techniques. There are two
fundamental methods of avalanche control: modification of terrain, and
modification of the snow cover.
Terrain modification may deflect
the sliding snow away from fixed facilities to be protected, or actually
prevent the avalanche release. Examples of deflecting structures are
snowsheds used to protect railways and highways. These must be strong
enough to support the dynamic load of sliding snow; hence most modern
snowsheds are built of reinforced concrete. Where sheds are impractical,
the sliding snow can be diverted laterally by wedges, pylons, or diversion
walls.
In favorable terrain the snow may be arrested by snow dams
or catchment basins. Avalanches are also arrested in the outrun, or
transition zone, of their paths by braking mounds conical earthen or
masonary mounds four meters or more high which axe arranged in a pattern
to break up the flowing snow into crosscurrents which internally dissipate
its kinetic energy. All of the passive deflection structures act
principally on snow sliding on the ground which may exert impact forces up
to 50 tons/m2. They have less effect on the dust cloud
accompanying a powder snow avalanche.
Active avalanche defense by
terrain modification is achieved with supporting structures in the
avalanche release zone. These are large walls, fences, or nets arranged to
retain snow and prevent avalanches from falling. Their size and spacing
are designed to (1) terrace the mountainside into discrete zones, each of
which has snow deposited to a surface slope less than the mean, (2) break
up the the continuity of the snow surface and prevent slab formation, and
(3) support snow on the mountainside in small, manageable sections. These
supporting structures, mostly massive fences in modern design, must be
strong enough to support creep pressures reaching tons per square meter,
while at the same time being light enough for economical transport and
erection high on a mountainside. Another type of defense used in the
release zone is the wind baffle, a wall or panel arranged to induce
irregular wind drifting which breaks the continuity of snow slabs. They
are not designed to withstand large creep pressures and are less effective
than supporting structures.
Avalanche control by snow modification
does not give the high degree of protection afforded by terrain
modification but is much cheaper. It commonly is used to reduce the hazard
to mobile entities, such as skiers or highway traffic, which may be
removed during periods of danger. The commonest technique is artificial
release, which brings down avalanches at a chosen safe time and inhibits
formation of large avalanches by relieving slopes of their snow burden
piecemeal in small ones. Slides on small paths are sometimes intentionally
released by skiing, but the preferred method is the detonation of a
brisant high explosive on the snow surface close to the expected fracture
line. One kilogram of TNT or its equivalent is considered the minimum
reliable charge. The charge may be placed by hand, but this can be
difficult and is sometimes dangerous. Artillery shells, armed with
superquick point detonating fuzes, are much more efficient, for a number
of targets can quickly and safely be engaged from a single gun
emplacement. Principal disadvantages of artillery are limitations to
military or government use and possible damage from shrapnel dispersion.
Mortars, light howitzers, and recoilless rifles have all been successfully
used for avalanche control; the 75mm recoilless rifle is the most
practical weapon for this purpose. Where frequent artificial release is
undertaken to protect a ski area or highway,, a fixed artillery
emplacement permits increased efficiency by blind firing during storms or
at night. Artificial release cannot be effectively employed at random. It
must be based on accurate appraisal of snow and weather conditions, and
careful selection of targets.
Another snow modification technique
is the application of mechanical disturbance to break up slab formation
(especially soft slabs) and induce stabilization through age hardening.
Skier traffic is the commonest available disturbance, while deliberate
packing of the snow by foot or ski is sometimes used. Depth hoar can be
satisfactorily stabilized only by intensive foot packing. Mechanical aids,
such as oversnow vehicles, can seldom be used at the slope angles existing
in avalanche release zones.
