PROBLEMS OF AVALANCHE RESEARCH
(Introductory Lecture)M. R. de QUERVAIN Davos.
Chairman of the Division of Seasonal Snow CO\er and Avalanches of ICSI and President of the organizing committee of the Symposium.
Avalanche research is not a science in itself but the search for answers on various questions about avalanches with all possible methods offered by modern science.
These questions may be classified as follows:
I. What kind of avalanches are there, where and how often do they occur?
2. Why and how do avalanches form?
3. How do avalanches move and what effects do they produce?
The first group of questions calls for a descriptive treatment. Sometimes descriptive science is rated low among certain scientists. This would certainly not be justified here. Any science has to start with reliable observations and with precise descriptions of the phenomena. With regard to avalanches, we may remember that an avalanche is first of all a process and not an object. All of us have seen innumerable avalanches- at rest, but how many have we seen breaking loose under natural conditions?
One task of the descriptive research on avalanches is to establish an inventory of the avalanches of a given area. I have the impression that this task has been neglected in many respects. At least it was neglected in Switzerland for a long time. J. CoAz (1822-1918) was one of the first to build up an inventory of the avalanches of a country, and his avalanche map is still of high interest (1). But after him the survey was carried on only sporadicly and in restricted areas.
Since, as mentioned before, an avalanche is a process, the inventory should involve besides the terrain the time and the time depe11di11g factors of the avalanches such as snow and weather conditions. If we want to draw any sllltistical conclusions from an "ai·ala11chc cadastre", as we call this kind of an inventory, it should be built up and maintained with high consistency and tenacity. Casual observations or obser•
vations covering only certain slopes and exposures may be worse than none at all.
But who can afford to pay an observer just for watching avalanches?
From a practical point of view it is most important to gel a complete survey of extreme avalanche situations like the one of 1951 in the Alps. The observations, taken during and after this disaster, are a substancial part of the Swiss avalanche cadastrc and represent in many places but not everywhere- what may be called the "envelopping conditions". In a modern way of processing observations, all data will be transferred to punch cards. It is obvious that for this kind of research avalanches have to be classified according to strict rules and that this classification should be based on observed facts and not on assumptions or vague ideas. I am convinced that a statistical evaluation of a homogenous material will reveal interesting correlations on the frequency and character of avalanches with respect to terrain, exposure and climatic conditions. Problems of special interest to be treated in this way arc the relationship between the frequency of avalanches and new snow depth with the air temperature as a parameter, or studies of the influence of ground conditions (roughness of the ground, vegetation, soil moisture etc.) on the avalanche activity. Who knows, for example, exactly what kind and size of shrubs- like rhododendron, alders, small pines etc.- do promote or check the formation of avalanches ? With this question we have already entered the second set of problems: why 011d how do ovala11chesform ? The historical development of avalanche research did not go along this line, since human curiosity is not satisfied with statistical correlations. In the years after the 1
first world war Wilhelm PAULCKE ( 2) and Gerald SELIGMAN (3) have studied the genesis of avalanches by direct observations in the fracture zone and have found the type of snow, or more precisely, the stratification of the snow cover to be the essential factor of avalanche formation.
Hence we have to distinguish two branches of genetic studies; the one is dealing with the problem of the genesis of certain types of snow or certain strata, and the other is engaged in the pure mechanics of avalanche formation.
Let us first look at a few problems of the mechanics of avalanche formation.
Thirty years ago ROBERT HAEFELI made the first approach to quantitative snow mechan- ics, and he has followed this line with a fruitful persistency up to the present day (4).
BUCHER (4) and others joined in not to forget the very active American group of SIPRE. As a result we have ideas today about the mechanisms of avalanche formation which seem to explain at least in a qualitative manner the observed phenomena. At the starting zone of avalanches we notice in most cases either the one type of fracture characterized by a point as origin or the other type with a sharp fracture line of hori- zontal extension. By the way, it can be demonstrated that under certain conditions both types may form simultaneously. The avalanche with the sharp fracture line, the slab avalanche as we call it, is the result of 4 kinds of fractures:
- one fracture under tensile stress above;
- two fractures under shear stress 10 the sides;
- one fracture in the area of the slope and;
- one fracture under pressure below.
The impressive tensile fracture was perhaps somewhat overestimated in its genetic significance. Certainly, this rupture and its position in a slope will often be a primary consequence of cumulated tensile stresses which in tum are related to a differential creeping movement along the slope. We would then speak of a primary tensile fracture.
But this is not the only possible mechanism.
If we start out from a primary shear fracture somewhere in the middle of a slope, the failure may expand upwards and downwards until the slab cannot be carried anymore by itselr, and the rest of the fractures are following instantly. There are good reasons to assume this to be the more frequent case.
The stability of a snow slab before its descent is a quality we are very keen to know. It could be defined as the ratio of the sum of the strength of all the prospective fracture areas to the weight component of the slab parallel to the slope. We notice that this ratio is decreasing with increasing size of the slab. But it has to be emphasized that the true and effective stability is smaller. In order to form the 1111c/eus of a fracture it will do if in one point the stress becomes for one or the other reason higher than the strength. Our attention has to be focussed on the formation of this nucleus on one hand, and on the expansion of the fracture on the other hand. In the course of this expansion, obviously quite stable layers may be involved in a slab avalanche. There is no need to say that any quantitative treatment must be troubled by a high rate of scattering.
The point fracture of the other type of avalanches, called a loose snow avalanche in our terminology, is rather a matter of micro mechanics than of macro mechanics.
One single snow crystal hitting its underlying neighbour and breaking it loose may start the movement. It is easy to establish a general equation for the propagation of the movement: The potential energy freed by the falling particles minus the energy consumed by internal friction must be equal or higher to the binding energy of the particles which are hit.
If a falling single crystal does not succeed to start the movement, a lump of snow may do it. It is just the matter of the energy of the initial shock. What we do not know too well, is how the snow is formed which behaves according to the said equation.
Very often the loose snow avalanches do not appear during a snow fall but only 2
after a certain period of grace. This is commonly and reasonably attributed to a superficial disaggregation or a loss of linkage between new snow crystals, but no one to my knowledge has clearly demonstrated it. There are also loose snow avalanches composed of old coarse granular snow. With these remarks we have switched over to the other fundamental line of genetic research, the one dealing with the formation of types of avalanching snow. From Paulcke and Seligman this line leads to BADER(4), EuoSTER (5), YosmA (O), and many others. They have solved a great number of problems around the process which is called snow metamorphism or diage11esis.
But first of all it is obvious that the highest and most frequent danger of avalanches is descending directly from the sky as fresh s11ow. The more fresh snow deposited in a short time, the higher the danger.
LA CHAPELLE (7) has given the influence of s11ow fall i11te11sity special attention.
He estimates a snow fall intensity above about 1,5 cm per hour to be favourable for avalanching.
In order to express our ideas on this phenomenon in a more concise manner, we try to cast the dependence of the avalanche danger upon fresh snow accumulation and time in a formula-although avalanche danger is not a physical concept.
Let us assume that in the time t (hours) s cm of fresh snow have been continuously deposited. Then, with k and II to be constants, we suggest that the avalanche danger D (a scale figure) would behave like.
s"
D =-k -( with 11
>
If we deduce the momentary development of the danger by differentiating the above formula
dD
=
k. s11-1( 11 ds _ ~)
dt r dt t
we notice that even with continuing snow fall (i.e.
di )
ds o) the danger does not necessarily increase.With more quantitative experience, it may be possible to establish the unknown parameters in correlation to other factors like wind and temperature, or to refine or replace the formula. In any case, there is a certain need for gradually consolidating the personal e.xperience by objective rules and laws.
Time is involved in the fresh snow avalanche problem, because of 2 metamorphic processes, namely:
- the instability of fresh snow crystals;
- densification of fresh snow under its own load.
But as important as these processes within the snow is - the metamorphic state of the old surface layer.
It still would be the subject of very interesting studies, considering the various atmospheric influences in different exposures.
A special type of change in the snow quality is caused by wi11d either during or after a snow fall. In transforming the snow into a brittle material and producing irregular deposits, the wind is the most active avalanche factor besides fresh snow.
There is no need to loose more words about this fact among avalanche specialists.
Just one point may be raised Seligman emphasizes the importance of moisture deposit from the air in connexion with the wind slab formation. It is evident that supercooled fog particles caught by drifting snow will serve as a special cement, but we do not know if a foggy wind flowing over a smooth snow cover would work down into the snow so as to influence its consistency to an appreciable depth. Some studies in this direction would certainly be of interest.
On metamorphism or the old snow layers I do nol wish to spe.ik this time. The bearing of this matter to the avalanche problem is too well known (8). Only one remark may be allowed: In view of further new snow deposits, the transformation of the snow surface layer is at least as important as the famous depth hoar formation.
The most complex problem in the avalanche genesis is probably the influence of the temperature, be it conductive or radiant heat. A change in temperature has the following immediate consequences on the bulk quality of snow:
Change of strength;
viscosity;
volume plasticity (i.e. the capability of being densified);
volume expansion or contraction.
These are, in principle, reversible temperature functions. In addition there are ,rreversible processes depending on temperature and time such as the densification itself, the change in the grain bindings, or generally the metamorphism which is governed not only by the temperature but also by the tc111pernr11re gradiellf in the snow. The complexity of the temperature influence is recognized when we try to predict the mechanical properties of a snow sample which is subjected to a given
\ariation of the surrounding temperature.
Who is able to predict precisely where we end? Sometimes it is even hard to say whether an increase or a decrease of strength will result. This trouble is often worrying us in our avalanche forecast.
Summarizing the basic features of avalanche genesis, we pay our ,tttention to the following four influences:
fresh snow deposit;
stratification of the old snow cover;
wind action;
temperature and its variation.
All four arc in the paws of research. But we are still far away from an avalanche forecast which can dispense with an experienced human brain.
We are gelling to the last group of questions, the ones on the movement and the effects of avala11cl1es.
From old sources some strange ideas have entered the common perception of avalanche movement, and they stay alife obstinately. Some of the first measured avalanche velocities were reported by M. OECHSLIN (9 ). By A. WAGNER (10) (coauthor of PAUi.CKE) and L. PRANDTI. (11) a professional standard was introduced in avalanche dynamics. The first measurements of avalanche pressures known to us including theoretical considerations were published by A.G. GOFF and G. E. OTTEN (12). Pro- bably the most profound exploration of avalanche dynamics we owe 10 A. V0EI.LMY (13).
He attacked the problem from two sides: backwards from observed damages and straight ahead from the hydrodynamic theory. But still we are living in an "eldorado"
of open problems.
Let us just take a look at the definition of avalanche velocities (fig. I). In an avalanche of mixed type, we have to distinguish between:
- a front velocity which is the usual observed velocity;
a mass velocity of variable amount over a depth profile which is lhe source of destroying forces and certainly higher than the front velocity;
- a velocity of a frontal shock wave, in principle, a phase velocity of sound speed.
In rather slow moving dry or wet avalanches, sometimes a shooi11g mavemem can be observed. Here the front velocity is a phase velocity and higher than the mass velocity. It is not impossible but not so easy to measure reliably the various avalanche
4
velocities. What I would like lo see once is a profile of lhe core velocity measured in a powder snow avalanche of mixed type. Special attention should be given the frontal shock wave. Its velocity and energy transport is questionabel as Voellmy and others have pointed out. In any case, there is no question about a hurricane running far ahead and carrying higher power than the avalanche cloud itself. Airborne avalanches are a source of further interesting phenomena like explosions, air cushions, protruding rockets etc. There is no doubt about the existence of these effects, but sometimes the given interpretation is not satisfactory. In order to be saved from non-realistic con- clusions the energy principle and the equation of continuity should always be kept in mind.
DEFINITION OF AVALANCHE VELOCITIES
!!!!. m11:ttd airborne, and flowing avalanche rigth ·shoving· avalanche
. \.: 't density m max mass velocity
of tort (mostly turbulent)
•t . front velocity
•• veloc,ty of shock wave (questionable]
Fig. I - Velocities in an avalanche.
Vm mass vtloc1ty vp phase velocity
Yp >Vm
Ti,. y,
It is difficult to carry out experiments with avalanches of natural size. As we will see, our colleagues in Japan have overcome a great deal of this difficulty. Nevertheless, it is not an easy affair. There are two ways to go around the problem. The one consists in confining the experiments to small slides. It allows one to study impact forces of moving snow and a number of other problems of engineering character. In Japan (14) as well as on the Weiss/lul,joch (15), investigations of this kind are in progress.
As another desirable way of investigating certain features of moving avalanches, rests in a model scale were suggested. It would indeed be wonderful if the path of an airborne avalanche could be simulated in a small scale model using a suitable powder instead of snow and replacing the air by another fluid. For simulating s110,0 drif1, the Americans have apparently found a practicable solution, but for avalanches, the prospect is not very promising.
Reynold's model law as well as Fraud's law have to be fulfilled, and this reduces the choice of parameters in an unpleasant manner·to
5
Index M : Model Index N: Nature a - Length
v
=
Kinematic viscosity of fluidLie
Difference in density between cloud and surrounding fluidFor a model scale of I : I 000 we need a scale in the kinematic viscosity of about I : 3.104. With available model substances like, for example, iron powder in hot water, a ratio of about I : 15 can be obtained. It yields quite good looking model avalanches, but who knows to what extent they behave like natural ones?
All dynamic studies are leading to the problem of friction, and in this respect snow is offering everything that can be sought of in the domain of internal and external friction. Fig. 2 is an attempt of a survey on the different kinds of friction we have to deal with. It is probably not even complete.
Good measurements are available for the friction of skis and runners of sledges
(BOWDEN (IO), KLEIN (17)) and for the internal friction of cohesive snow. Friction measure- ments for snow/snow and snow/ground are rather scarce and do not cover the full range of velocities. Moreover, the mechanism of this kind of friction is very complex.
On the internal friction of a snolli srream, we only dispose of vague ideas, but just this would be very useful to know. In a s11oru cloud finally, the known conditions of atmospheric turbulence are approached.
Two details are of a certain interest with respect to avalanche mechanics: the static friction of snow against snow or other materials is not a stable quality. It turns into cohesion even after a very short time of contact. As a consequence, a fracture may heal out in case the snow does not slide right away.
The other question is related to the wet friction between snow and ground. Why does a snow cover glide down slowly and peacefully and then all of a sudden speed up to a fast avalanche movement. ls it just a question of moisture? or has it to do with the whole stress situation in the slope?
Speaking of avalanche effects, we are thinking primarily of destroying forces. For many people, the airborne powder snow avalanche is the most powerful type.
But the simple formula
p ,..., ev2 e - density
v ~ velocity
(dynamic pressure proportional to the density and the square of the velocity) raises some doubts whether the light airborne avalanches really exert the highest pressures.
If we assume the mass velocity of an avalanche to be linked with the density somehow like v(e)
=
uo - f (e), the combination of the two formulae will probably yield a maximum pressure for a certain density in the range of the flowing avalanches. About the function v(e) we do not know much more than that the density of airborne avalanches will hardly exceed 10 kg/m3, whereas their velocity may reach the 100 m/s.Avalanches following the ground dispose of the full scale of densities between about 10 kg/m3 and maybe 400 kg/m3 with unknown maximum velocities.
Leaving untouched all the rest of the open problems of avalanche dynamics, 1 wish to mention some other avalanche effects which are not necessarily of destructive character. The erosion caused by avalanches is a phenomenon which probably escapes the attention of most of us short lived people. It needs the long term thinking of geologists and geomorphologists. We are pleased to hear more about this subject during the symposium. In saying that a number of avalanches- probably the majority- have hardly any erosive effect, we do not exclude that the rest of them cooperate actively with water and wind in modelling the surface of our planet.
Not of mechanical character are the hydrological effects of avalanches. The dislo- cation of snow from open slopes to the dense accumulations of avalanche deposits in lower zones offers various aspects. On one hand, the ablation of these snow masses 6
FRICTION OF SKIS.
snow/ smooth solid matter
v ~m s-1
FRICTION WET SNOW /GROUND.
gliding movement.
V~m•s-1
INTERNAL FRICTION OF OESAGGREGATED SNOW.
~ +air
-snow+ air• water . - snow+ water crystals touching each other.
laminar, streaky or turhulent movement.
v~m•s·1
INTERNAL FRICTION OF COHESIVE SNOW (VISCOSITY).
FRICTION WET SNOW/ GROUND.
slow gliding movement v~cm -day-1
FRICTION SNOW
I
SNOW.al static fr.-cohesion b) kinetic fr. { ~~fting
wet fast gliding movement v.-m,s-1
INTERNAL AND EXTERNAL FRICTION OF SNOW
CLOUD.
crystals not touching each other.
turbulent movement.
v-m-s--1
Fig. 2 - Different types of friction involved in moving snow.
7
is delayed by the reduction of surface; on the other hand, it is accelerated by the increase in the surrounding temperature and by higher absorption of radiation due to a reduced albedo. The hydrologists may tell us what influences are predominant and how finally run off is controlled by avalanches. Further research in this direction should be encouraged with reference to the hydrological decade.
The complementary problem lies in the de1111dmio11 of snow covered slopes by avalanches with the consequence of a higher heat exchange between ground and atmosphere. The soil deprived of snow is exposed to the access of frost and heat and suffers the loss of a water source. Botanists and foresters will certainly pay attention to this effect. Whether it is an important and limiting ecological factor for the deve- lopment of certain plant associations is not in the range of my judgment.
The hydrological effects of avalanches have been discussed by Albert HEIM (1 8) in the chapter dealing with avalanches in his book Hm1db11c/1 der Gle1scl1erk1111de published 1885. He says:
"Avalanches arc an importam factor lor equalizing climatic contrasts of different levels and seasons. Without avalanches it would be colder high up and hotter and dryer in lower levels ... Without avalanches the snow line would be lower and many beautiful pasture grounds and meadows would become permanent snow fields. The glaciers would grow and the climatic get rougher ... As a whole the avalanches arc a great blessing to organic life on earth ... their advantages are incomparatively higher than their damages.,.
Probably today we would not undersign all these sentences any more. But to us it is encouraging that the avalanches. the subject of our symposium, do not only offer negative and scientific aspects but po~itive aspect~ too.
SELECTED LITERATURE REFERENCES
(1) COAZ J., Die La\, inen der Sch,\cizcralpen, Bern, 188 I.
(2) PAULCKE W., Praktischc Schnee- und Lawinenkunde, Springer, Berlin, 1938.
( 3) SELIGMAN G., Snow Structure and Ski Fields, McMillan, London, 1936.
( 4) BADER H., HAEFELI R., BUCHER E. er al., Der Schnee und seine Metamorphose,
Beitr. Geo!. der Sc/11veiz, Geotec/111. Serie, Hydrologie 3, 1939.
( 5) EuosTER H.P., Beitrag zu einer Gefiigeanalyse des Schnees, Beirrag Geo/. der
Schweiz, Geotec/111. Serie, Hydrologie 5, 1952.
( 6) Yos!DA
z.,
A quantitative Study or, \.lctamorphosis of Snow crystal. Lo,r Temp.Sci. A 13. (1954) 11-28.
(7) LACHAPELLE E., Snow Ava,an~hes. Ag1ic11/111re Handbool,. No. /94. U.S. Dep1.
of Agric111lure. F?rcs1 Service. Jan. 1961.
,g) DE QUERVA!N M., Die Metamorphose des Schneekristalls. Verh. Sc/11vciz. Nar.
Ges. Dar·o.r (1950) 114-122.
(U) OECHSLIN M., Lawinengeschwindigkeiten und Lawinenluftdruck. Sc/11vciz. Zs.
fiir Forstwe.re11, 6. (1938) 153-160.
(10) WAGNER A., Luftbewegung bei Lawinenstiirzen. In (2) S. 126-131.
(11) PRANDTL L., Slromungslehre. Ver/. Vie/111;eg Brm111sc/11oeig (1944) S. 327.
(12) GOFF A.G. und OTTEN G.F., Experimentelle Bestimmung der Einschlagskraft von Lawinen, Mill. Akad. f. Wisse11sch. USSR., Geo phys. Ser. Nr. 3 ( 1939) Moscou.
(13) VO[LLMY A., Ober die Zerstorungskraft von Lawinen. Schweiz. Bauzei1r111g, 73 (1955) S. I 59-165 und S. 212-217.
(14) FURUKAWA I., Impact of Avalanche. J. Japanese Soc. Snow and Ice, Vol. 19. 5 (1957) pp. 12-13.
( 15) SALM B., Anlage zur Untersuchung dynamischer W1rkungen von bewegtem Schnee. ZAMP 15 (1964). Fasc. 4, 357-74.
( 10) BOWDEN F.P., The polishing of solids and the mechanism of sliding on ice and
snow. Proc. Soc. Chem. l11d. of Vicroria (1940) 240-250; Ski and snow: Friction between ski and snow. Neu• Scie111isr, 21 (1963).
117) KLttN G.J., The snow charactcristic5 of aircraft skis. Nut. Res. Council of Canada ( 1942).
(18) HEIM Alb., H,111db11ch der Gle1sc/1erk1111de: 1885, p. 38.
RELATION BETWEEN WEATHER SITUATION, SNOW METAMORPHISM AND AVALANCHE ACTMTY
ABSTRACT
Th. ZINGG
Federal Institute for Snow and Avalanche Research Weissfluhjoch, Davos
The current weather acts especially on the surface layer of a snow cover. The poor heat conductivity of snow is responsible for the thermal instability in a snow cover. This instability produces moisture transport in the upper levels and at the same time a metamorphism of the snow crystals into new shaped grains. This changes also the mechanical and physical properties of the snow cover. The phenomenon depends very much on the depth of the new fallen snow and of the whole snow cover. Especially the temperature gradient and the effective temperature of the snow determine the structure of snow and the grain shape.
The avalanche activity depends on the stratigraphy and on the current weather.
Important are: amount and kind of snowfall, wind action (separately or in connection with snowfall), temperature, radiation and rain (especially early in winter and in spring).
RESUME
Le caractere d'une couche de neige depend surtout des conditions meteorologiques regnantes. Les couches voisines de la surface surtout sont influencees par la temperature.
la radiation, le vent et l'humidite de l 'air. Les couches inferieures changent leur carac- terent plus lentement. La vitcsse du changement de la forme des grains ainsi que des proprietes mecaniqucs depend du gradient thermique et de la temperature effective.
L 'activite d'avalanche est fortement Iiee aux elements meteorologiqucs, specialement a la quantile et le genre de la chute de neige, au vent,
a
la temperature ct il la radiation et, chez nous avec moins d'importance, il la pluie.The snow cover at a given place is a climatic index and therefore in strong relation with different weather elements. On the other hand, relations exist also between avalanche activity and weather.
SNOW COVER AND WEATHER
A snow cover can be built up if the quantity of the fallen snow is great enough to persist until the next snowfall. The melting can be produced by air or soil temperature or by incoming radiation. The change of the snow cover is visible in the structure of the snow, and the shape of the ice grains; it can be measured with instruments like rammsonde, density and strength meters. Especially the density and the shape of the grains, their arrangement, the number of the connecting points and the surface of contacts depend on the weather. The original form of the crystals settled from the atmosphere has with a high probability an important value in the stratigraphy of the snow cover and in the avalanche activity.
Twice a month we investigate the snowfields at Weissfluhjoch (2 500 m), Davos ( I 550 m) and Klosters ( 1 200 m). The results demonstrate different relations between weather elements and the metamorphism of the whole snow cover.
In these profiles during several years the great variability of character of snow layers can be related to the great variability of weather development. Other elements also have an influence, i.e. the kind of the soil, the steepness and the exposition of the slope and the vegetation.
The first moment of an existing snow cover is very important for the subsequent development. Snow particles near the ground change their shape while partially melting into rounded grains. The radiation intensifies the melting process near the ground when the snow depth is below about 30 cm.
The poor heat conductivity of the snow cover causes a temperature gradient, in the bottom being near the freezing point and on the surface being colder. This temperature gradient is responsible, especially during cold weather, for a change of the snow crystals due to convective moisture transport within the snow cover.
A high temperature gradient, starting at O •C in the lower level produces a relatively rapid change of the grain shape and of the attached physical properties. Cups of hexagonal prisms are the result. The density and the strength remain small. This metamorphism is more or less finished within the first 14 days if the new snow layer has a depth less than about 50 cm. Higher amounts of freshly falen snow settles more rapidly. The finer the snow grains the more rapidly the density and strength increase.
We must consider however that heavy snow- falls leading to deep layers are more common in warm air masses than in cold ones, where the settling is delayed.
In the upper layers, the temperature gradient starts at a lower temperature than in the lower layer, especially early in the winter and during high winter period. It seems that in these beds the hexagonal cup forms are rare but the quantity of cups with rectangular forms increases, the diameter of the grains being smaller than for the well known basic cups. The growth begins very often directly from the branches of stellar crystals.
Normally the crystals in these layers have a very small cohesion, and the depth of the bed is in most cases less than 10 cm. Such layers are frequently responsible for avalanche activity. The structure of these snow beds breaks with the load of new snow.
All the important layers of new fallen snow change their original shapes in another manner. The stellar crystals become more or less irregular with a small diameter of I /4 to 1 /2 mm. The density increases rapidly but the strength more slowly up to the moment the entire layer reaches melting point. With increasing snow depth, the lower layers are protected from the current weather. The temperature gradient decreases in the lower part of the snow cover, and the metamorphism is less intense, the cup crystals in these layers beginning to fill up. The cohesion decreases by losses of material because of sublimation, but later on the strength increases again so that we can find cuplike forms with an important strength. Therefore it is better to speak of loose snow rather than of" Schwimmschnee" or depth hoar.
The current weather is an important factor determining the type of snow surface.
Outgoing radiation during the night decreases the surface temperature. If the air moisture is high enough, sublimation can take place and surface hoar will be produced as an additional very loose layer, which can be covered later on by new snow. These layers are very important for avalanche acitivity.
The kind of alternation of snowfalls, temperature, wind etc. produces a special stratigraphy of the snow cover which can be disturbed only by very important weather changes, such as the seasonal winterspring change, when the snow cover becomes wet.
The strength of the snow cover is influenced very much in the spring by the following weather factors: the temperature, the radiation, and rain. The true ablation begins when the snow cover is wet and with a mean daily temperature above the freezing point.
The development of a snow cover depends in the upper layer on the daily weather and in the deeper layers more and more on the mean influences of weather. The most important changes take place in the first weeks.
WEATHER AND AVALANCHE ACTIVITY
The snow cover is influenced very markedly by the weather conditions and the avalanche activity even more. The following weather elements are especially important:
IO
amount of snowfall (including the kind of snow crystals and the intensity of the snow- fall), wind as a drifting factor producing snow depth, the temperature and its change, radiation and rain.
In the Davos-Parsenn area, 428 avalanches came down in the winters 1955-56 to 1963-64 during the months December to April. These are only avalanches with natural triggering, not included is a great number of minor avalanches like superficial wet stuffs (especially in spring).
December January February March April
TABLE I
Number of avalanches attached to different weather elements.
Winter (1955-56 to 1963-64).
0 0
+
T TI •
unknownI
Total12 I 46
---
84%9 13 48
---
81%12 8 75
---
81%34 2 29
---
61%4 -
---
15%*
new snow+
snow drift 39 2
---
16%13 3
---
18%4 18
---
18%3 39
---
39%6 17 12
---
76%*+
new snow and snow drift O lnsolationT Temperature
OT Insolation and Temperature
• Rain
70
I 87
1%
I 118
1%
107
2 2 46
9%
-
428
Table I shows that 69% of the avalanches are highly correlated with new snow, wind transported snow or both. 30%are correlated with temperature and radiation and 1 % with rain and unknown causes. From December to February, 81 to 84% are caused by snowfall, wind or both. In March and April, the influence of temperature increases very much up to 76%. It is very interesting that about 90% of "snowfall" avalanches occur during the snowfall period, delayed at most by one day. In spring it often happens that the radiation (sometimes in combination with temperature) acts on unstable new snow and produces avalanches.
It seems that the snow load is responsible for avalanches in most cases. It is prac- tically not possible to examine every avalanche to find the gliding level. We have the following cases: fractures within the new bed itself, in most cases a very loose layer fallen without wind action; fracture on the old snow surface, surface hoar or smoothed hard surface; very often fractures on loose layers at depth (surface hoar, layer of depth hoar or loose layer built near cold old surfaces). Early in winter or in spring, the snow glides frequently on the ground (wet level).
TEMPERATURE AND RADIATION
In most cases both elements are combined, especially in the beginning of winter and in the ablation period. The immediate influence of temperature on avalanche production is very small in winter. The temperature increase or decrease is delayed in the snow cover and the radiation is active only in the upper layers. The two elements are more important when the snow cover has reached the freezing point, because the amount of free water has certainly a critical influence on avalanche activity.
The radiation has a great influence, but only the superficial layer to a depth of about 25 to 30 cm. These avalanches slide down especially in the afternoon. Radiation melts the snow on rocks, the water flows beneath the snow on the ground and makes avalanches possible in relatively high altitudes.
RAIN
In the higher mountains of alpine area, rain is not very important. There arc only two ca~es of the 428 avalanches, the number being higher at lower altitudes. Exaxt values are not availables just now. In high winter period, rain has nearly no influence on avalanche activity.
DISCUSSION
JOHN m LA MONTAGNE- The diagram by Dr. Zingg showing ram resistance through depth hoar crystals at the bottom of a snow pack indicated strength rather than weak- ness at this horizon. Our experience in Montana and in the Rocky Mountains shows the opposite relationship :that is, depth hoar is weak to the ram. The question was whether Dr. Zingg's diagram represented a special case in Switzerland.
12
LES VARIATIONS DE LA RESISTANCE DE LA NEIGE
ABSTRACT
Andre ROCH
lnstitut federal pour l'etude de la neige et des avalanches Weissfluhjoch Davos
A general relation is given to determine the influence of a change of temperature (below freezing) on the tensile strength of snow.
From a series of shear tests in which the snow was subjected to different pressures, perpendicular to the shejir plane, the intrinsic strength curves of various types of snow have been established.
Successive measures of the shear strength of different layers within the snow cover, show how it varies with time. It increases under pressure of new snowfalls and is reduced by metamorphism and/or a rise of temperature.
The relation between the shear strength parallel to the stratification and the tensile strength (measured in the same direction) varies greatly between strong and weak layers. This stems from the anisotropic qualities of snow which change with time and prevailing natural conditions.
RESUME
D'apres des tests, on deduit !'influence d'un changement de temperature (en dessous de 0·C) sur la resistance a la traction de la neige.
On a etabli Jes courbes intrinseques (enveloppes des ruptures de Mohr) des differentes neiges, d 'a pres des essais de rupture au cisaillement sous differentes press ions normales au plan de cisaillement.
Des mesures de rupture au cisaillement dans Jes strates de la couverture de neige, effectuees pendant trois hivers, montrent comment la resistance varie dans le temps.
Elle augmente sous l'effet de la compression de nouvelles chutes et diminue a cause de la metamorphose et par rechauffement.
La relation de la resistance au cisaillement dans un plan parallele a la stratification sur la resistance a la traction (traction appliquee dans le meme sens), montre une grande difference entre Jes strates devenues resistantes par compression et celles restees fragiles. Cette difference vient de l'anisotropie de la neige qui varie suivant les conditions auxquelles elle est soumise dans la nature.
INTRODUCTION
Les proprietes mecaniqucs de la neige decrites ici ne s'appliquent qu'a des neiges seches, en dessous du point de fusion. L 'influence de la fonte n 'est sign alee que qualitativement.
I. INFLUENCE D'UN CHANGEMENT DE TEMPERATURE SUR LA RESISTANCE A LA TRACTION DE LA NEIGE (fig. ))
D'apres de nombreux tests de rupture
a
la traction executes avec un appareil centrifuge, !'influence d'un changement de temperature sur cette resistance est pro- portionnelle a la valeur absolue de la temperature en degres centigrades I Ti, eleveea
la puissance 1/4.On peut ecrire :
alrl
~
I Ti114 (I)alrl est la resistance
a
la traction en kg/dm2a
la temperature T en dessous de 0 °C.- t - - t - - - - r - - - t - - - + - - - + - - - + - - - + 200
,., '---
--t-,..._,,_j _1~-~--~ . .----+.c..:..,__-.-, _ _ :,,,o. _ _ _ _ _j_ _ _ _ _ _ _ .j... _ _ _ __j_ 90
Q
' , \
---
r--.,.,-to
0 - 5 .Fig. I - Influence de la temperature sur la resistance
a
la traction de la neige, representee sur une double echelle logarithmique.14
n~e de
I
N'I
ykg/m3I
<1t=
Resistancea
la traction en kg/dm2 1e1ge2 < 200 T' - ] - 5,6 - 10 -17,4 -27,2
<Jt 2,6 3,8 4,55 5,2 5,9
- - - - - - - -
- -- -- - - - - - - -
3
>
200 T' - I - 5,6 - 10 - 17,4 -27,2<1t 4,8 9,3 11,2 9,15 18,6
- - - - - - - -
- - - -- - - - - - - -
4 216 T' - 2 - 4,8 - 10 - 20 - 33
<1t 6,4 6,88 10,85 11,0 15,7
- -
- -
- -- -- -- -
- -- -
- -- - 5 250 T' - 5 - 10 - 22 - 40<1t 20,2 22,8 23,2 24,8
- -
- -- -- -
- -- - - -
- -6 280 T' - 0,3 - 2 - 3,5 - 5 - 10 - 22
<1t 20,1 15,7 21,4 21,3 29,8 35,5
- - - -
- -- - - -
- -- - - -
- - 7 295 T' - 2 - 4,7 - 7 - 10 - 15 - 20 - 29 - 30 - 35 - 38<1t 36,3 47.5 48,8 50 51 67 65,4 77,8 83 70,5
- -
- - - -- - - -
- -- -
- -- -
- -"' 8 300 T' - 2,8 - 3,8 - 10,2 - 19,8 - 36,8
i:::
<C <Jt 23 38,2 34,4 41,0 49,7
Ill
i:::
- -
- -- - - -
- - - -- -
- -- - - -
-~
9 300-310 T' - 5 - 10 - 18 - 37,6<Jt 7 II 10,9 19,1
-~ a - - - - - -
- -- - - -- - - - - -
·.; 10 320-330 T' - 5 - 10 - 18
z
<1t 12,4 19,3 26- - - - - - - -
- -- - - - - -
II 330 T' - 5 - 10
<1t 23,6 32,2
- - - - - - - - - -- - - - - -
- -- -
12 346 T' - 10 - 22 - 25 - 38
<1t 81 115 120 156
- -- - - -
- -- - - - - - - -
- -- -
13 372 T' - ] - 5,2 - 7,2
<Jt 28,4 47,8 57,5
- -- -
- -
- -- - - - - -- -
- -- -
14 375 T' - 2 - 4,2 - 9,6 - 19,8 - 37
<Jt 19,4 25,4 30,6 34,6 48,6
- -
- -
- -- - - - - - - - - -
- -- -
15 390 T' - 1,5 - 10 - 30
<1t 9,9 37,7 69
- - - -
- -- - - -- -
- -- -- - - -
16 290-310 T' - 0,5 - 4,5 - 10 - 18 - 31
<1t 7,4 12,3 18,3 21,6 30,5
- - - - - -
- -- - - -
- -- -- -- - - -
17 420 T' - 2 - 10 - 20 - 30
ige
a
<1t 2,1 1,7 2,07 1,69- - - - - - - - - - - -
- -- - - - - -
ros
1ins 18 480 T' - 2 - 10 - 20 - 30
<1t 4,75 18 23,4 23
a
La resistance n'a pas ete mesuree pres de 0"C, de sortc quc cette relation approximative n'est valable que jusqu'il - 3 •C, pour tous les genres de neige.
On peut ainsi calculer la resistance
a
la traction d'une neigea
n'importe quelle temperature, dans la limite mentionnee, si l'on connait sa resistancea
une temperature donnee.De la figure I, on deduit les particularites suivantes : a) Plus la temperature est basse, plus la neige est resistante;
b) Sur deux neiges de resistance ditferente, l'influence d'un changement semblable de temperature est plus grande sur la neige la plus resistante;
c) Plus on sc rapproche de 0 °C, plus l'influence d'un changement de temperature est grande.
C'est ainsi que le rechauffement des derniers degres avant le point de fusion, provoquc la plus grande diminution de resistance.
La relation donnee s'applique plus ou moins bien
a
tous les genres de neige.L'exposanl variera suivant le genre du cristal et le poids specifique. Le Dr. Bucher montre cette influence (1). Le Dr. Fuchs a fait des tests de resistance
a
la traction sur de la neige mouillee et regelee (2). L 'exposant correspondanta
ses resultats est de 0, I au lieu de 0,25.fl. LES COURBES INTRINSEQUtS, LIGNtS tNVELOPPANTES DE COULOMB-MOHR, DE LA RESISTANCE DES DIFFERENTES NEIG[S
Des tests de cisaillement ont ete faits avec un cadre de I dm2 de surface, muni de deux lamelles transversales
a
la direction de la force pour repartir l'effort et d'un dynamometrea
ressort, figure 2. Pour appliquer une pression normale, on enfonce le cadre en dessous de la surface ct on place sur la neige, qui depasse le cadre, une plaque de verre et un poids. Le coefficient de frottcmcnt du verre sur la neige est tres petit, 0,01a
0,03, de sorte que quand la rupture sc produit, le cadre glisse sous la plaque de verrc qui reste en place.7
dynamomilr•
Fig. 2 - Croquis de la mesure de !'influence d'une pression normalc au plan de cisaillement sur la resistance au cisaillemcnt (test de cisaillemcnt).
La tres legere augmentation de resistance produite en posant momentanement le poids est negligee.
D'apres des tests fails avec des cadres de 0,25 dm2, I dm2 et 4 dm2 on a pu controler que la surface du cadre ne joue pas de role.
En revanche, la vitesse d'application de la contrainte de cisaillement joue un role.
Les tests ont ete effectues de fa~on
a
ce que la rupture se produise entre unea
deux secondes apres le debut de !'application de la force.D'apres les resultats, on deduit les particularites suivantes :
a) La courbe intrinseque de la neige fraiche part horizontalement vers la droite puis se redresse, figure 3a. Une petite pression normale semble ne pas augmenter la resistance de la neige fraiche. Une courbe intrinseque etablie par Haefeli (3) montre que la neige fraiche est meme affaiblie par une petite pression normale. Elle tend
a
briser les fragiles embranchements des etoiles eta
affaiblir ainsi la cohesion de feutrage. Quand la pression normale est plus forte, la neige fraiche, tres peu visqueuse, se tasse par ruptures successives et prend de plus en plus de resistance.Mais on a vite une autre neige, d'un poids specifique plus grand;
b) Plus les grains sont spheriques, plus la courbe intrinseque se rapproche d 'une droite, figure 3b;
-
ff-IS-
I
,
,,.;,,e
i,- .,,.
;,s~"' ,.I
.Poer_, _ocJP°.··
f 00 o'P
.. ~,,., ..
){I;) lo 00 0 ... • 1
.. ..
ov
--····-·· .
0000 · '
.,o'
.. •·
.DO
00
·'
5
' ..
~~ Cl ~;-·
.,
>2•"" ... ,.
19·-
.
f9 ·••',-r ,.,/ts•
i.•
,. "(c) ... i ~"'"•
3
2
,
""
A'',. t
12 ✓ . . « fz•~,.,)11-" .
..
I IA~. .,,. ... n... ,
.,. 4...
•ltJ ~~ns
finJ,__.
.
.. · .
• •· ,t,,
.i,.15
... t
1· A • .N
A"
",.,
...., .
• .,
(o)A Ao .I(
.,.,.,,;.
_ ...-~
~-iel,e~~
,.. ... .. ...
(J f 2 3 4
Fig. 3 - Courbes intrinseques (enveloppes de Mohr) de differents genres de neige, etablies par le test de cisaillement :
a) neige fraiche;
b) neige
a
grains plus ou moins spheriques;c) neige coulante en prismes et en gobelets et neige
a
grains tres fins.I I
Pression normale u en kg/dm2I
N•
I
OI I
2I
3I
5I
6I
8I
Genre de neigeI
- -
2 - -
3
- -
4
- -
--
5 6 7- -
8
- -
9
- -
10
- -
11
- -
12
- -
13
- -
14 15 16
- -
17
- -
18
- -
19
18
C Resistance au cisaillement kg/dm2 kg/
dm2
5,75 6,37 7,15
-
9,1 --
grains0 1,5mm- -
- -- - - - - - - - - - -
14 - 17.8
-
19,5- -
- - - -
5,12 7,3 8,6 10,15 9,6
-
- grains 0 0,5 mm- - - - - -
1,32 3,24 4,07 4,93
- - -
grains0 1,5mm- - - - - - - - - - - -
7,05 9,01 10,23
-
13,) --
grains0 Imm- - - -- - - - - - - -
2,33 3,84 4,15 4,65 5,8 - - grains0 2mm
- - - - - - -
4,65 5,57 6,77 6,80 8,90 -
-
grains0 1 mm- - - -- - - - - - - -
5,13 7,65 7,25 7,75 8,75
- -
gros grains0 2mm- - - -- - - - -
17 17,5 20,5
-
25 25 24- - - -
0 0,5-1 mm16,5 - - 21,5 29 - 32,5
- - - -
0,8 I, I 1,5 2,0
- - -
neige fraiche- - - -
- - -- - - -
2,2 3,3 3,5 3,3
- - -
- -
- -- - - -- -- - - -
0,55 1,75 1,8 3,0
-
- - grains fins- -
- - - -
- - -- - - -
0<
0,5 mm1,8 2,6 2,7 5,0
-
- -- - - - - -
0,45 0,75 1,35 2,25
- -
- neige fraiche- - - - - -
2,1 2,6
- -
5,0- -
grains fins- -
- - - - - -
1,3 3,1 4,1 4,9
-
--
0<
0,5 mm- -
- -- -
2,3 3 4,4 4,6
- - -
- -
- - - - - - - - - - - - -
neige coulante3,2 4,2 4,8 5,2
- - -
Diagramme des Fig. 3 et 4 - Influence de la pression normale u sur la resistance au cisaillement Ts
a ~ ., 2
-0
"'
·;
"'
"'
.,
<t -0
., .,
i::
.,
i::>, 0 ::'21
-
., ..
a
::s2 .,
-0
"'
·;
"'
:a
0
- .,
-0
.,
i::
.,
i::>, 0 ::'21
c) Les courbes intrinseques de la neige coulante, en prismes et en gobelets, de meme que celles de la neige en grains tres fins, s 'incurvent vers la droite, figure Jc. II est probable que plus la pression normale est forte, plus les angles des cristaux sont brises au moment de la rupture, ce qui donne une resistance augmentant de moins en moins.
D'apres !'ensemble des essais, figure 4, on remarque que plus la cohesion (resistance au cisaillement sans pression normale} est forte, plus I 'angle
e
du frottement interne est grand.1'--,L--,L-_...._ _ _ ----,''--- ...,£---4
..
. ."l-
..: a
Fig. 4 - Ensemble des courbes intrinseques mesurees dans la neige par le test de cisaillement.
Pour Jes mesures dans la couverture de neige et celles
a
la cassure des avalanches, on a neglige !'influence du genre de neige et on a ramene Jes courbes intrinsequesa
desdroites definies par une relation empirique simple :
1:8
=
c + (0,08 c+0,4) a (2)Ts est la resistance au cisaillement en kg/dm2 pour la pression normale au plan de cisaillement a en kg/dm2 ;
c est la cohesion (resistance au cisaillement sous pression normale) en kg/dm2•
L'equation theorique de la droite de Coulomb est :
Ts = C
+
atg(_)d'ou l'angle
e
tire de l'equation (2) devient : tgg=
0,08 C ~ 0,4(3)
(4) L'angle de frottement interne est ainsi uniquement une fonction de la cohesion c, ce qui est une simplification.
Pour ajuster les courbes intrinseques au genre de neige, on a change les coefficients et introduit un troisieme membre, ce qui donne :
neige granuleuse Ts = c
+
(0,08 c+
0,4) a;neige fraiche Ts= c + (0,08 c+0,I) a + 0,04 a2 ;
neige en gobelets T 8 = c
+
(0,08 c+
0,8) a - 0, I a2•Un test de cisaillement, avec une pression normale donnee, determine la pente sur laquelle la neige serait ii la limite de l'equilibre, figure 5a. Un second essai, sous pression normale ditferente, donne la courbe intrinseque simplifiee, figure 5b. Avec l'equation (2) un seul essai definit la courbe intrinseque simplifiee, car connaissant Ts et a, on tire c :
Ts - 0,4 a c= - - - -
1
+
0,08 a (5)Au moment de la rupture, l'etat de contrainte est defini par le cercle de Mohr tangent ii la courbe intrinseque au point d 'intersection avec la pente, figure 5c.
Ill. VARIATION DANS LE TEMPS DC LA RESISTANCE AU CISAILLEMENT DES DIFFERENCES STRATES DU MANTEAU DE NEIGE
Au cours des hivers 1949-50, 1950-51 et pendant les mois de janvier ii mars 1957, la resistance au cisaillement des ditferentes strates de la couverture de neige a ete mesuree periodiquement sur un champ horizontal au Weissfluhjoch.
Les mesures de l'hiver 1950-51, faites chaques semaines, sont representees ii la figure 6. On remarque grossierement deux genres de strates :
I) Les strates inferieures des grosses chutes de neige, couches 9, 10, 11, de meme que les strates restees peu de temps en surface et recouvertes par une grosse chute de neige, couches 7 et 8. Elles prennent une grande resistance, tout en restant compressibles, de sorte que plus tard, sous l'etfet de nouvelles chutes, elles aug- mentent davantage de resistance que :
2) Les strates qui restent longtemps pres de la surface sans fondre. Elles se trans- forment en neige granuleuse et leur resistance reste relativement faible, figure 6, couches 4, 5, 6, et figure 7, couche I. Ces couches sont peu compressibles et resistent
a
la pression des nouvelles chutes de neige. Dans cette categorie, entrent!es couches relativement peu epaisses du debut de l'hiver dans lesquelles un fort gradient de temperature provoque une metamorphose active des cristaux qui deviennent des prismes et des gobelets. Cette neige est peu compressible.
Les caracteristiques decrites, ressortent mieux encore ii la figure 7, representant les mesures dejanvier ii mars 1957, faites tousles trois ii quatrejours. Pour chaque couche, on a represente sa temperature, son gradient de temperature et la pression de la neige situee au-dessus.
20
a)
C
,;
+T
,,
b/
t::z-~Yz
~-yzeos2y,
T-
Y-z.sinyury
,,, /;,,, Ts ·. Z
p:
1::::.:.~:-:_-:_-:_-:i"4-l---<.,.~d
.,,
.,,
6.,, _,u,1,r)
/
,,,,
/. ·--J
/'y'
'
. . J ·
I .
_J01reu1on d~s seichons ..,es confr8inf~s maJ(imel~.s
.
Fig. 5 - Diagram me de Mohr, montrant l'etat de contrainte d 'une couche de neige sur une pente au moment de la rupture.
l
Ii,:
I
iii !
e I
f
.a II
0 20
0 2~
0 20 0 20 0 20 0 20
0 20 0 20 0 10 0 20
0 20 0 20 0 20 0 3
2
,
r
_y
4 / / ' ...,
:,
/
2, /
i - -
...
1 - I
1150/51
'- ~- .
-r..
~
r--,
~,'~
Hof. O,c.
o_A
... v
·r-,.- - / ...
~ IJ'C"" '\
, ~
""
'2~~ V . I \
-
I /
l i "
'
.I kl"\ ~ I/',.
' ,,
-/ II ""--"'\.
, ...
I' I I \',oJ V I
~, -
/ I \ \
/ /' I J \
9 / I
u...-
'
I/ q / - ' -
.... -
A/ \ .
/ I/ \ 7
/r'\
--
~
- .
V / \
I/ \ -
/
-
v'6 /
A IA ~ ...
-~
/ /
" -
/ \ ' /-
. I,... ...
'\ / ' - . I \ L,"/ l ' I ./"\.
I\. _ / \.
/ - '---""'
....
/ ~
A ~ /
'
--
._,, i....'
/
-
... . -~
'----
~~ •
,.
~ l/ r'I\
~ I\
~
IR l\ ~ ----.: i:::::::::
,__ ,L coudle•i ' - -
'ir----Z...
~
---:::::- -
-.Jo/Iv. lffr. lfrn Awl/ Hal Juln
Fig. 6 - Variation de la resistance au cisaillement mesuree chaque semaine dans la couverture de neige de I'hiver 1950-51.
22
I, 1,
'-..
~
'11
f
·!! ~
i °'
hiver 1956•57
"'
~i
3• -\i 3
2· ~ ~ 2
1· I! 1
~
,-11i
-to
-
~ -2'J
-20.3•
i
{!
:J,mvier
"' i
~ ... 20~ /0
~h_g ~ ~ ; ~~~~;~~~ ~~~~~/. \\~;IJ~~~~t~~
- --- -- - --
... ---___
--_ --- -- ---
J.
I ~I
n •
-
1. Fivrt'er
·--- ---
--- -- . --
·--- --- ------
t'f11111, • """""""'
.J .
I-
-I
1 . . .1. ff,r.s 1· Fl,;,.
r
..
Fig. 7 - Variation de la resistance au cisaillement de certaines strates du manteau de neige, mesuree tous !es 3
