[PDF] PROBLEMS OF AVALANCHE RESEARCH

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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. s 11 1 11 ds _ ~) dt r dt t ds we notice that even with continuing snow fall (i.e. di ) 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. 3 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 ( 1

2). 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

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