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1. Temporary defence

Once the planner responsible for the development of a watershed has at his disposal the most accurate map possible of the areas of avalanche risk, he will find that it will usually be impossible for him to forego all material development in areas recognized as dangerous, unless he abandons the whole project. This, for example, is often the case in resorts for winter sports, for it would be extraordinary if these could be reached by a road which avoided crossing any avalanche gulleys.

Measures must therefore be taken to ensure the safety of the equipment which has had to be set up in dangerous areas.

These measures can consist of permanent structures on the ground, that is permanent defence dealt with in Chapters V to VIII. Or otherwise these measures consist of interventions on a day-to-day basis, whenever meteorological or snow conditions require it. This is temporary defence, the subject matter of this chapter.

Preventive measures such as artificial consolidation of the snow constitute temporary defence, whereas when stabilizing or deviating structures are built, we are dealing with permanent defence.

"Permanent defence" is also used to describe braking and diverting structures designed to keep an avalanche away from sensitive areas; however, when the moment of an avalanche's release is determined artificially, this is considered a temporary defence.

1.1 Artificial consolidation

Artificial consolidation is powerless as a method of preventing the release of heavy snow avalanches at the end of the season. But such avalanches do not generally constitute a menace, partly because they occur at a time when skiable areas in winter sports resorts have already been deserted by the public, and partly because their path down to the valley is usually very well known so all precautions have already been taken to prevent them causing obstructions (bridges and tunnels).

On the other hand, if an entire zone of avalanche release is easily accessible, artificial consolidation can be an extremely effective means of control during the winter.

In the case of fresh snow, if one can intervene before the new layer exceeds 30 cm. sufficient consolidation can be simply obtained by opening the suspect gulley to skiers or by using the "snow cats" - the tracked machines used for maintenance of ski runs. Opening the gulley to the skiing public, after either a team of "pisteurs" or a snow cat has been through, has a beneficial effect and presents no danger.

If conditions have not allowed an early consolidation, and the layer of fresh snow is therefore over 30 cm. deep, consolidation by skiers will not always be sufficient. Consolidation under foot will have to be undertaken, be inning at the top of the gulley and taking adequate security precautions (roping-up)

If the formation of depth hoard is suspected (after a long period of cold weather over a thin snow cover, and with a large temperature gradient) artificial consolidation must be begun immediately in order to make the snow less porous and thereby block the circulation of air which causes the formation of depth boar. It is advisable to check that there is no fragile layer remaining by making a stratographic profile in several places.

Artificial consolidation has been used for a long time and is a valid method of prevention, but its use is relatively limited. We have seen that one must:

- not be concerned about heavy, end-of-season avalanches,

- have completely accessible gulleys,

- have numerous teams of experienced workers who must begin consolidation after the first few snowfalls,

- watch the development of snow conditions (metamorphosis) closely in order to intervene within a very short time (a few hours).

We will see that some of these suggestions will hold equally when it comes to artificial release of avalanches.

2. Preventive release

2.1 Limitations of the method

Preventive release of avalanches makes us masters only of the moment of release but not, as a rule, of the intensity or spatial magnitude of the avalanche. Thus we already see the essential limitations of this method; it cannot be used to protect property which could be damaged by the avalanche. It is only practical if used to ensure the safety of routes of communication (roads or ski runs) if traffic on these can be temporarily suspended during the release, and if these can then be re-opened quickly once the gulley has been completely "purged" and made harmless.

It is generally held that this method allows a succession of harmless avalanches to be released which could otherwise have turned into a large one causing major damage. This is generally the case: snow is released bit by bit by successive artificial releases, and the avalanches do not reach the road which used to be regularly covered; but we can never be absolutely sure just how large an artificially released avalanche might be. It is only under exceptional circumstances that specialists will undertake preventive releases when a natural avalanche could threaten an unprotected building, forest, pylon or work of art. The calculations for the maximum quantity of snow artificially released must be used in order to check that the avalanche will be too small to reach the threatened building or forest (see calculations for avalanches 4.2.2 Chapter III and in the annexe to Chapter V). As a rule this method will be reserved only for the protection of communication routes and ski runs.

2.2 Determining the appropriate moment to intervene

Whatever the size of the means used preventive release cannot be produced artificially unless the conditions are close to those for natural release. Preventive release is therefore closely related to the problem of forecasting naturally released avalanches.

2.2.1 Snow/weather forecasting

To start with, the meteorological services specialized in snow forecasting must be relied upon, and thus the organization must, in general, depend on:

- data from various snow/meteorological stations specialized in this field, including:

daily reading of precipitation, minimum and maximum temperatures, wind speed,

weekly reading of results of penetrometer and stratographic profiles;

- the interpretation by an expert of the synopsis of meteorological conditions broadcast several times a day.

Starting from these two sources the bulletins broadcast by the meteorological services can give an overall view of the risk, even on the limited scale of one mountain massif.

2.2.2 Local forecasts

However, this indispensable information is not sufficient. Within a general situation certain particular gulleys, because of their exposure, their altitude, or the manner in which the wind has loaded them with snow, are ready to release naturally whilst others are still stable. Delay in releasing the former would allow us to be taken by surprise, whilst attempts on the latter would be premature and will end in failure.

It is therefore indispensable that local authorities should have at their disposal: a small team of one to three specialists, a central station for gathering data, and various sub-stations for when there are differences of altitude of over 400 m. between potential starting zones and finally a minimum of technical equipment.

a) Necessary equipment

The main station is situated in a permanently accessible place and at an altitude approximately equal to that of the majority of starting zones.

It will have an anemometer recording wind speed and direction, a recording thermometer and a recorder of precipitation (heated rain-gauge) with the distinction between rain and snow being made by direct observation and by the study of the temperature trace on the recorder.

The sub-stations measure the same elements with simpler equipment for temperature and precipitation; accurate recording of wind speed and direction at altitude remains extremely desirable.

The team of forecasters should also have a telephone and a reliable and quick link with the meteorological services for forecasts, and also a portable kit of technical equipment:

- a standard penetrometer (Ramsonde),

- a lighter (100 g.) penetrometer with a mobile weight of 100 g. for investigations in fresh snow,

- a frame for measuring the shear strength of different strata of snow with its corresponding spring balance,

- a Canadian plate for measuring fresh snow,

- a shaking plate for measuring "in situ" the position of the weakest stratum in a layer of fresh snow,

- a powerful magnifying glass and a plate for determining the shape and size of the crystals,

- the necessary equipment for taking and weighing snow samples in order to determine its density.

Finally, each station be it main or sub-, should have some land nearby on which to carry out experimental releases. The land should include slopes of different exposures; they should be steep but short so that avalanches released on them by foot could not be dangerous in any way.

Figure 12
Graph of a snowfall period for continual assessment of avalanche danger. The observations are gathered over time; snow and avalanches are reported on the graph and kept up to date during snowfalls.

b) Local forecasting techniques

It is up to the local forecasters, with a few years of experience and adequate equipment, to develop their own techniques for forecasting adapted to the local conditions. They will remember the fact that the majority of natural avalanches occur immediately after heavy snowfalls. This is even truer when artificial release is undertaken regularly, unburdening the slopes as the snow cover builds up.

By way of example we will cite the forecasting method perfected by American and Canadian experts reported in the diagrams of Figure 12.

To begin with the periods of snowfall which produce dangerous avalanches are studied; for each period a graph of cumulative precipitation/ time is drawn and an arrow is used to indicate the instances of avalanches in one or more gulleys under observation.

Using only the periods of snowfall which provoked avalanches, one superimposes on a single diagram all the curves originating at the beginning of each snowfall period. One then traces the average curve which will be characteristic for the station and that type of snowfall period; this curve becomes the control curve. It will be seen that the avalanche gulleys will fall into two or more groups, depending on the interval of time between the beginning of the snowfall and the release of the avalanche. Certain precocious gulleys will avalanche after 15 hours, others after 36 hours, or others after 80 hours if snow continues to accumulate.

Often the first group of avalanche releases will coincide with the figures given by numerous authors as the threshold for the first releases of avalanches: 2.5 gr. of fresh snow per cm.

Starting from the point on the control curve corresponding to the average time of release for a group of gulleys a critical point can be established, after which the alarm should be given and explosive charges used. In practice one has to go back four to six hours from the critical point depending on how much time is needed to prepare for action. When a single curve for a period of snowfall approaches the critical point the alarm can be given and the individual gulleys which need dealing with will be known in advance.

Figure 12 also tells us if there is the need to worry about the formation of wind slabs of snow. It has been established that a windspeed of 25-50 km./hr. and temperatures between -120 and -1 constitute ideal conditions for the formation of such slabs. The wind direction will give us the geographic locations (down-wind slopes) of these slabs which are the most receptive to artificial release by explosion because of the easy transmission of fractures in wind-blown snow.

The third graph shows the evolution of the thickness of fresh snow (where the penetrometer penetrates without the assistance of the driving weight) and the index of stability which is the ratio between the shear strength of the weakest layer of fresh snow (layer determined by use of the light penetrometer) and the weight of snow over the weakest layer. An index of stability greater than or equal to 0.7 for fresh snow over 30 cm. thick indicates that the conditions are very close to the point of natural release of an avalanche. The total thickness of fresh snow gives us the approximate volume of snow which could be set in motion ¹/.

Often in practice, the local experience of those responsible supplants to a large extent many of these measurements. This experience allows avalanches to be released under the necessary conditions and with adequate safety; however progress needs to be made in Europe with the methods for determining the moment of release.

¹/ See following page

The indications of Graph II (Figure 12) (wind and temperature) and of Graph III (Figure 12) (thickness of fresh snow and index of stability) allow us to establish whether the time of alarm should be advanced or delayed (closure of road or skirun to be protected) and the time of release. Experiments can be carried out on slopes similar to those we alluded to above either by walking on them or skiing across them to see whether or not small avalanches are released.

2.3 Explosives

Due to its particular characteristics, snow "smothers" explosives, which will therefore not be effective unless used in relatively large quantities, especially when compared with blasting in mining operations, for example (Figure 13).

It is not so much the production of hot gasses as the shock waves, which create a sudden increase in pressure, which are important. They can:

- break up a slab of snow,

- break its anchorages,

- set a layer of snow vibrating thus diminishing the coefficient of friction with the ground or with another layer of snow (change from static to dynamic friction),

- break the dendritic crystals of fresh snow or dislodge crystals of depth hoard. The presence of this snow greatly enhances the effect of explosions and they are generally especially effective. Shattering explosives are normally used (speed of detonation from 2 000 to 6 000 m./sec.) in quite large quantities: the equivalent of 3.6 kg. of T.N.T. for surface work. In the United States charges of 1 kg. for fresh snow and 2-5 kg. for other cases are used.

Figure 15 shows that the high pressure engendered on the surface of snow is more widespread and therefore has maximum effect when the charge is placed at some distance above the surface. But only filling from an overhead cable allows this to be carried out (see Fig 15.)

When an explosion is set off on the surface we can calculate from Figure 15 the radius within which the high pressure waves will be adequate (approximately 30 millibars seems to be the lowest effective pressure for triggering an avalanche in snow which is practically ready to go) (Figure 16).

Lastly, in explosions at ground depth, where the depth of the charge is equal to the height of the snow cover the optimal charge is one which creates the largest crater on the surface: W = (h/0.75)³ If the gulley is artificially released in the normal manner the height h used will be that of fresh snow fallen during the "period of snow", at the end of which the avalanche is to be released.

2.3.1 Setting of explosives by hand

In periods of avalanche risk access to the starting zone is always dangerous. Generally one makes do, when possible without taking risks, with gaining access to the ridge dominating the starting zone in question. One can then use one of the following methods.

a) Blasting of cornices

One kg. charges are inserted every two metres either directly or after drilling into the snow, and joined by a detonating fuse and set off simultaneously. If the cornice is over 2.50 m. thick each hole will need two or three charges (two to three kg. of explosive). The aim of blasting cornices either is to cause an avalanche in the extra thickness of snow (scarp) below the cornice, or to pre-empt the natural fracture of the cornice and the subsequent avalanche.

Photo No. 11
Attempt at blasting a cornice (1 kg. charge buried deep in the snow, a failed attempt)

b) Throwing grenades

The charges are primed, lit and thrown in order to explode with a few seconds' delay. It is now advised to tie the charge to a length of thin rope of predetermined length in order to place the explosive exactly, and if need be recover it should it fail to detonate. Specially prepared charges do exist but hand made ones using food tins and a slow fuse can also be used.

Photo No. 12
Lighting a home-made charge using a food tin as a container. The thin rope tied to the charge can be seen, as well as the safety rope tied to the thrower.

Figure 13
Relation between incident pressure and reflected pressure for normal reflection on a rigid surface and on the surface of the snow cover (after Ingram)

Figure 14
Pressure wave caused by an explosion above the snow cover as a function of the height of the explosion and the distance from point zero (after Ingram)

Figure 15
Pressure wave for an explosion above-the surface of the snow (for a site at an altitude of 1 800 m.) (after M. Mellor)

c) Placing explosives by sliding them over the snow

The thin safety rope which is now regarded as desirable in all cases makes the actual "throwing" of the charge pointless. A cylindrical or conical charge can be set on its way simply by sliding it down the slope on the end of its string which can be anything up to 100 m. lon Otherwise a small sledge can be constructed (from ski-tips for example) so that the charge can be placed exactly at the chosen spot.

Although widely used, the placing of explosive by hand has the drawback of being risky for those carrying it out. In certain cases, due to particular atmospheric conditions, the use of explosive may have to be foregone. The gulleys then become heavily loaded with snow and those responsible may then be hesitant about "purging" the slope.

2.4 Release from a distance

Here we deal with the different means of causing an explosion in a predetermined area from a place which will undoubtedly be safe at all times.

2.4.1 The use of cablecars

Where a cablecar or chairlift goes over a starting zone charges can be thrown from above. Just as before, a thin rope is attached to the explosive so that it can be recovered should it fail to detonate. Furthermore, since there is seldom a shield for the cablecar (or chairlift) precautions should be taken to ensure that it is not damaged. The minimum distance required is, in metres:

D = W

where : W is the weight, in grams, of  T.N.T. or its equivalent.

2.4.2 Placing explosives in the ground before the first snowfall

This method consists essentially in laying a network of explosives underground, which can be used successively throughout the snow season. Experiments with this technique have been carried out in Switzerland; in France it has been used for a few years. Each line consists of several charges of explosive which are set off electrically by instantaneous antistatic detonators. A junction box connects all the lines, this is then connected to a remote control detonator, which can be placed in a safe place via multiconductor cable (Figure 16).

Figure 16
Sketch of a remote control release system

This system is of interest since it can be used whatever the weather and because the charges can be controlled at any time. There are, however, some problems in its use. From the technical point of view we see that, since the explosive charges are at round level, they have to be larger in order to have an equivalent effect (see paragraph 2.3 of this chapter). From the practical point, the technique is limited to areas with no rocks, as the cables have to be buried. On the safety side, we see that the placing of explosives has to occur at the last possible moment before the first snowfall in an area far from where tourists or hunters might wander.

Despite these limitations and its cost, this method has proved to be very useful and successful where other techniques cannot be used.

2.4.3 Use of a cable to place explosives

Experiments have been carried out in Austria, Germany and recently in France with a release technique using charges suspended from overhead cables. A circulating cable or a bogey on a single cable allows an explosive charge, lit at a distance, to be placed over a starting zone.

Experience has shown that providing the explosive is over 1.50 m. from the cable this will be undamaged. A long fuse can be lit in a safe place just before the charge is sent on its way. Alternatively electronic detonation using a single circular cable connected to a specially designed control box, which ensures maximum protection from the risks due to induced electric currents, can be used. Since the starting zones are all quite similar (high up in gulleys) it is quite easy to arrange a network of cables which covers each gulley successively. This method has the advantage of being above the snow level, which is the most effective position using the smallest charge. This method allows the explosive to be activated at the actual moment of utilization; it can be used in rocky gulleys; but a system of intermediate pylons placed on secondary ridges and out of danger from avalanches, whether they be natural or artificially released, is also needed. The use of these cables makes up for any deficiencies in underground networks; simultaneous use of both systems should be adequate for most situations. However, both systems entail the installation of permanent and costly equipment at the top starting zones which have themselves been carefully located and studied. Only launchers (of rockets, etc.) can make up for unforeseen situations.

2.4.4 Use of "launchers"

The use of launchers, rockets, mortars and cannons is now one of the classic methods of control and is widespread throughout many countries (especially the USA) provided there is no legislative problem (the holding and use of weapons of war by civilian authorities). The method has considerable advantages, primarily that of being quick and safe in its execution from safe places prepared beforehand. On the negative side, there is the problem of accuracy with some shots, but above all the permanent hazard presented by shells which have failed to explode.

a) Launchers

A detailed description of all the various types of launchers used is not possible here. We will have to make do with a brief summary.

Self-propelled missiles. rockets and bazookas have often been proposed. They are simple and cheap-, 'but their accuracy, range and the weight of explosive sent to the starting zone are not adequate. More sophisticated devices such as anti-tank weapons have not been tried because of their high price and the hollow charge with which they are equipped in the military version.

Mortars have been much used especially in Switzerland for the protection of -communication routes and, more recently, for winter sports resorts. The types of mortars that are used classically are 60 mm. and 81 mm. Experiments with 120 mm. mortars have shown them to be satisfactory, although all mortars have a lack of accuracy inherent to missiles with a curved trajectory.

Cannons are in current use in the United States where recoilless rifles of 75 and 106 mm. calibre are used and are completely satisfactory. These weapons have a great range, are extremely accurate and have a sighting system which is very easy to use. However, they have the disadvantage of the possibility of ricochet when fired at low angles, which could be very dangerous in the more densely inhabited mountains of Europe. The cannon and its ammunition, notably the 106 mm. , also have the drawback of being very expensive.

Pneumatic launchers The only launcher which has been specifically designed for artificial release of avalanches is a missile with a curved trajectory propelled by compressed gas (a cylinder of nitrogen); it is relatively light (100 kg.), it is easily handled but must be carefully set up. The initial speed of the missile is very low so the accuracy of the shot will decrease if there is a strong wind blowing. Its effective range is quite low (of the order of 1 100 m. horizontally for a 1 kg. charge of explosive, and of 900 m. for 2.5 kg. of explosive).

Clearly the ideal launcher is yet to be found, for it should be economical, accurate even in windy weather; it should have adequate range (at least 1 500 m. with 3 kg. of explosive), and should be easy to use. Lastly, the weapon must be entrusted to civilians, respecting the particular legislation of each country.

b) The projectiles

The use of projectiles developed for military uses is often the only option which is economically viable; however, these projectiles do not lend themselves readily to the requirements of avalanche release:

- from the point of view of weight the outer casing must be sufficiently strong to withstand the initial velocity, but it should have as little steel as possible (for it makes shrapnel) and as much explosive as possible;

- from the point of view of the security of the fuse: this, for safety reasons, should not be too sensitive, but fresh snow absorbs impacts progressively so too low a sensitivity could dangerously increase the proportion of failures to explode. On the other hand, it is vital in the mountains that the shell should explode even if its impact is with an oblique surface in relation to its trajectory;

- from the safety point of view it would be desirable that the detonating system self-destructed the charge if it had failed to explode on impact; such systems do exist but they are extremely expensive.

Only mass manufacture for a large market (Europe) would make the development of a specific projectile for avalanche release viable. Meanwhile we have to make do with what is available.

The number of different methods of avalanche release which we have examined in this chapter shows that no single one is perfectly suitable under all circumstances. However, the panoply is sufficiently varied so that wherever preemptive artificial release is required there is a suitable technique. Must we be reminded, though, that this preemptive release must be forbidden if the avalanche could reach property or people; if there is no good service for localizing avalanche risk; and finally if all the requisite safety measures necessary for handling explosives on the one hand, and avalanches on the other, have not been taken?

Naturally public authorities have been, little by little, led to taking a close interest in this field, in order to ensure careful elaboration and strict observation of safety instructions. The resulting regulations should not be considered a barely tolerable constraint but rather as an aid to the development of the method. Nothing could be more damaging to the diffusion of these methods than a succession of accidents caused by poor observation of regulations or a lack of them.

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