Just as avalanche defences aim to fix the snow cover to its slope, attempts are made with the appropriate structures to influence the mass of snow moved by the wind.
When snow falls in absolute calm, cold weather, it forms a very weak stratum which, if the gradient of the slope is appropriate, can collapse and form a powder snow avalanche. Conditions which allow such light snow to fall are virtually unknown above the tree line. Here the wind encounters few major obstacles and thus the snow crystals which have already been damaged during their fall form denser flakes. Wind picks them up when they reach the ground, if it has not already done so during their fall, and sweeps them along like sand in the desert. Snow transported in this manner (wind-blown snow) progressively levels broken ground and collects in hollows and depressions forming drifts, while prominent terrain is completely stripped of snow by the wind.
For snow to be blown by the wind (depending on its nature and the climatic conditions) a wind-speed of between 3 and 7m./sec. is necessary. Fresh dry snow is transported most easily.
The wind packs and bonds the crystals together, and consolidates them into ever thicker slabs which generally end up forming snow slab avalanches on avalancheprone slopes. These slabs evolve in a different way when the wind blows over obstacles: the slabs are then progressively transformed into cornices which protrude above the obstacle, especially if this is a ridge. The cornice is formed by a series of small strata of superimposed wind-blown snow, and these settle and knit under the effect of their own weight. Usually it is not the cornice itself that is dangerous, as this is made of compact snow, but the excess thickness of snow which builds up on the sheltered slope. On this slope snow slabs and noncohesive layers build up alternately until the snow slips away as an avalanche.
It is also possible that the collapse of a cornice releases an avalanche, but this is rarer.
Snow drifts as well as cornices in an area of defence works are evictence of the detrimental effects of wind.
The first structures devised to combat wind-blown snow sought to protect communication routes from drifts which disturbed traffic and made it more difficult to keep them clear. For this reason one finds almost everywhere, even at relatively low altitudes, and on flat but windy ground, hedges, trellis work and fences along roads. Train tracks have been protected in a similar manner either with galleries or with jet-roofs which accelerate the wind; the latter ensure selfclearing of points on some mountain railways, so long as the wind is constant.
As far as avalanche defences are concerned, the wind has been used unwittingly ever since the first free-standing dry-stone walls. Sharp corners in these soon proved to be structures which altered the deposition of snow. Thus, certain structures which had been built with rounded ends were soon altered to form sharp angles, or complementary structures in the form of isolated walls were erected. People even went so far as to build veritable anti-cornice walls on less steep slopes and on small terraces above a dangerous area.
Since the quality of the stone frequently left much to be desired# these structures were often too small; this led to the construction, over 30 years ago, of wooden baffles - either full or with openings (planks with a gap between them placed either vertically, horizontally or obliquely) - which had varying degrees of success. These first attempts showed that the structures had a considerable effect on the snow cover, particularly in provoking its deposition or preventing the formation of particular cornices. This effect is particularly marked above the tree line or on bare ground.
After the catastrophic winter of 1951, which indirectly provided an enormous stimulus to fabricated stabilizing structures, experts became more actively interested in wind deflecting structures. In effect, these fabricated stabilizing structures themselves influence the deposition of windblown snow; thus within some controlled areas the accumulation of wind-blown snow requires stabilizing structures of prohibitive size. Indeed, it is the structures themselves that, once in place, produce the formation of drifts which overload some of them and cause them either to collapse, or their foundations to be torn out. Various solutions have been found, but the fundamental principles are always as follows:
- use the wind to provoke an accumulation of snow in a suitable place,
- inhibit the accumulation of snow in unfavourable places.
However seductive these solutions might appear, wind deflecting structures cannot serve as the basis of avalanche defence in an area. In effect their use is a delicate matter, unless it always snows with the same prevailing wind - which is rare. Furthermore, when one wants to erect theme one seldom has sufficient knowledge of the local wind conditions; work often begins hurriedly in an area to be controlled, after a catastrophe or serious alarm and consequently, winter and meteorological observations are often seriously deficient.
On the other hand, when local conditions are better known, wind deflecting structures can provide an excellent service as a complement to classical structures. Above all, they allow structures in the starting zone to be contained within reasonable dimensions and prevent the formation of cornices above the controlled area.
These structures, which are not subjected to the pressure of snow but only have to be strong enough to stand up to the strongest winds, also have the advantage of being light and easy to transport, and do not require large works for their erection.
The guidelines give the goal as "the prevention of accumulation of wind-blown snow".
Under certain circumstances, stabilizing structures can be completed effectively with other types of defences.
Light deflecting structures such as screens, vertical or inclined faces, influence the deposition of wind-blown snow in such a way as to:
- inhibit the growth of cornices,
- reduce the deposition of snow in starting zones.
2.1 Wind baffles are designed to reduce the speed of the wind and thus, indirectly, cause the snow carried by it to fall to the ground.
Instead of having a wind baffle with the continuity of a wall, which would clearly be very effective but which would soon be filled up and thus be without further influence, gaps are left between its planks. By varying the size of the gap, the deposition desired can be produced down-wind of the structure, without its losing its efficacy as an obstacle. The height of the structure is determined in relation to the quantity of wind-blown snow which one wants to retain. A higher structure has a longer period of efficacy. Even more snow can be held back by staggering the wind baffles.
Materials that can be used in these structures are: wood, steel, aluminium (alloys). (See photos 79, 80 and 81.)
2.2 Eddy panels When one sees isolated trees in the mountains, particularly old arolla
pines and mountain firs with straight trunks, the gnarled branches of which spread
upwards, one notices that their base is usually free of snow. The wind is accelerated near
the obstacle and the snow is blown further on. This phenomenon does not occur with
spruces,
the branches of which reach the ground. The Austrian engineer Handl noticed
that old arollas are shaped like inverted trapeze and had the idea of building
eddy panels of the same shape: a panel three to four metres high, two metres
wide at its base and three metres at the top.
This system has the advantage of being simple, but nevertheless it is controversial. In effect such structures are not really suitable everywhere, and to place them one has to proceed by trial and error.
They can be used to inhibit the formation of cornices when there is no site available for a system of wind baffles. In the same manner, the load of snow built up by the wind on rakes and bridges can be reduced. Nevertheless, these panels must be placed by trial and error to find their optimum position. (See photos 82 and 83.)
The influence of wind baffles
Examples of profiles of snow deposited in the shelter of wind baffles:
No. 1: Average containing effect of a steel wind baffle with gaps (horizontal strips)
No. 2: Minimum and maximum containing effect for a light metal wind baffle with vertical strips (extremes taken over 10 years)
No. 3: As above, in wood (extremes taken over six years)
No. 4: As above for a full wind baffle. When this is covered the wind starts to carry the deposited snow away from the hollow of this frozen "wave".
2.3 Jet-roofs are the most effective wind-deflecting structures to prevent the formation of cornices and undesirable accumulations of snow just below a ridge. The structure is shaped like a flat roof inclined at 40 and made with planks with a gap of 1.5 em. between them to ensure selfclearing, and is usually built at the top of the ridge. The wind is diverted into the leeward slope and blows the snow that would normally accumulate there away f o it. (The lower edge of the structure is 1.20 m. above the ground) (See photo 84.)
2.4 Pulpits are a sort of prolongation of the windward slope over the downwind slope. These devices prevent the formation of cornices.
2.5 When the winds blow in several directions and when the climate is sufficiently inclement, communication routes, particularly railways, can be obstructed in a variety of ways. Simple wind baffles in such cases will not suffice to prevent the formation of snow drifts. Light galleries which will stand up to the wind, combined with wind fences, must be built. Such structures are not really relevant to the subject of avalanche defences and we mention them only for the sake of completeness of this nomenclature.
The research of Croce and T.R. Schneider showed that for wind baffles and snow fences along roads, structures with vertical planks and 50 percent gaps between each other, and a space 15 to 20 em. above the ground, give the best results.
If possible, the structures must be placed perpendicular to the wind direction (with a maximum tolerance of 20°)
Wide planks reduce the efficacy of wind baffles. Moreover, when leaving 50 percent gaps the width of the slats should not exceed 5 cm.
Wind baffles for roads should be placed at a distance equivalent to 15 times the height of the obstacle (created by the kerb) to be protected from snow drifts. This distance should be reduced to 10 times if the wind baffles are staggered.
According to Croce the distance on flat ground between the wind baffle and the object protected should be determined with the following formula:
where:
A = distance
h = height of wind baffle
k = variable between 0.8 and 1.35 according to the percentage of fill of the baffle, which should be between 35 and 75 percent
With regard to wind baffles built in areas of avalanche defences, practical experience shows that the formula just cited seems to give approximately correct results.
Various systems have been used successfully and the photographs nos. 78 to 81 show the results.
Planks nailed alternately on either side of the pole increase the turbulence of the wind (photo 112). Space between poles: 2 m. Height: 4 m. + 1 m. underground. Planks between 25 mm. and 16 em. wide. The lowest horizontal plank is 80 em. above the ground; 87 percent average fillin. Systems with vertical planks are better, and require less maintenance since they are not subjected to as much snow creep anglide. One metre of wind baffle can produce the deposition of 90 to 170 m³ of snow (max.220 m³)
Metal structures have also been built using the same criteria.
When one is building permanent structures precautions must be taken against permafrost which appears above a certain altitude.
Eddy panels can also be very effective. 2 An empty space of 103 m³ has been measured downwind of an eddy panel 7. 5 m in area. If one of these structures is placed on a slope (such as at the edge of an area of avalanche defences) it must be turned so that its face is parallel to the line of greatest gradient, for otherwise it will be destroyed by snow creep and glide. These structures must not be used where snow creep and glide is particularly pronounced nor where the water table is high.
Carefully placed wind deflecting structures have a constant effect in all weathers from the first snowfall until it eventually thaws. The most important condition is that there should be wind of sufficient strength and constancy over several days uninterruptedly. Such conditions are usually found in upper sub-Alpine regions and higher.
One should not worry too much about wind deflectors which take snow away from the starting zone and deposit it elsewhere in the defence area; one should take this into account but not excessively.
Isolated wind deflectors must definitively be built of durable materials if these are to be included in a worthwhile manner in a permanent complex of avalanche defences.
When an area is being controlled one must adapt to local conditions and combine the different structures rationally in such a way as to give the best protection against avalanche release. Thus. particularly with wind deflecting structures, the controlled area must be observed regularly, and visited in winter if possible and over several years before work is begun.
3.2 The direction of prevailing winds in the defence area must be known: both the winds that blow from the valley and the local winds that blow down the slope. Local winds that blow up the slope must be taken into consideration along ridges and on the windward side, wherever the gradient of the slope is gentle. Since the direction of the wind can vary considerably because of local relief, a wind gauge for speed and direction placed on the site is the best way to provide a better forecast of wind behaviour.
The director of works must be a good observer and must live near the area under control, for deflecting structures are erected temporarily to start with. Once one is certain that they are placed in the correct place, they are then fixed permanently. This does not exclude further controls and maintenance work, as well as the placement of complementary structures which might be necessary to complete the defences.
3.3 An important point is, of course, the resistance of the structures to the force of the - wind. Steel stays do not perform satisfactorily since they are too vulnerable to creep and glide and to the settling of the snow cover. Preferable to stays are raking shores.
One can also use anchoring cables placed longitudinally on the windward side instead of raking shores. In this case the posts of the wind baffle must be placed in the shape of a very flattened hyperbola so that tension is distributed equally on them and indirectly on the raking shores on the leeward side.
It is difficult to give accurate figures for the cost of these structures. One might say that they are relatively cheap compared to other systems (deviating or anchoring the snow). Here too the price of the superstructure varies according to the material used, with wood being the cheapest. Nevertheless, depending on the area, but especially if the defences are permanent, it is advisable to use more durable materials. It is also possible to run tests with wooden structures and on the basis of the results place permanent structures made of metal (or mixed materials).
Costs of transportation and erection can vary considerably, depending on ease of access and the type of ground encountered.
