Located in the centre of Europe, the Alps have been inhabited, even in their remotest valleys, for several thousand years. In France, the population maximum was reached during the nineteenth century. Since the beginning of the present century, however, the French Alps have experienced accelerating depopulation as the mountain people seek easier ways of life and higher living standards elsewhere. This process of emigration has resulted in a progressive loss of local knowledge concerning the historical experience of natural hazards (risques naturelles).
During the past 20 or 30 years, a new population has been attracted to re-occupy the mountains. These people are principally transitory and lack mountain-hazard experience: ski-ing is their primary attraction and they form a winter-time population maximum in new settlements high above the old villages. These new settlements take the form of high-density ski resorts, or dispersed mountain cottages, usually equipped with modern conveniences. Repeated accidents would indicate that many of these buildings have been sited without regard to, or knowledge of, areas under the threat of snow avalanches and other natural hazards. Public authorities now face the responsibility of controlling this hitherto unplanned and irresponsible growth pattern. Warning of the risks involved is a minimum requirement (cf. Reference AulitzkyAulitzky, 1974).
Historical knowledge: map of the probable location of avalanches (carte de localisation probable des avalanches – C.L.P.A.)
There is a pressing need to reconstitute “collective memory” based on the accumulation of group experience through time, and which is lacking for many areas because of mountain depopulation.
Since 1899 there has been a systematic attempt to assemble observations on activity in the better-known avalanche paths that occur in the proximity of inhabited settlements. This so- called “continuing avalanche enquiry” (enquête permanente sur les avalanches) is extremely important in the development of avalanche-hazard forecast bulletins published by Météorologie Nationale (de CréReference Crécycy, 1966). This approach was recently updated to ensure compatibility with the international avalanche classification (de Reference Quervain, Quervain, Chairman, Crécy, LaChapelle, Losev and ShodaQuervain and others, 1973). It is invaluable for avalanche-frequency studies but it covers only well-known avalanche paths and not new paths, or previously unrecorded paths that may threaten ski resorts.
The most important contribution to avalanche-zoning developments in the French Alps was made between 1970 and 1976. This work resulted in the publication of maps at a scale of 1 : 20000 known as “maps of the probable location of avalanches”. The areas covered included 600000 ha in the Alps and Pyrénees. Two approaches were employed.
Photo-interpretation
Air photographs were studied stercoscopically so that avalanche paths that have been active during the last 100 or 200 years could be identified (Reference CazabatCazabat, 1972). Starting zones arc indicated by topographic features: steepness of slopes, convex profiles, occurrence of cliffs, aspect, and so on. Avalanche tracks are usually conspicuous, especially when they cut through a forested area. Even when a century or more has elapsed since the occurrence of a significant avalanche event, variations in species composition and age structure are reliable diagnostic elements for the interpreter. Determination of the extent of the run-out zone is frequently more difficult. However, avalanche debris can often be identified, or a comparison of sequential air-photograph coverage will reveal progressive landscape modification such as to facilitate mapping of approximate run-out limits.
Field investigation
The air-photograph interpreter, accompanied by an avalanche specialist, can then undertake the second phase of the mapping; this will involve interviewing the local inhabitants. This method involves assembling local witnesses so that they confront one another, thus testing the accuracy of their recollections against each other or against any historical documentation. The field team will also map all landscape features of avalanches that may not have been observed during the photo-interpretation phase (Reference MartinelliMartinelli, 1974). These features will include:
(i) Areas of major snow accumulation: excessive thicknesses of snow along mountain lee crests or indications of snow deposits, such as avalanche debris remaining into the summer, and vegetation patterns.
(ii) Physical features such as waterlogged areas, badly kept pastures, or pastures being invaded by low shrubs; deformed or broken trees, and so on.
(iii) Sometimes there are anthropogenic indicators such as peculiarities in the shape, size, and orientation of fields as recorded on the cadastral register; the presence of a cross or an ex voto in a field, or even the remains of an old avalanche-deflecting structure originally built to protect a house which has since disappeared.
Data collected in this manner, after they have been submitted to verification by local witnesses, are recorded on the map. This compilation constitutes the second part of the map.
Map showing the probable location of avalanches: advantages and disadvantages
The two sets of data, air-photograph data and that derived from field work, are then superimposed on the same map, with differentiation being achieved by using an orange colour for the former and a pink colour for the latter. This becomes the map showing “the probable location of avalanches”, which is printed and distributed to various civic authorities responsible for public safety and land-use planning. The mayor of the relevant parish must ratify the map.
In each case where written evidence or verbal witnesses are available, a card is filed recording the avalanche-path situation and its features. The card is also used to record information on the time of each recorded avalanche event, the major damage incurred, and the name of the observer.
The main advantage of this type of map is that it can be prepared quickly and economically (two teams of two men can cover about 100 000 ha during one summer of field investigation at a cost of about 15 F/ha or 1.21 U.S.$/acre). Such maps are adequate to facilitate land-use decision-making on a broad scale, such as the siting of a new road, ski resort, or town. On the other hand, this approach does not enable assessment of avalanche frequency nor possible future occurrence. Therefore, it is not satisfactory as a basis for issuing licences for specific buildings.
“Avalanche-zone plans” (plans des zones exposées aux avalanches– P.Z.E.A.)
Article R.III.3 of the French “code de l'urbanisme” (town-planning code) stipulates that building restrictions may be applied to areas subject to natural hazards such as avalanches, and that these areas must be delimited by prefect decree. To meet such a legal requirement, a ministerial circular, dated 5 December 1974, was issued for organization of the preparation of avalanche-zone plans. These documents are drawn up at the request of the parish or prefect, and cover only those areas where town planning is in progress or is envisaged. The scale used is the same as that of the town plans (i.e. 1 : 2000 or 1 : 5000).
Typical features of avalanche-zone plans
They are comparable to their Swiss and Austrian counterparts in that they are multi-coloured :
Red zone: definitely dangerous and no construction permitted.
White zone: considered as most likely to be safe.
Blue zone: an intermediate area of uncertain safety.
To achieve this tripartite zoning, specialists must inspect every avalanche path and undertake a thorough enquiry. Generally, the “red zone” will correspond to the orange and pink areas of the maps of “the probable location of avalanches since it can be assumed that avalanches have occurred there in the past.
The “blue zone” is much more problematical. Personal evaluation by the author of the zone plan is of prime importance. However, this is supplemented by calculations using Voellmy’s formulae. Research is also undertaken into “model” avalanche flow by mathematical techniques (Reference Pochat and CarryPochat and Carry, 1978; Brugnot and Pochat, in press) for flowing avalanches and by the use of physical models in water channels for powder-snow avalanches (Hopfinger and Tochon-Reference Hopfinger and Tochon-DanguyDanguy, 1977; Reference Hopfinger and BeghinHopfinger and Beghin, 1978). These approaches are of great assistance in approximating blue-zone boundaries. Nevertheless, the problem of estimating maximum possible snow depth in the starting zone still remains. Similarly, the problem of establishing the return period of such a climax event remains unresolved. Thus the actual degree of hazard cannot be determined quantitatively.
Study of the card indexes of the “continuing avalanche enquiry” (“enquête permanente sur les avalanches”) will also be of assistance, in addition to the current research in numerical avalanche forecasting (Reference Bois and ObledBois and Obled, 1973; Bois and others, [1975]; Reference BovisBovis, 1977). Finally, it is necessary to decide on the frequency of occurrence between red- and blue-zone designation. In France two principles are taken into account:
(i) Extremely rare avalanches with a return period of the same magnitude as that of post-glacial climatic fluctuations (i.e. about 300 years) may be very severe. Yet, the long return period hardly warrants total prohibition of construction. Thus a blue-zone designation is made and security of inhabitants is achieved through a judicial decision to order evacuation by the civic authorities. Such may be ordered, for instance, during exceptional meteorological conditions, such as very heavy snow-fall, and circumstances that can easily be identified.
(ii) Small avalanches with a return period greater than 30 or 40 years are also regarded as falling within blue-zone delimitation. In this case, structural constraints on buildings can be used to ensure adequate protection.
Legal consequences of avalanche-zone plans
For the red zone no building permits will ever be issued, while construction in the white zone is not restricted from the point of view of protection against natural hazards.
In certain circumstances, building licences may be refused for blue-zone applications. If a licence is issued, it may be dependent on certain conditions determined by specialists. These include:
Structural conditions. Walls of buildings facing the path of an oncoming avalanche shall have no windows or, if windows are allowed, they shall have fixed frames only with strengthened glass such as to withstand pressures of 1000, 2000, or 3000 daN m-2 for one or two floor levels. These stipulations presume the use of concrete for both main and dividing walls.
Roofs must not protrude beyond the top of the wall that faces the direction from which the avalanche may come, nor can the pitch exceed 45°. In some cases, deflecting structures may be required for individual buildings.
Town-plan conditions. Limitations can be placed on building density. This is related to a land-occupation coefficient (“coefficient d’occupation des sols”) which is the ratio of total floor space of completed building to initial area of land. Similarly, building alignments may be controlled, according to the slope line and the spacing between alignments of buildings. Thus, the up-slope house on one alignment may be specially protected and the space between alignments determined so that the design avalanche can pass between buildings. In this way, avalanche protection for one group of buildings will be undertaken so as not to increase the risk of an adjacent group.
Conditions imposed by local authorities. There is the possibility of evacuation of houses during periods of determined high avalanche risk. For this purpose, blue-zone designation is very important. Also, the mayor of a community has the obligation each autumn to require a detailed forest inspection where such forests provide blue-zone protection. Thus, if the forest has disappeared, or has been damaged by fire or disease, the mayor can forbid winter-time occupation of the blue-zone buildings until appropriate responses have been made. These could include, for instance, erection of snow fences in the avalanche starting zones above the damaged forest. Similarly, the mayor is responsible for autumn inspection of snow fences that protect blue-zone structures.
Conclusion
At the present time in France there are about 50 mountain communities equipped with avalanche-zone plans; this is about half of the expressed needs. The requirement for additional zone plans is probably growing because of the general agreement that zoning is the best and least expensive form of avalanche-hazard protection and reduction. Such plans are subject to revision as knowledge about avalanche behaviour increases, or as local conditions change. However, it has been necessary to take these immediate preliminary steps rather than to wait for perfection, because of the rapid growth in pressure from town planning in relation to the continued growth in winter recreation. The mountains remain vast and inviting but the amount of safe land at high elevations is probably much less than originally envisaged by the ski-resort developers. Extensive areas still remain in France, as in other countries, for conquest by the skier. We hope our modest experience in avalanche-hazard zoning will be useful to other countries.
