U. S. DEPARTMENT OF AGRICULTURE
FOREST SERVICE

ALTA AVALANCHE STUDY CENTER

WASATCH NATIONAL FOREST

TRANSLATION NO. 11

Translated by Rolf Doebbling & E. R. LaChapelle

I. History of the Avalanche Zoning Plan

On March 6, 1928, the federal forest inspector Dr. F. Fankhauser delivered a lecture on avalanches and avalanche defenses in which he said, "If certain sites known to be endangered have recently been sold to strangers for house construction, so that in some cases an outlay of millions is required to correct the already-committed error by means of defense structures, then this should serve as a lesson for the future and induce community officials to deny construction permits where such danger exists." This was not only said but written down 40 years ago. Unhappily the words have had little of the apparent effect they should have had. This negligence has exacted a bitter revenge in many localities.

The situation is not a new one; buildings erected long ago in danger zones sooner or later fell victim to avalanches. There is no truth in the widely heard rumor that the ancients knew exactly where and where not to build. Sooner or later very old buildings have been destroyed by avalanches, and even whole villages wiped out.

The heavy construction activity in recent decades gave rise to local planning activities. In all cases these have to take avalanches into account. Avalanche zoning thus is a permanent part of local planning by means of which avalanche damage can be prevented. In this respect the expression "avalanche zone planning" is correct. Objectively better and more correct would be the designation "avalanche zone mapping", since avalanche zones are not the product of planning; on the contrary, they often come in conflict with planning, a factor which has great legal significance. Avalanche zones do not arise from the desires of planners, but are imposed on them as natural forces independent of human influence. The already adopted expression "avalanche zone planning" is justified if it is understood to include the whole complex of tasks involved in preventing avalanche damage which are addressed to technical as well as legal and administrative problems.

Localities available to foreigners in the mountains have not grown up only in recent times. The growth resulted in part from a lack of space in areas which were known to be avalanche districts. A period of avalanche inactivity sooner or later allows the hazard to sink into forgetfulness. O. Liitschg-Loetscher wrote about the health resort of Davos in 1943: "Because the health resort began to develop in the last quarter of the l9th century, more than a generation after the disaster of 1817, construction was most intensive on the sunniest, but most severely endangered, part of Davos. Warning voices should definitely have been raised against this, but, since most of the construction was undertaken by people unacquainted with the area who paid the natives high prices for the land, the warnings didn't get through."

For his part, Fankhauser spoke only of building permits which should have been denied. Later the Federal Forest Inspectorate took up the matter once more. After the great avalanche catastrophes of winter 1950/51, the Federal Department of the Interior published guidelines in which the necessity of avalanche zoning plans was emphasized. Seven years later, on July 9, 1959, the proclamation addressed to the governments of the mountain cantons was repeated, in which the instructions were recalled with the following words: "The guidelines of our Department of June 17, 1952, concerning afforestation and defense projects in tracts exposed to avalanche danger contained certain instructions intended to minimize the future exposure of human life and structures to avalanche danger. These instructions read as follows:

"The preparation of avalanche zoning plans and avalanche cadasters is indispensable if future loss of life and property is to be prevented.......... The Federal Government cannot underwrite contributions to resettlement or measures to protect structures against avalanches if the construction site was chosen without regard to the avalanche zoning plan and cadaster, or if such is lacking, warnings of building commissions were disregarded."

"Experience teaches that the avalanche catastrophes of 1951 and 1954 are all too soon forgotten and that today construction is beginning again with irresponsible carelessness in avalanche danger areas. The people involved must bear the responsibility for their own lack of foresight, for the Confederation, in conformation with the above-mentioned guidelines, cannot contribute to the cost of protective measures."

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Up to the end of the 50's the farmers of Davos and Verbier had used as they had for centuries their mowing meadows "auf den Boden", "Mayentset" and "PlanPra". In the autumn the cows grazed, the manure was put out to the meadows, and in summer the aromatic mountain hay was harvested. But the migration of mountain-people into the towns had long since begun. Again and again, "Maiensasse" were given up and even the less productive land was relinquished, for no more mowers could be found to cut the grass for poor compensation. The land became cheap. On the other hand, good economic conditions and the wealth of the lowlands brought more and more vacationers to the mountains. Among these were many who could afford a second, or vacation, house. By the early 50's, the hunger for land extended to mountain areas. The sale of stony acreage in a short time made the mountain farmers rich. Speculators smelled business, got into the act, and soon ferreted out the poorest and most remote corners of land. Areas bought en bloc were subdivided, designated as construction sites, and parcels sold at truly high connoisseurs' prices. The "construction sites" also included avalanche areas. But the buyers for the most part did not come from local areas and many were even foreigners. These people hardly ever noticed the serious drawback of their "piece of building land", or noticed it only after it was too late.

The situation of the construction land and real estate markets in the best-known vacation resorts can best be shown if we recall the measure against land speculation which, among other things, led to the "Mandatory Permission for the Acquisition of Land Parcels by Foreign Persons." The "sell-off of the homeland" was widespread in 1960. For instance, the Senate of the Grisons took up this matter. In the session of 27 May 1961 a member of the Senate declared: "Foreigners own 71 parcels of land in Davos 69 in St. Moritz and 99 in Arosa." In the 13 September 1960 session of the Senate of the Canton of Berne, Senator Gertsch inquired about which communities had avalanche zoning plans and which courts were competent to draw up and approve these plans. The reply was that not a single community of the canton had a finished plan. On 29 May 1962, Senator O. Largiadar from Pontresina posed a parliamentary question concerning protection against avalanche damage. This reads as follows:

"As a consequence of the current construction boom, vacation homes in some communities are being built without consideration and foresight in avalanche paths. The 17 June 1952 Guidelines of the Federal Department of the Interior concerning afforestation and defense structure projects in avalanche hazard zones contained certain instructions de signed to insure when possible in the future that lives and property were kept free of avalanche danger. In this same regulation, among other things the preparation of avalanche cadasters and zoning plans was required. The communities were also held responsible for suppressing construction planning in endangered areas. The most honorable Council is requested to impart information about who in the canton of the Grisons is responsible and ready to prepare these avalanche zoning plans. Does not the government accept the interpretation, that the required precautionary measures should be accorded more importance?"

We have taken a notice from the Neue Zurcher Zeitung of 3 November 1962, which is captioned "Expensive Vacation Homes are Built in Avalanche Paths". The notice reads as follows: "Chur, 2 November. The Council of the Grisons must presently act on the following: A scarcity of land suitable for construction has become apparent, especially in certain well-known resorts. This has led to the sale of avalanche hazard zones as construction sites. Furthermore, during the past summer numerous vacation homes have been erected on terrain which is not safe from avalanches."

The extent of construction activity in such resorts can be measured by the example of Davos (1950 permanent population: 10,500), for which the following figures on new housing starts have been taken from the newspaper "Die Volkswirtschaft": In 1959, 23 houses; in 1961, 47 houses; in 1963, 99 houses. The building activity thus has doubled every two years. Further examples are taken from a report of a regional planner who is familiar with conditions in the canton of the Grisons: "At one locality in the Upper Engadine with 221 residents, construction sites worth Sfr 2,000,000 were sold in 1964. At a thoroughly remote locality at the same altitude with 71 residents and situated around 700m above the valley road, a single corporation carefully acquired land for 30-40 houses. From other places whose names previously had been hardly known, similar alarming reports are arriving, accompanied by pleas for help, "what shall we do?".

Who should assume responsibility for the avalanche zone planning? This question was also posed by the Federal Forest Inspectorate, which has repeatedly pointed to the importance of avalanche zone planning. The Legal and Appeal Service of the Federal Department of the Interior discussed this question on 16 April 1961, in which it was stated: "Preparation of avalanche zoning plans is the affair of the mountain cantons as well as the communities endangered by avalanches. An expert opinion on the various legal aspects of avalanche zone planning points to concern for safety of the residents as primarily the responsibility of the local community. Since ancient times the community has been assigned to the local police by prescriptive rights. Inasmuch as the cantonal laws permit them to prepare zoning laws, the handling of avalanche zoning plans also falls within their competence." But only rarely do the local authorities have experts avail able who would be capable of executing the technical task of mapping avalanche zones. This is also clear from the proceedings of the 6 June 1962 conference of cantonal Forest Supervisors from the mountain cantons. The notice reads as follows: "The communities are not technically qualified for the preparation of a zoning plan; the forest services must assist them. The forestry officials as overseers of avalanche defense practice are obliged to serve the communities--when requested--as technical advisers on avalanche zone planning." This suggestion was especially appropriate because the forestry officials by reason of their professional expertise in delineating avalanche occurrence, especially in regard to timber damage, were the best prepared to offer objective opinions about avalanche dangers at individual areas.

"The Forest Inspectorate of the canton of the Grisons in the year 1872 had already issued a memorandum to the district forest officials assigning the task of collecting avalanche statistics and filling out the related forms. After the confederation in 1874 established a Forest Inspectorate, the federal department involved set out the project of accumulating avalanche statistics from the entire Swiss Alps. On 7 January 1878, the cantons were invited to have their forest personnel work toward this goal. These statistics included relevant historical facts about individual avalanche paths, such as unusual events associated with release and fall, size and character of effects and damages, accidents to persons and animals, unusual amounts of deposited snow debris, and similar data." This was written 89 years ago by the then Federal Forest Supervisor J. Coaz. The avalanche statistics were terminated in 1909 and published by Coaz in 1910. If we still had the original survey and if the statistics had been carried forward to the present, we would be close to a century-old avalanche cadaster, which would provide an outstanding foundation for avalanche zone planning. The two works of Coaz are interesting and valuable mines of information. Unfortunately, the accompanying avalanche map of the Swiss Alps to a scale of 1:250,000, even though a great achievement at the time, is not adequate for executing avalanche zoning plans. The original surveys in scales of 1:50,000, 1:25,000 and 1:100,000 cannot be located and were never continued. Another start was made following the avalanche winter of 1950/51. The Swiss Federal Institute for Snow and Avalanche Research (SLF Institute) began in 1955 the survey to build a new Swiss avalanche cadaster. In 1962 the cooperative work of the SLF Institute and the cantonal forest services was newly organized and the previously limited work was intensified.
It became necessary to train the needed specialists, not only for map ping avalanche areas, but also to introduce the representatives of local governments to their new responsibilities. The Federal Forest Inspectorate therefore directed the SLF Institute to give courses for people working on avalanche zone planning. The first course of this kind was given on 8-9 November 1962 in Davos. It was attended by 27 participants from all the mountain cantons except Schwyz and Obwalden. Representatives of local government and building departments, forest engineers, foresters, civil and cultivation engineers as well as a representative of an insurance dealing with damage caused by natural elements were among the participants. A second course was given 10-12 April 1967. While the first course was given to clarify questions on technical, administrative and legal aspects of avalanche zoning, the second course concentrated on mapping. For this reason, almost all of the 38 participants were forest service representatives, among whom were four Austrians. The Working Group for avalanche defenses, a dozen competent forest engineers working on avalanche control structures, was also interested in the problem of avalanche zone planning.


The first attempt at planning avalanche zoning was documented by the SLF Institute in regard to an extension and alignment plan for the settlement of Wengen on 30 September 1960. For the first time, problems dealing with the delimitation of avalanche-safe and avalanche-dangerous areas were discussed. Until 1968, most of the solutions then proposed were incorporated in more or less unchanged form in all new plannings. Even then it was obvious that an unquestionable separation of safe and dangerous areas could not be achieved. It became necessary to introduce a zone of transition between areas often proven to be swept by avalanches and areas proven to be safe from them. This zone of transition could not be considered totally safe from avalanche danger; however, the danger represented was so small that there was no justification for prohibiting construction in these areas. It became common practice to determine three zones. In definite avalanche areas, construction was prohibited and they were marked on maps in red color. The red zone was followed by a blue zone. In these areas, prohibition of construction was not justified; however, the possibility of danger was not excluded. An area that was definitely without danger from avalanches was designated as a white zone. The blue zone had in addition the special designation of an "imposition zone", which means that construction was permitted. However, building permits were issued with the understanding that the kind and form of building could be specified, as well as necessary reinforced construction or evacuation of the inhabitants in case of avalanche danger.
After the avalanche catastrophes on 26 January 1968 in Davos where the area "Boden" was especially hard hit, yet for which zoning maps existed, the question was raised if the SLF Institute practice of designating building zones was correct. Essentially, the researcher working on avalanche zoning maps should subdivide areas according to the degree of danger present. The government could then draw the line with respect to the use of the area for construction purposes and the kind of buildings that would be permissible.

At the conclusion of this historical survey, the question must be asked: How much has been achieved to this point? Unfortunately, avalanche zone planning was faced with the same problem as the planning of other projects such as street planning, village planning, etc.; that is, it was behind the actual needs. The people that were directly involved in this cannot be blamed, since they generally directed their efforts toward correcting this situation. It is the public that should be blamed, since it shows interest in a situation only after problems have developed, yet the planner is de pendent on its attitude. This is tragically demonstrated with water protection projects. In a lecture, "Building Politics in the Area of the Grisons--a prospect for Ten Years," given on November 23, 1966, Walz said, "A development has been noted for the last ten years that properly can only be called overcrowding." This proved to be true. Local governments of most involved communities, as well as building police and construction planning, were totally surprised and unprepared. Only under these circumstances was the project of avalanche zone planning initiated. Landowners, however, strongly objected, since they were interested in their property only as potential building sites. In many cases, reclamations were made before work was started on the project and hardly was planning finished for the priority areas when the first appeals were started by property owners. In perspective, the interests of property owners, as in most cases, is guided purely by the desire for making money. Speculation is mostly unchecked and it is shameful to see how wealth and money will turn people brutal and rotten. Questions dealing with ethics and human relations should be just as important as questions dealing with technology, legalities and administrative skill.

The problem could have been solved easier if notice would have been taken of the timely warning given by the Federal Forest Inspectorate. In any case, we should learn from our mistakes. Most avalanche zone planning has remained piece-work until today, mainly because extensive areas had to be neglected in order to deal with priority areas. We know of very few mountain communities that have planning which cover all the areas in question. In this respect, the outstanding efforts of the canton Nidwalden should be mentioned. Here the government systematically surveyed the whole area of the canton to determine the need for avalanche zone planning. This canton is the only one which has a detailed law concerning avalanche zoning plans, which went into effect April 24, 1964.

The serious avalanches in January 1968 drastically demonstrated the need for avalanche zone planning. On 5 February 1968, Senator Hubacher introduced the following bill/postulate to the Senate of the canton of Berne: "The government is asked to investigate the possible methods of constructing an avalanche zoning map for mountain areas. The latest avalanche occurrences have shown that such a map would not only be beneficial for the local people, but also for the tourist trade and for the building of tourist facilities, since it would give information on possible avalanche paths. In this connection, the question is raised if in areas designated as avalanche zones, construction should be prohibited, and if avalanche zoning could be used as a basis for an avalanche cadaster, which, analogous to the torrent cadaster, could be used for the promotion of structural avalanche control. The avalanche zoning map should also include every known avalanche path. Complete information is probably not possible, and no government responsibility could be linked to such a map. It should be used only as a guide."

In the press release, the Federal Forest Inspectorate also focused attention on avalanche zoning plans: "Because of observations made this winter, it is necessary to focus additional attention to organizations such as the Avalanche Warning Service in these regions, and to the so-called Passive Avalanche Defense that deals with the mapping of avalanche zones in which provisions have been made to restrict construction and which include building specifications."

As a result of a bill/postulate introduced by Representative Leu on 5 March 1968, the Diet (representatives of the cantons in the Parliament) will also have to deal with avalanche zone planning. This bill/postulate demands a thorough investigation on how law could be applied to inhabited areas to force the construction of avalanche defense structures and to prohibit construction in danger zones. A bill introduced to the National Council by Senator Grunig on 6 March 1968, states the following: "Thought should be given to a possible speedy establishment of an avalanche cadaster that would encompass all areas in Switzerland, especially since such a cadaster has already been demanded by different cantonal parliaments. Such an avalanche register would be prerequisite for avalanche zoning according to Article 32, Section 1. It would also constitute a solid basis for additional and necessary control structures. It would be an important part of indispensable national and regional planning in the mountains."

On the whole,avalanche zoning in Switzerland is still at its beginning. It can be proofed by some figures how big the task is which should be set about doing now. In the twelve mountain cantons of Berne, Uri, Schwyz, Ob walden, Nidwalden, Glarus, Fribourg, St. Gall, the Grisons, Tessin, Vaud and Valais there are 312 communities that need avalanche zone plans. Of these only 29 or 9% have such plans which often are not complete and extend only over part of the whole territory. These 312 communities extend over 1.5 millions of hectares. After deducting unproductive surfaces and the forested area there are still 664'280 hectares for which zoning plans must be prepared. The plans elaborated up to now comprehend a surface of 29'480 hectares or 4% of the total. The rest namely 96% have to be treated yet.


II. Concept and Description of the Avalanche Zone

An avalanche zone can generally be subdivided into three sections: a starting zone, a track and a runout-damage zone. Since the building authorities need to know only the conditions of the inhabited areas and potential construction sites, which are mostly located in the runout zone, the starting zone and the track are ordinarily not included in zone planning. How ever, workers compiling the avalanche zoning map must also judge the starting zone and the track. It is from the characteristics of these areas that the size, runout distance and frequency of expected slides can be estimated.

If the starting zone is uniform, chances are that the slide is released over the full extent of the zone, which results in a large avalanche. If it is separated by rock outcroppings, forest patches and ridges, small slides will frequently release, but the resulting avalanches will be small and harmless. Slopes with an inclination of 60% or more can generally be considered avalanche starting zones; however, a slide will break loose only under certain weather conditions. The most important factor which is necessary to release an avalanche is heavy snowfall. A large amount of snow fall will result in new snow avalanches that can be dangerous for inhabited areas. Under conditions of heavy snowfall, the rapidly increasing new snow layer cannot settle and stabilize enough to bond with the old layer of snow or the ground below it, so that after a given amount of time the new snow layer will slide off as an avalanche. Steep slopes with approximately 30cm of new snowfall represent a local slide danger. If the snowfall accumulates to about 50cm, the expected avalanches will be larger, but if it accumulates to about 70cm, avalanches release in areas that seldom experience them. An important factor in judging avalanche areas are slopes with an inclination of between 55% and 60%. Such slopes do not release immediately under heavy snowfall,allowing for exceptionally large avalanches, as was demonstrated in the winter of 1950/51.

Along with the new snow avalanches which usually occur in mid-winter and which are composed of dry and light snow, spring and ground avalanches should be mentioned. Their release is effected by warm and clear weather and by strong solar radiation or by the "Fohn", all factors which cause the entire meltwater-saturated snow layer to slide along the ground. The wet and heavy snow moves relatively slow and generally follows a well-known slide path. Ground avalanches are especially important to transportation and communication links, such as railroads and highways.

On slopes and in gullies with an inclination of about 30%, avalanches will not ordinarily stop in their downward movement. Slopes with such angles must still be counted as part of the track. The average inclination for most avalanche paths that extend for a long distance is between 40% and 70%. The direction and run out distance of an avalanche in the area of deposit is determined by the form, direction and slope of the lower area of the track. According to their type and by the corresponding shape of the track, avalanches can be thrown out of their path and can follow totally different directions. In one instance, it could be observed that the advanced fronts of ground avalanches and dust avalanches (a snow-air mixture) were deposited 400m apart, even though they released from the same zone. The different dynamics of two types of avalanches result in a much larger area of deposition than is observed with only one type of avalanche. The possibility of building on alluvial fans below the mouth of gullies must often be considered. Avalanches are channeled in the gullies and enter, at the beginning of the outrun, a relatively flat and wide terrain. Retardation often begins at the gully mouth. The outrun distance of large avalanches is long, despite the small inclination of the slope. At inclinations of 10% to 15%, outrun distances of 500m to 1000m are observed. On a steep and irregular track, dry snow avalanches develop into powder avalanches of respectable size. The resulting impact can lead to destruction at places which cannot be reached by the sliding snow. In narrow valleys, the air blast can reach far up the opposite slopes.

As a result of unusual weather conditions, avalanches can occur at places where it was never considered possible. A combination of low temperature and intense snowfall can lead to an unusually loose snow layer that can even re lease in forested areas. Mountain forests in general and especially larch forests show thin open stands which are not able to prevent the starting of avalanches. The avalanches from January 10-12, 1954, which especially affected the Grosse Walsertal in Austria are a case in point. Schilcher said following the event: "From the southern and partially from the northern valley slopes of the Grosse Walsertal, loose snow and dust avalanches of all sizes spread over 50% of the total area. The release areas existed along the length of the valley and were scattered in a southwesterly direction at any height between 800m and 1850m and were located in open country as well as in forests." Such possibilities must be strongly considered, since sunny ridges located under steep mountain forests make ideal locations for summer houses.

Another local source of danger is gliding snow. This is the slow and not often noticeable movement of the snowpack along the ground. It is observed that these phenomena occur most often on steep mowing meadows in the snowy foothills of the Alps. However, more gentle snow slopes are also subject to dangerous movement if they are water saturated. Early and heavy snowfall results in pronounced "glide winters." Recent examples are the winters of 1952/53 and 1965/66, in which many buildings were damaged.

The return interval of individual avalanches is the source of most difficulties for a sensible approach to an avalanche zone designation. We mean avalanches that are infrequent and extreme. It is these avalanches that are most dangerous to inhabited terrain, since they generally sweep large areas. In this respect, peculiar opinions have been expressed. For example, an appeal was filed against withholding a building permit for construction of a vacation house. The reason given was that a situation conflicting with safety laws only exists if "immediate" danger is present. Avalanches that only sweep down occasionally do not present immediate danger. It is possible to give information on the frequency and probability of avalanches by analyzing weather and avalanche statistics. However, since a slide event cannot be predicted in advance, this information is practically useless. This problem can be stated with an example as follows: Should an area that in a span of one hundred or several hundred years recorded only one avalanche be proscribed for building projects? We can imagine such an event occurring the following winter. The reader will arrive at the conclusion that construction would be irresponsible. But if the event is imagined in the distant future, a totally different reaction could be expected. Such problems were recently discussed openly in connection with the avalanche catastrophe "Auf den Boden" in Davos. de Quervain wrote: "To eliminate every possible risk by considering regular avalanche activity and every historical avalanche event as a basis for zone planning, several well-known resorts had to put under the ban entire quarters. A certain final risk in the future must be accepted even if only one serious catastrophe occurs in hundreds of years, or if more frequent but less serious damage is inflicted. The extent of the risk for the different area sections should be evaluated by people that have knowledge of the area or, if possible, by an avalanche expert. The final evaluation on what is reasonable has to be left to a judicial court."

The Avalanche Zoning Map should incorporate every danger associated with snow. Factors that should be considered are the type of avalanches, air blast caused by dust avalanches, the erratic path taken by wet snow slides, the creeping and sliding movement of the snow cover and unusual and extreme events. The thankless job of determining the extent of risks to be taken, by defining areas where building should be specified or construction prohibited, is left to the government. Together these constitute the avalanche zoning plan, which then presents the blueprint for local planners.


III. Avalanche Zone Mapping
Mountain communities are often scattered over extensive areas with much unproductive terrain which is characterized by boulder fields, rocks and glaciers. To illustrate this, the five largest communities have been selected:



Avalanche zoning maps that are mapped according to communities will later become an addition to community building regulations. Avalanche zone map ping ideally should include the total area of a community. However, little would be accomplished by including the large barren and uninhabited terrain. Mapping can then be limited to settled areas, potential building sites and regions with existing or potential commercial links. Border areas with uniform topography separating two communities should be dealt with according to principles that apply to planning in general; such areas should be considered as a unit. Mapping should not stop where the areas of two communities meet, despite a political division of uniform topography.

Sources of information available to researchers compiling avalanche zoning maps are:

-Topography and forested areas
-Avalanche scars in the terrain
-An eventual avalanche register (cadaster)
-An eventual avalanche chronicle
-Statements of local people

Practice has shown that the general plan for the register of land property at a scale of 1:10,000 can be used as an excellent foundation for studying the topography. Another valuable perspective can be gained by using the National Map with a smaller scale of 1:25,000, which is not yet obtainable for all mountain areas. At a contour interval of 20m, this map does not show detailed land forms; however, it can be used as a stop-gap if the general plan is missing. Large maps have the disadvantage of being hard to handle, especially if large areas must be worked. They lose a great deal of clarity, which should not be sacrificed for exaggerated details. It is easier to record on larger scales, as for example 1:5,000 or 1:2,000. Yet such maps with altitude contours exist only for small areas. It is advantageous to inspect survey maps for possible avalanche areas, record them approximately, and then verify their existence in the field. Under these circumstances, mapping can be done confidently and efficiently. The study of topography is the most important undertaking since all other sources of information can give only data on avalanches that actually have occurred. Topography shows all the potential avalanche areas, as well as those of avalanches which seldom occur whose scars have disappeared and which have not been recorded in a chronicle or avalanche register.

The forest is important for avalanche defense, especially at the starting zone. Its effect and importance with respect to the slide path and outrun area is often misunderstood. Forest growing in the slide path can divert small avalanches and if conditions are favorable, stop them. Recording small avalanches however for mapping purpose is useless. We need to know the area of the large ones. These will sweep the forest away, and much damage can be caused by the ram-effect of tree trunks carried down with the slide. The four tree species which occur in our mountain areas are of (have their) specific meaning for avalanche mapping. For example: avalanches seldom occur in areas forested with dense, large and old stands or spruce (Picea excelsa), cembra pine (Pinus cembra) and mountain pine (Pinus montana). On the other hand, caution must be exercised with scattered small stands or small strips of these species remaining on ridges. They convey the impress ion of being a closed barrier, which is, however, easily swept through by avalanches without causing any damage to the stands. In summer, a deceptive picture is projected by the shrub-like form of the mountain pine or dwarf pine. The dense and tall shrubs give the impression of constituting an effective barrier against the release of slides. Such stands are pressed to the ground in winter. Dwarf-pines escape damage even when swept by large avalanches. The larch (Larix decidua) is very resistant to slides because of its tough wood, its strong roots and its limberness in the sapling state etc. Trees ten meters tall can be bent to the ground and be overrun without breaking. Such trees will regain their erect position after the weight has been lifted. Similar observations can be made about resistance to creep and glide of the total snow cover. An open and old stand of larch can be swept by large avalanches without being damaged. More confusion is added by the fact that in summer, these trees have needles and give the impression of a closed stand, which is not true for winter when the needles are shed.

A forest is often scarred by avalanches. Stands of young trees of the same age which grow on the edge of the avalanche path indicate that avalanches occasionally occupy a much wider path. Trees without branches could have been growing at one time in the path of an avalanche. Remains of forests swept or carried down by avalanches can be used for avalanche mapping long after the snow has melted. These scars often show the extent of an avalanche better than the snow that has been carried down, since new snow fall and wind can obscure the latter. The forest is of additional importance with respect to construction, which becomes apparent only with the development of the avalanche zoning plan. Forested areas fall under different legal regulations. To build in the forest requires the clearing of a wooded area. Even if the building enthusiast could use an open area within a forest, it would be the equivalent of clearing, as is true of any claim to forest lots which become temporarily or permanently changed. Since foresters and community authorities of certain cantons may have no knowledge of this definite federal regulation, all mountain forests should be declared off-limits for construction and mapped in red.

We know of avalanche evidence which the layman would not consider possible. The large "Breitzug" avalanche that swept down in the area of Davos on January 26, 1968, dislocated rocks with a diameter of up to 1.6m. The rocks were picked up from the bed of the "Landwasser" and carried over a distance of 66m with an altitude difference of 14m up the opposite slope. The slope was covered with fist- and head-sized stones. A rock 9m long and 4m high can be found on the old Susten road, 500m behind the village of Obermad. An attached plate contains the following inscription: