by Art Mears

Thousands of snow avalanches fall from steep slopes each year. Most of these occur in remote mountain areas, do not affect man, and as a result go unnoticed. The avalanche phenomenon becomes a hazard when man and his works become exposed. Two broad categories of hazard can be distinguished. The first type affects travelers in mountain terrain who trigger or are reached by small avalanches during one of the many periods of unstable snowpack conditions during an average winter. The second type of hazard affects permanent facilities or residents who occupy areas reached only by extreme avalanches which occur at erratic and infrequent intervals.

The first type of hazard is not amenable to mapping. This hazard occurs many times during any given season primarily in response to rapid changes in snowpack stability induced by weather changes. The majority of victims trigger small slides which may occur on almost any steep slope and which often run very short distances. It is not possible to include all of these areas of potential hazard without resorting to a very "broad brush" mapping approach which would not distinguish between the vast number of potential danger areas and those areas completely safe. Users of the back-country should assume the responsibility of educating themselves in the techniques of avalanche avoidance and survival.

'Me second type of hazard can be effectively shown on maps. Mapping of the hazard is essential in areas of development or potential development because planning locations of permanent facilities requires knowledge of the magnitude and characteristics of the "rare event." Such events have seldom been observed in the United States primarily because of our short observational record in many locations. Thus the magnitude of the rare event must usually be determined through indirect analytical techniques. In this way avalanche zoning maps and plans can be made in areas subject to development.

Avalanche Hazard Zones

Two types of zones exist. The first depends on the dynamics of an avalanche of design size, the "design avalanche" being one of a size which should be considered in planning. The hazard zones can be defined in terms of the (1) magnitude/ frequency relationship, (2) types of avalanches possible, (3) discharges, (4) pressures, and (5) velocities. These zones of hazard intensity are facts of nature which man may choose to alter through avalanche defenses.

Although not fully understood, there is for any given path some relationship between the area and length of the runout zone, and the frequency or probability of the avalanche. This is called the magnitude/ frequency relationship, a concept well-known in flood plain determinations. The relationship is usually expressed as the return period (or annual probability) of an avalanche occurring that will travel some specified distance into the runout zone. There is no definable upper limit to avalanche size. One can always imagine an event slightly larger (and less likely) than the last one. Nevertheless, there does exist a practical upper size limit beyond which the probability of encounter with an avalanche is so small that it is similar to the probability of other risks we normally take in our activities.

It is evident that hazard depends on both the magnitude /frequency relationship and also on man's use of the area. For example, placing a public facility which concentrates large numbers of people for long periods of time within a hazard area creates a much more severe hazard than would be caused by periodic exposure of cross-country skiers at the same location. The response of the avalanche to weather and snowpack conditions does not change, but the hazard depends on both avalanche magnitude/ frequency relationships and total exposure time of people and facilities. The permanent facility is a fixed target.

Avalanche Zone Plans

When man arrives and defines the hazard zones to the best of his ability, a second, distinctly different problem occurs. He must now decide what degree of hazard is acceptable. In heavily developed areas the criteria for acceptable hazard in an avalanche zone is usually a government decision rather than a private one. In Switzerland the final decisions about land-use in hazard zones are usually made by local cantonal governments. In the United States final decisions are also usually made by local town and country governments.

Avalanche zoning plans emerge as a result of these government decisions. It has been generally recognized that, for permanent habitation, there exists a part of the avalanche path in which hazard is unacceptably high. In such areas avalanches may occur frequently or cause very high dynamic pressures when they do occur. In contrast, on the outer fringe of a design avalanche path, occurrence may be so rare that the probability of injury or structural damage is negligible, and the slight risk may be disregarded. Between these two zones there exists a transition zone of moderate hazard.

It has proven most useful to define these zones in terms of avalanche frequency and dynamic pressure, thus defining a magnitude/frequency distribution ill the runout zone. The first comprehensive avalanche zoning plans defined in these terms were developed by the Swiss and have been revised during the past few years. The Swiss government recommends the following zone definitions be applied by local governments:

Red (high hazard) Zone. This zone includes terrain exposed to frequent and/or large powerful avalanches that satisfy either of' the following:

1. Any avalanche with a return period of less than 30 years.
2. Any avalanche with a dynamic pressure of 3.0 metric tons per square meter (615 lbs/ft2) or more and with a return period of up to 300 years.

New buildings are not generally permitted in the red zone.

Blue (moderate hazard) Zone. This zone includes avalanches that are either small or infrequent:

1. Avalanches with dynamic pressures of less than 3.0 metric tons per square meter (615 lbs/ft2) and a return period of 30 to 300 years. The specification of return periods as long as 300 years is strongly questioned by some Swiss scientists who feel it is not practical to define avalanches with such small probabilities. The 300-year period is approximately equal to the oldest records of avalanches in Switzerland. Private homes are permitted in the blue zone if they are designed to resist avalanche forces or are otherwise protected by avalanche defense.

Yellow (very low hazard) Zone. This optional zone is not always used. It defines avalanches which are very rare or defines areas subject to the blast or low-density snow or air from powder avalanches.

1. A powder avalanche with dynamic pressures of 0.3 tons per square meter (62 lbs/ft2) or less having a periodicity of more than 30 years.
2. Extremely rare flowing avalanches with return periods of more than 300 years.

Swiss avalanche zoning guidelines suggest that buildings constructed according to standard Swiss construction norms would probably not be damaged by powder avalanches within this zone. This is not necessarily true of constructed in conformance with United States Uniform Building Code norms.

White Zone. White zones are considered to be hazard-free. Specifically, this means the zone is beyond the limit of the design avalanche as defined by the Swiss. It does not mean that it is outside of the range of all avalanches possible.

In Colorado, the Town of Vail has defined hazard zones most explicitly in terms of avalanche dynamics. Avalanche hazard zone definitions will be explicitly specified in the town comprehensive plan which is presently being developed. In the past, new buildings were reviewed on an individual basis and were permitted in hazard zones if they corresponded to the Vail definition of "moderate hazard." Construction was permitted within moderate hazard zones only if specifically designed avalanche defenses were constructed according to design criteria developed in a detailed avalanche dynamics study. Vail hazard zone definitions differ from those used in Switzerland in terms of avalanche frequency. Vail policy defines the moderate hazard zone as being affected by avalanches with return periods of up to "one of two centuries" but more than 25 years. This modification to the Swiss definition recognizes the uncertainty of specifying very long return periods in this area of short observational records. A powder blast zone is also defined in areas where it is applicable.

Map Detail and Hazard Zones

The maps discussed above require a detailed topographic base map and/or extensive on-site field investigations. The avalanche zone maps of the Town of Vail are at scales of 1:4800 (1" = 400') with 2-foot contour lines on the original map. The accuracy level of these maps correspond to the maximum possible level of accuracy obtainable from detailed analysis of a large avalanche path. Such detailed maps are rarely available in the United States. The scale most commonly available in mountain regions is 1: 24000 (1 " = 2000') with 40-foot contour lines. They do not provide sufficient detail for site-specific planning. Detailed avalanche hazards maps are sometimes shown on high quality air photographs, usually at a scale of 1 - 10000 or larger. Photographs have some advantages over maps in that they show trees, boulders, and recent buildings. These provide good reference points for hazard lines. Because of the lack of detailed maps in the United States it is required that careful field study be made of the starting zones, tracks and runout zones for all detailed avalanche studies.

Although detailed maps (or photo-maps) are necessary, there definitely exists an upper limit to the accuracy with which hazard lines may be plotted. Even after detailed studies have been completed there still exists some uncertainty about the limits and dynamics of the avalanche which must be planned for. Because of this uncertainty, it would be misleading to present runout zone details on maps of scales much larger than 1: 2000 unless the overall dimensions of the avalanche path are very small.

More generalized avalanche planning maps have been prepared for selected areas in Colorado by several sources. One set of maps were prepared by the Colorado Geological Survey. These maps were first prepared on 1: 24000 scale U. S. Geological Survey topographic base maps. They are similar to Swiss 1: 25000 scale avalanche planning maps in that they show three zones of hazard intensity. The definition of the high, moderate, and no hazard zones are given in the map section of the publication (Mears, in preparation). Delineation of zones of intensity serves a useful purpose as land planners can often determine if property is clearly located in one zone or another. Borderline cases cannot be determined from map inspection and require further study. If proper is clearly located in one zone or another the owner or prospective buyer can be advised of the situation. With some information about hazard intensity, informed land-use decisions and recommendations can be made. The maps included in the Colorado Geological Survey publication are intended for land-use purposes only. No attempt is made to warn the winter recreationalist of the hazard in the back-country, which, as stated earlier, fluctuates widely in response to weather conditions and may be extremely localized.

Additional avalanche maps of selected areas of Colorado have been prepared during the past five years by the U. S. Geological Survey, Colorado State University, and the University of Colorado. These maps differ significantly from each other in quality and apparent mapping objectives but were all prepared on 1: 24000 scale base maps. In some cases these maps have the stated purpose of warning the user of the back-country about avalanche hazard.

In general, the following points should be made about the preparation of avalanche maps for planning purposes:

1. The size of the runout zone should be related to the size of the starting zone and the elevation and climate of the mountain area.
2. The shape and length of the runout zone should take into consideration all types of avalanches thought to be possible over a long time period.
3. The dynamics and sequences of slab releases from the starting zone or zones, and how they combine should be considered as this strongly affects avalanche size. This is especially important in paths with complex or multiple starting zones.
4. Vegetation damage, while a good indicator of past events, does not necessarily show the size of an avalanche which must be planned for. In many cases damage may have been caused by other geological processes. Topography and potential avalanche velocity at various points in a path are better indicators of runout extents. Vegetation damage should be used whenever possible to estimate avalanche frequency.

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Last changed: July 11, 2002