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AVALANCHE ZONING
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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