The severity of avalanche hazard increases when humans move into avalanche areas; therefore, a recognition of the potential aerial extents of avalanches is necessary. This recognition is difficult to achieve when man has not had the opportunity to observe avalanche activity in any particular path over a long enough period of time so that a reasonable assessment of runout potential may be made.
The maximum measured impact pressure of an avalanche is 10 ton/ft while 1 ton/ft is more common. A typical range is form 0.5 to 5.0 ton/ft. Air blasts from powder avalanches commonly exert a pressure of 100 lb/ft of force. Pressures of only 20-50 lb/ft are capable of knocking out most windows and doors. Roads, highways, and railroads are blocked for hours, or sometimes days, every year due to avalanches. Many skiers, other winter sportsmen, and travelers have been injured or killed by avalanche activity.
Lack of recognition of avalanche runout potential has resulted in residential building construction within runout zones in Colorado. When the infrequent, large avalanche event occurs, damage to these buildings will occur unless measures are taken to protect existing structures.
General Criteria for Recognition
By far the most reliable way of locating avalanche areas is to study long-term, detailed records of past events when they are available. Such records are available for many localities in Europe, but unfortunately, compilation is just starting in Colorado.
Usually, data on the location, frequency, or severity of avalanche activity are completely lacking when new areas are considered for highways, winter sports, mining operations, or mountain home sites. Without adequate records of past events, the best alternative is to obtain what data are available, examine the area, map all recognizable paths, estimate the frequency and intensity of the avalanche action, and if possible, start a record of avalanche events.
Active or recently active avalanche paths are most easily identified on air photos or from low-flying airplanes or helicopters. The next best viewpoint is the slope or ridge across the valley from the suspected avalanche area. The entire path should be viewed from such vantage points so that there is less chance of misjudging the size of the path or of overlooking an indistinct or inconspicuous path. Such an overall view makes it possible to spot paths where the aspect of the starting zone and the track are different—an important feature in determining what wind direction causes deposition in the starting zone. Surveys from the valley bottom or lower slopes (the usual road location) are often very misleading. Crooked paths or those with a short, steep pitch in the lower track or runout zone often appear much shorter and smaller than they really are or may not even be recognized as avalanche paths.
Field Evidence of Avalanche
Avalanche paths in forested areas usually appear as strips straight down the mountain, characterized by a different type or age of the dominant vegetation. These vertical swaths through the trees can be very dramatic when the change is from natural timber to grasses and small herbs. They are less conspicuous but still obvious to most observers when the change is from conifer to aspen or brush. On the other hand, careful scrutiny and often a distant vantage point are needed to spot the change from mature timber to younger trees of the same species.
In some cases, avalanches run down slopes with only scattered trees or open park-like stands of trees. These paths are hard to see, and only long and complete records will reveal all of them. Suspected areas should be checked carefully for evidence of avalanche activity. Good indicators of avalanche activity are trees with scars or broken limbs on the uphill side, or trees that lean downhill. Leaning trees deserve a second look, however, to be sure avalanches and not snow or soil creep or a landslide causes them.
An accumulation of wood debris on lower slopes or in the valley may mark an avalanche run-out zone, as might a patch of aspen or young trees at the bottom of a likely avalanche path. Patches of downed trees all aligned in the same direction are a good indication of avalanche activity. Do not discount such patches of downed trees because their tops point uphill. They may mark areas of air-blast, or they may be the result of an avalanche that crossed the valley and ran part way up the opposite slope.
Summer identification of avalanche paths in non-forested areas is difficult and uncertain. Slope steepness, aspect, and surface roughness all offer clues but no proof. Other things being equal, avalanches will be more likely:
- On lee slopes than on windward slopes, because of wind loading
- On grass slopes than on brush-covered slopes, because of lower surface roughness
- On shaded northern slopes than on sunny southern slopes, because the snow stays loose and unstable longer
- On slopes between 30 degrees and 45 degrees than on steeper or gentler slopes because of their ability to accumulate sufficient snow on terrain steep enough to avalanche readily
Large patches of bare soil surrounded on the sides and above by vegetation, if located on slopes steep enough to avalanche, should be considered possible avalanche starting zones.This lack of vegetation is often due to deep snow accumulation.
Steep rock faces or cliffs that have numerous benches or pockets where snow can accumulate may also be the sources of avalanches in spite of the general statement that very steep slopes usually are not serious avalanche problems.
Many avalanche paths cross both non-forested and forested areas. In the Rocky Mountains, for example, many avalanches start above timberline, their track in the timber. In such cases, the swath through the trees is the most obvious identification feature, but the starting and run-out zones must be given full consideration when establishing size and estimating frequency and intensity of activity.
Not all avalanche paths run every year. Many run only once every five to fifteen years, and others even less frequently. Nor do all avalanches run the full length of their paths every time. Avalanches may stop in the starting zone, track, or run-out zone, depending on the amount and condition of the snow in the path. Field evidence—usually confined to the starting zone—that an avalanche has occurred includes:
A fracture line or fracture face where the unstable snow broke away as a slab avalanche from the remaining snow cover This is the most frequently observed and perhaps the most important, single, winter identification feature. The continuity of these fracture lines makes even small ones visible for great distances. However, new snowfall or drifting snow soon obscures shallow fracture lines and makes even large ones much less distinct.
A change in snow depth and in the texture and features of the snow surface, without a distinct fracture face All of these features, which mark the start of a loose snow avalanche, are quickly erased by snowfall and drifting snow, and may be missed even by a careful observer. Additional evidence of avalanches—features that may be located in the starting zone, track, or run-out zone, and whose size and location in the path are clues to the size of the avalanche – includes:
Blocks of snow in mounds Major concentrations usually mark the lower end of the avalanche. Lesser amounts may be scattered higher on the path, at breaks in the slopes, or curved in the track. This is the second most important winter identification feature.
Snow dirtier and denser than the surrounding cover At times, even after avalanche debris has been covered by fresh snow and all surface indications of avalanche debris are lost, a ski tip or pole or a probe rod can detect the harder, denser avalanche snow beneath. In late spring or summer, these deeper and denser snow deposits often persist after the surrounding cover has melted, and they make excellent identification features. It may be difficult, however, to tell if the debris is from one or more avalanches on the same path.
A clean white swath through gray or dust-covered snow in steep terrain After snow surfaces have become dust covered or modified by weather during long snow-free periods, the removal of these surface layers by avalanches reveals the clean, unmodified snow beneath. The change in color and texture is noticeable, even if the avalanche left little other evidence.
Accumulations of broken trees, limbs, twigs, leaves, and needles Entire trees may be uprooted, broken off, or bent over and are usually oriented parallel to the down-slope direction. Large amounts of timber in the debris indicate an avalanche that ran larger than usual or took a different route down the mountain.
Snow, mud, rock, or detached tree limbs plastered against uphill side of standing trees or rocks These signs often help mark the outer edges of the moving snow. They are most noticeable just after an avalanche has run and are quick to disappear.
Deep grooves in the snow and walls of snow; both usually oriented down the fall line These indicated avalanches in heavy, wet snow. Grooves and sides of walls are usually smooth and icy. These features are more common in spring avalanches than in winter ones.
“Flag trees” with fresh scars or broken limbs on uphill side of standing trees, and brush with healthy limbs confined to the downhill side Confusion with wind-damaged trees can be avoided by a complete investigation of the site containing such “flag trees.”
After an avalanche path has been located, it is important to know the size and frequency of avalanches on the path. Long-term observation is the best way to establish avalanche frequency and size. These are, however, available for only a few locations in the United States. The next best thing is to systematically observe the destructive effects of avalanches on the terrain during snow-free conditions. Sometimes, evidence may be found of multiple avalanche events of various sizes and ages through a careful analysis of destruction in the avalanche track and through the distribution of debris in the run-out zone. Additional sources of information may come from “old timers” in the area. Highway maintenance crews, power-line crews, ranchers, trappers, hunters, or fishermen should be quizzed. In more remote areas, ski touring, snowmobiling, or winter mountaineering groups may be a better source of information. Newspaper and other written accounts occasionally help in establishing the data of major events, but are selective toward very large avalanches or those that took lives or did extensive damage.
All incomplete records will be selective in one way or another, and must be used with caution. Highway crews will be most concerned with slides across the road and will seldom pay much attention to those that do not reach the road. Sportsmen will be more apt to see the early avalanches that run during hunting season or those that leave large debris cones that persist in the valley well into fishing season. Such accounts are not definitive in establishing avalanche frequency.
Consequence of Improper Utilization
Avalanches are not a hazard until human activity and land use are affected adversely by the avalanches. Possible conflicting land uses are recreation, residential, transportation, and mining. Examples of this conflict would include property damage, injury, deaths and excessive maintenance costs.