A snow avalanche is a mass of snow, ice, and debris; flowing and sliding rapidly down a steep slope.
Snow avalanches occur in the high mountains of Colorado during the winter as the result of heavy snow accumulations on steep slopes. When the snow pack becomes unstable, it suddenly releases and rapidly descends downslope either over a wide area or concentrated in an avalanche track. Avalanches reach speeds of up to 200 miles an hour and can exert forces great enough to destroy structures and uproot or snap off large trees. It may be preceded by an “air blast” which also is capable of damaging buildings.
Avalanche paths consist of a starting zone, a track, and a runout zone. In general the runout zone is the critical area for land use decisions because of its otherwise attractive setting for development. Avalanche-prone lands may pass many winters or even decades without a serious avalanche. Only part of an avalanche may release at once. Lack of vegetation or a predominance of quick-growing aspen and low shrubs often characterize active portions of an avalanche track and the runout zone, readily identifying the seasonal peril. Hundreds of snow avalanches happen each winter, most of them in remote places.
Man’s activities frequently trigger avalanche and certainly man’s activities create the hazard. The process only becomes a hazard when man interacts adversely with it. Where no structures exist or no recreational activity occurs, avalanches occur with no damage to structures or lives being lost. Building construction in an avalanche path eventually may result in destruction of property and the loss of life. Although most snow slides are initiated by natural causes, skiers frequently trigger the smaller avalanches that take their lives by breaking the snow surface while crossing an area prone to “run”. Avalanches can also be triggered by sounds from shouts, machine noises, and sonic booms.
West Riverside, Red Mountain Pass, Colorado,Photo by Don Bachman.
The West Riverside is a large path along US 550 in southwestern Colorado. It is located in the San Juan Mountains, about 7.7 miles north of Red Mountain Pass. The top of the starting zone is at 11,840 feet. Avalanches can fall 2,520 feet to Red Mountain Creek, but large slab avalanches such as this one can climb the 60 feet from the creek to the highway. During the winter of 1931-32, a huge avalanche buried the highway 53 feet X 1000 feet.
H.B. 1041, Part 1, 106-7-103 (2) “Avalanche” means a mass of snow or ice and other material that may become incorporated therein as such mass moves rapidly down a mountain slope.
Snow avalanches are the rapid downslope movement of snow, ice, and associated debris such as rocks and vegetation. “The forces generated by moderate or large avalanches can damage or destroy most manmade structures. The debris from even small avalanches is enough to block a highway or railroad” (Martinelli, 1974, p. 5). Avalanches occur in the mountainous areas of Colorado generally above 8,000 ft. elevation, and most commonly occur from November through April. Avalanche occurrence is directly related to topography, climate, vegetation and aspect* of the area. Much of the information in this report was extracted from “Snow Avalanche Sites – Their Identification and Evaluation” by M. Martinelli, Jr. (1974). Readers with particular interest in avalanches will find that publication quite valuable.
An avalanche site or area is a location with one or more avalanche paths. Avalanche path refers to the specific area where a snow mass moves. A complete path is made up of starting zone(s) at the top where the unstable snow breaks away from the more stable part of the snow cover, runout zone(s) at the bottom where the moving snow and entrained debris stop, and track (s) that run between starting zone, where damage occurs from the turbulent winds that accompany fast-moving powder avalanches. The air blast zone is usually in the vicinity of, but not necessarily continuous with, the lower track or runout zone. In some cases it may even run part way up the slope across the valley from the avalanche path.
Avalanche start most frequently on slopes with average gradients of 30 to 45 degrees. Slopes steeper than 45 degrees usually do not accumulate enough snow to produce very large avalanches in the Rocky Mountain climate. Avalanches may start on slopes of less than 30 degrees if the snow is highly unstable as the result of a prolonged warming trend, heavy snowfall, or unusual wind condition.
These starting zone slope angles are, however, merely the range in which most dangerous avalanches occur; do not assume that slopes outside this range are safe from avalanches.
The average gradient for the entire avalanche path will be more gentle than that of the starting zone. Average gradients of 20 degrees to 35 degrees are common for the tracks of Rocky Mountain avalanches while the slopes in the runout zones are often more gentle and sometimes completely flat, and may even extend up the opposite valley side.
Avalanches are not confined to specific terrain features: they may follow narrow gullies or ravines for all or part of their path; they may occur on broad, uniform slopes or even ridges and spurs. Longitudinal profiles of the paths may be concave, convex, or stepped. On stepped paths, small avalanches will often stop on a bench part way down the tract while larger ones run the full length of the path.
Severity of problem
The severity of avalanche hazard increases when the works of man extend into avalanche areas; therefore, the 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 (2) while 1 ton/ft (2) is more common. A typical range is form 0.5 to 5.0 ton/ft (2). Air blasts from powder avalanches commonly exert a pressure of 100 lb/ft (2) of force (Martinelli, speech November 8, 1973). Pressures of only 20-50 lb/ft (2) 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.
Criteria for Recognition — General
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 conifers 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; and
- 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 5 to 15 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:
- Mounds of blocks of snow. 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 form “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, snow mobiling, 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 man’s activities and land uses 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.
Avalanches can cause deaths whenever people are within the area affected by the avalanche. This area is the entire avalanche path including the air-blast zone. Death can be caused by impact and/or suffocation. In Colorado, from the beginning of the Colorado gold rush in 1859 to the winter of 2006 avalanches killed693 people, according to the paper “A HISTORY OF COLORADO AVALANCHE ACCIDENTS, 1859–2006”.This averages about 4.7 avalanche fatalities per year for the state.
In the late 1800’s and early 1900’s the number of fatalities caused by avalanches in Colorado was far greater due to the extensive mining activity in avalanche-prone areas. It has been reported that 119 people died in 1899 alone while it was not uncommon to have dozens killed each year. Now in the 1970’s, Colorado is again experiencing an increase of human activity in the high mountain area. H.B. 1041 provides government and citizens with the means to protect property and life from future high losses caused by snow avalanches.
Property damage can occur throughout the entire avalanche path. Impact (air or snow) damage ranges from minor to major structural damage to any structure within the path. Vehicles and equipment can be moved great distances and damaged. When deposited, the debris associated with the avalanche might cause damage and be expensive to remove. Roads and bridges may be damaged.
Roads, highways and railroads may become blocked by avalanche snow and debris. In addition to delaying highway and rail travel, it is costly to clear the transportation routes. In a few cases, where avalanches threaten access roads to mountaintop radio and microwave communication sites, emergency repairs and maintenance are delayed. In areas where efforts are underway to control avalanches, the maintenance of avalanche control structures and/or explosive control is costly.
In summary, man’s activities in avalanche-prone areas can be costly in both money and lives. Improper utilization of avalanche areas includes all uses that generate unacceptable costs in lives or property.