Avalanches occur as rapid, gravity-driven accelerations of different materials downslope at high rates of speed. All avalanches are caused by an overburden of material that is too massive and unstable for the slope that supports them. At least five different types occur, each type with numerous subtypes or relationships to other types: (1) snow avalanches, (2) ice avalanches, (3) slush avalanches, (4) debris avalanches, and (5) rock avalanches. Because of their common high velocities, all avalanches are greatly threatening to life, limb, and property, and the lives of tens of thousands of people have been lost in the many notorious occurrences. Because of the steep slopes and high potential energies of mountains that can convert their potential energy into high kinetic energy wherever snow, ice, slush, debris, and rock are detached from cliffs through climatic or seismic events, many mountain areas are the locations of all five of these types of mass movement. People who live in such areas are generally well aware of these natural hazards and face such risks with a certain fortitude, based on an understandable desire to beat the odds of what they hope are rare events. But, of course, the gambling odds are all too often not in favor of the people or their infrastructures in mountain regions where any or all types of avalanches can be common.
Snow falls and accumulates on mountain slopes as snowpack wherein the commonly light and fluffy flakes of powder snow pile up, become unstable in various ways, and eventually flow downhill as a snow avalanche. Because of the ubiquity of snow, and therefore of snow avalanches, in almost all high mountain regions, the science of analysis and prediction of such events has become quite sophisticated. Mountain weather, snow formation and snow pack, avalanche formation, and avalanche terrain, motion, and effects are all important elements in the prediction of snow avalanches wherein the elements of stability evaluation and snowpack observations enable better avalanche forecasting.
Avalanche-relevant mountain weather first concerns the deeper snows and relatively milder temperatures of the maritime snow climates of coastal mountains, as opposed to the lighter snows and colder climes of interior ranges, both of which types can set up different conditions of snow instability. Temperature conditions are an essential element in the consideration of avalanches—both while the snow is falling, being redeposited, or both, as well as much later in the heat exchange at and within the snowpack. The snow itself falls in many different forms of crystal type, from the light and fluffy powder flakes to the heavy, coarse graupel, or hail, with an enormous variation in between. Heat exchange at the snow surface occurs as heat enters or leaves the snowpack by conduction or convection of radiation as well as by condensation resulting from diffusion of water vapor. Metamorphism of the snow crystal results. Formation of feathery crystal of unstable surface hoar frost (e.g., frozen dew on a surface) can be a problem, especially where buried by later snowfalls. The melting and refreezing of moisture between snow crystals in contact with each other causes the formation of grain bonds, or sintering, which are also crucial elements in snow strength, so that the temperature, temperature gradients, grain geometries (i.e., orientation of grains in a deposit), and pore-space (i.e., gaps between grains in a deposit) configuration can all figure in avalanche formation. The highest crystal growth rates occur where there are large temperature gradients, higher temperatures, and large spaces between crystals. This produces unstable angular or faceted (i.e., grains with one or more smoothed surfaces) grains or depth hoar.
In addition to temperature and pressure in snowpack metamorphism as a factor in snow-avalanche formation, horizontal and vertical winds also control the amounts and distributions of snow as primary precipitation events or as secondary redistributions through blowing and drifting. Snow is eroded in acceleration regions and deposited in deceleration regions, which produces lee-zone (i.e., in the leeward side of an obstacle) accumulations, cross-loading, deposition in gullies and notches, and unstable cornices (cantile-vered snow structures formed by drifting snow) built out as overhangs off ridge tops, all of which can become potential avalanche sites as a result.
Snow-avalanche formation types are generally classified as loose snow avalanches that generally start at a point or the more dangerous slab avalanches that can propagate laterally across a slope. Loose snow avalanche formation occurs because such snow has little if any cohesion between flakes. A local loss of cohesion can result in a small movement that will propagate in a downhill direction into a bigger mass that can incorporate other snow layers of different moisture contents. Slab avalanches generally develop from a weaker layer such as faceted snow or depth hoar that fails and allows fractures of the snow to propagate both upslope and across slope to release the avalanche. The slope angles can control the types of snow avalanches so that dendritic (i.e., like the branching pattern of tree roots) or stellar snow crystal shapes have the highest stable angle of repose (up to 80°), while this may decrease to 35° for more rounded forms. As the water content increases, the angle of repose decreases, so that slush avalanches can occur on slopes of less than 15°. The normal range of slopes for release of slab avalanches is ∼25° to 55°.
Avalanche terrain is the area in mountains where certain features exist that warn of an avalanche hazard. An avalanche area is one in which various times, types, and geographies of snow avalanches occur throughout one or more avalanche paths. Any given avalanche path will have its own starting zone, track, and runout zone. Starting zones or zones of origin of avalanches are commonly higher on mountain ridges, slopes, gulleys, and other collection points for snow. Slope angles there range from 90° to 60° for small snow sloughs, because enough snow can never accumulate to form large avalanches. Dry snow types range from 30° to 60°, small slabs from 45° to 55°, multisized slabs from 45° to 35°, and large slabs or wet, loose snow avalanches from 35° to 25°. Infrequent wet snow avalanche types occur from 25° to 10°. Other controls include orientation to the wind, orientation to the sun, forest cover, underlying ground surface, altitude, slope dimensions, and crown and flank positions of fracture lines according to local terrain features. The track or zone of transition through which the moving snow mass flows is the area of highest velocity. These areas are either open slopes or channels, and the typical slopes are 30° to 15°, with significant deceleration on slopes below 10°. The runout zone or zone of deposition is where the avalanche comes to a stop and drops its load of snow and any entrained debris or trees and so forth. Determination of runout distances in avalanche hazard analysis includes long-term observations of avalanche deposits, observations of damage to vegetation, structures, and the ground surface, and historical records. Avalanche frequencies can also be determined by analysis of the disturbed vegetation.
Rapid avalanches of ice in mountains are generally caused when a glacier terminus (snout or end) is perched high on cliffs above, and as its front collapses through forward advance to the cliff edge, ice masses can be precipitated into the air or down steep slopes as huge and rapid masses of ice blocks. In some cases, where rock materials are incorporated as well and the internal ice melts, the moving mass can become a kind of rapid and highly mobile debris or rock avalanche. The two classes of ice avalanches resulting are thus based on size.
The smaller type is the result of calving or falling of ice blocks caused by internal flow (creep) and possibly also sliding of the glacier over its bed to a cliff edge where blocks break off and roll or bounce down the cliff as an analog to a rockfall. In many of the higher mountains in the world, these ice avalanches accumulate again at the cliff bottom to reconstitute a kind of glacier detached from its accumulation zone higher on the mountain.
The larger, rare types of ice avalanche occur where a huge piece of the glacier breaks off to form a massive ice avalanche, analogous to a large rapid landslide or rock avalanche, which can be greater than 106 m3 (cubic meters) in size. These are enormously destructive, having buried whole towns and thousands of people in the Andes, Alps, Caucasus, and elsewhere. Some of them have rock entrained in the failure as well, so that a continuum between large ice avalanches and rock avalanches probably exits.
These are a class of wet slab avalanches that are most common at high latitudes, generally due to rapid onset of spring snowmelt when the sun returns after long, dark winters. Starting zones can be 5° to 40°, but only rarely >25° to 30°. The snowpack is commonly partially or completely water saturated, and the bed surface of the failure is fairly impervious to that water. Depth hoar frost is common at the base of the snow cover, and failure release into a slush flow or “slusher” is commonly associated with intense snowmelt or heavy rain. Slush avalanches have exceptionally high densities, in some cases >1,000 kg (kilograms)/m3, so that impact forces are among the highest and most destructive of any snow avalanches known.
As their name implies, debris avalanches are usually mixed up masses of soil, gravel, rocks, trees, houses, cars, and whatever random materials exist on mountain slopes that can be mobilized and accelerated downslope in torrential rainstorms, rapid snowmelt, or collapse of water dams. Rainfall intensity or duration, along with the prior rainfall and general soil moisture conditions, is also a strong control in the triggering of debris avalanches.
Most well-vegetated mountain slopes have a thin veneer of sediment and soil on them that has been draped over them by glaciers, wind, and other processes and that is produced by long-term weathering of the bedrock beneath. Debris avalanches are common in topographic concavities or hollows in first-order watersheds. This geometry is conducive to the accumulation of colluvium and other sediment as well as the convergence of underground water necessary to cause the failure. Failure of this surficial cover is triggered by an unusual amount of water being delivered abruptly to the slope so that the water first infiltrates into the soil and sediment, saturates it to fill all pore spaces, and then, through return flow, begins to come back to the surface again lower down the slope. This greatly increased weight of water on the slope in the soil, coupled with the hydrostatic water pressure upward inside the slope cover and the seepage pressure of the return flow out onto the surface, lifts the sediment and soil mass and mobilizes it into the debris avalanche. Such rapid movements have a dominance of inertial forces with a high sediment concentration in a granular flow.
These rapid inertial granular flows are caused when a large volume of mostly dry rock fragments derives from the collapse of a slope or cliff and moves for a long distance, even on gentle slopes. These strurzstrom failures (i.e., failures that exhibit much greater horizontal movement than initial vertical drop) can be initiated by a sudden seismic acceleration, a melting of internal permafrost in the rock joints, a debutressing of the slope when nearby glaciers melt downward and remove support, or perhaps a simple freeze-thaw that wedges the rock mass outward. Once started, the fall height or sliding distance can impart a high velocity, which produces dilation and reduction of internal friction. These highly mobile masses of rock fragments can flow for long distances, apparently owing to a low effective coefficient of friction produced by some combination of fluidization produced by entrained air, vacuum-induced upward flow of air from the airfoil shape of the upper flow surface, trapped basal air-layer lubrication, buoyant dust suspensions, acoustic fluidization, or high-frequency vibration, steam generation from frictional heating, and various other hypothesized mobilization mechanisms. The result is that these long-runout-zone landslides are incredibly destructive, move with great rapidity, and obliterate everything in their path. In fact, as the size of the falling rock mass increases, the ratio of the fall height to the travel distance decreases, so that lower distances of fall can lead to further distances of travel. Lateral and frontal rides can develop when the rapidly traveling mass comes to an abrupt stop. These enigmatic features have also been observed on the Moon and Mars, where the gravity and atmospheres are very different, yet the physics of their motion must somehow be similar to produce near-identical surficial features.
Earthquakes, Geomorphology, Glaciers: Mountain, Ice, Landforms, Landslide
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