Lukas U. Arenson and Matthias Jakob
Mountain environments, home to about 12% of the global population and covering nearly a quarter of the global land surface, create hazardous conditions for various infrastructures. The economic and ecologic importance of these environments for tourism, transportation, hydropower generation, or natural resource extraction requires that direct and indirect interactions between infrastructures and geohazards be evaluated. Construction of infrastructure in mountain permafrost environments can change the ground thermal regime, affect gravity-driven processes, impact the strength of ice-rich foundations, or result in permafrost aggradation via natural convection. The severity of impact, and whether permafrost will degrade or aggrade in response to the construction, is a function of numerous parameters including climate change, which needs to be considered when evaluating the changes in existing or formation of new geohazards. The main challenge relates to the uncertainties associated with the projections of medium- (decadal) and long-term (century-scale) climate change. A fundamental understanding of the various processes at play and a good knowledge of the foundation conditions is required to ascertain that infrastructure in permafrost environment functions as intended. Many of the tools required for identifying geohazards in the periglacial and appropriate risk management strategies are already available.
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Natural Hazard Science. Please check back later for the full article.
Rock avalanches are very large (> 1 million cubic meters) landslides from rock slopes, which can travel much farther across the landscape than smaller events; the larger the avalanche, the greater the excess travel distance. Rock avalanches first became prominent in Switzerland in the 1800s, when the Elm and Goldau events killed people a surprisingly long way from the origin of the landslide; these events first posed the “long-runout rock-avalanche” problem. In essence, the long runout of these events appears to require low friction beneath and within the moving rock mass in order to explain their extremely long deposits; but in spite of recently intense research, this low friction still lacks a generally accepted explanation. Large collapses of volcano edifices can also generate rock avalanches that travel very long distances, albeit with a different runout-volume relationship to that of non-volcanic events. Compounding the puzzle is the presence of long-runout deposits not just on land, but also beneath the sea and on the surfaces of Mars and the Moon.
Numerous studies of rock avalanches have yielded some consistencies of material and behavior, for example, that little or no mixing of material occurs within the moving debris mass during runout; that the deposit material beneath a meter-scale surface layer is pervasively and intensely fragmented, with fragments down to sub-micron size; that many of these fragments are themselves agglomerates of even finer particles; that throughout the travel of a rock avalanche, large volumes of fine dust are produced; that rock avalanche surfaces are typically hummocky at a range of scales; and that there are definite trends in plots of runout distance against volume from rock avalanches in different environments.
Since rock avalanches can travel tens of kilometers from their source, they pose severe, if low-probability, direct hazards to societal assets in mountain valleys. Compounding this is their effect in the triggering of extensive and long-duration geomorphic hazard cascades.
Although large rock avalanches are rare, magnitude-frequency studies show that the proportion of total volume involved in large events is greater than the proportion in small events, so that a large proportion of the total sediment generated in mountains by uplift and denudation originates in large rock avalanches. Consequently, large rock avalanches exert a significant influence on mountain geomorphology by, for example, blocking rivers and forming landslide dams; these either fail, causing large dambreak floods and intense, long-duration aggradation episodes to propagate down river systems; or remain intact to infill with sediment and form large valley flats. Rock avalanches that fall onto glaciers often result in large terminal moraines being formed as debris accumulates at the glacier terminus, and these moraines may have no relation to any climatic change. In addition, misinterpretation of rock avalanche deposits as moraines can cause serious underestimation of hazard risk and misinterpretation of paleoclimate; for example, a deposit 28 km long in Kyrgyzstan was originally thought to be of glacial origin, but it is now known to be a rock avalanche caused by coseismic failure of a mountain slope.
Rock avalanche runout behavior poses truly fundamental scientific questions, and rock avalanches have important effects on a wide range of geomorphic processes. Better understanding of these awesome events is crucial for both geoscientific progress and for reducing impacts of future disasters.
Avalanches have long been a natural threat to humans in mountainous areas. At the end of the Middle Ages, the population in Europe experienced significant growth, leading to an intensive exploitation of upper valleys. At almost the same time, Europe’s climate cooled down considerably and severe winters became more common. In the Alps, several villages were partly destroyed by avalanches, forcing inhabitants to develop the first mitigation strategies against the threat. By the late 19th century, the development of central administrations led to the creation of national forestry departments in each alpine country, principally to tackle the dangers posed by avalanches. As a result, forest engineers conceived not only the science of avalanches but also the first large-scale techniques to alleviate avalanche risks (such as reforestation). However, with the steady growth of transport, industry, tourism, and urbanization in high-altitude areas, these earlier measures soon reached their limits. A new impetus was then given to better forecasting avalanche activity and predicting the destructive potential of extreme avalanches. Avalanche zoning, snowfall forecasts, avalanche-dynamics models, and new protection systems for the protection of structures and inhabitants have become increasingly more common since World War II.
With the advent of personal computers and the increasing sophistication of computational resources, it has become easier to predict the behavior of avalanches and protect threatened areas accordingly. The success of this research and the protection policies implemented since World War II are reflected in the drastic reduction in the number of disasters affecting dwellings in the Alps (most deaths by avalanche now occur during recreational activities). Significant progress has been made since the 1980s, leading to a better understanding of avalanche behavior and the mediation of associated risks. Yet we should not assume that this progress is steady or that our capacity to control such hazards is more advanced than it was two decades ago. Efforts to predict avalanches contrast with work in other sciences such as meteorology, for which forecasts have become increasingly more reliable with advancements in computational power. Explaining the difference is simple: in meteorology, the material is air, a substance whose behavior is well known. The main difficulty lies in the computation of enormous volumes of air encountering various flow and temperature conditions. For avalanches, the material is snow, a subtle mixture of water (in different forms) and air, whose behavior is remarkably complex. Modern models of avalanche dynamics are able to predict this behavior with varying degrees of success.