Observation and Spatial Modeling of Snow- and Ice-Related Mass Movement Hazards
Summary and Keywords
Snow- and ice-related hazardous processes threaten society in tropical to high-latitude mountain areas worldwide and at highly variable time scales. On the one hand, small snow avalanches are recorded in high numbers every winter. On the other hand, glacial lake outburst floods (GLOFs) or large-scale volcano–ice interactions occur less frequently but may evolve into destructive process chains resulting in major disasters. These extreme examples document the huge field of types, magnitudes, and frequencies of snow- and ice-related hazardous processes.
Mountain societies have learned to cope with natural hazards for centuries, guided by personal experiences and oral and written tradition. Historical records are today still important as a basis to mitigate snow- and ice-related hazards. They are complemented by a broad array of observation and modeling techniques. These techniques differ among themselves with regard to (1) the type of process under investigation and (2) the scale and purpose of investigation. Multi-scale monitoring and warning systems for snow avalanches are in operation in densely populated mid-latitude mountain areas. They build on meteorological and snow profile data in combination with a large pool of expert knowledge.
In contrast, ice-related processes such as ice- or rock-ice avalanches, GLOFs, or associated process chains cause damage less frequently in space and time, so that societies are less well adapted. Even though the hazard sources are often far from the society—making field observation challenging—flows travelling for tens of kilometers sometimes impact populated areas. These hazards are strongly influenced by climate change–induced glacier and permafrost dynamics. On the regional or national scale, the evolution of such hazards has to be monitored at short intervals through aerial and satellite imagery and terrain data, employing geographic information systems (GIS). Known hazardous situations have to be monitored in the field.
Physical models—applied either in the laboratory or at real-world sites—are employed to explore the mobility of hazardous processes. Since the 1950s, however, computer models have increasingly gained importance in exploring possible travel distances, impact areas, velocities, and impact forces of events. While simple empirical-statistical approaches are used at broad scales in combination with GIS, advanced numeric models are applied to analyze specific case studies. However, the input parameters for these models are uncertain so that (1) the model results have to be validated with observations and (2) appropriate strategies to deal with the uncertainties have to be applied before using the model results for hazard zoning or dimensioning of protective structures. Due to rapid atmospheric warming and related changes in the cryosphere, hazard situations beyond historical experiences are expected to be increasingly relevant in the future. Scenario-based modeling of complex systems and process chains therefore represents an emerging research direction.
Compared to many types of rocks, which may persist for billions of years, snow and ice—summarized as the cryosphere—are rather short-lived substances undergoing dynamic changes. Even though ice several million years old is found in Antarctica (Sugden et al., 1995), melting of valley glaciers sometimes becomes obvious within a few weeks. Snow may melt as soon as it reaches the earth’s surface, but it may also persist for hours, days, or months or even evolve into glacier ice over a few years.
The cryosphere provides many services to the society—be it natural beauty, the potential for tourism, water storage, or, in times of climate warming, the capacity of permafrost to retain carbon. Lack of understanding of the cryosphere, on the other hand, induces certain risks for society. Such risks may be related to slippery surfaces or thin lake ice, increased carbon emissions from thawing permafrost, jamming of lake and river ice, surging glaciers, snow load hazards, etc.
Not all of these aspects are covered in detail here; the present article instead focuses on various types of cryosphere-related mass movements in mountain areas. Hazards related to the seasonal snow cover are compared to those emanating from a changing glacial and periglacial environment. High-mountain environments are highly sensitive geosystems, in which the spatio-temporal distribution and evolution of snow, glaciers, permafrost, ecosystems, and the water cycle serve as early indicators for a fluctuating or even changing climate (Beniston, 2003; Harris et al., 2009; Huber, Bugmann, & Reasoner, 2005).
The observation and modeling techniques presented are thought to increase the awareness and understanding of ongoing processes, the capacity to foresee their possible consequences, and, therefore, the ability to reduce the related risks. Broad-scale spatial observation and modeling techniques are as important for prioritizing hot spots of hazard and risk as are fine-scale techniques for thoroughly analyzing specific events or situations.
Snow Avalanche Hazards
Snow is involved in various types of hazardous processes. The rapid melting of snow leads to saturation of the soil and causes landslides or debris flows. Such melting may be related to the climatic conditions (Cardinali, Ardizzone, Galli, Guzzetti, & Reichenbach, 2000; Decaulne, Sæmundsson, & Petursson, 2005) or to volcanic or geothermal activity. Further, snow cover may strongly influence the propagation of mass movements (Preh & Sausgruber, 2015). Here, however, the focus is put on snow avalanches.
Snow avalanche hazards are widespread throughout mid-latitude mountain areas. Various types of snow avalanches are known, depending on the physical characteristics of the snow and on the topography (De Quervain, De Crecy, LaChapelle, Losev, & Shoda, 1973). Major classification criteria include the type of rupture (slab avalanche vs. loose snow avalanche) and the form of movement (Fig. 1; powder avalanche vs. flow avalanche). A detailed account of snow physics and avalanche classifications would go beyond the scope of the present article and can be found in The Avalanche Handbook (McClung & Schaerer, 2006).
In Switzerland, the first protective measures for single buildings were implemented in the 17th century, while protective measures in the release areas were first implemented in the early 19th century (SLF, 1999–2015). The first avalanche inventory for Switzerland dates back to the end of the 19th century (Coaz, 1881). The first snow and avalanche guideline was published in the first half of the 20th century (Paulcke, 1938). At the same time, more specialized literature on snow physics (Bader et al., 1939) and avalanche dynamics (Oechslin, 1938) appeared. Two avalanche disasters in the early 1950s (much of the Alps was affected in 1950/51; in 1954 it was part of Vorarlberg in western Austria) induced broad-scale technical protection measures. Voellmy (1955) developed a mathematical model for snow avalanche motion which represents the basis for many of the computer models still used six decades later. Since the 1960s, spatial planning and hazard maps have aided the trend toward modern integrated hazard and risk management strategies for snow avalanches (SLF, 1999–2015).
Glacial and Periglacial Hazards
Hazards emanating from winter snow cover are often highly relevant for everyday life of the mountain population. Hazardous processes related to the glacial environment, in contrast, usually start far away from populated areas, and only high-magnitude, low-frequency events cause damage. While snow hazards are often associated with the cold season, glacial hazards are more relevant during summer when some water is available in its more active liquid state and the glacial systems become more dynamic and therefore potentially unstable.
Evidence for a worldwide accelerated retreat of glaciers over the last few decades is overwhelming (WGMS, 2008), involving the tropics (e.g., Kaser, 1999), arid and humid mid-latitudes (e.g., Aizen, Kuzmichenok, Surazakov, & Aizen, 2007; Lambrecht & Kuhn, 2007), and the polar regions (e.g., Cook, Fox, Vaughan, & Ferrigno, 2005). Most of this retreat has been attributed to the evident atmospheric temperature rise (IPCC, 2007, 2013). Even though the rates and characteristics of glacier retreat vary from place to place, the general trend is more than clear (Zemp et al., 2015). Locally, the dynamics of the glacial and periglacial environment disturb the equilibrium of the system, inducing an increased level of hazard (Dussaillant et al., 2010; Evans & Clague, 1994; Haeberli, Clague, Huggel, & Kääb, 2010a; Harris et al., 2009; Huggel, Haeberli, Kääb, Bieri, & Richardson, 2004a; Huggel, Kääb, & Salzmann, 2004b; Kääb et al., 2005; IPCC, 2007; Quincey et al., 2007). Ice hazards mainly include the following types of processes (Clague & O’Connor, 2014; Deline et al., 2014; Evans & Delaney, 2014; Fig. 2):
1. Ice avalanches and rock-ice avalanches, mainly starting from hanging glaciers or steep glacierized rock cliffs (e.g., Alean, 1985; Huggel, Kääb, Haeberli, Teysseire, & Paul, 2002; Noetzli, Huggel, Hoelzle, & Haeberli, 2006). In rare cases they pose a direct threat to people not involved in mountaineering or to infrastructures (Mahboob, Iqbal, & Atif, 2015). Furthermore, they may initiate destructive process chains when impacting a lake (e.g., Haeberli, Portocarrero, & Evans, 2010b) or entraining a large amount of snow, ice, or debris (Evans et al., 2009; Huggel et al., 2005).
2. Volcano–ice interactions, i.e., snow and ice melting due to volcanic eruptions or geothermal activity, possibly resulting in floods or volcanic debris flows (lahars) (e.g., Lowe et al., 1986; Pierson, Janda, Thouret, & Borrero, 1990; Wilson, Smellie, & Head, 2013). Such phenomena are well known in Iceland (Björnsson, 2002), where they are referred to as jökulhlaups (Icelandic “glacier runs”), a term that is also used for glacial lake outburst floods. The most notable disaster related to volcano–ice interactions in recent history occurred in Armero, Colombia (Table 1).
1941, Palcacocha, Perú
A glacial lake outburst flood (GLOF) from Lake Palcacocha devastated part of the city of Huaraz, claiming approx. 5,000 lives.
1954, Blons, Austria
Extraordinarily heavy snowfall caused a huge number of avalanches in the federal state of Vorarlberg. The most disastrous event resulted in 56 fatalities in the village of Blons.
1970, Huascarán, Perú
An earthquake-triggered rock/ice mass movement from Nevado Huascarán converted into a high-velocity flow of debris and mud, reaching velocities up to 85 m/s. The flow overtopped a lateral ridge and buried the town of Yungay. The main flow alleviated on a debris cone originating from a similar, but smaller event in 1962, but the debris flood reached the Pacific Ocean at a distance of 180 km. In contrast to earlier reports of up to 30,000 victims, Evans et al. (2009a) estimated a death toll of approx. 6,000.
Evans et al. (2009)
1985, Armero, Colombia
An eruption of the volcano Nevado del Ruiz led to melting of snow and ice, resulting in massive lahar flows destroying the town of Armero. The number of victims is given with >22,000.
1999, Galtür and Valzur, Austria
The accumulation of more than 2 m of new snow within one month resulted in an extreme avalanche cycle, the most disastrous events leading to 31 deaths in Galtür and 7 deaths in Valzur (federal state of Tyrol).
2002, Kolka-Karmadon, Russia
A 100 million m³ rock/ice avalanche, originating at the northern slopes of the Mt. Kazbek massif, mobilized almost the entire Kolka Glacier. The resulting mass flow continued for a total of 35 km, the last 15 km as a distal mudflow. The event caused the death of approx. 140 people along with massive destruction.
Huggel et al. (2005)
2002, Dasht, Tajikistan
A GLOF destroyed the village of Dasht 11 km downstream and killed dozens of people.
Mergili et al. (2012a)
2012, Siachen-Gayari, Pakistan
An ice avalanche buried an army camp along with 148 people.
Mahboob et al. (2015)
2012, Pokhara, Nepal
The Seti River debris flow, with estimated 72 deaths, originated as a rockfall 29 km away from and >3,400 m above the impact area. The actual debris flow evolved through the entrainment of snow, ice, and debris.
(*) Note that this list is not exhaustive.
3. Since the beginning of the 21st century research has strongly focused on GLOFs. Glacial lakes (Fig. 3) may be dammed by bedrock, moraines, or by the glaciers themselves. Such lakes often develop in areas influenced by permafrost. Tweed and Russell (1999) distinguished nine types of ice-dammed lakes. Some lakes are prone to sudden outbursts and pose a potential threat to the downstream communities. Studies of GLOFs or GLOF hazard cover most glacierized mountain areas in the world, such as the Himalayas of Nepal and Bhutan (Bajracharya, Mool, & Shrestha, 2007; ICIMOD, 2011; Richardson & Reynolds, 2000; Watanabe & Rothacher, 1996), the Karakorum (Hewitt, 1982; Hewitt & Liu, 2010), the Pamir (Gruber & Mergili, 2013; Mergili & Schneider, 2011; Mergili, Müller, & Schneider, 2013), the Tien Shan (Bolch et al., 2011; Narama, Duichonakunov, Kääb, Daiyrov, & Abrakhmatov, 2010), the Andes (Carey, Huggel, Bury, Portocarrero, & Haeberli, 2012; Emmer & Vilímek, 2014; Haeberli et al., 2010b; Vilímek, Zapata, Klimeš, Patzelt, & Santillán, 2005), the Alps of New Zealand (Allen, Schneider, & Owens, 2009), the North American mountains (Clarke, 1982), the Norwegian mountains (Breien, Blasio, Elverhoi, & Hoeg, 2008), and the European Alps (Emmer, Merkl, & Mergili, 2015; Haeberli, 1983; Huggel, Kääb, Haeberli, & Krummenacher, 2003; Huggel et al., 2002; Tinti, Maramai, & Cerutti, 1999). The anticipation of possible glacial lake development is seen as a first important step in hazard assessment (Frey, Haeberli, Linsbauer, Huggel, & Paul, 2010). GLOFs can evolve in various ways, for example, by any type of rapid mass movement (such as rock/ice avalanches or calving ice fronts) into lakes, rising lake levels leading to overflow, progressive incision, mechanical rupture or retrogressive erosion of a dam, hydrostatic failure or degradation of glacier dams, or ice-cores in moraine dams (Richardson & Reynolds, 2000; Walder & Costa, 1996). Peak discharges are often magnitudes higher than in the case of precipitation- or snow melt–related floods (Cenderelli & Wohl, 2001). Entrainment may considerably increase the magnitude of the event and convert the flood into a destructive debris flow. Another, particular type of outburst flood, often related to big lowland rivers, consists in the release of ice jams (Jasek, 2003).
4. Various types of mass movement triggered or facilitated by the degradation/thawing or fluctuations of permafrost (Haeberli, Schaub, & Huggel, 2016; Harris et al., 2009) or by debuttressing of slopes through glacier retreat. It is now well documented that permafrost dynamics are a prominent preparatory factor for the occurrence of mass movements, but until the 1980s and even 1990s, this fact had not been widely recognized (e.g., Haeberli, 1992). Thawing of permafrost may affect slope stability, for example, by fracturing of rocks during freeze–thaw cycles, by increased availability of physically active water, or by changed topography (e.g., Gruber & Haeberli, 2007; Haeberli, Wegmann, & Von der Muehll, 1997; Huggel, Clague, & Korup, 2012; Huggel et al., 2010; Krautblatter, Funk, & Günzel, 2013; Ravanel & Deline, 2011). Several studies on climate change–induced dynamics of permafrost and related geo-hazards have been conducted. Intensive and detailed research was carried out at selected sites in the Swiss and Italian Alps (e.g., Monte Rosa) including measurements and modeling of permafrost temperatures and related rockfall events, rock-ice avalanches, slides, and debris flows. Harris et al. (2009) provide a comprehensive list of references on permafrost, its changes, and related hazards.
The scientific community has paid most attention to GLOFs, phenomena which are—as all other ice-related hazardous processes—not new. Floods originating from the glacial Lake Missoula in the period between 16,000 and 12,000 years ago are considered the largest floods documented in the geological record (Baker, 1973; Baker & Bunker, 1985). Between the 16th and the 19th centuries, some valleys in the Alps were repeatedly devastated by large floods or debris flows, many of them observed during fine weather. This happened in the Saaser Tal (Switzerland), the Ötztal (Austria), and the Martelltal (today Italy). All of these valleys were heavily glacierized. Some glaciers blocked valleys, impounding lakes fed by the water of other glaciers. Failure of these glacier dams caused periodic or episodic GLOFs leading to those flood waves.
The earlier events were not documented in a scientific way, but they were written down in valley chronicles such as by the priest Josef Eberhöfer in the case of the Martelltal. Finsterwalder (1890) reported about a sequence of GLOFs in the Martelltal ( 1887, 1888, 1889, and 1891), causing destructive debris flows extending tens of kilometers down to the Venosta Valley. A dam was constructed in 1892/93 in order to protect the underlying villages from future events, which it successfully did in 1895 (Fig. 4).
Due to massive glacier retreat since the late 20th century, ice-dammed lakes and associated GLOFs have become rare in the Alps. Similar processes are observed in other mountain areas such as the Himalayas, the Karakorum, and the Hindukush, as nicely described by Richardson and Reynolds (2000) and Iturrizaga (2011). Other types of glacial lakes, however, have formed in front of the retreating glaciers in many areas worldwide (Tweed & Russell, 1999). Some of them are prone to sudden drainage by the impact of ice avalanches detaching from glaciers retreating over steep rock walls or of rock avalanches from glacially debuttressed lateral slopes and/or slopes with degrading permafrost (Haeberli et al., 2010b). Such process chains reflect the dynamics of the changing high-mountain environments. Other incidents such as the 1985 Nevado del Ruiz disaster (Lowe et al., 1986) illustrate the interaction of glacial environments with volcanic activities.
Snow and Ice Hazard Mitigation and Disasters
In terms of research and risk mitigation, snow hazards and ice hazards are commonly addressed by different scientific and technical communities. Snow hazards are often covered by physicists and meteorologists, technical protection by forest and civil engineers in some countries. The higher relevance of snow hazards—i.e., avalanches—for the everyday life of mountain people is reflected in the existence of country-level organizations responsible for avalanche research, technical protection, and warning, such as the Torrent and Avalanche Control WLV (Austria, founded in 1884) or the Institute for Snow and Avalanche Research SLF (Switzerland, founded in 1931). While in Switzerland, the SLF also takes over avalanche warning (targeted mainly at making outdoor leisure activities safer), in Austria this is done independently by the avalanche warning services at the provincial level. Other examples of organizations dealing with snow avalanche hazards include AINEVA (Italy), the American Avalanche Association (and avalanches centers in the mostly affected states), Avalanche Canada, and the New Zealand Avalanche Centre.
Research on glacial hazards, in contrast, is very often covered by geoscientists at the level of universities and other research institutions. While the standing group on Glacier and Permafrost Hazards in Mountains by the Commission on Cryospheric Sciences and the International Permafrost Association (GAPHAZ) collects the scientists working in this direction, more formal national-level agencies do not exist. In cases where glacial hazards result in flooding downstream, the water authorities are in charge of mitigation, such as the Autoridad Nacional de Aguas for the Cordillera Blanca (Perú). However, the implementation of mitigation measures often relies on stand-alone engineering projects and, in the case of developing countries, on initiatives launched by intergovernmental centers such as the International Centre for Integrated Mountain Development (ICIMOD, Himalayas) or nongovernmental organizations such as FOCUS Humanitarian Assistance (Pamir).
If the magnitude of the natural process is high enough and society fails to appropriately mitigate the associated risk, events may evolve into disasters. Table 1 summarizes some of the most notable snow/ice disasters in the past 100 years. The international database EM-DAT (Guha-Sapir, Below, & Hovois, 2015) lists 106 snow avalanche disasters with 5,230 fatalities in the period 1910–2015. It is not possible to identify the number of glacial disasters from this database, but the database of glacial hazards events produced within the scope of the Glaciorisk European Project (2001–2003) lists 672 hazard events in European history, including glacier surges (rapid increases in length without mass increase; Harrison et al., 2014; Post & Mayo, 1971).
Spatial observation and analysis techniques may help to anticipate hazardous events, to mitigate the associated risks, and to prevent possible disasters. In this sense, the following sections will focus on (1) remote sensing, i.e., the interpretation and analysis of satellite or aerial imagery and the products derived thereof, and (2) computer modeling.
“Essentially, all models are wrong, but some are useful.” This statement of Box and Draper (1987) is valid also—and particularly—with regard to computer models of geomorphic phenomena and, therefore, snow and ice hazards. While such models may help us to anticipate the occurrence, location, and possible impact of hazardous events in the future, the usefulness of any model strongly depends on the availability of data appropriate to feed or to validate the model. As a consequence, observation and spatial modeling cannot be covered separately (a methodological framework is illustrated in Fig. 5).
A particular issue consists in the spatial and temporal scales necessary to appropriately consider specific phenomena of snow and ice hazards. Figure 6 shows the time frames relevant for selected subprocesses. In principle, all those subprocesses may be considered at scales from local to global. While the appropriate observation and modeling methods strongly depend on the scale of investigation, most modern applications rely on geographic information systems (GIS), enabling an efficient workflow of spatial data management and analysis.
Observation, Monitoring, and Documentation of Snow- and Ice-Related Hazardous Processes
The broad spectrum of earth observation techniques summarized under the term remote sensing represents one of the most essential methodological approaches to the observation of snow and ice hazards. Methods of remote sensing are widely used in geo- and environmental sciences, but they are particularly important in the field of snow and ice hazards, often involving high-mountain areas hard or dangerous to access.
Remotely sensed data can be classified according to its spatial, temporal, spectral, and radiometric resolution. Countless applications in the geosciences rely on passive sensors capturing reflected radiation in the visible and near-infrared wave lengths. Passive sensors depend on the radiation reflected from features at the earth’s surface. This strategy largely fails in cloudy conditions and is limited to receiving spectral signals. Active sensors, emitting radiation by themselves, also work in cloudy conditions (with some limitations). They further enable deducing the time needed by the radiation to reach the earth’s surface, and therefore the distance between the sensor and the surface. Light detection and ranging (LiDAR—also referred to as laser scanning) and synthetic aperture radar (SAR) are the most relevant active remote sensing techniques in geosciences.
Most types of sensors can be mounted to satellites (space-borne), aircraft, helicopters, or unmanned aerial vehicles (air-borne) or terrestrial systems. Two types of remotely sensed data are of particular interest (1) satellite and aerial imagery and (2) digital elevation models (DEMs)
Numerous space-borne sensors yield optical and infrared imagery at various levels of spatial and temporal resolution (a non-exhaustive account of such sensors is provided by Mergili, 2015). Aerial images and derived orthophotos are further available for many countries and regions worldwide.
Space-borne, air-borne, and terrestrial stereo-photogrammetry has been used traditionally to produce high-resolution DEMs (Kääb et al., 2005; Mergili, 2007; San & Suzen, 2005; Toutin, 1995). However, there has been rapid progress in the first decade of the 21st century. While Kääb et al. (2005) still referred to LiDAR as a promising technique, this methodology has become the standard for producing high-resolution DEMs in many fields of geosciences, expressed by a large number of publications (French, 2003; Glenn, Streutker, Chadwick, Thackray, & Dorsch, 2006; Höfle & Rutzinger, 2011; Jones, Brewer, Johnstone, & Macklin, 2007; Liu, 2008). As laser radiation has a limited range, LiDAR sensors cannot be mounted to space-borne sensors and are therefore not suitable for producing global-scale DEM databases. Instead, SAR Interferometry (InSAR) is often used for this purpose. The Shuttle Radar Topographic Mission (SRTM; Jarvis, Reuter, Nelson, & Guevara, 2008; Van Zyl, 2001) and TanDEM-X (Krieger et al., 2007; Moreira et al., 2004) are the most notable initiatives in this direction.
Nevertheless, remotely sensed data are applicable to surficial features only, and they are limited by the spatial and temporal resolution of the sensor. Further, radar signals penetrate into snow or ice, so that the information provided may be inaccurate (Rignot, Echelmeyer, & Krabill, 2001). For many applications, remotely sensed data have to be complemented by ground-based methods such as geophysical explorations, digging or drilling, or field mapping.
All types of remotely sensed or directly measured data show the status of a system at a certain point in time (see Fig. 5). However, with regard to possible hazards it is often more important to explore the dynamics of the system. The deduction of changes from the analysis of multi-temporal data is referred to as monitoring. In this context, the temporal resolution—i.e., the frequency of observation—is essential and strongly depends on the relevant temporal scale (see Figs. 5 and 6).
Hazardous events by themselves rapidly modify the system involved. The documentation of events and the comparison of data obtained before and after the event strongly contribute to understand the underlying processes (see Fig. 5). Consequently, detailed analysis of past events plays an essential role in better understanding snow and ice hazards and anticipating future events. Event documentation is further important to feed or to validate computer models. In this context, inventories of various types of processes and landforms are particularly valuable. They represent a collection of observed cases (ideally, covering these cases in their entire spatial extent and attributing to them additional information such as the time of occurrence). Inventories are commonly produced and used in the field of mass movement research as a basis for spatial modeling (Galli, Ardizzone, Cardinali, Guzzetti, & Reichenbach, 2008; Guzzetti et al., 2012; Wieczorek, 1984).
However, questions regarding the quality, potentials, and limitations of such inventories remain a big issue (Pellicani & Spilotro, 2015; Steger, Bell, Petschko, & Glade, 2015). It has to be considered that high-mountain environments in many regions of the world are currently in a stage of transition at a level not experienced and observed in historic times. Consequently, hazard situations have the potential of being beyond historical experience and evidence. Highly dynamic glacial and periglacial environments require frequent updating of inventories of glaciers, glacial lakes, or other objects: new features such as emerging glacial lakes may be highly relevant from a hazard perspective (Emmer, Merkl, & Mergili, 2015). Obviously, such inventories cannot directly be used for assessing future situations, which may be different not only from the past but also from present-day conditions (Frey, Haeberli, Linsbauer, Huggel, & Paul, 2010; Haeberli, Schaub, & Huggel, 2016; Linsbauer, Paul, & Haeberli, 2012; Linsbauer et al., 2015, 2016). Large ice-related events are very often nonrepetitive, i.e., they may reduce or eliminate the hazard permanently (e.g., moraine breach and subsequent lake drainage) or at least temporarily (e.g., the detachment of an ice cliff).
Observation, Monitoring, and Documentation of Snow Avalanches and Related Hazards
While glaciers, permafrost, and the related geosystems commonly display their dynamics at time scales from months to decades (see next section), snow cover—and therefore also the related avalanche hazards—may change at time scales from hours to days, weeks and months. These dynamics imply a number of challenges with regard to observation: most importantly, status observations of the snow cover often lose their validity within a very short period.
Consequently, remote sensing of the development of the snow cover relies on sensors with at least daily resolution. A thermal infrared sensor has been used for monitoring global snow cover distribution with a frequency of 14 images per day (Armstrong & Brodzik, 1999). However, such a high temporal resolution can only be achieved at the cost of a low spatial resolution, 25 km in this case. Daily 1.1 km resolution snow cover and snow temperature data sets from the MODIS satellite have been available since 1999 (Hall, Riggs, Salomonson, DiGirolamo, & Bayr, 2002). Even though such data sets may be useful for a very broad-scale overview of snow avalanche hazards in terms of identifying snow climatic zones at regional scales (Negi, Thakur, & Mishra, 2007), the identification of snow avalanche hazards at finer scales would require high-frequency LiDAR and SAR application in order to capture snow depth changes. This requires terrestrial techniques, may suffer from mountain shadows or the presence of trees, and is usually only possible for test sites of limited size (Schaffhauser et al., 2008). The same issue arises with terrain data. While the general terrain features can be extracted from ordinary DEMs, high-frequency LiDAR applications are necessary to capture snow depth changes or to produce pre- and post-event DEMs. Examples of LiDAR applications in the field of snow avalanches are manifold (Bühler, Christen, Kowalski, & Bartelt, 2011; Deems, Painter, & Finnegan, 2013; Prokop, 2008; Sailer et al., 2008). Due to the high temporal variability of snow cover, broad-scale before-and-after comparisons are rarely available. Avalanche deposits can be detected by using space-borne SAR images due to the difference in backscatter from undisturbed snow and the rougher surface of avalanche deposits (Malnes et al., 2013).
Not all characteristics of snow can be captured through remotely sensed data. On-site investigations are particularly important in the following cases:
• The vertical structure of snow, which can hardly be captured by remote sensing methods, is a key factor for snow avalanche formation (Schweizer, Bruce Jamieson, & Schneebeli, 2003). Depth and vertical structure of snow, with particular regard to snow avalanches, requires digging of snow pits. This is a standard procedure regarding avalanche hazard analysis (Fig. 7). The temporal development of the properties and structure of the snow pack—also for forecasting purposes—is often explored by computer simulations (e.g., with the SNOWPACK Model; Bartelt & Lehning, 2002).
• Meteorological observations: snow avalanche hazard depends not only on the topography and the depth of the snow cover. Instead, it is also strongly influenced by the meteorological conditions such as precipitation, wind, and temperature (Davis, Elder, Howlett, & Bouzaglou, 1999; Lehning et al., 1999; Mock & Birkeland, 2000; Perla, 1970).
Remote sensing–derived inventories of snow avalanches are available for a number of countries and regions such as Norway (Frauenfelder et al., 2010) or the Catalan Pyrenees (Oller et al., 2006). However, as the traces of snow avalanches usually disappear rapidly, the preparation of inventories is highly challenging, compared to landslide inventories.
A robust basis of snow avalanche data is mainly available for particular test sites such as the Col du Lautaret in France (Prokop et al., 2013; Thibert et al., 2015), the Vallee de la Sionne in Switzerland (Ammann, 1999; Vallet et al., 2001), or the Seehore Peak in Italy (Maggioni et al., 2013). Those test sites are also employed to derive more specialized parameters such as velocities and impact pressures of snow avalanches (e.g., Baroudi, Sovilla, & Thibert, 2011; Gauer et al., 2007; Sovilla, Schaer, Kern, & Bartelt, 2008; Thibert, Baroudi, Limam, & Berthet-Rambaud, 2008). Physical model tests (full-scale experimental avalanches) are used to gain information on those parameters and to build a basis for the validation of mathematical models (Ammann, 1999; Issler et al., 1999; Lied, Moe, Kristensen, & Issler, 2001; Prokop et al., 2013; Sovilla et al., 2008). Impact pressures are particularly important for estimating the vulnerability of structures to an event (Barbolini, Cappabianca, & Sailer, 2004; Fuchs & Bründl, 2005; Grêt-Regamey & Straub, 2006; Keiler et al., 2006) and are part of the legal basis for avalanche hazard zoning, e.g., in Austria (Sauermoser, 2006). Estimates of impact pressures may also be derived from reported damage to structures.
Observation, Monitoring, and Documentation of Ice-Related Processes and Associated Hazards
A broad spectrum of remote sensing methods is employed for the observation and monitoring of the glacial and periglacial environment. Kääb et al. (2005) provide a comprehensive overview of applications of optical remote sensing for glacial and permafrost hazards. More specialized applications—particularly those where subsurface features are of interest—make use of thermal infrared (LeSchack & Del Grande, 1976), which has been employed to identify the presence and the state of permafrost (Ran, Li, Jin, & Guo, 2015) and the mapping of debris-covered glaciers and buried ice (Lougeay, 1974; Shukla, Arora, & Gupta, 2010). However, such applications are particularly challenging.
Radar remote sensing, an active technique, is a particularly useful but challenging method to detect small surface changes over large areas. It has been used for monitoring changes of glaciers (Akbari, Doulgeris, &Eltoft, 2014; Luckman, Quincey, & Bevan, 2007; Strozzi, Luckman, Murray, Wegmüller, & Werner, 2002; Vachon et al., 1996), in permafrost environments (Chen, Lin, Li, Chen, & Zhou, 2012; Chen, Lin, Zhou, Hong, & Wang, 2013; Delaloye, Strozzi, Lambiel, Perruchoud, & Raetzo, 2008; Grosse et al., 2014; Kenyi & Kaufmann, 2003; Wang & Li, 1999; Wang et al., 2015; Yoshikawa & Hinzman, 2003), for glacial lakes (Strozzi, Wiesmann, Kääb, Joshi, & Mool, 2012), and volcano-glacier interactions (Scharrer, Spieler, Mayer, & Münzer, 2008). LiDAR techniques are commonly used for monitoring changes in the glacier surface (Arnold, Rees, Devereux, & Amable, 2006; Jóhannesson, 2013; Rees & Arnold, 2007) and for high-mountain environments in general (Avian, Kellerer-Pirklbauer, & Bauer, 2009; Deline, Jaillet, Rabatel, & Ravanel, 2008; Fischer, Huggel, Kääb, & Haeberli, 2013; Hubbard et al., 2013). Some purposes require the combination of more than one type of remotely sensed data (Robson et al., 2015).
The glacial environment typically changes at lower rates than the snow cover. Inventories of glaciers, mainly derived from remotely sensed data, are available at a global scale, such as the World Glacier Inventory (WGI; Cogley, 2010), the Global Land Ice Measurement from Space (GLIMS; Armstrong et al., 2005), or recently the Randolph Glacier Inventory (RGI; Pfeffer et al., 2014), and for various countries (Kääb, Paul, Maisch, Hoelzle, & Haeberli, 2002; Lambrecht & Kuhn, 2007). However, those datasets need continuous updating to keep up with the changing glacial environments.
While glacier fluctuations may become evident within time frames of few weeks or months (Fig. 8), detecting long-term trends requires multi-year analyses of remotely sensed data. In accordance with the observation, monitoring, and documentation of snow avalanches and related hazards (see Fig. 5), monitoring of glacial hazards operates in a spatio-temporal field between two extremes:
• Broad-scale low-frequency (multi-annual) monitoring based on regionally available optical satellite imagery or orthophotos, resulting in inventories of relevant features such as glaciers (e.g., Abermann, Lambrecht, Fischer, & Kuhn, 2009; Abermann et al., 2012; Lambrecht & Kuhn, 2007; Stocker-Waldhuber, Wiesenegger, Abermann, Hynek, & Fischer, 2012 for Austria and parts thereof) or glacial lakes (e.g., Mergili, Müller, & Schneider, 2013 for the Pamir; Emmer et al., 2015 for western Austria).
• Fine-scale high-frequency (permanent to annual) monitoring of situations of particular interest by means of photogrammetry, terrestrial LiDAR, or InSAR is required to capture the evolution of hazardous processes. Monitoring initiatives of unstable permafrost rock walls serve as examples of such efforts (Fey et al., 2012; Ravanel, Deline, & Bodin, 2015).
Glacier and permafrost hazards are sometimes strongly influenced by interacting subannual, annual, and multi-annual trends (see Fig. 6). Ice avalanche hazard may increase by the continuous retreat of a glacier over a steep rock cliff, losing its abutment, but the actual triggering of the ice avalanche event may occur due to melting processes in summer or due to an earthquake. Surging glaciers may quickly block valleys and impound lakes (Harrison et al., 2014; Post & Mayo, 1971).
Monitoring of potentially hazardous glacial lakes is of particular concern. Lake fluctuations occur on subannual time scales to decadal ones. Multi-temporal orthophotos, being available back to the 1950s for some areas worldwide, are particularly useful to identify the long-term trends (Emmer et al., 2015; Merkl, 2015). Where long series of orthophotos are not available, declassified CORONA imagery from the 1960 and 1970s is often useful in combination with newer high-resolution satellite images. For broad-scale overviews, medium-resolution images such as ASTER or Landsat can be used, which, however, fail to capture some details and have a number of limitations (Mergili et al., 2013). When it comes to hazard analysis, it is essential to consider not only the lake and its surroundings, but also its catchment. GLOF hazard is influenced not only by the characteristics of the lake and the dam impounding it, but also by the possibility of ice avalanches or other types of mass movement possibly impacting the lake. Particularly the latter type of hazard has been observed to increase in a systematic way (Haeberli et al., 2016).
The glacial retreat during the past decades, in combination with the increased availability of remotely sensed data, has revealed a growth of pro-glacial lakes in many areas throughout the world. In the Pamir of Tajikistan, for example, lakes which did not exist or were very small ponds at the end of the 1960s have evolved to lakes of hundred thousands of square meters, posing a certain threat with regard to GLOFs (Fig. 9A). Similar trends are seen in other mountain areas such as the Himalayas and the Alps.
Glacial lakes, however, do not always evolve at constant rates. Lakes may remain stable or grow slowly for decades and then experience a sudden increase in area and volume within a few years, strongly coupled to the dynamics of the related glacier. Figure 9B illustrates the evolution of a lake in the Pamir of Tajikistan which suddenly appeared in summer 2001, then remained constant in size for approximately one year before suddenly draining in August 2002 without an obvious trigger, causing a major disaster with dozens of fatalities in the village of Dasht 11 km downstream (see Table 1; Mergili, Kopf, Müllebner, & Schneider, 2012a). From its surficial appearance, this lake would have been rated less hazardous than hundreds of other lakes in the same area.
The example of Dasht illustrates that remote sensing cannot capture glacial environments in their full complexity. A particular issue consists in the internal structure of glaciers, with particular regard to water pockets that might be prone to sudden drainage. Geophysical techniques such as ground-penetrating radar (GPR) are most frequently applied for this purpose (e.g., Murray, Booth, & Rippin, 2007; Vincent et al., 2012). Also, the depth of glacial lakes cannot be directly derived from remotely sensed data. Instead, echo sounding has been applied for this purpose (Cochachin, Huggel, Salazar, Haeberli, & Frey, 2015). Also, the presence and properties of permafrost at a particular site requires further investigation. Even though the presence of permafrost is often indicated by surface features, ground-based investigations are necessary to explore the depth of permafrost or of the active layer or the vertical temperature distribution (Hoelzle, 1992; Krautblatter, Huggel, Deline, & Hasler, 2012). Morris, Hassan, and Vaskinn (2007) have performed physical model tests of dam breaches, which can be relevant for the understanding of GLOFs.
While inventories of glaciers and, partly, also glacial lakes are readily available, inventories of recorded hazardous events exist in a rather fragmented way. GAPHAZ runs a worldwide database on glacier and permafrost hazards (Flubacher, Huggel, Kääb, & Zemp, 2007). The International Disaster Database EM-DAT (Guha-Sapir, Below, & Hoyois, 2015) explicitly lists snow avalanche disasters, but disasters related to ice hazards are assigned to mass movements instead of being listed in an explicit way. The GRIDABASE database (Glaciorisk European Project, 2001–2003) provides information on hazardous events related to European glaciers. Mool, Bajracharya, and Joshi (2001) prepared a GLOF inventory for the Himalaya-Hindukush Region, while the Glacial Lake Outburst Floods database (Vilímek, Emmer, Huggel, Schaub, & Würmli, 2014) aims at compiling information on recorded GLOFs worldwide. Schneider, Huggel, Haeberli, and Kaitna (2011) compiled data about 64 large rock/ice avalanches worldwide. It is, however, important to note that the interpretation of event inventories requires some care as the hazard connected to a slope, a glacier, a lake, etc., may change considerably over time, e.g., glacial lakes may appear or disappear within a relatively short period. Often it is even the event itself that changes the level of hazard.
Spatial Modeling of the Release of Snow- and Ice-Related Hazardous Processes
Any type of observation allows one to look into the past or (almost) the present. Even though it is widely accepted in geosciences that the past is the key to the future, observations do not necessary allow direct conclusions about what can happen in the future. With the widespread availability and the increasing capacity of computers, mathematical models have become the dominant tool employed to better understand, or even to anticipate, natural hazards processes. A primary goal of hazard modeling is to explore magnitude–frequency relationships, i.e., to predict the probability in space and time of a particular process of a given magnitude. However, lack of detailed information often makes it necessary to express the hazard in a less direct way. A particular challenge in terms of hazard analysis is posed by singular events such as GLOFs through the breach of a moraine. A clear distinction should be made between potentially repetitive events with somewhat stochastic patterns of occurrence (snow avalanches) and nonrepetitive events resulting from cumulative developments such as glacier retreat, permafrost degradation, or moraine breaching. For the latter cases, probabilities cannot easily be defined.
Various types of models do exist, implemented in a broad array of computer programs often relying on GIS. Recalling the statement of Box and Draper (1987), the question arises as to which type of model is useful for which purpose. In principle, simple but evocative models are assumed superior to those characterized by overparameterization (Box, 1976). Mergili (2015) distinguishes between statistical, rule-based, and physically based models, whereby all three types may either be fully deterministic, or may contain stochastic elements (Fig. 10). Each type of model is useful for a certain scale range and for a certain level of process understanding and data availability.
For practical reasons, two groups of models are distinguished in the present article:
1. Models attempting to delineate those areas tending to release hazardous processes.
2. Models capturing the propagation of hazardous processes in order to anticipate possible impact areas, flow velocities, depths, and impact pressures of hazardous processes.
Both groups may be applied at a broad range of scales, from rough rule-based overviews at regional, national, or even global levels (Gruber & Mergili, 2013; Nadim, Kjekstad, Peduzzi, Herold, & Jaedicke, 2006) to very detailed physically based investigations of particular situations (see Fig. 10).
While in classical landslide research, modeling the release is often given priority over modeling the motion, this trend is not so much observed in snow and ice hazards research. Because snow and ice hazards are diverse, there are—with the exception of snow avalanches—hardly any standard procedures accepted for computing their release: possible release areas of ice-related hazardous processes are often known a priori, e.g., a glacial lake in case of GLOF hazards or a hanging glacier, steep glacier section, or cliff in case of ice avalanche hazards. Modeling of the motion of snow- and ice-related hazardous processes commonly makes use of techniques similar to those used in landslide and debris flow research.
Mass movements start as soon as the destabilizing forces exceed the stabilizing forces in a given system, i.e., when the factor of safety becomes smaller than 1. The physically based concept of the factor of safety (limit equilibrium method) is extensively used in a direct way in local-scale geotechnical engineering (Duncan & Wright, 2005). In contrast, it is less commonly employed at broader scales and for snow and ice hazards. Instead, different types of surrogates are used. The factor of safety—particularly in terms of its spatial patterns—requires much detailed technical information on the properties as well as on the horizontal and vertical structure of the materials involved. At regional scales—but, in the case of complex processes, also at local scales—these parameters are highly uncertain or not known at all.
With the emergence of GIS it has become common to produce susceptibility maps for mass movement processes (generally referred to as landslide susceptibility mapping; Guzzetti, 2006; Van Westen, Van Asch, & Soeters, 2006). Nadim et al. (2006) produced a map of global landslide and avalanche hot spots, relying on a set of globally available spatial datasets and a largely rule-based approach. Usually, susceptibility mapping is applied at regional or catchment scales, whereby those efforts—mainly relying on statistical or physically based approaches—are much more evident for landslides than for snow avalanches.
The purpose of susceptibility maps for mass movement processes is to provide a general overview useful to prioritize areas requiring further investigation. They do not consider the dimension of time. While in landslide research, it is common practice to use physically based or statistical models in order to identify rainfall thresholds for landslide occurrence (Baum & Godt, 2010; Caine, 1980; Glade, Crozier, & Smith, 2000; Guzzetti, Peruccacci, Rossi, & Stark, 2007), in snow avalanche research the highly dynamic interaction of meteorological conditions with the structure of the snow is most relevant (Höller, 2007).
Spatial Modeling of Snow Avalanche Release
Ghinoi and Chung (2005) introduced a GIS-based statistical model for snow avalanche susceptibility mapping, mainly relying on terrain features. The resulting susceptibility maps are useful for identifying avalanche hot spots.
However, spatial modeling of snow avalanche release is most commonly performed in the context of avalanche warning systems. Avalanche warning systems aim at near-real-time predictions of the hazard level. They often consist in a combination of observations with physically based, statistical, and rule-based modeling approaches. The analysis procedures involved in such systems commonly result in bulletins targeted mainly at out-of-bounds skiers and snowboarders (Brabec, Meister, Stöckli, Stoffel, & Stucki, 2001). They have been established since the 1950s (Höller, 2007; Längle, 1977) and focus on meteorological (Höller, 2007) and—as far as available—snow data in order to estimate the hazard at the regional scale, without considering snow avalanche motion. Near-real-time avalanche warning systems do exist for countries such as Norway (Engeset, 2013) or on a provincial scale such as in Austria (Nairz & Kriz, 2005). Most commonly a five-step ordinal scale is used to describe avalanche hazard at the regional scale (Meister, 1995), which was developed in Europe but also used in North America, where it was later modified (Statham et al., 2010; Table 2). Most work has been published for Switzerland (e.g., Meister, 1995; Russi, Ammann, Brabec, Lehning, & Meister, 2003). The dynamics of the scow cover is computed by physically based models such as SNOWPACK (Bartelt & Lehning, 2002; Lehning, Bartelt, Brown, & Fierz, 2002b; Lehning, Bartelt, Brown, Fierz, & Satyawali, 2002a), attempting to predict snow layering, settlement, mass balance, energy exchange, and metamorphism. This model relies on meteorological and snow measurements. It is an integral part of the Swiss avalanche warning system, where predictions are made as a now-casting system for time horizons of a few hours, based on a dense network of automatic stations (Bartelt & Lehning, 2002; Lehning et al., 1999). The model results are highly valuable for estimating avalanche hazards in situations where direct measurements of the snowpack properties cannot be obtained due to limited accessibility or danger. Local-level avalanche hazard knowledge is complemented by direct snow measurements. The results are compiled into regional-level forecasts published daily during the main winter season (Regional Avalanche Information and Forecasting System RAIFoS; Brabec et al., 2001). The regional-scale maps are produced using a nearest-neighbor interpolation algorithm (NXD-VG; Brabec & Meister, 2001).
Table 2. The North American Public Avalanche Danger Scale
Features and advices
Natural and human-triggered avalanches are certain and may occur in many areas. All avalanche terrain has to be avoided.
Very dangerous avalanche conditions. Natural avalanches are likely, human-triggered avalanches are very likely. Large avalanches may occur in many areas, very large avalanches in specific areas. Traveling in avalanche terrain is not recommended.
Dangerous avalanche conditions. Natural avalanches are possible, human-triggered avalanches are likely. Small avalanches occur in many areas, large avalanches in specific areas, and very large avalanches in isolated areas. A careful snow pack evaluation, cautious route-finding, and conservative decision-making are essential.
Heightened avalanche conditions on specific terrain features. Natural avalanches are unlikely, but human-induced avalanches are possible. Small avalanches occur in specific areas, while large avalanches may still occur in isolated areas. Snow and terrain have to be evaluated carefully, features of concern have to be identified.
Generally safe avalanche conditions. Natural and human-triggered avalanches are unlikely, but small avalanches may occur in isolated areas or extreme terrain. Therefore, unstable snow on isolated terrain features requires attention.
Source: Modified after Statham et al. (2010).
Spatial Modeling of the Release of Ice-Related Hazardous Processes
Ice hazards are usually recorded at low frequencies or even as singular events. As a consequence, hazard maps exploring magnitude-frequency relationships are rarely supported. Spatial modeling of the release of ice-related hazardous processes therefore concentrates on susceptibility mapping. A common way employed to create susceptibility maps of snow and ice hazards is to use rule-based scoring schemes relying on field and remote sensing observations, and on statistical relationships:
1. The susceptibility of a glacier or a portion of a glacier to produce ice avalanching is derived from the statistical analysis of observed ice avalanches (Alean, 1985; Gruber & Mergili, 2013; Huggel, Haeberli, Kääb, Bieri, & Richardson, 2004a). Thereby, the susceptibility scores are determined through a two-dimensional scheme of glacier slope and mean annual air temperature (as a surrogate for the ice temperature). Steeper glaciers in warmer areas receive higher scores than less steep glaciers in colder areas.
2. For glacial lake outburst floods it is common to use rule-based models evaluating the various characteristics of a lake and its catchment in order to derive an index or a score for its susceptibility to sudden drainage. This is done for individual lakes (Emmer et al., 2015), but also at regional or national scales (Emmer & Vilímek, 2014; Gruber & Mergili, 2013; Mergili & Schneider, 2011). Empirical equations have been developed to estimate average lake depth from lake area (Emmer & Vilímek, 2014; Huggel, Kääb, Haeberli, Teysseire, & Paul, 2002) and to derive the expected peak discharge from the dam height or the lake volume (Costa, 1985; Costa & Schuster, 1988; Walder & O’Connor, 1997).
Ice hazard susceptibility models generally rely on DEMs at a level of detail corresponding to the scale of the study and on inventory maps of the features of interest (glaciers, lakes, permafrost). Information yielded by additional data sets such as seismic hazard (Giardini, 1999) or geological maps is sometimes included in the rule sets. It should be emphasized that the results of this type of models are often highly uncertain and mainly represent first-order analyses directed toward a basis for discussion or the identification of hot spots requiring further attention (Gruber & Mergili, 2013). Figure 11 illustrates some examples of susceptibility—or hazard indicator—maps and derived risk indicator maps for ice-related hazardous processes. Here, also, mass movement propagation is considered.
Also, regional, national, or global permafrost distribution maps are important in this context. Because permafrost is a highly complex phenomenon involving numerous variables (Harris et al., 2009), physically based models are only useful for improving the understanding of local-scale phenomena. The global permafrost zonation map of Gruber (2012), even though it includes different components, is most appropriately characterized as the result of a rule-based model (1 km resolution). Boeckli, Brenning, Gruber, and Noetzli (2012) produced a permafrost distribution map for the European Alps at a cell size of 25 m. Mergili et al. (2012a) describe the development and application of a map of potential permafrost areas for the territory of Tajikistan. A system of rules of thumb of lower permafrost boundaries established by Haeberli (1975) for Switzerland is adapted to the temperature regime in Tajikistan. Such an approach disregards local conditions such as snow cover or irradiation, but it can help to gain a rough idea on permafrost distribution at broad scales. In the study of Mergili et al. (2012a), potential future permafrost distribution maps are created for various assumptions of temperature changes in order to assess the possible response of permafrost distribution to scenarios of temperature increase established by the IPCC (2007, 2013).
Another effort developed recently is the detection of possible future glacial lake formation mainly deduced from the topography of the glacier bed (Frey et al., 2010; Köhler, Geilhausen, & Linsbauer, 2014; Linsbauer et al., 2012, 2015, 2016).
Spatial Modeling of the Motion of Snow- and Ice-Related Hazardous Processes
Gaining an idea about possible source areas of snow and ice hazards is sometimes sufficient as a basis to alert people or to protect objects located close to possible release zones. As soon as the consequences of snow avalanches, ice avalanches, GLOFs, or other types of mass movements for villages, roads, buildings, or humans in the valleys (i.e., the risks associated to these hazards) are of interest, the propagation of the movement has to be considered, be it for dimensioning of protective measures or for hazard and risk zoning. Hazard zoning in many European countries such as Switzerland, Italy, France, or Austria builds on return periods of events with defined impact pressures (Cappabianca, Barbolini, & Natale, 2008; Sauermoser, 2006). Damage records for potentially repetitive events, on the one hand, and models yielding impact pressures, on the other hand, are highly important in this regard.
Starting with the catastrophic winter of 1999 with the Galtuer disaster (see Table 1), substantial effort has been put in quantifying the risk related to snow avalanches, considering also the socioeconomic dimension (Cappabianca et al., 2008; Fuchs & Bründl, 2005; Fuchs, Bründl, & Stötter, 2004; Keiler et al., 2006). Impact pressures were derived from the results of physically based dynamic mass flow models such as AVAL-1D (Fuchs et al., 2005), SAMOS and ELBA+ (Keiler et al., 2006), or VARA1D (Cappabianca et al., 2008) and combined with vulnerability functions to predict the expected degree of damage, e.g., to buildings.
It should be emphasized that mathematical models employed for the propagation of snow- and ice-related hazardous processes are usually developed for a broader scope, i.e., for many types of geophysical mass flows. They can be used for various types of industrial flows, but also for flow-like mass movements such as debris flows or rock avalanches and largely go back to Voellmy (1955), who first formulated a physically based equation for the propagation of mass flows, an initiative resulting from the avalanche disasters in the Alps in 1950/51 and 1954. Powder-snow avalanches, however, require different types of modeling approaches (Norem, 1995; Sampl & Zwinger, 2004).
Two branches of mass flow models have developed out of the Voellmy model:
1. Physically based dynamic models make use of physical laws, assuming specific flow rheologies (Hungr, 1995; Iverson, 1997; McDougall & Hungr, 2004, 2005; Pitman & Le, 2005; Pudasaini, 2012; Pudasaini & Hutter, 2003; Pudasaini, Wang, & Hutter, 2005; Savage & Hutter, 1989; Takahashi, Nakagawa, Harada, & Yamashiki, 1992). A particular issue of importance is the entrainment along the path (Sovilla & Bartelt, 2002). This type of model is useful for the detailed backcalculation or prediction of specific events.
2. Mass point models relate the shear traction to the square of the velocity and assume an additional Coulomb friction effect (Pudasaini & Hutter, 2007). This approach considers only the center of the flowing mass, not its deformation and the spatial distribution of the flow variables (Gamma, 2000; Mergili, Schratz, Ostermann, & Fellin, 2012b; Perla, Cheng, & McClung, 1980; Wichmann & Becht, 2003). Mass point models are applied both at local and at regional scales. Even though they do not capture the processes in full detail, they are sufficient for certain purposes and are more comfortable to employ in combination with GIS than physically based dynamic models for reasons of the conceptual compatibility (i.e., type of coordinate system used) and the computational times.
A third type of model consists in empirical-statistical relationships. Since mass flow processes are often complex in detail, first estimates of travel distance and impact area often make use of statistical models. Threshold values of slope or horizontal and vertical distances (Burton & Bathurst, 1998; Corominas, Copons, Vilaplana, Altamir, & Amigó, 2003; Lied & Bakkehøi, 1980; McClung & Lied, 1987; Vandre, 1985) related to volume (Rickenmann, 1999) are used as criteria. The scatter in the relationships is usually large, and key parameters for design issues, such as impact pressures, are not provided (Hungr, Corominas, & Eberhardt, 2005). Some of the most important published relationships are shown in Table 3. Empirical-statistical relationships are also implemented in simple flow routing models (e.g., Gruber & Mergili, 2013; Mergili, Krenn, & Chu, 2015b; Huggel, Kääb, Haeberli, & Krummenacher, 2003; see Fig. 11).
Table 3. Selected Empirical Relationships Used for Estimating the Travel Distance of Various Types of Relevant Flow-Type Mass Movements
Lied & Bakkehøi (1980)
ωr = 11
Debris flows from GLOFs or periglacial debris flows, suitable mainly for coarse-grained debris flows
Worst case for debris flows from GLOFs
Floods from GLOFs
Noetzli et al. (2006)
ωr = 17
ωr = 7
Fine-grained debris flows
Zimmermann et al. (1997)
Notes: ωT = Angle of reach, ΔZ = total vertical displacement, y” = second derivative of avalanche slope, β = average gradient of avalanche track, θ = gradient of rupture zone, L = travel distance, V = volume, Qp = peak discharge.
Referring to Figure 10, mass point models contain elements of statistical and rule-based models while empirical-statistical relationships represent rule-based models as soon as applied to areas different from those where they were developed. In contrast to physically based dynamic models, mass point models and empirical-statistical relationships are mainly useful for delineating possible impact areas instead of flow pressures or energies. Their application in combination with GIS implies two problems:
1. While the direction of the flow is implicitly computed in physically based dynamic models, particular strategies of flow routing are necessary when applying mass point models or empirical-statistical relationships in combination with GIS. Using the steepest path through the DEM would disregard the lateral spreading. Therefore, either multiple flow direction algorithms (e.g., Horton, Jaboyedoff, Rudaz, & Zimmermann, 2013; Huggel et al., 2002) or constrained random walk approaches (Gamma, 2000; Mergili et al., 2015b; Pearson, 1905) are applied. This means that flow routing is repeated many times with randomly varied flow paths. Therefore, such computations contain a stochastic element.
2. Curves and bends in the flow path are not considered. It has to be known whether the relationship in question was developed based on true flow paths or on straight distances.
A particular challenge consists in modeling of process chains, i.e., including the impact of a mass movement on a lake. Appropriate modeling of process chains principally requires physically based modeling. Schneider, Huggel, Cochachin, Guillén, and García (2014) presented an effort to backcalculate a rock/ice avalanche–triggered GLOF by applying a model chain of three interacting, physically based models. The simulation of process chains represents an important emerging field of geoscience (Somos-Valenzuela et al., 2014; Worni, Huggel, Clague, Schaub, & Stoffel, 2014; Worni, Huggel, & Stoffel, 2013), with several challenges remaining. The two-phase flow model of Pudasaini (2012), and the—closely related—prospective software tool r.avaflow (Mergili, Fischer, Fellin, Ostermann, & Pudasaini, 2015a) are promising initiatives in this direction (Fig. 12).
In order to apply mathematical models to a specific problem, a software tool allowing appropriate functions for parameter input and output of the results is required. In general, there is still a lack of ready-to-use software applications employing the more advanced physically based models. Most notably, RAMMS (Christen, Bartelt, & Kowalski, 2010a; Christen, Kowalski, & Bartelt, 2010b) is widely used for modeling snow avalanches and other types of mass flows. Other software packages are SAMOS-AT and TITAN-2D. Particularly GLOFs often take an intermediate character between debris flows and floods. Therefore, it may be useful to compare the outcomes of more than one type of modeling software. In this sense, Mergili, Schneider, Worni, and Schneider (2011) have compared RAMMS and the river flow software FLO-2D for GLOFs. Table 4 lists some of the software applications used in the field of snow and ice hazard modeling.
Table 4. Selected Software Applications Suitable for Physically Based Dynamic Modeling of Snow and Ice Hazards
2D (depth-averaged) granular flow model for flow component and 3D turbulent mixture model for powder component.
Sampl & Zwinger (2004)
ArcGIS module; shallow water model.
Voellmy-fluid friction model, based on 2D shallow water equations.
Christen et al. (2002)
Mass and momentum conservation equations combined with probabilistic approach.
2D (depth-averaged) two-phase flow model for incompressible Coulomb continuum (shallow water model).
Pitman & Le (2005)
2D finite difference model for routing non-Newtonian flows.
O’Brien et al. (1993)
Model Validation and Parameter Optimization
Any type of model application relies on a set of input parameters. The type and number of parameters used depends on the type and complexity of the model. Friction parameters are most essential for physically based mass movement models and may significantly vary, depending of the type, physical characteristics, magnitude, and topography of the process under investigation. All spatial models for mass flow propagation rely on a digital elevation model (DEM) as well as the distribution or depth of the released mass. It should be mentioned that different purposes require different levels of DEM resolution. The highest possible resolution can cause unrealistic artefacts and is not a priori the best choice for hazard analysis purposes.
Many parameters are uncertain as they cannot be directly measured in their full spatial distribution. Strategies are needed to deal with those uncertainties. Therefore, modeling requires three main steps:
1. Optimization of the parameters in terms of the capability of the model to reproduce well-documented events in terms of impact area, flow depths, velocities, or pressures.
2. Validation of the model with another set of observed events of the same type and magnitude.
3. Forward calculations of possible future events (e.g., scenario-based modeling).
Traditionally, ad hoc approaches are used to optimize the parameters for well-documented events before applying the model for forward calculations, using the optimized set of parameters (Gruber & Bartelt, 2007; Jamieson et al., 2008). Validation is most commonly performed against observed deposition areas. Where available, other parameters are also used.
One-at-a-time parameter variation is frequently applied for optimization purposes, meaning that one parameter is varied—and optimized—while all the others are kept constant. This approach, however, has been criticized as inappropriate (Saltelli & Annoni, 2010) as it is not capable of considering interdependencies of the parameters. Fischer (2013) has attacked this gap with the tool AIMEC, building on random testing of a large number of parameter combinations, so-called Monte Carlo simulations. However, in the case of many-parameter models, a very high number of model runs is required. Mergili et al. (2015b) suggest considering not definite parameter values but instead combinations of parameter ranges, resulting in an impact indicator index which denotes the fraction of model runs predicting an impact on a given pixel (range 0–1).
Conclusions and Future Perspectives
The present article focuses on mass movements in mountain areas which are (1) related to snow and ice and (2) represent a potential threat for people, property, and infrastructures. However, modern perspectives prefer the view that damaging events, or even disasters, have social rather than natural reasons/sources, i.e., result from the failure to appropriate adaptation to or mitigation of the natural conditions or processes (Blaikie, Cannon, Davis, & Wisner, 2014; Carey et al., 2014). Even though this perspective may not be applicable to all “natural” disasters to the same extent, it is undisputed that strategies to reduce the losses should consider not only the natural processes but also the socioeconomic dimension, which is actually multi-dimensional. Hewitt (2016) underlines the fact that the vulnerability of a population to mountain hazards strongly depends on the economic capacities, i.e., “poverty seems the main source of vulnerability”. This issue is not further elaborated upon here; Hewitt (2016) provides a comprehensive account of the human ecology of disaster risk in cold mountainous regions. This type of viewpoint complements a more technical understanding of risk as, e.g., nicely outlined for the example of debris flows in Jakob, Holm, and McDougall (2016) or followed by Gruber and Mergili (2013) (see Fig. 11).
With regard to cryosphere-related mass movements, one should point out that risk evaluation is fundamentally different for snow avalanches than for most glacier- or permafrost-related processes. Damaging snow avalanches are potentially repetitive with a more or less stochastic temporal occurrence, while damaging ice-related processes are often nonrepetitive within policy-relevant time scales. As a consequence, probabilities of occurrence are much more difficult—if not impossible—to define for the latter than for the first. Moreover, the latter also often imply low-frequency/high-magnitude phenomena, which are politically difficult to treat from a risk management perspective.
Moreover, the terms “hazard” and “risk” relate to the future. Particularly under the prevailing climate-driven transition of virtually all cold mountain regions worldwide, the future will most likely be different not only from the past but even from present-day conditions. Modern, future-oriented climate change impact and adaptation research therefore relies on scenario-based hazard, vulnerability, and risk modeling in complex, highly interconnected systems and new landscapes. Haeberli et al. (2016) outline such research directions. First, they point to the impact of possible mass movements released from steep areas with degrading permafrost or retreating glaciers on possible future lakes forming on or in front of decaying or retreating glacier tongues and to the anticipation of the consequences of the resulting process chains throughout the 21st century. Second, they underline the importance of an appropriate “dialogue between scientists, engineers, policy makers and the public.” Such a dialogue is considered key for effectively reducing the risks from a changing cryosphere (Carey, Huggel, Bury, Portocarrero, & Haeberli, 2012; Carey et al., 2014).
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