Stefan Margreth

Predicting snow pressure forces exerted by the snow cover on structures on sloping terrain remains a challenging task. Especially with the increasing interest in alpine solar parks with little financial margin for safety, the demand for monitoring systems as well as accurate prediction models for snow pressure acting on slender supports is higher than ever before. This paper presents an experimental setup for measuring the reaction forces of the snow cover on various support and pressure bars of an snow supporting structure made of steel using strain gauges. In combination with the material properties and the entire static system of the structure, we can back calculate the pressure exerted by the snow cover. In order to interpret the measured forces correctly, additional field campaigns in winter are important to determine the local snow distribution, the snow temperature and the density of the snow cover. We extensively address challenges encountered during the measurement campaign, including eliminating temperature-induced strain, amplifying the signal in low strain ranges, handling combined bending and restraint stresses, and protecting sensors and cables against settlement and creep forces. Having overcome these challenges, the key advantages of using strain gauges are that the system is cost effective and non-invasive and therefore allows rapid installation on a large number of sections of interest. We discuss results obtained from the first winter of the campaign (2023/2024). The high measurement frequency of one measurement every five minutes provides an insight into the daily cycle of the forces. Most notably, on warm spring days with cold nights, we observed a continuous increase in forces starting at sunset, peaking at sunrise, followed by a rapid decline thereafter. The measurement data can be used to calibrate newly developed viscoelastic 3D snow creep models with various constitutive laws for the prediction of creep and glide forces on supporting structures. These models as well as the presented experimental setup show great potential to further optimise the design of supporting structures and installations of alpine solar parks.

With advancements in computational power and recent developments in particle-based numerical modeling, it is now possible to perform fully three-dimensional (3D) and real-scale snow avalanche simulations using methods such as the Material Point Method (MPM). These 3D MPM simulations explicitly model flow processes and velocity components in the slope-normal direction, allowing for accurate capture of phenomena such as flow detachment from the terrain in response to abrupt topography variations. In a recent case study where we applied MPM to simulate a snow avalanche at slope scale in 3D from release to run-out, we observed significant slope-normal velocity components, particularly in regions with pronounced variations in terrain curvature. This observation raised the question of the practical significance of slope-normal flow velocity components, especially in terms of their interaction with objects susceptible to uprooting, protection galleries, or other structures where slope-normal velocities might exert enough force to dislodge or damage roofs. To investigate this further, we use the flow conditions from a slope-scale MPM simulation of the 2019 “Salez” avalanche in Davos, Switzerland, which overflows a road gallery, as boundary conditions for Discrete Element Method (DEM) simulations. In these DEM simulations, we focused on the avalanche flow on the gallery roof, analyzing the resulting normal and shear forces as well as the velocity components normal and parallel to the roof. A comparison between the stresses obtained from DEM simulations and the calculations based on the Swiss Guidelines reveals that the guidelines provide conservative yet relatively accurate stress estimates. By distinguishing between the normal and parallel velocity components relative to the gallery roof, we find that the normal component is closely correlated with the resulting stresses, highlighting the potential significance of normal velocity components in determining the load on galleries.

In Switzerland, avalanche hazard zones must be considered in land-use planning. No new buildings can be constructed in the red hazard zone and modifications to existing buildings are severely restricted. When assessing building applications in the red hazard zone, no distinction is made as to whether a residential building in a settlement is used all year round or whether a cabin outside the settlement area, in the mountains, is only used in summer. A political initiative in the canton of Grisons called for a distinction to be made between year-round and summer use in the absence of avalanche danger. Therefore, we developed a methodology for defining the avalanche-free season. The avalanche-free season depends primarily on the length of the snow-free period, i.e. the period with-out continuous snow cover. For the snow climate of the canton of Grisons, the snow-free period at an elevation of 1500 m is about 163 days, and at 2500 m about 91 days. To better adapt the snow-free period to local conditions, we introduced adjustment factors that consider aspect, local climate, terrain roughness and the avalanche situation. Finally, to determine the avalanche-free season, we evaluated whether the location could be at risk of summer avalanches triggered by a heavy snowfall during the snow-free period. If a location is not at risk of summer avalanches, the avalanche-free season corresponds to the corrected snow-free period. On the other hand, if a location is at risk of summer avalanches, there is no avalanche-free season. The methodology is used by the canton of Grisons to determine the duration of temporary summer use of buildings in the red hazard zone.

Between January 6 and 15, 2019, South Tyrol, Italy, experienced a series of remarkably large and infrequent powder snow avalanches, marking some of the most significant occurrences in the region over the past five decades. These avalanches, characterized by their immense scale, decimated 200-year-old trees, left alpine huts in ruins and destroyed avalanche protection structures. Fortunately, no human casualties were reported. The relentless snowfall, which persisted for 17 days, was exacerbated by persistent northwesterly winds at high altitudes, resulting in an accumulation of 1.3 to 2.5 meters of new snow above the timberline along the eastern main ridge of the Alps. With consistently low temperatures in the first half of January and the lingering effects of unfavorable snow cover from early winter, the avalanche threat intensified. Several large avalanches occurred on January 14, when the perilous conditions escalated to an avalanche danger level of 4 along the northern border ridge with Austria. In nearby regions of Tyrol, Austria, the avalanche danger level reached 5.
In this paper, we compare 100-year calculation scenarios with an extreme powder snow avalanche from 2019. Our goal is to test existing hazard mapping procedures to determine how advanced avalanche dynamics models can be effectively employed to predict extreme avalanche hazards. Rather than  backcalculating the documented avalanche, we adopt the approach of a hazard engineer who must construct a hazard map without reference data, relying mainly on historical snow and weather data from the region. We conclude that it is possible to expand existing avalanche calculation procedures that utilize statistical extreme value snow cover data to include snow temperature and snow cover distribution.

Avalanche engineers use scenario-based simulations to predict avalanche runout and quantify impact  ressures. An important input parameter for such simulations is the avalanche return period. In Switzerland, 10-, 30-, 100-, 300- and > 300-year avalanche return periods are taken into account for hazard assessment. Another important input parameter is the fracture depth which is determined with a procedure developed by SLF in 1990. Presently, the procedure provides no methodology to include snow entrainment, snow temperatures or snow density. This limitation hampers the application of advanced models that incorporate snow entrainment.
The fracture depth is based on the determination of measured 3-day snow height increase using extreme
value statistical approaches. The statistical values are modified to include the effects of altitude, snow drift accumulations and terrain (slope angle). The procedure provides fracture depths d0 for e.g. 10-, 30-, 100- and 300-year avalanche return periods. In this paper, we modify this well-known SLF procedure to determine snow entrainment depths d0* (erodible snow) which are associated with measured 3-day snow height increase. The erodible snow along the track is modified including altitude gradients and the slope angle.
Within the framework of the proposed methodology, we demonstrate how to specify snow temperatures that are suitable for typical winter storms in the Alps, depending also on the altitude of the release area. These recommendations are based on snow profiles recorded during storm periods with heavy snow fall of February 1999 and January 2018, 2019 and 2021. The final component of the procedure is to provide a realistic snow density for the fracture slab.
The procedure is straightforward, easily integrated into existing workflows, and benefits from relying on measurement data, ensuring realistic predictions of entrainment and deposition heights. Crucially, this method enables the examination of avalanche runout in a shifting climate marked by rising temperatures.

Die räumliche Verteilung der Schneehöhen ist ein zentraler Parameter für die Planung von Lawinenschutzmassnahmen und Verwehungsverbauungen sowie für die Beurteilung derer Wirksamkeit. Bisher konnte die Schneehöhe nur punktuell oder bestenfalls entlang einer Linie (Transekt) gemessen und analysiert werden. Der Einsatz von Drohnen ermöglicht die Schneeverteilung über die Fläche zu ermitteln. Diese präzisen Schneehöhenkarten ermöglichen nicht nur die optimale Planung von neuen Anlagen, sondern auch deren Wirkungskontrolle. Weiter dienen sie dem Erkenntnisgewinn und dem Verständnis von bislang nicht geklärten Prozesszusammenhängen und Schadenbildern an bestehenden Anlagen.

The spatial distribution of snow depth is a central parameter for the planning of avalanche protection measures and drift control structures, as well as for the evaluation of their effectiveness. Until now, snow depth could only be measured and analysed at specific points or, at best, along a line (transect). The use of drones makes it possible to determine the snow distribution over an entire area. The resulting precise snow depth maps enable not only optimal planning of new facilities, but also assessments of their effectiveness. Further, they help us to gain knowledge and understanding of previously unexplained process interdependencies and patterns of damage regarding existing facilities.

The Swiss Alpine Club SAC operates 153 mountain huts in the Swiss Alps. Some of these huts are located at locations exposed to avalanches. In the past, several huts were hit by ava-lanches and in some cases destroyed. Due to climate change, the avalanche hazard may change. At higher elevations, winter precipitation and snowfall intensity are projected to intensify, in some cases increasing the avalanche hazard. The retreat of glaciers may change the topography which can also influence the hazard. For an initial risk assessment, the SLF studied the avalanche risk for all 153 huts on behalf of the SAC. The analysis was based on existing data such as the topography, event records and hazard indication maps; neither terrain surveys nor avalanche simulations were carried out. Each hut was assigned to one of three risk categories: 57% of the huts are not at risk, 24% might be at risk, and 19% of the huts have been proven to be at risk from avalanches. Based on this overview, the hazard was evaluated in detail in a second phase, where necessary. The same safety standards as for settlement areas were applied for this assessment. Scenarios with a return period of up to 300 years were considered, including possible effects of climate change. Where the existing protective measures were not sufficient, additional measures were proposed. Based on four examples, we show different aspects of the impact of climate change on hazard assessment.

Snow avalanche hazard mapping has a long tradition in the European Alps. Hazard maps delineate areas of potential avalanche danger and are only available for selected areas where people and significant infrastructure are endangered. They have been created over generations, at specific sites, mainly based on avalanche activity in the past. For a large part of the area (90 % in the case of the canton of Grisons) only strongly generalized hazard indication maps are available (SilvaProtect), not showing impact information such as pressure. This is a problem when new territory with no or an incomplete historical record is to be developed. It is an even larger problem when trying to predict the effects of climate change at the state scale, where the historical record may no longer be valid. To close this gap, we develop an automated approach to generate spatially coherent hazard indication mapping based on a digital elevation model for the canton of Grisons (7105 km²) in the Swiss Alps. We calculate eight different scenarios with return periods ranging from frequent to very rare as well as with and without taking the protective effects of the forest into account, resulting in a total of approximately 2 million individual avalanche simulations. This approach combines the automated delineation of potential release areas, the calculation of release depths and the numerical simulation of the avalanche dynamics. We find that between 47 % (most frequent scenario) and 67 % (most extreme scenario) of the cantonal area can be affected by avalanches. Without forest, approximately 20 % more area would be endangered. This procedure can be applied worldwide, where high-spatial-resolution digital elevation models, detailed information on the forest and data on the snow climate are available, enabling reproducible hazard indication mapping also in regions where no avalanche hazard maps yet exist. This is invaluable for climate change studies. The simulation results are validated with official hazard maps, by assessments of avalanche experts, and by existing avalanche cadastres derived from manual mapping and mapping based on satellite datasets. The results for the canton of Grisons are now operationally applied in the daily hazard assessment work of the authorities. Based on these experiences, the proposed approach can be applied for further mountain regions.

Gefahrenkarten bilden eine wichtige Grundlage für den Umgang mit der Lawinengefahr im Alpenraum. Allerdings existieren diese Karten nur im Siedlungsbereich, für einen Grossteil der Fläche sind keine oder nur sehr begrenzte Informationen zur Lawinengefährdung verfügbar. Aus diesem Grund hat das WSL Institut für Schnee- und Lawinenforschung SLF zusammen mit dem Amt für Wald und Naturgefahren (AWN) des Kantons Graubünden eine Methode entwickelt, um flächendeckende Gefahrenhinweiskarten automatisiert zu erstellen. Die Kombination von Anrissgebiets-Ausscheidung mit der numerischen Simulation der Auslaufstrecken mit der Software RAMMS macht dies möglich. Dabei halten wir uns so weit wie möglich an die Vorgehensweise, wie sie in der Schweiz zur Erstellung der Gefahrenkarten verwendet wird.

Hazard maps are a crucial base for tackling avalanche hazard in the Alps. But such maps are only elaborated within specific, mostly settled areas, for the large part none or only very limited information on avalanche danger is available. Therefore, the WSL Institute for Snow and Avalanche Research SLF and the office for forest and natural hazards (AWN) of the canton Grisons developed a methodology to calculate spatial continuous hazard indication maps for large Regions. The combination of release area delineation with the numerical simulation of avalanche runout with the software RAMMS makes this possible. We stick as much as possible with the procedure applied for the hazard mapping generation in Switzerland.

Im Lawinenschutz wird Holz insbesondere beim temporären Stützverbau sowie bei Gleitschneeschutzmassnahmen (Verpfählungen, Dreibein- böcken und Holzschwellen) eingesetzt. überdies wird Holz im Verwehungsverbau und vereinzelt bei Ablenkbauwerken verwendet.

Dans la protection contre les avalanches, le bois est utilisé en particulier pour les ouvrages de soutènement temporaires ainsi que pour les mesures de protection contre les glissements de neige (pieux, trépieds et seuils en bois). En outre, le bois est utilisé dans les ouvrages de protection contre les congères et, de manière isolée, dans les ouvrages de déviation.

In avalanche control, wood is used particularly for temporary supporting structures and for measures to counteract gliding snow (stakes, tripods and wooden logs). Wood is also used for snowdrift control
structures and, occasionally, for avalanche deflectors.

The actions of wind, earthquakes and snow on roofs have already been standardised throughout Europe. However, very few regulations exist how structures should be designed in case of floods, landslides, debris flows, rock fall, avalanches, snow pressure and hail. The Swiss Society of Engineers and Architects SIA therefore supplemented the existing standard with detailed information regarding the design of structures endangered by natural hazards. The revised Swiss code SIA 261/1 “Actions on Structures – Supplementary Specifications” has been published in 2020.

In January 2018, heavy snowfalls caused an extraordinary avalanche situation in Switzerland with an extent not observed since the avalanche winter of 1999. The amount of precipitation was far above the long-term average und in parts of the Swiss Alps even twice the average. For the first time since 1999, the highest danger level 5-Very High was forecast for a 36-hour period and for most regions of the Swiss Alps. We present key findings of the analysis of this extraordinary event and show that the mitigation efforts taken after the catastrophic events in February 1999 were effective. Despite very high avalanche activity, there was no loss of life and damage to property was limited.

Snow avalanches can pose a risk to persons, buildings, and infrastructure in mountain regions. As long as people have settled in the Alps, they had to deal with avalanche hazard. Structural mitigation measures, protection forests, and land-use planning reduced the hazard and the risk in past decades and contributed thereby to the development of mountain regions. In many countries, decisions on dealing with the consequences of avalanches are increasingly based on the assessment of risks and not only on the reduction of hazard. In the first part of this chapter, we present different elements of an integrative risk management of avalanches with a focus on Switzerland. In the second part, we present an example of how this concept can be applied to handle the avalanche risk in the City of Juneau, Alaska.

Tall structures play an important role in the economic well-being of mountainous regions. They serve to distribute hydroelectric energy (power line transmission towers), generate tourist income (cableway pylons) and protect important infrastructure from natural hazards (trees). It is not uncommon for these structures to be destroyed by the air-blast of a powder avalanche. In this paper we investigate the shock response spectra of an asymmetric steel structure subjected to powder avalanche impact. We perform a spectral analysis of measured accelerations in the in-plane (avalanche flow) and cross-plane (normal to avalanche flow) directions. The structural eigenfrequencies in each direction are likewise calculated using a simple finite element model consisting of three-dimensional beam elements. Mode superposition is then used to reconstruct the measured accelerations, velocities and displacements in order to identify the magnitude and duration of the powder avalanche loading. From the analysis we find that the powder avalanche must contain high-frequency, short-duration blasts (f > 2 Hz) of moderate magnitude (p ≈ 1-10 kPa). High frequency blasts are also present in the cross-plane, transverse flow direction. This result, however, does not exclude the existence of longer duration blasts, which, because of the frequency mismatch, cannot excite the measurement pylon. It appears the high-frequency content of the cloud attenuates to endanger low-frequency structures (f ≈ 1 Hz) such as trees at the lateral edges of the flow, or hanging cables located well above the core. The shock response of a specific structure therefore depends on its location relative to the avalanche core, depending on how the blast-structures dissipate in time and space. Because the powder cloud contains a wide range of blast frequencies, tall structures are vulnerable to the action of the powder cloud, even if they are located well outside the avalanche core.