Mark Schär

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.

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.

Observations of the deposits, flow marks, and damages of three mixed-snow avalanches of widely different size were analyzed with regard to flow regimes, velocities, pressures, densities, flow depths, erosion modes, and mass balance. Three deposit types of different density and granulometry could be clearly discerned in these avalanches. They are attributed to dense, fluidized, and suspension flow regimes, respectively. Combining observations, we estimated the density in the fluidized layer as 35–100 kg m⁻³, in good agreement with inferences from pressure measurements. Upper bounds for the suspension layer density, arising from the run-up height, velocity, and damage pattern, are about 5 kg m⁻³ at the valley bottom. An approximate momentum balance of the dense layer suggests that the snow cover was eroded to considerable depth, but only partly entrained into the flow proper. The suspension layer had largely lost its erosive power at the point where it separated from the denser parts of the avalanche. Our estimates shed doubt on collisions between snow particles and aerodynamic forces at the head of the avalanche as sole mechanisms for creating and upholding the fluidized layer. We conjecture that the drag from air escaping from the snow cover as it is being compressed by the overriding avalanche could supply the missing lift force.

The assessment of avalanche loads depends partly on well-founded facts such as observed avalanches, but the avalanche expert generally also has to make many assumptions that are difficult to base on objective evidence. This is particularly true for avalanche tracks with complex release zones and for the assessment of avalanches with a long return period, where little empirical data is available. However, an engineer requires precise load values for example for the design of a snow shed. We present a study where we analysed a well documented avalanche track in Davos that has a snow shed at the bottom. To estimate the uncertainty of the avalanche loads on the snow shed, we varied the decisive input parameters to perform numerical avalanche simulations systematically and asked three experts to estimate the probability of each parameter value. Several thousand avalanche-dynamic simulations with the software RAMMS were performed to cover all possible parameter combinations and the avalanche load on the snow shed was computed for each run. Using the probabilities that were estimated for the input parameters, we calculated the probability distribution for the avalanche load and the probability for the load exceeding the bearing capacity of the structural system of the snow shed. The main conclusions are: (a) Although the involved experts have all a long experience and a similar professional background, their estimations varied by a factor of 2.8 for the maximum load related to a 300-year return period. (b) For the snow shed, we got a failure probability in the order of 1E-3 per winter. This number is compatible with the low number of documented snow shed collapses, but below the requirements of European building codes which require an annual failure probability of 1E-5 or lower, depending on the consequence class.

The assessment of the impact pressure exerted by avalanches on structures is an important task in avalanche engineering. Impact pressures of more than 50 avalanches were measured at the Vallée de la Sionne experimental site, in Switzerland, in the last 20 years. Measurements performed on a 20m high and 0.6m wide pylon show that the highest long-lasting bending moment is exerted by warm/wet avalanches, characterized by velocity of around 10ms⁻¹, which however can develop very high flow depths, up to 7m in one of our data sets. In spite of the relatively low absolute value of pressures these avalanches exert a constant load for tens of seconds, over large depths. Conversely, the intermittency region, coupled with a dense basal layer, which characterized the front of large powder avalanches exerts the highest short-lasting bending moment. Mesoscale coherent structures which are present in this region can move with velocities up to 60ms⁻¹ and produce peak loads of 800–1000 kPa, which however, only last for a fraction of a second. The dense basal layer of large powder snow avalanches can also exerts a very large long-lasting pressure (up to 1000 kPa), which, however is concentrated in a thin layer close to the base of the flow, and thus contributes marginally to the overall bending moment. Finally, heavy objects transported into the flow, such as rocks, can exert large local pressure peaks that can be larger than 1000 kPa. We compare these measurements to standard calculation procedures and discuss their relevance in term of structure design.

Mountain cableways are regularly endangered by avalanches and snow pressure due to the slow movement of the snowpack on slopes. It is therefore important that the risks from avalanches and snow pressure are already taken into account adequately during the planning of a cableway. In Switzerland, the Federal Office of Transport (FOT) requires the elaboration of an avalanche expert report within the framework of the approval procedure for cableways; its contents are described in this guideline. In particular, the scenarios are defined that have to be considered for the analysis of avalanches and snow pressure hazard, in order to design the cableway installations and to plan the mitigation measures. On the basis of the guideline, the avalanche expert determines the nominal values of the actions and the necessary structural and temporary mitigation measures. This information is the base for the planning engineer to design the cableway in detail. To do this, the most important formulae for the calculation of avalanches and snow pressure, as well as for the determination of the maximum snow depth are specified. The relevant combinations of actions for the design are then defined. This guideline contains all the relevant information which has to be considered for the plan approval, the operating approval and the evaluation of cableways with regard to the risk of avalanches and snow pressure.

Powder snow avalanches are a common hazard in high alpine regions. Steep tracks and cold snow temperatures facilitate the formation of the powder suspension cloud developing from the dense, fast-moving avalanche core. Considerable damage to buildings and power lines is possible even when the dense core of the avalanche stops before reaching the infrastructure. An efficient simulation tool that calculates powder pressures in three-dimensional terrain would help engineers plan mitigation measures. We present a novel two-layer powder avalanche model that couples the cloud to the flowing core, allowing the simulation of mixed flowing/powder avalanches. The model predicts endangered areas outside the reach of the dense core. We apply this model to a well-documented case study in southern Switzerland and discuss the potential for future engineering applications.

Mountain cableways are regularly endangered by avalanches and snow pressure due to the slow movement of the snowpack on slopes. It is therefore important that the risks from avalanches and snow pressure are already taken into account adequately during the planning of a cableway. In Switzerland, the Federal Office of Transport (FOT) requires the elaboration of an avalanche expert report within the framework of the approval procedure for cableways; its contents are described in this guideline. In particular, the scenarios are defined that have to be considered for the analysis of avalanches and snow pressure hazard, in order to design the cableway installations and to plan the mitigation measures. On the basis of the guideline, the avalanche expert determines the nominal values of the actions and the necessary structural and temporary mitigation measures. This information is the base for the planning engineer to design the cableway in detail. To do this, the most important formulae for the calculation of avalanches and snow pressure, as well as for the determination of the maximum snow depth are specified. The relevant combinations of actions for the design are then defined. This guideline contains all the relevant information which has to be considered for the plan approval, the operating approval and the evaluation of cableways with regard to the risk of avalanches and snow pressure.

Wet snow avalanches interact with infrastructures around the world but their significance on the structure design is frequently neglected due to the low velocity, which characterize the flow and thus the expected low impact pressures. Recent pressure measurements performed at the Swiss Vallée de la Sionne full-scale test site show that wet avalanche pressures, measured on a 20 m high tower, are considerably higher than those predicted by conventional avalanche engineering guidelines, thus potentially becoming relevant for the design of tower-like structures. In order to understand under which circumstances wet avalanches can become more relevant than their dry counter part and in order to establish simple rules to evaluate the pressure the avalanche exerts on a tower-like object, we analyse pressure and velocity data collected at the Vallée the la Sionne on obstacles of different shape and dimension.

We report impact pressures exerted by three wet-snow avalanches on a pylon equipped with piezoelectric load cells. These pressures were considerably higher than those predicted by conventional avalanche engineering guidelines. The time-averaged pressure linearly increased with the immersion depth of the load cells and it was about eight times larger than the hydrostatic snow pressure. At the same immersion depth, the pressures were very similar for all three avalanches and no dependency between avalanche velocity and pressure was apparent. The pressure time series were characterized by large fluctuations. For all immersion depths and for all avalanches, the standard deviations of the fluctuations were, on average, about 20% of the mean value. We compare our observations with results of slow-drag granular experiments, where similar behavior has been explained by formation and destruction of chain structures due to jamming of granular material around the pylon, and we propose the same mechanism as a possible microscale interpretation of our observations.

The snow surface height was precisely measured, with a laser scanner, before and after the passage of two dry‐mixed avalanches in Vallée de la Sionne during the winter of 2005–2006. The measurements were used to calculate the depth of the deposited snow along each entire avalanche path with a height resolution of 100 mm and a horizontal resolution of 500 mm. These data are much more accurate than any previous measurements from large avalanches and show that the deposit depth is strongly negatively correlated with the slope angle. That is, on steep slopes the deposit is shallow, and on gentle slopes the deposit is deep. The time evolution of the snow depth, showing the initial erosion and final deposition as the avalanche passed, was also observed at one position using a frequency‐modulated continuous wave radar. Measurements at a nearby position gave flow speed profiles and showed that the avalanche tail consists of a steady state subcritical flow that lasts for about 100 s. Eventually, the tail slowly decelerates as the depth slightly decreases, and then it comes to rest. We show that the dependency between the slope angle and the deposition depth can be explained by both a cohesive friction model and the Pouliquen h_stop model.

Ropeways in ski areas are often exposed to avalanches. The new ropeway guideline fixes principles and procedures how to account for avalanche and snow pressure hazards. The two main subjects of the guideline define firstly standards to determine the avalanche and snow pressure forces acting on the structures, and assure secondly safety for the public. To ensure structural integrity, the guideline defines ‘normal’ and ‘accidental’ design avalanches, as well as static snow pressure, which have to be considered in the structural analysis. Temporary safety measures are a commonly used method to avoid avalanche hazards on ski-runs. However, avalanche incidents on ski runs show that there is always a residual danger, which cannot be neglected. Therefore, for exposed areas where people gather (e.g. terminal stations), the guideline requires an additional safety margin: it must be ensured that spontaneous avalanches with a return period of 10 years will not reach these areas. Ten years application of the guideline in consulting work for new and existing ropeways show that the defined standards can be regarded as appropriate for the vast majority of cases. The winter 2008/09 provided some avalanche events that allowed for a first full-scale test of ropeways designed according to the guidelines. We will show typical examples of hazard assessments and proposed safety measures for ropeways.

A fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures. This task is complicated because snow avalanches flow in a variety of regimes, primarily depending on snow temperature and moisture content. In this paper we address this problem by analyzing measured impact pressures, flow velocities, and flow depths of five Vallée de la Sionne avalanches. The measurements are made on a 20 m high tubular pylon instrumented with high-frequency pressure transducers and optoelectronic velocity sensors. In the observed avalanches, we find both subcritical and supercritical flow regimes. Typical Froude numbers were smaller than 6. The subcritical regime (Fr < 1) is characterized by a flow plug riding above a highly sheared basal layer. The measured pressures are large and velocity-independent in contradiction to calculation procedures. Pressure fluctuations increase with flow depth, indicating a kinematic stick-slip phenomena which is largest at the basal layer. Supercritical flow regimes (1 < Fr < 6) are characterized by a sheared flow all over the avalanche depth. In this regime the impact pressure is velocity-dependent. We derive relationships governing impact pressure as a function of the Froude number, and therefore flow regime, encompassing all the observed avalanches. 

Impact pressures of eight snow avalanches measured at the Swiss avalanche test site Vallée de la Sionne are reported. Avalanche typologies varied between dense and powder. Measurements were performed on obstacles of different shape and dimension. High-frequency pressure sensors were mounted on a 20 m high tubular pylon and on a 5 m high steel wedge. To interpret the influence of sensor dimension on impact pressure measurements, the total pressure exerted on the steel wedge was recorded using two bi-axial sensors and compared to the pressure recorded by the high-frequency sensors. On a small concrete wall, a 1 m² pressure plate mounted with 4 load-gauge bolts measured normal and tangential forces. Along the tubular pylon (between 1–6 m above ground) optoelectronic sensors recorded the avalanche flow velocity. Flow depths were measured by mechanical sensors. Analysis of high resolution impact forces in combination with velocity measurements allowed us to reconstruct the flow structure. We combined impact pressure with observation on avalanche structure to obtain load distribution and size effects for different avalanche typologies. Measured pressures are compared to the existing Swiss calculation procedure. It is shown that actual calculation formulas are not able to properly reproduce the measured pressure values and the load distribution.

In winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1-19 in above ground were carried out in the upper run-out zone of the avalanche test site at Vallee de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and >3 in. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow), particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peal, detection and correction for grazing impacts, pressure measurements will become a premier toot for testing granular flow models.