Wet Snow Avalanches Research Articles

Лавинопрояви у природних територіальних комплексах ландшафту Боржава

The territory landscape structure is considered. Main basic avalanche activity research tasks give an example. The landscape Borzhava climate conditions in avalanche hazard period are research. Main climatologically factors of the territory (temperature, wind direction, snowfall) that are affect on formation avalanche situation are describe. Number of snowfall. Investigation the snow cover allocate on different slope orientation. The main type of the avalanche, that are typical for research landscape are analyzing. The number of dry snow and wet snow avalanche, number of avalanche on different slope orientation are showed. Depicted landscape Borzhava avalanche activity for different time period. Relief and vegetation influence on slide down snow mass is research. The landscape structure affect on formation avalanche hazard territory is describe. The main avalanche activity natural terrain complex are illustration on landscape Borzhava orogidrographical model. The main avalanche risk terrain locate are determine. Key words: climate, avalanche, avalanche activity, natural terrain complex, Ukrainian Carpathians.

Sediment Transport and Bedrock Erosion by Wet Snow Avalanches in the Guggigraben, Matter Valley, Switzerland

Wet snow avalanches breaking through to the base of the snowpack or overriding snowfree surfaces can entrain basal material and act as important agents of sediment transport in steep alpine catchments. Here we present results from 4 years of measurements at the Guggigraben catchment (Matter Valley, Canton Valais), quantifying the volume of sediment transported by avalanches each winter season. Sediment load was estimated by measuring the debris content within representative areas and then extrapolating the cumulative volume. Results reveal a total transported sediment volume of 70 m3 in 2009, 23 m3 in 2010, 15 m3 in 2011, and 35 m3 in 2012, which correspond to catchment-wide erosion rates of approximately 0.05, 0.02, 0.01, and 0.03 mm/a, respectively. Sediment appears to be sourced predominantly from within the main channel or one its tributaries, originating first from rockfall or landslides. Avalanches thus play an important role at the Guggigraben in transporting loose sediment from temporary storage sites to the fan and main river system. Within the bedrock gully and in the avalanche source region above, signs of abrasive wear were evident on exposed bedrock surfaces. These included rounded and scoured bedrock, fresh boulder impacts, and scratch marks on the gully walls.

Analysis of the snow-atmosphere energy balance during wet-snow instabilities and implications for avalanche prediction

Abstract. Wet-snow avalanches are notoriously difficult to predict; their formation mechanism is poorly understood since in situ measurements representing the thermal and mechanical evolution are difficult to perform. Instead, air temperature is commonly used as a predictor variable for days with high wet-snow avalanche danger – often with limited success. As melt water is a major driver of wet-snow instability and snow melt depends on the energy input into the snow cover, we computed the energy balance for predicting periods with high wet-snow avalanche activity. The energy balance was partly measured and partly modelled for virtual slopes at different elevations for the aspects south and north using the 1-D snow cover model SNOWPACK. We used measured meteorological variables and computed energy balance and its components to compare wet-snow avalanche days to non-avalanche days for four consecutive winter seasons in the surroundings of Davos, Switzerland. Air temperature, the net shortwave radiation and the energy input integrated over 3 or 5 days showed best results in discriminating event from non-event days. Multivariate statistics, however, revealed that for better predicting avalanche days, information on the cold content of the snowpack is necessary. Wet-snow avalanche activity was closely related to periods when large parts of the snowpack reached an isothermal state (0 °C) and energy input exceeded a maximum value of 200 kJ m−2 in one day, or the 3-day sum of positive energy input was larger than 1.2 MJ m−2. Prediction accuracy with measured meteorological variables was as good as with computed energy balance parameters, but simulated energy balance variables accounted better for different aspects, slopes and elevations than meteorological data.

Snowpack estimations in the starting zone of large-scale snow avalanches in the Makunosawa valley, Myoko, Japan

AbstractThe Makunosawa valley, Myoko, Japan, experiences frequent avalanches and is therefore ideally suited to study how meteorological elements influence avalanche activity. Since 2000, five large-scale snow avalanches with running distances >2000 m have been observed and some characteristics of these avalanches have been obtained. However, the characteristics of the snowpack in the starting zones could not be observed because they are too difficult to approach and no snow-pit observations have been carried out. We simulated the variations in the snowpack in the starting zone using the numerical snowpack model SNOWPACK with local meteorological data. The results indicate a layer of faceted crystals with low shear strength followed by rapid loading from snowfall was the cause of three avalanches in February. Conversely, no layer of faceted crystals was shown by the model before a January avalanche and we assume the sliding surface of the avalanche to be precipitation particles. The only wet-snow avalanche is attributed to a decrease in shear strength due to infiltration of meltwater and an increase in liquid water content in the boundary of two layers of different grain sizes.

Wet-snow avalanche interaction with a deflecting dam: field observations and numerical simulations in a case study

Abstract. In avalanche-prone areas, deflecting dams are widely used to divert avalanches away from endangered objects. In recent years, their effectiveness has been questioned when several large and multiple avalanches have overrun such dams. In 2008, we were able to observe a large wet-snow avalanche, characterized by an high water content, that interacted with a deflecting dam and overflowed it at its lower end. To evaluate the dam's performance, we carried out an airborne laser scanning campaign immediately after the avalanche. This data, together with a video sequence made during the avalanche descent, provided a unique data set to study the dynamics of a wet dense snow avalanche and its flow behavior along a deflecting dam. To evaluate the effect of the complex flow field of the avalanche along the dam and to provide a basis for discussion of the residual risk, we performed numerical simulations using a two-dimensional dense snow avalanche dynamics model with entrainment. In comparison to dry dense snow avalanches, we found that wet-snow avalanches, with high water content, seem to be differently influenced by the local small-scale topography roughness. Rough terrain close to the dam deflected the flow to produce abrupt impacts with the dam. At the impact sites, instability waves were generated and increased the already large flow depths. The complex flow dynamics around the dam may produce large, local snow deposits. Furthermore, the high water content in the snow may decrease the avalanche internal friction angle, inducing wet-snow avalanches to spread further laterally than dry-snow avalanches. Based on our analysis, we made recommendations for designing deflecting dams and for residual risk analysis to take into account the effects of wet-snow avalanche flow.

Monitoring of snow avalanches using a seismic array: Location, speed estimation, and relationships to meteorological variables

Monitoring snow avalanches is necessary in order to better understand their triggering mechanisms and ultimately improve forecast performance. Seismic monitoring has been developed by several groups over the last 20 years and holds great potential to detect, locate, and characterize snow avalanches. During the 2009–2010 winter, a seismic antenna was installed in the French Alps close to the village of Saint‐Christophe‐en‐Oisans (1700 m above sea level). The array of seven sensors operated during 50 days in October and November 2009 under snow‐free conditions and during 40 days in January and February 2010 in presence of snow. It recorded different types of seismic events including snow avalanches, rockfalls, shots, and regional and local microearthquakes. Eighty avalanche signals were visually identified. Using a beam‐forming method, we were able to locate snow avalanches on slopes of various orientations in a radius of about 3 km and track their propagation. The location technique allowed for the estimation of avalanches' front speed, which ranged between 12 and 32 m s−1. The method can also distinguish dry and wet snow avalanches. Durations of avalanches can be as long as 380 s because of the length of the slopes in the area. Seismic monitoring provides a catalog of avalanches with precise times, which can be used to analyze the impact of meteorological forcings on the avalanche triggering. Snowfall is found to be the dominant forcing of avalanche activity during this period, as revealed by the strongest correlation. For the period of study, our results suggest that the impact of precipitation on the snowpack instability lasts for about 6 days.

Examining spring wet slab and glide avalanche occurrence along the Going-to-the-Sun Road corridor, Glacier National Park, Montana, USA

Abstract Wet slab and glide snow avalanches are dangerous and yet can be particularly difficult to predict. Wet slab and glide avalanches are presumably triggered by free water moving through the snowpack and the subsequent interaction with layer or ground interfaces, and typically occur in the spring during warming and subsequent melt periods. In Glacier National Park (GNP), Montana, both types of avalanches can occur in the same year and affect the spring opening operations of the Going-to-the-Sun Road (GTSR). We investigated the timing of wet slab and glide avalanche occurrence along the GTSR from 2003 to 2011 using meteorological and snowpack data from two high-elevation weather stations, one SNOTEL site, and an avalanche database to characterize 55 wet slab and 182 glide avalanches. Daily wet slab and glide avalanche occurrence were combined to represent an avalanche day and were compared to non-avalanche days (no avalanche occurrence) for 60 variables (both direct and derived measurements) using a univariate analysis. A classification tree (CART) was then trained to capture the most important variables for examining specific meteorological and snowpack variables that contribute to these types of wet snow avalanches. The CART was 10-fold cross validated using the data for 2003–2010 seasons and resulted in overall predictive accuracy of 73%. We then used the statistically optimal CART as a predictive model for the spring avalanche season of 2011, which resulted in an overall predictive accuracy of 82% for both avalanche and non-avalanche days, and a predictive accuracy of 91% for avalanche days. The results suggest that the role of air temperature and snowpack settlement appear to be the most important variables in wet slab and glide avalanche occurrence. When applied to the 2011 season, the results of the CART model are encouraging and they enhance our understanding of some of the required meteorological and snowpack conditions for wet slab and glide avalanche occurrence.

Change in snow strength caused by rain

AbstractRain-on-snow events can cause wet snow avalanches. Laboratory experiments were carried out to investigate the change in snow strength with increasing water content through rainwater percolation. Snowpack was artificially prepared consisting of a thin ice layer and fine compacted snow, and rainfall (2mmh–1) was artificially applied 22–25.5 and 49–52 hours after the snowpack was formed. Snow hardness was measured with a push–pull force gauge to indicate the snow strength before and after each rain-on-snow event. After the first rainfall, the upper half of the snowpack became wet and a rapid decrease in snow hardness was observed. After the second rainfall the rainwater penetrated the ice layer, high water content was observed above the ice layer but the hardness exceeded that estimated from an empirical relationship between hardness and water content. Micrographs of the snow particles suggest that the delay in grain coarsening observed near the wetting front induces the harder than estimated snow condition.

Meteorology, Topography and Snowpack Conditions causing Two Extreme Mid‐Winter Slush and Wet Slab Avalanche Periods in High Arctic Maritime Svalbard

ABSTRACTMid‐winter wet avalanche cycles in High Arctic Svalbard occurred during January 2010 and March 2011, allowing studies of slush and wet slab avalanche deposits. Both cycles represented extreme events in magnitude and frequency and were caused by the passage of low‐pressure atmospheric systems with positive air temperatures, high wind speeds and 100‐year record monthly rainfall. Slush avalanches were confined to river‐cut gorges, with low starting‐zone inclinations, and deposits consisting of flow lobes and levées. Wet slab avalanches were not confined topographically, started anywhere on open mountain slopes and displayed tongue‐shaped debris deposits. During both of the two wet avalanche periods analysed, snowpack conditions favoured the release of slush avalanches, as the snowpack consisted of a coarse‐grained middle section above a water‐impermeable ice layer. Such snowpack conditions are typical for central Svalbard. The resulting slush and wet slab avalanches were extreme in their size and runout distances, crossing frequently used snowmobile tracks at 20 locations and posing a threat to traffic and infrastructure. Four additional potential large‐scale slush avalanche periods were identified from analysis of the meteorological record from Longyearbyen (1912–2011). They cluster in the mid to early 1990s, with comparable meteorological conditions to the January 2010 and March 2011 wet avalanche cycles. It is concluded that the frequency and duration of low‐pressure weather systems are the dominant controls on wet snow avalanches, and that mean snow season air temperature (October–May) is of little importance. Copyright © 2011 John Wiley & Sons, Ltd.

Point observations of liquid water content in wet snow – investigating methodical, spatial and temporal aspects

Abstract. Information about the volume and the spatial and temporal distribution of liquid water in snow is important for forecasting wet snow avalanches and for predicting melt-water run-off. The distribution of liquid water in snow is commonly estimated from point measurements using a "hand" squeeze test, or a dielectric device such as a "Snow Fork" or a "Denoth meter". Here we compare estimates of water content in the Swiss Alps made using the hand test to those made with a Snow Fork and a Denoth meter. Measurements were conducted in the Swiss Alps, mostly above tree line; more than 12 000 measurements were made at 85 locations over 30 days. Results show that the hand test generally over estimates the volumetric liquid water content. Estimates using the Snow Fork are generally 1 % higher than those derived from the Denoth meter. The measurements were also used to investigate temporal and small-scale spatial patterns of wetness. Results show that typically a single point measurement does not characterize the wetness of the surrounding snow. Large diurnal changes in wetness are common in the near-surface snow, and associated changes at depth were also observed. A single vertical profile of measurements is not sufficient to determine whether these changes were a result of a spatially homogeneous wetting front or caused by infiltration through pipes. Based on our observations, we suggest that three measurements at horizontal distances greater than 50 cm are needed to adequately characterize the distribution of liquid water through a snowpack. Further, we suggest a simplified classification scheme that includes five wetness patterns that incorporate both the vertical and horizontal distribution of liquid water in a snowpack.

Wet-snow instabilities: comparison of measured and modelled liquid water content and snow stratigraphy

AbstarctWet-snow avalanches are difficult to forecast, as the change from stable to unstable snow conditions occurs rapidly in a wet snowpack, often in response to water production and movement. Snow stratigraphy plays a vital role in determining flux behaviour. Capillary barriers or melt–freeze crusts can impede and divert water horizontally over large areas and thus may act as a failure layer for wet-snow avalanches. We present a comparison of measured and modelled liquid water content, θw, and snow stratigraphy during periods of wet-snow instabilities. Special attention is given to the reproducibility of capillary barriers, ponding of water on melt–freeze crusts and the timing of first wetting and of water arrival at the bottom of the snowpack, because these factors are believed to play a major role in the formation of wet-snow avalanches. In situ measurements were performed in the vicinity of automatic weather stations or close to recent wet-snow avalanches in order to compare them with model results. The simulations are based on two different water flux models incorporated within the 1-D snow-cover model SNOWPACK. The comparison of the two model runs with observed θw and stratigraphy revealed that both water-transport models reproduced the ponding of water on melt–freeze crusts. However, in both models melt–freeze crusts were transformed to normal melt forms earlier than observed in nature, so still existing ponding was not captured by the models. Only one of the models was able to reproduce capillary barriers in agreement with observations. The time of the first wetting at the surface was well predicted, but the simulated arrival time of the wetting front at the bottom of the snowpack differed between the simulations; it was either too early or too late compared with the observation.

Effects of flow regime and sensor geometry on snow avalanche impact-pressure measurements

AbstractImpact pressures of snow avalanches have been measured at the Swiss Vallée de la Sionne experimental test site using two kinds of sensor placed at different locations in the avalanche flow. Pressures measured in a fast dry-snow avalanche and a slow wet-snow avalanche are compared and discussed. The pressures recorded using the two types of sensor in the dense flow of a dry-snow avalanche agree well, showing negligible dependence on the measurement device. On the other hand, significantly different pressures are measured in the slow dense flow of a wet-snow avalanche. This is attributed to the slow drag and bulk flow of this type of avalanche, leading to the formation and collapse of force-chain structures against the different surfaces of the sensors. At a macroscopic scale, limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor, according to a Mohr–Coulomb failure criterion. The proposed model explains (1) how impact pressure can be up to eight times higher than hydrostatic snow pressure in wet cohesive slow avalanches and (2) its dependence on sensor geometry.

Microstructural resistance of snow following first wetting

Abstract One of the main causes for the formation of wet snow avalanches is the decrease in strength following the introduction of liquid water. The influence of liquid water on initially dry snow was investigated in twelve field experiments in the Eastern Swiss Alps. For this, a variety of different snow types were artificially wetted and liquid water content (using the Snow Fork), microstructural penetration resistance (Snow Micro Penetrometer, SMP) and snow density were measured. Measurements were repeated up to eight times resulting in a total 1100 liquid water content, 355 penetration resistance, and 737 snow density measurements. Following wetting, microstructural penetration resistance decreased in layers consisting of faceted crystals or depth hoar at low liquid water content. Layers consisting of precipitation particles or small round grains, on the other hand, showed no significant trend at a similar water content. The results indicate that first wetting of faceted crystals and depth hoar layers has an impact on snow strength, and consequently contributes to a decrease in snow stability.

Numerical modeling of liquid water movement through layered snow based on new measurements of the water retention curve

Abstract Liquid water movement in snow is an important aspect of wet snow metamorphism and is vital for forecasting wet snow avalanches. However, despite its importance, liquid water movement is over-simplified in the current version of the numerical model SNOWPACK, yielding an inadequate simulation of the water content profile. In general, estimations of liquid water flux in porous materials are based on hydraulic conductivity and suction. This paper presents a water transport model based on the van Genuchten formulation, with parameters obtained from gravity drainage column experiments. Simulations that try to describe the capillary barrier between layers of different grain sizes lead to long computation times because of the small time increments required. In this study, a new algorithm was developed and incorporated in the water transport model in order to simulate the capillary barrier using relatively long time increments (60 s). The model was then used to simulate water movement between snow layers of different grain size. The results confirm that a water-saturated layer forms at the boundary between fine and underlying very coarse snow, which is consistent with observations. The new water transport model was incorporated in SNOWPACK, which then produced an unstable water-saturated layer at the boundary between different grain sizes. Simulation results from the SNOWPACK model were compared with measurements. The main achievement in this study is that the natural capillary barrier was documented and then reproduced using the modified SNOWPACK model. Nevertheless, much work has still to be done to get fully satisfactory results for the reproducibility of grain size and liquid water content.

High-speed video recording of basal shear layers in snow chute flows

Abstract Run-out distances and flow velocities of snow avalanches are mainly determined by frictional processes originating from the interaction with the ground. At the SLF snow chute at the Weissfluhjoch near Davos, a setup was developed which allowed us to record high-speed movies of the basal shear layer of small-scale avalanches with a frame rate of 1000 frames per second. Shear processes could be observed in high-resolution slow motion. Downstream velocity profiles were extracted by a pattern matching algorithm. The comparison of computed profiles with velocity profiles obtained from optical sensors showed good agreement. However, the temporal and spatial resolutions are much higher for the high-speed video data. Because the optical velocity sensors are one-dimensional, we found that they overestimate the velocities when a flow-normal velocity component exists as well. All measured velocity profiles exhibited very high shear rates near the ground. The maximum shear rates were up to 600/s for dry snow and 200/s for wet snow avalanches. The observations of the video images suggested a turbulent motion of the snow in the basal shear layer.

Velocity measurements of wet snow avalanche on the Dhundi snow chute

AbstractWet snow avalanches in India are common during the mid- and late winter in the Pir Panjal Range (2000–3000ma.s.l.) and during the late winter in the Great Himalayan Range (3000 ma.s.l. and above). Although it is well known that the presence of liquid water in snow makes the flow behaviour of wet snow avalanches different from that of dry snow avalanches, there exist few actual flow measurements with wet snow. The aim of this investigation is to understand the dynamics of wet snow avalanches by conducting medium-scale experiments (volumes of 3, 6 and 11 m3) on the Dhundi snow chute in Himachal Pradesh, India. We measured flow velocities using video data, as well as optical velocity sensors installed on the side walls and running surface. Measurement results relating to the slip velocity of the front and tail of the moving snow mass, as well as the average slip velocity, are presented. In addition, we use the results of the vertical velocity profile measurements to calculate the effective viscosity of snow at two locations within the flow. We identified a shear thinning type of behaviour, suggesting that a single avalanche rheology cannot describe wet snow avalanche behaviour.

Slow drag in wet-snow avalanche flow

AbstractWe 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.

Measured shear rates in large dry and wet snow avalanches

AbstractWe present estimates of internal shear rates of real-scale avalanches that are based on velocity measurements. Optical velocity sensors installed on the instrument pylon at the Swiss Vallée de la Sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches. Possible sources of error in the correlation analysis of the time-lagged reflectivity signals measured by optical sensors are identified for real-size avalanches. These include spurious velocities due to noise and elongated peaks. An appropriate choice of the correlation length is essential for obtaining good velocity estimates. Placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data. Coherent signals are found only in the dense flowing cores. We observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche. At the front, large shear rates are found throughout the depth; at the tail, plug flows overriding highly sheared layers near the bottom of the flow are observed. The measured velocities change strongly with height above the ground and fluctuations around the measured mean velocity can be identified. We find that the dense flows are laminar, undergoing a transition from supercritical to subcritical flow behaviour from the head to the tail. Furthermore, we provide real-scale experimental evidence that the mean shear rate and the magnitude of velocity fluctuations increase with the mean discharge.

Characteristics of wet-snow avalanche activity: 20 years of observations from a high alpine valley (Dischma, Switzerland)

The occurrence of wet-snow avalanches is, in general, poorly understood. For 20 years (winters of 1975–1976 to 1994–1995), the avalanche activity has been observed in the Dischma valley near Davos (Eastern Swiss Alps). The study area comprises a large starting zone of north-easterly aspect (2,300 m a.s.l.) with several avalanche paths. We have analyzed the occurrence data in combination with meteorological and snowpack data collected at an elevation of 2,090 m a.s.l. During the 20-year observation period, almost 800 wet-snow avalanches were observed, about 4.5 times more loose snow avalanches than slab avalanches. Considering both types of avalanches jointly, snow depth, precipitation and air temperature showed the highest correlation with avalanche activity. Most loose snow avalanches occurred when air temperature was high and/or after a precipitation period. Slab avalanches occurrence was primarily related to warm air temperatures and snowpack properties such as the isothermal state and the existence of capillary barriers. Radiation did not show up as a significant variable. The results suggest that in a transitional snow climate wet-snow avalanches are, as dry snow avalanches, often related to precipitation events, and that wet slab instability strongly depends on snowpack properties in relation to warming of the snowpack and melt water production.

Inferences on flow mechanisms from snow avalanche deposits

AbstractThe deposit structure of 20 very small to large avalanches that occurred in the Davos area, eastern Swiss Alps, during winters 2004/05 and 2005/06 was investigated. Snow-cover entrainment was significant in the majority of events and likely to have occurred in all cases. Evidence was found both for plough-like frontal entrainment (especially in wet-snow avalanches) and more gradual erosion along the base of dry-snow avalanches. Several of the dry-snow avalanches, both small and large, showed a fairly abrupt decrease in deposit thickness in the distal direction, often accompanied by changes in the granulometry and the deposit density. Combined with other observations (snow plastered onto tree trunks, deposit-less flow marks in bends, etc.) and measurements at instrumented test sites, this phenomenon is best explained as being due to a fluidized, low-density flow regime that formed mostly in the head of some dry-snow avalanches. The mass fraction of the fluidized deposits ranged from less than 1% to ∼25% of the total deposit mass. Fluidization appears to depend rather sensitively on snow conditions and path properties.