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Perry Bartelt - One of the best experts on this subject based on the ideXlab platform.

  • Point release wet snow Avalanches
    Natural Hazards and Earth System Sciences Discussions, 2015
    Co-Authors: C. Vera Valero, Yves Buhler, Perry Bartelt
    Abstract:

    Abstract. Wet snow Avalanches can initiate from large fracture slabs or small point releases. Point release wet snow Avalanches can reach dangerous proportions when they (1) initiate on steep and long avalanche paths and (2) entrain warm moist snow. In this paper we investigate the dynamics of point release wet snow Avalanches by applying a numerical model to simulate documented case studies on high altitude slopes in the Chilean Andes (33° S). The model predicts avalanche flow temperature as well as meltwater production, given the thermal initial conditions of the release mass and snowcover entrainment. As the release mass is small, avalanche velocity and runout are primarily controlled by snowcover temperature and moisture content. We demonstrate how the interaction between terrain and entrainment processes influence the production of meltwater and therefore lubrication processes leading to longer runout. This information is useful to avalanche forecasters. An understanding of wet snow avalanche dynamics is important to study how climate change scenarios will influence land usage in mountain regions in the near future.

  • Chapter 12 – Snow Avalanches
    Snow and Ice-Related Hazards Risks and Disasters, 2015
    Co-Authors: Jurg Schweizer, Perry Bartelt, Alec Van Herwijnen
    Abstract:

    Snow Avalanches are a major natural hazard in most snow-covered mountain areas of the world. They are rapid, gravity-driven mass movements and are considered a meteorologically induced hazard. Snow Avalanches are one of the few hazards that can be forecast, and in situ measurements of instability are feasible. Advanced hazard-mitigation measures exist, such as land-use planning based on modeling avalanche dynamics. The most dangerous snow Avalanches start as a dry-snow, slab avalanche that is best described with a fracture mechanical approach. How fast and how far an avalanche flows is the fundamental question in avalanche engineering. Models of different levels of physical complexity enable the prediction of avalanche motion. Although the avalanche danger (probability of occurrence) for a given region can be forecast—in most countries with significant avalanche hazard, avalanche warnings are issued on a regular basis—the prediction of a single event in time and space is not (yet) possible.

  • MODELLING SMALL AND FREQUENT Avalanches
    2014
    Co-Authors: Lisa Dreier, Thomas Feistl, Marc Christen, Yves Buhler, Walter Steinkogler, Perry Bartelt
    Abstract:

    Numerical simulation tools are commonly used to model extreme events, that is Avalanches with return periods of 30 years or more. Recently, a new demand has arisen in avalanche engineering practice: the modelling of “small”, frequent Avalanches. These Avalanches with release volumes between 1,000 10,000 m 3 often threaten traffic infrastructure and ski runs. In this paper we apply a new physical avalanche model to simulate “small”, frequent Avalanches using high spatial resolution DEM data. The case studies consist of Avalanches documented in the Swiss accident database. For these Avalanches, we have reliable data concerning release location, fracture height, run-out distance and snow temperatures at time of release. Photographs provide information regarding snow cover entrainment. A set of model parameters was determined which depends on the avalanche flow type and hence on snow temperature. We explicitly avoided changing parameters according to avalanche size. The Avalanches were simulated according to the temperature classification scheme we established. We analyzed the impact of the release location, release height and entrainment on the avalanche run-out. Our results highlight the importance of release zone definition, release height, snow temperature and the difference between summer and winter terrain models for small-scale Avalanches. We plan to apply the findings of this study to produce a small-scale avalanche simulation tool intended to support persons in charge of ski resorts and traffic infrastructure.

  • Stopping Behavior of Snow Avalanches in Forests
    2012
    Co-Authors: Thomas Feistl, Marc Christen, Peter Bebi, Yves Buhler, Michaela Teich, Perry Bartelt
    Abstract:

    A longstanding problem in avalanche science is to understand how forests stop small and medium sized Avalanches. Avalanche dynamics models have traditionally been employed to calculate extreme avalanche runout and have assigned a minor role to forests in dissipating flow energy. In this paper we quantify the important effect of forests in stopping small avalanche events, crucial for road and ski-run safety. We performed field studies of several Avalanches where trees affected the runout. We gathered information concerning the starting location, deposition heights, runout distance and forest structure. These studies were made during the 2011/12 winter where many gliding snow Avalanches released in forested areas in Switzerland and Germany. Using the field observations as a guide, we hypothesized that mass detrainment due to tree-avalanche interaction led to a significant deceleration of the Avalanches. This effect is important for physical based avalanche dynamics models which reveal that avalanche mobility is strongly linked to mass entrainment/detrainment. We tested this hypothesis with a numerical experiment and simulated the documented avalanche events using a velocity dependent detrainment model to reconstruct the braking effect of forests. For the numerical investigations we used high spatial resolution digital terrain models. The results highlight how forests influence mass and energy fluxes at the front and sides of Avalanches. Of particular importance is the distribution of velocity across the flow width of the avalanche, as flow mass can be easily stopped at the flow boundaries.

  • POTENTIAL IMPACTS OF CLIMATE CHANGE ON SNOW Avalanches STARTING IN FORESTED TERRAIN
    2012
    Co-Authors: Michaela Teich, Natalie Zurbriggen, Melanie Ulrich, Perry Bartelt, Adrienne Grêt-regamey, Christoph Marty, Peter Bebi
    Abstract:

    Frequency and magnitude of Avalanches starting in forested terrain (forest Avalanches) are likely to be affected by climate change. We addressed two important developments which will influence the forest avalanche regime: 1) trends in the occurrence of favorable snow and weather situations which increase the probability of forest avalanche releases, and 2) changes in the extent, composition and structure of mountain forests. We applied a logistic trend analysis over 41 years to investigate past changes in the occurrence of snow and weather conditions which are associated with forest avalanche releases in the Swiss Alps. We found negative trends for two typical situations, 'new snow forest Avalanches' and 'other forest Avalanches'. In combination with the currently observed increase in forest cover extent and density, it is thus likely that avalanche releases in forests will become less frequent. For Avalanches started in forested areas, we found that higher densities of small-diameter trees (

Karl W Birkeland - One of the best experts on this subject based on the ideXlab platform.

  • meteorological variables associated with deep slab Avalanches on persistent weak layers
    International Snow Science Workshop 2014 Proceedings Banff Canada, 2014
    Co-Authors: Alex Marienthal, Karl W Birkeland, Jordy Hendrikx, Kathryn M. Irvine
    Abstract:

    Deep slab Avalanches are a particularly challenging avalanche forecasting problem. These Avalanches are typically difficult to trigger, yet when they release they tend to propagate far and can result in large and destructive Avalanches. For this work we define deep slab Avalanches as those that fail on persistent weak layers deeper than 0.9m (3 feet), and that occur after February 1. We utilized a 44year record of avalanche control and meteorological data from Bridger Bowl Ski Area in southwest Montana to test the usefulness of meteorological variables for predicting seasons with deep slab Avalanches. While previous studies often exclusively use data from the days preceding deep slab cycles, we include meteorological metrics over the early months of the season when persistent weak layers form. We used classification trees for our analyses. Our results showed that seasons with Avalanches on deep persistent weak layers typically had drier early months, and often had maximum snow depth greater than 88cm in November, which provided ideal conditions for persistent weak layer development. This paper provides insights for ski patrollers, guides, and avalanche forecasters who seek to understand the seasonal conditions that are conducive to deep slab Avalanches on persistent weak layers later in the season.

  • storm snow Avalanches characteristics and forecasting
    Proceedings 2012 International Snow Science Workshop Anchorage Alaska, 2012
    Co-Authors: Edward H. Bair, Ron Simenhois, Karl W Birkeland, Jeff Dozier
    Abstract:

    At ski areas, a majority of Avalanches fail in storm snow. We investigate these Avalanches using stability tests and avalanche observations from California and Alaska. Collapse amplitudes during fracture, measured using particle tracking, were 1 mm for a failure layer of precipitation particles and 7 mm for a layer of unrimed sectored plates. Stability test results showed little dependence on slope angle, suggesting that both precipitation particles and older faceted crystals (persistent weak layers) fail as described by the anticrack model, with collapse providing energy. Using observations from avalanche control work at Mammoth Mountain, CA USA, a large coastal ski area where 9/10 Avalanches fail in storm snow, we examined Extended Column Test (ECT) results and their relation to avalanche activity. ECT propagation was a powerful predictor; days with ECTs that propagated had significantly more and larger Avalanches. Since other studies have shown that the ECT is an effective predictor of Avalanches involving persistent weak layers, we suggest that the ECT is an effective test to predict both types of Avalanches, those that fail in storm snow and those that fail on persistent weak layers.

  • meteorological and environmental observations from three glide avalanche cycles and the resulting hazard management technique
    2010 International Snow Science Workshop, 2010
    Co-Authors: Ron Simenhois, Karl W Birkeland
    Abstract:

    Glide Avalanches are a significant hazard that threatens people and property in many snowy climates. They are hard to control, poorly understood, and extremely challenging to forecast. This paper presents meteorological and environmental data associated with three glide avalanche cycles. It also discusses hazard reduction techniques from an operational perspective and provides possible explanations why previous attempts to artificially trigger glide Avalanches rarely succeed. During Southeast Alaska’s winter of 09/10, we witnessed three glide avalanche cycles with over 35 total Avalanches. During those cycles we collected data on snowpack, precipitation, temperature, relative humidity, sky coverage and streamflow, as well as slope aspect, elevation, steepness, shape and ground cover. We also recorded visual snow surface observations leading to the transition of some of the glide cracks to Avalanches. Although glide avalanche activity is clearly somehow related to atmospheric events, we found no direct correlation between meteorological data and avalanche occurrences. However, we did find a rough correlation between snowpack, terrain and avalanche time distribution in two out of the three cycles. Our lack of reliable forecasting and control tools for glide Avalanches implies that limiting the potential destructive size of glide Avalanches throughout the entire winter may be the most effective approach to managing the hazard for some operations.

  • avalanche survival strategies for different parts of a flowing avalanche
    Proceedings Whistler 2008 International Snow Science Workshop September 21-27 2008, 2008
    Co-Authors: Karl W Birkeland, Theo Meiners, Perry Bartelt
    Abstract:

    Swimming in Avalanches has recently been questioned, with detractors stating that “swimming leads to dying”. Since no direct scientific evidence exists to either refute or support the idea of swimming, we combine the practical experience of avalanche survivors with our emerging knowledge of avalanche dynamics to arrive at possible survival strategies for different parts of flowing Avalanches. Practical experience and avalanche dynamics theory are largely consistent and suggest the following strategies: 1) Once an avalanche is released, every effort must be made to get off the moving slab, 2) After being caught, the victim must do everything possible to try to get toward the back, or tail, of the avalanche since this is where Avalanches run out of mass and where a victim is more likely to be left behind by the slide, 3) Experience shows that in some Avalanches a backstroking and log rolling motion may help the victim stay near the surface and move toward the flanks of the avalanche, and 4) If at all possible, the head of the avalanche should be avoided since the turbulent flow and large forces in this area increase the odds of injury and deep burial. Though it cannot be definitively proven, experience and avalanche dynamics theory suggest that swimming – or as some call it, “struggling” – is part of a viable strategy for surviving an avalanche once you are caught.

  • avalanche frequency and magnitude using power law exponents to investigate snow avalanche size proportions through time and space
    Proceedings Whistler 2008 International Snow Science Workshop September 21-27 2008, 2008
    Co-Authors: Adam Naisbitt, Richard Forster, Karl W Birkeland, William L Harrison
    Abstract:

    Power-laws provide a means for investigating snow avalanche frequency-magnitude relationships and their contributing factors. This research uses power laws to explore variations in avalanche size proportions through space and time, as well as investigating factors which may contribute to these variations. Data utilized for this work includes the Westwide Avalanche Network data from the western United States for regional analyses, with path-specific analyses focused on data from Utah's Little Cottonwood Canyon. Results show power-law exponents vary through space both at the regional level and between individual avalanche paths. Avalanche size proportions, with respect to space, are the product of terrain based variables at both the mountain range and the path levels, with alpha angles significantly correlated to the proportion of small to large Avalanches. This research also indicates that variation in exponents through time is indicative of changes in seasonal weather and snowpack characteristics, with mean snow height also significantly correlated to the proportion of small to large Avalanches. Knowledge of power-law exponents for particular avalanche paths, and their relationship to seasonal snowpack depth, may be helpful for managing Avalanches along highway corridors, in ski areas, or in backcountry forecasting operations.

Jurg Schweizer - One of the best experts on this subject based on the ideXlab platform.

  • On the relation between avalanche occurrence and avalanche danger level
    2019
    Co-Authors: Jurg Schweizer, Christoph Mitterer, Frank Techel, Andreas Stoffel, Benjamin Reuter
    Abstract:

    Abstract. In many countries with seasonally snow-covered mountain ranges warnings are issued to alert the public about imminent avalanche danger, mostly employing a 5-level danger scale. However, as avalanche danger cannot be measured, the charac-terization of avalanche danger remains qualitative. The probability of avalanche occurrence in combination with the ex-pected avalanche type and size decide on the degree of danger in a given forecast region (≳ 100 km2). To describe ava-lanche occurrence probability the snowpack stability and its spatial distribution need to be assessed. To quantify the rela-tion between avalanche occurrence and avalanche danger level we analyzed a large data set of visually observed ava-lanches from the region of Davos (Eastern Swiss Alps), all with mapped outlines, and compared the avalanche activity to the forecast danger level on the day of occurrence. The number of Avalanches per day strongly increased with increasing danger level confirming that not only the release probability but also the frequency of locations with a weakness in the snowpack where Avalanches may initiate from, increases within a region. Avalanche size did in general not increase with increasing avalanche danger level, suggesting that avalanche size may be of secondary importance compared to snowpack stability and its distribution when assessing the danger level. Moreover, the frequency of wet-snow Avalanches was found to be higher than the frequency of dry-snow Avalanches on a given day; also, wet-snow Avalanches tended to be larger. This finding may indicate that the danger scale is not used consistently with regard to avalanche type. Although, observed ava-lanche occurrence and avalanche danger level are subject to uncertainties, our findings on the characteristics of avalanche activity may allow revisiting the definitions of the European avalanche danger scale. The description of the danger levels can be improved, in particular by quantifying some of the many proportional quantifiers. For instance, ‘many Avalanches’, expected at danger level 4–High, means on the order of 10 Avalanches per 100 km2. Whereas our data set is one of the most comprehensive, visually observed avalanche records are known to be inherently incomplete so that our results often refer to a lower limit and should be confirmed using other similarly comprehensive data sets.

  • Forecasting snow Avalanches using avalanche activity data obtained through seismic monitoring
    Cold Regions Science and Technology, 2016
    Co-Authors: A. Van Herwijnen, Matthias Heck, Jurg Schweizer
    Abstract:

    Abstract Accurate avalanche occurrence data are of crucial importance for avalanche forecasting, since recent avalanching provides direct evidence on snowpack instability. We therefore explore how avalanche activity data obtained through seismic monitoring can be used for avalanche forecasting. By visually inspecting data from a seismic sensor deployed in an avalanche starting zone, we obtained three avalanche catalogues for two entire winters and one period of 10 days with intense wet-snow avalanche activity. Avalanche activity was clustered in time for all catalogues, and diurnal periodicity was clearly present during spring. In winter, when dry-snow Avalanches predominantly release, rather weak long-term correlations on the order of several days were found between past and future avalanche activity. We investigated the performance of a simple model to predict future Avalanches based on past avalanche activity. Model performance was better in spring than in winter, especially for very short time scales of up to 3h , and for time scales around 24 h. Furthermore, the performance of our very simple model was comparable to the performance of more sophisticated models to forecast wet-snow avalanche release based on meteorological input variables. While it is clear that for operational avalanche forecasting automatic avalanche detection still has to be developed, overall this work shows that avalanche activity data obtained through seismic monitoring would yield very valuable data for wet-snow avalanche forecasting.

  • Chapter 12 – Snow Avalanches
    Snow and Ice-Related Hazards Risks and Disasters, 2015
    Co-Authors: Jurg Schweizer, Perry Bartelt, Alec Van Herwijnen
    Abstract:

    Snow Avalanches are a major natural hazard in most snow-covered mountain areas of the world. They are rapid, gravity-driven mass movements and are considered a meteorologically induced hazard. Snow Avalanches are one of the few hazards that can be forecast, and in situ measurements of instability are feasible. Advanced hazard-mitigation measures exist, such as land-use planning based on modeling avalanche dynamics. The most dangerous snow Avalanches start as a dry-snow, slab avalanche that is best described with a fracture mechanical approach. How fast and how far an avalanche flows is the fundamental question in avalanche engineering. Models of different levels of physical complexity enable the prediction of avalanche motion. Although the avalanche danger (probability of occurrence) for a given region can be forecast—in most countries with significant avalanche hazard, avalanche warnings are issued on a regular basis—the prediction of a single event in time and space is not (yet) possible.

  • Detecting Avalanches Using Seismic Monitoring Systems
    2014
    Co-Authors: Alec Van Herwijnen, Matthias Heck, Jurg Schweizer
    Abstract:

    Meteorological, snowpack and avalanche activity data provide the essential building blocks for avalanche forecasting. While in recent decades the availability and coverage of meteorological data has dramatically improved, the same has not happened for avalanche activity data. The reason is that data on avalanche activity are generally obtained through visual observations, which are imprecise and impossible when visibility is limited. This leads to large uncertainties in the number and exact timing of Avalanches, resulting in rather poor correlations between avalanche activity, meteorological parameters and estimated avalanche danger. To improve avalanche forecasting, remote detection of Avalanches is therefore required to obtain accurate and near real-time avalanche activity data. Seismic monitoring systems are very well suited for this task and typically rely on one or several sensors in an avalanche track or at valley bottom to detect Avalanches. We employ a different approach, consisting of continuously recording seismic signals in an alpine start zone. Avalanches can then visually be identified in the seismic data to obtain an avalanche database. Based on measurements from our field sites above Davos (Switzerland), we show how seismic monitoring can provide high resolution avalanche activity data, and how these data can provide new insights into avalanche formation processes. While for large-scale operational avalanche forecasting automatic avalanche detection still has to be developed, we will further show that seismic monitoring can already effectively be used to remotely detect artificially triggered Avalanches below fixed avalanche control installations.

  • Artificial avalanche release and the probability of triggering secondary Avalanches
    2013
    Co-Authors: Lukas Stoffel, Stefan Margreth, Jurg Schweizer
    Abstract:

    Today, Avalanches are often artificially triggered to protect ski areas and to some extent, transportation routes and residential areas. During avalanche control, unwanted secondary ava- lanches may also be triggered in adjacent avalanche paths. Damage potential and secondary ava- lanches are two important factors to consider when evaluating the practicality of artificial release in an avalanche path or elaborating a safety concept. A too high risk of secondary Avalanches can be a no- go criterion for the application of artificial release in a specific area. In 2012 we elaborated a guideline with evaluation criteria how to estimate the likelihood of triggering a secondary avalanche. The proba- bility of triggering secondary Avalanches can be classified in three classes which are mainly based on topographical features, snow distribution and climatic conditions (main wind direction). Secondary ava- lanches are most often triggered as a result of crack propagation in the snowpack. Other trigger pos- sibilities are the effect of the air blast wave and of ground vibrations.

Jacob A. Maas - One of the best experts on this subject based on the ideXlab platform.

  • Tree-Ring Dating of Snow Avalanches in Glacier National Park, Montana, USA
    Advances in Global Change Research, 2010
    Co-Authors: David Butler, Carol F. Sawyer, Jacob A. Maas
    Abstract:

    Snow Avalanches are major hazards to humans occupying or visiting mountain ranges around the world. Accurate dating of past high-magnitude snow Avalanches is important for a better understanding of their frequency, extent, and climatic driving factors. As climates change, prediction of shifts in avalanche frequency and/or magnitude are better enabled when a thorough understanding of past avalanche occurrences exists.

Peter Bebi - One of the best experts on this subject based on the ideXlab platform.

  • Snow avalanche activity in Żleb Żandarmerii in a time of climate change (Tatra Mts., Poland)
    Catena, 2017
    Co-Authors: Bogdan Gądek, Zofia Rączkowska, Elżbieta Rojan, Alejandro Casteller, Ryszard J. Kaczka, Peter Bebi
    Abstract:

    Abstract This paper reports from a survey of the occurrence of large Avalanches in Żleb Żandarmerii. This couloir is known to be one of the most hazardous avalanche paths in the Tatra Mountains and has one of the longest histories of avalanche observation. This survey looked at the runout distance, return period, dynamics and geoecological implications of Avalanches in the context of current climate change. The study took advantage of the longest record of meteorological data available in the Tatra Mountains, as well as archival avalanche observations, topographical maps, orthophotomaps and a high-resolution digital terrain model. Avalanche data were obtained using geomorphological and dendrogeomorphic methods and through modelling with the RAMMS numerical avalanche dynamics simulation software. The largest Avalanches reach the foot of its counter slope. Their length, release volume, flow velocity and pressure can exceed respectively 1000 m, 80 000 m3, 45 m/s and 600 kPa. The results of our study suggest that current climate warming has been accompanied by thinning and shortening of the duration of snow cover, as well as by an upward expansion of the timberline (including in the large-avalanche runout zones) of up to 80 m since the mid-1920s. No distinct temporal trend was identified in the large avalanche return period since 1909, but their mass and intensity have declined. Forests and timberline expansion were found to have no influence on the extent of the Avalanches in our study, while ground relief could determine both their downward extent and lateral expansion.

  • Explicit avalanche-forest feedback simulations improve the performance of a coupled avalanche-forest model
    Ecological Complexity, 2014
    Co-Authors: Natalie Zurbriggen, Peter Bebi, Michaela Teich, Julia E. M. S. Nabel, Heike Lischke
    Abstract:

    Abstract Many temperate and boreal mountain landscapes are strongly affected by snow Avalanches. Forests can reduce avalanche release probability, leading to a positive feedback between forests and Avalanches. The effects of this feedback, especially when influenced by changing environmental conditions, make the projection of the future developments of mountain forests and Avalanches challenging. In order to study this feedback under a wide range of environmental situations, we coupled a forest landscape model with a new probabilistic avalanche module. The coupled model TreeMig-Aval allows yearly spatially explicit simulations of climatically driven forest dynamics, with species-specific growth, mortality, and reproduction. Simulated spatially explicit avalanche release is driven by climate, topography, forest type and density. These factors, together with additional factors increasing tree mortality, influence the strength of the positive feedback between forests and Avalanches. We investigated (a) the influences of the three environmental factors temperature, slope steepness, and additional mortality on the simulated dynamics of mountain forests and Avalanches, (b) the plausibility of TreeMig-Aval , and (c) whether the complexity of TreeMig-Aval could be reduced. The sensitivity of avalanche release probability to environmental changes was thus compared between TreeMig-Aval and two simplified model versions. The three environmental drivers had strong and often nonlinear influences on the simulated forest and avalanche dynamics. The simulated avalanche release probability showed linear to sigmoidal decreases with temperature, a peak-shaped response to slope steepness, and steep sigmoidal increases with additional mortality. However, these response shapes of avalanche release probability to each environmental factor changed along the axes of the two other factors studied. These interactions suggest that future mountain forest simulation studies should explicitly account for the influence of environmental drivers on the avalanche-forest feedback. The simulations showed that the behavior of TreeMig-Aval is plausible and comparable to expert knowledge and previously published literature. Moreover, large differences in the sensitivity of the avalanche release probability to the environmental factors were apparent between TreeMig-Aval and the simplified model versions, revealing that for plausible simulations of avalanche-prone mountain regions it is necessary to explicitly account for the avalanche-forest feedback in TreeMig-Aval . In particular the simulated treeline was sensitive to changes in model structure and prone to underestimation of the avalanche release probability in the simplified model versions. When the feedback is explicitly accounted for, TreeMig-Aval is a useful tool for simulation studies of mountain forests including spatially explicit disturbances.

  • Stopping Behavior of Snow Avalanches in Forests
    2012
    Co-Authors: Thomas Feistl, Marc Christen, Peter Bebi, Yves Buhler, Michaela Teich, Perry Bartelt
    Abstract:

    A longstanding problem in avalanche science is to understand how forests stop small and medium sized Avalanches. Avalanche dynamics models have traditionally been employed to calculate extreme avalanche runout and have assigned a minor role to forests in dissipating flow energy. In this paper we quantify the important effect of forests in stopping small avalanche events, crucial for road and ski-run safety. We performed field studies of several Avalanches where trees affected the runout. We gathered information concerning the starting location, deposition heights, runout distance and forest structure. These studies were made during the 2011/12 winter where many gliding snow Avalanches released in forested areas in Switzerland and Germany. Using the field observations as a guide, we hypothesized that mass detrainment due to tree-avalanche interaction led to a significant deceleration of the Avalanches. This effect is important for physical based avalanche dynamics models which reveal that avalanche mobility is strongly linked to mass entrainment/detrainment. We tested this hypothesis with a numerical experiment and simulated the documented avalanche events using a velocity dependent detrainment model to reconstruct the braking effect of forests. For the numerical investigations we used high spatial resolution digital terrain models. The results highlight how forests influence mass and energy fluxes at the front and sides of Avalanches. Of particular importance is the distribution of velocity across the flow width of the avalanche, as flow mass can be easily stopped at the flow boundaries.

  • POTENTIAL IMPACTS OF CLIMATE CHANGE ON SNOW Avalanches STARTING IN FORESTED TERRAIN
    2012
    Co-Authors: Michaela Teich, Natalie Zurbriggen, Melanie Ulrich, Perry Bartelt, Adrienne Grêt-regamey, Christoph Marty, Peter Bebi
    Abstract:

    Frequency and magnitude of Avalanches starting in forested terrain (forest Avalanches) are likely to be affected by climate change. We addressed two important developments which will influence the forest avalanche regime: 1) trends in the occurrence of favorable snow and weather situations which increase the probability of forest avalanche releases, and 2) changes in the extent, composition and structure of mountain forests. We applied a logistic trend analysis over 41 years to investigate past changes in the occurrence of snow and weather conditions which are associated with forest avalanche releases in the Swiss Alps. We found negative trends for two typical situations, 'new snow forest Avalanches' and 'other forest Avalanches'. In combination with the currently observed increase in forest cover extent and density, it is thus likely that avalanche releases in forests will become less frequent. For Avalanches started in forested areas, we found that higher densities of small-diameter trees (

  • Avalanche simulations in forested terrain
    2012
    Co-Authors: Michaela Teich, Thomas Feistl, Perry Bartelt, Peter Bebi, Irene Vasella, Adrienne Grêt-regamey
    Abstract:

    Avalanche dynamics models are used for hazard zoning and engineering to predict runout distances and impact pressures of snow avalanche events. The effect of mountain forests as an effective biological protection measure against Avalanches has rarely been addressed in this context. Avalanche runout distances of small to medium Avalanches are strongly influenced by the structural conditions of forests in the avalanche path; however, this varying decelerating effect has not yet been implemented in avalanche models. Within the two-dimensional avalanche dynamics program RAMMS the standard Voellmy-Salm model can be applied to predict runout distances, flow velocities and impact pressures in complex three-dimensional terrain. Currently, the occurrence of forests is realized by increasing but constant friction parameters μ (dry-Coulomb type friction) and ξ (velocity squared friction) compared to open unforested terrain. Back-calculations of 41 well documented small Avalanches which released in forests of the Swiss Alps emphasize the need for a further calibration dependent on differences in forest structure. Since the friction parameters are more conceptual than physical, they must be fitted by matching model results and recorded data which basically involves solving an inverse problem. A way of providing probabilistic statements about unobservable information is Bayesian inference. Therefore, we present a framework for a Bayesian probabilistic model calibration of the friction parameter ξ accounting for differences in forest structure in the avalanche path. Considering different forest characteristics within avalanche simulations will improve current applications for avalanche models, e.g. in mountain forest and natural hazard management.