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Release areas of snow avalanches: new methods and parameters


Veitinger, Jochen. Release areas of snow avalanches: new methods and parameters. 2015, University of Zurich, Faculty of Science.

Abstract

Location and extent of avalanche starting zones are of crucial importance in order to correctly estimate the potential danger that avalanches pose to roads, railways or other infrastructure. Presently, release area assessment is based on terrain analysis, combined with expert judgement. Tools for the automatic definition of release areas are scarce and exclusively based on parameters derived from summer topography, such as slope and curvature; this leads to several limitations concerning the performance of such algorithms. One reason for this is that they neglect the effect of the snow cover on terrain morphology. In alpine terrain, the snow-covered winter surface deviates from its under- lying summer terrain due to the progressive smoothing caused by snow. It is assumed that this may change the potential release area size and location – especially in the case of surface slab avalanches; therefore, it seems very likely that release area calculations performed on a summer terrain may strongly differ from calculations on a more realistic winter terrain. Introducing the snow distribution into release area definition therefore may significantly improve the definition and partitioning of potential release areas for hazard mapping purposes, as well as going towards closing the gap concerning the implementation of avalanche dynamics simulations in short-term hazard assessment. The main aim of this thesis is therefore centred on assessing the effect of snow distribution on surface morphology, and accordingly integrating this into a new algorithm with the objective to estimate the size and location of slab avalanche release areas.
In an effort to quantify terrain smoothing, high-resolution lidar measurements of the snow distribution in a high alpine catchment were performed throughout three winter seasons, comprising several acquisitions per winter season. A method was developed geared towards quantifying terrain smoothing by combining roughness calculations of snow surfaces and their corresponding underlying terrain with snow depth measurements. The evaluation of terrain smoothing showed that surface roughness at scales larger than fine-scale drift features is, to some degree, persistent between winter sea- sons. Further, the degree and scale of terrain smoothing processes in a topographical basin could be linked to snow distribution, namely mean snow depth and its variability. This allows approximating the winter terrain from the summer terrain as a function of snow depth.
In a next step, the effect of snow distribution on the potential size and location of slab avalanche release areas was assessed. On the one hand, examination centred on establishing the extent to which a homogeneous terrain surface is more favourable to produce larger avalanche release areas than irregular ones. Slab properties, such as thickness and its variability, as measured by airborne laser scanning both prior to and after artificial slab avalanche release, were compared to surface roughness of underlying bed surface and snow-free terrain. The results show quantitatively, that the potential size of avalanche release areas may be linked to the surface roughness of the winter terrain. Further, winter terrain roughness appeared to be highly capable of delimiting release areas. On the other hand, the relevance of terrain smoothing was statistically assessed by relating point snow depth measurements to observed avalanche release area sizes in single avalanche paths. We observed that very large surface slab release areas occurred only in relatively thick snow covers, whilst very large slab release areas, running on weak basal layers, also occurred in shallow snowpacks. This suggests that the effect of terrain smoothing is mainly relevant for surface slab avalanches.
Based on the scale-dependency of terrain smoothing processes, we finally integrated a snow depth-dependent roughness parameter into a new fuzzy logic framework for the estimation of potential slab avalanche release areas. The validation process demonstrated an improved estimation of avalanche release areas – especially for more frequent avalanches; several case studies further illustrated the practical usefulness of this approach for hazard mapping and short-term hazard assessment. The algorithm allows capturing fine-scale topography and its attenuation under snow influence, thus providing valuable information on the partitioning of potential release areas. In addition, a wind direction-dependent sheltering parameter enables the user to define release area scenarios as a function of single storm or drift events; yet, the application of the algorithm in real case situations remains limited as snowpack stability is not integrated. In order to make this approach amenable, future research activity could therefore focus on the coupling of the algorithm with snowpack conditions.

Abstract

Location and extent of avalanche starting zones are of crucial importance in order to correctly estimate the potential danger that avalanches pose to roads, railways or other infrastructure. Presently, release area assessment is based on terrain analysis, combined with expert judgement. Tools for the automatic definition of release areas are scarce and exclusively based on parameters derived from summer topography, such as slope and curvature; this leads to several limitations concerning the performance of such algorithms. One reason for this is that they neglect the effect of the snow cover on terrain morphology. In alpine terrain, the snow-covered winter surface deviates from its under- lying summer terrain due to the progressive smoothing caused by snow. It is assumed that this may change the potential release area size and location – especially in the case of surface slab avalanches; therefore, it seems very likely that release area calculations performed on a summer terrain may strongly differ from calculations on a more realistic winter terrain. Introducing the snow distribution into release area definition therefore may significantly improve the definition and partitioning of potential release areas for hazard mapping purposes, as well as going towards closing the gap concerning the implementation of avalanche dynamics simulations in short-term hazard assessment. The main aim of this thesis is therefore centred on assessing the effect of snow distribution on surface morphology, and accordingly integrating this into a new algorithm with the objective to estimate the size and location of slab avalanche release areas.
In an effort to quantify terrain smoothing, high-resolution lidar measurements of the snow distribution in a high alpine catchment were performed throughout three winter seasons, comprising several acquisitions per winter season. A method was developed geared towards quantifying terrain smoothing by combining roughness calculations of snow surfaces and their corresponding underlying terrain with snow depth measurements. The evaluation of terrain smoothing showed that surface roughness at scales larger than fine-scale drift features is, to some degree, persistent between winter sea- sons. Further, the degree and scale of terrain smoothing processes in a topographical basin could be linked to snow distribution, namely mean snow depth and its variability. This allows approximating the winter terrain from the summer terrain as a function of snow depth.
In a next step, the effect of snow distribution on the potential size and location of slab avalanche release areas was assessed. On the one hand, examination centred on establishing the extent to which a homogeneous terrain surface is more favourable to produce larger avalanche release areas than irregular ones. Slab properties, such as thickness and its variability, as measured by airborne laser scanning both prior to and after artificial slab avalanche release, were compared to surface roughness of underlying bed surface and snow-free terrain. The results show quantitatively, that the potential size of avalanche release areas may be linked to the surface roughness of the winter terrain. Further, winter terrain roughness appeared to be highly capable of delimiting release areas. On the other hand, the relevance of terrain smoothing was statistically assessed by relating point snow depth measurements to observed avalanche release area sizes in single avalanche paths. We observed that very large surface slab release areas occurred only in relatively thick snow covers, whilst very large slab release areas, running on weak basal layers, also occurred in shallow snowpacks. This suggests that the effect of terrain smoothing is mainly relevant for surface slab avalanches.
Based on the scale-dependency of terrain smoothing processes, we finally integrated a snow depth-dependent roughness parameter into a new fuzzy logic framework for the estimation of potential slab avalanche release areas. The validation process demonstrated an improved estimation of avalanche release areas – especially for more frequent avalanches; several case studies further illustrated the practical usefulness of this approach for hazard mapping and short-term hazard assessment. The algorithm allows capturing fine-scale topography and its attenuation under snow influence, thus providing valuable information on the partitioning of potential release areas. In addition, a wind direction-dependent sheltering parameter enables the user to define release area scenarios as a function of single storm or drift events; yet, the application of the algorithm in real case situations remains limited as snowpack stability is not integrated. In order to make this approach amenable, future research activity could therefore focus on the coupling of the algorithm with snowpack conditions.

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Additional indexing

Item Type:Dissertation
Referees:Purves Ross S, Sovilla Betty, Weibel Robert
Communities & Collections:07 Faculty of Science > Institute of Geography
Dewey Decimal Classification:910 Geography & travel
Language:English
Date:2015
Deposited On:28 Jan 2016 13:10
Last Modified:05 Apr 2016 20:02
Number of Pages:161
Free access at:Official URL. An embargo period may apply.
Official URL:http://opac.nebis.ch/ediss/20152539.pdf

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