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Textural and Mechanical Variability of Mountain Snowpacks

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Textural and Mechanical Variability of Mountain Snowpacks Textural and mechanical variability of mountain snowpacks Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakultät der Universität Bern vorgelegt von Christine Pielmeier von Deutschland Leiter der Arbeit: Prof. Dr. Peter Germann, Geographisches Institut der Universität Bern Dr. Martin Schneebeli, Eidg. Institut für Schnee- und Lawinenforschung SLF, Davos Ko-Referent: Dr. Andreas Papritz, Institut für terrestrische Ökologie, ETH Zürich Von der Philosophisch-naturwissenschaftlichen Fakultät angenommen. Bern, 06.02.2003 Der Dekan Prof. Dr. Gerhard Jäger 1 Contents Chapter 1 Introduction 2 Chapter 2 Developments in snow stratigraphy C. Pielmeier, M. Schneebeli 7 Chapter 3 Measuring snow profiles with high resolution: Interpretation of the force-distance signal from a snow micro penetrometer C. Pielmeier, M. Schneebeli 55 Chapter 4 Snow texture: A comparison of empirical versus simulated Texture Index for alpine snow C. Pielmeier, M. Schneebeli, T. Stucki 69 Chapter 5 Soil thermal conditions on a ski slope and below a natural snow cover in a sub-alpine ski resort M. Stähli, T. Keller, F. Gadient, D. Gustafsson, C. Rixen, C. Pielmeier 84 Chapter 6 Snow stratigraphy measured by snow hardness and compared to surface section images C. Pielmeier, M. Schneebeli 103 Chapter 7 Synopsis 123 Acknowledgement 126 2 Chapter 1 Introduction The variability of physical properties in porous media is an area of strong interest to the environmental science community. Most porous media occurring in nature show a distinct spatial variability. To have a tool in the mathematical treatment of this phenomenon the science of spatial statistics has been developed (Matheron, 1965). Cressie (1993) describes the methods and applications in his monograph. Fenton and Vanmarcke (1991) developed an efficient method for the simulation of random fields and their use in finite-element models. Snow is in this respect a model material, typical to the properties of many different sedimented porous media. Snow is also a very special medium as it consists only of a single mineral, ice. This property may simplify many types of investigations but is at the same time sometimes cumbersome due to the inclination to rapid metamorphic changes. The spatial variability of the snow is of prime interest to: − Snow hydrology (generation of preferential flow path and water flow), (Carroll and Cressie, 1997) − Snow chemistry (solute transport of adsorbed chemicals) − Snow mechanics (development of simulation models used in avalanche forecasting and vehicle trafficability), (Smith and Sommerfeld, 1985; Conway and Abrahamson, 1988; Föhn, 1988; Jamieson and Johnston, 1993; Birkeland et al., 1995; Sturm et al., 1997) − Snow ecology (food availability and resistance to movement), (Huggard, 1993; McRoberts et al., 1995) In the following, the state of the art of methods available to investigate spatial variability of snow, and the controversies created by uncertain data (Smith and Sommerfeld, 1985; Conway and Abrahamson, 1988; Föhn, 1988; Jamieson and Johnston, 1993) are explained. The initial cause for spatial variability of snowpacks is created by the terrain and weather during snow precipitation and deposition. Subsequent precipitation events are forming layers. Layers deposited during stronger winds may change in their physical properties within a few 0.1 m. The structure and texture1 of the snow crystals are then modified due to rapid thermodynamic processes, summarised under the term „metamorphism“. It is supposed that certain metamorphic processes, especially the formation of subsurface hoar, are highly variable in spatial extent (again a typical scale of a few 0.1 m is supposed, but this number is very uncertain). This causes many controversies in the avalanche forecast community, because the spatial distribution of so-called weak spots is extremely relevant to natural avalanche release. However, the investigation of these properties and spatial variation could not be answered with the detail necessary to apply the concepts of random fields. This was caused by a serious lack of an instrument resolving layers with an accuracy of about 1 mm that is at the same 1 Texture is defined as the size, shape and arrangement of grains in a specimen of snow, structure defines the properties of the grains. 3 time field portable, rapid, and permitting a physical interpretation of the measurements. Johnson and Schneebeli (1998) and Schneebeli and Johnson (1998) developed the SnowMicroPen (SMP), a new type of a high resolution snow penetrometer. The displacement resolution of this instrument is 0.004 mm. The SnowMicroPen force signal can be interpreted and correlated to the texture of snow. This correlation is based on measurements of more than 40 well-defined laboratory snow samples. The correlation to snow texture was done by statistical methods (Pielmeier, 1998, Schneebeli et al. 1999) and by developing a micro- mechanical model (Johnson and Schneebeli, 1999). Parallel measurements in sieved and homogeneous natural snowpacks show excellent correspondence. This is an improvement in vertical resolution of about a factor 100. Two persons are able to measure more than 100 profiles (of 1.5 m depth) within a day, about 20 times faster than classical snow profiles. This instrument has therefore the potential to overcome the limitations typical to all formerly used methods and based on it a systematic spatial investigation of stratified snowpacks at different scales is now possible. Table 1 presents an overview of available methods in snowpack investigations and their limitations. The methods are of varying relevance. The classical snow profile, based on the International classification for seasonal snow on the ground (Colbeck et al., 1990), has a high relevance today. It delivers an integral description although its objectivity and resolution are limited. Instrument Type of Objectivity z-resol. horizontal-resol. Time Comment measurement [m] radius [m] [min] Shear Frame mechanical High 0.01 0.1 20 destructive, only single layer Stuff Block mechanical Medium - 0.3 10 destructive, only first weak layer Rutsch Block mechanical Medium - 1.2 30 destructive, only first weak layer Rammsonde mechanical Medium 0.04 0.02 20 SnowMicroPen mechanical High 0.001 0.003 5 Snow Profile descriptive Low-Med. 0.01 0.1 40 destructive Translucent optical High 0.005 0.005 40 destructive Profile Near-Infrared optical High 0.005 0.005 40 destructive Profile Radar electro- High 0.05 ? 0.05 60 Interpretation magnetical difficult Serial Sections optical High 0.00001 0.00001 50 destructive Density High 0.04 0.2 3 destructive Of all methods, only the SnowMicroPen, has enough spatial resolution and is at the same time quick enough to measure the spatial variation of a snowfield. Due to the lack of appropriate field investigation methods, the spatial structure of the snowpack has only been investigated in a very limited way (Conway and Abrahamson, 1988; Föhn, 1988). None of the investigations were able to generate sufficiently large datasets to calculate the one- or two-dimensional autocorrelation functions. This information is a necessity to assume the correct random distributions of properties, as it is necessary for finite-element simulations of water infiltration, snow creep or avalanche formation. Hence, the spatial variability of snow texture and snow mechanical properties is badly understood. The development of more 3 refined snowpack simulations will be of little scientific and practical value without more highly detailed investigations of the textural and mechanical properties and their variations. The main goals of the thesis are to measure and analyse the micro structural and micro mechanical properties of the complete natural snowpack using the SMP, and to establish a systematic method for the investigation, analysis and interpretation of the small scale spatial variability of mountain snowpack properties. To meet these aims, the following steps have been taken: - Application and development of interpretation methods of the SMP force-distance signal to field profiles. - Comparison of measured snow micro properties with simulated snow micro properties. - Characterisation of the properties of natural versus artificial snowpacks on ski slopes. - SMP force signal calibration and improvements of the sensor technology for field applications. - Systematic measurements, analysis and interpretation of the spatial variability of the textural and mechanical snowpack properties. The thesis consists of this introductory chapter, five main chapters containing two published papers and three papers submitted for publication in reviewed journals, and the final synopsis chapter: Chapter 2 is a literature review on the developments in snow stratigraphy over the last 150 years. The paradigms in snow stratigraphy shifted from a slope-scale, geomorphologic- sedimentologic approach in the beginnings to a one-dimensional approach to the physical and mechanical properties in “homogenous” layers in the 1940’s. The concept of well-defined layers gets accepted and this paradigm still dominates today. Avalanche formation is related to the spatial variation of snow properties. New instrumental developments show that the perceived strict layering may be a too simple model and modern snow stratigraphy has to integrate different scales. In this review the different directions taken in snow stratigraphy are followed and their suitability to meet the requirements in modern snow research is discussed. Chapter 3 introduces the method of
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