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Lawinen treten in vielen Gebirgsregionen der Erde als lebensbedrohende Naturgefahr in Erscheinung und werden in den Alpen treffend als "Weißer Tod" bezeichnet. Für die Sicherung des Lebensraums und der Verkehrswege steht eine breite Palette technischer Schutzmaßnahmen (Lawinenverbauung, künstliche Auslösung, Monitoring- und Warnsysteme) zur Verfügung, die Teil einer eigenständigen Ingenieurdisziplin sind. Das Werk bietet dem Leser einen umfassenden Überblick über die Grundlagen des technischen Lawinenschutzes (Analyse, Bewertung und Darstellung von Lawinengefahren und -risiken) und stellt im Detail die Methoden der Planung, Konstruktion, Bemessung und Erhaltung von Schutzbauwerken und temporären technischen Maßnahmen dar. Die Bedeutung des technischen Lawinenschutzes weltweit wird im Überblick dargestellt. Die Beiträge zu diesem Handbuch wurden von führenden europäischen Experten des technischen Lawinenschutzes erstellt. Die dargestellten Schutzsysteme und Methoden entsprechen dem aktuellen Stand der Technik und basieren auf den normativen Standards in Österreich und in der Schweiz. Das Werk richtet sich an Ingenieure und Planer, die mit der Konzeption und Ausführung von technischen Lawinenschutzmaßnahmen betraut sind, aber auch an interessierte Leser anderer Fachdisziplinen, die mit Fragen des Lawinenschutzes konfrontiert sind. Die Autoren sind führende Fachleute für technischen Lawinenschutz in Deutschland, Frankreich, Italien, Island, Japan, Kanada, Norwegen, Österreich, der Schweiz, Spanien und in den USA.
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Cover
Title Page
Copyright
Preface
List of Contributors
Chapter 1: Introduction
1.1 Avalanche Hazards
1.2 Technical Avalanche Defense: Classification
1.3 Avalanche Disasters, Development of Avalanche Defense: Historical Overview
1.4 History of Avalanche Defense
Chapter 2: Avalanches: Evolution and Impact
2.1 Characteristics of Avalanches
2.2 Meteorological Principles of Avalanche Evolution
2.3 Nivological Principles of Avalanche Evolution
2.4 Frequency and Magnitude of Avalanche Events
2.5 Morphological Principles of Avalanche Evolution
2.6 Avalanche Protection Forest
Chapter 3: Avalanche Dynamics: Models and Impact
3.1 Principles of Avalanche Dynamics
3.2 Numerical Avalanche Models and Simulation
3.3 Avalanche Action on Objects (Obstacles)
Chapter 4: Avalanche Hazard Assessment and Planning of Protection Measures
4.1 Avalanche Hazard (Risk) Assessment and Mapping
4.2 Mapping of Avalanche Hazards and Risks
4.3 Planning of Avalanche Defense Structures
Chapter 5: Structural avalanche protection: defense systems and construction types
5.1 Principles of structural avalanche defense
5.2 Structural avalanche defense in the starting zone
5.3 Structural avalanche defense in the avalanche path and deposition zone
Chapter 6: Structural avalanche defense: design and construction
6.1 Normative Bases of Design
6.2 Design of Avalanche Defense Structures in the Starting Zone
6.3 Design of Snowdrift Protection Structures
6.4 Design of Avalanche Catching, Deflection and Retarding Structures
6.5 Design of Avalanche Breakers
6.6 Design of Avalanche Galleries (Tunnels)
Chapter 7: Construction Work and Maintenance of Structural Avalanche Control
7.1 Construction Work (Avalanche Defense Structures)
7.2 Maintenance of Avalanche Defense Structures
Chapter 8: Building Protection (Direct Protection) Measures
8.1 Structural Building Protection Measures
8.2 Safety Concepts for Buildings Endangered By Avalanches
Chapter 9: Artificial Release and Monitoring Technology for Avalanches
9.1 Methods of Temporary Avalanche Defense
9.2 Artificial Release of Avalanches
9.3 Avalanche Monitoring Technology
Chapter 10: Technical avalanche protection international: facts and figures
Literature
Index
Advertiser Index
EULA
Table 1.1
Table 1.2
Table 2.1
Table 2.2
Table 2.3
Table 2.4
Table 2.5
Table 3.1
Table 3.2
Table 3.3
Table 3.4
Table 3.5
Table 3.6
Table 3.7
Table 4.1
Table 4.2
Table 4.3
Table 4.4
Table 4.5
Table 4.6
Table 4.7
Table 4.8
Table 4.9
Table 4.10
Table 4.11
Table 4.12
Table 4.13
Table 4.14
Table 4.15
Table 5.1
Table 5.2
Table 5.3
Table 5.4
Table 5.5
Table 5.6
Table 5.7
Table 5.8
Table 5.9
Table 6.1
Table 6.2
Table 6.3
Table 6.4
Table 6.5
Table 6.6
Table 6.7
Table 6.8
Table 6.9
Table 6.10
Table 6.11
Table 6.12
Table 6.13
Table 6.14
Table 6.15
Table 6.16
Table 6.17
Table 6.18
Table 6.19
Table 7.1
Table 7.2
Table 7.3
Table 7.4
Table 7.5
Table 7.6
Table 7.7
Table 7.8
Table 7.9
Table 7.10
Table 7.11
Table 7.12
Table 7.13
Table 7.14
Table 7.15
Table 7.16
Table 7.17
Table 7.18
Table 7.19
Table 7.20
Table 7.21
Table 7.22
Table 8.1
Table 8.2
Table 9.1
Table 9.2
Table 9.3
Table 9.4
Fig. 1.1
Fig. 1.2
Fig. 1.3
Fig. 1.4
Fig. 1.5
Fig. 1.6
Fig. 1.7
Fig. 1.8
Fig. 1.9
Fig. 1.10
Fig. 1.11
Fig. 1.12
Fig. 2.1
Fig. 2.2
Fig. 2.3
Fig. 2.4
Fig. 2.5
Fig. 2.6
Fig. 2.7
Fig. 2.8
Fig. 2.9
Fig. 2.10
Fig. 2.11
Fig. 2.12
Fig. 2.13
Fig. 2.14
Fig. 2.15
Fig. 2.16
Fig. 2.17
Fig. 2.18
Fig. 2.19
Fig. 2.20
Fig. 2.21
Fig. 2.22
Fig. 2.23
Fig. 2.24
Fig. 3.1
Fig. 3.2
Fig. 3.3
Fig. 3.4
Fig. 3.5
Fig. 3.6
Fig. 3.7
Fig. 3.8
Fig. 3.9
Fig. 3.10
Fig. 3.11
Fig. 3.12
Fig. 3.13
Fig. 3.14
Fig. 3.15
Fig. 3.16
Fig. 3.17
Fig. 3.18
Fig. 3.19
Fig. 3.20
Fig. 3.21
Fig. 4.1
Fig. 4.2
Fig. 4.3
Fig. 4.4
Fig. 4.5
Fig. 4.6
Fig. 4.7
Fig. 4.8
Fig. 4.9
Fig. 4.10
Fig. 4.11
Fig. 4.12
Fig. 4.13
Fig. 5.1
Fig. 5.2
Fig. 5.3
Fig. 5.4
Fig. 5.5
Fig. 5.6
Fig. 5.7
Fig. 5.8
Fig. 5.9
Fig. 5.10
Fig. 5.11
Fig. 5.12
Fig. 5.13
Fig. 5.14
Fig. 5.15
Fig. 5.16
Fig. 5.17
Fig. 5.18
Fig. 5.19
Fig. 5.20
Fig. 5.21
Fig. 5.22
Fig. 5.23
Fig. 5.24
Fig. 5.25
Fig. 5.26
Fig. 5.27
Fig. 5.28
Fig. 5.29
Fig. 5.30
Fig. 5.31
Fig. 5.32
Fig. 5.33
Fig. 5.34
Fig. 5.35
Fig. 6.1
Fig. 6.2
Fig. 6.3
Fig. 6.4
Fig. 6.5
Fig. 6.6
Fig. 6.7
Fig. 6.8
Fig. 6.9
Fig. 6.10
Fig. 6.11
Fig. 6.12
Fig. 6.13
Fig. 6.14
Fig. 6.15
Fig. 6.16
Fig. 6.17
Fig. 6.18
Fig. 6.19
Fig. 6.20
Fig. 6.21
Fig. 6.22
Fig. 6.23
Fig. 6.24
Fig. 6.25
Fig. 6.26
Fig. 6.27
Fig. 6.28
Fig. 6.29
Fig. 6.30
Fig. 6.31
Fig. 6.32
Fig. 6.33
Fig. 6.34
Fig. 6.35
Fig. 6.36
Fig. 6.37
Fig. 6.38
Fig. 6.39
Fig. 6.40
Fig. 6.41
Fig. 6.42
Fig. 6.43
Fig. 6.44
Fig. 6.45
Fig. 7.1
Fig. 7.2
Fig. 7.3
Fig. 7.4
Fig. 7.5
Fig. 7.6
Fig. 7.7
Fig. 7.8
Fig. 7.9
Fig. 7.10
Fig. 7.11
Fig. 7.12
Fig. 7.13
Fig. 7.14
Fig. 7.15
Fig. 7.16
Fig. 7.17
Fig. 7.18
Fig. 7.19
Fig. 7.20
Fig. 7.21
Fig. 7.22
Fig. 7.23
Fig. 7.24
Fig. 7.25
Fig. 7.26
Fig. 8.1
Fig. 8.2
Fig. 8.3
Fig. 8.4
Fig. 8.5
Fig. 8.6
Fig. 8.7
Fig. 8.8
Fig. 8.9
Fig. 8.10
Fig. 8.11
Fig. 8.12
Fig. 9.1
Fig. 9.2
Fig. 9.3
Fig. 9.4
Fig. 9.5
Fig. 9.6
Fig. 9.7
Fig. 9.8
Fig. 9.9
Fig. 9.10
Fig. 9.11
Fig. 9.12
Fig. 9.13
Fig. 9.14
Fig. 9.15
Fig. 9.16
Fig. 9.17
Fig. 9.18
Cover
Table of Contents
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Edited by
Florian Rudolf-Miklau
Siegfried Sauermoser
Arthur I. Mears
All books published by Ernst & Sohn are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2015 Wilhelm Ernst & Sohn, Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Rotherstraße 21, 10245 Berlin, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-433-03034-9
ePDF ISBN: 978-3-433-60386-4
ePub ISBN: 978-3-433-60387-1
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oBook ISBN: 978-3-433-60384-0
Large, high-energy snow avalanches can have high destructive consequences in developed areas. Each year, avalanche catastrophes occur in many mountain regions around the globe. This causes a large number of fatalities and severe damage to buildings and infrastructure. In some mountain areas, especially in the European Alps, a high level of safety for settlement areas is attained by application of technical avalanche defense construction. Simultaneously, new risk potentials continue to emerge in mountain regions from building in endangered areas, the establishment of new roads and railway lines across the mountains and development of tourism (skiing, alpine resorts). These are sometimes located partially or entirely outside protected areas. Consequently the demand for technical avalanche protection in these regions is constantly increasing.
During the last decades technical avalanche protection has evolved – especially in the Alpine countries Austria, Switzerland, Italy and France as well as Norway, Iceland, USA and Canada – from a specialist field to a stand-alone engineering branch. Currently avalanche defense structures and protection systems are established in practically all inhabited mountain regions worldwide. With this engineering handbook the editors are able to provide the first comprehensive overview of the field of technical avalanche protection in the English language and establish a common state-of-the-art. The book is based on the German edition, which was published in 2011, and comprises all relevant facts on fundamentals of avalanche protection technology as well as of planning, dimensioning, construction and maintenance of defense structures.
Technical avalanche protection denotes structural measures (defense structures), which are predominantly applied to protect inhabited areas. In such areas frequent and/or large avalanches may occur and cause significant risks to humans and material assets. The structures may consist of steel, concrete, earth, rock or wood material. Planning of defense structures is based on an intensive analysis and assessment of avalanche hazards and risks. Structure design usually considers a design event, which takes into account avalanches with a certain probability of occurrence and the applicable mass and energy associated with this design event. An unusual aspect of design, construction and maintenance is the enormous force of impact by avalanches and the extreme environmental and climatic conditions (alpine high altitude areas, subarctic climate) to which the structures are exposed. The extreme terrain and climatic conditions at the construction sites also bring about extraordinary challenges to workers and engineers.
However, several decades of experience in avalanche protection engineering have demonstrated the limits and usefulness of structural avalanche defense systems. Alternatively new technologies were developed in the field of artificial avalanche release, supported by sophisticated methods of avalanche monitoring. One of the starting points for emerging new technologies was the large avalanche cycle in the Alps in 1999. The new methods can be combined with classical defense structures and applied together with other kinds of protection measures (e.g. avalanche warning, closure, evacuation) for the purpose of an integrated avalanche risk management procedure. Temporary avalanche protection systems – in the wider sense of the term technical avalanche protection – are also comprehensively presented in this book.
Until recently the state-of-the-art of technical avalanche engineering was available in several normative documents; however most advances in this field result from empirical developments in engineering practice. The highest stage of development and standardization was reached in the field of snow supporting structures in the starting zone. The oldest and best established standard in this field is the Swiss guideline on ‘Defense structures in avalanche starting zones’ (in its current version 2007) [194], which represents one of the most important sources of this handbook. Recently in several European countries standardization processes took place which lead to the publication of normative documents, partially in order to adapt the Swiss Guideline to national framework conditions: for example France: Norme Française (1992) [219]; Iceland: Jóhannesson und Margreth [148]; Austria: ÖNORM-Regeln 24805 ff. [244–246]. In other countries such as Norway, USA, Canada or Japan still no specific national standards are available. One of the most important steps was the adaptation of norms to the regulation of the Eurocode (unified European standardization). This handbook includes a comprehensive overview of the relevant standards and guidelines of technical avalanche protection at the current status. The Eurocode refers to Swiss (SIA), Austrian (ON), German (DIN) and US standards.
In Chapter 1 the reader is introduced to the system of technical avalanche protection and its historical development based on a fundamental classification of protection measures. Chapter 2 deals with the fundamentals of avalanche formation and the criteria for frequency, magnitude and risk assessment. Subsequently Chapter 3 presents the physical principles of avalanche dynamics impact on objects and the numerical avalanche process models best established in engineering practice. Chapter 4 is dedicated to the system of hazard and risk mapping, based on hazard and risk assessment, and shows the planning processes for structural avalanche defense. The most important protection concepts and goals are also provided in Chapter 4 as well as criteria of a sustainable planning according to technical, economic and environmental principles. Chapter 5 provides a comprehensive and systematic overview of defense structures in the avalanche starting zone as well as the avalanche path and runout zone. All relevant, applicable and historic construction types are presented by technical description system sketches and photographs. The construction and dimensioning of avalanche defense structures, with special respect to supporting components, building material and geotechnical fundaments of foundation are dealt with in Chapter 6. This chapter also comprises all relevant information for dimensioning and technical calculation of required in engineering practice. Chapter 7 presents the fundamentals of construction works and maintenance for avalanche defense structures and with special respect to the Alpine environment. Details on construction methods, construction site infrastructure, transportation systems and construction equipment is included as well as the system of monitoring (inspection) and maintenance for avalanche defense structures over their useful life. Chapter 8 gives a comprehensive overview of the methods of building protection (object protection) in areas endangered by avalanches. Finally Chapter 9 comprises the fundamentals and technology of temporary avalanche protection by artificial release, avalanche warning and monitoring. In this chapter current developments and best practice examples of artificial avalanche release technology from Switzerland and Austria were added (referring to the chapter in the German edition). Chapter 10 finally presents an international overview (table) of avalanche protection in the most endangered countries (based on the German edition).
During the writing of this handbook the editors were able to bring together an international team of leading experts in technical avalanche protection. Authors from Austria, Switzerland, USA, Norway, Canada, Iceland, Japan, France and Italy have directly contributed to this book or supported it with essential information. The book represents a sequel of publication in the field of natural hazard engineering in the framework of Wiley/Ernst & Sohn Berlin publishing house. The main purpose of this publication is to share specialized engineering knowledge and experience in avalanche protection among experts worldwide and contribute to more safety in mountain regions exposed to avalanche risks.
Special thanks go to the Federal Ministry of Agriculture, Forestry, Environment and Water Management in Vienna, the Austrian Service for Torrent and Avalanche Control, the Austrian Standards Institute, the WSL Institute for Snow and Avalanche Research SLF in Davos, the Tyrolean Avalanche Warning Service in Innsbruck, the Austrian Research Centre for Forests, the Austrian Meteorological Service, the Icelandic Meteorological Office (Reykjavík), the American Avalanche Association (AAA), the South East Alaska Avalanche Center (AAC) and the Canadian Avalanche Association CAA (Revelstoke), who have actively supported the creation and elaboration of this handbook. The publication of this handbook would not have been possible without the intensive translation work by DeAnn Cougler (Munich; MB eurocom international languages Vienna) and the critical review by Emily Procter (Bolzano) as well as the design work of Andreas Herbert (Innsbruck). We also thank the legion of colleagues, who have given technical advice and the companies in the field of avalanche protection, who have supported us by latest information on new technologies. Finally special appreciation goes to the team of Ernst & Sohn in Berlin, especially Claudia Ozimek and Ute-Marlen Günther, for the support, patience and engagement to bring avalanche protection technology to the global engineering community.
Vienna, Innsbruck and Gunnison, October 2014
Florian Rudolf-Miklau, Siegfried Sauermoser, and Art Mears
DI Dr. Florian Rudolf-Miklau
Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management
Austrian Service for Torrent and Avalanche Control
Marxergasse 2
1030 Vienna
Austria
DI Siegfried Sauermoser
Austrian Service for Torrent and Avalanche Control, Section Tyrol
Wilhelm-Greil-Str. 9
6020 Innsbruck
Austria
Arthur I. Mears
Arthur I. Mears, P.E., Inc.
555 County Road 16
Gunnison, CO 81230
U.S.A.
Dr. Karl Gabl
Austrian Central Institute for Meteorology and Geodynamics ZAMG
Branch office Innsbruck
Fürstenweg 180
6020 Innsbruck
Austria
PhD. Peter Gauer
Norwegian Geotechnical Institute NGI
Sognsveien 72
0806 Oslo
Norway
DI Matthias Granig
Austrian Service for Torrent and Avalanche Control
Staff Unit for Snow and Avalanches
Wilhelm-Greil-Str. 9
6020 Innsbruck
Austria
Dr. Robert Hofmann
State authorised and certified chartered engineer
Consulting engineer for structural engineering
Hochstraße 17/2
2380 Perchtoldsdorf
Austria
Dr. Karl Kleemayr
Federal Research and Training Centre for Forests, Natural Hazards and Landscape BFW
Institute for Natural Hazards
Rennweg 1
6020 Innsbruck
Austria
Dipl.-Bauing. ETH Stefan Margreth
WSL Institute for Snow and Avalanche Research SLF
Organisational Unit Snow Avalanches and Preventiontion Measures
Flüelastr. 11
7260 Davos Village
Switzerland
Mag. Michael Mölk
Austrian Service for Torrent and Avalanche Control
Staff Unit for Geology
Wilhelm-Greil-Str. 9
6020 Innsbruck
Austria
DI Patrick Nairz
Avalanche Warning Service of Tyrol Tyrolean Provincial Government, Department for Civil Protection and Disaster Prevention
Eduard-Wallnfer-Platz 3 (Landhaus 1)
A-6020 Innsbruck
Austria
DI Wolfgang Schilcher
Austrian Service for Torrent and Avalanche Control, Section Vorarlberg
Oberfeldweg 6
6700 Bludenz
Austria
DI Christoph Skolaut
Consulting Engineers Skolaut Naturraum
Herzog-Odilo-Straße 1/1
5310 Mondsee
Austria
DDI Dr. Jürgen Suda
alpinfra, consulting + engineering gmbh
Kuefsteingasse 15-19
1140 Vienna
Austria
Dipl.-Bauing. ETH Lukas Stoffel
WSL Institute for Snow and Avalanche Research SLF
Organisational Unit Snow Avalanches and Preventiontion Measures
Flüelastr. 11
7260 Davos Village
Switzerland
Dr. Markus Stoffel
University of Bern, Institute of Geological Sciences
Dendrolab.ch
Baltzerstr. 1-3
3012 Bern
Switzerland
DI Gebhard Walter
Austrian Service for Torrent and Avalanche Control
Section Tyrol
Wilhelm-Greil-Str. 9
6020 Innsbruck
Austria
MSc Emily Simone Procter
European Academy of Bolzano EURAC, Institute of Mountain Emergency Medicine
Viale Druso, 1
39100 Bolzano
Italy
DI Arnold Kogelnig
Wyssen avalanche control AG
Reimmichlgasse 5
6020 Innsbruck
Austria
Mag. Roderich Urschler
SUFAG Snowbusiness GmbH
Hans-Maier-Strasse 9
6020 Innsbruck
Austria
Marco Larghi
Inauen Schätti
Tschachen 1
8762 Schwanden
Switzerland
Francois Rapin
National Research Institute of Science and Technology for Environment and Agriculture IRSTEA
Research Unit ADRET
BP 76
Domaine Universitaire
38402 St. Martin D'Heres Cedex
France
Bernhard Zenke
Bavarian Environment Agency
Unit 87 - Avalanche Warning Center, Avalanche Protection
Heßstr. 128
80797 Munich
Germany
Yasuo Ishii
Public Works Research Institute
Erosion and Sediment Research Group
Landslide Research Team
2-6-8, Nishiki-cho, Myoko-shi,
Niigata-ken, 944-0051
Japan
Dr. Rudolf Pollinger
Autonomous Province of Bolzano-Bozen
Department 30 − Flood Control
Cesare-Battisti-Straße 23
39100 Bozen
Italy
Krister Kristensen
Norwegian Geotechnical Institute NGI
P.O. Box. 3930 Ullevål Stadion
806 Oslo
Norway
Tomas Johannesson
Iceland Met Office
Veðurstofa Íslands
Bústaðavegi 7- 9
108 Reykjavík
Iceland
Pere Oller
Institut Cartogràfic i Geològic de Catalunya
ICGC Territorial Support Center (CST) Pyrenees
Passeig Pompeu Fabra, 21
25620 Tremp
Spain
Chris Stethem
Stethem & Associates Ltd.
409, 8 Avenue
Canmore, AB T1W2E6
Canada
Siegfried Sauermoser, Florian Rudolf-Miklau and Stefan Margreth
Avalanches are defined as large masses of snow or ice that move rapidly down a mountainside or over a precipice. The term snow avalanche is more accurate to make the conceptual demarcation from other types of avalanches such as rock avalanches or mud flows. According to ONR 24 805, 3.34 [202], snow avalanches are characterized by rapid movement of snow masses that were triggered from the snow cover. Snow avalanches that cause human losses as well as severe property and environmental damage are classified as natural catastrophes.
Throughout history, avalanches have had a major impact on the development of settlements in mountain regions (Figure 1.1). This influence is obvious from the location and structure of historical villages and traffic routes. Typical toponyms like Lähn or Lavin indicate old avalanche paths and are probably derived from the Latin terms labi (gliding down) and labes (falling) [7]. For many centuries, humans were not able to protect themselves effectively from avalanche hazards and resorted to simplistic solutions such as avoiding areas at risk. Despite the sparse population in Alpine regions, major avalanche disasters with numerous victims occurred repeatedly in history, as people were not able to assess the risk of these infrequent but catastrophic events.
Fig. 1.1 Alpine living space, shaped by avalanches (© Sauermoser)
In the last century, increasing populations in the Alps (1870: 7.8 million; 2010: 13.6 million) in combination with growing demands for mobility and leisure activities in Alpine terrain have increased avalanche risk significantly. Traditionally, Alpine valleys were scarcely populated apart from mountain farms, whereas today there are a wide range of competing interests in land use such as settlement developments, traffic, trade and industry, tourism and recreation facilities. This has created progressive consumption of land and use of higher risk areas for building. Some Alpine valleys in well-developed regions are subject to urban sprawl and in areas where tourism is the only profitable economic branch, intensive development of higher elevation areas has occurred, especially for skiing. Though depopulation has been reported in infrastructure-poor mountain regions (mountain escape), the Alps will be subject to intensive land use in the future as well since mountains are a sustainable source of natural resources (timber, water, renewable energy and mining).
Increasing traffic density and volume of transportation have resulted in a growing demand for efficient and safe transit corridors across the Alps (e.g. Tenda, Fréjus, Mont Blanc tunnel, Simplon pass, Lötschberg tunnel, St. Gotthard, San Bernadino, Arlberg, Reschen pass, Brenner, Felbertauern, Tauern and Katschberg tunnel, Tauern railway Böckstein/Mallnitz, Gesäuse railway). Outdoor leisure activities and sports (mountaineering, mountain biking, skiing, hunting) have increased human activity in higher elevation areas. In the last decades, the majority of avalanche victims have been skiers off marked slopes as well as ski tourers and free riders.
Increased human impact is noticeable in the European Alps and can be expected in the future in other mountain regions around the world. Avalanche risk and safety expectations have increased significantly while the risk acceptance of a modern society is constantly decreasing. Consequently, the demand for technical avalanche protection in the Alps increased within a short time and prompted rapid development in defense technology. The diverse technological innovations included both new types of avalanche defense structures with permanent protection effects and high-tech systems with temporary protection effects, especially for monitoring and detection of descent or artificial release of avalanches. The establishment of the field of technical avalanche defense as a stand-alone engineering discipline shows the central role avalanches play in mountain regions.
An avalanche hazard refers simply to a source of potential harm, and is a function of the likelihood of triggering and the destructive size of an avalanche. The different dimensions of avalanche hazards are expressed in the five-point European Avalanche Hazard Scale [79] (Table 4.1). Avalanche risk must relate to a specific element at risk, for example people, buildings, vehicles, or infrastructure. Avalanche risk is determined by the exposure of that element and its vulnerability to the avalanche hazard. Avalanche hazards are not necessarily related to catastrophic events. Most of the avalanche accidents causing loss of human life occur in unsecured areas where the people involved actually triggered the avalanche. These so-called tourist avalanches happen frequently but generally do not affect settlement areas, traffic routes or infrastructure and thus are not considered target areas for permanent technical defense structures (also for economic reasons). As avalanche size increases, the probability of occurrence decreases but settlements and traffic routes may also be affected. For example, a so-called hundred-year avalanche represents an event that occurs – from a statistical point of view – on average once every 100 years.
Snow avalanches can occur anywhere where sufficient snowfall occurs within a short time on slopes with an inclination of more than 30 degrees. Avalanches occur throughout the Alps and many other mountain ranges in the world including the Pyrenees, Apennines, Norwegian Fjordland, Iceland (Figure 1.2), Rocky Mountains, Andes, Japanese and New Zealand Alps, Elbrus mountains, Hindu Kusch, Pamir mountain range, Russian Altai and Baikal mountains, Chinese Tianshan or Himalayas (Figure 1.3). In ancient times, the Greek geographer Strabon (63 BC to 23 AC) documented avalanche events in the Caucasus Mountains in his scriptures ‘Geographica’. In Austria, more than 6000 avalanche paths have a potential impact on settlement areas [35] and countless other avalanches occur in undeveloped mountain areas or remote, seasonally used regions. In Switzerland, more than 20 000 dangerous avalanches are known. The capital of Alaska, Juneau, is an example of an urban area at high-risk from avalanches [60] (Figure 4.5).
Fig. 1.2 The Icelandic village Seydisfjördur is a high-risk area for avalanches (© Sauermoser)
Fig. 1.3 Global overview of mountain regions with potential avalanche hazards (originally elaborated by Glazovszkaya [78]) (The map is only a rough presentation, as no exact survey was carried out)
An avalanche hazard is not absolute, but is relative to an element at risk. Avalanche defense measures are also designed relative to a specific scenario, and several such measures are presented in this book. In countries where avalanche risk is considered substantial, avalanche defense should use a holistic approach that considers various relevant protection goals and possible measures.
Avalanche defense refers to any measure in the catchment area of an avalanche used to achieve the targeted protection goal [202], and is classified as follows [161]:
–
Active
defense measures prevent avalanches from starting or act directly on the flow process, and
–
Passive
defense measures mitigate the consequences of a potential avalanche hazard.
Active measures are appropriate to reduce the frequency of hazardous avalanches or directly decrease the intensity of the avalanche process. In contrast, passive measures reduce either the damage potential or the vulnerability of objects at risk.
Avalanche defense measures provide either permanent (constantly effective) or temporary (time-limited effect, adjusted to a specific situation) protection [222]. Table 1.1 gives an overview of the classification scheme of avalanche defense measures.
Table 1.1 Classification scheme of avalanche defense measures
Defense measure
Permanent effect
Temporary effect
Active
Precautionary effect
Reducing the disposition for an event
Forest and bioengineering measures (protection forest, high-altitude afforestation)Avalanche defense structures: snow supporting structures, snowdrift control structures
Artificial release of avalanches
Acting directly on the avalanche process
Avalanche defense structures: dams, breakers, tunnels, galleries
Closure for roads Evacuation (of buildings at acute risk)
Reaction to an event
Emergency measures (after an event)Catastrophe management
Passive
Precautionary effect
Legal measures (regulations, prohibitions)Hazard mappingPlanning measures (land use planning)Administrative measures (building permission, relocation of buildings at risk)Structural building (object) protectionCatastrophe management plans
Information (risk communication)Avalanche monitoring and predictionAvalanche commissionsAvalanche warning service
Reaction to an event
PreparednessCatastrophe management
Another classification of avalanche defense measures uses the risk cycle of the natural hazard management [209] (Figure 1.4). According to [222], the hemisphere of precaution comprises prevention, preparation and preparedness; the hemisphere of response (to catastrophes) integrates intervention, assistance and restoration. Most of the measures presented in this book are among the sectors of prevention and preparation.
Fig. 1.4 Risk cycle for natural hazard management (© AdaptAlp)
Holistic systems for avalanche defense have been established in most Alpine countries (Austria, Switzerland, France, Italy, Germany, Slovenia), as well as in other European countries (Norway, Iceland), furthermore in Canada, USA, Japan and New Zealand. Avalanche defense is generally a public service (task of the state), though the degree of responsibility and actual duties varies substantially. This holds true especially for the organization, financing and execution of technical avalanche defense. Furthermore, in other mountainous countries in Europe and around the globe, such as in Poland, Slovakia, Romania, Bulgaria, Spain, Great Britain, Russia, Turkey, China, Andean states, Himalaya and the Caucasus region, avalanche defense has gained in importance due to major events.
In the relevant technical standard literature (e.g. Margreth [165], ONR 24805 [202]) the term technical avalanche defense is equated with structural (constructional) defense measures with permanent effects – in contrast to the technical avalanche defense measures with temporary effects (Section 1.2.3 and chapter 9). The protection effect of these measures is constant, that is independent of the actual avalanche risk or season.
Technical defense measures typically refer to avalanche defense structures, meaning constructed works (sometimes including mechanical and electronic components) and are termed avalanche defense structures in the engineering field (Figure 1.5 a and b).
Fig. 1.5 Examples of structural avalanche defense structures: (a) snow nets in a starting zone (© Sauermoser); (b) avalanche retarding dam in the municipality of Galtür (Tyrol) (© Rudolf-Miklau)
According to [165], structural avalanche defense is based on one of two strategies:
– hinder initiation or propagation of an avalanche by stabilizing (support) the snow pack in the starting zone or by reducing snow drift (snow displacement by wind), or
– break, decelerate, retard, deflect or retain avalanches in motion (deflection or retarding structures).
Measures based on the first strategy are used in the starting zone of avalanches (Figure 1.5a), whereas measures based on the second are constructed in the avalanche path or runout zone (Figure 1.5b). Table 1.2 gives an overview of the classification and function of structural avalanche defense structures. A third group of measures includes structural building (object) protection, whereby the protection effect is defined for a single object (e.g. residential house, towers of a cable car, electricity pole) (Chapter 8). Object protection measures are amongst the avalanche defense structures.
Table 1.2 Overview of avalanche defense structures classified by the function and location in the catchment area, according to [161]
Structural avalanche defense
Avalanche defense structures
Object protection
Category of defense measure
Snow drift control structures
Snow supporting structures
Avalanche catching and retarding structures
Avalanche deflection structures, snow sheds and tunnels
Structural building protection
Function (protection effect)
Structures that control the snow drift and snow accumulation in the starting zone.
Structures that stabilize and sustain the snowpack in the starting zone and prevent the release of avalanches.
Structures that stop or decelerate the motion of avalanches or dissipate the energy in order to reduce the run out distance.
Structures that deflect avalanches in motion from objects at risk or to by-pass them from traffic routes (roads, railway lines).
The building at risk is enforced in a way that it is able to withstand the impact (stress) of avalanches with little damage.
Type of defense structure
Snow drift fenceWind baffleWind roof(Jet roof)
Snow bridge/rake/netCombined snow bridge (steel/wood)Terrace
Avalanche catching or retarding wall (dam)Avalanche moundAvalanche breaker
Avalanche deflecting dam (wall)Gallery (shed)Tunnel
Avalanche splitting wedgeRoof terraceImpact wall
Location in catchment area
Starting zone
Starting zone
Avalanche pathRunout zone
Avalanche pathRunout zone
Avalanche pathRunout zone
In this book the term technical avalanche defense is used in a broader sense and also comprises active and passive measures with temporary protection effects. These are measures with effects that are limited in time and that require additional assessment of the actual avalanche danger (e.g. by an avalanche commission).
Technology for the artificial release of avalanches is amongst these temporary defense measures (Section 9.2) and includes structures or facilities adapted to specific avalanche hazard situations. These usually supplement other protection structures but may in a few special cases substitute them. To initiate avalanche release, additional loads are applied to the snowpack and fracturing occurs at natural weak zones/layers. According to [263], a wide range of new technologies for artificial release of avalanches is available on the market (example system Gaz.Ex; Figure 1.6). Artificial release minimizes the duration of traffic routes closures (roads, railway lines, cableways, ski slopes) or evacuation of buildings (public places). In Europe, this measure is applied predominantly for protection in ski areas and traffic routes, albeit with some limitations, but is rarely used for the protection of settlements and buildings due to legal and safety concerns.
Fig. 1.6 Artificial avalanche release facilities with the system Gaz.Ex (© Interfab Snowbusiness GmbH)
Technical systems (facilities) for avalanche defense are also used for avalanche monitoring, prognosis and warning (alert) (passive defense measures with temporary effect). Avalanche monitoring and prognosis (warning) requires digital measuring technology, remote sensing and computer-based models. According to [111], these technical systems facilitate assessment of avalanche hazards, recording of snow layering and compilation of relevant meteorological data with the aim to create daily updated avalanche reports (warning) with regional relatedness. Computer-based models for prognosis are appropriate to assess the actual avalanche hazards on a local level, if sufficient documents and data on historical avalanches are available (Section 1.3.1). All these technologies represent an indispensable support for the work of regional authorities and avalanche commissions for a specific emergency situation (blockage, evacuation, closure). In comparison, direct remote sensing and detection of moving avalanches (e.g. by high-speed cameras, geophones or radar) is primarily of scientific interest but will gain importance in the future with improvements in technology (Section 9.3).
Avalanche events have been recorded throughout the history of settlements in the Alps. Livius [7] reported heavy losses caused by avalanches during Hannibal's crossing of the Alps (loss of 18,000 men and 2,000 horses). Famous Alpine villages such as Heiligenblut or St. Christoph/Arlberg owe their origin to avalanche disasters. One of the most disastrous avalanche winters was in February 1689 with a toll of 265 people including 80 people in the Swiss villages Saas and St. Antönien [117]. In 1667 in Switzerland, the village Anzonico was totally destroyed and 88 people died. Avalanches played a major role during World War I along the Dolomite front. Extraordinary snowfall and low temperatures led to catastrophic avalanches that buried several thousand soldiers on both sides of the front. On the Austrian side of the front alone, some 6000 soldiers were killed on December 16, 1916 on black Thursday.
Extreme avalanche events in the twentieth century include (see Figure 1.7):
1909
The most severe avalanche catastrophe of the European Railway history happened during the construction of the Tauern railway when 26 workers were killed [117].
1916
During World War II, Italy and Austria had military bases in the Alps and these troops were soon to find that bombs and enemy fire were not the only threats – heavy snow instigated a series of avalanches in the Tyrol region causing the death of 10 000 soldiers on what became known as
White Friday
1916.
1924
Avalanches caused 9 deaths in Styria [117].
1935
Heavy snowfall (12.5 m snow depth in Langen am Arlberg, Austria) led to numerous avalanches in Austria, Switzerland and South Tyrol. More than 100 people were killed by avalanches, 50% of them were skiers [117].
1950/51
In Austria in more than 1000 avalanches, 135 people were killed. In Switzerland 98 people were killed.
1954
An avalanche catastrophe in Vorarlberg, Austria, caused 143 deaths [117].
1968/70
There were 24 avalanche victims in the region of Davos, Switzerland [5]. In 1970, several enormous avalanches occurred in Austria (
Figure 1.8
) and France with 39 deaths in Val d'Isère. An avalanche killed 74 people on the Plateau d'Assy in Savoyen.
1998/99
The most recent avalanche catastrophe occurred in February 1999 and caused 70 deaths in France, Switzerland and Austria; 31 persons died in a large avalanche in Galtür, Austria.
Fig. 1.7 Annual number of avalanche victims in Austria from 1945 to 2010 (© Austrian Board for Alpine Security Surveillance)
Fig. 1.8 Wiestal Avalanche (municipality Bichlbach, Tyrol) after the avalanche event in 1970 (© WLV Tyrol)
Statistical classification of the recurrence probabilities of historic avalanche cycles is limited because of a lack of long observation periods. For the avalanche event in Galtür in February 1999, the return period has been retrospectively estimated as 100–200 years [169] and the recurrence probability of the snow precipitation as 300 years [68].
Avalanches are observed worldwide. The highest number of fatalities ever recorded in one event was the catastrophic mudflow in Huascaran, Peru, in 1970 where 25 000 people died [88].
Other notable avalanche events include:
Norway
From 1836 to 1998: 1510 people were killed in avalanches (in 1679, there were 130 fatalities in Western Norway; in 1868, there were 161 fatalities) [143].March 5, 1986: In Vassdalen in Nordland county, a snow avalanche was released from Storebalak. 31 men from the North Norway Brigade were involved, 16 men were killed, 15 survived.March 1909 and March 1956: 51 fatalities on Lofoten islands.February 1928: 45 persons were killed mainly by slush flows in Hordaland, Sogn.
Iceland
1995: 34 avalanche fatalities in two avalanche events in Sudavik and Flateyri [125].1974: An avalanche killed 12 inhabitants in Neskaupstadur.1910: An avalanche killed 20 inhabitants in Hnifsdalur.1919: An avalanche killed 18 inhabitants in Siglufjördur.
USA/Canada
1910: Two passenger trains were buried by avalanches at Stevens Pass, Wellington, 97 passengers died [8, 142]1910: 62 persons died at Rogers pass, Canada.1965: 40 persons died in the Granduc mine disaster.
Turkey [87]
1975/76: 9 avalanche events with 170 fatalities.1991/92: 112 avalanche events with 328 fatalities.1992/93: 31 avalanche events with 135 fatalities.
Chile
The most disastrous avalanche event within the last 100 years happened in August 1944 in the working class district of the copper mine El Teniente with 102 casualties [150].
Pakistan
February 2010: 102 fatalities in Kohistan.April 2012: An avalanche hit a military camp near the Siachen glacier in the Karakoram branch of the Himalaya mountains, 135 soldiers died.
India
1979: A series of avalanches buried the valley leaving at least 200 victims in Lahaul Valley.
Afghanistan
2010: 166 fatalities at Salang Pass.2012: Several avalanches in the Daspai area killed 201 people.
Corsica
1934: An avalanche from Castagniccia caused 37 fatalities in Ortiporia.
Romania
April 1977: 23 fatalities in Balea lac in the Southern Carpatians.
Slovakia
1924: Vel'ká Fatra avalanche in the Low Tatra mountains destroyed half of the village and killed 18 people.1956: Vajskorska dolina avalanche caused 16 fatalities.
Japan
1993–1998: 143 avalanche disasters reported, these caused 50 fatalities.
The first structural avalanche defense structures were built in the form of earth or rock walls and were positioned directly above the endangered object (by hand, since machinery was not available). In Austria, the first known direct defense structure was a rock fill wedge erected in 1613 in the village of Galtür to protect the houses in the area called Birche. In Switzerland, technical defense structures have existed since the sixteenth century. One example is the defense wedge on the church Frauenkirche in Davos (Figure 1.9).
Fig. 1.9 Historical avalanche defense structures: Endangered by the avalanche Frauentobellawine, the church Frauenkirche was protected by an avalanche splitting wedge, constructed most probably in the 16th century (© Margreth)
Avalanche defense walls in Austria were known as Schneearchen, Spaltecken or Sauköpfe [122]. Many houses had shed-like roofs to guide avalanches over the house. Although these first technical defense structures were primarily direct defense structures, organizational measures were also common. Houses located in safe areas were designated as meeting spots during evacuations and high-risk periods. It is still possible to find these cellars, called Lahngrube, Lahnkeller or Lawinengruften in old farmhouses today.
The need for a more systematic approach to defense structures arose in the 19th century, for example on a larger scale in the starting zones of avalanches. Emphasis was placed on the maintenance and restoration of protection forests and, through this, the frequency of avalanches was effectively reduced. Coaz [42] (the first Swiss forest inspector and pioneer in avalanche defense) reported the first avalanche defense structure in the 18th century; these were earth terraces with a length of 100–200 m, depth of 0.8 m and distance between the rows of approx. 20 m.
The erection of stone or earth terraces was the beginning of systematic protection in starting zones (Figure 1.10). However, Coaz noted already in 1910 that the height of the terraces was too low to support the accumulated snow cover and prevent avalanche release. Newer rock walls were built with a height of up to nine meters, and between 1876 and 1938, approx. 100 km of stone walls had been erected in Switzerland [64].
Fig. 1.10 Historical avalanche defense structures: rock fill terraces at the Schafberg mountain in Pontresina (Switzerland), constructed around 1900 (© Margreth)
In Austria, the first systematic avalanche defense structures in a starting zone was built by the Austrian Service in Torrent Control at the Rax Mountain in Lower Austria. In the starting zone of the avalanche Lahngrubenlawine 770 m of rock fill walls and terraces and 23 wooden snow rakes were built [33].
A vast network of technical avalanche defense structures was necessary at the end of the 19th century during the construction of trans-Alp roads and railways. During the construction of the Arlberg railway from 1880 to 1884, serious avalanche accidents occurred on the western part of the pass. The first defense structures were built by Pollack (an employee of the railway company) (Figure 1.11a). In Switzerland, the first avalanche gallery was built on the Splügen and Simplon passes (Figure 1.11b). In Austria, the first avalanche gallery to protect a road was built by Karl Ritter von Geha in 1854. By 1888, 21 avalanche galleries had been constructed with a total length of 1.6 km.
Fig. 1.11 Pioneer technical avalanche defense structures: (a) avalanche defense structures in the starting zone included rock fill terraces and snow rakes (Arlbergrechen) for the Arlberg railroad (© ÖBB); (b) avalanche tunnel with barrel vault, constructed in 1824 at the Splügen pass in Grisons (© Cantonal department of monument preservation Gisons, Switzerland)
The avalanche disasters of 1951 and 1954 intensified efforts to establish widespread technical avalanche protection in Alpine countries. In comparison to the vertical earth or stone terraces that had been used until this point but were ineffective because of rapid filling with falling or drifting snow, supporting structures made of steel, rope wires or wood erected perpendicular to the slope were thought to be more efficient. Steel snow bridges were developed by the Austrian Alpine Montan Union in 1955 and were used for the first time at the protection site on the mountain Heuberg in Häselgehr, Austria (Figure 1.12).
Fig. 1.12 First avalanche defense structures in the starting zone with prefabricated snow bridges in steel, Heuberg, Häselgehr (© WLV Tyrol).
The first types of steel snow bridges were constructed with concrete foundations. This method carried high transportation costs since the concrete and steel parts had to be transported with a cable crane to the starting zone and from there with a narrow gauge railroad along the hillside to the construction site. To reduce costs, tests were made with anchors and blasting anchors. Nowadays foundations consist of ground plates for the supports and micropiles or anchors for the girders (Section 5.2.3).
Already in 1955, the first edition of the ‘Swiss guideline for defense structures in avalanche starting zones’ was published [50, 51]. After some revisions, the Swiss guidelines [163] have become the technical reference for supporting structures in many countries. Technical measures that influence the distribution of snow in or around the starting zone were also investigated. In Switzerland, the first snowdrift measures were erected in 1908 at the avalanche site Faldumalp in Valais, where rock fill walls were used to influence the snow distribution within the starting zone. In Austria, wind roofs and wind baffles were erected at the avalanche site Heuberg in Häselgehr (Sections 5.2.4.3 and 5.2.4.4).
If construction of protection measures in the starting zone was not feasible because of costs or unfavourable conditions for foundations, technical defense was implemented along the path or runout zone, usually in the form of deflecting or retarding dams or walls. Historical retarding mounds are visible along the Penzenlehner and Arzleralm avalanches above Innsbruck (Tyrol, Austria). After two severe avalanche cycles in Austria in 1935 and 1951, different kinds of retarding measures (concrete splitting wedges, earth or stone masonry retarding mounds) were erected on the mountain Nordkette near Innsbruck (Tyrol) to reduce the runout distance of the avalanches that endangered the town.
Many of these historical protection measures are still used today, which in part indicates that the basic principles of technical avalanche defense have not deviated far from the original principles introduced in the middle of the 20th century.
Patrick Nairz, Art Mears, Siegfried Sauermoser, Karl Gabl, Markus Stoffel and Stefan Margreth
An avalanche is a rapid flow of snow, ice, rock or mud along a slope with a volume of more than 100 m3 and a path length of more than 50 m. Depending on the material involved, it is classified as a snow, ice, rock or mud avalanche. The term avalanche implies the whole process of movement, from release in the starting zone to displacement (flow) through the path and deposition in the runout zone. The runout zone is an area with an average inclination less than 10°, that is where the inclination is insufficient for further movement of snow.
The load on the snowpack may be caused by gravity, in which case failure results either from weakening in the snowpack or from increased load from precipitation, or from other external factors, for example skiers, snowmobilers or explosives. Slow movements of the snow cover (on a scale of mm to m per day) are described as snow creep or snow glide.
Precise classification of avalanches is necessary for standardized reporting and analysis, however there are several classifications. The first attempts at avalanche classification were simple schemes aimed at alpinists. Flaig [58] developed the avalanche table that distinguished dry-snow avalanches, wet-snow avalanches and ice avalanches, and Zsigmondy & Paulke [286] distinguished dry and wet new-snow avalanches and old-snow avalanches.
The first detailed avalanche classification was published by the UNESCO in 1981 (Avalanche Atlas) [273]. The purpose of this was to observe and record avalanche characteristics in a condensed form suitable for the evaluation of statistical and physical principles governing these phenomena. A secondary purpose was to enable users to describe an observed or expected avalanche in simple terms, easily understood by others familiar with the classifications. This classification defines genetic (i.e. the origin or precipitating conditions) and morphologic features of an avalanche (Table 2.1) and this classification is still commonly used for avalanche reporting.
Table 2.1Morphological avalanche classification (Avalanche Atlas, 1981 [273], modified in ONR 24805 [202]): the classification is based on the manner of starting, form of movement, form of the path, and manner and form of deposition
Zone
Criterion
Alternative characteristics
Starting zone
Manner of starting
Starting from a point (loose-snow avalanche)
Starting from a line (slab avalanche) (
Figure 2.1a
,
2.12
)
Position of sliding surface
Within snow cover (surface-layer avalanche)
On the ground (full-depth avalanche)
Liquid water in snow
Absent (dry-snow avalanche)
Present (wet-snow avalanche)
Avalanche path
Form of path
Open slope (unconfined avalanche)
Gully or channel (channelled avalanche)
Form of movement
Snow dust cloud (powder snow avalanche) (
Figure 2.1b
)
Flowing on ground (flow avalanche)
Mixed (mixed avalanche)
Runout zone
Surface roughness of deposit
Coarse (>0.3 m) (coarse deposit)
Fine (<0.3 m) (fine deposit)
Liquid water in snow debris at time of deposition
Absent (dry avalanche deposit)
Present (wet avalanche deposit)
Contamination of deposit
None apparent (clean avalanche)
Present (contaminated avalanche)
Fig. 2.1 Types of avalanches: (a) Fracture of a slab avalanche and (b) powder snow avalanche in motion (© WLV Tyrol)
A minimum inclination for snow accumulation and avalanche formation is approx. 30°. At slopes with inclination higher than 50° sufficient snow accumulation is not likely. In the Alps, most avalanche starting zones are situated above the potential timberline, which is between 1800 and 2000 m depending on the position within the Alps (inner or periphery regions). The actual timberline is on average 200–400 m lower because of historical clear-cutting practices in subalpine areas for Alpine pastures [63]. Uniform and trough-shaped slopes promote snow accumulation and avalanche formation. The inclination of slopes above the tree line in typical glacial-formed alpine valleys is generally between 30° and 50°, thus typical for starting zones. Heavier precipitation in periphery areas of the Alps causes higher avalanche activity than in drier, inner alpine regions.
Fig. 2.2 Typical avalanche path in Alpine valleys (© Sauermoser)
Frequent avalanche activity seems to occur in so-called avalanche cycles in which widespread areas are affected concurrently by avalanches due to the spatial extent of precipitation events. Weather conditions conducive for avalanche formation mostly concentrate on the northern or southern regions of the Alps. The avalanche cycle in 1999, for example, covered only the northwestern part of the Austrian mountains, and the avalanche cycle in 1954 covered a small area in the westernmost part of Austria.
To a large degree, avalanches occur because of extreme weather phenomena (snowfall, temperature, wind). Strong snowfalls, warm and cold air occurrences, strong winds, on both the northern and southern sides of the Alps, are caused by specific weather conditions.
Depression areas, which determine the weather in Europe, mainly occur over the Atlantic Ocean and travel westward in the direction of the continent. Because of the temperature inversion in the valleys, the temperature in the high mountain areas cannot be calculated directly based on the valley temperatures. If the Alpine area is located at the edge of a high-pressure area over Russia, high-altitude continental cold air flows into the Alpine area. When snow volumes are low, these weather conditions lead to the formation of depth hoar. However, if the Alpine area is being affected by a high-pressure area centred over the Alps or Mediterranean, the eastern Alps will be influenced by tropical warm air.
Especially during the winter, warm fronts carry intensive precipitation with them, because, physically speaking, warmer air also can absorb more moisture than colder air. After a cold front has passed through, the current often turns from northwest to north. This results in strong build ups of precipitation along the northern reaches of the Alps.
The following weather conditions are relevant in the Alps in connection with the formation of avalanches.
Between a depression area reaching from Poland to Northern Scandinavia and a high-pressure area over the Atlantic (Azores to the Gulf of Biscay), there forms a northwestern upper airflow in the Alps. While the depression area on the western side guides the Arctic cold air from the Arctic Ocean in the direction of central Europe, the high-pressure area over the Atlantic distributes heat and, especially, moist air from the northwest to the Alps. Cold and warm fronts (Figure 2.3) occur at the border of these air masses.
Fig. 2.3 Northwestern area on 20.02.1999 (500 hPa area) (© ZAMG, EZMWF)
The northwestern current results in a lifting on the northern rim of the Alps, and buildup of the air in the Alps. Under these conditions, the northern borders of the Alps and the buildup areas experience large amounts of new snow. The Alpine regions receive smaller amounts, and it is predominantly dry on the southern side of the Alps (the north-foehn side). Under such weather conditions, 222 cm of new snow fell in Rauz in Arlberg within 2 days on January 1954 (125-year event), or in Galtür in February 1999, 254 cm in 10 days (300-year event).
The large amounts of new snow in the Alpine region from the northwestern areas are due to warm fronts and vorticity. Warm air from the northwest glides on the cold air, and, furthermore, turbulences in the northwestern current result in heavy precipitation caused by the strong vertical movement. Strong snow drifting can be deduced from the weather maps (isobars).
Depression areas with connected fronts pass the Alps from west to east. The quick interchange of warm and cold fronts results in larger temperature fluctuations.
As already mentioned, the strong lifting effects in the atmosphere are responsible for the heavy precipitation. Besides the described lifting in the mountains (damning up), on the fronts (frontal lifting), as well as upgliding warm air, the strong high-altitude current (jet stream) and turbulence formation (vorticity) play a major role. During cases of heavy precipitation during the winter, the presence of the jet stream must be taken into account when seeking meteorological advice. Turbulence in the atmosphere may occur due the velocity differentiation in a large current area, and result in the air mass either lifting or sinking. By evaluating the numerically calculated turbulence size, significant liftings – at least in large areas – can be recognized and used to forecast extreme snowfall.
As opposed to the northwestern area when the depression is located in Eastern Europe, for a southern current the controlling depression area is located above Western Europe. In Figure 2.4