Field experiments and numerical modelling of mass

8,61

MB

entrainment and deposition processes in snow avalanches

201

stron

6392

ID

Swiss Federal Institute of Technology Zurich

2004

rok

Contents

Contents i

Abstract iv

1 Introduction 1

1.1 History and motivation 1

1.2 Avalanche classi¯cation and de¯nitions 4

1.2.1 Avalanche characterization in the release zone 5

1.2.2 Avalanche characterization in the °owing zone 7

1.2.3 Avalanche characterization in the deposition zone 11

1.3 Avalanche hazard maps in Switzerland 13

1.4 Mathematical modelling of dense avalanches 15

1.5 The importance of the avalanche mass balance 20

1.6 Goals and outline of the study 23

2 Avalanche investigation techniques 25

2.1 Introduction 25

2.2 Determining avalanche mass evolution 26

2.2.1 Conceptual model of data collection in the ¯eld 26

2.2.2 Measurement techniques 30

2.2.3 Field measurement methods and their use 32

2.2.4 Photogrammetry and ¯eld measurements 35

2.2.5 Digital orthophotos and ¯eld measurements 39

2.3 Internal avalanche processes 39

2.3.1 Flow depth sensors 40

2.3.2 FMCW radar 42

2.4 Avalanche-snow cover interaction 44

2.4.1 FMCW radar to measure entrainment location and rate 44

2.4.2 Field measurements to de¯ne mechanical properties of the snow cover 45

2.4.3 Seismic signals 46

3 Field experiments and observations 49

3.1 Avalanche indices and general de¯nitions 49

3.2 Experimental sites and other data sources 54

3.2.1 The Pizzac avalanche test site: instruments and methods 54

3.2.2 The Vall¶ee de la Sionne avalanche test site: instruments and methods 56

3.2.3 Data from the catastrophic winter 1998/99 57

3.2.4 Other avalanches 61

3.3 Avalanche experiments at the Monte Pizzac test site. 61

3.3.1 Mass balance analysis 61

3.3.2 Avalanche mass distribution 72

3.3.3 Discussion of experimental survey 75

3.4 Avalanche experiments at the Vall¶ee de la Sionne test site 78

3.4.1 Avalanche mass 78

3.4.2 Entrainment location and entrainment rate 84

3.4.3 Discussion of experiment results 89

3.5 Further investigated avalanches of the catastrophic winter 1998/99 91

3.5.1 The Obergoms avalanches 91

3.5.2 The Obergoms avalanche data 91

3.5.3 The Aulta avalanche 96

3.5.4 The Aulta avalanche data 97

3.5.5 Discussion of survey results 100

3.6 The Braemabuel avalanche 104

3.6.1 Avalanche data 104

3.6.2 Braemabuel avalanche mass balance 107

4 A theory of mass entrainment and deposition 109

4.1 Conditions for entrainment and deposition 109

4.2 Basic physical processes in snow entrainment and deposition 115

4.2.1 The snow cover strength 115

4.2.2 Entrainment at the avalanche front 121

4.2.3 Entrainment along the avalanche basal surface 122

4.2.4 Deposition at the avalanche tail 126

4.2.5 Summary of the basic entrainment processes 126

4.3 A theory of entrainment 126

5 Modelling dense snow avalanche °ow with entrainment 131

5.1 A one dimensional depth-averaged model with entrainment 131

5.1.1 The Voellmy-°uid model 133

5.1.2 The Norwegian NIS model 135

5.1.3 Numerical implementation 136

5.2 Model sensitivity 139

5.2.1 E®ect on critical avalanche quantities 143

5.3 Model validation 149

5.3.1 The Braemabuel avalanche 150

5.3.2 Small avalanche: Pizzac simulations 154

5.3.3 Large avalanche: Vall¶ee de la Sionne simulations 159

5.3.4 The extreme avalanches of the winter 1998/99 163

6 Conclusions 171

Notations 175

Acknowledgements 181

Bibliography 182

Curriculum Vitae 190