Dunes

Acknowledgements xv

Introduction 1

Part One <10 m2; <10 years 5

1 Wind and Sand 7

Wind versus Bed 7

The Law of the Wall 8

Improving the wind/bed model 9

Lift-Off 12

Holding down by gravity 12

Holding down by cohesion 12

Raising by lift 13

Raising by drag 13

Raising by bombardment 14

Thresholds 14

Grain size 16

The slope of the bed 17

The dynamics of water content 17

Crusts 19

Pellets 20

Sand in Motion 20

Saltation 20

Streamers and other medium-scale patterns of saltating sand 22Reptation 22

Creep 23

Other near-surface activity 23

Suspension 24

The vertical distribution of load and grain size 24

The saturation length 24

The fetch effect 26

The response of a loose bed to erosion by the wind 27

The Transport Rate 27

Shapes, densities and mixtures of size 29

Hard surfaces 30

Rough surfaces 30

Moisture, temperature and humidity 31

Rain 31

References 31

2 Ripples 32

Subtypes 35

Models 36

Flow response 36

Gravity wave 36

Saltation length 37

Shadow zone 37

Mathematical 37

Pattern 38

3 The Form and Behaviour of Free Dunes 39

Definitions 39

Early Stages 39

Start 39

Minimum size 40

The Profile of a Fully Grown Dune 41

Toe 41

Windward slope (or ?stoss slope?) 43

Crest 45

Lee slope 46

Movement 53

Turnover time, bulk transport 56

Size 56

Flow-hierarchy models 57

Grain-size models 57

The time/supply model 58

References 58

Part Two 1000 to 10,000 m2; 100 to 1000 years 59

4 Pattern in Free Dunes 61

Definitions 61

Wind-Directional Regimes 62

Global winds 62

Local wind systems 62

The Classification of Wind-Directional Regimes 65

Wind-Directional Regimes and Dune Pattern 66

Transverse Dunes 66

Two-dimensional pattern: vertical and downwind 67

Two-dimensional pattern: horizontal and transverse to the wind 68

Self-organisation 69

Barchans 71

Quasi-transverse patterns 75

Linear Dunes 80

Introduction 80

Models of formation 82

Sand Sheets 88

Dunes with Distinctive Sand 90

Gravel dunes 90

Zibars 91

Clay dunes 92

Lunettes 92

Gypsum dunes 93

Diatomite sands 93

Volcanic sands 93

Snow and ice dunes 94

Niveo-aeolian deposits 94

References 94

5 Forced Dunes 96

Dunes Built around Bluff Obstacles 96

Climbing and echo dunes 96

Flanking and lee dunes 97

Cliff-top and falling dunes 99

Dunes on Gently Sloping Terrain 99

Reference 99

6 Dunes and Plants 100

Wind, Sand and Plants 100

Rigid objects 100

Spatial pattern 101

Porosity 102

Flexibility 102

Plants as living things 103

The broader time/space framework 104

Dunes among Plants 104

Nebkhas 104

Blowouts 107

Parabolic dunes 109

References 111

7 Coastal Dunes 112

Coastal Dunes and Climate 112

The Beach?Dune System 114

Exclusively Coastal Dunes 117

Embryo dunes 117

Fore-dunes (?frontal dunes? or ?retention ridges?) 118

Tsunamis 120

Coastal sand sheets 120

References 121

Part Three >0.3 mm; <2,200,000,000 years 123

8 Sand Seas 125

Terms 125

Large Sand Seas 127

Growth and Development 127

Sand Seas in Tectonic Basins 129

Topographically Unconfined Sand Seas 131

Transfer between Sand Seas 133

9 A History of Dune Sand 134

Provenance 134

Recycling 137

Maturation 139

Mineralogy 139

Size characteristics 141

Shape and surface texture 141

Redness 144

Relationships between Dune Fields and the Sources of Their Sand 145

Source-bordering dune fields 145

Dune fields that have migrated away from their source 145

Sand seas that have taken sand from many local sources 146

The Australian sand seas and some aeolian sandstones 146

References 146

10 A History of Inland Dunes 147

Very Ancient Dunes: Siliceous Windblown Sandstones 147

The Emergence of Familiar Spatial and Dynamic Patterns 151

Dune Historiography 153

Dating 153

Dune-building environments 156

The long-term development of sand seas: sediment state 160

Quaternary Dune-Building Climates 160

Dunes in the Early- and Mid-Pleistocene 162

Late Pleistocene Dunes 163

The main theatres of dune formation in the Late Pleistocene 165

Dunes in the Holocene 175

The deglaciated North 176

The mid-latitudes 177

The semi-arid tropics 178

The present deserts 178

References 179

11 A History of Coastal Dunes 181

Long Sequences 181

Sea Level 181

Other Controls 182

Calcareous Aeolianite 184

Reference 185

12 Mars, Venus, Titan 186

Similarities 186

Differences 187

Sand 188

Ripples and Transverse Aeolian Ridges 190

Dunes 191

Mars 191

Venus 194

Titan 194

Reference 195

Part Four Care 197

13 Local, Short-Term Care (<1000 m2; <10 years) 199

Dunes in Deserts 199

Folk science 199

New approaches 200

Stabilised Dunes in Semi-Arid Areas 204

Coastal Dunes 204

References 207

14 Sustainability (>100,000 m2; >10 years) 208

Constraints 208

Complexity 208

Uncertainty 210

Environmental change 210

Sustainability 211

Coastal dunes 211

Stabilised inland dunes 212

References 213

Index 214