Geomorphology Of Desert Dunes
By Nicholas Lancaster
London and New York
Synopsis
Sand dunes cover up to a quarter of many desert regions. Understanding the factors that determine their shape, size and dynamics is important to an understanding of many aspects of the geomorphology and ecology of desert regions, as well as to resource management in these areas. This book offers a comprehensive understanding of how desert dunes are formed, how they change and their environmental importance. Analyzing dune types, patterns, sand seas and sediments, and dune dynamics and processes at different temporal and spatial scales, the author draws on extensive research from the deserts of South Africa, North America, India, Northern Europe and Australia. Investigating the role of climatic change,The Geomorphology of Desert Dunesconcludes that a better understanding of dune processes and dynamics is vital for effective and appropriate mitigation of environmental problems in arid regions.
Excerpt
This book is a personal view of the geomorphology of desert dunes that has developed from my research on dune sediments, processes, and dynamics over the past two decades. This work has been made possible by research support from a variety of organizations, including the Royal Geographical Society, the Royal Society, and the National Geographic Society; the Universities of Cambridge, Witwatersrand, and Cape Town; the Transvaal Museum; the Foundation for Research Development (South Africa); NASA, NATO, and the National Science Foundation.
In the course of my work, I have interacted with many individuals who have provided assistance and discussion of problems and results. I thank them all for their company and time. I would like to single out A.T. (Dick) Grove (University of Cambridge), Mary Seely (Desert Ecological Research Unit of Namibia), Ron Greeley (Arizona State University), Gary Kocurek (University of Texas at Austin), and Bill Nickling (University of Guelph) as individuals who have made major personal contributions to my thinking on aeolian processes and dunes.
Many individuals have helped me in producing this book. In particular, Dale Ritter of the Quaternary Sciences Center, Desert Research Institute, provided institutional support; Rob Bamford did all the drafting for the figures; and Liane Alessi assisted with proof reading.
CONTENTS
List of figures viii
List of plates xiv
List of tables xvi
Preface xvii
Acknowledgements xviii
1 INTRODUCTION 1
Problems and challenges in dune geomorphology
Development of modern dune studies 4
Aspects of dune studies 6
Contemporary paradigms for dune studies 7
2 SAND TRANSPORT BY THE WIND 10
Introduction 10
The surface wind 11
Entrainment of sand by the wind 15
Saltation processes 23
Sand transport equations 27
Controls on sand transport rates 31
Wind ripples 36
Conclusions 42
3 DUNE MORPHOLOGY AND MORPHOMETRY 44
Introduction 44
Dune classifications 45
Aeolian bedform hierarchies 48
Morphology and morphometry of major dune types 50
Conclusions 83
4 DUNE SEDIMENTS 85
Introduction 85
Dune sands 88
Sedimentary structures of major dune types 93
Grain size and sorting characteristics of dune sands
Interdune deposits 118
Conclusions 119
5 DUNE DYNAMICS 122
Introduction 122
Dune initiation 123
Airflow over dunes 127
Erosion and deposition patterns on dunes 146
Relations between erosion and deposition patterns and winds 154
Development of dune profiles 157
Conclusions 162
6 CONTROLS OF DUNE MORPHOLOGY 164
Introduction 164
Factors that may determine dune morphology 164
Controls on dune size and spacing 179
Aerodynamic models for dune spacing 185
Development of compound and complex dunes 190
A general model for dune size and spacing 193
Controls on dune alignment 195
Conclusions 197
7 SAND SEAS 198
Introduction 198
sand sea accumulation 198
Dune patterns in sand seas 209
Grain size and sorting patterns in sand seas 222
Modes of sand sea accumulation 225
Conclusions 226
8 PALAEOENVIRONMENTS AND DUNES 228
Introduction 228
Dune systems and palaeoclimates 230
Palaeoclimatic information from dune systems 237
Interdune deposits 243
Effects of climatic changes on sand sea and dune development 245
Conclusions 254
9 REVIEW AND PROSPECT 255
Conclusions 256
References 258
Index
PLATES
1 Landsat image of the central parts of the Namib Sand Sea 3
2 Oblique aerial view of compound crescentic dunes in the Namib SandSea. Note two sets of dunes: the main forms and the superimposed forms 4
3 Landsat image of the Gran Desierto Sand Sea of northern Mexico 5
4 The Kelso Dunes aeolian sediment transport system. Sediment istransported from fluvial source areas to the west at the terminal fan-deltaof the Mojave River toward the depositional sink, Kelso Dunes 8
5 Wind ripples:
(a) Ripples in medium-fine sand in the Gran Desierto Sand Sea, Sonora,Mexico 38
(b) Granule ripples on the Skeleton Coast, Namibia 38
6 Simple crescentic dunes:
(a) Barchans on the upwind margin of the Skeleton Coast dune field,Namibia 54
(b) Simple crescentic ridges, Skeleton Coast dune field, Namibia 55
7 Compound crescentic dunes in the Algodones dune field, California 57
8 Simple linear dunes: 59
(a) Vegetated linear dunes in the northern Gran Desierto, Mexico 60
(b) Sinuous crested dunes in the northern Namib Sand Sea 60
9 Compound linear dunes in the southern Namib Sand Sea 64
10 Complex linear dunes in the northern part of the Namib Sand Sea 66
11 Star dunes: (a) At Sossus Vlei, Namibia; (b) Gran Desierto 72
12 Parabolic dunes on the Snake River Plains, Idaho 77
13 Nebkha anchored by !nara in the Kuiseb delta region, Namibia 78
14 Zibars between linear and crescentic dunes, Algodones dune field,California
15 (a) Sand sheet developed in coarse, poorly sorted sand, Elizabeth Bay,Namibia 81
(b) Sparsely vegetated sand sheet in the northern Gran Desierto Sand Sea 81
16 Climbing and falling dunes:
(a) Climbing dunes in the Mojave Desert 82
(b) Falling dune: the Cat Dune, Mojave Desert 82
17 Primary sedimentary structures in dunes:
(a) Wind ripple laminae on linear dune plinth 86
(b) Grainflow cross strata 86
(c) Grainfall laminae and grainflows, Great Sand Dunes, Colorado 86
18 Sand sheet structures in the northwestern Gran Desierto 103
19 Sediments of dry interdune area in the Namib Sand Sea dominated bywind ripple laminae 119
20 Late Pleistocene west interdune (seasonal or ephemeral lake) in theNamib Sand Sea 120
21 Dune lee side processes:
(a) Avalanching 138
(b) Lee side flow separation 138
(c) Along dune transport reworking grainflow deposits 139
22 Examples of relict, dormant, and active dunes at Kelso Dunes,California: (a) Active dunes, (b) Dormant dunes (last active 150 yr ago),and (c) Relict dunes (stabilized 3.5 to 4 ka) 232-3
23 Landsat image of relict linear dunes in the western Kalahari in Namibia
TABLES
2.1 Typical values of aerodynamic roughness (z0) for desert surfaces 13
3.1 Morphometry of simple crescentic dunes 56
3.2 Morphometry of compound crescentic dunes 58
3.3 Morphometry of linear dunes 61
3.4 Morphometry of star dunes in different sand seas 73
4.1 Terminology of graphical statistics of grain size and sorting 105
4.2 Comparative grain size and sorting for different dune types in varioussand seas 105
6.1 Orientation of dunes with respect to resultant and gross bedform-normal transport directions 194
7.1 Distribution of major dune types in Saharan sand seas
PREFACE
This book is a personal view of the geomorphology of desert dunes that has developed from myresearch on dune sediments, processes, and dynamics over the past two decades. This work hasbeen made possible by research support from a variety of organizations, including the RoyalGeographical Society, the Royal Society, and the National Geographic Society; the Universities ofCambridge, Witwatersrand, and Cape Town; the Transvaal Museum; the Foundation forResearch Development (South Africa); NASA, NATO, and the National Science Foundation.
In the course of my work, I have interacted with many individuals who have provided assistanceand discussion of problems and results. I thank them all for their company and time. I would liketo single out A.T. (Dick) Grove (University of Cambridge), Mary Seely (Desert EcologicalResearch Unit of Namibia), Ron Greeley (Arizona State University), Gary Kocurek (University ofTexas at Austin), and Bill Nickling (University of Guelph) as individuals who have made majorpersonal contributions to my thinking on aeolian processes and dunes.
Many individuals have helped me in producing this book. In particular, Dale Ritter of theQuaternary Sciences Center, Desert Research Institute, provided institutional support; RobBamford did all the drafting for the figures; and Liane Alessi assisted with proof reading 212
INTRODUCTION
Quartz sand dominates aeolian deposits in most arid regions, as dust-sized particles are carriedout of the desert entirely or are trapped by vegetation or rough surfaces on desert margins (Tsoarand Pye 1987). The greater part (>95 per cent) of this sand occurs in accumulations known assand seas or ergs (Wilson 1973) that comprise areas of dunes of varying morphological types andsizes, as well as areas of sand sheets. Smaller dune areas are known informally as dune fields.Major sand seas occur in the eastern hemisphere arid zones of the Sahara, Arabia, central Asia,Australia and southern Africa, where they cover as much as 45 per cent of the area classified asarid (Figure 1.1). In North and South America there are no large sand seas and dunes cover lessthan 1 per cent of the arid zone.
PROBLEMS AND CHALLENGES IN DUNEGEOMORPHOLOGY
Plate 1 shows a Landsat image of the Namib sand sea, in southwestern Africa. Even with aground resolution of 80 m per pixel, this image shows a wealth of information about dune formand patterns, and raises many fundamental questions about the geomorphology of the area.
Dominating the image are large south-to-north oriented linear dunes. Why are these dunesoriented in this direction, and what factors determine their size and very regular spacing? Whatdetermines the very different linear dune orientation to the east of the sand sea? Dune typechanges from crescentic through linear to star dunes from the coast to the inland edge of thesand sea. Why does this occur, and what factors determine dune type? Other questionsimmediately arise. What are the relations between the dunes and the river valleys that enter thesand sea from the east and how has the sand sea reacted to changes in sea level and climate? Acloser aerial view of some of the coastal crescentic dunes (Plate 2) reveals further questions: whyare there two sets of dunes: the main crescentic ridges and the smaller superimposed dunes?
On the same scale, the Landsat image of the Gran Desierto sand sea in northern Mexico (Plate 3)shows a much smaller sand sea, but one with a very
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