Geomorphology of Desert Dunes

Nicholas Lancaster

• Publisher: Routledge
• Year: 2013
• Topic: Physical Sciences
• eBook ISBN:
  9781134927258
• Subtopic: Geography
• Index: Geography

How desert dunes are formed, how they change, their environmental significance and the role of climate change - these issues are examined through extensive case studies drawn from South Africa, India, Northern Europe and Australia.

1
Introduction

Quartz sand dominates aeolian deposits in most arid regions, as dust-sized particles are carried out of the desert entirely or are trapped by vegetation or rough surfaces on desert margins (Tsoar and Pye 1987). The greater part (>95 per cent) of this sand occurs in accumulations known as sand seas or ergs (Wilson 1973) that comprise areas of dunes of varying morphological types and sizes, 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 as arid (Figure 1.1). In North and South America there are no large sand seas and dunes cover less than 1 per cent of the arid zone.

Problems and Challenges in Dune Geomorphology

Plate 1 shows a Landsat image of the Namib sand sea, in southwestern Africa. Even with a ground resolution of 80 m per pixel, this image shows a wealth of information about dune form and 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 dunes oriented in this direction, and what factors determine their size and very regular spacing? What determines the very different linear dune orientation to the east of the sand sea? Dune type changes from crescentic through linear to star dunes from the coast to the inland edge of the sand sea. Why does this occur, and what factors determine dune type? Other questions immediately arise. What are the relations between the dunes and the river valleys that enter the sand sea from the east and how has the sand sea reacted to changes in sea level and climate? A closer aerial view of some of the coastal crescentic dunes (Plate 2) reveals further questions: why are 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 different set of dune patterns. Unlike the Namib sand sea, the Gran Desierto shows a series of discrete areas of different dune types. Again, the questions of controlling factors and formative processes arise, but with the additional consideration: why does this sand sea appear different from the Namib? Why do large star dunes occur next to small crescentic dunes? Is this an effect of dune size and process-form interactions? Do different dune areas represent multiple generations of dunes formed at different times and in different conditions, and therefore does the climatic, tectonic, and sea level history of this area play a role in determining dune patterns? What is the role of different sediment source areas and how have these changed over time?

These images point to some of the challenges in dune geomorphology today. Whereas we have quite a good knowledge of the main aspects of dune morphology and are beginning to understand the processes

Figure 1.1 Location of major sand seas and dune fields (after Thomas 1989).

that maintain different dune types, we still are a long way from understanding the fundamental controls of dune size and spacing, and the processes by which dunes and sand seas develop.

Plate 1 Landsat image of the central parts of the Namib Sand Sea.

Development of Modern Dune Studies

Early work on desert dunes was dominated by descriptions of dune form, based largely on exploratory investigations. Important contributions were made by Hedin (1903) in central Asia, Gautier (1935) and Newbold (1924) in the Sahara, Blandford (1876) in the Indian subcontinent, Thesiger (1949) in Arabia, and Passarge (1904) in the Kalahari. Following this exploratory phase, scientific investigations of desert regions by Capot-Rey (1947), Monod (1958), Beadnell (1910), and Bagnold (1933) in the Sahara; Madigan (1946) in Australia; Kaiser (1926) in the Namib, and Hack (1941) in the United States, produced a wealth of important information on the occurrence of dunes and their basic forms. Attempts to relate form to process were very few. One exception is the pioneering work of Cornish (1914), but it was not until the seminal work of R.A.Bagnold (1941) in the Egyptian desert that the relations between the characteristics of the surface wind and sand processes. transport processes emerged and were quantified.

In recent years, the focus of studies of dunes has changed radically. Three main developments have stimulated investigations of dunes: (1) Satellite images of desert regions have become widely available, focusing attention on the development of dune patterns and their relations to sediment sources and wind regimes; (2) Orbital images of Mars (Mariner and Viking missions) (Greeley, Lancaster et al. 1992) and Venus (Magellan) (Greeley, Arvidson et al. 1992) have shown that aeolian processes occur on other

Plate 2 Oblique aerial view of compound crescentic dunes in the Namib Sand Sea. Note two sets of dunes: the main forms and the superimposed forms.

terrestrial planets, and have resulted in sharpened concepts of the fundamentals of the physics of sediment transport by the wind; and (3) The discovery of oil and gas in Mesozoic sandstone in the western United States and northwest Europe has stimulated a series of detailed studies of aeolian sediments and sedimentary

Aspects of Dune Studies

Studies of dunes are concentrated in three main areas: (1) the description of dune forms and patterns, (2) analyses of dune sediments, and (3) investigations of dune dynamics and processes.

Description of dune forms and patterns

The starting point for all aspects of desert dune geomorphology is the identification and description of different dune types. Initially, descriptions of dunes were made in the course of ground investigations, often during the exploration of desert regions. Names for the forms (e.g. self, silk, barchan, zibar, etc.) were derived from the rich terminology for desert landforms employed by the local population (Bagnold 1951; Breed et al. 1979). Later investigations utilized aerial photography to great advantage in their descriptions (e.g. Wilson 1972). As a result, data on dune patterns and trends became more widely available. In the 1970s, the availability of satellite images of desert regions made it possible to study the patterns of dunes throughout sand seas with relative ease and led to the realization that basic dune forms in different sand seas are remarkably similar (McKee 1979a). The realization that dunes of similar characteristics occur in widely separated sand seas has focused attention upon the factors which control their morphology and morphometry.

Plate 3 Landsat image of the Gran Desierto Sand Sea of northern Mexico.

Studies of dune sediments

Studies of dune sediments have concentrated upon investigations of grain shape, colour and mineralogy; grain size and sorting characteristics; and sedimentary structures in dune sediments. Many investigations of dune sediments have been linked to attempts to positively identify sands in the rock record as aeolian and to characterize their depositional environments (Ahlbrandt 1979). This has frequently involved comparisons of aeolian sands with those deposited in marine, coastal, fluvial, or glacial environments (Mason and Folk 1958). However, recent work has shown that the use of textural parameters (grain size, sorting) is unreliable as an indicator of depositional environments. Consequently, attention has turned to studies of the variability of grain size and sorting parameters of dune sands over individual dunes and within sand seas and dune fields, as well as to the analysis of primary sedimentary structures.

Studies of sedimentary structures in dunes were pioneered by McKee and have provided much valuable information on the ways in which dunes accumulate (McKee 1966). However, the logistics of carrying out such studies in most desert regions have prevented their full potential from being realized. Recent work has been concerned with the identification and description of small-scale sedimentary structures associated with primary aeolian depositional processes as well as experimental and theoretical investigations of aeolian deposition (Hunter 1977).

Studies of dune processes

Studies of the physics of grain movement by the wind provide the basis of our knowledge of aeolian processes and sand transport rates. Many investigations are still strongly influenced by the seminal work of Bagnold (1941). In recent years debate on the nature of aeolian processes on other terrestrial planets, notably Mars and Venus, has led to a re-examination of the physics of grain movement and the nature of aeolian saltation. Consideration has been given to the effects of grain density and mineralogy and grain shape on sand transport rates.

Investigations of dune processes have concentrated on documenting the movement and dynamics of dunes in terms of airflow and sediment transport. Studies of the movement of dunes, especially those of barchan type, have had a long history. In recent years, there have been important advances in the knowledge of dune processes through careful study of winds and sand movements on individual dunes (e.g. Tsoar 1978; Havholm and Kocurek 1988; Lancaster 1989a; Livingstone 1989). These studies have given rise to a new understanding of the factors which influence dune dynamics and morphology of barchan and linear dunes, and demonstrate the importance of secondary airflow in controlling dune morphology.

As observed by Warren (1969), investigations of dunes are geographically clustered. In part there is also a temporal pattern. For example, field investigations of the Saharan sand seas, mostly by the French, peaked in the 1950s and 1960s. Since then there have been few field studies, but many investigations using remote sensing imagery. In recent years the best-studied sand seas and dune fields have been in Australia (especially the Simpson-Strzelecki), the Namib and Kalahari in southern Africa, the Thar Desert of India, and the deserts of the USA. Many Middle Eastern and Asian sand seas are almost unknown to modern investigators, except through remote sensing investigations.

Contemporary Paradigms for Dune Studies

Today, the combination of the regional perspective of remote sensing data with detailed field studies has resulted in a new set of paradigms that emphasize studies of dune dynamics and processes on different temporal and spatial scales, in contrast to the descriptive approach employed by many earlier workers. Valuable perspectives are also coming from the realization that Quaternary climatic and sea level changes have had a major effect on the accumulation of many sand seas.

Dune-forming processes operate at three main spatial and temporal scales (Warren and Knott 1983) which correspond approximately to the steady, graded and cyclic time scales of Schumm and Lichty (1965). Processes operating at the steady scale, which involves very short or even instantaneous amounts of time and small areas, include the formation of wind ripples. The graded scale involves periods of 10⁻¹ to 10² years, and particularly concerns the dynamics and morphology of dunes, which tend towards an actual or partial equilibrium with respect to rates and directions of sand movements generated by surface winds. Form-flow interactions and feedback processes are important at this scale, which is probably the most important for determining the morphology and dynamics of dunes. The cyclic time scale involves periods of 10³–10⁶ years and a spatial scale corresponding to that of sand seas and their regional physiographic and tectonic setting. Processes at this scale involve those responsible for the accumulation of sand seas as sedimentary bodies.

At all scales of investigation, aeolian processes operat...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• CONTENTS
• List of figures
• List of plates
• List of tables
• Preface
• Acknowledgements
• 1 INTRODUCTION
• 2 SAND TRANSPORT BY THE WIND
• 3 DUNE MORPHOLOGY AND MORPHOMETRY
• 4 DUNE SEDIMENTS
• 5 DUNE DYNAMICS
• 6 CONTROLS OF DUNE MORPHOLOGY
• 7 SAND SEAS
• 8 PALAEOENVIRONMENTS AND DUNES
• 9 REVIEW AND PROSPECT
• References
• Index