Mankind and Deserts 3

Wind in Deserts and Civilizations

Fernand Joly, Guilhem Bourrié, Guilhem Bourrie

• Publisher: Wiley-ISTE
• Year: 2021
• Print ISBN:
  9781786306326
• eBook ISBN:
  9781119823988
• Edition: 1
• Topic: Biowissenschaften
• Subtopic: Ökologie

The wild beauty of deserts has always been a source of fascination the world over. Mankind and Deserts 3 – the third and final volume – focuses on wind, frequently present in all deserts, either hot or cold. Wind plays a major role in aridity and landscapes bear numerous forms due to its action, erosion, transportation and surface formations, some discreet and others spectacular, such as vast expanses of towering yardangs. Aeolian dynamics lead to dune formation, simple or associated with sand ridges or ergs, as in the Sahara. Mankind has attempted, to varying degrees of success, to cope with sand accumulation; ignoring aeolian dynamics has led many development projects to failure. This is developed by Yann Callot, a Professor at Lyon University who studied aeolian dynamics in the Sahara.

Traditional societies have adapted to live in deserts, establishing vibrant civilizations with original ways of living, managing water resources and creating routes for trade, especially for salt. In a changing environment, useful lessons can be drawn from the genius of mankind?s adaptation to such diverse and fragile environments. This is explained by Marc Côte, who was a Professor at Constantine (Algeria) and Aix-en-Provence Universities.

From ancient, almost mythical, exploration to modern scientific studies, deserts have come to be better known yet still hold great appeal. This book traces the history of their knowledge while providing a basis for understanding their features and the tools needed for their protection, in an ever-changing world.

1
Aeolian Landforms in Deserts

1.1. Forms and surface states

The wind generates specific forms. Their features are often related to the processes that created them. Starting from the postulate that transport itself does not create forms, we can make a distinction between two types: those related to the action of the transported material (ablation forms) and those produced by the accumulation of this material.

The term “forms” or “landforms” will be used in a broad sense here to resolve one of the major problems encountered in earth sciences: the impossibility of classifying the objects being described into species, as in the biological sciences. This implies that the continuum of these objects will be broken up into “forms” (whenever they can be characterized). Particularly in the study of “aeolian processes”, the ubiquity of wind action on the earth’s surface, and the absence of any hierarchy of the forms through the convergence of elementary objects (as is the case in hydographic networks, organized by gravity flow), leads to the study of objects of such varying sizes that not all of them can be categorized into relief forms.

These small objects do, however, play an essential role through their number and how they are organized, often covering vast stretches without any notable thickness, which implies that they cannot be characterized by volume.

This has led to researchers prioritizing the concept of a “surface state”, a highly rigorous concept used by researchers studying the physical chemistry of the atmosphere in order to characterize the mobilization capacities of material on the soil surface. This characterization is sometimes based on a geomorphological approach that makes it possible to model the ruggedness of the desert surface. The model that is thus obtained is itself applied to other models for the creation of desert aerosols, greatly improving the quality of the results (Marticorena et al. 1995; Callot, Marticorena, and Bergametti 2000).

This distinction between surface states and landforms makes it possible to be more rigorous in considering the variety of wind effects, while respecting the varying scales across which this is observed: quite often an aeolian landform will be covered by one or more surface states; on the other hand, “aeolian” surface states may exist without being overlaid onto aeolian landforms.

1.2. Ablation forms

Ablation forms are much larger due to the vast stretches of surface states that are generated by ablation, rather than the landforms themselves, which actually cover a more limited area on the scale of global deserts.

1.2.1. Desert pavement

A desert pavement is a surface state that affects a variety of substrates: hard or soft rock, superficial formation.

This feature is sometimes confused with a deflation pavement, but the term must be used with caution as it designates a single, unique cause to the formation of the pavement. While wind is the dominant process at present in arid and semi-arid regions, many other processes can, or could have, contributed to the formation of such pavements or could even have been the principal cause of their formation. These include weathering, which prepares the material for the wind, and fluviatile action, which could have brought in the constituent elements of the pavement and which is often responsible for the flattening of surfaces and the spreading out of material. Observations in the Mojave Desert (Wells, Dohrendwend, et al. 1985) even indicate that certain pavements may have resulted from an accretion process: a pavement of volcanic debris was separated from its original flow by a meter of windblown sediments. The sediments were only able to be deposited by the “infiltration” of these sediments between the constituent elements of the pavements, and then by the coarser sediment rising up through the windblown material, like pebbles and blocks rise to the surface of a field.

Figure 1.1. Reg. For a color version of this figure, see www.iste.co.uk/bourrie/deserts3.zip

There are three names for this pavement, depending on the size of the constituent material:

• – the reg (Figures 1.1(a) and 1.1(b)) is the best-known. It is made up of a series of blocks or pebbles, which are more or less closely arranged, with dimensions varying between a few centimeters and, sometimes, 1 or 2 dm. A reg may have a hydrological origin, especially along a visible slope, but the aeolian dynamic cannot, of course, be excluded. This origin may be verified based on the shape of the constituent elements: if they are rounded, a fluviatile origin may be assumed, unless the substrate is made up of puddingstone. But we must then study the processes that lead to the release of these elements, as fluviatile pebbles may have been released by aeolian processes. On certain surfaces that are almost perfectly flat, like the large hamadas in the Northern Sahara, or the vast Central Saharan plains, the absence of any slope makes it possible to state that the wind plays the dominant role in these formations;
• – a serir (Figure 1.2) is less well-known and is made up of much smaller constituent elements than a reg, their dimension being smaller than 5 cm. Often rounded by the wind and/or water, these elements are also more or less spaced out. Although it is made up of smaller elements, the serir may also offer remarkable continuity and can look identical for over a thousand square kilometers, without the causes leading to these formations being clearly identified;
• – the granulometric continuum of elements covering desert surfaces makes it difficult to define an exact line separating serirs from surfaces made up of large sand grains that have undergone little or no displacement by the wind, and wh...

Table of contents

• Cover
• Table of Contents
• Title Page
• Copyright
• Foreword
• Introduction: Aeolian Dynamics and Processes
• 1 Aeolian Landforms in Deserts
• 2 Humans and Winds in Deserts
• 3 Living in Deserts
• List of Authors
• Index
• End User License Agreement