Introduction: Aeolian Dynamics and Processes

I was in a world of stark, pure sand dunes stretching away to infinity on either side in strict geometric forms, somewhat affected by the winds but only inasmuch as the same shapes were re-formed (from that point all is calm).

Jean Giono, "Noé", in OEuvres romanesques complètes tome III, R. Ricatte (ed.), Gallimard, Paris, 1974, p. 839.

The action of the wind is perceived to be so omnipresent and preponderant in arid regions that it is still sometimes considered to be even more emblematic of deserts than dryness. Is this why there are so many errors and false information around wind and its best-known manifestation, the dunes? The uniqueness of wind-formed shapes and wind action is the most likely reason for this fascination, leading to some researchers (sometimes quite influential ones) succumbing to the mirage of the wind and overestimating its actions. As we will see, however, the facts are more complex: striking effects are not necessarily synonymous with effectiveness.

Aeolian processes ("aeolian" comes from Aeolus, the Greek god of the wind) are among the most original geomorphological processes in the world. The reason behind this is simple: the circulation of material through the air partially frees the wind, especially over a medium scale, from topographical constraints (Figure I.1). The wind thus enjoys relative freedom vis-à-vis gravity and, therefore, vis-à-vis pre-existing slopes and relief. For example, along with glaciers, it is the only entity capable of moving material upwards, as certain developers have learned the hard way!

Figure I.1. A sandy spit that curves around relief in the SE of the Tibesti range, in a region with almost unidirectional wind (image from Google Earth). For a color version of this figure, see www.iste.co.uk/bourrie/deserts3.zip

Given the unique feature of wind actions, we will proceed as follows: we will first more closely examine the conditions and processes that make it possible to form and, sometimes, remake wind formations or aeolian landforms; we will then take a differentiated approach based on the environment and associated formations. Finally, we will conclude with a few representative examples of the interactions (often disastrous!) between the winds and humans.

I.1. Conditions and environments in which wind acts

I.1.1. Where is the wind effective?

The wind is undeniably unique: it blows across the entire surface of the earth, with varying intensity that is dependent on a number of criteria. These differences can be observed on a global scale. For example, the vast stretches of the Southern oceans are swept by atmospheric depressions that are specific to certain Southern latitudes: the Roaring Forties and Furious Fifties, which are much sought after in sailing races. On the continental level, there are calmer regions, such as central Siberia. The great Canadian north, which is less cold than Siberia, is sometimes considered to have a more difficult winter as it has more violent winds. There may also be marked differences at a local level. A remarkable example of this is the rhodanian corridor: when the northern mistral blows, the Rhone valley, oriented N-S, or the plateaus that ring it, may be battered by an intolerable mistral, while tributary valleys (such as those with rivers that descend from the Cévennes), which are oriented W-E, experience an almost summery micro-climate.

The omnipresent nature of the wind may lead us to conclude that aeolian landforms exist on all continents. However, this is not the case in reality except over very small areas. Indeed, a few conditions must be met in order for wind dynamics to be effective:

• 1) Strong and frequent winds are a favorable condition, but not essential. If all other conditions are satisfied, even weak winds can create large aeolian landforms. However, there is a threshold velocity for aeolian erosion below which the wind cannot displace any material. The displacement threshold for sandy materials can vary according to local conditions and, notably, depending on the size of the material being transported. However, it most often lies between 4.5 and 6 m/s, at a height of 10 m. This will be examined in greater detail when we study the quantities of sand that can be displaced by the wind.
• 2) However, this first condition is not sufficient. In many regions, the wind by itself is not very effective on firm land except during exceptional storms, such as the storms that hit France in December 1999 or Cyclone Xynthia in 2010. It is the substances themselves, which are transported within very precise granulometric limits, that carry out significant morphogenic actions for accumulation and ablation. However, the wind must find available material to transport. Materials that can be picked up and carried are sand, dust (see below), and also volcanic ash, as is sometime seen in Iceland.
• 3) The previous condition presupposes the existence of at least some portion of bare ground. It thus implies the absence, or at least discontinuity, of plant cover, which usually binds materials and blocks wind action through the roughness and cohesion that it introduces. The reason for this is the strong relationship between green cover and the availability of material. Vegetation causes the soil surface to develop a kind of "roughness" that is so effective at disrupting and preventing displacement by wind that when there is any plant cover over 30%, the wind becomes completely ineffective (Marticorena and Bergametti 1995). The effectiveness of this green cover can be seen along the Britanny coast where tempests simply ruffle through the tips of the grassy, prairie covering. The difference is all the more marked if this prairie abuts a cultivated plot of land with bare, exposed soil!

There are any number of situations that fall between covered soil and partially or totally bare soils, and which govern the extent to which aeolian activity occurs. Hot and cold deserts are regions where aeolian activity prevails. A study was carried out in conditions of pronounced aridity, in north-west Mauritania, to examine the fight against sand accumulation on the railroad tracks linking the Zouerate iron mines with the Nouadhibou port and to measure how effective green cover was. It was seen that over one particular zone, measuring a few hundred meters, sand never accumulated on the railroad tracks through wind action. This was an area covered by a few acacias that are too scattered to be effective, and a steppe that is not very dense and almost mono-specific, with a tall, perennial grass that is locally called Markouba (Panicum turgidum) (Figure I.2). A series of 20 wind-speed measurements in this Markouba zone gave an average of 9.1m/s at a height of 2 m and 1.5m/s at a height of 30 cm; just behind a green knoll, the wind was almost zero, between 0.4 and 0.1m/s.

These conditions may be present-day conditions or may have existed in the recent geological past, during the last quaternary climatic variations, or much further in the past: Paleozoic aeolian sediments are of interest for their hydrocarbon potential (Glennie 1970; Alsharhan et al. 1998). It is often useful, therefore, to distinguish between actual, functional formations (such as active dunes) and ancient formations (such as loess or certain covering sands).

I.1.2. The environments where wind action occurs

The necessary conditions, listed above, limit wind action to areas where they are present and where several causes may combine.

I.1.2.1. "Natural" regions

This term denotes regions where natural conditions are the principal factor allowing wind action. This does not exclude human interventions as a factor that limits or, alas!, exacerbates aeolien action. These regions are as follows:

• 1) areas in littoral zones where beaches combine the absence of vegetation with an inflow of sand that may be large;
• 2) areas around volcanoes where there may be abundant ashes and where unique aeolien phenomena may occur, linked to the shockwaves associated with the explosion;
• 3) regions where harshness of climate either prevents or slows vegetation growth. These are where aeolian landforms are most often seen.

Figure I.2. Spontaneous Markouba steppes protecting the Nouadhibou - Zouerate railroad, which can be made out in the background. Even such a loose formation is remarkably effective at preventing aeolian action. For a color version of this figure, see www.iste.co.uk/bourrie/deserts3.zip

In this last case, the limiting factors may have two origins:

• - cold glacial and periglacial climates. The formations could be inherited, like European loess, or present-day formations, further north, in areas where plant cover does not protect the soil;
• - arid and semi-arid environments in hot or temperate regions, which are the subject of study in this book, with very little precipitation and high evaporation. Such regions are found in several parts of the world (Figure I.3): Australia, with its interior deserts, the North American West and the arid diagonal through South America; the western part of southern Africa and, above all, the vast arid arc from the Atlantic Sahara to West Africa.

Figure I.3....