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11.1: Introduction

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  • Everyone knows that winds on the Earth are commonly strong enough to erode, transport, and deposit sediment. What is perhaps less obvious is that the modes of sediment transport by the wind are greatly different from those of sediment transport by water flows. This great difference does not arise from any great difference in the structure of the wind at the lowermost levels in the atmosphere: you saw in Chapter 7 that low in the atmospheric boundary layer the dynamics of flow are the same in all essential respects as in turbulent shear flows above a solid boundary in water. The difference lies in the greatly different ratio of sediment density to fluid density, which is almost eight hundred times greater in air than in water; go back and look at Figure 8.1.5 in Chapter 8 to see where the point for \(\rho_{s}/\rho\) lies for quartz-density particles in air, relative to the point for quartz-density particles in water. This difference has profound effects on the nature of particle movement in the two fluid media. As discussed briefly in Chapter 8, the very large ratio of particle density to air density means that the trajectories of particles that are in transport by the wind are largely independent of the fluid turbulence, except for fine particles, in the silt and clay size range.

    Another important difference between sediment transport by wind and sediment transport by water is that the wind is a more efficient size-sorting agent. For transport by water, it is broadly true that larger particles are more difficult to move than finer particles—silts are moved much more readily than gravels, for example—but the weakness of this effect is highlighted by the nearly equal mobility of a wide range of sand to gravel sizes in many flow settings, as discussed in Chapter 14. By contrast, the wind entrains dust and silt much more readily than sand, provided that the sediment is not bound to the substrate by cohesive forces, and gravel is much more difficult to move than sand. Except in the very strongest winds, all but the finest gravel sizes are invariably immobile, whereas water flows, even leaving rheological flows like debris flows out of account, can move even large boulders if the flow is sufficiently strong.

    It is not an exaggeration for me to say that the modern era of study of sand movement by the wind started with R.A. Bagnold’s work in the deserts of North Africa in the 1930s, which culminated in the publication of his little book (literally “little”: 265 pages in a book measuring 22 cm by 14 cm) The Physics of Blown Sand and Desert Dunes in 1941. It is a classic, in the fullest sense of the term: it is an outstanding example of a magisterial work that sets the course of future work in a field of science for many decades. It is by far the most widely cited work on eolian sediment movement, and it remains essential reading for anyone who is seriously interested in the topic. Also, several extensive early wind-tunnel studies of eolian sand transport, with results that are still valuable today, are worthy of mention (Kawamura, 1951; Zingg, 1952, 1953; Horikawa and Shen, 1960; Belly, 1964). Chepil, in a long series of papers, (see especially Chepil, 1945, 1958, 1959), was the pioneer in modern studies of wind erosion of soils; some of his work bears directly upon the transport of loose sand by wind. After the appearance of a multitude of papers on saltation from the mid-1970s to the mid-1990s, in large part from just a few groups of researchers (Greeley and co-workers; Willetts and co-workers; Anderson, Haff, and co-workers; see the list of references at the end of the chapter), the frequency of published works on saltation has decreased somewhat. You are likely to get that impression if you scan the list of references. For clear reviews of the eolian sediment movement, see Greeley and Iversen (1985), Anderson (1989), Anderson et al. (1991), and Willetts (1998).

    Research in the field of eolian sediment transport, over the past several decades, has fallen fairly naturally into three overlapping areas: soil erosion; transport of sand by saltation; and the nature and dynamics of eolian bed forms (wind ripples and eolian dunes). (The adjective eolian, meaning produced, eroded, carried, or deposited by the wind, and spelled aeolian in British-style English, comes from the name of a minor Greek god, Aeolos, who was the keeper of the four winds; see the Encyclopedia Mythica or the Wikipedia on the Internet for more information.) This chapter deals with the second of those areas. Loess—deposits of windblown silt that is carried in suspension far from its source, for tens or even hundreds of kilometers—covers a far larger percentage of the Earth’s surface than eolian sand, and it is important for agriculture in many parts of the world, but the topic of loess deposition is beyond the scope of these notes.