12.7: Wind Ripples and Eolian Dunes
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Wind Ripples
Introduction
When a sand-moving wind lows across a surface of loose sand, wind ripples soon make their appearance. In their classic manifestation, wind ripples are almost perfectly straight-crested low ridges extending for long distance transverse to the wind. In places, a wind ripple ends abruptly, and in other places there are “tuning fork” junctions at which a single ripple branches into two. Ripple spacing range mostly between a few centimeters and ten centimeters—although in coarser particle sizes the spacing increases up to a few meters and the ripple become much less regular in their geometry. Such ripples have been called granule ripples. Upwind (stoss) surfaces of common wind ripples have slope angles of \(X\), and downwind(lee surface have slopes of \(X\), much less than the angle of repose for loose sand. Crests as well as troughs are rounded. As with subaqueous current ripples, wind ripples move downwind at speeds orders of magnitude slower than the driving wind. In contrast to subaqueous current ripples, particle size at the crests of the ripples are coarser than in the troughs. It is in the troughs that finer particles—of very fine sand size down into silt size—find resting places, sheltered from the wind.
As with so many aspects of eolian sedimentation, modern study of wind ripples began with Bagnold (1941), who studied them both in the field and in laboratory wind tunnel. (It is especially easy to make wind ripples even in a short wind tunnel.) A later classic paper is that by Sharp (1963). Two of the most extensive wind-tunnel studies of wind ripples are those of Seppälä and Lindé (1978) and Walker (1981). In what to my knowledge is the most extensive and systematic wind-tunnel study of wind ripples to date, Walker (1981) found that ripple spacing increases with both mean particle size and wind velocity, and, for a given particle size, ripple spacing increases as the sorting become less good.
The dynamics of wind ripples has had a long history of controversy. Bagnold theorized that the spacing of wind ripples was set by a certain “characteristic” saltation jump length. Later workers, beginning with Sharp (1963), rejected Bagnold’s concept and emphasized the role of surface creep, driven by saltation impacts, in forming and maintaining the ripples. This line of thought culminated in a stability analysis of ripple development by Anderson (1987). A rather different approach to wind ripples was taken by Werner and Gillespie (1993) and by Landry and Werner (1994).
In recent years, physicists and applied mathematicians have been attracted to the dynamics of wind ripples, perhaps in part because it is such an intriguing example of dynamical self-organization, and perhaps in part because it lends itself to theoretical and numerical modeling in which the messiness of turbulence does not have a direct effect on the process. This interest has resulted in numerous papers, published mainly in physics periodicals; see, in particular, papers by Nishimori and Ouchi (1993), Ouchi and Nishimori (1995), Prigozhin (1995), Stam (1996), Hoyle and Woods (1997), Hoyle and Mehta (1999), Valance and Rioual (1999), Terzidis et al. (1998), Kurtze et al. (2000), Valdewalle and Galam (2000) Miao et al. (2001), Niño et al. (2002), and Yizhaq et al. (2004). In contrast, observational studies of wind ripples seem to have been scarce in recent times; see Andreotti et al. (2006).
Eolian Dunes
Introduction
In areas covered widely by movable sand, the wind shapes the sand into large-scale features called dunes. In contrast to the subaqueous case, for which there is controversy about the dynamical distinction between ripples and dunes, it is clear that there is a fundamental dynamical distinction between wind ripples and eolian dunes.This was first made explicit in a widely cited paper by Wilson (1972) (Figure \(\PageIndex{1}\)). Eolian dunes range in spacing from many meters, at a minimum, to thousands of meters. There seems to be no upper limit to dune size, given sufficient sand and a sufficient reach on which the wind can do its work. For a thorough exposition of eolian dune types, see Pye and Tsoar (1990).
In sharp contrast to subaqueous dunes, the shapes of eolian dunes, and their orientation elative to the sand-moving wind, range very widely. Features that are classified under the term dune range from those that are strictly transverse to the wind, to those that are extremely regular in geometry and are closely parallel to the wind—hence the distinction between transverse dunes and longitudinal dunes. In regions where the winds are highly variable in direction, star dunes, with arms oriented in various directions, form. Smaller dunes can be superimposed upon larger dunes.
A thought experiment seems in order here. In the case of subaqueous dunes, much of what we know comes from studies of dunes generated by unidirectional flows of water under equilibrium conditions in flumes. In the case of eolian dunes, no experimental programs of that sort have ever been conducted, to my knowledge at least. The basic problem is that because of the minimum size of dunes is so large, it would take an extraordinarily large wind tunnel to make experiments on the equilibrium characteristics of eolian dunes. And even then, of course, the presence of the roof in the wind tunnel would make the results less applicable to the natural environment, in which, in the context of eolian dunes, is effectively unlimited in height.
What would we find if we built a long quonset-hut-like building, perhaps a large fraction of a kilometer long, with a roof a few tens of meters high, over a deep bed of loose sand, and passed a controlled, steady wind through the tunnel, perhaps by means of a propeller driven by an old-fashioned airplane engine mounted at the downwind end of the tunnel, while at the same time adding new sand at the upwind end of the tunnel? Presumably, dunes would develop; how would their spacing depend on wind velocity and sand size? Would they grow to the point of constriction by the height of the tunnel for all wind speeds, or would their spacing increase with wind speed? Would dune size vary with sand size? The answers to those questions, which are fairly clear for subaqueous dunes, are not known.
Nature provides us with much less controlled conditions: everywhere on Earth, even in the least variable climatic conditions, the wind varies in both speed and direction. That variability makes any conclusions about how dune geometry depends on wind conditions fraught with uncertainty.
In areas where the availability of movable sand is limited, eolian dunes take the form of barchans: crescent-shaped dunes, with horns pointing downwind, that move across a non-moveable surface. Sand is supplied to the barchans from upwind; the barchans lose sediment, at about the same rate, from the downwind tips of the horns. Barchans are not restricted to eolian environments: it is easy to make miniature barchans in water flows in a flume in which limited quantities of fine sand or silt move across a the rigid floor of the flume.
Are eolian dunes and subaqueous dunes identical, in terms of the fundamental dynamics? This question is not explicitly addressed in the literature, to my knowledge, but I would speculate that the specialists, if asked, would say that they indeed are. The only way to know for sure would be to make a systematic series of observations over the range of intermediate ratios of particle density to fluid density—and that has never been done and is unlikely ever to happen.