9.5: Eolian Ripplers and Eolian Dunes

You learned earlier that when a current of water flows over a bed of loose sand, if the current is strong enough to move some of the sediment several kinds of geometrical features, called bed forms, are molded by the flow. The same is true for flows of air over loose sediment. And in fact, just as in water flows, two different kinds of bed forms, ripples and dunes, are formed at widely different scales. A dynamical instability of some kind, whereby a planar sand surface is unstable to small disturbances, is involved in both cases, water and wind. The dynamics behind these instabilities is just as mysterious in the case of wind as in the case of water. It’s not even known for sure whether subaqueous and subaerial ripples are the same dynamical kind of feature, and whether subaqueous and subaerial dunes are the same dynamical kind of feature.

Here’s another home experiment for you to try. (I promise you that this will be the last home experiment I propose this semester!) It would be easy to build a home wind tunnel. (See Figure 9-2 above.) Just nail four wide pine boards together to make a duct several feet long, and arrange the downwind end to pass into a hole in a large box. In the other wall of the box, mount a window fan. Put a layer of sand in the duct, turn on the fan, and you have eolian transport. (It would help to improve the design in three ways: put a couple of Plexiglas windows in the duct so you can watch the sand move; make the upper surface of the duct removable so you can get at the sand and photograph it easily; and arrange a flap board against the downwind side of the fan to control the wind speed.)

Gradually increase the wind speed over a planar sand surface in your duct until the sand grains just begin to move. Very soon after that, fully developed saltation sets in. If then you gradually decreased the wind speed again, you would find that the minimum wind speed needed to maintain already existing saltation is much less than the wind speed needed to initiate saltation. A physicist would call this a hysteresis effect. (Can you recall an earlier example of hysteresis in this course?) These two wind speeds represent what are called the saltation threshold and the fluid threshold, respectively.

Crank up the wind again until there’s saltation. You would have to wait only a matter of minutes until the originally planar sand surface becomes molded into a strikingly regular series of wind-transverse rounded ridges and troughs called wind ripples or eolian ripples. The typical spacing of wind ripples is of the order of several centimeters, although they get bigger in poorly sorted and/or coarser sediment. They shift very slowly in the downwind direction, by movement of sediment up the upwind flanks and deposition of that sediment on the downwind flanks.

There’s a tendency for the particle size to be slightly coarser on the crests of the ripples and slightly finer in the troughs. Long ago R.A. Bagnold, a pioneer in the study of eolian sediment transport, devised a spectacularly visual 422 demonstration of this effect by making wind ripples in a thoroughly mixed sand of two different colors in a wind tunnel: as the ripples develop, the bed surface resolves itself into alternating stripes of the two different colors right before one’s eyes!

Eolian dunes are not as easy to make in experimental wind tunnels, because their minimum scale is too big. Almost everything we know about dunes is from studies in the field. One of the big problems in the studying of eolian dunes is that there aren’t many places in the world characterized by really steady winds: obviously the wind speed varies with time, but in most places the wind direction changes substantially too. Nobody really knows what the reference case of dunes formed by a steady wind over a full sand surface of effectively infinite extent looks like.

Eolian dunes take many different shapes, and there is no generally accepted classification. One reads about linear dunes, longitudinal dunes, crescentic dunes, seif dunes, oblique dunes, barchan dunes, star dunes, and many others. I think a fairly satisfactory but simplified way of classifying dunes is to recognize three kinds of dunes formed in full sand beds depending on orientation relative to a dominant sand-moving wind:

• transverse dunes, oriented normal to the wind
• longitudinal dunes (also called seif dunes), oriented parallel to the wind
• oblique dunes, oriented oblique to the wind.

It’s not even clear whether these three basic kinds are dynamically different from one another or not. Dunes formed by winds blowing from various directions without a dominant sand-movement direction are called star dunes. Star dunes are found where sand-transport paths converge to form a sand “sink” or sand-storage area. Finally, dunes formed in areas where there is not enough sand to keep troughs from being floored by immobile substrate take a characteristic crescent shape are called barchan dunes. There’s a continuous transition in geometry from barchan dunes to transverse dunes as the thickness of the sand increases relative to dune dimensions, so that less and less of the troughs expose immobile substrate.

Of course, dunes are found not just in deserts. Coastal dunes are common along many shorelines where onshore winds pick up beach sand and carry it inland to form belts of dunes, narrow or wide. The prerequisite for any area of sand dunes is a source of sand. In deserts that source might be fluvial sediment freshly deposited where a river finally loses all of its water by infiltration into the river bed, or just an area where sand from sandstone bedrock is freed by slow weathering.

If the sand supply is abundant and the transport is consistently in one direction away from the source, enormous areas of deserts can be covered with mobile sand. Such areas are usually picturesquely called sand seas. Sand seas are especially prominent in North Africa and the Arabian Peninsula.

Readings

Allen, P.A., 1997, Earth Surface Processes. Blackwell Science, 404 p. (Chapter 10)

Bagnold, R.A., 1941, The Physics of Blown Sand and Desert Dunes. Chapman & Hall, 265 p.

Bloom, A.L., 1998, Geomorphology; A Systematic Analysis of Late Cenozoic Landforms, Third Edition. Prentice Hall, 482 p. (Chapter 13)

Easterbrook, D.J., 1999, Surface Processes and Landforms, Second Edition. Prentice Hall, 546 p. (Chapter 17)

Greeley, R., 1985, Wind As a Geological Process, on Earth, Mars, Venus, and Titan. Cambridge University Press, 333 p.

Pye, K., and Tsoar, H, 1990, Aeolian Sand and Sand Dunes. Unwin Hyman,