Dunes as Physical Systems - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 241

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Dunes as Physical Systems

-visible rhythms of blue and brown,
sea and sand dance upon my strings.

The balance of complexity and regularity is something that
human beings, a species adapted to exploit niches in a
dynamic environment, are attuned to detect. Dunes are often
rather regular landforms, but rarely exactly regular, a fea-
ture that our brains are evolved to trigger pleasure from.
One of the earliest chapters on the formation of sand
dunes was by the British geographer, Vaughan Cornish, in
1895. He also studied snow drifts and waves in the sea and
in rivers, clearly struck by the analogy between wind-
formed waves in a sand surface and wind-driven waves on
the surface of a liquid. That same analogy also inspired
astronaut Story Musgrave to compose this poem upon
seeing the Namib from the Space Shuttle (see Fig. 6.19 ):

While poetic, the analogy is not predictively useful.
While some of the mechanics of airflow separation, etc. is
common to the worlds of sand and sea, the ripples on those
two media are defined by altogether distinct processes.
That said, the fact that ripples and dunes can form with
similar patterns and arrangements over scales from ripples
of a few centimeters to dunes several kilometers apart (e.g.,
Fig. 20.1 ), suggests some fundamental processes at work,
and the planetary perspective—which shows, for example,
very different environments on Earth and Titan producing
identical landforms—only reinforces this.
As we have described in Chap. 2 to 10 , and especially in
Chap. 10 , modern physics can now largely capture the essential

Now, Namibia, desert streaming into ocean,
waves of bright sand diving into dunes of dark water

Fig. 20.1 Ripples on the surface
of dunes at the Lencois
Maranhenses in Brazil. The dune
surface is damp and packed hard,
but bright dry sand has started to
move across it, and a pattern of
transverse ripples is seen at left.
But towards the right, where sand
is less abundant, the ripple
pattern is two-dimensional, with
much more crescentic
morphology where the ripples are
sparse. In other words, at this
10-cm scale (the case of a
leatherman tool is present for
scale), the pattern has many
similarities with sand-starved
barchan dunes at the 10-100 m
scale. Photo R. Lorenz