Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 52

Fig. 5.1 Streamers on a linear dune (no slip face) at the Dumont
dunes, California. Wind is blowing from left to right, as shown by
streamers on sticks at stations 1-4 (stakes are about 70 cm high). The
strong left-right wind resumes at stations 9 and 10 where the flow has

re-attached. At stations 5-8, the streamers droop, indicating low wind,
and at stations 6 and 7 indicate an uphill flow due to the recirculating
vortex in the lee of the dune. Photo R Lorenz

Fig. 5.2 Airflow vectors over a transverse dune, simulated using computational fluid dynamics (CFD—see Chap. 19 ) showing the flow
separation at the crest and the recirculating vortex in the lee. (From Schatz and Hermann 2004, image courtesy of Hans Jurgen Herrmann.)

can be derived by measuring the angle, which is readily
done by drizzling sand to form a conical pile. This exper-
iment was performed (e.g., Moore 1987) by the sampling
arm of the Viking lander on Mars in 1976 (see Fig. 5.4 ).
Experimental studies have demonstrated that changing the
strength of the acceleration of gravity has a minimal to non-
detectable effect upon a dynamic angle of repose, as is the
case within a rotating drum.
The grains on the slip face are usually loosely packed
and thus have little cohesive strength. The combination of
this weak packing and the steep slopes makes it difficult to
climb the slip face on foot or in vehicles (see Sect. 1.4 ). On
the other hand, it is easy to slide down—dune skiing and
surfing is possible (Fig. 5.5 ).
Because the entire slip face is essentially at the avalan-
ching threshold, it is a 'critical' system wherein avalanches
can take place over the whole size range from a single grain

Fig. 5.3 'Smoking dune': wind from left to right is launching sand
which initially follows the separating streamline at the crest, but
drizzles down in clouds onto the lee side. Lee deposits are thus often a
layered combination of these airfall deposits and avalanche flows. See
also Fig. 12.13 . Dune near Palen: Photo J. Zimbelman