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Fig. 9.5 A GPS track obtained
simply by carrying an ordinary
handheld receiver while walking
the perimeter and slip faces of
four dome and two barchan dunes
south of Liwa in the United Arab
Emirates. The track was made in
December 2012, while the
satellite image was obtained in
November 2010. The smallest
dunes have moved some 10-
14 m, while the barchans have
moved by 6-8 m. Image
generated with Google Earth,
with data from Lorenz and
Radebaugh
Fig. 9.6 Migration rate versus size for very different sizes of dune at
a single location (Liwa, United Arab Emirates) plotted on logarithmic
axes. On such a plot a constant sand flux Q is a straight line (compare
with Fig. 9.3 ), and it can be seen (as discussed in the text) that the
migration of dunes varying two orders of magnitude in size are
consistent with the same sand flux. Note that the dome sand fluxes
appear slightly smaller than those for the barchans, perhaps because
some sand saltates over the dome without contributing much to its
movement. This plot highlights the fact that a dune migration rate is
more or less meaningless without also knowing the height of the dune
Fig. 9.7 A HiRISE image of dunes in Niili Patera (see also Fig. 12.19 )
from which ripple and slipface migration rates have been measured. The
key features are the intrinsically high resolution of the image, and a
pattern of marks on the ground next to the dune to act as reference markers
against which to refer the slipface movement. http://photojournal.jpl.nasa.
gov/catalog/PIA14878 . Credit NASA/JPL/U.Arizona
Historical migration rates can also be inferred from dune
cores. For example, a core of a megabarchan in the Rub' Al
Khali desert has sand exposure ages from optically-stimu-
lated luminescence dating (see Chap. 17 ) that increase by
1 kyr per 20 m depth. If we imagine this accumulation rate
is due to an advancing slip face, then it implies an advance
standing on a road or other easily-noted image feature), then
a dune migration can be compared by measuring the dis-
placement of outline coordinates from the dune position in a
satellite image.
 
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