Geoscience Reference
In-Depth Information
Fig. 9.3 A compilation of
barchan migration rates: numbers
after the dataset authors indicate
the years over which migration
was observed. The Tatooine data
point is a barchan at the Star
Wars film set in Tunisia (see
Chap. 24 , and Lorenz et al.
(2013)). A candidate reciprocal
function as described in the text,
with Q = 300 m 2 /year and
Ho = 3 m is shown. The scatter
of points at a given location
about such a curve is small—the
scatter of the points in the plot
overall is largely due to the dif-
ference in transport rate (Q)
between different sites (Hasten-
rath and Finkel measurements are
in Peru; Long and Sharp at the
Salton Sea, USA, Slattery in
Namibia)
discriminate against the fluid sand background. Similarly,
stakes or other reference marks may be easily buried. Star
dunes are, by and large, static. Linear dunes similarly
accumulate in part because of a lack of a net transport
direction, although may undergo a slight lateral migration.
Over the course of a year, the change in direction of wind
(that leads to the linear/longitudinal arrangement) may lead
to a back-and-forth shift of the position and orientation of
the crest, but the dune overall may not move much. The first
quantitative dune migration rates (cited by Bagnold) are
those of barchans at the Kharga Oasis in Egypt (see Fig. 6.5 )
in 1910. Bagnold notes the reciprocal relationship with
height and that, at that migration rate, those barchans may
have taken about 7000 years to reach their current location
given an assumed source point at the beginning of the bar-
chan stream.
Dune migration is now often measured on Earth from
aerial photographs and, increasingly, satellite images can be
used to measure movement on a larger scale. It is essential
to have fiducial markers—in a great sea of sand where all
the dunes move a comparable amount, there may be no
fixed reference points to detect the motion. Modern methods
use automatic correlation techniques to determine fixed
references, an example being the analysis by Vermeesch
and Drake (2008) showing migration rates of *25 m/year
for quite large dunes (tens of meters high) and an implied
sand flux of 600 m 3 /m/year. It should be noted that this very
high value may be due to the sand: a fine diatomite that
likely has a lower density (and thus lower threshold speed)
than typical quartz sands.
In some cases, bedrock exposures may be present, but
often the most convenient fiducials are roads, railways or
pipelines. Buildings are also effective (see, e.g., Fig. 9.4 ).
Remote sensing is discussed in Chap. 18 . One other remote
technique, that merits further exploitation for dune and
ripple migration studies, is radar interferometry, which can
detect cm-scale changes in surfaces.
The traditional field approach to measuring dune
movement is to embed stakes in the ground as reference
points and measure the dune movement over the course of
some years with a tape measure or theodolite, or perhaps
with field photographs (see Chap. 16 ). Four studies of this
sort are particularly notable. First, after using the astro-
nomical identification of its location and the identification
of some empty food and fuel tins to identify Camp 18 of
Bagnold's 1930 expedition in the Sudan, it was possible to
construct a 57-year record of a single barchan (Haynes
1989). This 16 m-high dune was observed to have a con-
stant movement of *7 m/year. Another observation, by
Lettau and Lettau (1969) of a barchan in Peru is notable in
being time-resolved. They observed the barchan for two
days, making contemporaneous wind measurements, and
observed advance of 2-3 cm over a few hours in that
period when the wind speed and direction was favorable.
There is ample scope for repeating this type of observation
using timelapse cameras today (see Sects. 9.4 and 16.2.2 ).
 
Search WWH ::




Custom Search