Basic Types of Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 72

Fig. 6.3 HiRISE image of dome
dunes in Noachis Terra, Mars.
These appear to be grading into
linear forms, perhaps suggesting
the domes are either forming into
linears, or perhaps are the last
remants of some linears that have
now all but disappeared. Some
small ridges appear at the bottom
of some of the domes (which lack
slip faces) and a few dust devil
tracks cross the dunes. Image
NASA/JPL/U.Arizona

changes in sand or wind conditions. Dome dunes are also
widely encountered on Mars (Fig. 6.3 ). They have not been
identified on Venus or Titan, but since they are the smallest
type of dune, this may be more a function of the resolution of
observations rather than an indication of their absence.

a given area is inversely proportional to the height of the
dune—see Chap. 8 .
Because they are common in Egypt, barchans feature
prominently in Bagnold's book—they may have thus
received more attention than they deserve on an area- or
volume-weighted basis worldwide. Nonetheless, as the most
dynamic of dunes, and with an appealing 2-dimensional
planform, they are important and interesting. More recently,
barchans have also been the subject of extensive study in
water tanks/flumes (Taniguchi et al. 2012; see Fig. 6.8 )as
well as in numerical simulations (see the discussion in
Chap. 19 , and especially Andreotti et al. 2002). That work
is largely stimulated by observations of barchans on Mars
which have a variety of forms (Figs. 6.9 , 6.10 and 6.11 ),
from teardrop domes with two slipfaces and a tail, to
wing-shaped reversing barchans (Fig. 6.12 ), to hooked
barchans which may be intermediate between barchan and
linear (longitudinal) forms. Note that barchans may develop
superposed small ('elemental') duneforms (e.g., Elbelrhiti
et al. 2008); while morphologically similar to the much

6.2 Barchan

The barchan is a common crescent-shaped dune form
(Figs. 6.4 , 6.5 , 6.6 and 6.7 ) found in regions with both a
limited sand supply and with a unimodal wind regime. The
name is originally from Turkistan. A single slip face is
present along the crest of the dune, and the horns of the
dune point downwind. The horns of barchans can some-
times be the local source for subsequent barchans, where
sand leaving the horn becomes the source of another bar-
chan dune, and barchans often form extended corridors.
Barchan movement downwind has been documented at sites
around the world; the rate of movement of barchans within