Dune Fields, Sand Seas and Transport Pathways - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 101

Fig. 8.5 Small linear dunes,
stabilized by vegetation in the
Arica Hills, California. Photo
J. Zimbelman

the roughness height represented by vegetation; the effect
on the wind is a complex interplay between both the height
of individual plants and their spacing on the surface (see
also the discussion in Sect. 1.3 on deliberate introduction of
vegetation to control dunes). Once vegetation becomes
sufficiently abundant, the surface becomes effectively
insulated from the wind, except perhaps where turbulence
may be generated immediately around large obstacles, but
such localized scour is not capable of carrying out regional
transportation of the sand at the surface. Vegetation also has
the effect of introducing organic matter into the sand,

making it more cohesive.
It is possible to immobilize dunes without invoking
vegetation. Enough moisture alone can reduce transport
substantially. Other mechanisms on Earth include indura-
tion by ice (see Chap. 5 ) and cementation by soluble min-
erals such as gypsum.
It was speculated for some time that many dunes on Mars
are immobilized via similar mechanisms—the fact that a
few examples of dune and ripple migration ( Chap. 9 ) have
now been documented does not detract from the fact that
dune migration has not been generally observed, and indeed
observation of blocky eroding barchan forms (Fig. 12.25 )
shows that some dunes have been indurated.
On Titan it is assumed that the large equatorial sand seas
have active dunes, but some outlier features at higher lati-
tudes have been observed (with different orientations from
the main sand seas, and somewhat different radar appear-
ance) and it has been speculated that these may be fossil
dunes from a previous climate epoch.

Fig. 8.6 Farmland in Australia seen from an airliner, productive in
today's moister conditions in spite of the evident presence of sand
dunes (linear dunes, in south part of the Simpson Desert). Photo Jani
Radebaugh

may substantially remobilize. The change in sand mobility
can also lead to a change in morphology; e.g., Wolfe and
Hugenholtz (2009) show that what are now parabolic dunes
on the Canadian prairies were barchan dunes in a drier
period about 200 years ago.
Vegetation can become the dominant 'roughness
elements' of the surface, causing the height of zero wind
speed in the boundary layer to rise high enough so that sand
grains can no longer be induced to move (and thus sand
accumulates in the lee of individual plants, forming neb-
khas; Fig. 7.18 ) . Many field experiments have documented