Geoscience Reference
In-Depth Information
Dune formation commences where sand accumulates
on landing, either beneath a slower part of the air flow or
where its effective velocity is reduced by friction over the
embryonic dune. Further growth superimposes zones
of faster and slower flow on the general wind field and
also initiates vortices as the air tumbles over lee slopes.
A relationship develops between dune morphology,
regional (primary) and local (secondary) air flow which
is responsible for the family of distinctive dune shapes.
The direction of primary and effective winds may change
seasonally, or over longer periods, and complex forms
reflect these multidirectional influences.
Where sand is relatively scarce wind shapes the sides
as well as the crest into a classic crescentic dune or
barchan , but linear dunes form in larger coalescing sand
beds. Asymmetrical extension of one horn may draw
barchans out into longitudinal or seif dunes . They may
develop in any case where sand is less abundant, or
coalesce transversely in aklé form. Draa are generally
developed from longitudinal dunes. Transverse dunes
develop where air flow itself acquires wave motion.
Troughs in the wave approach the surface and set sand in
motion, forming dunes in their lee and below crests where
air diverges from the surface. Air flow and vortices rarely
stay constant or symmetrical, and a number of systematic
irregularities readily develop in either form of linear dune.
Vortex convergence between longitudinal dunes draws
two parallel ridges together into parabolic junctions.
Individual parabolic dunes form at blow-outs in trans-
verse dunes where the windward slope experiences
accelerated erosion and breaches the dune crest or,
conversely, receives a diminished onward supply. Non-
uniform motion of transverse crests may superimpose
barchanoid (barchan-like) or parabolic elements ( Plate
16.4 ). Longitudinal dunes subjected to a diverse range of
secondary wind directions acquire star-shaped patterns,
known as rhourds ( Figure 16.5 ).
Plate 16.3 Dune field (draa) in the Tengger Shamo desert,
Inner Mongolian Autonomous Region, northern China.
Photo: Mike Fullen
drives sand forward by creep, saltation and ballistic impact
on the windward side of surface irregularities. Grains
avalanche or slide en masse down lee slopes oversteepened
above the dry sand friction angle. Fast-moving grains
catch up with those moving more slowly to form a more
prominent ridge transverse to air flow - the ripple -
which then tends towards an equilibrium form. At the
other end of the scale, draa are megadunes , i.e. very
large dunes or dune complexes 20-400 m high and at
wavelengths of 0.3-3 km. Individual dunes may be
superimposed on them ( Plate 16.3 ). Dune height is
limited by the maximum particle size capable of resisting
higher (exposed) crest velocities. Fixed dunes develop in
the lee of obstacles where air flow is reduced below fall
velocity and, although sand is still lost and gained at their
perimeter, the landform is metastable. By contrast, free
dunes develop independently in open flow and are
intrinsically unstable and mobile. Dunes and ripples
advance, by particle deflation from the windward slope to
leeward slopes, more slowly than the movement of
individual particles through them. Advance rates are
inversely related to bed form size, with barchans fastest at
5-20 m yr -1 and entire sand seas slowest. With growth
rates of 1-10 km 3 Myr -1 , the modern form and
distribution of active ergs fit well into the Plio-Pleistocene
global climatic context.
Coastal dunes
Coastal dunes depend for their formation and nourish-
ment on the predominance of effective, onshore winds
and deflation of sustained sand supplies in the backshore
zone. Free-draining sands and vigorous coastal air streams
create physiological drought even in humid climates, as
their occurrence in mid- to high latitudes and more
stormy belts shows. The windward edge of the backshore
behind a broad beach is susceptible to deflation through
onshore sand movement, regular desiccation and minimal
vegetation cover. Thereafter the dune system is dependent
on the development of biogeomorphic processes. Sand
 
 
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