The work of the wind - Fundamentals of the Physical Environment - page 522

and flow patterns over the sand bed. The most common unit is the sand dune , 1-30 m
high with wavelengths of 10-500 m, which occurs in a variety of shapes and sizes. Much
smaller ripples form on its upwind face, 0·1-5 cm high with wavelengths of 0·02-2 m.
Wind 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 airflow - 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 16.2). 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 a −1
and entire sand seas slowest. With growth rates of 1-10 km 3 Ma −1 , the modern form and
distribution of active ergs fit well into the Plio-Pleistocene global climatic context.
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.