Desert dunes - Climatic Change in Deserts - page 118

Figure 8.5. Wind velocity and sand movement. (After Williams et al., 1998 , modified
from Bagnold, 1941 .)
origin was closely tied up with periodic influxes of fluvial sand. The last two models
are, of course, not mutually exclusive, because some lateral accretion can also occur
in dunes characterised by mainly vertical accretion.
8.4 Dune form and movement in relation to wind speed, sand supply
and vegetation cover
As noted in Section 8.3 , dune form varies in relation to wind speed, sand supply and
vegetation cover. As a broad generalisation, transverse dunes (i.e., dunes oriented
perpendicular to the dominant wind direction) are common in areas where winds are
strong and sand supply is abundant, overriding the presence of any vegetation. They
are therefore a poor indicator of aridity. Linear dunes are associated with moderately
strong unidirectional or bidirectional winds, sparse to absent plant cover and moderate
sand supply. Crescentic, or barchan , dunes are aligned with their horns oriented
downwind (the reverse of transverse dunes), so the downwind slip face is at a right
angle to the dominant wind direction. They can advance quite rapidly, at rates of up