Geoscience Reference
In-Depth Information
(a)
(b)
Figure 19.7
Parabolic dunes: (a, b) White Sands, New Mexico.
19.4.1
The stoss or windward slope
with a thickness equivalent to the surface roughness (1/30
of the grain diameter for a sand surface), and (2) a shear
stress layer less than 1 m thick in which shear stress effects
decrease with height above the surface. The shear stress
developed in this part of the boundary layer generates sand
transport on dunes. The basic features of this conceptual
model have been confirmed by field studies of air flow
over dunes (Livingstone, Wiggs and Weaver, 2007) and
reproduced in numerical models (Parsons, Walker, and
Wiggs, 2004; Weng
et al.
, 1991).
Winds approaching the upwind toe of a dune stagnate
slightly and are reduced in velocity (Livingstone, Wiggs
and Weaver, 2007; Wiggs, 1993). On the stoss, or wind-
ward slope of the dune, streamlines are compressed and
winds accelerate up the slope. The magnitude of the ve-
locity increase varies with dune height and dune steep-
ness or aspect ratio (Figure 19.9) and is characterised by
the speed-up ratio
s
or amplification factor
Az
(
U
2
/
U
1
,
}
}
Outerlayer
U
0
OuterRegion
Planetaryboundary
layer
}
Middlelayer
}
Innerregion
Shear stress layer
Innersurfacelayer
