Geoscience Reference
In-Depth Information
5
Deposits of Sand: Ripples Versus Dunes
We have seen that sediment can form mounds on the
surface and that these mounds can grow through the insta-
bility associated with the saturation length. However, sev-
eral effects lead to more complex structures—the
recirculation bubble of airflow, the avalanching slip face of
the dune, and the possibility that multiple particle sizes or
densities may be present. These lead to some distinct types
of structures—ripples rather than dunes. The way sediment
is deposited can also leave traces in the dune (or in a
sandstone if the dune becomes lithified), or on the surface
on which the dune sits.
the surface roughness upwind, and the sharpness of the
separation point.
The recirculating flow can sweep sediment back towards
the slip face of the dune if the floor is smooth. The sepa-
ration bubble can strongly influence how dunes mutually
interact, notably in the case of barchans—once the down-
wind barchan enters the lee of the upwind one, the airflow
on it will be considerably disturbed.
5.2
Angle of Repose: The slip face
The second effect involves the sand, whether or not an
atmosphere is present. Sand generally has small cohesion,
and the sideways forces at sand-sand contact surfaces is due
to friction alone. This means that a pile of sand will tend to
have slopes that do not exceed the angle of repose—in fact,
when granular materials, whether sand, gravel, cobbles, or
boulders, come to rest on or adjacent to comparable parti-
cles, they tend to form piles with uniform slopes. If sand is
added to the top of a pile or slope, it may steepen initially,
but soon an avalanche will occur, where the weight of sand
has overcome friction and allows the sand to move until the
slope has shallowed to the angle of repose once more. The
progressive deposition of sand on the lee side of a dune
means that the lee side sees continual avalanching and is
generally at the angle of repose.
For well-rounded, dry sand grains, the angle of repose is
around 33. Accumulations of irregular coarse particles
(pebbles or cobbles) can attain an angle of repose of 41,
but such a slope is not typical for sand accumulations that
do not also include a considerable quantity of silt or clay.
The strength of the acceleration of gravity is not a promi-
nent factor in determining the angle of repose, which is
more dependent upon systematic trends in particle shape or
particle surface roughness. Since the angle of repose is
weakly diagnostic of grain shape, constraints on the latter
5.1
Flow Separation and Recirculation
First, airflow going around a surface will try to follow it.
However, like tea poured from the spout of a well-designed
teapot, if the surface peels away too sharply, the airflow
cannot follow it and detaches from it. This flow separation
means the lee side of the dune can be rather sheltered,
experiencing much less wind stress. Often there is a recir-
culating flow behind the dune, in the so-called separation
bubble, before the airflow re-attaches downwind. This re-
circulating vortex leads to a reverse flow on the lee side,
which can sweep sand back towards the dune. This recir-
culating vortex and its role in dune dynamics was recog-
nized over a century ago (Cornish 1897).
This recirculation can be studied with field instruments,
or visualized with smoke or streamers (e.g., Fig. 5.1 ), or
sometimes shown by the sand itself. It has also been studied
in wind tunnels and with computer simulations (Fig. 5.2 ;
Schatz and Hermann 2005; Hermann et al. 2005; see also
Chap. 19 ) . Occasionally, the sand itself acts (imperfectly) to
visualize the flow, as saltating sand flies off the crest of a
saltating dune (Fig. 5.3 ). Typically, the reattachment point
is downwind of the dune toe by 4-6 times the dune height,
although a wider range may be encountered depending on
 
 
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