Geoscience Reference
In-Depth Information
Backslope
(windward slope)
Slip face
(lee slope)
Former dune
position
Crest
Wind
Future dune
position
position
Erosion by
saltation
Deposition
and slumping
Angle of
repose
(a)
Figure 18.16 Dune components and migration.
(a) When viewed in cross section, a freely active dune
has three primary components: the backslope, crest,
and slip face. Dunes migrate downwind through the
combined processes of sand saltation on the back-
slope and slumping on the slip face. (b) This large
sand dune in central Brazil has a beautiful example
of a slip face, which slopes down toward the river.
Airflow is left to right across the image. Note the pin-
nacle in the background and the person (arrow) for
scale.
(b)
Figure 18.16a illustrates how these processes collectively
operate. The best way to visualize these patterns is to begin
on the windward slope of the dune where the compressed and
accelerated airflow shown in Figure 18.16 causes erosion. No-
tice in Figure 18.16a that this slope is called the
backslope
and has the shallowest slope of the feature, ranging between
about 10° and 15°. Once wind begins to move sand grains on
this surface, they bounce up the slope by saltation. When they
reach the top of the dune, which is called the
crest
, they fly
into the zone of calm air in the lee of the dune. As they fall,
they are deposited on an evolving slope that lies at the angle
of repose. Remember from Chapter 14 that this angle for sand
stands between about 30° and 34°. This slope on a dune is
called the
slip face
because sand on this relatively steep slope
sometimes slides downhill under the force of gravity after it
is deposited. Due to the combined effects of erosion on the
backslope and deposition on the slip face, the dune migrates
downwind. This migration is depicted in Figure 18.16a by the
dashed red lines that represent the former and future position
of the dune. A great example of an actual dune slip face can be
seen in Figure 18.16b.
The extent of sand dune migration depends on the bal-
ance between erosion on the backslope and deposition on the
lee slope. As a result, maintenance of the dune form depends
on the forward movement of the entire feature because the
amount of backslope erosion is balanced by the amount of
deposition on the lee slope. In reality, variations in fluid dy-
namics and other factors result in complex cross-sectional
The gradual slope of a dune that faces the prevail-
Slip face
The steep slope that lies on the leeward side of a
sand dune at the angle of repose.
Backslope
ing winds.
The highest point of a dune.
Crest
