Geoscience Reference
In-Depth Information
environments, the typical response is sand dunes. After all,
these beautiful and exotic landforms are frequently used in
popular culture to epitomize some of the most extreme places
on Earth, such as the Sahara Desert. Sand dunes are also fas-
cinating from a scientific perspective because they are great
indicators of environmental change. In certain places, they
can form or reactivate even when a slight decrease in rain-
fall or change in wind speed and direction occurs. Given this
sensitivity, dunes are fun to study because they can literally
change shape overnight, whereas most landforms associated
with rivers and glaciers usually require years, decades, or even
centuries to change.
The formation of dunes is dependent on the interactions
of sand supply, wind speed and direction, and vegetation,
as these factors relate to the erosion and deposition of sand.
Given that these relationships are complex and not always
easily explained, let's look at a simplistic example that is
easy to visualize. Imagine first that a bare, sandy patch is
exposed to wind flowing over it and is thus a source for a
dune that is about to form (Figure 18.15a). This patch might
be present because a lot of sand accumulated along a beach
or because drought reduced vegetation cover. During the
initial phase of sand movement, the airflow over the patch
is essentially laminar, which means that air molecules are
moving parallel to one another downwind with no turbu-
lence. Now imagine that the moving sand grains encounter
some obstruction, such as closely spaced clumps of grass,
that results in the deposition of sand and the formation of
a small mound. Such accumulation of sand can also be
caused by a change in surface texture that slows air speed.
As deposition increases, wind speed at the top of the
mound accelerates because the vertical space in which the air
is flowing compresses (Figure 18.15b). At the same time,
airflow slows and begins to separate in the lee of the mound
because the wind can no longer follow the downwind slope.
Given this reduced wind speed, sand deposition begins to oc-
cur downwind of the growing mound. This separation of air-
flow increases as the mound grows further, creating an area of
erratic air movement in the lee of the mound and even reversal
of flow (Figure 18.15c). Although gusts of wind can occur in
this area, this zone is relatively calm compared to the flowing
air above.
Once the relationships between airflow and topogra-
phy develop fully, the mound of sand transitions into a fully
active sand dune, with distinct patterns of erosion and de-
position that result in downwind migration of the landform.
Wind
Sand patch
(a)
Airflow compresses and accelerates over crest
Wind
As mound grows, airflow
begins to separate in the
lee of dune.
(b)
Wind
Airflow compresses and accelerates over crest
Zone of relatively
calm air.
Patrick Hesp
(c)
Figure 18.15 Formation of a sand dune.
(a) Sand dune formation begins when a bare patch of sand sediment is eroded by the wind.
(b) Moving sand encounters an obstruction or change in surface texture that traps sediment, causing a mound of sand to grow. Airflow
is compressed and accelerates on the windward side of the mound. At the same time, airflow on the opposite side of the mound begins
to separate and slow. As a result, the mound slowly grows and begins to move downwind. (c) The mound grows further, resulting in full
separation of flow and the development of a relatively calm zone in the lee of the dune. Note also that the dune has migrated further
downwind.
