Environmental Engineering Reference
In-Depth Information
Transportation
Winds with velocities of 20 to 30 mi/h can cause medium sand grains to go into suspen-
sion; velocities of only 10 mi/h can cause fine sand to move. Grains of fine sand or silt
move in suspension by saltation (hopping along the surface) or by rolling, depending
upon wind velocity.
Erosion
Deflation
The removal of loose particles from an area leaving a denuded surface covered with coarse
gravel and cobbles (lag gravels) is referred to as deflation. These deposits are characteris-
tic of many true deserts and can cover many hundreds of square miles as they do in the
Sahara and the Middle East. Deflation also excavates large basin-like depressions, some of
which are immense, such as the Qattara in Egypt. The bottoms of these depressions are at
or near the groundwater table (Holmes, 1964); the wind cannot excavate lower (deflate)
because of the binding action of soil-moisture capillary forces on the soil particles.
Abrasion
Abrasion is a significant cause of erosion in an arid climate. Most of the abrasive particles
of quartz sand are carried along at a height of 1 to 2 ft above the surface; therefore, erosion
is frequently in the form of undercutting. The results are often evident as balancing rocks
(Figure 7.62).
Ventifacts are pebbles or cobbles that have had facets eroded into their sides
by wind abrasion while lying on the desert floor.
7.5.2
Dunes and Sand Sheets
Occurrence
Dunes and sand sheets occur in arid regions or along the shores of oceans or large lakes.
Dunes
Dunes are depositional features of wind-blown sand and include any mound or ridge of
sand with a crest or definite summit. Deposition begins when wind-blown sand encoun-
ters a surface irregularity or vegetation. As the sand builds a mound, a long windward
slope is formed; the sand blows over the crest to come to rest on the leeward side at its
angle of repose (30-35
). As the base continues to spread, the dune can be mounded to
heights of over 300 ft, creating sand hills such as shown in
Figure 7.63.
Unless dunes become stabilized by vegetation, they continue to migrate inland, moving
very significantly during storms. Dune migration covering a distance of about 25 km is
shown in
Figure 7.64.
°
Classes of Dunes
Several classes of dunes are illustrated in
Figure 7.65.
Transverse dunes
extend at right angles to the wind direction; they are common to the lee-
ward side of beaches (see Figure 7.64).
Longitudinal dunes
form ridges elongated parallel to the wind direction, and are thought
to be the result of crosswinds in a desert environment (Thornbury, 1969).
Barchans
, or crescent-shaped dunes, are common to desert environments (
Figure 7.66)
.
U-shaped dunes
or parabolic dunes are stabilized or partially stabilized. They are typical
of the eastern shore of Lake Michigan (
Figure 7.67)
.
