Environmental Engineering Reference
In-Depth Information
FIGURE 7.67
Topographic expression of parabolic (U-shaped) sand dunes along shoreline of Lake Michigan. (From USGS
Dune Acres quadrangle sheet, Indiana; scale 1:24,000.) (Courtesy of USGS.)
Sand Sheets
Sand sheets are typical of desert environments and can cover enormous areas. They vary
in landform from flat to undulating to complex and can include various forms of barchan,
longitudinal, or transverse dunes, as shown in
Figure 7.66,
which can extend for hundreds
of square miles. In the United States, much of Nebraska is covered with ancient sand
sheets of postglacial origin
(
Figure 7.68)
.
Dune Activity
Active dunes
are typical of desert regions. If winds blow at velocities of 25 to 30 m/h for
long enough intervals, small barchans (to 20 ft height) can move at the rate of 60 ft/year,
and the larger barchans (40 ft in height) can move about 30 ft/year.
Inactive dunes
are typical of moist climates and are stabilized by natural vegetation. In
the United States, very large areas of stabilized dunes are common to Nebraska. Figure
7.68 shows the very irregularly eroded features of these relict dunes.
Semistabilized dunes
are common along coastlines in moist climates.
Dune Stabilization
Stabilization may be achieved by planting tough binding grasses such as bent or marram
in climates with adequate moisture. The harsh tufts prevent the movement of the under-
lying sand, trap wind-blown particles, and continue to grow upward through the accu-
mulating sand.
Migration may also be prevented by light open fences, or by treating the windward side
of dunes with a retardant or agent such as calcium chloride, lignin sulfite, or petroleum
derivatives.
