Controlling Sand Movement Through Mechanical Measures: China’s Experience - Combating Desertification in Asia, Africa and the Middle East - page 29

Prevailing wind

Fig. 2.6 The effect of dense structural and windproof sand barriers on sand-driving wind

The degree of density of a sand barrier suitable for a given area can be determined
in accordance with the wind force and the abundance of the sand in the area. Under
normal conditions, ventilating sand barriers whose degree of density is between 25
and 50 % are usually adopted in dune control. In an area where the wind force is
strong and the sand is abundant, the degree of density of the sand barrier should be
greater than normal; conversely, in an area where the wind force is weak and the
sand not so abundant, the degree should be smaller than normal.
The sand control function of the standing sand barrier is satisfactory because it
not only can stop the sand carried by the sand-driving wind, but also can stabilize
the sand which is originally located around the barriers. The standing sand barriers
can gradually gather many small sand dunes or sand mounds into one big dune and
form a “sand-hold-back” sand dam or sand mountain. Standing sand barriers are
often adopted to protect farmlands, canals, transport lines and wind gaps, which are
seriously threatened by the moving sand, In order to prevent the protected objects
from being buried by sand dropped from the sand-driving wind, a suitable distance
must be kept between the sand barriers and the protected objects. This distance can
be determined according to the degree of density and the range of dropped sand of
the sand barriers.

4.2 Dense Structural Sand Barriers

A strong vortex flow will form around the sand barrier that is completely windproof
or structurally dense because the sand-driving wind is raised upon the windward
side and descends sharply on the leeward of the barrier when the sand-driving
wind is obstructed by such a sand barrier. Because the vortex flows formed around
the sand barrier affect each other, the kinetic energy of the blown sand is greatly
consumed and both the wind velocity and the sand-transporting capacity of the wind
are reduced. The sand carried by the sand-bearing wind will heap up around the sand
barriers (Fig. 2.6 ).
If the sand is very abundant, the height of the dropped sand will be rapidly equal
to the height of the sand barrier. In this case, the sand barrier is liable to be buried
by the dropped sand, so the service life of a barrier, in this condition, is very short,