Controlling Sand Movement Through Mechanical Measures: China’s Experience - Combating Desertification in Asia, Africa and the Middle East - page 28

Prevailing wind

Fig. 2.5 The effect of ventilation sand barrier on sand-driving wind

be divided into different types on the basis of the materials, structure and patterns.
These are ventilating sand barriers and dense structure sand barriers that have
different functions in combating sand damage.

4.1 Ventilating Sand Barriers

When the sand-driving wind passes through ventilating sand barriers, the frictional
drag is increased and will form separate turbulent flow and vortex flow around the
barriers. The sand-transporting capacity of the sand-driving wind will be reduced
because of the decrease in the kinetic energy and in the velocity of the wind through
the interaction of the two kinds of flow. The sand will heap up on the windward
and leeward sides of the sand barrier. The dropped sand on the windward side of
the sand barrier is much less than that on the leeward side, so the sand barrier is not
apt to be buried by the dropped sand. On the leeward side of the sand barrier, the
mound of the dropped sand gently spreads with the wind and the mounded range.
Grain size of the dropped sand is much larger than that of the windward side. So the
ventilating sand barrier can act on sand-control for a long time and can stop a great
deal of sand on the leeward side of the sand barrier (Fig. 2.5 ).
The phenomenon of sand-control differs with the different densities of the
ventilating sand barriers, the space on the barrier and the level of ventilation. In
general, the ratio of the area of the space on the barrier to the total area of the barrier
is called the degree of density of the sand barrier. The degree of density of a sand
barrier is generally regarded as the index of the sand-control efficiency of the sand
barrier. When the degree of density of a sand barrier is about 25 %, it can heap
up sand, on the leeward of the barrier, within a range of 7-8 times the height of the
sand barrier. When the degree of density reaches 50 %, the range of the dropped sand
on the leeward of the barrier is around 12-13 times the height of the sand barrier.
As the degree of density of the sand barrier increases, the range of the dropped
sand, the amount of dropped sand mound and the sand-control capacity of the sand
barrier will rise. But when the degree of density of a sand barrier exceeds 50 %,
wind erosion at the windward side of the sand barrier may occur, and under these
circumstances, the sand barrier may be destroyed from the base. In order to make
optimum use of sand barriers, the height, the row spacing and the degree of density
of sand barriers should be determined in the light of specific conditions.