Controlling Sand Movement Through Mechanical Measures: China’s Experience - Combating Desertification in Asia, Africa and the Middle East - page 42

If the run of the sand barriers is wrong, secondary sand barriers should be set
up so that erosion can be controlled. If some parts of the sand barriers have been
destroyed, they must be repaired immediately.
Initially, clay sand barriers can improve the moisture conditions of sandy land
because the depressions among the sand barriers can collect the run-off of any
rain. According to observation: with a precipitation of 25 cm in 4 days, the water
permeates to depths of between 28.5 and 35.6 cm on the sand dunes protected by
the clay sand barriers and to a depth of only 18.5-23 cm on bare sand dunes. The
moisture contents in the depth of 0-15 cm are that, 13.35 mm for the dunes protected
by high standing sand barriers, under 10 mm for the low standing sand barriers and
16-16.65 mm for the clay sand barriers. The moisture status of the sand protected
by the clay sand barriers is greatly improved. The depths of the dry sand layers of
sand dunes protected by the clay sand barrier and by the straw sand barrier are 8-10
and 18-20 cm, respectively, therefore for the sake of afforestation, the clay sand
barrier is better than others.
However, at a later period after installation of the clay sand barrier, thin crusts
will form on the sand surface because of the washing of the rainwater. This can
prevent the rainfall seeping into the sand dunes. In this case, the moisture conditions
of the sand dunes will have harmful effects on afforestation. Therefore afforestation
should be undertaken immediately in the spring following the setting up of the
barrier.
At 20 cm above the ground, the clay sand barrier can reduce the wind velocity
by 27-33 %, the high standing straw sand barrier by 42 % and the low standing
straw barrier by 8-17 %. At 2 m above the ground, the clay sand barrier can reduce
the wind velocity by 40 % or more, and the straw sand barrier can reduce the wind
velocity by 10-40 %. Therefore, in respect to the function of protecting seedlings,
the clay sand barrier is better than other barriers. The cost of the clay barrier is the
lowest among the various sand barriers because it only consumes manpower. The
materials -the clay- are located on the low land which is just on the foot of the sand
dunes. Therefore the clay sand barrier can not only reduce the cost, but also save
a lot of other materials, such as straws, stalks, and branches, etc. So in many areas
where a clay source is available and the straws or branches are in short supply, the
clay sand barrier is the best choice.
The cost of the man-power for the clay sand barrier is around 150 man days/ha,
but for covering a sand dune with earth, this cost is about 1,500 man days/ha, The
cost for labor of the clay sand barrier ,the straw sand barrier and the branch sand
barrier are about RMB 0.60, 1.05 and 1.05 Yuan/m, respectively. So the cost of the
clay sand barrier is the lowest one. In general, the service life of the clay sand barrier,
if it has been set up correctly, is around 4-5 years. The plants such as Haloxylon
persicum , which are planted in the sand barriers, can exercise a function of sand-
control very effectively within 2-3 years of planting. At this point plants can replace
the clay sand barrier to control moving sand.