Controlling Sand Movement Through Mechanical Measures: China’s Experience - Combating Desertification in Asia, Africa and the Middle East - page 34

Fig. 2.13 The disposing
patterns at the head
of longitudinal dunes

lines should be shortened. The undulations on the semi-lunar dunes show that the
winds here are various and unsteady. In such circumstances the check-shaped sand
barriers should be used. The patterns of sand barriers on longitudinal dunes are
illustrated in Fig. 2.13 .
The disposal patterns of sand barriers at the head of longitudinal dunes are the
same as the patterns on the windward slope of semi-lunar dunes. The patterns on
the body of longitudinal dunes are complex. The body of a longitudinal dune can be
divided into a windward slope and a leeward slope. The patterns on the windward
slope of longitudinal dunes are similar to those on the semi-lunar dunes. A main
sand barrier on the ridge of a longitudinal dune, along the run of the dune, should
be established first. And then secondary barriers that are vertical to that main sand
barrier are set up. In order to eliminate the vortex flow, the run of the secondary
barriers should be inclined in the direction of the prevailing wind. To revise the
distance between two barrier lines and the run of sand barriers, some short barrier
lines among the secondary barriers should be set up. For the longitudinal dunes
whose surface is gentle and smooth, fishbone-shaped or double fishbone-shaped
sand barriers are usually adopted.
For the complicated irregular sand dunes, the fishbone-shaped sand barriers are
usually used by local inhabitants of the sandy area (Fig. 2.14 ).
Fishbone-shaped sand barriers are composed of a main barrier and many
secondary barriers. The latter consist of two kinds. One is a longer barrier from
ridge of sand dunes to the foot of the dunes, and the other is a shorter barrier inter
spread among the longer ones as needed to adjust the pattern.

4.3.2 Row Spacing and Standard Dimensions of Sand Barriers

The row spacing of high standing sand barriers must be appropriate for the
conditions. If it is too wide, the sand barrier is liable to be destroyed by blown sand.
Conversely, if the row spacing is too close, man-hours and materials are wasted.
From the experiences of local people, the row spacing of sand barriers should be