Controlling Sand Movement Through Mechanical Measures: China’s Experience - Combating Desertification in Asia, Africa and the Middle East - page 22

Fig. 2.1 Relationship between mechanical and biological measures

2.1 Types of Mechanical Measures and Their Uses

Modern classification of mechanical measure is generally based on the materials
used, the methods of installation, the disposal pattern for the trapped sand, and the
height, structure and function of the mechanical measures. Mechanical measures can
be categorized into surface (covering) sand barriers and standing sand barriers. The
types of mechanical measures and their functions are summarized in the Fig. 2.2 .

3 Surface Sand Barriers

The movement of sand dunes needs two prerequisites. They are (1) power source -
wind and (2) substance source - sand. When the wind (velocity 4.5 m/s) blows

over the loose surface of the sandy land or sand dune, it will become sand-driving
wind because some sand is entrained (lifted by the wind). This sand-driving wind is
harmful as it will bury or damage crops or other objects. Measures to prevent such
damage can be taken by covering the surface of the sand dunes. The principle of the
surface barrier is to eliminate one of the above two prerequisites - substance source
sand. When the wind passes over a sandy area, where a surface barrier has been set
up, there will be no sand-driving wind formed.
The materials of surface barriers include straws and stalks of some crops,
branches of trees, cobble stone, clay, earth, emulsified asphalt, asphalt belt and many
kinds of complex polymers. The function of the barriers is to stabilize the moving
sand on the spot because the sand is fixed and the passing wind is separated from
the sand.
In many sandy areas of China, sections of sand dunes and oasis are distributed in
a checker board pattern. These scattered sand dunes seriously threaten the farmlands
of the oasis. In these circumstances, the laying of sand barriers, which are made

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