Geoscience Reference
In-Depth Information
Fig. 2.8 The run of sand
barrier on the windward slope
of dunes
and the leeward slope is shorter and steep. From this phenomenon, the direction of
the prevailing wind can be easily seen but for micro landscapes, the directions of
the wind are various, and in this case we need to consult patterns of sand ripples.
The run of the ripples formed by the coarse sands on the surface of sand dunes
is perpendicular to the direction of the prevailing wind. These ripples can clearly
show us the direction of the prevailing wind, so sand barriers can be set up along
the run of the ripples of coarse sands. Ripples of fine sand formed between the
ripples of coarse sands, caused by partial changes of the direction of the prevailing
wind should not be regarded as indicators of the direction of the prevailing wind. In
general, the third indicator, the run of sand plaits, is more reliable than the previous
two. Because the spreading direction of the sand plait is identical with the direction
of the prevailing wind, this direction can be recognized through an observation of
the run of sand plaits.
The run of the sand barrier should be kept at right angles to the direction of the
prevailing wind. When a sand barrier is planned for the windward slope of existing
sand dunes, a line, which goes along the axis of the sand dune, should be first drawn
in line with the prevailing wind. For practical purposes, the angle between the run
of the sand barrier and this line should be more than 90 ı because of the distribution
patterns of the wind on the sand dune. The wind is stronger in the middle (top) than
on the two sides. If the angle between the run of the sand barrier and the direction
of the prevailing wind is acute, the wind will be concentrated in the middle part
of the sand dune, and therefore the sand barrier is liable to be destroyed in this
area. If the angle is between 90 ı and 100 ı , the wind will go along the two sides of
the obstruction. The effectiveness of the sand-control of this kind of sand barrier is
satisfactory (Fig. 2.8 ).
The disposing patterns of sand barriers are mainly as follows: linear-shape,
parallel lines, trellis (check) shape and fishbone shape. In an area where the direction
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