Page 95 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
90 Geotechnical Engineering

4.8.2 Movement of Sand
Whereas silt is fine enough to be carried suspended in air as dust, sand grains
quickly settle out and bounce along on the ground, sand-blasting exposed rocks
and ankles, and clipping off old fence posts close to the ground. The transport
mechanism is called saltation, and requires that sand dunes be connected to
a source. The connection can either be more dunes or a barren ‘‘desert pavement’’
that is covered with wind-polished stones. Desert pavements may show thin,
straight sand strips or streaks after a single storm.

4.8.3 Cliff-Head Dunes

Eddies created by winds crossing a river floodplain can carry sand to the top of
the adjacent river bank where it is deposited as ‘‘cliff-head dunes.’’ These dunes
are analogous to snow drifts, and normally remain anchored to their source areas,
growing ever larger and extending inland. In arid areas, isolated dunes called
‘‘barchans’’ may break away and migrate downwind as the ‘‘children’’ of
cliff-head dunes.

4.8.4 Dune Shapes

Dunes may exhibit many shapes indicative of the directions of prevailing winds.
Most common where there is a generous supply of sand are transverse dunes that
resemble gigantic ripples. Where there is less abundant sand, transverse dunes
may become partitioned into barchans, which are the classic crescent-shaped
dunes that are a favorite with photographers. The tails or horns of a barchan
sweep off downwind, a tail from one dune leading to the head of the next.

A British Army engineer, R. A. Bagnold, made an extensive study of dunes in
the Sahara during World War II. He found that with two instead of one dominant
wind direction, one tail of a barchan tends to grow longer and give a classic
‘‘seif’’ dune, named for its shape like an Arabian sword. The long tails of seifs
may link into a continuous chain or ‘‘longitudinal’’ dune with a succession of
peaks and saddles. Whereas the maximum size of barchans is limited to about
30 m (100 ft) because of blowing off at the crest, longitudinal dunes, being
alternately blown at from both sides, may continue to build to a height of 200 m
(650 ft), and become a very prominent landform extending downwind for many
kilometers.

4.8.5 Dune Slip Face and the Angle of Repose

A characteristic of all active dunes regardless of shape is the occurrence of a slip
face on the leeward side, as illustrated in Fig. 4.13. The slip face is the main area
of deposition, as sand grains bouncing up the windward side drop into the
wind shadow behind the dune. Sand grains sprinkling down a slip face adjust
to a constant angle that depends on the frictional characteristics of the sand.

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