Page 94 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 89

4.7.7 Breakers and Barrier Beaches
The drag of wind on the water causes a near-circular motion where the diameter
of the circle represents the height of the waves. As a wave progresses into shallow
water, frictional drag at the bottom distorts the circular orbit and reduces the
wavelength so that water piles up. The wave breaks over at the top when the depth
is about l.5 times the wave height.

Breaking dissipates energy, so sand deposition occurs to the landward side of the
break zone and builds a submerged offshore bar. The bar can be built above sea
level during storms, in which case it becomes a barrier beach that is separated from
the shore by a lagoon. The lagoon then tends to trap fine-grained sediments
brought in by streams and rivers.

Barrier beaches are common along many coastlines, including the eastern and
southern coasts of the United States, and are shown on maps. They tend to erode
on the seaward side and build up on the landward side, so a beach slowly migrates
landward and covers the associated clayey lagoon sediments. The clay helps to
prevent intermingling of fresh and salt water in the groundwater supply, but also
forms a soft zone for foundations.

4.7.8 Fossil Beaches
In the long term most land masses are not stable relative to a sea level that also
is not stable. An emerging coastline may preserve beaches as part of the upland.
Such beaches often are marked by sand dunes. A submergent coast is more likely
to be marked by narrow beaches, rugged cliffs, and estuaries.

4.8 EOLIAN SANDS

4.8.1 Dunes
Sand dunes are among the most easily recognized sedimentary deposits because of
their sweeping curves and frequently blowing sand. Less obvious are sand dunes
that no longer are active and are referred to as stable dunes. Stable dunes support
protective vegetation but nevertheless often are pock-marked with wind-eroded
blowouts.

Active dunes require a continuous source of sand, and therefore occur adjacent
to beaches or alluvial sand, particularly along braided rivers. Desert dunes derive
from the extensive alluvial fans in deserts. Contrary to popular conceptions, sand
dunes cover only about one-fourth to one-third of desert areas, the rest being
mainly exposed rock and alluvium.

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