Page 96 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 91

Figure 4.13
Anatomy of a sand
dune.

This is the angle of repose, and normally is less than 358, although it can
give the impression of being much steeper.

4.8.6 Internal Structure of a Sand Dune

Continual bombardment of the windward side of a dune by saltating sand
grains develops a crust that slowly creeps up over the dune, sometimes
extending out over the edge of the slip face. The dune itself also migrates as
sand grains are dislodged from the windward side and carried across to
the slip face. The migrating dune therefore encompasses former slip surfaces,
as shown in Fig. 4.13. The interior of a dune therefore remains relatively
soft and is an invitation for vehicles to become stuck if tires break through
the surface crust.

4.8.7 Migration Rate
As dunes migrate they cover and encapsulate whatever is in the way—trees,
roads, houses, whatever. As a general rule one should not place anything
close to the lee side of an active sand dune unless it is on wheels.

The rate of dune migration is predictable based on empirical measurements.
Large dunes migrate more slowly than small ones because more sand is
required to extend the slip face. While data are limited, measurements of several
barchans in Egypt gave an average rate of advance of about R ¼ 180/H 20 m/y,
where R is meters per year and H is the dune height in meters.

Example 4.1
Your brother-in-law asks to borrow money to invest in a beach motel that has a swimming
pool and tennis court, and is about 200 m downwind from a beautiful, active, 10-m-high
sand dune. How long will he have before the motel is part of the scenery and he has
to apply for government aid?

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