Page 90 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 85

Figure 4.10
Illustration of
graded bedding
and cross-bedding.

Figure 4.11
The large alluvial
fan at the center
has been cut off at
the toe by the
river. A large,
steep talus slope is
at the right of the
fan, and a road
may be seen
running across the
fan. Indus River,
Karakorum Range,
Pakistan.

susceptible to wind erosion, and silt may be observed billowing upward from
exposed sand bars in Alaska. The silt then is deposed as loess.

Terraces left by braided streams are important sources for aggregate, and may
be recognized on airphotos from darker, branching channels enclosing lighter
almond-shaped sand bars.

4.6.16 Alluvial Fans

A sediment-laden stream can become braided if there is a sudden decrease in
gradient, the most common example being when a tributary stream flows out
onto a floodplain. Sediment is deposited and clogs the stream channel so it finds
another route, which then becomes clogged. This process is repeated and develops
a radial pattern that builds up a fan-shaped deposit appropriately called an
alluvial fan (Fig. 4.11). As alluvial fans are relatively coarse, open-graded material,
in arid areas they are important reservoirs for groundwater.

Coalescing fans from adjacent streams constitute an alluvial plain or ‘‘bahada.’’
This can be a preferred site for a road or other construction, as it is not subject
to flooding by the river. However, it still is subject to flash flooding and erosion by
the tributary.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.