Page 84 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 79

Meandering presents a dynamic equilibrium between erosion and deposition.
A stream that has reached this balance is said to be mature. There is an important
implication in this, because straightening a meandering river makes it shorter,
and increases its gradient and potential for erosion. Straightening a river upstream
from a bridge therefore can be a bad idea unless the river banks are protected
from erosion. This normally is accomplished by driving steel sheet pile or placing
large rocks called ‘‘rip-rap.’’

4.6.5 River Meandering and Property Lines
Shifting river channels undercut soil along the outside of a meander so that it
slides off into the river, and simultaneously deposits sand bars on the inside of
the meander. Courts make a legal distinction between a rapid change caused
by channel abandonment, referred to in legal terms as an ‘‘avulsion,’’ and a slow,
gradual shift due to bank erosion, referred to as an ‘‘alluvion.’’ An avulsion
occurs when one meander loop catches up with another, and leaves legal
boundaries intact, whereas a slow channel migration takes property lines with it.
An avulsion in 1898 left the town of Carter Lake, Iowa, on the Nebraska side
of the Missouri River, so Omaha residents drive through Iowa to get to their
airport, which would seem an ideal place to require a toll.

4.6.6 Floodplains of Meandering Rivers
Lateral meandering creates relatively level floodplains that, as the name implies,
are subjected to flooding. Floodplains nevertheless are favored industrial
sites because they are close to river transportation.

River meandering and associated sorting activities create a host of different
sedimentary floodplain soils that vary from relatively clean gravel and sand to
heavy, soft clay. Most deposits are readily identifiable from their landforms
and their appearance on airphotos, as shown in Fig. 4.7. The most important
deposits are as follows:

Point Bars
Probably the most conspicuous deposit of a meandering river is sand that
occupies the inner area of each meander loop. The term ‘‘point bar,’’ like many
other terms used to describe rivers, comes from river navigation. As meander
loops migrate downstream the point bars are like footprints forming a line
down the river. Because the point bar inside one meander loop is directly
across the river from the next one, they form a continuous band of sand that
is criss-crossed by the river channel. Bridges across meandering rivers there-
fore, at least in part, are supported on point bars. After they are deposited,
point bars soon are covered by vegetation, but their identity still can be deter-
mined from their position relative to existing or former channels of a meandering
river.

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