Page 87 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
82 Geotechnical Engineering

repeatedly in the Mississippi River above New Orleans, each time forming
a new delta. As the delta extends outward, the river level must rise in order to
preserve a downhill gradient, so the system becomes unstable and the river will
seek to escape the confines of its meander belt and levees. This is the situation at
New Orleans.

4.6.9 Overbank Deposits

Water spilling out over the banks of a river slows down and deposits sediment.
Fine sand and silt are deposited close to the river channel in low ridges called
natural levees, which are preferred building sites. Exploration borings must extend
through the natural levee because it normally will be on top of softer floodplain
clay and may conceal a buried clay plug.

Some floodplain areas will have a lighter color on airphotos and appear to have a
braided pattern indicating a sand deposit, but this is misleading because the sand
may only be a meter or so thick. This is a sand splay, which occurs where a levee
has been breached at some time in the past.

Clay carried overbank or through a breach is slow to settle out, and forms
a continuous blanket on top of older deposits. This clay is called a ‘‘backswamp’’
or ‘‘slackwater’’ deposit, and can be meters thick. It frequently is expansive clay
and therefore can be of critical importance in engineering. Desiccation at the
surface creates a harder crust that can be utilized for supporting lightly loaded
foundations.

4.6.10 Natural and Artificial Levees

The first response to a flood is to build or increase the height of a levee. This
confines the river from a breakout and increases its water level and gradient.
Breaching a levee on one side of the river can save the other side, which is why
levees are patrolled during flood stages to prevent the use of dynamite.

As a delta builds out into a sea, the river is extended and natural levees are
built up higher during flood. A river level that is higher than the adjacent
floodplain is a recipe for disaster. The most famous example of delta extension
is the heavily loess-laden Yellow River (Huang Ho) in China, considered to be
the muddiest major river in the world. In some locations the delta has extended
as much as 8 km (5 miles) in one year, and natural levees have been built up to
the extent that the river as much as 21 m (70 ft) above the adjacent floodplain.
Sudden breakouts are almost impossible to contain, and have killed hundreds
of thousands of people, giving the river its name ‘‘River of Sorrow.’’ Dams are
being constructed to control flooding, but a river carrying such a heavy load of
sediment will quickly silt up the reservoirs and reduce their effectiveness. One plan
is to allow the river to flow on through during periods when it is most heavily
laden with silt.

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