Page 92 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 87

electrical charge that normally keeps particles repellent to one another changes
so they attract one another; the clay becomes flocculated into silt-size particles
that settle out and make up most of the foreset beds. In fresh water, clays remain
suspended and slowly settle out into the lakebed and as bottomset beds in a delta.
Foreset beds in a freshwater delta are mainly sand and silt.

Topset beds in both types of deltas are similar to river floodplain deposits, being
extensions of the channel sands, natural levees, and backswamp clays. A ‘‘bird’s
foot’’ delta, such as that of the Mississippi, is defined by natural levees that
continue and channelize the river for a distance out into the sea.

4.7.3 Estuarine Deltas
An estuary is a river valley that has been drowned by the postglacial rise of sea
level and where tides are present. Sedimentation that is confined within an estuary
constitutes ‘‘estuarine delta,’’ which is mainly tidal mud flats.

4.7.4 Deltas in Artificial Lakes
The several benefits from dams, namely flood control, irrigation, power genera-
tion, recreation, and navigation, are not without environmental consequences.
Some are predictable from the river dynamics: a dam is the base level for the
portion of the river that is upstream, causing the stream to pond as a reservoir.
Delta building then initiates where the river enters the lake, and people who are in
the market for a lakeside lot are advised to avoid the upper reaches of the reservoir.

As the delta grows and the reservoir becomes silted in, uses of the lake gradually
will be compromised. A clue is growing mud flats that eventually will become
floodplain. In the U.S., a minimum reservoir design life of 100 years has been
considered acceptable, but sedimentation rates often show that estimates that
were used as a basis for dam building were overly optimistic. Reducing soil
erosion in the drainage area has obvious benefits, and where feasible smaller check
dams can be built upstream from a main reservoir to catch sediment in areas
that can more easily be cleaned out. Adding height to a dam is far more difficult,
and dredging is expensive and may cost more than the dam while posing
a problem of where to put the spoil.

The seriousness of the coming problem is shown by reservoirs that already have
become silted in 15 years after construction. By the time a dam was completed on
the Yellow River in China in the 1970s, the capacity of the reservoir for power
generation had been reduced by three-fourths. If sedimentation continues at the
present rate, the reservoir will be completely filled with loess-derived silt by 2050,
80 years after completion of the dam

The most promising solution to the sedimentation problem may be to bypass
sediment-laden water, particularly during floods that carry the bulk of

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