Page 93 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
88 Geotechnical Engineering

the sediment. After completion of the Three Gorges Dam on the Yangtze River in
China in 2009, the plan is to draw the reservoir down during the flood season, which
will allow 90 percent of the annual sediment load to pass through with 60 percent
of the annual inflow of water. The procedure undoubtedly will be rife with
controversy, because managing a dam to prevent sedimentation also means
managing it to allow flooding. Even though it should extend the reservoir life by
a factor of 10, the anticipated life still is only 100 years. Thus, while hydroelectric
power is widely regarded as a renewable resource, the reservoirs created by
damming are not readily renewable.

4.7.5 Downstream from a Dam

Removal of sediment makes a river more erosive, so the channel immediately
downstream from a dam generally deepens and works with increased energy
against its banks, increasing the need for bank protection to prevent landslides.

Sediment depletion by a reservoir also impacts the delta downstream that holds
a tenuous balance against wave erosion. The delta of the Colorado River guards
the Imperial Valley of California, one of the richest agricultural areas in the world,
against invasion by the sea, but the sea has been advancing since completion of the
Hoover Dam in 1934.

Wave erosion of deltas is the primary source for beach sand that is distributed
by longshore currents. Thus as a delta becomes depleted, beaches may require
protection from wave erosion. Current policy in many areas is to prohibit building
closer to the sea than the anticipated beach position after 50 or 100 years.

In summary, changes that are worked on a river, whether by shortening, confin-
ing it with levees, damming, or other measures, all affect the natural balance of the
river system, its delta, and the associated beaches. This does not mean that there
is no net gain, but only that engineers and planners should be fully aware of the
eventual consequences and incorporate them into their feasibility studies.

4.7.6 Beach Deposits
After they are derived from deltas or other sources such as erosion of headlands,
beaches are subjected to continuous, unending wave action that grinds away soft
minerals and concentrates hard minerals, in particular quartz. Beach sediments also
are well sorted (all one size), well rounded, and can vary in size from gravel to sand.

Longshore currents that carry sands along the lengths of beaches are products of
winds blowing toward the beach at an angle. With certain wind directions two
opposing longshore currents can come together in a bay and create a dangerous
rip tide that flows offshore. Swimmers or boaters caught in a rip tide should move
parallel with the shore instead of trying to fight against it, which is a losing battle.

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