Indirect Techniques for Erosion Protection
Advantages are: Little or no bank preparation is involved for indirect
protection. This reduces costs and riparian environmental impacts, simplifies
the acquisition of rights-of-way, eliminates material disposal problems, and
usually allows existing overbank drainage patterns to remain undisturbed.
Existing channel alignment and/or geometry can be modified, although the
changes may not always be beneficial or predictable.
Indirect approaches usually increase geotechnical bank stability by inducing
sediment deposition at the bank toe, although this process may not be rapid
or reliable enough to meet project goals.
Disadvantages are: Where geotechnical bank instability or erosion from
overbank drainage is a major factor, the fact that indirect protection does not
immediately relieve these problems can be a serious and often unacceptable
shortcoming.
Because significant changes in flow alignment, channel geometry, roughness,
and other hydraulic factors often result from indirect protection structures,
special attention must be given to the stream's morphological response.
Some types of indirect protection structures may be a safety hazard if the
stream is used for recreation or navigation, and the aesthetics of some types
often leave much to be desired, although vegetative growth may ultimately
reduce the visual impact in most regions.
Since indirect methods extend into the stream channel, their construction may
be difficult, especially during high flow. Also, the structures may be subjected
to severe hydraulic conditions throughout their lifespan, and should be closely
monitored to insure that maintenance is performed as necessary.
8.1 DIKES AND RETARDS
The following paragraphs outline the general description, advantages, disadvantages,
typical applications, and design considerations for dikes and retards used in bank
stabilization methods:
"Dikes" are defined as a system of individual structures which protrude into
the channel, generally transverse to the flow. Other terms which are often
used are "groins," "jetties," "spurs," "wing dams," and if they protrude only
a short distance into the channel, "hard points." The term "dikes" is also used
in some regions to refer to earthen flood-containing structures, which are also
called "levees," but that usage is not relevant here.
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