Indirect Techniques for Erosion Protection
Where long-term funding is provided, they are often built in increments in
order to reduce costs by modifying the river's form gradually, and taking
advantage of subsequent deposition to reduce total project cost.
Dikes and retards are often used on large rivers to increase depth for
navigation, in addition to improving the alignment and stabilizing the banks.
They can be used to stabilize the channel alignment upstream and downstream
of armor revetments in bends, since the shallower depths, moderate velocities,
and less concentrated drift loads upstream and downstream of bends are more
suitable to in-channel structures than is the bend itself.
Dikes and retards can be used where establishment of riparian vegetation is
a high priority. Initial plantings and natural establishment of native species
can be supplemented by later plantings on sediments deposited within and
behind the structures, or by sloping and vegetating the upper bank slopes once
lower bank stability has been attained.
No formal and widely tested design criteria for dikes and retards exist,
although design concepts based on experience and model tests have been
developed for some applications. A study performed for the U.S. Federal
Highway Administration and reported by Brown (1985) is one of the most
comprehensive analyses of dikes. That report is based on model tests, a
literature review, and a survey of several hundred field installations. Studies
by the U.S. Army Corps of Engineers (USACE, 1981) also provide
observations on design parameters. Some findings from these and other
studies, and from practice, are discussed later under specific headings. The
following general concepts apply to the design of both dikes and retards:
(a) Because there are so many variations in design, one must be cautious of
becoming so engrossed in the details of materials and construction that
the importance of the basic layout is overlooked. If the basic principles
in 5.1 and 6.1 are followed, then there are many specific designs that will
work equally well, but if basic principles are neglected, the most
painstaking attention to detail will be in vain.
(b) Simplicity should be a design goal. The principles of value engineering
are particularly applicable for dikes and retards. Other factors being
equal, a design with fewer components and mechanical connections will
be more durable and less costly than a more complicated design.
(c) Basic decisions on materials and structural design for a specific project
are inherent in the selection process discussed in Chapter 5. Other
aspects are covered below under more specific headings. An
exhaustive investigation by the engineer of all design alternatives for a
specific project is neither practical or necessary. Many of the