Selection and Design of Channel Rehabilitation 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.
Retard is defined as a continuous structure approximately parallel to the streamflow. It can be a
single structure or two, or more, adjacent and parallel structures, in which case the space between may be
filled with various materials. Other terms that are sometimes used are longitudinal dikes, parallel dikes,
jetties, guide banks, and training walls. Most designs have occasional tiebacks extending from the bank
out to the main structure. These tiebacks have the appearance of dikes. In fact, many retard designs can
be viewed as being a dike system with a longitudinal component connecting the ends of the dikes.
Dikes and retards can be applied to a wide range of conditions. However, the most common use
is on shallow, wide streams with moderate to high transport of suspended bed material. Shallow channel
depths reduce the required height of structures, a wide channel provides room for the channel alignment
and geometry to adjust, and a heavy supply of suspended bed material accelerates the rate of induced
deposition.
Where long-term funding is provided, dikes and retards are often built in increments in order to
reduce costs by modifying the river form gradually, and taking advantage of subsequent deposition.
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.
Advantages: Dikes and retards provide a means to modify the channel alignment, are well suited
to the incremental construction approach, and are amenable to the establishment of woody vegetation.
Also, many designs use locally available material.
Dikes and retards offer the opportunity for incorporating a wide variety of environmental features
by increasing the diversity of aquatic and terrestrial habitat, although subsequent sediment deposition may
be detrimental to shallow water habitat. The reduction of water surface area due to deposition within the
dike or retard system will reduce evaporation rates, which may be considered to be a benefit in semi-arid
areas.
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