Indirect Techniques for Erosion Protection
"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.
Advantages are: Dikes and retards provide a means to modify the channel
alignment if that is a project requirement. They are also 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. They may increase 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.
Disadvantages are: Those designs which involve "perishable" materials or
mechanical connections are susceptible to gradual deterioration and to
damage by debris, fire, ice, and vandals.
Channel capacity at high flow is decreased initially when dikes or retards are
constructed, although the channel will usually adjust by forming a deeper,
though narrower, cross-section, and the ultimate result may even be an
increase in conveyance capacity. However, the extent of the adjustment
cannot be always be predicted reliably, even with physical or numerical
models. Since conservative assumptions on future deposition and vegetative
growth would be necessary, extensive use of dikes or retards must be
approached with caution on projects where channel flood conveyance is a
concern.
Typical applications are: 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, because 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.
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