Selection and Design of Channel Rehabilitation Methods
Dikes are usually less expensive than retards for a given situation, and will not interfere with access
to the stream. Also, after the stream has adapted to the initial project, dikes can be extended farther into
the stream if necessary to fully achieve project objectives, whereas with retards, modification of the initial
alignment is likely to be much more expensive.
Disadvantages: Those designs that 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, 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.
Dikes are more vulnerable to floating debris than are retards, since dikes present abrupt obstacles
to flow, whereas retards, being approximately parallel to flow, will allow much of the floating debris to pass
through the project reach. Also, erosion between the dikes in a system will often be more severe and of
longer duration than erosion within a retard system.
188.8.131.52 Other Flow Deflectors
Structures other than dikes and retards may provide a means of altering hydraulic conditions in
order to resist bank erosion in bends. One of the most intractable problems of river engineering is posed
by the coupled processes of deposition of sediment on point bar faces and scour in the thalweg of bends.
Several approaches have successfully addressed these coupled processes in some cases. These
approaches alter secondary currents so that sediment transport away from the toe of the bank is reduced.
This results in a more uniform cross-section shape, with shallower thalweg depths and a wider channel at
low flow. These approaches include Iowa vanes, and bendway weirs. Because these are recently
developed techniques, the long term success of these structures as a bank stabilization scheme is not
known. Further research and monitoring of existing structures is needed to document the long-term
performance and to develop more definitive design criteria.
VEGETATIVE METHODS FOR EROSION CONTROL
Vegetation is the basic component of bioengineering (Schiechtl, 1980) or biotechnical engineering
(Gray and Leiser, 1982; Gray and Sotir, 1996). Schiechtl (1980) states that bioengineering requires the
skills of the engineer, the learning of the biologist, and the artistry of the landscape architect. The
concept of bioengineering is ancient, but there has been much recent research and documentation of the
topic. The publications just cited, as well as Coppin and Richards (1990), provide comprehensive
coverage, and many other works provide discussion of specialized aspects of the subject.