Indirect Techniques for Erosion Protection
overwhelming number of possible variations are described by California
State Department of Highways (1960) and FHWA (1985). Beyond their
practical value, these publications provide testaments to the wide variety
of river stabilization problems encountered in practice, and to human
imagination in problem solving.
(d) The need for toe and local scour protection may be less obvious than for
armoring techniques, but is still important (see 6.3). Using a permanent
scour protection material, such as stone, in conjunction with dikes or
retards of a less durable material will allow the designer to be less
concerned about dike and retard durability, if woody vegetation will
eventually provide the same erosion protection to the middle and upper
bank as the dikes or retards provided in the beginning.
(e) Since mechanical connections cannot be made underwater, river stages
during the construction season will affect some aspects of design,
dictating that prefabricated elements or a launchable material such as
stone be used for the portion of the structure which will be built
Advantages of dikes as compared to retards is that they will usually be less expensive
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 of dikes as compared to retards is that they will usually be less effective
in eliminating bank erosion. 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.