Indirect Techniques for Erosion Protection
retard. The top elevation of tiebacks is commonly made the same as the retard, although
a lower elevation can be used for a less costly, but less conservative, design.
The spacing of tiebacks can be designed according to the concepts discussed
previously for spacing of dikes. However, such a design would often be overly
conservative, since the tiebacks are simply used to reinforce the main protection device,
the retard itself. The permissible increase in spacing can be determined for a specific site
only by applying judgement, experience, and the factors discussed in 6.6, "Safety factor."
General practice is to place tiebacks on the shortest line from the retard to the
bank. This is the least costly approach, and provides a compromise between them being
perpendicular to the realigned flow and perpendicular to the existing bankline. The lack
of agreement regarding the optimum angle that transverse structures should have with
respect to direction of flow is less troubling for tiebacks than for dikes, since the tiebacks
are not the primary component of the work.
Tieback bankhead design should follow the same principles as for dike
bankheads, but can be less conservative in many cases since the retard itself will usually
decrease erosive forces at the tieback bankhead.
8.1.5 PERMEABLE RETARDS
The advantages of permeable retards as compared to impermeable retards are that
they are equally, if not more, effective when used on streams with relatively high
concentrations of suspended sediment, and are often less costly to construct, since materials
are usually available locally. Typical permeable retards are shown in Figure 8.3.
The disadvantages are that they are less durable than stone retards and some of the
other impermeable retard materials, and are usually considered less aesthetic. They also
interfere to a greater degree with access to the stream channel.
Most aspects of materials and structural design are the same as for permeable dikes
(see 8.1.2). Other design considerations beyond those discussed previously for retards are
(a) Double-row retards are sometimes used to increase structural stability and to further
reduce flow behind the retard. A double-row design also gives the impression of better
toe protection, but that may be illusory for rigid retards, since if the first row fails from
toe scour, the second row is likely to fail eventually also. However, the outer row of
flexible double-row retards, such as jacks, can displace downward into a scour hole and
still provide protection to the inner row.