General Principles of Erosion Protection
6.3.2.5 Dikes
The provision of toe protection for dikes is a more complex problem than that for
armor revetments. The complexity arises when trying to distinguish among three cases:
(1) General toe scour which immediately endangers the overall stability of the bank
and threatens to flank or fail the dike system.
(2) Local scour which can threaten the integrity of part of a dike, and which
ultimately may fail local portions of the bank.
(3) Local scour which may cause minor damage to a dike or minor bank instability,
but which can be accepted.
In practice, it is difficult to separate these three processes. Conceptually, however,
case (1) must be prevented, and case (2) must be addressed if the consequences of it
occurring are high. Acceptance of case (3) is inherent in the choice of dikes as the method
of erosion control.
The alternative toe protection treatments, which can be used separately or in
combination, are to:
Extend the dikes into the channel to move general scour far enough away
from the bank to prevent major geotechnical instability.
Provide separate protection at the toe of the bank with an adjustable armor or
flexible mattress. With this approach, the dikes will limit the velocity and
associated general scour near the bank, theoretically allowing a less substantial
toe protection than without dikes. This approach may not be cost effective
for preventing general scour, since the effect of dikes on general scour cannot
be reliably predicted, requiring a more conservative design for the separate toe
protection than is theoretically necessary. However, it is often used to protect
against local scour induced by the dike itself.
Provide separate protection riverward of the bank toe, perhaps along a line
connecting the ends of the dikes. This is a conservative, but costly approach,
which may negate the cost advantage that dikes might otherwise provide. In
the extreme case, this approach would more properly be termed a type of
retard. In this case the dikes would simply serve as tiebacks, and would be
the secondary component of the work.
Some permeable dike designs, such as tire-posts and "Palisades" (a commercial
product) allow components of the structure itself to be displaced downward, maintaining
contact with the bed as scour occurs. With these designs, the same cautions that are stated
below for flexible retards are applicable.
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