Quantcast Design Considerations (Cont.)

Indirect Techniques for Erosion Protection
beyond which dikes are not economically feasible. Fortunately, that limiting height is
often greater than that required for successful performance, since stabilization of the toe
and lower bank slopes are the key to success in most applications. Also, the incremental
construction approach discussed in 5.3.3 can sometimes be used to reduce the additional
cost of increased height.
As a very broad generalization based on past experience, an acceptable range of
dike heights in many situations is between 1/3 and 2/3 of bank height, or in the case of
incised streams, 1/3 to 2/3 of the distance between low water elevation and the elevation
of a flow with a return interval of one to two years. The lower figure will certainly not
be a conservative design, or even as conservative as designing a retard to the same
elevation, but dikes are not as suitable as retards for a situation requiring conservative
design in any case.
As a design refinement, the height of a dike can vary from the bankhead to the
riverward end, i.e., be sloped downward. This provides two advantages:
It creates less constriction of flow as flow increases, because the riverward
portion is submerged at higher flows. This is particularly important for
impermeable dikes.
It results in maximum economy, because the structure can then more closely
follow the contour of the bank and channel bottom, reducing the required
size of structural components of permeable dikes, and reducing the volume
of impervious dikes. This in fact is the only feasible approach when
prefabricated components of a single size, such as jacks, are used.
A combination of sloped and level profiles is often used when the channel is to
be shifted away from the bank significantly.
A dike profile can be "notched" for environmental purposes, allowing some flow
to enter the dike system to enhance habitat diversity and water quality, while still
diverting sufficient flow to provide erosion protection to the bank.
Physical model studies reported by Franco (1982) indicated that a system of dikes
having successively lower elevations in the downstream direction tended to accumulate
more deposition than other designs. However, that finding is not usually pertinent to
bank protection dikes. The model studies were for long structures in a wide channel,
designed to deepen the crossing between two bends. Following that scheme for dikes
in a typical eroding channel would require either that the upstream structures be
relatively high, or the downstream structures relatively low, choices which would
respectively either increase the cost of the upstream dikes substantially, or reduce the
effectiveness of the downstream dikes. In a bend, the hydraulics of flow would likely
overcome whatever beneficial effect a stepped-down system might have, resulting in the
strongest attack on the bank being where the dikes would be the lowest.
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