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Indirect Techniques for Erosion Protection
8.1.1.3 Typical Applications
Typical application of dikes is in straight reaches and long radius bends, since as bend
radius decreases, spacing must decrease, and the required number of dikes soon reaches a
point where a retard could be built for the same cost or, if channel realignment is not required,
an armor technique could be used.
8.1.1.4 Design Considerations
Design considerations for dikes beyond the general factors discussed in 8.1 is one of
the most complex issues in design of erosion protection works. There is general agreement
on some aspects, but considerable diversity, even controversy, on others. A complete reading
of the Federal Highway Administration report is recommended to obtain full understanding
of the complexities involved in dike design (Brown, 1985).
Design involves the following major parameters:
(a)
Permeability;
(b)
Length;
(c)
Spacing;
(d)
Angle with respect to flow;
(e)
Height;
(f)
Bankhead design; and
(g)
Structural scour protection.
(a) Since permeability affects some of the other design parameters, it is appropriate to
discuss it first. Permeability is defined as the ratio of the area of openings in the dike to
the total projected area of the dike, and is expressed as a percentage. If the stream
carries only a small amount of debris, or the dikes are low enough that debris will pass
over them during most flows, the permeability can be assumed to be the as-built
condition. However, if debris loads are moderate to high, then some reduction in
permeability with time should be assumed.
FHWA (1985) suggests that where a large reduction in at-bank velocity is
required, such as in sharper bends, permeability should not exceed 35 percent. Where
a moderate reduction in velocity is sufficient, such as in bends with mild curvature and
less easily erodible bank material, permeabilities up to 50 percent can be used. In mild
exposures such as straight reaches with low erosion potential, permeabilities up to 80
percent may be successful. However, permeabilities greater than 50 percent are not
recommended unless success under conditions similar to the project at hand can be
documented.
The U.S. Army Corps of Engineers (USACE, 1981) suggests that permeability
should decrease with decreasing size and quantity of sediment carried by the stream in
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