Quantcast Design Considerations (Cont.)

 
  
 
Indirect Techniques for Erosion Protection
As in determining dike spacing, any future change in alignment of flow due to
channel migration upstream should be considered when designing the angle of dikes with
respect to flow direction.
(e) General factors affecting the optimum height of erosion protection works were discussed
in Section 6.2.5, "Top elevation of protection." Although the term "elevation" is more
precise than "height," the term "height" will be used in the discussion below because it
is more commonly used in dike design practice.
Height of dikes in a system is often related to bank height which, in turn, can be
related to some recurrence frequency of river stage. In humid areas, bank height is often
a one or two year return interval for streams that are neither aggrading or degrading.
Unfortunately, any design relationship of dike height to bank height is more conceptual
than quantitative, and no generally accepted precise guidance can be stated.
In spite of the uncertainties involved, some general guidance can be stated
regarding the determination of appropriate dike height. FHWA (1985) states that dikes
need be only high enough to protect the bank zone of active erosion, but follows that
general axiom with the following three specific guidelines:
Dike height should be no higher than top bank, but no lower than 3 feet
below "design flow."
Impermeable dikes should be submerged 3 feet at the most severe expected
flow condition, because the local scour associated with submerged dikes
seems to be smaller and located farther from the bank than that associated
with unsubmerged dikes.
Permeable dikes should be lower than flow stages that carry significant
debris loads.
Application of these guidelines will often result in a fairly conservative design,
which is understandable, since the guidelines were developed for application to the
protection of highway facilities from channel migration. However, the latitude which
exists in the determination of the design flow and the most severe expected flow
condition still leaves considerable latitude for the engineer to be more or less
conservative as appropriate for a specific project, even if dike height is based on these
guidelines.
In practice, the uncertainties of the physical effects of height often become moot,
because the economics of dike construction often dictate that dikes be considerably
lower than top bank elevation. For permeable dikes, the rapid increase in cost as the
height increases is due to structural factors, as discussed below under "permeable dikes."
For impermeable dikes, the rapid increase in cost is due to the exponential increase in
structure volume as height increases. For a specific project, there will usually be a height
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