Quantcast Possible Exceptions

 
  
 
General Principles of Erosion Protection
stable. Therefore, any channel realignment project undertaken to alter one of the situations
listed above should avoid extreme values of those variables. Determining the preferred range
of values for a particular stream will be especially difficult if the stream has recently aggraded
or degraded, and the planform is still adjusting, especially if the threshold between meandering
and braided has been crossed.
(1) Specific considerations regarding channel realignment in very sharp bends are as
follows:
Realignment of the channel in a very sharp bend may be justified in order to
prevent a major channel avulsion in the future. Such an avulsion, in the form of
a natural cutoff or development of a major chute channel through the point bar
of the bend, might cause serious bank stability problems in downstream reaches.
The radius of curvature of the bend can be increased, that is, the bend made
flatter, either by using indirect protection, or by making a well-planned cutoff.
An increase in bend radius may result in a decrease in maximum channel depth
in the bend, thus improving bank stability.
Use of an indirect protection method in this situation will ideally result in the
maximum depth of scour occurring farther away from the toe of the bank than
under natural conditions, thus improving bank stability with respect to mass
failure.
The disadvantages of using an indirect protection technique in this situation are
that (a) much of the construction has to be done in the deepest part of the
existing channel, thus increasing the cost and difficulty of construction; and (b)
it may be necessary to excavate the opposite point bar to relieve the initial
constriction caused by the stabilization structures. Otherwise, local velocities,
and perhaps even backwater effects in extreme cases, may be unacceptably high
in the interim period that it takes the channel to adjust to the work.
Realigning the channel by constructing a cutoff across the neck of the bend
amounts to the channel relocation approach discussed in Chapter 4. Erosion in
the bend can be eliminated by a cutoff, but erosion will continue elsewhere if
channel migration is characteristic of the stream. Therefore, even a cutoff may
need to be accompanied by bank protection, and the prediction of subsequent
long-term channel behavior is more uncertain in the presence of a cutoff.
Detailed considerations for designing a cutoff are presented by Petersen (1986).
(2) If the natural bankline alignment is highly irregular, and it is determined that a
more uniform flow and planform would best accomplish project purposes, then
the bankline can be smoothed either by (a) placing indirect protection on a
smooth alignment riverward of the natural bankline, as discussed above for a
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