General Principles of Erosion Protection
6.1.2 CHANNEL ALIGNMENT
The preferred choice for the alignment of a stabilized channel is straight-forward.
However, exceptions to the preferred choice are rather common. Following is a discussion
of the preferred choice and some exceptions.
6.1.2.1 Preferred Choice
The preferred alignment in most cases is to accept the existing general channel
alignment, because significantly changing the alignment makes it more difficult to predict the
ultimate equilibrium planform and channel geometry. This uncertainty carries risk not only
for the success of the work, but also for assessing the potential for detrimental effects caused
by the work.
Relocation of a bank which is to be armored or vegetated requires costly and
time-consuming excavation and/or filling. Also, the environmental effects of removing and
disposing of large amounts of bank material may be unacceptable. The work would be
vulnerable to damage from high flows during construction, causing both contractual and
engineering difficulties. Banks constructed totally of fill material would be highly susceptible
to settlement and scour even after being armored, unless the fill is well-compacted during
construction and a well-designed filter is provided. Both of these measures would add
substantial cost and difficulty to the project. Also, vegetation may not provide adequate
protection for banks newly constructed of fill material, which would further limit the potential
for a cost-effective and environmentally acceptable design.
Indirect protection methods (Chapter 8) can more easily be used to modify the
existing alignment, but the same basic principle applies - the existing stream, especially if it
is in a condition of dynamic equilibrium and has developed a stable alignment, so changing
that alignment may generate system-wide instability and should be approached with caution.
6.1.2.2 Possible Exceptions
Exceptions to the preferred choice of accepting the natural alignment are sometimes
justified in situations other than that of limited foreshore. Three potential exceptions are:
(1) At very sharp bends;
(2) Highly irregular banklines; and
(3) Straight reaches with unstable planforms.
Channel realignment in these situations is more likely to be required on projects with
navigation aspects than on projects with only bank stability aspects.
Most alluvial rivers have a range of values of radius of curvature, meander wave
length, or in straight reaches, pool and bar spacing, within which the planform is dynamically
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