General Principles of Erosion Protection
sharp bend; or by (b) placing trenchfill or windrow revetment landward of the
protruding bankline points, and allowing the stream to erode the irregularities
away. This approach offers simple design and construction, since the operation
is removed from the active channel. However, the bank retreat which occurs
before the channel reaches the stabilization structure may not be greeted
enthusiastically by property owners. Conversely, if erosion is slower than
anticipated, a navigation project with a schedule to meet may require costly
dredging of the uneroded foreshore.
(3) In a straight reach with an unstable planform, or on a braided stream, it may be
desirable to increase the sinuosity of the main channel in order to stabilize the
location of scour pools and bars. This can also provide better channel alignment
and a deeper channel for navigation. However, an accurate assessment of
channel migration tendencies, the stable range of values for pool and bar spacing,
and the ratio of radius to width, is especially critical in this situation. Such a
realignment often involves both armor revetments and indirect protection in
combination, depending upon the bank and channel topography along the
proposed realigned bankline.
6.2 HYDRAULICS
Having used applied fluvial geomorphology to decide on the location of bank
protection work, the next step is to apply a fluvial hydraulics analysis to decide how deep,
how high, and how strong to make the work. The concepts of fluvial hydraulics presented
in Chapters 2 and 3 apply to the following factors:
Design discharge;
Tractive force and permissible velocity;
Secondary currents;
Variations in river stage;
Top elevation of protection;
Wave, vessel, and ice forces; and
Prediction of toe scour.
6.2.1 DESIGN DISCHARGE
It is important to recognize the distinction between design discharge and dominant
discharge. Design discharge usually refers to an extreme event, and is often used in
connection with flood control channel analyses. A tractive force or velocity associated with
the design discharge is also commonly used to compute stone size for riprap armor (see
6.2.2), and a similar approach can be used to design many commercially available armor
materials. The design discharge can be defined quite precisely using hydrologic analyses.
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