Selection of Site-specific Stabilization Techniques
placing the armor is usually preceded by grading or smoothing the bank. Roughness factors
for commonly used materials are given in hydraulic handbooks, and roughness factors for
many commercial armors have been determined by the manufacturers.
Dikes and retards, however, could be expected to reduce conveyance, as a result of
obstruction of flow by the structures themselves, and as a result of subsequent deposition of
sediments and vegetative growth induced by the structures. However, channel adjustments
will usually occur, and the ultimate effect may even be a more hydraulically efficient channel
if sufficient deepening occurs in the stabilized channel. A precise prediction of the ultimate
effect of comprehensive stabilization with indirect methods is not possible. However, the
sensitivity for a particular project can be defined by computing flow profiles with and without
the project. The most sophisticated approach is to use a coupled flow-sediment numerical
model to account for possible deepening of the channel after the initial constriction, but the
most conservative approach is to assume that:
No deepening will occur in the stabilized channel.
Significant deposition will occur within the structures.
Stabilized banks will eventually become vegetated in a manner similar
to naturally stable banks, or in the case of works designed to trap
sediment, more heavily vegetated than normal.
Protruding structures are significant roughness elements.
Anticipated channel maintenance activities, such as removal of vegetation and
sediment deposits from the channel, will have a bearing on the evaluation of ultimate channel
conveyance.
5.1.6.2 Low Flows
A less obvious concern focuses on the potential impact of a comprehensive bank
stabilization project on the relationship of river stage to low-flow discharge. A lowering of
stages at a given low discharge may occur following implementation of the project, if a
significant length of channel becomes narrower and deeper, and thus more efficient at low
flows. Elliott et al. (1991) discuss the lowering of low flow stages due to complex
interrelated factors, including stabilization works, on the lower Mississippi River. The degree
of such impact can be estimated, although again not precisely, by the same approach
discussed above for flood flows.
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