General Principles of Erosion Protection
6.3.2.7 Other Flow Deflectors
Iowa vanes and bendway weirs are similar to dikes and retards in that they function
by inducing deposition at the bank toe rather than permitting scour to occur. Since they
significantly alter secondary currents, rather than simply relocating the secondary currents,
they should be less demanding of toe protection than dikes or retards. However, since these
are relatively new techniques, long-term field experience is not yet available.
6.3.2.8 Vegetative Bank Protection
The importance of toe protection for successful bank stabilization using vegetation
cannot be overemphasized. Vegetation alone is unlikely to be successful as toe protection
unless velocities during design flows are so low that little toe scour is predicted, and climate,
inundation conditions, and soils are conducive to a vigorous growth at the toe.
Selection of a toe protection technique should assume that the vegetated portion of
the bank is in effect a rigid armor, which dictates that either a self-launching material or a
flexible mattress be used at the toe. In practice, vegetation is usually used as a cost-saving
or environmental feature in conjunction with a structural technique, and appropriate toe
protection will be an integral part of the design of the structural technique. Typical examples
are vegetative plantings between dikes, behind retards, and on the upper bank slope above
one of the many armor materials.
6.3.2.9 Retaining Wall
If a retaining wall is part of the solution to geotechnical instability, then the approach
to toe protection should be the same as for rigid retards. The alternative of designing the wall
to be stable under maximum scour is likely to be more costly than limiting the scour. It also
introduces the risk of a sudden, and, perhaps, catastrophic mass failure in the event of
miscalculation of the maximum scour depth, since underdesign of toe protection is more likely
to manifest itself gradually and is more easily detected in time for remedy than is excess scour
during high flows in the absence of toe protection. Since retaining walls are often used in
situations where consequences of failure are high, increasing the safety factor by using toe
protection as well as extra structural strength is advisable.
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