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 General Principles of Erosion Protection
If topography is such that significant amounts of surface drainage enter the channel
in the vicinity of the work, it is necessary to collect the overland flow. This can often be
accomplished by small unlined ditches if drainage areas are small, slopes are flat, and the soil
is erosion resistant. The ditches should usually include vegetative treatment or
soil-stabilization. If grading of the bank is part of the stabilization work, the natural levees
should be rebuilt using material from bank grading or ditch construction, as shown in Figure
6.5.
The provision of controlled outlets into the stream is sometimes a simple matter of
leaving a natural outlet undisturbed, if the flow carried by the outlet is not increased by
alterations to the topography during construction. Otherwise, or if the natural outlet shows
signs of instability, a lined outlet or culvert should be provided. Steep drops can be
accommodated by a drop culvert, or by providing energy dissipators at the ends of lined
outlets or culverts. The detailed design will be site-specific. Schwab et al. (1981) treat the
subject thoroughly, and Schiechtl (1980) provides information on successful techniques.
More specific guidance based on site conditions can usually be obtained locally.
Rigid armor is more susceptible than most armors to undermining by surface drainage
or destabilization by excess hydrostatic pressures due to the trapping of sub-surface water
behind the armor. Therefore, special care should be taken in collecting surface water and
providing outlets into the stream. "Keying in" the top of the armor, or providing a "collar"
of adjustable armor, is a common practice.
When indirect bank protection methods are used, surface drainage is often not a
consideration, since the work usually does not significantly alter existing drainage conditions.
A reduction in erosion from surface drainage may be an incidental benefit of the work if
deposition behind the bank protection structure raises the base level of existing outlets. This
may, in fact, present a problem if deposition is high enough to block local drainage outlets.
Usually, however, the only drainage treatment necessary when using indirect protection is to
treat areas disturbed during construction. Treatment for local surface erosion can be designed
separately if it is a significant problem to be addressed under the project.
6.5 MANUFACTURERS' RECOMMENDATIONS
Manufacturers and distributors of the various patented or commercially available
erosion protection products may not be completely objective, since they have a vested interest
in their product. However, they also want their product to perform well, and their experience
with it is likely to be extensive. While their design methods should not be accepted
uncritically, when they are supported by a service record under comparable conditions they
may obviate the need for a duplication of effort by the designer. However, the procurement
policies of some governmental agencies may make it difficult to specify a particular product
by brand name. Adding the phrase "or equal" to the specification may
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