Selection of Site-specific Stabilization Techniques
Some species seem to prefer eroding vegetated sites to riprap protected
banks, and, therefore would prefer indirect protection to armor. Thus the
environmental difficulty of preventing channel migration while still providing
optimum habitat for these species may be impossible.
Dike and retard structures provide excellent habitat for some species, and are
often productive fishing sites. They are amenable to "notching" (constructing
low points in the profile) in order to provide habitat diversity and to reduce
longterm adverse impacts on aquatic habitat from excessive sedimentation.
Ideally, the notches would be designed so that enough flow passed through
the structures to retain high quality aquatic habitat without causing
unacceptable bank erosion, unacceptable loss in navigation channel
dimensions, or undesirably high velocities within the aquatic habitat itself.
However, such a delicate balance is difficult to achieve in practice, even if
numerical or physical models are used in design. Design details of the notches
may be overwhelmed by the overall geomorphic and hydraulic conditions in
the area. Nevertheless, notches may be worthwhile in many cases for
providing boat access into the dike field, for allowing movement of fish and
other organisms between the main channel and the dike field, and for
maintaining water quality in the dike field pools, even if the impact on long-
term sedimentation is uncertain.
Materials such as slag may contain chemicals that degrade water quality by
leaching, and should not be used if this risk is unacceptable.
Selection considerations relating specifically to terrestrial habitat are:
If earthwork is a part of the selected method, diversity can be provided by
disposing of the excavated material in an irregular fashion, to create local
Backfill can be placed over stone and seeded or vegetated with desirable
species. This is especially appropriate when stone is placed in an excavation
behind top bank, as with stone dike roots.
Backfilling over stone and other irregular armor materials on the upper bank
slope will expedite the growth of vegetation, and enhance the natural
deposition which sometimes occurs within the interstices of the armor. By
backfilling during the construction process, seeding of desirable species at that
time may be successful.
Placement of fill on the top of dikes that protrude into the channel sometimes
succeeds in inducing the growth of native vegetation that is tolerant to the
inundation frequency of the top of the structure. This will of course be
unsuccessful if high streamflows scour the fill from the structure.
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