Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
6 SUMMARY AND RECOMMENDATIONS
Bioengineering can be a useful tool in controlling bank erosion, but should not be
considered a panacea. It needs to be performed in a prudent manner and in consonance with
good planform and channel bed stability design. It must be done with the landscape and
watershed in mind, particularly with respect to erosion that has occurred as a result of both
broad basinwide activities and local, site-specific causes. Nevertheless, bioengineering must
be done at the reach level. This must be done in a systematic way with thought given to its
effects both upstream and downstream and it may have to be done incrementally to overcome
seasonal time constraints. For instance, woody plants must be planted in the dormant season.
There are numerous questions that must be answered prior to bioengineering implementation.
Answering these questions and designing a project must be an integrated process that starts
with the planning phase and continues through the construction phase. There are obviously
feedback loops from the design and construction phases back to the planning phase.
Additional information may have to be retrieved that calls for more planning actions.
Bioengineering must be accomplished with enough hardness to prevent both undercutting
of the streambank toe and erosion of the upper and lower ends (flanking) of the treated reach.
This can be done with one or both of: (1) hard toe and flanking protection, e.g., rock riprap,
refusals, and (2) deflection of water away from the target reach to be protected through
deflection structures, e.g., groins, hard points, vanes, and dikes. With both of these methods,
only appropriate plant species should be used in a manner consistent with their natural
habitats. This is often done by using streambank zones that correspond with micro-habitats
of native plant species in local stream environments. Where possible, both herbaceous and
woody species are used with grass or grass-like plants, e.g., sedges, rushes, reed grasses, in
the lower-most zone, then shrubby, woody vegetation in the middle zone, and for the most
part, larger shrubs and trees in the upper-most zone. These zones are respectively called the
"splash, bank, and terrace zones."
Careful planning must be done to acquire the kinds of plants in the amounts needed. This
may take up to one year before installation of the various treatments because plants either
have to be grown in sufficient quantities in nurseries or they have to be located in the wild and
either collected or grown from wild plant stock.
Bioengineering treatments have been noted, depending on the type of treatment, to resist
up to 12 fps local flow velocities. It is recognized, however, that local flow velocities during
peak discharges are difficult to obtain during those events because of safety considerations.
B-88
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