Selection and Design of Channel Rehabilitation Methods
Vegetation can function as either armor or indirect protection, and in some applications, can
function as both simultaneously. Grassy vegetation and the roots of brushy and woody vegetation function
as armor, while brushy and woody vegetation function as indirect protection. The roots of vegetation may
also add a degree of geotechnical stability to a bank slope through reinforcing the soil.
Some factors that affect the success of a bioengineering approach, such as weather and the timing
and magnitude of streamflows, are beyond the control of the designer. Therefore, expert advice, careful
planning, and attention to detail are critical to maximizing the probability of success.
Many streambank protection projects include vegetation without conscious thought by the designer,
since native vegetation often establishes itself once the processes of bank failure are stopped by structural
means. However, if the potential for utilizing vegetation is considered from the beginning, then the
effectiveness, environmental aspects, and economy of a project can often be significantly improved.
Vegetation is most often used in conjunction with structural protection. Exceptions may be made
for very small waterways, for areas of low erosion activity, or for situations where the consequences of
failure are low and there is provision for rehabilitation in case of failure. Vegetation can have a particularly
important role in the stabilization of upper bank slopes. Vegetation is especially appropriate for
environmentally sensitive projects, whether benefits to recreation, esthetics, or wildlife is the object.
Vegetation is well-suited for incremental construction, either to wait for more favorable planting
conditions for specific types of vegetation or to wait for deposition of sediments in the area to be planted.
Vegetation is also suitable for inexpensive reinforcement or repair of existing erosion protection works in
some situations.
Woody vegetation is useful in preventing or repairing scour at or behind top of bank, especially if
the scour resulted from an infrequent flood event that is not likely to recur before the vegetation becomes
effective.
Woody vegetation is sometimes used to prevent floating debris from exiting the channel during
floods and becoming a nuisance in the floodplain.
Advantages: The two obvious advantages of vegetation as erosion protection are environmental
attractions and relatively low cost. A third and less obvious attraction is that it can increase the safety factor
of structural protection by enhancing the level of performance. Because many types of vegetative treatment
are labor intensive, the cost advantage will be especially prominent in regions where labor is inexpensive,
skilled in agriculture, and conscientious.
Disadvantages: Some characteristics which make vegetation effective and desirable in most
situations may be disadvantages in other situations. However, many of the following concerns will either
not be applicable for a specific project, or will be acceptable as compromises in light of vegetation's merits.
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