General Approach to Bank Stabilization
The preceding account illustrates the fact that, even though reservoir construction may
change a stream's water and sediment supply in a relatively straight-forward manner, reliable
predictions of the ultimate effects on bank stability and stabilization procedures are elusive.
4.1.2 BED STABILIZATION
Assessing the need for bed stabilization measures requires not only a quantification
of the active processes of degradation, but also knowledge of the erodibility of bed and
substrate materials throughout the entire system, because the rate and magnitude of
degradation is very sensitive to bed erodibility. This presents a difficult task if the geologic
and morphologic history of the basin is complex. Even with ample data, the erodibility of
cohesive soils and weak rocks cannot be accurately predicted. Numerical models do not
account for cohesive materials well, and often the best approach is an empirical one, based
on the known historical behavior of the particular system in question. If a proposed project
will significantly change either the inputs of water or sediment, or the channel slope, then even
channel history is not a reliable guide, and design safety factors should be large.
If significant bed degradation is occurring or is expected, then a project should include
bed stabilization measures. The only exception is if the requirement or authority for bank
stabilization is limited to a very few sites. Local stabilization can be achieved without bed
stabilization by designing the toe of the bank protection to function despite general bed
degradation. However, this protects only the immediate area of the project. Also, when
applied to several sites, the cost of heavy toe protection can exceed the cost of bed
stabilization measures without yielding the broader benefits of bed stabilization. A detailed
discussion of bed stabilization techniques and design guidance is given in Chapter 12.
4.1.3 SITE-SPECIFIC BANK STABILIZATION
This approach is a simpler concept to implement, carries a relatively low risk of
induced channel system instability, and is the most immediate and tangible solution.
However, unless properly planned and designed, the risk of failure is high. It will be the only
component required if project scope is limited to a particular site or local reach of stream, or
if the initial conceptual analysis has determined that the stream is in dynamic equilibrium and
the predominant cause of bank failure is of local origin. Chapters 5 through 9 discuss the
planning and design of site-specific bank stabilization works in detail.