General Principles of Erosion Protection
alleviate that difficulty, but even if a contractor's proposed substitute is considered by
engineering personnel to not be "equal," documenting that to the satisfaction of administrative
personnel may be difficult.
6.6 SAFETY FACTOR
Most engineering analyses for the design of structures such as buildings and bridges
provide for a "safety factor," even when the physical laws governing the behavior of the
structure are well-defined and readily quantifiable. Because rigorous design procedures are
lacking for many aspects of streambank protection, the need for a safety factor is even more
apparent. However, there is a significant difference in the analogy, in that failure of buildings
and bridges invariably carries the risk of loss of life, whereas that is often not the case with
streambank protection works. Also, the safety factor for buildings and bridges is usually
governed by statutes and by codes of practice, which is also not usually the case for
streambank work.
The safety factor is influenced by the:
Designer's level of experience
with the protection method being used
with the stream itself or comparable streams;
Difficulty of constructing the work to specifications;
Sponsor's capabilities
to perform routine maintenance
to perform emergency reinforcement; and
Consequences of failure.
The designer's experience with the protection method and with the stream itself is a
measure of the confidence that can be placed in a prediction of the performance of the work
and in a prediction of the consequences of failure of the work. The reliability of available data
on the stream also affects the level of confidence which can be placed in the designer's
assessment of the causes of bank instability.
The difficulty of constructing the work to specifications affects the possibility that
undetected construction flaws will leave vulnerable points. The timing of construction affects
the likelihood that bad weather and high flows will extend the work period, making
incomplete portions of the work more vulnerable, especially if vegetative treatment is an
important component. Also, construction delays which result in changes in the channel may
make the design itself unsatisfactory in extreme cases.
The competence of the construction personnel and the capabilities of their equipment
are also factors. However, these factors may be unknown at the time the project is being
designed.
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