Selection of Site-specific Stabilization Techniques
5.1.1.1 Required Project Lifespan
This factor will determine the degree of importance of the durability of alternative
methods to a particular project. Required project lifespan is seldom truly quantifiable, even
though those projects which require a formal economic analysis must be assigned a specific
project lifespan. In practice, the selection of techniques usually involves only a qualitative
assessment of required project lifespan, a choice between a "short-term" or a "long-term"
lifespan.
Two examples of situations involving only a short-term required project lifespan are:
Emergency stabilization during an unusual flow event, which requires
immediate action under conditions not permitting the design and construction
of a permanent solution, conditions which may be so rare as to not justify a
permanent project.
Local stabilization on a rapidly migrating stream where the attack at the area
of concern will be of short duration, and the probability of severe attack
occurring at the same point in the foreseeable future is low, or at least
acceptable.
The extreme case of required project lifespan being long-term is the most common
situation. When in doubt, a long-term project lifespan should be assumed, since labor and
equipment costs are usually the most expensive part of the project, and a small premium for
durable materials will usually provide a cost-effective increase in the factor of safety.
A classification of intermediate project lifespan may be appropriate in special cases:
When an eroding channel is to be stabilized with the expectation that a future
project will result in the relocation of either the stream channel or the
endangered structure.
When a project for stabilizing a local erosion problem will eventually be
endangered from downstream by channel degradation or from upstream by a
migrating bend. This factor is, therefore, related to the determination of
project components, and to the determination of upstream and downstream
limits of the project, discussed in 6.1.1.
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