EP 1165-2-1
30 Jul 99
be developed without the project and benefits are based on savings in
future flood proofing costs or reduction in damages to future
development.
(3) If no project or separable project increment can be
economically justified to protect existing development, interspersed
vacant property and/or property that would be developed without the
project, there is ordinarily no budgetary interest in expanding the
area of protection to achieve land development (location) benefits
even if net benefits are increased and economic justification can be
achieved.
(4) A limited exception to policy principles (1) through (3)
above can be considered in the case where the cost of protecting
existing development can be substantially reduced if some vacant
property that is not interspersed with existing development is
included in the protected area. This situation typically exists where
an existing levee or floodwall is being raised to provide a higher
degree of protection. These exceptions will be considered on a case-
by-case basis. Compatibility with EO 11988 still must be
demonstrated. It also must be clear that the primary objective of the
project is not land development but the minimization of the cost of
protecting existing development.
e. Benefit Determination Involving Existing Levees. Problems
have often arisen in the benefit evaluation of flood damage reduction
studies when there are existing levees of uncertain reliability.
Specifically, the problem is one of engineering judgment but has
implications for benefit evaluation: engineering opinion may differ or
be uncertain on the ability of the levees to contain flows with water
surface elevations of given heights. This may lead to difficulty in
arriving at a clear, reasonable and agreed upon without project
condition.
(1) General. Investigations for flood damage prevention
involving the evaluation of the physical effectiveness of existing
levees and the related effect on the economic analysis shall use a
systematic approach to resolving indeterminate, or arguable, degrees
of reliability. Reasonable technical investigations shall be pursued
to establish the minimum and, to the extent possible, the maximum
estimated levels of physical effectiveness. Necessary information and
summary of analyses shall be included in report presentations of plan
formulation and shall be documented in appropriate supporting
materials.
(2) Sources of Uncertainty. Studies involving existing
levees will focus on the sources of uncertainty (likely causes of
failure). Other than overtopping, levees principally fail due to one
or a combination of four causes: surface erosion, internal erosion
(piping), underseepage, and slides within the levee embankment or
foundation soils. Reasonable investigations, commensurate with the
level of detail suitable to the planning activity underway, shall
determine the condition of existing levees with respect to the factors
that can lead to failure, if this information does not already exist.
(3) Performance Record. Existing levees either have or have
not failed during previous flood events or have shown evidence of
distress such as various degrees of piping, underseepage and
sloughing. Information regarding their performance is relevant and
vitally important in forming judgments regarding future performance.
However, it should not be assumed that because a levee has passed a
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