EP 1165-2-1
30 Jul 99
relationships for the continental U.S. are published by the National
Weather Service in a series of hyrometeorological reports .
d. Probable Maximum Flood (PMF). The PMF is the flood that
may be expected from the most severe combination of critical
meteorologic and hydrologic conditions that are reasonably possible in
the drainage basin under study. A PMF is developed from PMP.
Assumptions concerning rainfall losses, snowmelt runoff, channel
efficiency, etc. are adjusted to produce the largest flood reasonably
possible. The PMF is used to design high hazard structures (top of
dam, outlet and spillway capacities) where failure cannot be
tolerated.
e. Inflow Design Flood (IDF). The IDF for a dam is the flood
hydrograph used in the design or evaluation of a dam and its
appurtenant works (ER 1110-8-2(FR)). In some older documents, this
may be referred to as the spillway design flood. The upper limit of
the IDF is the PMF.
f. Project Performance. The analysis will quantify the
project reliability and performance by explicitly incorporating the
uncertainties associated with key hydrologic, hydraulic, and other
engineering variables. This reliability and performance will be
reported as the protection for a target percent chance exceedance
flood with a specified reliability. For example, the proposed project
is expected to contain the one-half percent (0.5 percent) chance
exceedance flood, should it occur, with a ninety percent (90%)
reliability. This performance may also be described in terms of the
percent chance of containing a specific historic flood should it
occur. To fully define how a project is expected to function requires
describing project impacts at several flood levels and locations.
There is no minimum level of performance or reliability required for
Corps projects; therefore, any project increments beyond the NED plan
represents explicit risk management options. It is, therefore, vital
that all participants understand the performance, reliability and
costs of the NED plan, as well as, increments and decrements of the
plan, in order to fully participate in an informed decision-making
process.
13-5. Risk-Based Analysis. The risk-based analysis framework is
defined as an approach to evaluation and decision making that
explicitly, and to the extent practical, analytically, incorporates
considerations of risk and uncertainty. These risks and uncertainties
arise from measurement errors, short data records, and from the innate
variability of complex physical, social, and economic situations,
particularly those dealing with future occurrences. Because it
captures and quantifies the extent of the risk and uncertainty in the
various planning and design components of an investment project, this
approach has been found very useful. Each of the components can be
examined and conscious decisions made reflecting an explicit tradeoff
between risk and costs. Risk-based analysis can identify which plans
are more robust and can be used to compare plans in terms of their
likely physical performance and economic success.
13-6. Structural Measures. Different types of structural flood
damage reduction measures have different primary and secondary impacts
on flooding. Plan formulation and impact assessment should take into
account all impacts, and residual flooding from all sources. (The
dominant flooding may be from a different source under without and
with project conditions.) In project planning, both the primary
beneficial effects and the secondary effects of the alternatives must
be borne in mind and appopriately accommodated.
13-3
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