EP 1110-2-13
28 Jun 96
significant reduction in the hazard, with and without
(loss of life and economic damages) over the hazard
dam failure, is achieved.
which would have existed if the dam had not failed.
Recommendations for any modifications that would
(4) Measures to accommodate floods larger
accommodate floods larger than the flood identified as
than the BSC may be warranted in some cases. When
the BSC must be supported by an analysis that presents
the project benefits that would be lost, and repair costs
the incremental costs and benefits of the enhanced design
for failure are large enough, costs for structural
in a manner that demonstrates the merits of the
modifications to prevent failure may be economically
recommendation. Such enlargement of project scope
justified in spite of the low probability of the floods
may require Congressional authorization.
involved.
(2) Determination of the flood that identifies the
(5) Conduct of the analysis requires careful
BSC will require definition of the relationship between
application of professional judgment for determining
flood flows and adverse impacts (loss of life and
those parameters where data and modeling capability
economic damages) with and without dam failure for a
are limited. Therefore, the importance of documenting
range of floods that fully utilizes the existing structure up
the logic assumptions, critical to the conclusions and
to the probable maximum flood (PMF). Selection of a
recommendations drawn from the analysis, cannot be
BSC predicated on the hazard to life from dam failure
over-emphasized.
Also, the evaluation should
will require supporting information to demonstrate that
produce a significant amount of information needed
the safety of the population would actually be threatened.
throughout the decision making process, particularly
The evaluation should distinguish between total
in those cases where it is appropriate to proceed
population downstream of a dam and the population that
beyond the base condition. The information must be
would likely be in a life threatening situation given the
displayed in a format that assists the decision maker
extent of prefailure flooding, warning time available,
when evaluating the important trade-offs involved.
evacuation opportunities, and other factors that might
affect the occupancy of the incrementally inundated area
c.
Policy on seismic criteria. The following
at the time the failure occurs. Appropriate freeboard
policy is used to make decisions on the merits of dam
necessary to accommodate potential wind and wave
safety modifications related to current earthquake
conditions will be included for all flood evaluations. The
design criteria (ER 1110-2-1155).
evaluation consists of two phases. Phase I is a
comparative hazard analysis in which the threshold flood
(1) Projects that retain or have the potential to
and the BSC are established. Phase II is the risk-cost
retain a permanent pool, failure of which would result
analysis required if modifications for a flood larger than
in loss of life, substantial property damage, or indirect
the BSC is recommended (ER 1110-2-1155). Examples
loss such as the loss of essential emergency services
of the analysis required to develop the base condition are
provided by the dam, are required to survive and
illustrated in "Guidelines for Evaluating Modifications of
remain safe during and following the maximum
Existing Dams Related to Hydrologic Deficiencies,"
credible earthquake event. Such projects shall
Institute for Water Resources Report 86-R-7 (Stakhiv
additionally be capable of remaining operational with
and Moser 1986).
only minor repair during and after an operating basis
earthquake (OBE). In the case of projects intended for
(3) Selection of a recommended level of
short-term temporary flood storage, including those
modification also should reflect concern for economy.
with low permanent pools, risk based assessments may
Modification costs in the vicinity of the scale of
be warranted. Combining a rare earthquake event
improvement identified as the BSC should be examined
with a rare hydrologic event demonstrates extremely
for sudden increases in the cost/scale of improvement
low risk and therefore unwise use of funds.
relationship. This type of change could occur, for
instance, when a costly highway relocation is
(2) Technical requirements for selecting
encountered near the scale of improvement identified as
seismic design values and performing design analyses
the BSC. An adjustment in the level of fix recommended
are contained in ER 1110-2-1806 (see also Federal
may be warranted under these conditions. On the other
hand, the large increase in costs may be justified if a
5-2
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