Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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Proposed Hydrologic Criteria
Proposed Hydrologic Criteria
Pages 97-105
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From page 97... ...
In recognition of these problems, the proposals set out in the following paragraphs seek to strike reasonable balances between what is theoretically desirable and what is practical based on current technologies. SAFETY EVALUATION FLOOD The committee recommends the adoption of the term safety evaluation flood (SEE)
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From page 98... ...
Also, decisions in design for a new dam based solely on economic analysis without regard to who bears costs and risks could violate principles of equity. Such considerations indicate that the design of a new dam for a location upstream from an urban-type development, which would introduce potential for excessive damages and loss of life in the event of dam failure, should incorporate the maximum reasonable level of protection against failure during extreme floods, unless it can be shown that the failure of the dam during such floods would not increase the potential for loss of life and damages downstream.
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From page 99... ...
to meet one safety standard or criteria, be retrofitted or modified to meet newly adopted criteria unless the existing facility is currently judged, given new evidence, to expose the public to unacceptably large and immediate risks. These factors suggest that some criterion in addition to the PMF should be
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From page 100... ...
If the PMF is ruled out, the following types of alternatives may be considered as bases for a safety evaluation for an existing dam: · a flood having some selected estimated annual probability or average return period; · a project-specific evaluation based on trial analyses of effects in downstream areas of potential dam failures during floods of various magnitudes (this risk analysis approach is discussed further below) ; and · a flood that is some arbitrary fraction of a PMF (or perhaps derived from a fraction of the PMP)
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From page 101... ...
Appendix D outlines procedures for estimating probabilities that events will occur within definite time periods once estimates of the annual probabilities or average return periods are established. As indicated by the above, the committee considers that there is no single, universally correct approach to evaluating the safety of all existing highhazard dams against extreme floods.
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From page 102... ...
RISK-BASED ANALYSES Risk-based analysis when used to determine spillway capacity requirements provides the opportunity to weigh objectively the relative merits of alternative modifications embodying either variation in scope or variation in design concept. The difficulties associated with this approach relate to (1)
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From page 103... ...
Second, although the relative cost positions in a ranking of alternative mollification schemes may be affected by the frequencies assigned to extremely rare events, sensitivity studies should bring out this dependence and provide basis for judgments among the schemes. For the present, the committee recommends development of the frequency curves for average return periods in excess of 100 years in accordance with procedures described in Appendix E
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From page 104... ...
Similarly, the nonfederal owner is confronted by the obvious trend toward full liability for damages, as demonstrated by recent court decisions exhibiting strict liability concepts. However, there are major differences between
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From page 105... ...
In the meantime, nonfederal owners, regulators, and designers will need to keep abreast of potential research findings relating to risk-based methods. Until the risk analysis approach can be extended to the nonfederal field, the potential overall savings to society that it appears to offer will not be realized.
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