Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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Considration of Risk in Dam Safety Evaluations
Considration of Risk in Dam Safety Evaluations
Pages 74-83
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From page 74... ...
This chapter compares the risks of dam failure with other man-made risks and includes discussions on the nature of risks from extreme floods and earthquakes, the attempts to cope with such risks, and how society has handled other types of risk with similarly potentially serious consequences. RELATIVE IMPORTANCE OF RISK OF DAM FAILURES There are some data that compare the impacts of dam failures to the impacts of other man-made and technological hazards in terms of a number of risk-related parameters.
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From page 75... ...
as from several other hazards, such as fireworks accidents, skyscraper fires, train crashes, smoking, toxic effects from asbestos spray, and radiation from nuclear wastes. Further, information indicates that the maximum number of people who could be killed in a worst event is probably greater for dam failures than most any other kinds of hazards.
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From page 76... ...
From available evidence, estimates can be made of the probable maximum limits of future floods and earthquakes, but the size and timing of extreme floods and earthquakes cannot be certain. Hence, any such design involves an unknown, a risk factor.
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From page 77... ...
However, experience has shown that, as more data become available, estimates of such extreme events also change. The probabilistic approach seeks, by statistical study of past historical events, to estimate the return periods or annual probabilities of occurrences of extreme hypothetical flood or earthquake events of various magnitudes.
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From page 78... ...
Much of the agency risk statements are not goals so much as statements of what is a de minimus risk level. The Supreme Court vacated the OSHA benzene standard in 1980 on the grounds that OSHA had not found that occupational exposure to benzene constituted a "significant risk" at the prior standard.
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From page 79... ...
RETROFITTING TO MEET NEW STANDARDS Many dam owners, including federal agencies, have found that dams built years ago fail by considerable margins to meet current agency standards for new dams. Many spillways at existing dams are deficient in light of such current standards.
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From page 80... ...
The same situation is usually found when considering upgrading an existing dam to meet current earthquake criteria. The question arises, then, whether safety standards for new dams should be applied to retrofitting existing dams.
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From page 81... ...
The Federal Aviation Administration comes closer to using a formal method than any other agency surveyed. If a risk is determined to mean a failure rate of 1 in 1 billion hours (1 in 114,155 years)
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From page 82... ...
Recall of automobiles to correct deficiencies related to safety, smoke detectors in residences, sprinkler systems for hotels, and correction of design deficiencies in commercial aircraft are some examples of such required retrofitting. The actions of the National Highway Transportation Safety Administration (NHSTA)
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From page 83... ...
O ¢¢ O These examples suggest that different agencies handle the problem differently, that most of them do not have a general formal criterion for distinguishing risk acceptabilities, but that all of them do in fact recognize the need to be responsive to the greater costs of applying new safety standards to what exists than of applying these standards to what we do in the future. A different approach to evaluating risk may sometimes be appropriate to decisions regarding an existing dam.
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