FLOOD08791

• County: Statewide
• Basin: Statewide
• Title: ANCOLD Guidelines on Risk Assessment
• Date: 8/1/1998
• Prepared By: ANCOLD Working Group on Risk Assessment
• Floodplain - Doc Type: Educational/Technical/Reference Information

The New Societal Risk Criteria

The new Societal Risk criteria curves are given at Figure 2.

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Intolerable risks.

Limit of tolerability

Objective. for new
dams and major
storage augmentations

Risks are generally
acceptable: but
acceptability is subject
to the marginal cost of
further risk reduction.

lE.8
1 10 100 1000 10000
N, number of fatelities due to dam failure

Important note: Where fatalities are expected, as part of a risk-based
decisIon at a specific dam, consultation with the affected public Is
required as part of the final decision process.

Figure 2 Revised Societal Risk Criteria Curves

The basic change to these curves is the truncations at IE-06 for the Limit and IE-07 for the Objective/the
use of a slope of minus I (that is, a constant expected value of life loss) until the truncation levels are
reached and a narrowing of the band between Limit and Objective. The truncations reflect a view that it is
not possible, at a practicable cost, to provide lower probabilities of failure given the uncertainties of materia.1
properties and geotechnical.conditions; or at least, it is not possible, given the state of the art, torelilibly
assess probabilities of failure to such very low values. Once truncations at the~e levels are accepted, there is
little point in providing a steepening of the curves above the truncation levels, because such a steepening
would make only a small difference at those higher levels.

Some points in support of the proposition that risks lower than the truncation levels cannot reasonably be
delivered, are:

. the average historic chance of dam failure is about IE-04 per damper annum (Whitman 1984 and Von
Thun, 1985), The Limit truncation is thus 1% ,and the Objective is 0.1%, of the average historic risk. It
seems hardly credible that even lower risks could be reliably provided with existing dam design and