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Last modified
1/25/2010 6:47:55 PM
Creation date
10/5/2006 1:10:26 AM
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Floodplain Documents
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Title
Guidelines on Selection of Acceptable Flood Capacity for Dams
Date
3/1/2000
Prepared By
Australian National Committee on Large Dams
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I i <br />II <br />I' <br />I' <br /> <br />I <br />I <br />I <br /> <br />1.3.4. Risk to Life Criteria. <br />(Refer Appendix 3, sub-section A3. I) <br /> <br />The risk to life criteria proposed in ANCOLD <br />(1998) relate to total risk from all loads and <br />, <br />as such, these criteria cannot be applied solely <br />to hydrologic risk. This contrasts with the <br />1986 prescriptive approach which tended to <br />require a PMF standard (as a notional upper <br />limit) where lives were at risk. <br /> <br />For some dams it may not be feasible, or <br />practicable, to meet the risk to life criteria, <br />but the As Low as Reasonably Practicable <br />(ALARP) principle can be used to provide a <br />general basis to assess practical limits for risk <br />reduction, in conjunction with deterministic <br />standards. <br /> <br />1.3.5. Determin istic Stan dards. <br /> <br />In application of the risk process it will still <br />be necessary to use traditional deterministic <br />procedures in parallel for the initial <br />consideration of potential inadequacies and <br />for consideration of risk reduction options. <br /> <br />Prescriptive standards are based o~ judgment, <br />lessons from dam failures, and models, <br />generally with a factor of safety, which can <br />only attempt to represent natural and material <br />properties with many uncertainties. Neither <br />prescriptive standards (nor the risk approach) <br />can guarantee an absolutely safe dam. <br /> <br />The dam industry has not previously <br />identified this. Nor has it educated the <br />community that dam engineering is not an <br />exact science. The community may well have <br />the expectation that large dams caimot fail, <br />without realising this cannot be achieved, <br />although the possibility may be extremely <br />low. <br /> <br />The' standards based design does not, and <br />cannot in many cases, identify the order of <br />probability of failure, even though it may <br />provide an acceptably low risk of failure. <br />Because of this, and the community <br />intolerance of public structure failures, the <br />deterministic approach has tended to be <br />conservative. <br /> <br />1.3.6. Risk Studies <br />consideration. <br /> <br />Matters for <br /> <br />(i) Context. The owner should set the <br />context as business policy with <br />overall acceptable risk criteria in <br />regard to business 'risk and <br />community, legal and political <br />responsibilities. <br />(ii) Scoping Studies. It is advisable to <br />do a preliminary scoping <br />(screening) study to assess the <br />order of inadequacy of spillway <br />provisions, the potential risk <br />reduction options, and <br />practicalities, incremental costs <br />and benefits. This will also <br />identify the need, and or extent, of <br />further detailed risk assessment. A <br />relatively low cost, practical <br />solution may be evident, <br />(iii) Site Specific. Each dam is unique <br />in location, incremental cost of <br />upgrading, and consequences of <br />failure,' and must be assessed <br />individually \vith the key factors <br />of relative probability of failure, <br />incremental consequences, and <br />incremental costs, in conjunction <br />with risk and deterministic criteria. <br />(iv) Sanity Check. It is essential that <br />sanity checks be carried out during <br />each phase of a risk study to assess <br />results, having regard to the <br />uncertainties in the data, <br />assumptions and risk <br />methodologies. This relates to the <br />estimates of probability in <br />particular as they are based on <br />probability theory and can only <br />provide an estimate of the <br />likelihood of the occurrence of an <br />uncertain future event; they are not <br />exact or absolute. <br />(v) Risk to Life criteria, There is <br />uncertainty within the dam <br />industry in the selection and <br />application of risk to life criteria, <br />and the legal liability if decisions <br />are based on risk criteria, rather <br />than established deterministic <br />standards. <br /> <br />ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams <br /> <br />3 <br />
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