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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 <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />6.1.3. Relative Probabilities. <br /> <br />Historical data on causes of dam failure show <br />that floods and spillways are a major cause, <br />followed by piping and seepage; on the other <br />hand, very few failures have been recorded due <br />to earthquakes, <br /> <br />Risk studies to date indicate there may be <br />incompatibilities in the assigned probabilities <br />between: <br />(i) the relative probabilities assigned to floods, <br />based on statistical exceedance; piping/seepage <br />based on historical failure; and earthquake <br />based on statistical exceedance, due to <br />different methodologies and assumptions, and <br />(ii) the assignment of conditional probabilities <br />of failure for each type of load. Historical data <br />indicates the conditional probability for <br />overtopping failure of an embankment dam <br />can be orders of magnitude higher than for <br />failure under earthquake loading <br /> <br />6.1.4. Iterative Process. <br /> <br />The risk process is iterative, and can be <br />lengthy and costly; it should be phased, and at <br />each phase the necessity and cost effectiveness <br />of continuing in greater detail should be <br />assessed, <br /> <br />This can include consideration of the size of <br />dam, consequences of failure, site and dam <br />suitability, and incremental costs for spillway <br />upgrading versus incremental benefits. <br /> <br />6.1.5. Sani(v Checks. <br /> <br />It is essential that a sanity check be made on <br />the outcomes at each phase of the risk study. It <br />must be realised that there are uncertainties in <br />the estimates from the different methodologies <br />and mathematical models as all are based on <br />probabilistic theoI)' with limited data and <br />uncertainties in modelling and assumptions. <br /> <br />The sanity check should also include a <br />comparison with assessments using the <br />traditional deterministic standards, including <br />the PMF event. <br /> <br />6.2. Indicative Procedure for AFC. <br /> <br />An indicative sub set of a basic risk flow chart <br />procedure for hydrologic safety is shown in the <br /> <br />flow chart, Fig 6. L This should be considered <br />in relation to procedures in the current <br />ANCOLD Guidelines on Risk Assessment. <br /> <br />The procedures below highlight the flood <br />considerations, with the factors relevant to <br />hydrologic safety in italics, as distinct from <br />factors relative to overall risk studies . <br /> <br />(i). Review data and inspect dam site and <br />downstream area. <br />This applies to both existing dams and <br />proposed dams, and is essential for a <br />preliminary understanding of potential risks, <br />priorities and risk reduction options from all <br />load events. <br /> <br />I <br />t <br />J <br />t <br /> <br />(ii). Prepare a flood frequency estimate to <br />current methodology (Book VI, ARR99). <br />. Existing dams: review the original <br />design flood estimates and any <br />subsequent reviews, and prepare <br />updated flood frequency estimates as <br />necessary. <br />. Proposed dams: prepare flood <br />frequency estimates to the current <br />methodology. <br /> <br />-1 <br />1 <br />~ <br />i <br />, <br />, <br /> <br />(iii). Assess the DCF. <br />. Initial assessment: the DCF can be <br />usedfor initial dambreak studies and <br />assessment of failure consequences; <br /> <br />. Existing dams: the DCF with an <br />associated AEP can indicate the <br />order of potential flood capacity <br />deficiencies. This will help to <br />consider options for increasing flood <br />capacity in relation to risk criteria. <br />Assess the impact on dam safety of <br />the assumptions for pre flood level. <br /> <br />. Proposed dams: an iterative study is <br />required of alternative spillway and <br />dam arrangements with the <br />associated DCF and AEP. <br /> <br />(iv) Prepare a consequence assessment. <br />. Dam break analyses provide <br />indicative flood mapping, <br />inundation areas, flood wave height <br />and travel times, within the <br />limitations of methodology and <br />representation of dynamic wave and <br /> <br />16 ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams <br />