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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 />3.2. Flood Estimation. <br /> <br />Book VI includes procedures for estimating <br />design rainfalls and AEP neutral <br />transfOlTImtion to estimates oflarge to extreme <br />floods. The transfonnation of PMP is tenned <br />the "PMP Design Flood". <br /> <br />Book VI includes a procedure for assignment <br />of probability to the operational estimate of the <br />PMP (as distinct from the theoretical PMP), <br />and development of the associated rainfall <br />frequency curve to provide a basis for risk <br />assessment. <br /> <br />3.3. AEP of the PMP. <br /> <br />The basis for assigning probabilities to the <br />operational estimate of the PMP in Book VI is <br />related to catchment area, The assigned AEP <br />varies from 10.7 for small catchments to 10" <br />for large catchments. <br /> <br />While this basis implies there are different <br />levels of risk depending on the catchment area, <br />from a pragmatic point of view the PMP <br />estimates are based on the estimated upper- <br />limit rainstonn for the area. <br /> <br />3.4. Associated Procedures. <br /> <br />Book VI also provides procedures for: <br />. Joint probability analysis of inflow <br />and pre-flood storage level and <br />volume; <br />. Concurrent tributary flows; <br />. Treatment of uncertainty . <br /> <br />It also contains worked examples. <br /> <br />3.5. Probable Maximum Flood (PMF). <br /> <br />It is noted that PMF (ARR87) is well <br />entrenched as the notional "upper limit" <br />prescriptive flood safety standard where lives <br />are at risk, <br /> <br />Book VI also includes similar procedures for <br />estimation ofPMF, still as a notional limiting <br />value of an e".-treme flood that could <br />reasonably be expected to occur, without <br />superimposing risks of vel)' low probability, <br /> <br />but incotpomting some degree of <br />conservatism. The estimation of PMF allows <br />more scope for selection of temporal and <br />spatial patterns and losses, and hence more <br />variability, than estimation of the PMP Design <br />Flood, <br /> <br />The magnitude of PMF, as estimated, should <br />be greater than, or at least equal to, the PMP <br />design flood, However it is noted that it is not <br />possible to assign a probability to PMF. <br /> <br />PMF can still be considered as a pragmatic <br />"upper limit" alternative to the PMP design <br />flood, particularly where comprehensive risk <br />analyses may not be justifiable (Chapter 8). <br />However it should be recognised that, from a <br />risk perspective, it is not a consistent upper <br />limit or level of safety <br /> <br />I <br />, <br />I <br />T <br />". <br />4 <br /> <br />The use of PMF could apply as a traditional <br />alternative for large catchments where, with <br />the assigned probability for the PMP design <br />flood, the feasible risk reduction options <br />cannot meet the risk criteria. Meeting the risk <br />criteria in that case would imply extrapolating <br />beyond the PMP design flood, <br /> <br />4. ACCEPTABLE <br />CAPACITY (AFC) <br /> <br />FLOOD <br /> <br />4.1. The Acceptable Flood Capacity <br />(AFC) <br /> <br />The Acceptable Flood Capacity (AFC) for <br />a specific dam is the overall flood <br />capacity, including freeboard as relevant, <br />which provides an appropriate level of <br />safety against a flood initiated dam failure <br />, <br />to protect the community and environment <br />to acceptable risk levels, within the total <br />context of overall dam safety from all load <br />cases. <br /> <br />The potential for floods greater than the AFC <br />should be recognised, as well as the fact that <br />the failure of the dam would then be a possible <br />outcome. <br /> <br />Follmving consideration of business risks and <br />responsibilities to the community, the owner <br />may well decide to adopt a larger AFC (Le, a <br /> <br />6 <br /> <br />ANCOW Guidelines on Selection of an Acceptable flood Capacity for Dams <br />