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DRAFT D I I I I I I I I I I I ,I I ,I I I I I I addltionaleftert required to derive information relevant to design storm losses could nDt be justified. . Hydrograph modelling considerations: the discussion presented in Sections 4.3.1 to 4.3.3 regarding the use of hydrograph models is generally focused on those issues that most need to be considered when extrapolating to conditions well beyond those encountered during calibration to observed floods. While in general these considerations encourage a sound understanding of model features, they are less important if the models are applied to conditions s,imilar to those found during calibration. Aside from the above, there are a number of issues addressed in this Book that may impact upon the derivation of floods more frequent than 1 in 100 AEP: . Revised areal reduction factors: revised information on areal reduction factors (referred to in Section 32b) is available for some regions Df Australia for AEPs as frequent as 1 in 2. This revised information should be more appropriate to use than the factors presented in Book II Section 1. . Incorporation of spatial trend in large catchments: the discussion in Section 3.10 (b) concerning the incorporation of significant variation in design rainfalls by dividing the catchment into two or more sub- catchments is generaily applicable to fioods of all magnitudes. . Additional design considerations: there are a number of additional design considerations discussed in this Book that are potentially applicable to floods with AEPs more frequent than 1 in 50 AEP. These issues (mostly presented in Section 5) include the derivation of seasonal design floods, the joint-probability treatment of initial reservoir drawdown and concurrent tributary flows, and the treatment of uncertainty. Most of these considerations are not specifically addressed elsewhere in ARR, and thus are not likely to conflict with other recommendations. 2 PROCEDURES FOR ESTlMA T1NG LARGE TO EXTREME FLOODS 2.1 Summary of Procedures The procedures for estimating Large, Rare, and Extreme floods can be summarised in the following three main categories depending on the probability of the flood to be estimated: (i) Floods with AEPs between 1 in 50 and 1 in 100 (Large floods): Estimates should be based on flood frequency anaiysis, or on design rainfalls, as described in earlier Books. A choice must be made between the available methods (see Book III Section 2). (ii) Floods with AEPs between 1 in 100 AEP and the credible limit of extrapolation (Rare floods): Estimates should be based on design rainfalls derived using the regional methods described in Section 3.3, or else on regionallpaleohydrological flood frequency analysis (Section 4.9). (iii) Floods with AEPs beyond the credible limit of extrapolation (Extreme floods, including the PMP Design Flood): These flood estimates may be required for direct use in design situations of high risk, either in terms of Book VI - EstimatIon or Large to t:.xtreme I-Iooas risk tll l>uman life or economic losses. or where soci<ll or political considerations require a very high level of safety. Estimates should be based on the use Df a flood event model with design rainfalls obtained by interpolation between the credible limit of extrapolated rainfalls and the PMP. An additional category of procedures which differs from the above in the design objective: (iv) Probable Maximum Flood (the limiting value flood that can reasonably be expected to occur): This may be required for comparison with estimates derived from previous studies or for some other design objective that requires a nDtional upper limiting value of flood without an associated AEP. In practice, the magnitude of the PMF will be greater than (or possibly equal to) the magnitude of the flood derived from the PMP using an AEP-neutral transformation (see 2.2b). To avoid confusion with the PMF, it is suggested that the flood derived from the PMP using AEP-neu al assum ions sho Id be terme e P Desi n Floo . A brief summary of the recommended Rrocedures and references to the relevant sections is presented in Tables 2 and 3. It should be recognised that these tables represent a summary of procedures that are described in detail in later sections, and they are not intended to be self- explanatory. 2.2 Theoretical and Practical Implications of the Procedures Several theoretical Dr conceptual assumptions are implied by the range of procedures outlined in Section 2.1, above. Recognition of the implications of these assumptions is important to informed and sDund design. (a) Basis of interpolation procedure for Extreme rainfalls Estimates of Extreme floods beyond the credible limit of extrapolation are based on the following two pragmatic considerations: (i) the magnitude and AEP of the PMP; and, (ii) the shape of the frequency curve between the credible limit of extrapolation and the PMP. Estimates of the AEP of the PMP are subject to a high degree of uncertainty as they relate to events beyond the realm of experience and are based on methods whose conceptual foundations are unclear. The currently recDmmended estimates of the AEP of the PMP (detailed in Section 3.5) are based on the interpretation of the PMP values as operational estimates that can be exceeded, rather than upper limiting vaiues Df rainfall. This interpretation of the PMP also implies that the frequency curve should not be tangential to the horizontal at the estimated PMP, but rather should extend through the PMP at a slope consistent with the shape of the lower sections of the curve. The shape Df the rainfall frequency curve between the credible limit of extrapolation and the PMP is based on arbitrary but plausible assumptions and provides a consistent and reasonable basis for design rainfalls within this range. Further information on the interpolation procedure is provided in Section 3.6. The practical implication of this is that estimates of Extreme events have a greater level of uncertainty than the events within the credible limit of extrapolation.