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I I I I I I I I I 'I I I I I I 'I I I between these two baseflow values is of minor importance then a representative, fixed value could be used for all intermediate AEPs. However if deemed appropriate, the magnitude of the baseflow could be varied linearly on a plot of baseflow versus 10g(AEP) between the value adopted for the 1 in 100 AEP event and that adopted for the flood resulting from the PMP (alternatively Equation 7 could be used). 4.5 Derivation of Complete Design Flood Frequency Curve Construction of a flood frequency curve provides information on the variation of flood peak with AEP. The curve is generally derived by drawing a smooth line through selected design flood estimates. The shape of the curve can be defined by one of two methods: (i) Design rainfalls: Use of a hydrograph model to convert design rainfalls of any AEP into design floods of the same AEP, based on an AEP-neutral selection of design inputs. With this approach, design rainfalls are derived using any of the procedures described in Section 3, and are converted to design floods using a hydrograph model based on the preceding recommendations in Section 4. The design flood of specified AEP is generally adopted as the one which gives the highest peak discharge for a range of different rainfall durations. Minor smoothing of design inputs or derived design floods may be required to obtain a smooth flood frequency curve. (iil Shape factors: Derivation of design floods based on the PM P and the 1 in 50 AEP and 1 in 100 AEP design rainfalls, with intermediate values determined by use of shape factors. With this approach, the three design floods are derived as described in item (i), and intermediate flood estimates are derived by application of the shape factors presented in Table 7, using the same procedure described in Section 3.6.1. The approach based on design rainfalls is considered to be more robust and to have a more rational basis than the use of shape factors based on design floods. Design rainfalls are derived using regional analyses of data that have long records compared to flow data, are generaliy more reliable, and are skewed within a relatively narrow range. Importantly, the credible limit of rainfall extrapolation is generaliy well beyond that of floods, and thus the 'gap' over which interpolation is required is generally less than that required for floods. The shape factor approach may be considered more appropriate for those regions in which the Annual Summer Spring AUt~" wonte, ! ~ c .~ ~ I I 2000 50000 10 e ExceedaflC8 Probability (1ln V) 10' 50 100 Figure 8 Schematic diagram illustrating the conversion of seasonal exceedance probabilities into annual estimates. .........""" .. -~....,-..~".... .......'1::f...~.... ..."..,...",.... . ............ credible limit of rainfall extrapolation is 1 in 100 AEP (Section 3.6.1), and for which losses must be derived without the use of pre-storm temporal patterns (Section 4.1.3). By avoiding discontinuities that arise from the discrete nature of many design inputs and changes in critical rainfall duration, it tends to produce smoother flood frequency curves, but there is little evidence to support the adopted shape factors. The derivation of a frequency curve of outflows from a reservoir raises additional issues that are addressed in Section 5.2. 4.6 Seasonal Design Floods (aJ Need for seasonal estimates In some situations Large to Extreme design floods may be required for speciflc seasons within the year. Seasonal estimates may need to be investigated if it is suspected that the design factors of interest do not have an equal chance of occurring throughout the year. For example, seasonal estimates may be required to assess the consequences of dam failure when the population at risk may be dependent on the time of year (e.g. summer holidays). The likelihood of snowmelt is an obvious example, though this will only need to be considered if a large proportion of the catchment lies above the snowline. Perhaps the most commonly encountered example is related to the evaluation of spillway adequacy, where the largest seasonal floods may coincide with the largest expected drawdown in the reservoir. As discussed in Section 3.7, there are a number of conceptual and theoretical problems associated with the derivation of seasonal design rainfalls. Accordingly, seasonal design floods should only be derived if preliminary investigations indicate that the seasonal factors of interest have an appreciable impact on the required design outcome. (b) Theoretical and practical issues Seasonal frequency curves can be derived using similar procedures to those required for annual frequency curves, though careful consideration needs to be given to the determination of losses and the manner in which design flood estimates are validated. Given a set of seasonal frequency curves, care needs to be given to converting the seasonal exceedance probabilities to annual estimates. The AEP of a specific event (e.g. a dam overtopping event, 00) which is not conditional on the time of year can be approximated by summing the seasonal exceedance probabilities of the selected event. As an example, if the year was divided into two seasons, then two separate events could be considered: a summer event O. (0)00) and a winter event Ow (0)00). If these events are regarded as being independent, then the unconditional AEP of an event 0>00, i.e. of Q, or Ow, is: AEP[O,] = SEP.[O,] + SEPw[Oo] (8) where SEP.[Oo] and SEPw[Oo] are respectively the summer and winter seasonal exceedance probabilities of the selected event, and AEP[Ool represents the probability of one or more events of magnitude 0 " 00 occurring in a single year. The computation of the AEPs from seasonal distributions for more than two seasons is analogous, and is Hluiltrated in Figure 8. The SEPs can be simplv added to give AEPs, if the seasons are deflned such as to form an exhaustive set of mutually exclusive events (i.e. they are non-overlapping and cover the whole year).