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11/23/2009 10:40:45 AM
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Title
Australian Rainfall and Runoff 1998, Revision of Book VI - Estimation of Large to Extreme Floods
Date
11/28/1998
Prepared By
Rory Nathan, Sinclair Knight Merz
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />I <br />I <br />I <br /> <br />I <br /> <br />1 <br /> <br />1 <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />DRAFT D <br /> <br />Book VI - Estimation of large to Extreme Floods <br /> <br />References to the temporal and spatial patterns to be <br />used with the GTSM method are provided in Sections 3.9 <br />and 3.10. <br /> <br />3.4.5 Longer Durations in the Transition Zone <br /> <br />For catchments located close to the boundary between <br />the GTSM and GSAM there is uncertainty regarding which <br />method to apply. Both methods have their weaknes~es <br />(Minty et aI., 1996): the GSAM lacks storms of tropical <br />origin, and the GTSM has a rudimentary treatment of <br />topographic effects which may be important at these <br />locations. In order to accDunt for this uncertainty the PMP <br />estimates for catchments within this transition zone are <br />calculated as the distance-weighted average of the PMP <br />estimates from both the GTSM and the GSAM. Oesign <br />temporal and spatial patterns from both the GTSM and <br />GSAM may be applied separately to the depths and used to <br />determine the maximum resulting flODd. The temporal and <br />spatial patterns corresponding to each method should be <br />used together, but patterns from different methods should <br />not be mixed; for example a GTSM spatial pattern should <br />not be used with a GSAM temporal pattern, nor vice versa. <br /> <br />3.4.6 <br /> <br />Laurenson and Kuczera used three important pieces of <br />information to formulate their recommendations: <br /> <br />(i) the reasoned insights of Kennedy and Hart (1984); <br /> <br />(ii) the test results obtained for a 1700 km' area near <br />Sydney using a joint probability approach (Pearse and <br />Laurenson, 1997); and, <br />(iii) the results obtained by Nathan et aL (1998) based on <br />an extrapolation technique similar to one used by <br />Schaefer (1994). <br />Of these three pieces of information, the method used by <br />Pearse and Laurenson was regarded as the most thorough, <br />using more relevant data, and relying less on extrapolation <br />than the other methods. However, the reviewers ccnsidered <br />that certain elements of the approach cDuld not be <br />satisfactorily validated. Of the other two approaches, the <br />results obtained by Nathan et aL were found to be based <br />on better data, more thorough analysis, more careful and <br />systematic extrapolation, and a more up-to-date PMP <br />estimation method. While Laurenson and Kuczera <br />considered Nathan et aL's approach to be more reliable <br />than Kennedy and Hart's, they also recognised that the two <br />sets of results are not greatly different Since the latter have <br />been in use by the profession in Australia since 1984 and <br />are slightly more conservative, there was no justification to <br />change from the original Kennedy and Hart <br />recommendations. <br />The AEP of PMP estimates are considered to vary <br />solely as a function of catchment area, and the <br />recommended relationship is shown graphically in Figure 4. <br />Laurenson and Kuczera stress that there is considerable <br />uncertainty surrounding these reccmmendatt.:>ns, and that <br />the estimates should be interpreted as follows: <br /> <br />. the reccmmended AEP values plus or minus two orders <br />of magnitude of AEP should be regarded as the notional <br />upper and lower limits for the true AEPs: <br /> <br />. the recommended AEP values plus or minus one order <br />of magnitude of AEP ShDUld be regarded as the <br />ccnfidence limits with about 75% subjective probability <br />that the true AEP lies within these limits; and, <br /> <br />. the recommended AEP values should be regarded as <br />the best estimates of the AEPs. <br />The notional 75% confidence and upper and lower limits <br />are shown On Figure 4. While the recommended error <br />t bands are undoubtedly wider than is desirable, they are <br />regarded as a realistic assessment of the true uncertainty. <br />In order to inccrporate this uncertainty into a risk <br />analysis, Laurenson and Kuczera recommend the <br />construction of a probability mass function that provides a <br />75% chance that the true AEP lies within one-order-of- <br />magnitude of the recommended AEP, and a 10.0% chance <br />that the true AEP lies within two-orders-of-magnotude of the <br />recommended AEP. Table 5 presents an example of a <br />probability mass function which meets these requirements. <br />For example if the reccmmended AEP were 1 in 10', then <br />there is a 11.0% chance that the true AEP lies between 1 in <br />10' and 1 in 10"", and there is a 42A% chance that it lies <br />between 1 in 10" and 1 in 1 0"'; the first example <br />ccrresponds simply to a single probability interval adjacent <br />to the mid-point of 0.00 in Table 5, and the second example <br />corresponds to the central four probabBity intervals. <br />Although the probabilities are subjective, they do reflect the <br />considerable uncertainty in the AEP estimates. The <br />uncertainty can be directly incorporated into a risk analysis <br />by performing an assessment for each of the AEPs in Table <br />5 and weighting the results using the associated subjective <br />probabillty. <br /> <br />Intermediate Durations <br /> <br />Estimates of PMP depths for durations between the <br />upper limit of the GSOM and the lower limit of. the <br />GTSMIGSAM are not directly covered by the generahsed <br />procedures referred to in Sections 3A.2, 3.4.3, and 3AA. <br />In the tropical areas, it is possible that there ~ay be <br />some inccnsistencies between GSOM and GTSM estimates <br />at the shorter durations, and accordingly it may be <br />necessary to adjust the values to obtain a smooth <br />relationship between burst depth and duration. The Bureau <br />of Meteorology make the necessary adjustments when <br />providing both GSOM and GTSM estimates for a particular <br />site and their advice should be sought if the GSOM <br />esti;"ates were derived independently. <br />For southeast Australia, estimates for intermediate <br />durations need to be obtained by interpolation. <br />Considerable judgement is required to derive a relationship <br />between burst depth and duration and advice should be <br />sought from the Bureau of Meteorology. <br /> <br />3.5 Assigning an AEP to the PMP <br /> <br />Assigning an AEP to the PMP is inccnsistent ~ith the <br />"upper limiting" concept of the PMP (see definotlon In <br />Section 3.4.1). It is necessary, however, to reccgnose that <br />operational estimates of PMP are estimates only, and their <br />accuracy is crucially dependent on the validity of both the <br />method and the data used to derive them. Thus operational <br />estimates of PMP may ccnceivably be exceeded. <br />The method proposed to assign an AEP to the PMP Is <br />based on the review by Laurenson and Kuczera (1998) of <br />the procedures recommended in the 1987 edition of ARR, <br />as well as subsequent work done both in Australia and <br />overseas. The recommended AEP values are subject to <br />ccnsiderable uncertainty as they are for events beyond the <br />realm of experience and are based on methods whose <br />ccnceptual foundations are unclear. laurenson and <br />Kuczera concluded that at present there is no conceptually <br />sound, defensible basis upon which to make <br />recommendations for design practice. Therefore, the <br />recommendations made below must be viewed as interim, <br />pending the outccmes of ongoing research. It is important <br />to note, however, lI'iat these recornmendalions are intended <br />to replace earlier recommendations provided by the Bureau <br />of Meteorology (Bureau of Meteorology, 1994; Minty et aI., <br />1996). <br />
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