6 Summary In New Zealand little progress has been made in flood estimation techniques over <strong>the</strong> last 25 years despite an upsurge in <strong>the</strong> amount of streamflow data that has been col- Iected over this period. This study has attempted to improve this situation by al flood frequency analysis procedu <strong>the</strong> available annual and historical flo I catchments' The procedure, known as <strong>the</strong> Regional Flood Estimation (RFË) method, is applicable to both gauged and ungauged iural catchments which in general <strong>are</strong> greater than 20 km' in <strong>are</strong>a. Since <strong>the</strong> method was developed by averaging <strong>the</strong> sampling variation that exists in individual flood records, it should provide a more reliable design flood peak estimate than that determined by fitting a frequency curve to a relatively short record. The RFE method comprises a set of eight regional flood frequency cu vs T, and a set of niné regionat when <strong>the</strong>re is little or no flood ¡ S <strong>the</strong> T-Year flood, and Q is <strong>the</strong> mean annual flood. The most important independent variables in <strong>the</strong> equations <strong>are</strong> catchment <strong>are</strong>a and an index of <strong>the</strong> catchment rainfall. The regional curves may be used up to <strong>the</strong> 200-year return period to estimate a design flood peak, except in <strong>the</strong> Otago-Southland region where <strong>the</strong> upper limit on return period is restricted to 100 years because of <strong>the</strong> limited data in this <strong>are</strong>a that were available <strong>for</strong> analysis. The curves <strong>are</strong> defined by <strong>the</strong> straight-line extreme value type I (EVl) distribution <strong>for</strong> all but two of <strong>the</strong> regions <strong>the</strong> Bay of Plenty - and South Canterbury regions, where <strong>the</strong> extreme value type 2 (EV2) tlistribution was found to give a better definitión of <strong>the</strong> regional trend in <strong>the</strong> data. Although <strong>the</strong> general extreme value (C gional curves, th of <strong>the</strong> log-Pears tion tests carried that <strong>the</strong> LP3 distribution may well have given an equally good description of <strong>the</strong> curves. It is evident, both from <strong>the</strong> regional mass probability plots and from <strong>the</strong> standard error equations derived <strong>for</strong> <strong>the</strong> iegional curves, that <strong>the</strong> variability in <strong>the</strong> regional Qr/Q data is well within acceptable limits. A quantitative indication of <strong>the</strong> confidence that may be placed on values of Q1/Q estimated fiom a regional curve is obtainable from <strong>the</strong> standard error equations which give estimates comparing very favourably with those given by <strong>the</strong> equivalent NERC (1975) equation. A feature of this study is <strong>the</strong> dependence of <strong>the</strong> results on climate. This is illustrated by <strong>the</strong> regions, which <strong>are</strong> partially consistent with recognised climatic boundaries, and by <strong>the</strong> difference in slope of <strong>the</strong> western and eastern regional curves. The latter curves have greater slopes, which ãre uttributable to <strong>the</strong> greater variability in <strong>the</strong> flood peak data <strong>for</strong> <strong>the</strong> eastern regions where <strong>the</strong> climate is drier and <strong>the</strong> antecedent conditions more variable. Fur<strong>the</strong>r indication of <strong>the</strong> climatic influence is given by <strong>the</strong> regional equations <strong>for</strong> estimating Q . ¡.lo physical characteristics, o<strong>the</strong>r than catchment <strong>are</strong>a, <strong>are</strong> included in <strong>the</strong> equations, <strong>the</strong> only o<strong>the</strong>r important parameters being catchment rainfall estimates. This suggests that climate may be <strong>the</strong> dominant factor affecting flood peaks with magnitude equal to or greater than <strong>the</strong> mean annual flood. O<strong>the</strong>r factors often considered important, such as geology and topography, have been accounted <strong>for</strong> to some extent in <strong>the</strong> regionalisation of <strong>the</strong> country. The country is divided into two sets of regions' one set <strong>for</strong> estimating Qr/Q and one <strong>for</strong> estimating Q. <strong>These</strong> <strong>are</strong> very similar t tempts were e purposes, b t regions is a a Q1/Q and Q. The application_of <strong>the</strong> method to a catchment <strong>for</strong> <strong>the</strong> estimation of Q1/Q and ted in Chapter 5 with four <strong>the</strong> advantage, when only a of combining <strong>the</strong> estimates uation and <strong>the</strong> flood record to obtain a weighted average "best" estimate of Q . t¡e precision of each equation is expressed of record and, àepend estimate of Q from a he error of estimating od record. Thus when an important waterway project is being considered, a recorder should be installed as soon as possible to record <strong>the</strong> flood peaks. In addition to <strong>the</strong> regional curves of Q1/Q which extend up to a maximum of 200 years, generalised curves, one <strong>for</strong> <strong>the</strong> west and one <strong>for</strong> <strong>the</strong> east, <strong>are</strong> given. <strong>These</strong> curves were derived from all <strong>the</strong> flood peak data collected <strong>for</strong> this study, excluding four extreme flood events, and <strong>the</strong>y can be applied from beyond <strong>the</strong> limit of <strong>the</strong> regional curves up to <strong>the</strong> 1000-year return period. Of interest is <strong>the</strong> marked similarity of <strong>the</strong>se curves with those derived by Stevens and ing too many regions. lt is envisaged that as more flood p.ãk dut" become available, revisions and refinements will te made to <strong>the</strong> RFE method' especially <strong>for</strong> <strong>the</strong> estimation of Q. In all cases, we recommend that o<strong>the</strong>r methods <strong>for</strong> estimating design flood magnitude also should be used and <strong>the</strong> results comp<strong>are</strong>d be<strong>for</strong>e a final figure is selected' Water & soil technical publication no. 20 (1982) 83
Water & soil technical publication no. 20 (1982)
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WATER & SOIL TECHNICAL PUBLICATION
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Regional flood est¡mat¡on in New
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Tables 1.1 Risk ofexceedence for sp
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Preface Water & soil technical publ
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annual flood Q (the mean of the ann
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Water & soil technical publication
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Water & soil technical publication