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o SOUTH AFRICA to o ,s.E. BRtTAtN .,,' EASTERN N.Z. WESTERN N.Z. -N.W. BRITAIN 23 Reduced Voriote 567 2.33 5to2050 -ffi roo 200 500 rooo Return Period ! yeors Figure 3.26 Comparison of the New Zealand generalised curves with those f rom the British lsles and South Af rica. - cords with the longest reliable one. The aim of this correlation is to remove the effect of any climatic variation that might exist amongst the records of differing length. However, the correlations are often so poor that there appears to be very little advantage in attempting this form of extension. Further it is uncertain at the present time whether there are significant climatic trends in annual flood peak records (ICE 1975, pp.76-80). For example, Cunnane (NERC 1975, pp.l25-32) performed a number of statistical tests on 28 long records of annual flood peaks. The tests suggested that the peaks were largely random, and therefore contained no noticeable climatic trend. A similar con- clusion was reaclned by Beard (1977) in an analysis of annual flood peaks in 300 long records for stations in the United States. In this New Ze:aland study it was assumed that there was no climatic trend in the annual flood peak records. This same assumptiorn was made by NERC (1975), and in Beard's (1977) flood frequency analysis work for the U.S. Water Resources Council. The possibility of there being climatic variations in the flood records is not ruled out, but this is an area ol'hydrology that requires further research before proper account can be taken of such variation in a regional study like this. Water & soil technical publication no. 20 (1982) 5l
3.5.5 Extension method The NERC (1975) extension method proved very useful in developing the Bay of Plenty regional curve and the generalised curves. However, it was not considered necessary to use the method for the development of every regional curve. For instance, in a test on the South Island tr)Vest Coast regional data, the extension method produced a curve almost identical with that obtained by the usual curve-fitting procedure (section 3.3.2). There was also uncertainty in the method concerning the statistical dependence between the original and the new set of average values. In a sense it appeared that the same information on the large Q/Q values was being used twice. This would weight the curve-fitting process in favour of the historical flood peaks, the estimat€s of which are generally less reliable than those for the annual flood peaks. In addition, there is considerable difficulty in grouping the stations of a region such that neighbouring catchments are not represented in a group. While this condition can normally be satisfied, there are necessarily some catchments represented which are in close proximity to one another, which raises doubts about treating the flood peak data of a group as independent sample items. 3.5.6 Catchment s¡ze Catchments less than 20 km'? were omitted from the study, except in the Northland-Auckland area, on the prior belief that they would have steeper flood frequency curves than the larger catchments (section 3.2.1). This was found to be true at times, especially for the very small catchments of the order of I km' in area. However, the initial curve for the Northern North Island region (Figure 3.8) refutes the general argument against the omission of small catchments. The curve was based on the flood peak däta for catchments ranging in area from 2.48 to 1606 km':(see Table 3.2), yet it is not steep nor does the corresponding regional probability plot show greater scatter than that for other western regions. The curve is a typical western one and flatter than any eastern regional curve. While it was quite evident that the flood peak data for some small catchments could not be tolerated in the derivation of a regional curve, it does appear from the Northern North Island example that small catchments could possibly have been considered in the derivation of some of the curves. 3.6 Summary Eight flood frequency regions have been defined for New Zealand, and an average Q/Q vs T curve derived for each. It is suggested that these regional curves can be used to estimate Qr/Q for rural catchments within the region concerned for return periods up to 200 years. An exception is the Otago-Southland curve; it is only tentative and should not be extrapolated beyond the 100-year return period. For return periods exceeding the recommended upper limit of the regional curve, one of the two generalised curves developed for the east and the west of New Zealand can aid the estimation of Q1/Q. With all the curves it is important that they are not applied to catchments whose areas are too far outside the range of areas used in the derivation of the curve concerned. An indication of the reliability of a Q1/Q estimate taken from a curve is provided by standard error equations, which give values somewhat less than a similar equation determined by NERC (1975) for use with the British Isles curves. The most notable characteristic of the curves is that those for the east have steeper slopes than those for the west, reflecting the greater variability in the annual flood peak data for the eastern catchments. The same characteristic is evident in the British Isles regional curves. Also of note is that the two New Zealand generalised curves resemble their Great Britain counterparts (Stevens and Lynn 1978). 52 Water & soil technical publication no. 20 (1982)
Page 1 and 2: WATER & SOIL TECHNICAL PUBLICATION
Page 3 and 4: Regional flood est¡mat¡on in New
Page 5 and 6: Tables 1.1 Risk ofexceedence for sp
Page 7 and 8: Preface Water & soil technical publ
Page 9 and 10: annual flood Q (the mean of the ann
Page 11 and 12: Water & soil technical publication
Page 13 and 14: Doto ovoiloble on TIDEDA Figure 2'1
Page 15 and 16: P=45 O= 15 x4 (l) '= o o t.25 2'.O
Page 17 and 18: Êv2 ( ko) Reduced Voriote y I'O t1
Page 19 and 20: turn period. A plotting position fo
Page 21 and 22: 'o ,l\, I zl l- I Water & soil tech
Page 23 and 24: Table 3.2 Flow stat¡ons used. SITE
Page 25 and 26: e process of examining the simtrend
Page 27 and 28: Iì_ t_ I I I I I I I t-. I I tñ I
Page 29 and 30: Þ I"IEST COFST 0 o I E 6o o o 25 3
Page 31 and 32: SOUTHEBN REDUCEO Y VRBIßTE NETURN
Page 33 and 34: SOUTH ISLFND HEST COFST I]ÊTR èo
Page 35 and 36: SOUTH CÊNTERBURY DRTÊ NEOUCEO Y V
Page 37 and 38: COMB I NED ,,IE5T CORST DRTR o rt
Page 39 and 40: BÊY OF PLENTY DRTF o o o NEOUCEO Y
Page 41 and 42: NOBTH ISLRND ßEGIONÊL CUBVES 1.50
Page 43 and 44: data for this area are required bef
Page 45 and 46: NELSON REGIONFL CURVE BEOUCEO Y VRR
Page 47 and 48: 46 EÊSTEFìN NEI,,I ZEFLÊND DÊTI
Page 49 and 50: Table 3.9 The grouping and the grou
Page 51: allowing curves rather than just st
Page 55 and 56: Table 4.1 South lsland catchment ch
Page 57 and 58: I 2 t l¡ 5 5 7 I 9 l0 t1 ,12 I5 1{
Page 59 and 60: decided instead to use estimates of
Page 61 and 62: \Í l{ I t, ìj( Fþurc 4.4 Distrib
Page 63 and 64: 62 Fþurc 4,6 Logarithmic rcsidual
Page 65 and 66: Table 4.6 Final equations for South
Page 67 and 68: I ++ ++ * E 4.6 Analysis of North l
Page 69 and 70: Non Pumice = õ = 3-24xto'6 AREA'7
Page 71 and 72: Northlond f C"ro^ondel f Eost Cope
Page 73 and 74: Table 4.11 Comparable equations for
Page 75 and 76: Poo led cv Pooled o.40 F¡guro 4.12
Page 77 and 78: mates of Cp typically range between
Page 79 and 80: -J æ 'll o Eo !¡ Assemble llood p
Page 81 and 82: Year 1958 1959 1960 l96r t962 1963
Page 83 and 84: The estimate of Q from the regional
Page 85 and 86: Water & soil technical publication
Page 87 and 88: Maguiness, J.A.; Blackwood, P.L.; B
Page 89 and 90: The liequency factor K is a functio
Page 91 and 92: e.9., Linsley et ol. (1915); lrish
Page 93 and 94: Maguiness, J.A.; Blackwood, P.L.; B
Page 95 and 96: sltt 930 t [rûltRÀict ¡ l? s;¡t
Page 97 and 98: 3950't flrlÀRt R tr îlt¡Ît ;tP
Page 99 and 100: S ITI 29202 RuNttiltcl I ¡t tllEtt
Page 101 and 102: ïI-'_____ ::19: lt¡IPt0r n À1 i
Page 103 and 104:
9100r GNE? R ¡T DOBSO| s¡18 93207
Page 105 and 106:
6800 1 sBlrri R tT SrITEC¡,tttS 6I
Page 107 and 108:
APPENDIX C Summary of tho data for
Page 109 and 110:
oæ ON LAT. STATION NA ATION LAT. N
Page 111 and 112:
o NUI,IBER HHAKAIIARU 854801 38 L7'
Page 113 and 114:
l9 AT NU¡IB ËR AR',tËtlANA ÂRAI
Page 115 and 116:
A ON LAT. STATION NA NUMBER AHUNA 5
Page 117 and 118:
â NUI,IBER Lü{G.E ON NU14EER I,IU
Page 119 and 120:
æ TATI ¡rouNT sor.tERS 7L740? 43
Page 121 and 122:
N) o STATIoN NAr{gmLfï3---ffij NUI
Page 123 and 124:
APPENDIX E Compadson of Regional Fl
Page 125 and 126:
Table F.2 Flood peak data. NEW OTAG
Page 127 and 128:
F.3 Analysis and results tatively p
Page 129 and 130:
ét êt REOUCEO VßFIHTE 2. 33 5 l0
Page 131 and 132:
SOLITH I EA l') COAST soUTH CANTERB
Page 133 and 134:
0tiftflt a itÉitut5 ü¡. il/5 tg?
Page 135:
Water & soil technical publication
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