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The regions that were chosen initiatly reflect the difference in rainfall behaviour in New Zealand, and each region can generally be distinguished by its own rainfall characteristics. Where there are pronounced regional rainfall characteristics, and where there are sound topographical reasons for this, the boundary line between regions was easily decided upon. For instance, in the South Island West Coast region, where the rainfall is greatest and is dominated by the orographic influence of the Southern Alps, the ridge line of the Alps was the obvious dividing line between this region and the other South Island regions. However, it is probable that the effect of the West Coast rainfall extends somewhat east of the ridge line, as indicated by the probability plots for the Shotover and upper Wairau catchments. The actual boundary line for the South Island West Coast therefore includes these catchments and some eastern headwater catchments in the Alps, e.g., the catchments of the Wilkin, Makarora and Matukituki Rivers. While the rainfall characteristics of different parts of New Zealand helped to identify the regions, the actual definition of the regional boundaries was not always as straightforward as in the South Island West Coast example the topographical features were not always as dominant - as the Southern Alps in influencing the rainfall. ln addition, there was sometimes a poor areal distribution of flow stations, so that often the definition of the regional boundaries was rather subjective. A striking outcome of the regionalisation work was the small number of flood frequency regions. In earlier work Toebes and Palmer (19ó9) divided the country into 90 hydrological regions according to climatological, geological and topographical factors. However, in this study eight regions were considered adequate to define the variation in flood frequency behavioui in the country. This is an order of magnitude less than the number used by Toebes and Palmer, and suggests that climate is the dominant factor influencing floods in New Zealand and that geology and topography generally play relatively minor roles. 3.4.4 Homogene¡ty test Some flood regionalisation studies have used the statistical test described by Dalrymple (1960) to identify the stations that form a hydrological homogeneous region. This homogeneity test involves the graphical or analytical fitting of a frequency distribution to each station's flood peak data and the estimation of peak discharge values for the 2.33 and lO-year return periods, i.e., Qt ,, and Q,o respectively. The ratio of Q,o/Q, ,¡ is then formed, which is an index of the straightJine slope of the fitted frequency curve between the two return periods. The test places confidence limits on the ratio Q'o/Q, ¡¡ and stations with ratios lying within the limits are taken as being part of the homogeneous region. Although the test Provides a quant ing a region, it was not relied uPon a insensitive when tested on South Is served that the test was unable to detect even major differences in individual frequency curves past the lO-year return period. This deficiency ofthe test was also noted by Benson (r962a\. 3.5 Flood frequencY discussion 3.5. I Regional d¡fferences The most prominent characteristic of the regional curves is that the curves for the western regions of an island have a Water & soil technical publication no. 20 (1982) regions receive more rainfall more regularly than their eastern counterparts. Thus, the ratio of runoff to rainfall is comparatively high for the western catchments and does not vary markedly for the storms that produce the annual floods. The mean annual flood is therefore fairly large and there is little variability in the annual flood peak data (section 4.9). As a consequence, the difference between the 100-year and mean annual flood is not very great, e.g., in the curves for the two western regions the 100-year flood peak is less than 2.35 times the mean annual flood. On the other hanLd, the catchments in the eastern regions are usually drier and have a lower runoff to rainfall ratio. The mean annual Ilood for an eastern catchment is therefore less than that for a western one of the same size (section 4.5). There is eLlso a greater range in the runoff to rainfall ratio fo¡ an eastern catchment and this is reflected in the greater variability of the annual flood peak data for such a catchment (section 4.9). The difference between the 100-year and the rnean annual flood is therefore significantly greater than that for a western catchment, as is borne out by eastern regional curves being steeper than the western curves. It is worth noting that the wettest region (the South Island West Coast region) has the flattest regional curve and one of the driest regions (the South Canterbury region) has the steepest curve. While the slope of a regional curve is an indication of the variability in the individual data samples for the region, the curvature may be taken as an index of the skewness in the samples. As indicated by NERC (1975, p.42,47), the EVI distribution has a coefficient of skew of 1.14, and skew values greater or ler;s than this figure correspond to the EV2 and EV3 distributions respectively. The majority of the regional curves in this study are described by the straight-line EVI distribution, implying that the average skewness in the data samples for these regions was small. This in fact was the case: for the regions with straight-line fits to the data the average skew of a region's annual flood peak samples was in the range 0.39 (S.I. West Coast) to 1.09 (Central Hawke's Bay) and the EVI distribution was found to give a good approximation to the regional data up to the 200-year return period. In the Bay of Plenty, South Canterbury and Otago-Southland regions the average skew of a region's data samples was greater than 1.2. For the first two of these regions, the EV2 dtistribution was found to give a good fit to the regional dat¡1. Because of the smaller number of data samples for the third region, however, less significance can be attached to the average skew (1.23) of the region's data samples, and the IlVl distribution approximated the data satisfactorily up to the 100-year return period. It is interesting to note that the two regions with EV2 regional curves can both be regarded as dry areas; the South Canterbury region because of its low rainfall, and the Bay of Plenty region because of its absorbent pumice soils (see also section 4.6.2) and its relatively low rainfall in comparison with its regional neighbour to the west. The pooling of the regional flood peak data to construct the generalised curves produced mass probability plots (Figures 3.24 and 3.25) that show very little scatter in the data up to about the l0-year return period. Beyond this the scatter is greater, but not excessively so, and is due in part to some of the uncertainty associated with the historical flood peaks. The averaging of the pooled data and the application of the extension method (NERC 1975) clearly defined the flood frequency trend for the fitting of each generalised curve. rühile the curves require some refinement using more and especially longer flood records, they should still provide the designer with a reasonable guide as to the magnitude of flood peaks past the 2(Ð-year return period, the maximLum upper bound of the regional curves. An assessment of the reliability of each curve can be made from an inspectiorr of the scatter in the corresponding mass probability plot eLnd by applying the appropriate North Island group CF equation (section 3.4.1). 49
allowing curves rather than just straight lines to be fitted to the regional data with confidence. In this New Zealand curve and the eastern curve is the straight-line EVl. This is d generalised curves ynn (1978) for Creat given in Figure 3.26. curve and the corresponding regional ones, results from in_ clusion in the development of the generalised curve of some of the extreme flood peaks which had been excluded from the derivation of the corresponding regional curves. While there is a valid argument for the inclusion of these extreme e still been too thereby weightperiods. been described New Zeatand curves bear a remarkabl|t','"tJJr:i*t;tÏ:: to their Great Britain counterparts. Although the two countries have broadly similar cli_ mates, New Zealand has greater extremes of wet and dry. That the New Zealand curves do not reflect this with túe western and eastern curves be - tively, in relation to the corre - is possibly due to the ave ment of the curves, i.e., the i treme wet or dry climates is largely nullified by the lumping together of these areas with others which are noticeãblt drier or wetter, respectively. The fact that the western New was used to describe the generalised curve. ntinuity between Oing generalised " . ;;,f.tïi'l"i::; expectation. This may be viewed as taking a weighted aver_ age of the estimates. 3.5.3 Vadation within a reg¡on 3.5.2 Compar¡son with rhe Br¡t¡sh lsles ; f these curves reveals some remarkable similarities with the curves obtained in this New Zealand study. First of all, the British Isles regional curves display the same trend of an increase in the slope of the curves between those for the western regions and those for the eastern re_ gions the curve for the western-most - region, the whole of Ireland, has the smallest slope, whereas the curve for the eastern-most region, East Anglia, has the greatest, Further, the range of the ordinates of the British Isles and New Zealand curves is almost the same at high return periods. For example, the South Island West Coast regional curve, which has the smallest slope of all the New Zéaland very dry in relation to is likely to be steeper catchment which is ve tend to have a flatter An extension of this argument leads to the concept of sub-regions which, although geographically part of a làrger surrounding region, display a e/Q frequency trend of thiir is E b 50 3.5.4 Secular climatic variat¡on -In th9 regionalisation procedure described by Dalrymple (19@), it is recommended that all the flood records should be brought to a common base length by correlating the re- 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: Table 3.9 The grouping and the grou
Page 53 and 54: 3.5.5 Extension method The NERC (19
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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