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correlations between (logs oÐ FOREST and 1224, and FOREST and MARAIN were 0.90 and 0.65 respectively; this was one region where estimates of l2?A were suspected to be unreliable. The preferred equations of Table 4.10 are shown in Figure 4.10 with their appropriate regions, and also the residual errors for logarithms. In general the equations seem more reliable in the north and east of the island, a situation also noted in the South Island. The distribution ofresiduals generally appears reasonably random in space. The region for the West Coast includes a number of catchments tributary to the Wanganui River and a number originating on the slopes of the central North Island volcanoes. Many of the larger residuals for the island are clustered here, presumably because of the variety of soil types, including pumice, and the sparse coverage of rainfall intensity measurements. The equations for the East Coast and West Coast regions suggest that they should be combined into one, but doing so resulted in regional biases. Hence the separate regions should be maintained. 4.7 Discussion of results The equations derived in sections 4.5 and 4.6 are intended for application to rural catchments where flood storage is not excessive or where other dampening effects are not dominant. All the nine regional equations use catchment area, and all but one use area and one or two of the rainfall statistics; other physical catchment characteristics used in this study appear to be of little consequence. This is an important finding, since results obtained in overseas countries (see section 4.8) have suggested that these other physical characteristics are relatively important. The dominance of rainfall statistics is possibly due to the generally steep nature of New Zealand catchments (see section 4.8). The two sets of rainfall statistics considered in the study have a large range of values; both cover more than one order of magnitude in the South Island, though the range is less in the North Island. Since this study was completed, an updated analysis of rainfall intensity data has become available (Tomlinson 1980; Coulter and Hessell 1980). Tomlinson used 16000 years of data from 940 manual daily raingauges and 3500 years from 180 recording raingauges. Comparison of revised 1224 estimates with those from Robertson (1963) for a sample of 87 stations did not reveal statistically significant differences. This suggested that the revised estimates may also be used for estimating 1224. Sirce the revision used more than twice the number of stations, more accurate estimates of 1224 lor individual catchments should be possible. For convenience, revised 1224 estimates for the 94O daily gauges used by Tomlinson are included as Appendix D. - The revised intensity data were also mapped by Tomlinson. However, in high altitude areas, especially along the Southern Alps, use of the mapped values is not recommended. In mapping, rainfall is assumed to increase with altitude, This increase was not allowed for when catchment estimates for l2A were made, so the maps will give catchment estimates of 1224 greater than the estimates used in deriving the equations for Q. Thus inflated Q estimates may result from The 2-year rec uration intensitY estimated from auges was used principally because of the better national coverage of daily iead manual gauges for observing this duration rainfall compared with shorter duration rainfall statistics estimated from automatic rainfall recorder data. As noted in section 4.4, estimates of catchment mean annual rainfall have a low accuracy for many South Island and this is possibly why inclusion of mean annual rainfall improves the estimate in three of the five North Island regional equations. Little can be saicl of the physical significance of the exponents for the regional equations. Whilst values of the intensity exponent around unity for two of the South Island regions might have some physical interpretation, the meaning of exponents of the rainfall statistics which exceed 2.0 is unclear, even though their statistical significance is undoubted. Obviousl),, in using the equations, particular care must be given to the estimation of rainfall statistics, because estimates outside of the range of the values of the sample used in developing the equations could result in severe errors in estimates of Q. Tables 4.1 and 4.2 are a guide to typical valtues of the rainfall statistics. Where a catchment lies near a regional boundary and each regional equation gives different estimates, the precise location ofthe boundary is bound to create difficulties. For some regions, dominant flood-producing weather patterns are thought to apply, and a decision about which region a catchment fits into could be based on the weather conditions which are believed to produce the most flooding. An example is a number of Marlborough and Canterbury rivers in the Inland region, where the flood-producing weather conditions are possibly southerly. Their headwaters near the divide flood in nor'westerly weather condi tions and fit in the West Coast region. The rivers flow through the East Coast region where flood-producing weather patterns are thought to be easterly. In the case of the North Island Pumice region, catchments have been included where pumice was thought to have a dominant influence on the flood hydrology, but two coastal Bay of Plenty catchments were tentatively included here because this was where they fitted best, even although pumice is not dominant on these small catchments. rÙVithin the Pumice region, there appears to be a gradation in the effect of the pumice. For instance, pumice lies to great depths on the Kaingaroa Plateau within which much of the catchment of the Rangitaiki River lies. The 28 years of record for this river at Murupara (AREA : ll84km', Station 15408), but which was not used because it exceeds ll00 km', gives_a Qous : 41.6 m'/s but the estimate from the equation is Q"rt = 202 m3/s, representing a residual error of - 0.69. Clearly, the dampening effects of pumice are severe in this case. The annual flood regions delineated in this chapter are' in general, very similar to the flood frequency regions defined in Chapter 3. An exception to this is in the eastern area of the South Island (see Figure 4.7). From a design characteristics, on the other hand, is also concerned with the coefficients of variation and skewness of the flood record, and these can be regarded as the slopes and curvatures respectively of the individual dimensionless-magni- Therefore it was not unexof the flood frequencY chargions that differed in Places from the set deveioped for estimating Q. 4.8 Compar¡son with other results Similar equations given in Table 4.ll l. favourably in terms ent of determination this needs to be balanced against the use of a relatively small range of catchment areas (0'2 to ll00 km'); poorer fits may be obtained if larger catchments are used' The other notable feature of Table 4'll is the range of expon- estimated with reasonable accuracy for the North Island, ents for AREA. The values tend to be less than the values Water & soil technical publication no. 20 (1982) 7l
Table 4.11 Comparable equations for other countries, Est¡mat¡ng eqn R2 SC est Region Area Range (km,) Min. Max. Reference o o = 0.56 1 AREA 8s : 0.0765 AREAi o€ S1O85o s' o.841 o.922 o.1 94 o.142 New England New England : 2.42 AREA ss Texas and Sthn = o.o589 AREA 68110241 New Mexico 50 250OO Benson ('l 962b) 90000 Benson (1 964) O :a AREAb 0.267
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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
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Preface Water & soil technical publ
Page 9 and 10:
annual flood Q (the mean of the ann
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Water & soil technical publication
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Doto ovoiloble on TIDEDA Figure 2'1
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P=45 O= 15 x4 (l) '= o o t.25 2'.O
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Êv2 ( ko) Reduced Voriote y I'O t1
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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 and 52: allowing curves rather than just st
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: Northlond f C"ro^ondel f Eost Cope
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
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APPENDIX E Compadson of Regional Fl
Page 125 and 126:
Table F.2 Flood peak data. NEW OTAG
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F.3 Analysis and results tatively p
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ét êt REOUCEO VßFIHTE 2. 33 5 l0
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SOLITH I EA l') COAST soUTH CANTERB
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Water & soil technical publication
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