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5 Application 5.1 lntroduction This chapter collates the applicable results and findings from the preceding two chapters and formulates them into what is subsequently reierred to as the Regional Flood Estimation (RFE) method. Rules for the applicability of the RFE method are given, a design strategy for estimating the T-year flood peak is suggested and a number of examples are given which demonstrate the use of the method. The RFE method is intended as a procedure to be used for estimating design flood magnitude in situations where insufficient flood records are available for conventional fiequency analysis. It is one of several design flood estimation methods in such situations and other methods should be used and compared with it. It has been derived from tlood records by: (¡) defining regional flood frequency curves of Q1/Q vs T, where Q1 is a design flood with return period T and Q is the mean annual flood; and (ii) developing a set of equations for estimating Q based on catchment area and measures of rainfall. A comparison with Technical Memorandum No' 6l (TM 6l) is reported in Appendix E. 5.2 Applicability The applicability of the RFE method is necessarily constrained by the restrictions that were applied to the data used in deriving the method. The following constraints therefore apply. The method should only be used for rural catchments. The method should not be applied to catchments in which snowmelt, glaciers, springs, lake storage or ponding significantly affect the flood peak characteristics. The ranges of catchment areas for which the regional flood frequency curves and the regional mean annual flood equations were derived are listed in Table 5.1. These are a guide for the size of catchment to which the method should be applied. Because of the subjective and rather broad definition of regional boundaries, it is suggested that, for catchments near boundaries, floöd frequency curves and annual flood equations for the regions either side of the lines.should be used in estimating Q1/Q and Q respectively. As in a design situation where different methods yield different estimates, the different Q1/Q and Q estimates then need to be 'compared', i.e., the merits of each should be assessed and the choice of an estimate should be made after a rationalisation of the relevant facts. Alternatively, a belief probability can be attached to each estimate and their expectation calculated, which is akin to taking a weighted average of the estimates. 5.3 Design Strategy 5.3.1 General The strategy for the use of the RFE method in design is dependent on two main factors: N, the length in years of the flood record if a record is available, and T, the design return period. The influence of these factors on the two parts of the RFE method (i.e. the regional mean annual flood equations and the regional flood frequency curves) is explained in the two following sections and summarised in Figure 5.1 . 5.3.2 Estimat¡on of O (¡) N(Nu Where there is a flood record and its length N is less than Nu, which is the is equivalent to the prãcision of the r on (see Table 4. l2)' the mean annual imated from the regional equation and the available flood record. In applying the equation it is particularly important to estimate values for the rainfall variables 1224 and MARAIN in a similar manner and from the same data base as used in the equation's derivation. Specific points to note in estimating values for 1224 and MARAIN are outlined below. 1224 Estimates ol the 1224 rainfall intensity statistic used in deriving the equations for Q were obtained by taking the arithmetic mean of the 2-year 24-hour data listed by Robertson (1963, Table 9) for rainfall stations located within, or near to, the catchment concerned. Estirrates can be made from the tabular results (Appendix D) obtained by Tomlinson (1980) in a recent revision of the frequency an- Table 5.1 Ranges of catchment areas used to derive regional flood frequency curves and mean annual flood equations. Flood frequency reglon (Fis. 3.6, 3.7) Combined N.l. West Coast Central Bay of PlentY N.l. East Coast Central Hawke's Bay S.l. West Coast S.l. East Coast South Canterbury Otagoi Southland Catchment area (km2) (Table 3.2) fntn max 2.5 6643 28.2 2893 171 2370 24.3 2424 48 6350 74.6 3430 22.4 899) lo9 ) 18321 Mean annual flood feglon (Fig.4 7,4.1O) Northland/Coromandel/ East Cape West Coast Manawatu/WairaraPa/ Wellington Pumice Northland/Coromandel/ East Cape East Coast West Coast lnland Marlborough/ Canterbury East Coast (Mackenzie, lnland (Otago, Southland (East Coast Water & soil technical publication no. 20 (1982) Catchment area (km') (Tables 4.1, 4.2) min max o4 640 3.1 1075 9.4 734 2.6 534 o.4 640 o5 997 4.O 998 o.2 1070 2.2 464 36.8 1088 2.2 464
-J æ 'll o Eo !¡ Assemble llood prák dåta lncludlng âll hletoslcat flood ln?ornatlon. lrha¿ lr th. I.ngÈh N of th. evallsblr flood rscord? .õ -3 J ! o { o t o f o*æ o CL o 2. c¡ f Øñ o @ c 2. f (o è o Ðo e. o to, f! o cl m ø d. :t 0t 4. o f o ê , oo. Apply the Generallsed Curve Esttnato0arauelghtad o? tho âstlnãtls fron3 I Calculating th€ n€an ol avall¿ble annual serleo ( graphically interpolatlng 0¡.¡¡ fron an histo¡ical :cri ee ) 2 Applylng the roglonal squation Ooes I exc.ed th! llnlt of th6 Rsglonal Cu¡ve? obtaln 0t snd det€rñiñs the stãndg¡d arror ol oltlnata NsNu Apply the RBglonåI Cu¡ve N>Nu E¡tlnate D ¡¡ th¡ arlthfr.tlc taån o? th¡ annual ae¡L¡s fo¡m e frequency anelysia ui tuo-Farsm6tsr distributioñs except for sitEÊ j.n rhe South CanteEbuDy snd 8ay of Plsnty flood l'¡6au€ncy rBgioñs. uhers I. hl6torlca¡ ftood p.ak infq¡ñatlon 6våilablo Is N ¿20 o! 1>100 ? EeÈlnate õ uy grephic;Ily lnt!¡pol3t¡ng Ê¡.¡¡ frm th. historlcal s.¡l3s Perforn a frequgñcy enalysis ulth tuo and th¡3e-paranrtsF di BtriSuti,oñ3 ConpBr€ ths frsqu6ncy analyale rs6ult! uith ths rstinatr obtafn€d from the R€gional Curv¡ (o¡ the Gener€lla6d Curva \¡hen I excs€ds the reglonaÌ curve llnlt) ônd obtðin s uelghted flood p.!k ..tlnatr' .Epeclslly ll I > 2N 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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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
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Table 3.2 Flow stat¡ons used. SITE
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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 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: mates of Cp typically range between
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
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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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