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3.3.3 Bay of Plenty region Missing from Figures 3.8 to 3.16 is the plot of the regional curve for the Bay of Plenty region (Region 2). In defining the average curve for this region it was found that four large historical floods appeared to produce an unrealistically high degree of upwards curvature at the top end of the fitted curve. Fitting a curve to all the plotted data instead of just the average values made very little difference. Therefore in an effort to obtain a more realistic regional curve, an extension method was employed (NERC 1975, pp.l7t-2). The method involved splitting the dimensionless flood peak data for the region into three groups (Table 3.3a). Each group contained the data for stations whose catchments were not close neighbours, so that it could be assumed that a group's sample items were statistically independent. The largest four Q/Q values in each group, irrespective of station, were then treated as the four largest values in a random sample of size M, where M was the total number of station years spanned by the data in a group, and not simply the total number of flood peaks for the stations in a group. Hence, where there was an historical series containing a flood peak known to be the largest in N + J years, the number of station years for this particular station . was N * J years, the historical record length. In the special case where the historical series did not contain the largest historical peak in N + J years but, say, only the second or third largest in that period, the number of station years was not so straightforward and an equivalent length in years had to be determined. The return period of the second or third largest historical peak was calculated using the Gringorten formula (Equation 3.20) and substituted back into the formula, this time using a rank of one instead of two or three as before. The value of N resulting from this back substitution was taken as the equivalent length of station years. The number M varied from group to group, but an attempt was made to keep it reasonably constant. Values for y were calculated for the four largest Q/Q values in each group by taking M as the record length and using the Gringorten formula and Equation 3.16 (Table 3.3b). The 12 pairs of Q,zQ values and their corresponding y values, i.e., the four pairs from each ofthe three groups, were averaged over the 0.5 class intervals of y (Table 3.3c), and the three largest averages were plotted along with those obtained from the original flood peak data. There is some statistical dependence between the two types of average values, but it was thought that, with M being fairly large for each group, the dependence would be small. Finally the regional curve was defined by fitting Equation 3.14 to the combined set of original and new average values. The probability plot of the regional data is shown in Figure 3.19 (the four historical flood peaks are indicated with circles and the corresponding site numbers are given alongside). Also shown is the regional curve that resulted from fitting Equation 3.14 to the original and new average values. The latter values are indicated with squares. It was difficult to finalise the Bay of Plenty's southern boundary line which, from the probability plots, appeared to lie somewhere near the mountains in the volcanic plateau area of the North Island. Besides the problems caused by the geology and the uncertain Ìveather pattern in this area, there were also complicatións arising from the Tongariro Power Development Scheme which had altered the natural flow of some of the rivers. While the chosen southern boundary is reasonably consistent with the geology of the area and with the trend in the probability plots for the stations concerned, some further definition of the boundary line may be necessary at some later stage. The definition of the Bay of Plenty on its eastern boundary posed a problem of a quite different nature. The probability plots clearly indicated that the eastern boundary line TaHe 3.3 Calculatk¡ns for extending the set of average values for the Bay of Plenty region. (a) Grouping of statio,ns Group 1 Group 2 Group 3 Mangatepopo @ Ketetahi t=8 Utuhina @ S.H. 5 Bridge r=9 Tongariro @ Turangi t=26 Waiotapu @ Reporoa f=38 Wanganui @ Te Porere l=10 Kaituna @ Te Matai r=21 Tongariro @ Puketarata t--26 Tahunaatara @ Ohakuri t=12 Wanganui @ Headwaters ( = 11 Mangorewa @ Saunders Farm f=9 Rangitaiki @ Murupara t-_38 Pokaiwhenua @ Puketurua ¿ = 13 M=81 M=69 M=71 / = the length in ye,ars spanned by the data for a station. M = the total length of station years spanned by the data ¡n a group. (bl The maximum O/O and y values Group 1 Group 2 Group 3 o/o o/o O/o y 2.870 4.972 2.464 3.941 2.044 3.440 1.746 3.104 2.649 4.811 2.994 4.840 2.462 3.780 2.261 3.809 2.008 3.277 1.993 3.306 1.944 2.940 1.960 2.969 (c) Classification and averages of the maximum O/O and y values. y interval 4.5 - 5.O 4.O - 4.5 3.5 - 4.O 3.O - 3.5 2.5 - 3.O Water & soil technical publication no. 20 (1982) No. of values Average O/O Average y 3 3 4 2 2.84 2.40 1.95 1.95 4.87 3.84 2.28 2.96 should be near th€ Rangitaiki River, with the catchments either side of the river displaying a noticeably different flood frequency trend. Support for this difference in trend can be found in the geology of the area. The area west of the river is a pumice and rhyolite zone, whereas the area to the east comprises sedimentary rocks, e.g., sandstones and greywackes. The dividing line between the two geological areas is abrupt and coincides almost exactly with the line of the Rangitaiki River: The problem then was not so much where to locate the boundary line, but in which region to put the flow stations for the Rangitaiki River itself, since the flow in the riverr represents the integral effect of the two geological areas on the runoff process. However, the trends in the probability plots for three stations on the river (i.e., sites 15408, 15410 and 15432) were consistent with the flood frequency trend of one of the regions either side of the river, and the bourrdary line was drawn such that each station was included in the most appropriate region. The same approach could not be applied to a fourth flow station on the river at Te Teko (site 15412), which is downstream of the othe¡ three stations. The trend of the Q/Q probability plot for this station lay in between the trends exhibited in the Bay of Plenty and North Island East Coast regional plots. This was presumably because the peak flow at the station contains very significant contributions from 37
BÊY OF PLENTY DRTF o o o NEOUCEO Y VRBIßTE s r0 ¿0 30 50 7s r00 FETUFN PEHIOO (YEßRS) BÊY ÚF PLENTY BEGIONÊL CUBVE o o lo o Þ Þ è t.so 2.25 3.00 3.75 NEOUCEO Y VRNIßTE r.ô¡r 2.3! s ro ¿o !o 50 75 ¡oo 2oo nETUBq FEnI00 tYERnS) Flgure 3.19 Region 2: regional plot and curve. Water & soil technical publication no. 20 (1982) 38
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: COMB I NED ,,IE5T CORST DRTR o rt
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 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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