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4 Estimation of mean annual flood 4.1 lntroduction This chapter covers estimation of the mean annual flood 1Q¡ tor sites with no flood record. A procedure is presented which estimates Q as a function of catchment and climatic characteristics. Where necessary, Q so estimated is designated Q "s distinguishing it from Q o5, calculated from observed flood records. The flood records used to derive the procedure were selected as outlined below. This selection was based on the length of record, the size and type of catchment, and the quality of record. Selection of the characteristics and their abstraction from maps and other published information are detailed. It was found that improved estimates of Q were obtained by forming regions. Delineation of these regions and obvious boundary problems are discussed. Catchments with records not fitting into regional patterns are identified and reasons for some anomalies are suggested. Best fitting equations are summarised with accompanyinig standard error estimates. 4.2 Proposed method The relation between Q and measurable catchment characteristics was assumed to have the fo¡m of Q: a X' b'X, b' 4t where a, b,, b, ... are constants to be estimated and X', X, ... are characteristics of the catchment having an influence on the mean annual flood. After taking logarithms, log a, b,, b, ... were estimated by standard multiple linear regression. There is good precedent in the literature for this type of multiplicative function (NERC 1975; Benson 1962b); thus other possible forms were not investigated. 4.3 Records used Catchments used in the study were selected from those having an area within the range 0.22 to I 100 km'. The three smallest catchments have areas 0.22, 0.52 and 2.18 km'. Smaller catchments were excluded because flood discharges from very small (often ephemeral) catchments result from short duration rainfalls and are sensitive to infiltration and vegetation effects (Campbell 1962). The upper bound was imposed because estimates of mean rainfalls over large catchments can be unrealistic. In any case, since most large catchments in New Zealand are monitored, most ungauged catchment estimation problems occur in relatively small catchments. Records for catchments with significant impoundments or diversions were not used. This meant excluding a number of lake outflow records. However, outflows from all larger lakes are monitored for hydroelectric purposes and flood estimates for them are best derived directly from these records. Urban catchments, and others with areas of limestone country, or permanent snowfields or glaciers, were also excluded. Four years of re_cord were considered the minimum necessary to estimate Qou, md with the above constraints, data for 63 South Island (Figure 4. I and Table 4.1) and 97 North Island (Figure 4.2 atd Table 4.2) catchments were assembled for the study. The distribution of these catchments throughout the country was reasonable, except that there were too few in the southern part of the South Island' Tables 4.I and 4.2 contain the data used in this chapter. These are the mean annual flood Qo6, , the coefficient of variation Cu of annual maxima, the length of record, catchment area, and other catchment and climatic characteristics. Generally, flow data are complete to the 1976 or 1977 calendar year. 4.4 Collect¡on of character¡st¡cs A number of indiLces can be used as characteristics X,, X, ... in Equation 4.1. However, if the relation to be developed is to be useful as a design procedure, the cha¡acteristics must be rea'dily obtainable from published information such as maps, climate summaries, etc. The characteristics can be subdivided into two groups; those which cha¡acterise the phvsical catchment, and those representing the climate over the catchment. The physical characteristics included the size arnd shape of the catchment and the embedded stream channel, the vegetation, and the hydraulic properties of the soil. The NZMS I (l:63360) map series is the most detailed map series giving a national coverage and was used to providr: measurements of the area, mean elevation, channel density, main channel length and slope, and percent forest cover. Hydraulic properties of the soil (e.g. permeability) are less easily defined; these depend on soil type, surface slope, vegetation rooting characteristics and underlying geology, and vary widely over most catchments. Much of this latter information is now available nationwide from the National Land Resource Inventory Iùy'orksheets which are plotted at the same scale as the NZMS I map series, but this was not available when the present study was undertaken. Climatic data available in Meteorological Service reports and maps include rnean annual rainfall, rainfall intensity of specified probability, distance of the catchment from important meteorological barriers, and measures related to the time and rate of snowmelt. Snowmelt is generally not an important flood-producing mechanism in New Zealand and in this study two rainfall parameters were used: mean annual rainfall for the catchment, and the 2-year return period 24-hour duration rainfall estimated for the catchment. Thus the following characteristics were estimated for each catchment: Catchment area. Water & soil technical publication no. 20 (1982) (i) (it) Main channel length. (iii) Main charurel slope. (iv) Mean catchment elevation. (v) Stream frequency. (vi) Percentagecatchmentforested. (vii) Mean annual rainfall over catchment. (viil) 2-year return period, 24-hour duration rainfall. The first six of these are physical and vegetation characteristics which were extracted from the NZMS I maps. Although recent nraps in this series are photogrammetric maps, earlier maps are based on survey records and plane table sketch surveys. Others are provisional without contours, so channel slope and mean elevation could not be defined for every catchment. The considerable variation from one map to anothcr in the definition of streams meant that estimates of the sl:ream frequency, that is, the number of stream junctions lper unit area, were unlikely to be consistent from one area to another. It was felt important to utilise all available information so stream frequency was included but, in the event, it did not assist in explaining the variation of Q between catchàents' The procedures for estimating the physical characteristics which were adapted from Benson (1962b, 1964) and Newsom (1975), and acronyms by which these cha¡acteristics will subsequently be known are given here. (i) Catchment area: (AREA) (km') Catchment boundaries were drawn on maps and the enclosed at'ea measured by planimeter. These areas are normally available with recording station information. 53
Table 4.1 South lsland catchment characteristics. 1rì 11 2It 2(¡ 27 28 21 30 51 32 51 Jlr 55 l5 37 38 lq h0 41 lr2 ll'¡ qq b5 46 lr7 q8 49 50 51 52 53 54 55 56 57 58 59 60 61 62 6l o ^.- STATIoN tm{il 5?tì16 tlÏ-.nni 5 7008 l¡37.00 57106 6S.20 5750! 83t.00 59001 77.nr\ 6fìl 10 1161 .74 601tli 260.00 6fì1I6 59.f10 62!.0t 133.f'n 62104 -1.6.80 621rì5 187.fllì 64t01 524.n0 6460C 89.2fì 64610 1I.30 651n4 540.00 65q02 56.n0 66409 0,27 6660J 1.05 66604 1,42 67601 lt.?0 6Î001 35.00 68806 108.00 69506 260.00 696I lr 25 0 .00 61 618 I 20 .00 61621 72,7i 717î2 tJ.0n 7!LO3 11 2. 70 71106 91.60 71109 100,I0 71116 227 .nn 7lr2t 67.5tì 71f22 7,Jtt lLlz? 16.10 711 29 2b . nn 711t5 6l .fio 7!702 148.0n 73501 37,80 7rr5l,h 55,2rì TttSItE 22.F,rt 74t5i 2.87 7tt7Ol ,1-0.00 75259 22.20 7527 6 ll5 6. 0 n 7850-\ t0.00 73625 46.60 78801 5.09 80201 18.70 th701 5h9,00 86802 3725.00 906 0tt 2132.0î 90605 27 .iî 91101 1588.00 911102 lq.10 9!lr0h 692.00 91407 119tì.00 9!206 1680.00 91207 460.00 9t'2on 7rB.no 932u 974.00 91212 196.00 932t7 181.00 94502 1 251 .flo cv 0.1n 0.85 0.79 0.tfj o.7n o.lrb 0,¡0 0 .10 0.55 0 .95 n.lr8 0.70 0.27 | ,17 ll.t!7 0.70 1.54 1.1i I .01 0.fi8 0.5'r 0.71r 0.75 0¡98 r.l5 0.61 0.81 0.77 0.58 0.1 I n.5lr I .14 o.70 0.h7 0,11 n.75 1.nt 0.1 1 n,56 0.59 r) .96 0.30 o.26 0.lrl 0.1)0 0.81 n.78 o .2P, 0.!.6 0.10 o.2tl 0 .15 o.2t o .27 0.r4 0.40 0.28 0.59 o .?.7 0.20 o.32 l-ength Record (yrs) 8 6 15 5 16 25 11 18 5 73 t2 10 3 10 It 5 t2 I2 6 16 I 40 lll 10 5 1l (; 6 L2 7 6 q 11 10 6 72 10 10 L3 7 7 11 7 It ll 7 9 6 9 1l 11 13 IJ 16 11 5 AREA MARAIN 1224 LENGTH 51O85 {km'¿) {mm) (mm} (kml (m/m) 5lEû0 287r !.10 16.50 0.0198 161.00 2100 Rl 25.10 0.(tr00 B1 .60 1140 81 20,50 0.0134 1164.00 1500 124 16.50 0. CI79 _l1,fta 7620 9t 6.50 0,n210 76b.00 14Lo 3t G1.so oi'riis 505.00 L6L0 69 4C.00 0.0118 192.00 24!0 69 29.40 0.0140 q9?.00 1570 76 68.90 0.0092 2n.00 7200 76 9.rlo 0.0600 440.00 L720 7l) st.tO 0.0097 464 .00 L23\ Lt2 49. q0 0.0109 7r,.60 4300 7ß L7.tt} 0.0290 41.60 030 74 10,50 0.0120 1070.00 2000 0t 76.10 o;00i2 tlz.2o 7\0 77 li,40 0.0270 o.22 L4C0 8l O.lO 0.20r¡0 ?.13 1GO 63 2.50 0.1280 1,2t |to GJ 2.70 0.1ir0 5.20 l.rr00 !.04 Z.trO 0.1t¡00 16r; .0 0 1100 6 9 10 .7 0 O, (07 9 q?1.q0 L35o 6c 54.90 0.0140 t20.00 1010 80 t,6.BO o.oijO 456.00 1075 61 r¡6.40 0.0262 41 2 .0 0 777 61 tti .ZO 0 .010 6 2?.\9 800 52 7.eo UNDEF 78.70 ejo q1 2t: (o úi¡óEÊ 899.00 720 48 62.30 uilDEF 150.00 s70 q1 29.50 0.0070 23,7i lr30 3\ 9.10 0.0470 11.40 tz(J bi 10.10 0.0160 122.00 [00 J5 25.t0 0.0420 108C .00 7022 75 06.00 0.005 6 1!!.90 1010 rrr 2e.10 0.0020 109.00 10t0 51 17.10 0.01ió 36.80 10r!0 b6 17.!.0 0.0030 , u 1.90 lt t0 66 L7 .70 0,0070 155.00 6520 520 16,30 o:fì'riõ 842.00 6920 tO2 7rr,00 0.0168 152.00 731,0 ttl ,1.40 lt.0290 3.99 1500 18s 5.20 0.0560 99q.00 5700 226 65.90 0.0067 _lq.qq 27\3 104 8.90 0.0190 642.00 4010 81 66.s0 o.noõé 790.00 l¡1r70 8l 62.50 0.OOBO 99e.00 3580 81 69,50 0.0082 254.00 2520 81 15.60 o,ollo 980.00 2990 64 95.00 0.0091 E:7.SS i250 64 75.20 0.0120 -2!!.gg 1610 6b 1e8.00 2t.Eo ö:diii t070 64 th.1o 0:ótÉ,, 694.00 \720 LS2 116-10 0.0170 FOREST {%) 4t 4t 62 57 22 7 19 2 0 1 2 5 25 1 32 0 0 0 9 0 11 0 0 0 1 0 0 0 0 0 3 0 0 0 0 0 0 29 0 0 0 8 0 2 0 0 99 26 49 2f 78 56 16 76 58 76 77 7t 72 76 67 77 STMFCY ELEV (km-'z) (m) 0.16 Ln' 0.48 1019 0.89 52tt 0 .71 587 0.80 594 0.87 785 0 .2\ 1 qll 0,21 t392 0.62 1510 0.02 11q0 0,r5 1200 0,72 500 0,27 rb20 0.41 5t5 0. b6 960 0.55 UNDEF 0 .0 9¡¡5 0.0 2E'1 0.0 r18 1 .60 t 0ll 0.78 647 0 .1¡r 100O 0,51 .820 o.ql 917 0 .25 5lr0 ].10 UNDEF 1.80 UNDEF O. 85 UNDEF l.tt 827 L .72 1128 0.85 12b0 0.46 l0llr 0.11 840 o.25 970 o.22 lt85 0 .3t 1l¡90 1 . 1O UI,¡DE F 0. t5 288 1 ,71 83t r.r1 913 2 .62 3lr 2 5. (l) L75 L.3t 1t68 0.28 tt88 0,08 50 1 .05 38' 0.0, 69 o.z\ 162 0.15 8b0 0,¡2 1050 0.56 1150 0.0 2\0 0 . 74 8l+t 1.08 L\2 0.17 7 20 0.21 710 0.t3 610 0 .26 860 tr zE 'TtÌ 0.25 971¡ 0.50 679 0.2[ 1168 0.50 700 (ii) (iii) (iv) Chsnnel length: (LENGTH) (km) The main channel of the stream was defined and extended to the catchment divide, and its length meas_ ured with an opisometer. Channel slope: (51085) (m/m) Two points were chosen at l0 per cent and g5 per cent of the channel length from the outlet. Channel slope was determined by dividing their difference in elevation by % of the channel length. Catchment mesn eleyrtion: (ELEV) (m) A grid was overlaid on the map, the grid size being selected such that at least 20 points lay within the catchment boundary. The elevation of each point within the area was noted and the mean of these elevations was taken as the catchment mean elevation. (v) Stream frequency: (STMFCY) (tns/km,) The number ofjunctions for all stream channels de_ fined on the catchment map was counted and expressed as the number per unit area. (vi) Forest cover: (FOREST) (Vo) The area of catchment defined as forest on the NZMS I maps was measured and expressed as a percentage of the total area. 54 (vii¡ Me¡n annual rainfall: (MARAIN) (mm) Mean annual rainfall for the catchment was estimated from records for rainfall stations within and adjacent to it. Where catchments had no such rainfall records, mean values were obtained by planimetry from l:500 000 isohyetal maps of l94l-1.} annual rainfall normals (NZ Met. Ser. (1977) ..Mean annual rainfall maps (1941-70),' unpublished). In mountainous areas having few raingauges and large rainfall gradients, the isohyets are believed to indicate only general trends and do not provide accurate estimates of catchment rainfall, particularly in the catchments draining the Main Divide of the Southern Alps. (vüi) Rainf¡ll intensity: (1224) (mm) At the time of the study the most recent source of published rainfall intensity data for New Zealand was Robertson (1963). This provides frequency analyses of rainfall intensity information available in the early 1960's, using data for 46 recording rainfall stations and 468 daily-read stations. As there were relatively few records from recording gauges available to provide short period rainfall data, the areal extrapolation of this information to catchments remote from gauges was considered unwise. It was 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 and 52: allowing curves rather than just st
Page 53: 3.5.5 Extension method The NERC (19
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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