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Appendix F
Flood frequency analysis for Otago and
Southland
F.1 lntroduction
At the time of developing the flood frequency curves for
the eight regions covering New Zealand, there were only six
relatively short flood records available for the
Otago/Southland region. This region's flood frequency
curve was therefore treated as provisional and was only extended
to the 100-year return period; for the other regions
the curves were drawn up to 200 years. The tentative nature
of the analysis is best illustrated by the fact that one of the
records used was for the Pomahaka River at Burkes Ford
(Station 75232) for the period 1963-1975. The flood peak
for 1972 of 1088 mtls was omitted because it appeared to
be an extreme outlier, yet this peak has been exceeded three
times over the period 1978-1980.
The number of notably large floods in the area in the
period 1978-1980, and the availability of substantially more
data, suggested that reassessment of flood frequencies in
this area was appropriate. This appendix gives the results
of the reassessment, which has been completed just in time
to be published as a supplement to the main study.
F.2 Data collection
With assistance of staff of the Otago and Southland Catchment
Boards, annual maximum flows for stations with at
least l0 years of reliable flow record were extracted. The
catchments are listed in Table F.l and their locations are
indicated in Figure F.l. The flood peak data are given in
Table F.2.
Additional historical data were sought. These data took
two forms. The first was estimates of large floods which
occurred before recording commenced and which were the
largest for a known period. For example, the estimated
p"it of 220 m3/s in tÈe Leith on 19-20 March 192í is the
largest known in this area since settlement, which is taken
as dating from 1850. The second form was when the largest
recorded flood was also the largest known peak in a
preceding interval. For example, the peak of 505 m',/s
recorded on the Makarewa River on 15 October 1978 is
known not to have been exceeded since 1895.
Sources of historical information are "Hydrology Annuals"
No. 3 No. 17 published by the Ministry of Works
(1955-1969), and
-
"Floods in New Zealand (1920-1953)"
published by the Soil Conservaton and Rivers Control
Council (1957), supplemented by information from catchment
boa¡ds and some early newspaper reports. As early
historical estimates are of uncertain accuracy, only data
considered to be reliable were used.
Annual maximum l2-hr duration lake inflows calculated
from records of levels and outflows (Gilbert 1978) for
Hawea, Wanaka, Wakatipu and Te Anau were used. Local
inflows to Manapouri were not used because they are
calculated as Manapouri outflow, minus inflow from Te
Anau, minus change in lake storage, and since inflow from
Te Anau is 6690 of Manapouri outflow the residual local
inflow is subject to large errors. Although the l2-hr maxima
inflows are less than instantaneous maxima, the
reasonable assumption that the instantaneous maxima are
a constant ratio (say 1.2) of the l2-hr maxima suggests
these data can usefully supplement the maxima recorded on
rivers for frequency analysis purposes.
Flow records were also derived for two contributing
areas in large catchments by subtracting, from downstream
flows, the upstream flows lagged to allow for time of
travel, Records of annual maxima were thus derived for the
Waiau River between Tuatapere and the Mararoa confluence,
and the Clutha River between Clyde and the
Shotover and the three lakes. Because these data are derived
by differencing hydrographs, after making assumptions
about the travel times of the upstream hydrograph, the errors
in the estimated peaks are significantly greater than for
other records. They were therefore not used in deriving the
regional curves; however, they provide a useful independent
check on the curves derived from the other data.
Table F.1 L¡st of catchments.
Number of
Stat¡on
Catchment number
in Fig. 1
Name of River and Recording Station
Year
Record
starts
Catchment
atea
(km2l
1
2
3
4
5
6
7
I I
10
11
12
13
14
15
16
17
18
19
20
21
22
73501
74308
74310
74314
74346
75259
75232
77504
77505
7A502
78607
78625
78633
78803
78906
75276
75253
23
24 84701
Water of Le¡th at University Footbridge
Taieri at Outram
Taier¡ at Sutton
Taieri at Patearoa-Paerau Bridge
Loganburn at Paerau
Fraser at Old Man Range
Pomahaka at Burkes Ford
Mataura at Gore
Mataura at Parawa
Waihopai at Kennington
Oreti at Lumsden
Otap¡ri at McBridges Bridge
Makarewa at Freezing Works Bridge
Middle Creek at Otahuti
Aparima at Dunrobin
Lake Wakatipu inflow
Lake Wanaka inflow
Lake Hawea inflow
Lake Te Anau inflow
Shotover at Bowens Peak
Manuherikia at Ophir
1 964
1 955
1 961
1 968
1 967
1 969
1 962
1 957
1 956
1 958
1 957
1 963
1 955
1 970
1 963
1927
1 930
1 931
1 926
1 968
197 1
Clutha tributaries above Clyde, and below
Shotover and the lake outflows (see McKerchar,
1981. Table 4.1) 1963
Waiau tributa¡ies above Tuatapere and below
Mararoa. {see McKerchar, 1981, Table 4.1} 1969
Cleddau at M¡lford 1964
45
4 705
3 066
738
150
122
1 924
3 465
766
152
1 160
108
1 040
27
215
3 133
2 624
1 384
3 124
1 088
2 036
3 839
2 336
155
Water & soil technical publication no. 20 (1982)
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