WATER & SOIL - These are not the droids you are looking for.

data for this area are required before a decision can be
made whether to make the area a region in itself or to include
it in the surrounding region.
The second sub-region identified is the area south-west of
Nelson in the South Island West Coast region (see Figures
3.7 and 3.22). lt was of interest in that the flood peall data
for two flow stations (sites 57008 and 57101) in the area did
not conform at all to the regional flood frequency trend;
these stations were omitted from the derivation of the regional
curve. The non-conformity of the data was thought
to be due to the fact that the area is in a rain-shadow, caused
mainly by the Southern Alps to the west, and receives
significantly less rainfall than the other parts of the region.
These factors encouraged a detailed look at the flood frequency
behaviour in the whole area,
The flood peak data that were collected for the area according
to the criteria in sections 3.2.1 and 3.2.2,together
with extra data that did not meet these criteria but were
nevertheless thought to be of some use here, were pooled
together to form a probability plot for the area. All the data
used in the plot are summarised in Table 3.5, while the extra
data are listed in full in Appendix C.
The probability plot that was obtained for the Nelson
area, and the curve that was fitted to the plotted data, are
shown in Figure 3.23. Despite the inadequacies with the
data and the small number of flow stations used it would
seem, both from the trend in the probability plot and from
the large difference between the fitted curve and the South
Island West Coast regional curve (shown as the dashed line
in Figure 3.23), that there is some justification for treating
the area south-west of Nelson as a separate sub-region, and
not part of the surrounding region. A greater amount of re-
Iiable data is needed to confirm this point and to define the
area's own regional curve with confidence. In the meantime,
however, it is suggested that the regional curve for the
South Island East Coast region should be used for the area
rather than the one for the South Island West Coast region.
The former region is drier and has a steeper frequency
curve and hence is more in keeping with the Nelson area.
3.3.7 General¡sed flood frequency curves
From all the flood peak data assembled for this study,
two generalised flood frequency curves extending to high
return periods were developed. In recognition of the differences
in the characteristics of the regional curves for the
west and east of New Zealand, one generalised curve was
developed for the western areas (Regions I and 5) and one
for the eastern areas (Regions 2,3, 4,6, 7 and 8). The development
was based on the principle utilised by Stevens
and Lynn (1978) of pooling regional data together to obtain
more stable flood estimates for high return periods. With
the large base of pooled data for each generalised curve, it
was hoped that the curves could be extended to the 1000-
year return period with sufficient accuracy to be useful in
design.
These curves incorporated many of the historical flood
peaks that were excluded from the regional analyses under
Rule (lli) of section 3.2.2. ln general, this rule was invoked
in a regional analysis because a flood peak was an extreme
outlier and the available length of flood record at the station
concerned was insufficient for the computation of a
plausible return period for that flood peak. However, there
were good reasons for including here some of the previously
excluded historical peaks: the large base of data for
each curve, together with the use of the extension method,
would help to prevent these peaks from exerting undue bias
on the shape of the curves; and as the curves were to be extended
to the 1000-year return period, the flood peaks with
return periods approaching this limit should be used, where
possible.
All but four of the previously excluded extreme peaks
were considered suitable for the development of the generalised
curves, The remaining four peaks were truly extreme
events, and even tbe large bases of data associated with the
generalised curves were insufficient to enable realistic return
periods to be ascribed to them. Two of the peaks occurred
in the same storm in 1938 in the adjacent Mohaka
Table 3.6 Calculations for extending the sets of average values
for the generalised curves.
(a) The Maximum O/O and y values.
Western New Zealand
Group 1 Group 2 Group 3 Group 4
o/o
4.58 7.13
2.94 6.11
2.91 5.61
2.74 5.28
o/o
4.84 7.O7
2.62 6.O5
2.59 5.55
2.45 5.22
yo/OyO/Oy
M= 7OO 661 659
No. of Stat¡ons
: 23 15
Eastern New Zealand
2.99 7.O7 3.90 7.13
2.58 6.05 3.25 6.1 1
2.46 5.55 2.81 5.61
2.40 5.22 2.49 5.28
700
Group 1 Group 2 Group 3 Group 4
o/o o/o o/o o/o
3.12 6.68
2.87 5.65
2.74 5.15
2.54 4.42
4.31 6.68
4.06 5.66
3.65 5.1 6
3.55 4.83
4.62 6.69 3.85 6.83
3.82 5.66 3.75 5.80
2.99 5.17 3.53 5.31
2.A7 4.83 3.06 4.98
M= 444 448 450 517
No. of Stations
= 14
19
(bl Classification and averages of the maximum O/O and y values.
t5
t3
Table 3.5 Summary of flow stations in the Nelson a¡ea.
Site No. Flow Station Catchment No. Annual
Area (km'zl (and historical)
flood peaks
56901 Riwaka River at Moss Bush
57002 Motueka River at Baton Br.
57006 Wangapeka River at Swing Br.
57OOB Motueka River at Gorge
57009 Motueka River at Woodstock
571O1 Moutere River at Old House Rd.
571OG Stanleybrook River at Barkers
48
'1647
373
163
1750
60.7
81
10
18
I
9
8
10
7
y lnterval
Wefem NZ:
7.O - 7.5
6.5 - 7.0
6.0 - 6.5
5.5 - 6.0
5.0 - 5.5
Eaetem NZ:
6,5 - 7.O
6,0 - 6.5
5.5 - 6.0
5.0 - 5.5
4.5 - 5.O
No. of values Average O/O Average y
4
4
4
4
4
4
4
4
4.O8
2.85
2.69
2.52
3.98
3.63
3.23
3.O2
7.10
6.08
5.58
5.25
6.72
569
520
487
42
Water & soil technical publication no. 20 (1982)