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3.5.5 Extension method
The NERC (1975) extension method proved very useful
in developing the Bay of Plenty regional curve and the generalised
curves. However, it was not considered necessary
to use the method for the development of every regional
curve. For instance, in a test on the South Island tr)Vest
Coast regional data, the extension method produced a
curve almost identical with that obtained by the usual
curve-fitting procedure (section 3.3.2). There was also uncertainty
in the method concerning the statistical dependence
between the original and the new set of average values.
In a sense it appeared that the same information on the
large Q/Q values was being used twice. This would weight
the curve-fitting process in favour of the historical flood
peaks, the estimat€s of which are generally less reliable than
those for the annual flood peaks. In addition, there is considerable
difficulty in grouping the stations of a region such
that neighbouring catchments are not represented in a
group. While this condition can normally be satisfied, there
are necessarily some catchments represented which are in
close proximity to one another, which raises doubts about
treating the flood peak data of a group as independent sample
items.
3.5.6 Catchment s¡ze
Catchments less than 20 km'? were omitted from the
study, except in the Northland-Auckland area, on the prior
belief that they would have steeper flood frequency curves
than the larger catchments (section 3.2.1). This was found
to be true at times, especially for the very small catchments
of the order of I km' in area. However, the initial curve for
the Northern North Island region (Figure 3.8) refutes the
general argument against the omission of small catchments.
The curve was based on the flood peak däta for catchments
ranging in area from 2.48 to 1606 km':(see Table 3.2), yet it
is not steep nor does the corresponding regional probability
plot show greater scatter than that for other western regions.
The curve is a typical western one and flatter than
any eastern regional curve.
While it was quite evident that the flood peak data for
some small catchments could not be tolerated in the derivation
of a regional curve, it does appear from the Northern
North Island example that small catchments could possibly
have been considered in the derivation of some of the
curves.
3.6 Summary
Eight flood frequency regions have been defined for New
Zealand, and an average Q/Q vs T curve derived for each.
It is suggested that these regional curves can be used to estimate
Qr/Q for rural catchments within the region concerned
for return periods up to 200 years. An exception is
the Otago-Southland curve; it is only tentative and should
not be extrapolated beyond the 100-year return period. For
return periods exceeding the recommended upper limit of
the regional curve, one of the two generalised curves developed
for the east and the west of New Zealand can aid
the estimation of Q1/Q. With all the curves it is important
that they are not applied to catchments whose areas are too
far outside the range of areas used in the derivation of the
curve concerned. An indication of the reliability of a Q1/Q
estimate taken from a curve is provided by standard error
equations, which give values somewhat less than a similar
equation determined by NERC (1975) for use with the British
Isles curves.
The most notable characteristic of the curves is that those
for the east have steeper slopes than those for the west, reflecting
the greater variability in the annual flood peak data
for the eastern catchments. The same characteristic is evident
in the British Isles regional curves. Also of note is that
the two New Zealand generalised curves resemble their
Great Britain counterparts (Stevens and Lynn 1978).
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Water & soil technical publication no. 20 (1982)