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

mates of Cp typically range between 2OVo and 44go for different
regions. They are generally somewhat greater than
Cp, and are generally not greatly influenced by the values
assumed for p.
Values estimated for Nu given in the right-hand column
of Table 4.12 range from one year for the West Coast of
both islands to about seven years for the East Coast of the
South Island. Such results are in accord with intuition.
Where the Cy is low_(as on the West Coast) a reasonably accurate
estimate of Qo5, may be obtained from a relatively
short period of rec
on
is of limited valu
is
greater, a longer
to
estimate Qo6. with
on
equation estimate, and the regression equations may be of
greater utility.
tühere only a short period of record is available for a site
for which a design flood estimate is required, a decision
var (Q)
weights for combin
in a best estimate. If
its variance can be e
_l
var (Qo6.)
I
var (Q.,x)
4t0
4.11 Summary
_ The country was divided into nine regions lbr estimating
Q using regression analysis. The physical justification lor
these regions was discussed. Apart from the south of the
South lsland, the study used a good distribution ol catchments,
and the range of values covered for Q, area, and
other parameters was very large.
The results demonstated that generally catchment area
and the rainfall parameters considered are sufficient to predict
large differences in flood magnitudes within the nine
regions delineated and that the other physical characteristics
used for the catchments do not improve that prediction.
Apart from the Southern Alps of the South lsland,
the mean annual rainfall can be estimated with reasonable
confidence from isohyetal maps. Rainfall intensities were
estimated from data available in Robertson's (1963) publication.
Updated intensity data are now available (Tomlinson
1980; Coulter and Hessell 1980) and, with more extensive
intensity information, better estimation equations are
anticipated.
Preliminary results of the study enabled identification ol
catchments which did not fit into regional trends. Where
reasons for anomalies could be identified, the catchments
were excluded from the final analyses since their inclusion
could have led to biased results. About 790 of catchments
were in this category. Designers should be aware of factors
likely to modify flood peaks and if in doubt seek specialisr
advice.
16
Water & soil technical publication no. 20 (1982)