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5 Application
5.1 lntroduction
This chapter collates the applicable results and findings
from the preceding two chapters and formulates them into
what is subsequently reierred to as the Regional Flood Estimation
(RFE) method. Rules for the applicability of the
RFE method are given, a design strategy for estimating the
T-year flood peak is suggested and a number of examples
are given which demonstrate the use of the method.
The RFE method is intended as a procedure to be used
for estimating design flood magnitude in situations where
insufficient flood records are available for conventional
fiequency analysis. It is one of several design flood estimation
methods in such situations and other methods should
be used and compared with it. It has been derived from
tlood records by:
(¡) defining regional flood frequency curves of Q1/Q vs
T, where Q1 is a design flood with return period T and
Q is the mean annual flood; and
(ii) developing a set of equations for estimating Q based
on catchment area and measures of rainfall.
A comparison with Technical Memorandum No' 6l
(TM 6l) is reported in Appendix E.
5.2 Applicability
The applicability of the RFE method is necessarily constrained
by the restrictions that were applied to the data
used in deriving the method. The following constraints
therefore apply.
The method should only be used for rural catchments.
The method should not be applied to catchments in
which snowmelt, glaciers, springs, lake storage or
ponding significantly affect the flood peak characteristics.
The ranges of catchment areas for which the regional
flood frequency curves and the regional mean annual
flood equations were derived are listed in Table 5.1.
These are a guide for the size of catchment to which
the method should be applied.
Because of the subjective and rather broad definition of
regional boundaries, it is suggested that, for catchments
near boundaries, floöd frequency curves and annual flood
equations for the regions either side of the lines.should be
used in estimating Q1/Q and Q respectively. As in a design
situation where different methods yield different estimates,
the different Q1/Q and Q estimates then need to be 'compared',
i.e., the merits of each should be assessed and the
choice of an estimate should be made after a rationalisation
of the relevant facts. Alternatively, a belief probability can
be attached to each estimate and their expectation calculated,
which is akin to taking a weighted average of the estimates.
5.3 Design Strategy
5.3.1 General
The strategy for the use of the RFE method in design is
dependent on two main factors: N, the length in years of
the flood record if a record is available, and T, the design
return period. The influence of these factors on the two
parts of the RFE method (i.e. the regional mean annual
flood equations and the regional flood frequency curves) is
explained in the two following sections and summarised in
Figure 5.1 .
5.3.2 Estimat¡on of O
(¡) N(Nu
Where there is a flood record and its length N is less than
Nu, which is the
is equivalent to the
prãcision of the r
on (see Table 4. l2)'
the mean annual
imated from the regional
equation and the available flood record. In applying
the equation it is particularly important to estimate values
for the rainfall variables 1224 and MARAIN in a similar
manner and from the same data base as used in the equation's
derivation. Specific points to note in estimating
values for 1224 and MARAIN are outlined below.
1224 Estimates ol the 1224 rainfall intensity statistic used
in deriving the equations for Q were obtained by taking the
arithmetic mean of the 2-year 24-hour data listed by Robertson
(1963, Table 9) for rainfall stations located within,
or near to, the catchment concerned. Estirrates can be
made from the tabular results (Appendix D) obtained by
Tomlinson (1980) in a recent revision of the frequency an-
Table 5.1 Ranges of catchment areas used to derive regional flood frequency curves and mean annual
flood equations.
Flood frequency
reglon
(Fis. 3.6, 3.7)
Combined N.l.
West Coast
Central Bay of PlentY
N.l. East Coast
Central Hawke's Bay
S.l. West Coast
S.l. East Coast
South Canterbury
Otagoi Southland
Catchment area
(km2)
(Table 3.2)
fntn max
2.5 6643
28.2 2893
171 2370
24.3 2424
48 6350
74.6 3430
22.4 899)
lo9
)
18321
Mean annual flood
feglon
(Fig.4 7,4.1O)
Northland/Coromandel/
East Cape
West Coast
Manawatu/WairaraPa/
Wellington
Pumice
Northland/Coromandel/
East Cape
East Coast
West Coast
lnland Marlborough/
Canterbury
East Coast
(Mackenzie, lnland
(Otago, Southland
(East Coast
Water & soil technical publication no. 20 (1982)
Catchment area
(km')
(Tables 4.1, 4.2)
min max
o4 640
3.1 1075
9.4 734
2.6 534
o.4 640
o5
997
4.O 998
o.2 1070
2.2 464
36.8 1088
2.2 464