FLOOD01798

• County: Statewide
• Title: Australian Rainfall and Runoff 1998, Revision of Book VI - Estimation of Large to Extreme Floods
• Date: 11/28/1998
• Prepared By: Rory Nathan, Sinclair Knight Merz
• Floodplain - Doc Type: Educational/Technical/Reference Information

DRAFT D

Book VI . Estimation of Large to Extreme Floods

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Table 13 CRC-FORGE point rainfall estimates (AEP~1 in 2000 AEP)
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1.130
1.269
1.470
1.635
1.814

1.131 1.135
1.271 1.282
1.474 1.496
1.642 1.674
1.825 1.867
182 211
206 239
231 271
268 316
299 353
332 394

136
154
173
200
222
247

of the information presented in Table 7 are presented in the
10~ and 11th rows. Finally, the design rainfalls
corresponding to AEPs of 1 in 2000 and 1 in 50000 are
presented in the last two rows of Table 12. A plot of the full
frequency curves for durations of 6, 24, and 72 hours is
shown in Figure 11 (curves labelled "Based on shape
factors").

AEP design point rainfalls are the same as derived for the
preceding example, as shown in Table 12.

(ii) Extrapo/afion of regional information to of her
durations

The results presented in Table 13 only relate to the
standard durations for which CRC-FORGE estimates are
directly available. Estimates for other durations can be
obtained using the procedures discussed in Section 3.6.3
(and as illustrated in Figure 6).

Estimates of the 18 and 36 hour durations may be
derived by fitting a line of best fit through the log-
transformed values of storm duration (the independent
variable) and rainfall depth (the dependent variable). The
log-transformed values of the 24, 48, and 72 hour depths
are shown in the 2" to 4th columns of Table 14, and the
slope and intercept coefficients of the corresponding lines
of best fit are shown in the 5th and 6th columns (these are
easily derived using standard functions available in
spreadsheet software). The estimates derived using the
regression equations are shown in the last two columns of
the table (these have been transformed back into the
arithmetic domain). Note that for consistency with other
examples these calculations are presented for rainfall
depths, though identical answers would be obtained if
applied to rainfall intensities.

Estimates of the shorter duration events are derived
using the ratios between the 1 in Y AEP 24 hour event to
the 1 in 100 AEP 24 hour event. These ratios, and the 1 in
100 AEP rainfalls for the short duration events (from Table
12), are shown in Table 15. The estimated rainfall depths
for AEPs rarer than 1 in 100 are shown in the last four rows

6.2,2 Upper Limit of Extrapolation is
1 in 2000 AEP

In this example, it is assumed that regional (CRC-
FORGE) rainfall information is available, and thus the
rainfall frequency curve is obtained by the following three
design procedures:
(i) derivation of the design rainfalls up to the credible
limit of 1 in 2000 AEP (Section 3.3.2)
(ii) extrapolation of the regional design information to the
required shorter durations (Section 3.6.3)
(iii) extrapolation of the 1 in 2000 AEP rainfalls to the PMP
(Section 3.6.2)
The following sub-sections illustrate each of the above
steps.

(i) Derivation of design rainfalls up to 1 in 2000 AEP

The CRC-FORGE procedure for this region uses the 1
in 50 AEP design rainfall as the index variable, and design
estimates are available directly for durations of 24, 48, and
72 hours. Growth factors for the standard durations were
derived for a range of AEPs from a digitised map of the
catchment. The values of the index variable and the derived
growth factons are shown in Table 13. Note that the 1 in 50

Table 14 Long duration point rainfall estimates (AEP<;:1 in 2000 AEP),

24 48 72 18 36
1.380 1.681 1.857 1.255 1.556
2.133 2.260 2.324 0.403 1.578 121 161
2.186 2.313 2.379 0.407 1.626 137 182
2.236 2.364 2.432 0.411 1.670 153 204
2.300 2.426 2.499 0.418 1.725 177 237
2.347 2.475 2.548 0.423 1.764 197 264
2.392 2.521 2.595 0.427 1.802 218 293
------ -- --------------------- ---------- ------ - -- ------ - --------

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