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

0.760 0.796 0.632 0.843 0.866 0.896 0.912 0.928
0.760 0.796 0.832 0.841 0.866 0.893 0.908 0.924
0.760 0.796 0.832 0.839 0.863 0.890 0.905 0.920
0.760 0.796 0.832 0.836 0.860 0.886 0.900 0.915
0.760 0.796 0.832 0.833 0.857 0.883 0.897 0.911
0.760 0.796 0.832 0.831 0.855 0.880 0.S94 0.907
50 65 83 101 118 145 166 196
56 71 91 115 133 162 187 221
62 80 102 129 149 182 209 249
72 92 118 148 172 210 242 289
80 103 131 164 191 233 268 322
89 114 146 181 211 258 297 357

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Table 15 Short duration point rainfall estimates (AEP;'l
in 2000 AEP)

154 1.000 73 89 109
173 1.123 82 100 122
200 1.301 95 116 142
222 1.447 106 129 158
247 1.605 117 143 175

ofTable 15.
(iii) Extrapolation of 1 in 2000 AEP rainfalls to the PMP

The above examples provide design point rainfalls for all
durations more frequent than 1 in 2000 AEP. These depths
can be extrapolated to the AEP of the PMP using the
procedure presented in Section 3.6.2, but first the point
rainfalls need to be converted to areal values by application
of areal reduction factors. Table 16 lists the areal reduction
factors for the required storm durations and AEPs using the
equations derived by Siriwardena and Weinmann (as
described in Section 6.2.1). The lower half of Table 16
shows the areal rainfalls resulting from the product of the
point rainfall estimates (Tables 12 and 13) and the areal
reduction factors.
The areal rainfall estimates listed in Table 16 are
extrapolated to AEPs between the 1 in 2000 using the
procedure developed by Siriwardena and Weinmann
(Section 3.6.~. Table 17 lists the input design rainfalls (in
the 2"" and 3 rows), where, with reference to Equations 1
to 6, the values used in the procedure are as follows:

Lower end point of linear segment:
P" 1 in 1 000 AEP areal rainfall depth

y, 1000

Starting point of interpolation:

Table 16 Areal rainfall estimates

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-~-"~. -...,....-..-.. -. --'::1- -- -~"-"'-' .----

Py, 1 in 2000 AEP areal rainfall depth
Y, 2000
Upper end point of interpolation:
PPMP - PMP
YPMP - 2.28x10'
The values of the variables Ry, and gy obtained using
Equations 2 to Equations 6 are shown for an AEP of 1 in
10000.
The full rainfall frequency curves for durations of 6, 24,
and 72 hours are shown in Figure 11 (labelled as "Based
on CRC-FORGE").

6.2,3 Estimation of Preliminary PMP Estimates

In order to illustrate application of the regional
prediction equations, preliminary estimates of the PMP are
derived using the equations relevant to the GSAM region
(Table 10). The 1 in 50 AEP 72 hour event ("in) point
rainfall intensity is 2.93 mm/hr, and the catchment area is
439 km'. Input of these vaiues into the equations provided
in Table 10 yield the PMP estimates listed in the 2" row of
Table 18. It is seen that in this case the preliminary
estimates are up to around 10% higher lhan the actual
design estimates provided by the Bureau of Meteorology
(note that the regression estimates are rounded to the
nearest 10 mm since the data used to derive the equations
were provided to that level of accuracy).

6.3 Derivation of Inflow Flood Frequency
Curve

Inflow flood frequency curves are derived for four design
situations. The first two design situations considered
correspond to use of the design burst rainfall information
derived in Sections 6.2.1 and 6.2.2, in conjunction with
design burst loss rates. Next, it is assumed that pre-burst
temporal patterns are available, and design storm losses
are used with the more accurate rainfall data derived in
Section 6.2.2. Lastly, a preliminary estimate of the inflow
frequency curve is derived using only regional data. A
runoff-routing model is used to derive the results for the first
three design situations, and the first section below
describes seiection of the design parameters.