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

I
I
I
I
I
I
I
I

rainfall represents around 27% of the burst total, distributed
over a period of time approximately equivalent to that of the
burst. A~hough the total rainfall depth of a design storm is
greater than the depth of the corresponding burst, the
associated initial and continuing losses are also higher.
Figure 14 also shows the proportion of the design rainfalls
that are specified as losses for both the design burst and
design stonn situations, as well as the resulting design
hydrographs.
It is seen that adoption of low loss rates for use with the
PMP design burst yields a design flood that is slightly
higher than the corresponding design storm hydrograph,
though the corresponding volume of the hydrograph is
approximately 5% less. The full frequency curve for this
design approach is shown in Figure 13 as filled triangles.

6.3.5 Preliminary Estimates Based on Regional
Information

A preliminary frequency curve may be derived solely
from regional information. For this example, the 1 in 50
AEP and 100 AEP flood estimates are derived using the
probabilistic rational method (Book IV Section 1) and are
found to be 370 and 485 m'ts, respectively. The PMP
design flood is conservatively approximated to be the PMF,
which in turn is derived using the prediction equation
derived by Nathan et al. (1994):

QPMP QPMF

129.1 A""

5500 m'ts

Estimates of the design floods between the 1 in 100
AEP and PMF events are obtained using the shape factors
obtained from Table 7, as described in Section 6.2.1. The

=

I
I

(a) 1 in 100 AEP event

I
I

40

c=J Total rainfall
_ Rainfallloss

Design burst

20

I
I
I
I

o
40

20

Design storm

o

400

Design burst
Design storm

I

200

I
I

. 0

2lJ

60

so

40

o

___n.. ~",....._.._.. _. __.~_._ _n........_ _ .____

Table 20 Regional estimates of intermediate values.

~l:~;t~$~~!l~,~6~I~r7'
PMF;(m
I~g'(~;~;
If@l(2U
I~~(

11.122
0.361
0.766

1;'i~:.
..;irF

1200
3100

relevant calculations and results are summarised in Table
20, and the resulting frequency curve is shown in Figure 13
(as filled circle symbols).

6.4 Joint Probability Analysis of Initial
Reservoir Level

The example provided here illustrates application of the
Laurenson (1974) method to derive a frequency curve of
outflows from a reservoir under conditions of variable
drawdown. It is assumed that the following information has
been derived for the reservoir:
(i) inflow frequency curve;
(ii) the relationship between inflows and outflows, for
different initial reservoir levels;
(iii) the frequency distribution of storage volume.

(b) PMP design flood event

200

c=J Total rainfall
_ Rainfallloss

Design burst

100

o
200

Design storm

100

o
4000

~

2000

Design burst
Design storm

o

o

20

40

60

so

Figure 14 Pre-burst and burst design temporal patterns and hydrographs for the 1 in 100 AEP and PMP design
flood events.