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

50 20 10
Annual Exceedance Probabili %

Figure 12 Comparison of runoff-routing and flood frequency results.

DRAH U

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1000

+

Observed annual maxima
Fitted flood frequency curve
Calibration loss params. used
Hill et al. loss params. used
Fitted loss params. used

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.,
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<1l
0.
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100

+

+

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10

95

90

80

indicated that design losses do not generally vary with AEP
for this range of events, and thus there is little justification
for adopting different loss rates for the 1 in 50 AEP and 1 in
100 AEP events. The fixed loss values obtained by fitting to
these estimates are thus considered a reasonable
compromise.

(b) Based on regional prediction equations
and reconciliation with flood frequency
estimates

In order to illustrate application of regional loss
equations, it may be assumed that the characteristics of the
catchment were investigated and found to fall within the
range of conditions used by Hill et al. (1997) to derive
regional estimates of design storm and burst losses. The
loss equations derived by Hill et al. (1997) for initial storm
loss (IL., mm), initial burst ioss (14, mm) and continuing
loss (CL, mrn/hr) are as follows:

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IL. = -25.8 BFI + 33.8

(r" = 0.55, s.e. = 5.1 mm)

CL = 7.97 BFI + 0.00659 PET - 6.00
(r" = 0.60, s.e. = 1.5 mm/hr)

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". - "-l' '.'~.f.]

where BFI is the baseftow index (the proportion of total
streamflow that is estimated to be from groundwater
sources), PET - is the mean annual potential
evapotranspiration (mm), 0 denotes the storm duration
(hours) and MAR the mean annual rainfall (mm).

(r" = 0.43, s.e. = 18%)

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tsOOK VI - t::stlmatlon ot Large to t:xtreme "'loads

++++

(a) Burst loss results.
- - - - -, - - -'- - ~
1 . t.
1. t
t t
t + t
t . t
t . t
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+ I 1
I_________~t
/ \
/ \
I \
I \
b Storm loss results \

800
.'
.
+
.
.
200
5 2 1 0.5

5

2

1 0.5 0.2

Analysis of streamflow data yielded a BFI value of 0.33,
and the annual potential evapotranspiration was estimated
from Grayson et al. (1996) to be 1100 mm. Inspection of
Bureau of Meteorology records indicates that the mean
annual rainfall is around 650 mm. Thus, the different loss
values may be estimated using the above equations are as
follows:

IL$ ::;:

=

(-25.8 x 0.33) + 33.8
25.3 mm

ILb = 23.5 [1- 1+14:.l[]
650
= 13 mm for a 24 hour storm.

CL = 7.97xO.33 + 0.00659'1100 - 6.00

= 3.9 mm/hr

The magnitudes of the design floods obtained using the
Hill et al. losses are shown in Figure 12 as filled, square
symbols.

It is seen that the Hill et al. estimates of the loss values
fall slightly below the results obtained using flood frequency
analysis. These loss values are considerably closer to the
values found by trial and error (discussed above) than are
the values obtained from calibration to historic events.

6.3.2 Estimates Based on Design Rainfall Bursts
With a Limit of Extrapolation of 1 in 100
AEP

With this example, the design burst loss values derived
in Section 6.3.1 are used in conjunction with the design
rainfalls obtained without the use of CRC-FORGE