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
1

I
I
I
I
I
I
I
I
I
I
I

ORAFT 0

defJiglt rainfalla (or pa/eaflQod Informatiollt bill no
detailed PMP estimates are available:
The procedures applied in these situations are
similar to (ii) but the use of additional information
(from regional pooling of rainfall data fDr rainfall-based
methods or from paleohydrological flood frequency
analysis for flood-based estimates) allows a
considerable reductiDn of uncertainty in the range of
Rare floods. Where possible, these procedures
should be applied in preference to the simple
extrapolation described in (iI), above. Guidance on the
estimation of Large to Rare design rainfalls is
provided in Sections 3.2 and 3.3, and procedures
relevant to the conversion of these design rainfalls to
design floods are given in Sections 4.1 to 4.5. Direct
estimates of Rare floods may be obtained using
regional flood frequency analysis (Book IV, Section 11)
and paleohydrological analyses (Section 4.9).

(iv) Design of floodplain management or flood protection
schemes based on risk management principles:
While risk-based design requires the assessment
of the contribution of Rare to Extreme floods to the
total expected flood damage figure, the low probability
of floods in this range means that the contribution
from this range of floods to the total expected flood
damage is relatively low in most situations. A lower
degree of reliability of flood estimates in the Rare to
Extreme range is therefore acceptable for these
applications.
The further the AEP range of interest extends
beyond the range of interpolation (Large floods), the

CIl
Cl
l- I.
C'Il
(/) -I
W
e::: Z(I)
w e>~
I- 000
Z UJ::>
01--
u.. CIl wE::
0 >-~ Co
I. ~ ::l
W C'Il <(_0
e::: z--'~
Cl -jjje>
Z :2_~
-(I)
i2 illiij
~11.
0 a.
0 CIl
0 E
-I
u.. ~
><
W
lowest
required

Book VI ~ Estimation of large to Extreme Floods

mOTlf important it is for the practitioner t. consider in
detail the guidelines in Section 4 Dn extrapolation of
loss and hydrograph model characteristics. For all
applications where the range of interest extends to
extreme floods, and where large uncertainty in flood
estimates would impact significantly on design
decisions, detailed flood studies ere justitied, based
on the current-best-practice guidelines provided in this
Book.

3 ESTIMA TION OF DESIGN RAINFALLS

3.1 General

(a) Required design information

In general, design rainfalls are required to derive all
Large to Extreme flood estimates (possible exceptions to
this are discussed in Section 4.1). Information is required
on the depth of rainfall, as well as its distribution in both
space and time. In general design rainfalls are required for
a range of durations. Design floods are calculated for each
duration, including routing through any reservoirs or other
storages, to determine the critical rainfall durations that
produce the maximum flood discharge, flood volume or
possibly duration of flooding. Short duratiDn design rainfalls
may be required even on large catchments to check that
their occurrence Dn only part of the catchment area does
not produce a critical flood, and to check that the
magnitudes of the calculated floods vary in a regular
manner as the duration of the rainfall increases.

(I)
~
gj(l)~
>1-0>
>Z8
~UJ<:
0:2",
(I)::l
-'(1) <T
<(UJ '"
:2(1)~
l1.(I)8
0<(
z>-~
e>~~
Ci5::>J!!
UJO'<::
o UJ .2'
--'0'<::
<(<(~
Z
u:

highest
achievable

DESIRABLE DEGREE OF RELIABILITY
.

Increasing data requirement
Increasing amount of effort
Increasing level of expertise

Figure 2 Qualitative indication of the range of flood magnitudes and the relative degree of reliability required for
different applications (see text in Section 2.3 for further explanation).