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

Selection of design spatial patterns for different regions, durations and AEPs
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Table 9

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Descriptive'
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GSDM spatial
patterns

Section 1, and they incorporate the combined effect of
variations in elevation, slope, aspect and geographical
location. These patterns should not be rotated or
translated.

. Rare to Extreme long duration design rainfalls in
tropical regions. The spatial patterns provided with
GTSM estimates are elliptical, and they should be
applied to all Rare to Extreme events. The spatial
pattern should be positioned to maximise the rainfall
depth within the catchment

. Rare to Extreme long duration design rainfa/ls in the
transition zone. In the transition zone between the
GSAM and GTSM regions, both sets of spatial
patterns should be used (in conjunction with the
corresponding temporal patterns) and the highest
resulting flood should be adopted.

. Short duration PMP rainfalls. It is important to note
that a supplement to Bulletin 53 was issued in
December 1996. The supplement contains a
substantially revised method of spatially distributing
rainfall, and it is important that practitioners no longer
use the method described in the original Bulletin 53.

. Long duration PMP rainfa/ls. The GTSM provides
spatial patterns in which the rainfall depth varies in an
elliptical fashion, though the patterns do not
incorporate the influence of topography as does the
GSAM approach.

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3.11 Preliminary Estimates of PMP

Preliminary estima~ of PMP mav be required for
feasibility studies. For catchments less than 1000 km',
values can be obtained from the final design data in Bulletin
53 of the Bureau of Meteorology (1994) for durations up tD
3 hours or 6 hours depending on the relevant zone. For
longer durations, "'gional prediction equations have been
derived based on the results of detailed studies unaertaken
by the Bureau of Meteorology for catchments located in
South Australia, Victoria, New South Wales, and

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Uniformly distributed
(unless clear evidence for spatial trend)

Both GSAM and
GSDM patterns

GSAM spatial
patterns

G1SM spatial
patterns

Queensland (Sinclair Knight Merz, 1998). These equations
may be used to obtain preliminary estimates of the PMP in
those States, but for final design purposes it is necessary to
obtain more accurate estimates from the Bureau of
Meteorology. Regional prediction equations for preliminary
PMP estimates are shown in Tables 10 and 11 for both the
GSAM and GTSM regions.

4 ESTlMA TION OF DESIGN FLOODS

4.1 General Approach

The recommended procedures required to derive final
estimates of design floods depend on the probability of the
flood to be estimated and the design objective. The
following section provides guidance on final design
procedures, while Section 4.7 describes quicker, less
accurate procedures for preliminary assessment purposes.

(a) Selection of basic procedure

The available procedures can be divided into two main
groups: those based on fitting a frequency curve to flood
maxima, and those based on design rainfalls. FloDd
frequency methods (Book IV, Section 2) are used to
provide estimates of peak discharge, but perhaps their
most valuable role in the context of this book is to provide
information that can be used to validate, or even calibrate,
rainfall-based procedures. The limit of credible
extrapolation for flood frequency analysis based on regional
gauged da\ll is perhaps 1 in 500 AEP (Table 1), though
paleoflood analysis (Section 4.9) can be used to
considerably extend this limit The credible limit of flood
frequency analysis that can be typically obtained using at-
site data is perhaps only 1 in 50 AEP (Table 1).

Rainfall-based procedures use loss models and
hydrograph models to Iransform design ralnfaillnputs into
design flood estimates. Final design estimates of Rare to
Extreme floods, beyond the credible limit of extrapolation