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

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DOO..... VI - t:511mallOn 01 Large to ~reme t-looas

Preface to Book Vf

The revision of Australian Rainfall and Runoff (ARR) is an ongoing process, and the revised format of ARR98 was
designed to facilitate the progressive update and revision of the guidelines_ The revision Df this book arose because the
following three conditions were satisfied:

(i) the need for revision was voiced by industry through the receipt of survey information, discussion at professional 'f;
forums, and the parallel revision of the AN COLD guidelines Dn the selection of acceptable flood capacity for dams
(ANCOLD,1998);

(iI) further advances were made in regional data analysis techniques related to the estimation of large and rare rainfall
and flood events, resulting in new procedures and improved design data;

(iii) the necessary in-kind and cash support required to undertake the revision were provided by industry bodies, and
cooperative arrangements between industry, research institutions, and the Federal government (see
Acknowledgments),

This revision is largely based on the philosophy and procedures presented in Chapter 13 of ARR87, but certain of the
procedures have been revised to reduce inconsistencies and there is an increased focus on the derivation of rare
design floods_ The salient changes introduced in this revision include:

- a shift of emphasis away from a standards-based approach to design to a risk-based one, in which the focus of
interest is on deriving design flood estimates of specified annual exceedance probability (AEP) rather than estimates ~
of flood magnitudes related to the Probable Maximum Flood (PM F);
. an allowance for incorporation Df new procedures for the estimation of large to rare events;
. the abandonment of the upper limit concept associated with operational PMP estimates; *
_ differences in estimation of the AEP of the Probable Maximum PrecipitatiDn (PMP); *
. the incorporation of PMP estimates obtained using the Generalised SDuth-east Australia Method
. a new interpolation procedure to estimate design rainfalls in the extreme event range; -j(
_ increased focus on the AEP-neutral requirements for transformation between rainfalls and floods;
. a new approach to the incorporation of design losses;
. clarification of differences between those procedures based on methods that can be tested, and those based on our
broadest understanding of the physical limits Df hydrometeorolDgical processes; and,
. more detailed guidance on the treatment of jDint probability issues, seasonal design information, and the treatment
of uncertainty.

In addition, this revision includes references to recent research outcomes that impact upon selection of design inputs
related to areal reduction factors, design losses for large events, regional model parameters, and rainfalls antecedent tD
rare and extreme rainfalls_

The final recommendations have been prepared and reviewed by many practitioners and academics experienced in
flood estimation practice_ While it is not possible to attain unanimous agreement on all aspects of the revisions, the final
recommendations are nevertheless based on a consensus view.

During preparation of the revision, a number of the procedures related to design practice were benchmarked by
agencies with extensive experience in application of the guidelines_ Overall, it can be stated that the main impact of the
revisions is on rare floods, and that in general estimates of rare floods made using the current recommendations will be
of lower magnitude than those obtained using the 1987 version of the guidelines_ It is worth noting that no changes have
been introduced, apart from the impact of generalised PMP estimates fDr south-eastern Australia, that would
systematically increase the magnitude of the estimated PMF.

These guidelines have been delayed somewhat by the need to prove that the salient procedures can be applied in
practice_ For example, design information is now available in some states that allow practitioners to derive rainfall
estimates with AEPs as low as 1 in 2000. While work is currently underway to provide similar estimates for other
regions, it is likely that the improved design information will not be available for all States for some years to come_
Guidance is provided for practitioners working in regions without the new design information, though it should be
recognised that where appropriate information does not exist the recommended procedures are biased towards
providing conservatively high estimates_ It is thus hoped that the positive approach in these guidelines towards the
adoption of innovative procedures and new design data will provide the impetus to continue investment in research that
will further reduce the uncertainty assDciated with estimates of large to extreme floods_

Dr Rory Nathan
'Convenor
ARR87 Chapter 13 Revision Committee