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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Il is important to note here that the event whose AEP is
being analysed needs to be clearly defined in terms of a
magnitude (e.g. Q ;, 100 m'/s) father .than in terms of a
concept (e.g. "PMP") that does not directly relate to a
magnitude. This means that the above equation cannot be
directly applied to PMPs for different seasons but only to
rainfalls or floods of a specified magnitude occurring in
different seasons.

4.7 Preliminary Estimates of Design Floods

There are some design situations where it is desirable
to derive approximate design flood estimates by applying a
"quick" method. Examples of situations where preliminary
estimates are desirable include:

. flood estimates for preliminary assessment of spillway
adequacy of existing dams;

. determination of priorities for the undertaking of detailed
studies;

. estimation of concurrent floods of minor importance for
the analysis of incremental consequences arising from
dam failure;

. preliminary evaluation of different dam sites for planning
studies; and,

. determination of hydrologic loads in a portfoliO risk
analysis of a group of storages.
The overall requirement for these types of analyses is
that estimates can be derived quickly, and that given the
large uncertainty the estimates should be biased towards
conservatively high values.
Preliminary estimates should not be used for final
design purposes, nor should the resulls be relied upon for
making decisions about long term levels of acceptable risk.
Practitioners are encouraged to use any information and
methods that they consider appropriate, and the following
recommendations are provided for general guidance only.
Generally two types of preliminary design estimates are
required:

. Peak discharge: estimates of peak discharge are
directly suitable for the preliminary design of bridge
waterways and spillways for those storages where it can
be conservatively assumed that attenuation of the inflow
hydrograph can be ignored;

. Flood hydrograph: estimates of the hydrograph are
required where it is necessary to obtain an estimate of
flood volume as well as peak discharge, for example the
sizing of detention basins or the assessment of spillway
adequacy for storages which appreciably attenuate the
inflow hydrograph.

4.7.1 Preliminary Estimates of Peak Discharge

One possible approach to deriving a frequency curve of
peak discharges is to derive preliminary estimates of the 1
in 50 AEP, 1 in 100 AEP design events, and the PMP
Design Flood. These estimates can then be used to
construct a frequency curve based on the shape factors
provided in Table 7, using the procedure described for
rainfalls in Section 3.6.1.

Preliminary estimates of the 1 in 50 AEP and 1 in 100
AEP events can be derived from a variety of sources,
including any of the methods discussed in Book IV, Section
1. The probabilistic rational method is a quick way of
determining preli"linary design flood estimates from design
rail\falle, \hoijgh it is I\~ssary to ensure \hat the method' is
only applied to catchments up to the maximum size to
which the particular relations apply. Estimates of floods

.............. --.....-..-.. -. --';:1....- -....-...-. .----

based solely on regional information are also often
available (e.g. Grayson et al. 1996).

Preliminary estimates of the PMP Design Flood can be
conservatively approximated by estimates of the PMF.
Regional prediction equations for the PMF are availabie for
some regions (Nathan et aI., 1994), though envelope
curves for world floods may also provide useful information
(Rodier and Roche, 1984).

4,7.2 Preliminary Estimates of Design
Hydrographs

Estimates of the complete design hydrograph can also
be obtained in a variety of ways. Such estimates generally
require more time and effort in application than estimates of
peak discharge, particularly as the estimated inflow
hydrographs often need to be routed through a structure to
assess the degree of attenuation.

Estimates of the volume of the hydrographs can easily
be determined from estimates of design rainfalls and
losses. The volume of the hydrograph can simply be
determined as the average depth of rainfall excess over the
catchment multiplied by the catchment are~. Alternatively,
prediction equations for hydrograph volume are available
for some regions (e.g. Nathan et aI., 1994).

Appropriate hydrograph shapes can be derived by
scaling hydrographs obtained from more detailed studies
on similar catchments, though prediction equations are
available for synthetic hydrographs (e.g. Book V Section 2;
Nathan et aI., 1994). Alternatively, a more time-consuming
approach would be to derive design rainfalls partially based
on the preliminary PMP estimates (Section 3.11) and use a
flood event model with parameters obtained from regional
prediction equations (Sections 4.3.5 and 4.3.6).

Approximate routing techniques are availabie for
assessing the ability of the structure to attenuate the peak
inflow. Some approximate routing techniques are available,
though some of the simpler numerical routing techniques
are easily computed using spreadsheet tools (e.g. Book V,
Section 1).

4.8 Estimation of the Probable Maximum
Flood

*

The PMF concept originates from a period when
structures were designed to withstand a loading of a given
standard, rather than a level of acceptable risk for a specific
situation. With the more recent shift towards risk-based
design the PMF has become increasingly less relevant.
Nevertheless it is recognised that in some situations it is
desirable to estimate the limiting magnitude of floods that
could reasonably occur (for example in certain situations
the PMF can be used to obviate the need for risk-based
estimates; ANCOLD, 1998), and accordingly it is a concept
that still has some relevance.

The 1987 edition of ARR deflned the PMF as the
limiting value of flood that could reasonably be expected to
occur. Superimposing risks of very low probabilities was not
considered reasonable, but it was considered prudent to
incorporate SOme degree of conservatism. While it is
possible to derive an upper limiting value of flood
magnitude, it is not possible to assign an AEP to this event.
Conservatively estimated (reasonably possible) values of
the faclors involved in the transformation of the PMP to the
PMF introduce a shift in probabilily but, because the phrase
"reasonably possible" is a qualitative description of
probability, the AEP 01 \he resulting flood defies
quantification. In practice, the magnitude of the PMF will be
greater than the magnitude of the flood derived from the

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