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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(c) Storm sequences in tropical regions

The nature of Large to Extreme .rainfall sequences in
the tropical region is not clearly understood. The issue of
storm sequences will be addressed in the foreshadowed
review of the GTSM. In the meantime, advice on Extreme
rainfalls for very long durations in the tropical regions Df
Australia should be sought from the Bureau of Meteorology.

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3.9 Temporal Patterns

3.9.1 General

(a) Background

Temporal patterns are required to distribute the design
rainfall depths over the duration of the rainfall burst They
are usually expressed as fractions of the total burst rainfall
in each time interval and allow the derivation of design
burst hyetographs. The adopted pattern can have a major
effect on the computed flood. Wood and Alvarez (1982),
Brown (1982) and Nathan (1992) give examples of
differences of over 50% in flood peaks for spillway design
floods resulting from different assumed temporal patterns.
As discussed in Section 2.2, for rainfall-based estimates
of floods it is necessary to adopt an AEP-neutral approach,
where the objective is to derive a flood with an AEP
equivalent to its causative design rainfall. The most
appropriate solution to the problem is to adopt an explicit
joint probability approach, in which the temporal pattem is
treated as a random variable (e.g. Bloschl and Sivapalan,
1996; Weinmann et aI., 1998). Alternative approaches
which aim to incorporate the variability of temporal patterns
are discussed in Weinmann et aL (1998). Appropriate joint
probability procedures are still the subject of ongoing
research, and practitioners are encouraged to adopt
suitable procedures once they become available.

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(b) Derivation of typical patterns

In the absence of more sophisticated procedures, it has
been argued (e.g. French, 1986) that in order to avoid the
introduction of bias it is necessary to adopt a sin91e design
temporal pattern of average variability. This is strictly
correct only if the rainfall was transformed linearly into

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runoff, a characteristic that is (unfortunately) not observed
in practice. Given that the degree of non-linearity for Large
to Extreme events is largely unknown, tt is usually
considered sufficient to select a single temporal pattern
characteristic from the central range of appropriate
observations.
The patterns given in Book II Section 2 for rainfall
frequency estimates of relatively high probability were
derived using the average variability method developed by
Pilgrim et aL (1969), a technique developed specifically to
yield "typical" pattems suitable for design purposes. There
is evidence to suggest (e.g. Rowbottom et aI., 1986;
Nathan, 1992) that the temporal patterns of more extreme
events are more unifDrm, and accordingly the GSAM PMP
temporal patterns developed by Nathan (1992) were
smoothed to ensure a gradual transition between storms of
different areas and durations, as well as moderate
smoothing from one time increment to the next. The
temporal patterns provided in Book II Section 2 were
derived using point rainfalls though in practice they are
applied to areal design rainfalls. Effects of spatial averaging
are also likely to make areal rainfall patterns more uniform.

(c) Pre-burst temporal patterns

Information on design rainfall depths for a given AEP
relates to the depth of rainfall that can be expected to occur
within design bursts of specified duration; it does not
represent complete storms. One of the main difficulties
associated with using design bursts rather than complete
stDrms is that it is difficult to know what desi9n losses to
adopt, particularly for the derivation of Large to Extreme
floods (see Section 4.2).

Traditionally, temporal patterns were derived for use
with design bursts of rainfalls, not complete storms. In order
to avoid the problems associated wtth design loss
selection, guidance is now provided on. the derivation of
temporal patterns for rainfalls antecedent to design bursts.
Thus, temporal pattern information can be used to derive
hyetographs of complete storms that incorporate design
bursts of specified duration and AEP. At present pre-burst
rainfall temporal patterns are only available from the
Bureau of Meteorology for southeastern Australia (guidance
on the use of these pre-burst temporal patterns is provided
in Section 4.2.3c). Pre-burst patterns are alsD planned to

Table 8 Selection of design burst temporal patterns for different regions, durations and AEPs

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GSDM patterns

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Book II, Sect. 2
patterns

Both unsmoothed
24hr GSAM aRd
longest duration
GSDM patterns

Unsmoothed
GSAM patterns

GTSM patterns

Both smoothed
24 hr GSAM and
longest duration
GSDM pattems

Smoothed GSAM
patterns