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

DRAFT 0

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Book VI - EstimatIon ot Large to I:xtreme t-IOOdS

Table 4 Summary of the sources of design rainfall infonnatlon.

Temporal pallern

Spatial pattern

Between 1 in 100 to
the credible limit
of extrapolation

Depth

Temporal pattern
Spatial pattern

Between the
credible limit of
extrapolation and
the PMP

Depth

TempDral pattern

Spatial pattern

Depth

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Temporal pattern

Spatial pattern

(b) Sources of design rainfall information

The sources of information used to derive estimates of
the design rainfalls vary with AEP. For design rainfalls with
AEPs equal to or more frequent than 1 in 100, all the
necessary design informatiDn is provided in Book II
Sections 1 and 2. The absolute Iimij of extrapolation of this
source of design information for selected cases is 1 in 500
(see pDint ii in Section 2.3). Rare design rainfalls are
preferably derived using appropriate regional design
Information based on the analysis of observed rainfall
maxima, though if necessary less reliable (and probably
more conservative) rainfall estimates may be derived using
an arbitrary interpoiation procedure based on the PMP and
the slope between the 1 in 50 and 1 in 100 AEP design
rainfalls. Extreme design rainfalls beyond the credible limij
of extrapolation must be derived using a pragmatic
interpolation procedure between this limit and the AEP of
the PMP. A summary of the different sources of design
rainfall information is presented in Table 4.

(c) Uncertainty in design rainfall estimates

It should be recognised that the magnijude of the
uncertainty in design rainfall estimates increases with
decreasing AEP. Unfortunately there is little detailed
information on the uncertainty associated wijh the rainfall
estimates provided in Book II Section 1, and there is eVen
less informatiDn available on the uncertainties associated
with PMP estimates. Some information is available on the
more recent estimates of large to Rare rainfalls (e.g.
McConachy et at;"'1997; Nandakumar et at, 1997), and this
may be used to provide some indication of the uncertainties
involved in other design rainfall estimates.

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Genere~sed \:k1lrign data (Book 11 Section 1;
Section 3.2; Section 3.8)

For GTSM region generally as per Book II, Section 1 (see
Section 3.10)
For GSAM region see Section 3.9

Generally as per Book 11, Section 1 (see Section 3.10)

RegiDnal information (Section 3.3)
or interpolation (Section 3.6)

As for the generalised PMP methods (Section 3.9)

As for the generalised PMP methods (Section 3.10)

Interpolation procedures (Section 3.6)
As for the generalised PMP methods (Section 3.9)

As for the generalised PMP methods (Section 3.10)
Generalised PMP methods

GSDM
(see Sections
3.4.2,3.9,3.10)

Interpolation
and/or adjustment
between GSDM
and GSAM/GTSM
(Section 3-4-6)

GSAM or GTSM
(Sections 3-4-3,
3-4-4, and 3-4-5)

The uncertainty of design rainfall estimates reflects the
level of information available, wijh significantly increased
uncertainty in areas of sparse rain gauge coverage, such
as mountainous areas. There is little that practitioners can
do to reduce the uncertainty associated with the design
rainfall estimates, as in general the estimates are derived
from regionalised information. Apart from random errors
introduced during collation of the design inputs, the
uncertainty involved in estimation of design rainfalls by
practitioners with differing levels of skill will generally be
similar. However, the differences between the subjective
practice adopted by practitioners will become evident in the
manner in which uncertainty is incorporated into achieving
the final design objective.

3.2 Estimation of Design Rainfalls With an

AEP Between 1 in 50 and 1 in 100

(a) Point design rainfall depths

The informatiDn and procedures related to the
estimation of design rainfalls with an AEP equal to or more
frequent than 1 in 100 are detailed in Book II Section 1. In
some regions efforts are underway to revise this
information using the additional rainfall data that has
become available since preparation of the 1987 edijion of
ARR (ARR87), and practitioners are encouraged to take
advantage of this more recent infDrmation if it has been
shown to be soundly based. It is important to recognise that
considerable hydrometeorological analysis and judgement
was used to prepare the design isolines for the maps
presented in Volume 2 of ARR87; ij is likely that there will
be noticeable differences between design estimates
derived from a frequency analysis of gauged data at an