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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Table 6 Summary of interpolation procedure for estimation of rainfalls between the credible limit of extrapolation
and the AEP of the PMP,

1 in 100 AEP design events

Defined by 1 in 50 and 1 in 100 AEP
design events

Durations for which
design estimates
directly available
(e.g. 18 hrs and
longer)'

Rainfall depth for
credible limit of
extrapolation (e.g. 1
in 2000 AEP)

Durations for which
design estimates not
available (e.g. less
than 18 hours)

Extrapolated from
design estimates for
longer durations
(credible limit of
extrapolation depends
on duration)

Defined by two estimates close to limit of
extrapolation

Operational estimate of PMP using assigned AEP
(Sections 3.4 and 3.5)

Prescriptive, no credible basis for
departure from procedure

Shape factors (Table 7)

Log-normal

Range limits as specified in Table 1

Fixed procedure requiring subjective
sketching

Not prescriptive, practitioners are encouraged
to take advantage of improvements as they are
developed

2 parameter parabolic function

Log-log

Ratio of S,JS,,,, ranging from 0.25 to 2.00 (see
Figure 5)

Mathematical function; easily adaptable to any
starting and end points.

Uncertainly increases
for shorter durations
as greater degree of
extrapolation required.
Subjective judgement
needed when applying
procedure to durations
S 3 hours.

1. Examples of duration and AEP limits are provided for Victorian eRG-FORGE regional design estimates as described by Siriwardena
and Weinmann (1998).

Highly conservative in most design
situations, large uncertainly in
intermediate AEP range.

Reduced uncertainty
asthe'gap'of
interpolation is
relatively small.