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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still be drawn and steps 5 to 7 completed to give an overall
view of the assumed frequency relationship. An example
illustrating the derivation of intermediate design rainfalls
using this procedure is given in Section 62.1

(c) Differences between recommendations
and the procedure in the 1987 edition

The above interpolation procedure differs from the
method presented in the 1987 edition of ARR in two
respects:
(i) the AEP of the PMP is rounded to the nearest order of
magnitude to select the appropriate shape factors but
the PMP is plotted at its actual assigned AEP; and

(ii) the frequency curve is not tangential to the horizontal
at the PMP.

These two differences are introduced so that the
resulting frequency curve is consistent with the concepts
used to construct the frequency curve when regional design
information is available. In practice, for the same AEP of
the PMP, the differences between this and the 1987 version
of the method will be negligible except for events very near
the PMP.

3.6.2 Interpolation Between the Credible Limit of
Extrapolation and the PMP When Rare
Estimates Are Available

(a) Basis of procedure

Siriwardena and Weinmann (1998) have developed a
procedure suited to the interpolation between regional

Table 7 Values of [log(XvlX,,,/Iog(x,,MPIX,oo))

~-....... ..........,~.....,...., --''3-'''' ~........,...... .----

estimates of Rare rainfalls and the PM P. The procedure
was developed and tested on Victorian data using design
information derived using the CRC-FORGE procedure.
While it is possible that other procedures may be
developed for other regions, the procedure developed by
Siriwardena and Weinmann is described here as it is
considered to have generic applicability.

The procedure is appficable to 'gaps' of different ranges
corresponding to differences in both the AEP of the credible
limit of extrapolation and to the assigned AEP of the PMP.
The procedure involves the fitting of a 2-parameter
parabolic function in log-log space to ensure a smooth, well
behaved function when design rainfalls are plotted against
AEP on logarithmic scales. The following boundary
conditions are adopted:

. at the starting point of interpolation, the slope of the
interpolated curve matches the slope defined by design
estimates from the upper segment of the frequency
curve bounded by the credible limit of extrapolation;
and,

- the slope of the interpolated curve through the PMP
estimate is not constrained to the horizontal but is
determined by the shape of the frequency curve at
higher AEPs.

It needs to be emphasised that the interpolation is
entirely determined by estimates of the conditions at the
two end points; no additional information is introduced in
fitting the curve. Details on the derivation of the procedure
can be found in Siriwardena and Weinmann (1998).

0.877 0.976 0.928 0.989 0.948 0.994 0.956 0.995
0.711 0.936 0.802 0.970 0.850 0.982 0.893 0.987
0.585 0.880 0.690 0.944 0.752 0.967 0.814 0.975
OA89 0.816 0.600 0.911 0.667 0.945 0.734 0.960
OA19 0.783 0.527 0.862 0.597 0.915 0.663 0.941
0.372 0.766 OA67 0.811 0.539 0.874 0.597 0.916
0.345 0.756 OA20 0.784 OA88 0.827 0.539 0.882
0.328 0.749 0.385 0.770 OA47 0.797 OA91 0.838
0.316 0.745 0.360 0.762 OA13 0.781 OA51 0.808
0.306 0.742 0.342 0.757 0.385 0.771 OA19 0.789
0.298 0.740 0.330 0.753 0.363 0.766 0.394 0.778
0.291 0.738 0.321 0.751 0.347 0.762 0.375 0.771
0.286 0.737 0.314 0.749 0.335 0.759 0.360 0.765
0.281 0.736 0.307 0.147 0.327 0.756 0.348 0.761
0.276 0.735 0.301 0.746 0.320 0.754 0.338 0.759
0.272 0.734 0.950 0.745 0.314 0.753 0.329 0.757
Q.265 0.733 0.286 0.744 0.304 0.752 0.314 0.755
0260 0.732 0.279 0.743 0.294 0.751 0.303 0.754