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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DRAFT D

3.4.2 Small Areas and Short Durations

The Generalised Short Duration Method (GSDM)
provides complete information on PMP estimates for areas
up to, 1000 kin' and durations up to I; hours in tropical and
subtrDpical coastal areas, and up to 3 hours in inland and
southern Australia. DetailS of the method are published in
Bulletin 53 by the Bureau of Meteorology (1994). The
method is based on adjusted United States data, as the
available information from the relatively sparse Australian
rain gauge and pluviometer network indicates that the short
duration rainfall potential in Australia is similar to that in the
United States where the data base is much larger.
Only one set of depth-duration-area curves is given for
durations of one hour and less where the type of terrain
does not affect the PMP. For durations greater than one
hour. two separate sets of depth-duration-area curves are
given for "smDoth" and "rough" terrain. Bulletin 53 states
that the latter applies to catchments within which elevation
changes of 50 m or more within horizontal distances of 400
m are common. More generally, the term "rough terrain"
refers to areas in which slopes of 1 in 8 or greater are
common. Catchments which satisfy this criterion should be
classified as "rough". Rugged terrain, as well as triggering
more convective storms than flat terrain. tends to hold them
in place. thus increasing the rainfall over a given area. It is
also possible for a storm to be held in position over flat
terrain next to an escarpment Catchments in almost any
part of the Great Dividing Range will be in the "rough"
category. if there is "smooth" terrain within the catchment
that is further than 20 km from generally "rough" terrain, an
areally weighted factor of "rough" and "smooth" terrain
should be calculated such that the factors sum to unity. If a
catchment proves difficull to classify using these
guidelines, it should be classified as "rough".
Rainfalls are reduced in accordance with the
precipitable water above the average elevation of the
catchment On the basis of U.S. data, no reduction is made
below an elevation of 1500 m. and only relatively small
reductions are made above that
Bulletin 53 provides guidance on the temporal and
spatial distribution of design rainfalls, as discussed in
Sections 3.9 and 3.10.

3.4.3 Longer Durations in Southeast Australia

For durations between 24 and up tD 120 hours in
southeast Australia, PMP estimates should be derived
using the GSAM (Minty et aI., 1996); approximate estimates
are also available for storm durations down to 12 hours.
Practitioners should obtain the required design estimates
from the Bureau of Meteorology.
The GSAM provides estimates of PMP for the standard
storm areas of 100, 500. 1000, 2500, 5000. 10000, 20000,
40000 and 60000 km', and for stDrm durations of 24, 36.
48. 72. 96 and 120 hours. With the GSAM, a PMP is
estimated by combining catchment-specific features with
regionalised information. The catchment-specific features
include storm type, moisture content and topographic
influences. The regional information is based on an
envelDpe of the standardised. maximised components of
extreme historic storms. I.e. an envelope of the
characteristics of the most extreme storms on record which
have had their site-specific components removed. In
summary. a PMP estimate is derived from the following
steps:
1. Derive the s/;lndard maximised convergence depth for
a given storm area and duration (this represents the
component of the PMP at a standard, hypothetical

Book VI - EstImation or large to I:.xtreme ~Ioods

Iocat..... the result oJ synoptic-scale atmospheric
disturbances unaffected by terrain).

2. Determine the moisture adjustment factor as the ratio
of the precipitable water at the extreme dewpoint
temperature for the catchment of interest to that at the
standard. hypothetical location (this represents a
transposition factor based on the moisture potentialS
at the two locations).

3. Compute the convergence component of the PMP at
the catchment of interest by multiplying the standard
convergence depth (step 1) by the moisture adjust-
ment factor (step 2).

4. Multiply the convergence component of the PMP at
the site of interest (step 3) by a topographic
enhancement factor based on the modified 72-hour 1
In 50 AEP rainfall intensity field (the topographic
component is that proportion Df the rainfall that is
attributable solely to topographical influences).

The GSAM was deyeloped using a catalogue of data
compiled from the largest observed rainfalls at all stations
within the GSAM storm search area (Meighen and
Kennedy. 1995). The rainfall durations analysed range from
1 tD 7 days. although most of the storms haVe durations of
5 days or less. Some single events were significant at a
number of durations and these are listed separately in the
catalogue. The total areas covered by the rainstorms vary
from 10000 km' to over 60000 km'. In total. information on
110 rainstorms was carefully analysed for inclusion in the
storm catalogue. The majority of the storms occurred along
the east coast of southeast Australia, though a significant
number occurred inland and over South Australia.

References to the temporal and spatial patterns to be
used with the GSAM method are provided in Sections 3.9
and 3.10.

3.4.4 Longer Durations in Tropical Areas

For the larger areas and/or IDnger durations for regions
of Australia affected by tropical conditions. the Bureau has
develDped the Generalised Tropical Storm Method (GTSM).
This method involves wide transposllion of recorded major
storms as a basis for PMP estimates over about 75% of
Australia. covering the Northern Territory, Queensland,
most of Western Australia and northern New South Wales
(Kennedy and Hart. 1984).

With the GTSM. an initial PMP value is obtained for the
area and duration required from a set of depth-duration-
area (DDA) CUrves. Adjustment factors are then calculated
for the extreme persisting dew point temperature for the
catchment. its distance from the sea, intervening barriers
and the local topography. The method recognises two
distinct zones (see Figure 3): the East Coast Tropical Zone
and the Remaining Tropical Zone. The Queensland Coastal
region is differentiated from the rest of Australia because
troughs along the Queensland and northern New South
Wales coasts can cause heavy rainfalls that persist for
much longer than elsewhere. The DDA curves were
determined using isohyetal maps derived fDr the seven
most extreme tropical storms recorded in Australia. The
limited number of storms in the data base used to derive
the DDA curves is of concern. and accordingly initiatives
are currently underway to undertake a detailed review of
the GTSM method.

GTSM estimates are available for durations of 6 to 96
hours in the East Coast Tropical Zone, and 6 to 72 hours in
the Remaining Tropical Zone; if storm sequences are
considered. estimates are available for durations up to 168
hDurs in the East Coast Tropical Zone only.