Geoscience Reference
In-Depth Information
shows the highly localized nature of the event. Within
5 km of the escarpment, rainfall amounts had dropped
to less than 400 mm; within 10 km, amounts were less
than 200 mm. Because any area on this map can be
subject to this type of flooding - albeit rarely - it has
been estimated that the Wollongong urban area could
experience a Dapto-type event every 25 years. In the
20 years since 1984, five flash flood events of similar
magnitude have occurred. Emergency organizations
here should be prepared to handle high-magnitude,
localized but rare floods in this region, with a much
shorter expectancy rate than shown by individual
events.
The Sydney metropolitan flooding that followed was
not a single event, but rather a series of intense
thunderstorms, which struck various parts of the metro-
politan area on 5-9 November 1984. Accompanying
the synoptic weather situation was a series of tornadoes
and waterspouts that caused over $A1 million damage.
The floods themselves cost the insurance industry
$A40 million. One death resulted from lightning. This
death toll is small considering the fact that most
observers would class the lightning activity on the night
of 8 November as the worst in living memory. Over
the five-day period, rainfall totalled 550 mm south-
south-east of the Sydney central business district.
Most of the eastern and northern suburbs, as well as
the Royal National Park south of Sydney, received
totals in excess of 300 mm.
The floods originated from a low-pressure cell
centered south-east of Brisbane on 5 November. This
low moved slowly down the coast, rotating counter-
clockwise around a strong, blocking high-pressure cell
(maximum pressure 1032 hPa) that drifted from
Adelaide to New Zealand over the period (Figure 6.8).
The high-pressure cell directed consistent, moist,
zonal flow onto the coastline. Concomitant with the
movement of the high, a low-pressure trough devel-
oped parallel to the coastline. This trough spawned
low-pressure cells that forced airflow over Sydney,
N
Wollongong
Mullet Cr.
Lake
Illawarra
Dapto
Tasman Sea
0
2
4
6
8
10 km
Forty-eight-hour isohyets for the Dapto flood, Wollongong, Australia, 18 February 1984 (based on Nanson & Hean, 1984).
Fig. 6.7
