Geoscience Reference
In-Depth Information
Nov 1984
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Synoptic pattern for Sydney, Australia, flash floods, 5-9 November 1984.
Fig. 6.8
resulting in intense atmospheric instability. The
flooding occurred as three separate events during this
period.
The first event on Monday 5 November began as
thunderstorm activity at Cronulla (Figure 6.9a) and
slowly moved northward over the city center. This
thunderstorm activity originated as a series of intense
cells associated with a low-pressure trough parallel to
the coast and a separate low-pressure cell 50-100 km
south of Sydney. A second trough existed in the interior
of the continent (west of the Dividing Range), and
drifted over Sydney in the early evening. Both of these
troughs were responsible for separate rainfall events.
Over 220 mm of rainfall fell within 24 hours over
Randwick. Rainfall intensities for time spans under
nine hours exceeded the 1:100-year event. The most
serious flooding was caused by a storm drain at
Randwick racecourse that was unable to cope with the
runoff. Streets in adjacent Kensington bore the brunt of
substantial flooding, which entered houses to depths of
0.5-1.0 m. Evening rush hour was thrown into chaos.
In the second event, on 8 November, a low-pressure
trough was again present, this time directly over
Sydney. Airflow converged into this trough over the
northern suburbs. Seventeen convective cells were
responsible for rainfall during the morning with the first
two producing the heaviest falls. Over 230 mm of rain
fell at Turramurra, in the northern suburbs, with falls
of 125 mm between 7:15 and 8:15 am (Figure 6.9b).
This latter intensity exceeded the 1:100-year event. The
20-minute rainfall intensity record at Ryde had a
recurrence interval of between 50 and 500 years.
Flooding during the second event began within
20 minutes of the commencement of heavy rainfall.
Peak discharges occurred within five minutes of
flooding. Many of the catchments that flooded were no
more than 1 km long; however, flood depths up to 1.5 m
became common, both in channels and in shallow
depressions forming part of the drainage network. Many
residents were unaware that these depressions were part
of the flood stream network. Flooding occurred in the
middle reaches of catchments as stormwater drains
reached capacity and surcharged through access-hole
covers. This
surcharging
capacity is deliberately built
into Sydney's stormwater system, as it is prohibitively
expensive to build an underground drainage network
large enough to cope with rare events. Some of the
flooding was due to storm inlet blockage by trees, loose
debris or, in some cases, cars transported in floodwaters.
It was even found that backyard paling fences could sig-
nificantly block, divert, or concentrate flows, increasing
flood levels locally by as much as 40 cm.
The third event occurred on the night of
8 November. Synoptically, it was very similar to the
second event, with a trough cutting through Sydney
and converging air into the city region. Rainfalls of
249 mm were measured at Sydney's Botanic Gardens,
and values near 300 mm in and around the Royal
National Park to the south (Figure 6.9c). Again, several
cells were involved, and again peak intensities for time
