Geoscience Reference
In-Depth Information
related to ENSO - illustrate the broad catastrophic
nature of flooding in eastern Australia that substantiate
those Aboriginal legends.
Flooding in the summer of 1973-1974 coincided
with one of Australia's wettest La NiƱa events in
the twentieth century. Rainfall was torrential and
continuous throughout most of January 1974, as the
intertropical convergence settled over tropical
Australia. On 25 January, Cyclone Wanda moved into
the interior of Queensland and New South Wales,
dumping in excess of 300 mm of rain in 24 hours over
a wide area, and triggering massive flooding of all river
systems. Rain in the catchment of the Brisbane River
produced one of the worst urban floods in Australian
history. These January floods represented the largest
natural disaster to occur in Australia to that point in
time. Flooding covered an area of 3 800 000 km 2 ,
larger than the drainage basin of the Mississippi River.
From Alice Springs to the Pacific Ocean, and from
the Gulf of Carpentaria to the Murray River, military
airlifts had to be arranged to supply isolated towns, cut
off by floodwaters, with emergency food and stock
fodder. Around the Gulf of Carpentaria, the tributaries
of the Flinders River amalgamated to form a river
150 km wide. In northern New South Wales, torrential
rainfall continued week after week, raising river levels
in excess of 20 m. Floodwaters slowly flowed down
the Diamantina and Coopers Creek drainage into the
interior, filling Lake Eyre for only the fourth time this
century. At Wentworth in New South Wales, farmland
remained flooded for two years as successive floods
came down the Darling and Murray in 1974-1975.
Stock losses to sheep alone in New South Wales
totalled 500 000. The scale and magnitude of flooding
was unprecedented.
The Brisbane flood hit an expanding city that had not
witnessed a major flood since 1893. Few suspected the
flooding potential of the Brisbane River. Synoptically,
the flooding of the city was due to the persistent recur-
rence of cyclones tracking over eastern Queensland in
the space of four weeks, culminating in Cyclone Wanda.
However, many of the flood's effects were exacerbated
by human factors. Somerset Dam, built after the 1893
flood to contain similar events, was totally inadequate
for this purpose; yet, many people believed that the
dam protected them from flooding. The sprawling
nature of growth in the Brisbane River Basin had also
led to large-scale clearing of forest in rugged terrain,
which enhanced runoff during the high-intensity
rainfall periods of the 1974 flood. The character of the
drainage basin was also altered. Urban development
had seen some creeks filled in, while others had been
lined with concrete. These latter channels were
designed to contain only the 1:10-year flood. Roads,
driveways, and houses had sealed large sections of the
landscape. These modifications resulted in calculated
flood discharges being twice that of the 1:100-year flood
for adjacent vegetated catchments. Some urban
catchments experienced the 1:1000-year flood event.
Additionally, there was very little delay between time of
peak rainfall intensity and peak flooding. Flash flooding
occurred in urban catchments with flood peaks cresting
within one hour of peak rainfall intensities. Many areas
also had inadequate rain gauging stations, so that
the exact amount of rain falling in some parts of the
catchment was unknown, and the forecasting of flood
levels impossible to determine.
The Brisbane flood also saw the collapse of
organized disaster warnings. There was no central data
processing authority, so that local flooding in key areas
went unnoticed. Over 70 per cent of residents ques-
tioned afterwards about the flood had received no
official warning. The media, in their efforts to report a
major story from the field, sparked rumors and clouded
the true picture of flooding. The rapid series of flash
floods, in isolated parts of the catchment 24 hours prior
to the main flood, led to public confusion and commit-
ted the Disaster Relief Organisation to what afterwards
were evaluated as only minor disasters. The major
flood peak was disastrous. All bridges across the
Brisbane River were damaged or destroyed and
35 people drowned. At its height, the river broke its
bank and ran through the central business district of
Brisbane. In Ipswich, 1200 homes were destroyed.
Overall, 20 000 people were made homeless. Only
11 per cent of residents who experienced the main
flood received evacuation assistance from emergency
organizations, and only 30 per cent acknowledged
the clean-up help from any relief organization. Most
relief came from friends and community groups.
Over 40 per cent of victims received help from
church contacts, and over 30 per cent of volunteers
surveyed stated that they belonged to no organized
group. Generally, most people applied themselves to
evacuation, alternative accommodation, clean-up and
rehabilitation with little reliance on government or
social organizations for help. This raised serious
questions about the efficiency of such organizations in
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