September 13 2002
The greatest River
Murray flood eclipses 1956 levels
Described as the
greatest catastrophe in South Australia’s history, the 1956 River Murray
flood is the largest ever recorded in our state.
Now UniSA researchers are causing more than a ripple with a new
wave of investigations revealing that a prehistoric River Murray flood of
much greater magnitude surpassed the biggest flood on record.
dating back only to recent times, honours students Amanda Sulley and Derek
Snowball from the School of Environmental and Recreation Management
undertook research, using the Black Box gum (Eucalyptus
largiflorens) to interpret the pre-1840 salinity and flood history of
While data that
can extend the flood records is valuable, little work on these important
phenomena has been undertaken in Australia.
Derek’s research, funded by UniSA’s Division of Information
Technology, Engineering and the Environment, uncovered valuable data that
more fully reflects the Murray’s flooding history, according to Adjunct
Professor Robert Bourman who supervised the study with Ms Joan Gibbs and
Dr Fleur Tiver from the School.
To establish the
reliability and timing of pre-historic floods, known as palaeofloods, the
researchers looked at the distribution of the Black Box Gum in the River
“Black Box is
considered to be a reliable biological indicator of past flood levels
because it grows in distinct horizontal lines on the River Murray
floodplain. Its seeds germinate in the debris deposited on the floodwater
fringes of the riverbank,” Professor Bourman said.
dating of samples collected from existing gums revealed that the trees
were of a modern age, with establishment in the last 250 years. This gives
us an indication of the possible timing of the pre-historic flood of
around the year 1750.
researchers also undertook a survey to obtain the heights of individual
trees at their bases. This showed that the palaeoflood reached a maximum
height on the River Murray at Overland Corner of 18.01 metres, making it
greater than the largest flood on record, rising 2.11 metres above the
1956 flood height.
measured the cross-section of the river, they applied the Manning Equation
to determine the discharge of the prehistoric flood. This was estimated to
be 7,686 cubic metres per second, almost double the discharge of the 1956
flood, which measured 3,950 cubic metres per second,” Professor Bourman
calculated discharge and proposed age of the flood, the students were able
to calculate when on average a palaeoflood might return. This was measured
at 1000 years. This means that every year there is a 1000 to one chance of
a flood of that magnitude occurring, as it certainly will, at some time in
of this research have major implications for the wider community. They
show that houses, shacks and other structures even above the 1956 level
are at risk.”
Bourman believes that policies should now be put in place that limit
construction of permanent structures within the flood prone area from the
river channel extending to the palaeoflood level, to avoid possible damage
and costs caused by extreme flooding.
So what’s the
good news on floods?
not be seen as disastrous for the River Murray but as an important part of
the natural system, and as a partial solution to salinity, according to Ms
Gibbs. “Extreme floods are nature’s way of flushing out the salt,”
river should have salinity levels of less than 25,000 parts per million.
Medium level flows of water through the River Murray are needed to control
River Murray has high salinity levels averaging 34,000 parts per million
caused by the removal of 80,000 to 100,000 megalitres of water each day
for irrigation. This is severely affecting vegetation, causing salt stress
and destroying the river’s aquatic ecosystem.
studied the effects of varying salinity and flooding levels on the seed
germination, root growth rates and seedling development of three Black Box
Gum varieties. This was necessary to establish the conditions required for
the gums to germinate and grow,” Ms Gibbs said.
showed that while adult trees were tolerant of saline conditions, juvenile
trees were very salt intolerant. The Black Box seeds germinated in
different saline conditions but even at low saline levels the seedlings
were not healthy until flushed regularly with fresh water. Flooding is
essential for juvenile Black Box trees to grow, to flush salt from their
“If you think
that the prehistoric River Murray flood was the greatest catastrophe in
South Australia’s history, the River Murray’s high salinity levels
could be an even greater catastrophe affecting all of us in the near
future,” Ms Gibbs said.
contact: Geraldine Hinter (08) 8302 0963 or 0417 861832