The southern part of the city closest to the river was the most severly affected. The river flooded twice more within a fortnight of the main flood, both times due to tropical cyclones crossing the coast.
The floods which occurred on the 27th of January are considered to be the largest floods to affect the city of Brisbane in the 20th Century. However, it was Cyclone Wanda that caused mm of rain to fall on the city during a space of 36 hours. The floods peaked at 6. So what can we do to plan better? Floods are the most expensive type of natural disaster in Australia. This does not include the incalculable cost of declining water quality and ecosystem health in offshore ecosystems such as Moreton Bay.
To manage flood risk, we have to understand the chance of different-sized floods occurring. The chance of a flood event can be described using a variety of terms, commonly including the average recurrence interval ARI. However, the preferred method is now annual exceedance probability AEP. Initial estimations of the event, based on 31 years of gauging records in the Upper Lockyer, indicated an AEP of 0.
But another extreme event in , with five more years of data, reduced this to 1. This illustrates a major problem with calculating flood risk: flood prediction is extremely dependent on the amount of data. With the passing of time, memories of the last flood fade or become clouded.
Those who do remember, either from their own, first hand experience, or from what they have read or been told, tend to think of the last flood as the worst that has ever occurred.
The flood was the worst experienced by Brisbane during the 20th century however, however both the and floods were, based on existing records, significantly higher. There are also references to even older, more severe floods. For instance, in , John Oxley, during his explorations, mentioned seeing evidence of a severe flood in the area north of the junction of the Brisbane and Bremer Rivers. This he estimated at around fifty feet above the normal level of the two rivers.
Based on grass and other debris high on tree branches, his estimate was a flood of some one hundred feet above the normal river level. If we take the official record of the flood level on 4 February at the pumping station, 94 feet Therefore, we have a number of major floods in the Brisbane River, that we can describe as being catastrophic in size and severity, , , , and , well spaced for sure, but not evenly or predictably spaced.
With five huge floods over a period of some years, we can say that the river experiences a major inundation, on average, every 37 years or so. Therefore descriptions such as a one in fifty year flood tend not to adequately portray the reality of the historical record. Of course, in terms of potential damage, there are mitigating factors such as new dams and improved drainage, however the growth and greater density of Brisbane also progressively increases the risk.
As well, any mitigating factors can always be negated by a few more feet more of flood waters, unpredictably or rapidly delivered. In any examination of flooding in the river, we are faced with the rather stark evidence of the historic record. Regardless of the way in which planners, commentators and others attempt to describe the height and severity of floods, as well as their frequency, nature operates in accord with its own timetable. Unfortunately, it seems that the only certainty is that the river, from time to time, will flood in a devastating fashion, with our only defence being the ways in which we plan and build in the future.
We welcome relevant, respectful comments. Please read our Comment Policy before commenting. We also welcome direct feedback via Contact Us. Descriptions of the flooding reveal a sensory overload. The floods changed the human relationship with the river. Previously viewed as a benign economic resource, the Brisbane River now became the enemy of progress. The floods accelerated a program of engineering interventions that included dredging, truncating bends, building training walls, and ultimately the completion of Somerset Dam in the catchment in Controlling the river by dam construction in response to these floods, rather than implementing land use management for hazard reduction, set a path of dependency on dams that continues today.
Cook, Margaret. Rachel Carson Center for Environment and Society. This work is licensed under a Creative Commons Attribution 4.
Please click on an image to view its individual rights status. Skip to main content. Arcadia Collection: Water Histories. Greater Brisbane area inundated in the flood of Victoria Bridge, connecting north and south Brisbane, destroyed by floods.
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