2011 Floods in Brisbane, Australia: How Much Water?

Brisbane, the third-largest city in Australia and the capital of Queensland, has been seriously flooded, a natural disaster not seen here since 1974. As I write this post, thousands of volunteers are helping affected people and businesses to clean up and get back on their feet.

The media has been providing extensive coverage of the disaster and its aftermath for weeks now, providing opportunities for math teachers to investigate with their students some of the statistics being quoted. This article explores one statistic in particular, the amount of water released from the Wivenhoe Dam in one day. There are many similar opportunities for the math teacher who wishes to incorporate some "real math" with relevant, interesting connections with these current events.

Brisbane Flood, 1893

Over 100 years ago, in 1893, Brisbane suffered serious flooding of the inner city and low-lying suburbs, when the Brisbane River broke its banks in February 1893 after a tropical cyclone: The 1893 flood was just one of several floods around that time, there being other floods in 1887, 1890 and two others in 1893 a fortnight after the major flood.

Brisbane Flood, 1974

In January 1974, after weeks of heavy rain, Cyclone "Wanda" developed off the Queensland coast, which then turned into a rain depression and dumped huge amounts of water onto the city and the catchment of the Brisbane River. The result was flooding of many riverside and low-lying suburbs of the city, and over 6,700 houses being flooded.

Wivenhoe Dam

After the water had gone and city planners had a chance to analyze what caused the flood and how they might prevent a repeat of the devastating inundation of so many homes and businesses, it was decided to build a large dam upstream from Brisbane, as a flood mitigation measure. That dam is the Wivenhoe Dam: This dam, which is presently spilling huge quantities of water over the above spillway, is an impressive structure with some impressive statistics:

    * Height: 50 meters
    * Length: 2.3 kilometers
    * Capacity: 2,640,000,000 cubic meters
    * Surface Area: 109.4 square meters
    * Length of Shoreline: 462 kilometers
    * Catchment Area: 7.020 square kilometers
    * Average Annual Rainfall: 904 millimeters
    * Maximum Capacity: 225%

The 2011 Flood, Brisbane, Queensland, Australia

At the time of this article, Brisbane is slowly recovering from yet another serious flood, the peak of which was on 13 January 2011. Even though the peak of the flood was around 1 meter below the 1974 peak, this time 20,000 houses have been inundated, and the economic cost is much higher. In the media coverage of this natural disaster, Wivenhoe Dam has attracted an understandable amount of attention. Specifically, the management of the water kept behind the dam and the water released over the spillway is of great interest to the people of Brisbane. At the peak of water release, over 645,000 megaliters of water were released in one day. This is around half of the flood abatement part of the storage at Wivenhoe, which is 1,450,000 megaliters on top of the water stored for drinking and supply to Brisbane.

How Much Water is That?

Six hundred and forty-five thousand megaliters sounds like a lot - it is. But this quantity of water is very difficult to visualize. How can a teacher help students understand this quantity?

Step 1: How big is a liter?

One liter is a familiar quantity to students who buy milk in liters. It may be converted into 1,000 milliliters, or 1,000 cubic centimeters. This is the same volume of a place value thousands block, which is a 10 centimeter cube.

Ask students what would have the same volume as one liter - one liter of milk, obviously, a medium bottle of soft drink, less than one old-style house brick.......
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