Most people are surprised to hear it, but flooding is the most dollar-costly natural hazard that affects Australia. It doesn’t kill as many people as heatwaves and bush fires do, but the damage it does to property, infrastructure and production is greater than that wrought by bush fires, severe storms, tropical cyclones, landslides or earthquakes.
That’s the bad news. The good news is that flooding is the most easily mitigated of the natural hazards we face. We know, by and large, where it will happen, we usually get advance notice and there is much we can do to contain the damage that it brings. One of the questions that arises from the floods that have assailed eastern Australia virtually constantly since September is why we have done so little and suffered so much.
The answer is twofold. Part of it is that not all the damage can be prevented: it is hard to see how the loss of crops and coal production could have been avoided in this month’s Queensland floods. Another part of it is that public and political will to act has been lacking. A recent illustration of this was the reaction to the offer in 2006 by the Mayor of Brisbane, Campbell Newman, to buy back hundreds of severely flood prone properties in that city at market value. More than four years on, very few property owners have taken up his suggestion.
Only one Australian state - NSW - has ever put a major effort into flood mitigation. There, the equivalent of well over a billion dollars in today’s terms has been spent on it since the early 1960s. That sum amounts to an average investment of more than $20 million per year. The effort has dropped off lately, but for more than 40 years NSW led the nation in investing in creating flood-resilient communities.
Queensland’s government refused to support Newman’s voluntary buy-back initiative. That is consistent with Queensland’s approach to flood mitigation over the long term: relatively little has been done. One result is that very few towns in Queensland have levee protection. Dozens in NSW have it, and NSW has done many other things to mitigate the costs wrought by floods as well. The two states each account for about 40% of the nation’s average annual flood damage, but their responses to dealing with the losses have been radically different.
So what can be done to mitigate the costs of flooding? First, it has to be recognised that the best flood mitigation - or to give it its modern, more inclusive name, floodplain management - almost always consists of a mixture of strategies. Some of these may involve engineering solutions like levees (to keep floodwaters out of certain areas), flood bypasses (which "train" water to travel past rather than through towns), flood mitigation dams (which pond floodwaters and release them slowly after downstream river levels have fallen) and flood retention basins (which also hold floodwater temporarily).
But these "structural" measures can only do part of the job. Levees can almost never be built to keep out the very worst floods. Sooner or later they are overtopped with great damage and loss resulting in the "protected" areas. This is not to decry them. At Grafton, in the northern rivers of NSW, the levees built in the early 1970s have kept out six floods which would have entered residential and/or commercial areas; in the 130 years of Grafton’s existence before 1970 some 18 separate floods did damage in the town. The comparison of the periods before and after the levees were built is a strong testament to their worth.
But none of the floods which Grafton has experienced has been anything like as big as nature could produce there. One day, a flood will overwhelm the levees and flood the town. The benefit will have lain in damage avoided in the many floods that were kept at bay, and the occasional big one which is not excluded will not deny that benefit. It will simply prove that levees cannot solve the whole problem or make Grafton flood-free.
Or take mitigation dams. They can lower the downstream peak levels, but they can only fully pond relatively small floods. Genuinely big ones cannot be totally controlled unless there are many more dams than we now have and, of course, provided that the heavy rainfall does not fall downstream of them. To fully mitigate big floods at, say, Rockhampton, all the tributaries of the Fitzroy River would have to be dammed as well as the main stem of the river upstream of the city - with huge losses of agriculturally productive land, the abandonment of many farms and several towns and an enormous cost in construction. It would be totally unfeasible, economically and environmentally.
Engineering structures have limits, and their place is easily overstated. We need to recognise that all floods cannot be made benign. Genuinely big ones will inevitably overwhelm the structural measures we institute or will outstrip their mitigative capacities. Put simply, big floods contain far too much water to hold or direct.
Structures alone can never do the whole job, and floodplain management must therefore be about more than "controlling" floodwaters. We must recognise that we need to adapt to flooding and learn to live with it rather than trying to overpower it.
Fortunately there are many ways we can adapt to flooding. Houses can be raised, dwellings and commercial premises can be removed from the most frequently and dangerously inundated parts of floodplains, and planning regimes can be instituted that prevent rebuilding in such areas or the expansion of urban areas onto land that is certain to be inundated. All of these things have been done in Australia, especially in NSW, since the great floods in that state during the mid-1950s.