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Flood management is not mostly about grand engineering schemes

By Chas Keys - posted Tuesday, 12 February 2013


Lloyd Hamilton (On Line Opinion, 11/2/2013) makes some useful points about the management of flooding in south-east Queensland, especially when he says there is no single solution and that several different measures are needed. But the general direction of the solutions he advances should be rejected. Mitigation is not solely, or even largely, about capturing floodwaters or moving them elsewhere.

We need to see flooding as a problem caused by human actions and behaviour, not as a problem caused by nature that needs to be controlled. And we need to stop trying to 'flood-proof' Queensland: it simply can't be done. For one thing it would be prohibitively expensive, and for another it would have disastrous consequences environmentally. It could deny floodplains the occasional soaking they need to be healthy, and it could lead to the loss of productive land and valued wilderness.

When people experience the horrors of bad floods, it is natural that they seek relief. In general, they think of engineering solutions ─ like dredging the rivers or building more dams. But dredging is rarely a useful approach: just look at the huge volumes of water that are stored on floodplains and the comparatively small carrying capacity of river channels. And there is no point in making rivers deeper close to sea level. That will simply mean more sea water in the lower reaches.

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Dams can play a part in flood mitigation, but not as large a part as is commonly supposed. The coastal rivers on which most of our big towns and cities are situated have many tributaries, and to prevent flooding they'd all have to be dammed. This would be enormously expensive. In any case most of the best sites for dams have already been utilised and the amount of land that would be drowned by building more dams would be prohibitive. And there would still be flooding when heavy rain fell downstream of the dams.

Hamilton's solutions suffer ─ as his piece recognises several times ─ from several weaknesses. Digging big tunnels to move water from one river to another would be extremely costly, and it would have little worthwhile impact if all the catchments he mentions (the Brisbane, North Pine and Logan) were in flood at the same time as happens periodically. And he notes correctly that low-lying, flat areas are not the best areas for storing water behind dams.

Straightening the Brisbane River would be disastrous. The steeper gradient and faster flows would exacerbate erosion, banks would slip into the river and the channel would become wider, shallower and more silted. These were the results when the Hunter River, in NSW, was straightened as it was from the late nineteenth century. Productive land was lost to slumping and the river's ability to carry flood flows was reduced. There was no useful gain except that boats had shorter journeys ─ but that benefit was lost when the river silted up and became too shallow to navigate in times of low flow.

Far better to look first not at modifying the environment but doing something about what humans have done and keep doing. Over the decades we have built our towns and cities largely on floodplains ─ and we have done little to protect them from the flooding they will inevitably have to bear. Few councils in Queensland have ever studied floods properly, and many have not mapped where floodwaters flow. The result is that subdivisions have been built in places that are certain to be inundated. In recent years we have seen large areas of new houses ─ Emerald is one place among several that come to mind ─ flooded well above their floors. They have been built in the wrong places.

Developers have been allowed to focus on areas where development is easy rather than where it is appropriate. Yet we have no shortage of land that could be developed away from the reach of flooding.

And because of the lack of flood studies, information has not been gathered to allow councils to advise people of the risks they take in buying or building on floodplains. Thus they are shocked when floodwaters overwhelm them. They are also shocked when their insurance premiums grow to become unaffordable, which in some cases means they stop taking out insurance. That creates the potential for personal financial ruin.

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The legacy of decades of development on floodplains is that we have allowed the problem to grow, slowly but massively. In the best of all worlds governments would simply buy the properties in flood-liable areas and resettle people ─ but that would cost hundreds of billions of dollars. It might happen on a piecemeal basis, as it has in Grantham in the Lockyer Valley where the town is being virtually relocated, but to buy back much of Townsville, Bundaberg, Rockhampton, Brisbane or the Gold Coast is simply untenable. The most we could afford to do is to buy the most seriously flood-prone properties and turn them over to recreation, farming or parkland.

In some cases we can protect what is already there, and we can certainly stop further development that will exacerbate the problem. Many things could be done by way of protecting people from past developmental mistakes. In some locations levees could be built, and in others floodwaters could be 'trained' away from urban areas. Retention basins could be constructed to 'store' stormwater or creek flooding and let it go slowly once downstream levels have fallen. We could raise the floors of houses.

And we could work at ensuring that people understand that their properties are on floodplains, understand flood warnings and know when a warning means they should act to raise their belongings and evacuate. We could raise low sections of roads so they can function as evacuation routes rather than trapping people when floodwaters flow over them. Raising roads could also help prevent communities from becoming isolated during floods, which always causes great inconvenience, economic disruption and sometimes distress.

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About the Author

Chas Keys is a flood consultant, an Honorary Associate of Risk Frontiers at Macquarie University and a former Deputy Director-General of the NSW State Emergency Service.

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