On 21 January 2010 The Daily Examiner ran articles pointing to the findings of a paper presented at 47th Annual Floodplain Management Authorities Of NSW Conference on 27 February–2 March 2007 at Gunnedah, titled BIG LEVEES – ARE THEY A GOOD IDEA?, authored by Drew Bewsher & John Maddocks of Bewsher Consulting Pty, Ltd, Sydney and Ian Dinham of Clarence Valley Council, Grafton.
One of these newspaper articles was careful to inform Clarence Valley residents that overtopping existing levees would have a warning period of only hours:
The paper states that the amount of time communities had to respond varied from town to town. In Maclean, modelling suggests the 100-year flood would overtop the levee there within just three hours.
Grafton comes out a bit more fortunate, taking about 10 hours before the town became inundated with water.
While the conference paper in question did point to some levee wall risk factors (see below), it finally came out in favour of the idea of levee walls in the final paragraph; This is not to say that we shouldn’t build big levees. Depending on site limitations.
Almost as a matter of course it totally ignored the fact that these upriver levee walls make unprotected downriver small villages like Iluka and Yamba more vulnerable during major flooding.
I think it was no accident that at least one Clarence Valley shire councillor made a rather gullible local journalist (renowned for rarely seeking alternative viewpoints) aware of this conference paper – it certainly paves the way to lengthen or create new upriver levees, despite the numerous qualifications it contains.
The former of these two gentleman would be well aware that community pressure on the back of national debate will demand more, not less, physical protection as populations unrealistically squat on ancient floodplains and, it is highly unlikely that either he or his fellow councillors will deny these demands with that last paragraph cop out just waiting to be quoted in any debate within the Chamber.
The Impact of Levees on the Flood Risk
Levees are built to reduce the flood risk to a community. They may be particularly useful in eliminating small or nuisance floods, and depending on their height, may also havesome success in mitigating larger floods. The flood risk to the community ‘protected’ bythe levee is reduced – up to the point that the levee is overtopped or it fails. After thispoint, there may be rapid inundation of the previously ‘protected’ area and deep inundation depths, resulting in a very high flood hazard to residents and occupiers of the area. In some cases, evacuation routes may be cut at an early stage, leaving occupants isolated and trapped in extremely dangerous conditions. In smaller catchments, there may be little warning that the levee will overtop, and virtually no time for the community torespond. There is also the threat of catastrophic levee failure, either before overtoppingor shortly afterwards.When the levee does overtop, the risk and threat to life will nearly always be greater (and often significantly greater) than when there was no levee. When the probabilities and consequences of all sizes of floods are considered, those thatovertop the levee and those that don’t, it may be that in some levee situations where the consequences of overtopping are disastrous, that the levee actually represents a netincrease in flood risk, not a reduction. Clearly where high levees are already ‘protecting’ extensive urban areas andcommunities are complacent about the consequences of overtopping, public awarenessinitiatives are essential to initially establish, and to then maintain the community in a ‘floodready’ state so that the flood risks can be mitigated.
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