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September 25, 2022

What would extreme flood in Murwillumbah be like?

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After 22 hours’ heavy rain in the catchment above Murwillumbah, the CBD Levee is about to be overtopped. If it continues to rain heavily for three more hours, the residential area behind the CBD levee will be under more than four metres of water.

Cyclone Debbie in 2017 took lives, destroyed homes and businesses and left a swath of destruction throughout the region. This was a one-in-100-year flood, that is there’s a one per cent chance of it happening in any year.

In Murwillumbah CBD the protective levee began to be overtopped at the Murwillumbah High School during Cyclone Debbie but the heavy rain then stopped, saving the town from a truly catastrophic event. Lismore was not so lucky with the levee being topped and leaving the CBD devastated with many businesses being destroyed.

Tweed Council’s Floodplain Management Committee (TFMC) have been looking at the effectiveness of the three levee system that has been installed to protect the Murwillumbah CBD and the possible impact of what is called a 500-year average recurrence interval (ARI) flood. That is, a flood that is likely to occur once every 500 years, a 0.2 per cent chance of it happening in any year.

The simulation of a one-in-500-year flood shows the Murwillumbah CBD levee overtopping near Murwillumbah High School after 22 hours of heavy rain in the catchment above the town. Three hours later, after heavy rain has been falling for 25 hours, the residential area behind the Murwillumbah CBD levee is under more than four metres of water. Residential areas behind the Dorothy / William Streets Levee are also under water up to four metres deep and the water is rising behind the East Murwillumbah Levee.

Council Chair of the TFMC Danny Rose said the video graphically illustrates the protection limits of the levees.

‘We haven’t yet seen a 500-year ARI flood in Murwillumbah but it could happen,’ said dMr Rose.

‘All residents are encouraged to watch the simulation and make their home emergency plan now as we are officially in storm season.

‘Council, the State Emergency Service (SES) and the TFMC are working hard to educate all Tweed residents on their flood risk and ensure that everyone knows their trigger to act to protect life and property.’

The best safeguard against storm and flood is preparation. Plan now for you, your family and your pets to limit your damages and losses if a storm or flood hits your area. Talk your plan through with your family and then put it on paper. The State Emergency Service has a template to guide you through making your plan.

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  1. This is misleading. With climate change 1 in 100 year and 1 in 500 year floods are likely to occur much more frequently, so the chances of them occurring in any single year are higher than the figures quoted.. This should be mentioned in the article to give context.

  2. When the Lismore CBD levee was being debated the hydrological engineers made it clear that a levee increases the danger. A given flood that would have filled the Lismore basin in 12 hours will now fill it in less than one hour. That means fast-flowing water rather than a gradual rise.
    Levees are dangerous.(and only keep out minor nuisance floods).
    Knowing this LCC went ahead with the levee.

  3. Spot on Paul. Levees keep out minor floods & allow the collective memory to fade. Development in the catchment often renders flood recurrence modelling obselete. Then comes a devastating major flood. Read Isaac Smith’s spin-doctoring after last flood.
    Construction of a levee or similar project requires the expertise of 2 types of engineers. Civil engineers to build, and consultants who engineer a pre-determined outcome from a consultation process.


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