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Latest models take guess work out of Grafton's floods

One in 100 year flood level.
One in 100 year flood level.

HI-TECH computer modelling that takes into account river topography, forecast rainfall, the amount of water in a catchment, tides, sea conditions and a host of other variables is now the primary source of information for flood forecasting, but that was not always the case.

Flood modelling used as part of the design process for the flood levee that has protected Grafton for the past 50 years was based on a physical model made from cement that was installed in a special facility in Manly.

SUPERMODEL: Researchers in Manly in 1967 put the finishing touches to a concrete model used to predict flood heights around Grafton. The modelling was used to help with the design of the Grafton flood levee we see today.
SUPERMODEL: Researchers in Manly in 1967 put the finishing touches to a concrete model used to predict flood heights around Grafton. The modelling was used to help with the design of the Grafton flood levee we see today.

Coincidently it was when this model was being developed in 1967 that the last flood entered Grafton.

Clarence Valley Council local emergency management officer, Kieran McAndrew, said it was important to have confidence in flood model, and while the model was skilfully made and made to the best standards of the time, its ability to provide highly accurate information was limited.

"Conversely, contemporary flood modelling is of course a highly technical and complex process, undertaken by highly-trained specialists,” he said. "The current Clarence River flood model is able to generate some quite accurate information, which is why council and the SES have a high level of confidence in the current digital flood model.

"On council's website there are flood model animations for public viewing, like the 100-year flood for the whole Clarence Valley from Mountain View down.

"The digital flood model can provide us with much more than flood extent maps. For example, the model can predict the location of anticipated levee overtopping and advise the expected heights when this overtopping would occur relative to the gauge at the end of Prince Street.”

An example sheet is provided on the web page for the intersection of Victoria and Prince streets, near the famous White Fig tree.

FROM LEFT: Cnr Prince and Victoria Sts. In these modelling examples you can see this area is dry in a 20-year flood, has both dry and flooded land in a 50-year and 100-year floods and is completely underwater in an Extreme flood event, below. Note that an extreme flood is both extreme in flood level and rarity.
FROM LEFT: Cnr Prince and Victoria Sts. In these modelling examples you can see this area is dry in a 20-year flood, has both dry and flooded land in a 50-year and 100-year floods and is completely underwater in an Extreme flood event, below. Note that an extreme flood is both extreme in flood level and rarity.

"In this you can see this area is dry in a 20-year flood, has dry and flooded land in 50 and 100-year floods and is completely underwater in an extreme flood event.

"An extreme flood is extreme in flood level and in rarity. It is a one in several thousand year event, and statistically very unlikely occur.”

The June 14 edition of The Daily Examiner features a 12-page flood feature you can hold onto to use as a handy guide to not only prepare for the next flood but also remind you why you need to. The edition will continue to be available at The Daily Examiner office located at 55 Fitzroy St, Grafton.



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