Would an extreme flood make proposed Dunoon dam risky?

SOME residents at The Channon fear a future extreme flood event could be exacerbated if the proposed Dunoon Dam is built.

Building a 50-Gigalitre dam in Dunoon is being considered as one of the options raised by Rous County Council (RCC) as part of its Future Water project 2060.

In her submission to Rous, The Channon resident and botanist Nan Nicholson cited the 2012 Environmental Flows Assessment (EFA), commissioned by RCC.

"(It) makes clear that the most extreme floods may be exacerbated in the 3km immediately below the dam wall."

In her submission, Mrs Nicholson also confirmed the proposed dam could keep properties safe by minimising some flooding events.

"The Dunoon dam would indeed mitigate many floods," she said.

But in the case of an 'extreme flooding event', the dam could exacerbate the flooding impact downstream, she said.

"The additional impact is explained by the surface area of water in Dunoon Dam," she said.

" 'When full, the dam acts to increase catchment run-off efficiency, with all rain that falls directly onto the water surface delivered directly into downstream flow when the dam is spilling.' ", Mrs Nicholson quoted from the document.

"In discussion with Rous, I have been assured that improved modelling might effectively discount the risk, and anyway the risk is low, given the frequency of extreme flood events.

"This does nothing to reassure me," she said.


The catchment and buffer zones of a proposed Dunoon Dam.
The catchment and buffer zones of a proposed Dunoon Dam.


"Dams can mitigate flooding when they are not full, but in extreme events they are already full and cannot mitigate - they worsen the flooding."

Rous County Council general manager Phillip Rudd disagreed with the statements made by Mrs Nicholson.

"Her statement does not consider the whole report's findings and recommendations," he said. "The report states that the model used is not of sufficient accuracy to predict the magnitude of any increases in flood peak discharge and hence cannot reliably be used to predict additional impacts on the downstream environment.

Mr Rudd said that the document recommends that further hydrological analysis be completed to determine likely instantaneous peak discharges from the dam.

"Should the Dunoon Dam be preferred as council's long term water supply source option, council would undertake those specific assessments," he said.

"This analysis would adhered to the nationally-recognised standards for this type of assessment which was not completed in the Environmental Flow Assessment report, as that was not the intention of that study.

"It is noted that where flood studies have been completed, by using appropriate flood modelling analysis techniques, those studies show significant reductions in the instantaneous peak discharges."


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