Warragamba, which accounts for 80 per cent of Sydney’s dam storage capacity, was sitting at 98 per cent full as of Thursday. The operating rules dictate the reservoir can only start to spill when water reaches about 1 metre below the top of the wall, with the equivalent of 70 gigalitres of so-called airspace.
Those rules sparked a debate among some ministers in the Berejiklian government during the worst of the flooding, with Emergency Services Minister David Elliott among those saying the dam should have released more water ahead of the predicted rainfall.
A spokeswoman for Water Minister Melinda Pavey said the minister “supports considering all options to minimise the impacts and severity of floods across NSW”. The Herald has approached Minister Pavey to ask whether any review is planned now the emergency has receded.
The spokeswoman said last month that the Hawkesbury-Nepean Valley Flood Management Strategy had considered several options for flood mitigation.
“Amongst these were lowering the water levels down to either 5 metres or 12 metres,” she said. The latter amounts to reducing the dam level by 800 gigalitres, or about 40 per cent of its capacity.
“The government has considered opportunities to bring forward the lowering of the water level, to partially reduce flooding risks for minor events,” the spokeswoman told the Herald last month.
Stuart Khan, a professor at the University of NSW’s School of Civil & Environmental Engineering, said the desalination plant would remain operational out of concerns for water quality.
He noted that during the 2013 Brisbane floods, so much mud flowed that the city water treatment facilities were overwhelmed, requiring supplies from a desal plant on the Gold Coast to ensure Brisbane didn’t run out of drinkable water.
“It comes down to having multiple independent water sources to draw from,” Professor Khan said.
In Sydney’s case, about 80 per cent of the water treatment is done at the Prospect plant.
Professor Khan said changing Warragamba’s operating rules would not necessarily resolve the issues since releasing a lot of water in the face of a coming rainfall event would “take the possibility of a flood to make it a sure thing”.
An advantage, though, of maintaining a large airspace at Warragamba would be to reduce the need to spend potentially several billion dollars to raise the wall by 14 metres. Other ways to increase the flexibility of Sydney’s water supplies would include expanding the desalination plant or building additional ones, and increasing water recycling, Professor Khan said.
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Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.