Warragamba Dam is spilling a Sydney Harbour's worth of water each day into the already swollen Sydney basin.
Key points:
- The dam has been filling steadily since February 2020
- Warragamba would have needed to be taken down to 25 per cent of storage capacity before the rain in order to cope with the volume, WaterNSW said
- Modelling by WaterNSW showed that in the last seven days, 1,500 gigalitres of water had flowed into the dam
WaterNSW reported yesterday that the spillway was releasing 450 gigalitres a day (450 billion litres), a rate that could go up if the dam storage levels continue to rise due to the heavy rain.
That daily spill equates to nearly a quarter of the dam's total capacity of 2,000 gigalitres.
It is not much less than the 567 gigalitres that the Sydney population drinks in one year.
The levels peaked overnight at 500 gigalitres and have now dropped down to 300 gigalitres, WaterNSW said.
The dam had been filling steadily since the drought-breaking rains in February 2020, and by March 19, was 94.6 per cent full.
Modelling by WaterNSW showed that in the last seven days, 1,500 gigalitres of water, about 75 per cent of its total storage, had flowed into the dam.
Stuart Khan, professor of civil and environmental engineering at the University of NSW, said that the large volume of water flowing over the spillway would struggle to get through the choke points downstream, particularly around Sackville on the Hawkesbury River.
A spokesperson for WaterNSW said in order to have had enough capacity in the dam to capture the inflows from this event, Warragamba would have needed to be taken down to 25 per cent of storage capacity before the rain.
ABC News: Brendan Esposito
)Ian Wright, a senior lecturer at Western Sydney University, described the claim, which was repeated by NSW Premier Gladys Berejiklian, as hyperbole.
"The dam was not built to hold back floodwater to protect the Hawkesbury Valley. The dam was built as our water supply for Sydney," he said.
A spokesperson for Minister for Water Melissa Pavey said that it was unlawful for WaterNSW to release water on the basis of a forecast of a potential rain event.
"Until Thursday 17 March 2021, this was not forecast as a major rain event. The BoM (Bureau of Meteorology) upgraded the forecast Thursday afternoon, almost doubling predicted rainfall," she said.
"Based on BoM’s original forecast, there was only a small probability that Warragamba would have a minor spill."
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)The Warragamba Dam was completed in 1960 but was never built with flood mitigation abilities, which is a feature of the Wivenhoe Dam in Brisbane.
"There's no way on earth technically, they would have been able to do that (build flood mitigation)," he said. "I don’t think it was a realistic prospect that they had the engineering at the time," Dr Wright said.
Dr Wright said the only safe option WaterNSW had was to open the gates at the top of the wall to allow excess water to flow out.
The lowest level Warragamba Dam has ever been was 38.8 per cent in 2004 during the Millennium drought.
Dr Wright described the Warragamba River that feeds the dam as a "freak of nature".
The Warragamba river flows into a short, deep narrow gorge and collects water from the Wollondilly river in the Southern Highlands and the Coxs river in the Blue Mountains.
Professor Khan said extending the dam wall 17 metres, which has been proposed to control flooding in the Hawkesbury-Nepean valley, could allow the dam to hold about 1,000 gigalitres of inflow to the dam.
"That would delay the flooding events by a couple of days, it probably would not have breached quite yet," he said.
But another 500 gigalitres would still need to have been released, which could have delayed the flood peak, which Professor Khan said comes down to timing.
Professor Khan said other water supply strategies, such as desalination and water recycling that weren't available when the dam was built, should be considered first so that the water level of the dam can be kept lower without affecting water supply.
About 73,000 people are living in flood-prone areas in the Hawkesbury-Nepean valley.
Dr Wright said that the infrastructure was not in place to allow people to live on the floodplain.
"I think we're losing that cultural awareness of what life on a flood plain is like, while we have had rampant expansion of areas of Sydney that perhaps they shouldn't have," he said.
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2021-03-22 05:07:24Z
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