Recent downpours in Sydney have stirred up debate about a $690 million plan to mitigate flood risk by raising the wall of the Warragamba Dam, a project that could threaten Aboriginal heritage sites and endangered species. By Drew Rooke.

Flood dangers and the Warragamba Dam project

Warragamba Dam on February 10, after heavy weekend rainfall increased water levels by more than 20 percentage points.
Warragamba Dam on February 10, after heavy weekend rainfall increased water levels by more than 20 percentage points.
Credit: AAP Image / Joel Carrett

“It has flooded in Penrith before,” the huge billboards warned. “It will happen again.”

The public safety campaign, launched in September last year by the New South Wales government and the State Emergency Service, at a cost of nearly $800,000, aimed to raise awareness about flood risk in Western Sydney.

As the state sweltered through a dry summer, buffeted by devastating bushfires, the admonitions seemed out of place.

Then, two weeks ago, they came to fruition as a huge storm battered Australia’s east coast. Some 400 millimetres of rain fell across Sydney in three days, increasing the capacity of Warragamba Dam from 43 per cent to 71 per cent. Flood evacuation orders were issued as the Hawkesbury River reached a peak of more than nine metres above mean sea level at the town of Windsor and major bridges were closed in the Hawkesbury–Nepean Valley.

Due to its unique topography, this valley – stretching from the foothills of the Blue Mountains and across Western Sydney – is vulnerable to extreme flooding. In it, a series of narrow sandstone gorges along the river system constrict the flow of floodwater from the five major tributaries, causing it to rapidly back up and spill across the floodplain. There are few evacuation routes for the 134,000 people who live and work in the area, a figure forecast to double over the next 30 years.

The recent downpour has stirred debate about the government’s flood strategy for the valley; namely, the $690 million plan to raise the Warragamba Dam wall by 14 metres. According to the strategy, this would reduce overall flood damage in the valley by 75 per cent.

But there is strong resistance.

If the current plan proceeds, up to 4700 hectares of the World Heritage-listed Blue Mountains – and 65 kilometres of wild rivers upstream of Warragamba Dam – could be underwater for up to five weeks in the event of a major flood.

This area holds at least 50 recognised heritage sites of the Gundungurra traditional owners, who lost much of their ancestral land when the Warragamba Dam was originally built, causing flooding in the Burragorang Valley. According to Gundungurra elder David King, the fact the government is even considering raising the wall, given the damage it will cause to his remaining ancestral lands, is “a slap in the face”.

“They do token acknowledgements and get us in for welcomes and then they just go and flood your Country,” he says. “Culturally, it hurts. There’s no respect.”

The area at risk of flooding is also critical habitat for at least 48 threatened animal and plant species, including the regent honeyeater and Camden white gum. Even temporary inundation of habitat could push many of these species closer to extinction.

In light of these issues, a group of scientists, former Environment ministers and conservationists wrote to New South Wales Premier Gladys Berejiklian in October 2018, calling on her to dump the plan to raise the dam wall. They came with their own flood warning: an inundation of the kind being proposed would result in “extensive and irreversible damage to the integrity of the greater Blue Mountains world heritage area”.

The United Nations’ World Heritage Committee echoed this view in July 2019, expressing concern at the plan and its impact.

But the government also has to weigh the cost of not acting. Its own flood strategy says that if an event equivalent to the 1867 flood – which created an inland sea in the valley and claimed 20 lives – were to occur, about 12,000 properties in the Hawkesbury–Nepean Valley would be affected. Some 90,000 residents would be forced to evacuate.

Stuart Ayres, the minister for Western Sydney, told The Sydney Morning Herald that the latest flooding was “a warning shot”. If the dam had been fuller, he said, residents in the valley below would have “been in a lot of trouble”.

But flood expert Professor Jamie Pittock from the Fenner School of Environment and Society at the Australian National University says the latest flooding is a reminder that the plan to raise the dam wall is “foolish”.

“What we’ve seen even with this modest flooding is serious disruption to the residents of Western Sydney, even though not a single drop of floodwater flowed past Warragamba Dam,” he tells The Saturday Paper.

Pittock says raising the dam wall will only mitigate small-to-medium sized floods that originate upstream. If an extremely large flood occurs, as is increasingly likely due to climate change and altered rainfall patterns, there is still a high chance the dam will rapidly reach capacity and need to be spilled.

“No dam will ever stop all floods forever,” Pittock says, calling the plan a “classic example” of relying too heavily on infrastructure to mitigate flood risk.

He points to the 2011 Brisbane floods as a similar planning failure.

Before that disaster, which affected more than 200,000 people and cost insurers more than $2.5 billion, Queensland’s Wivenhoe Dam had been kept at half capacity to reduce the risk of severe flooding. But it wasn’t enough: the dam quickly filled during heavy rainfall and had to be spilled.

At one point, the equivalent of 6000 swimming pools of water was pouring through the wall every second, causing the Brisbane River to swell rapidly to over 10 metres.

Pittock fears the plan to raise the Warragamba Dam wall will be “used as an excuse for more suburban development” in low-lying, high-risk areas of the Hawkesbury–Nepean Valley.

Barry Calvert, mayor of Hawkesbury City Council, shares Pittock’s concerns. At a recent hearing of a parliamentary committee currently inquiring into the Warragamba plan, Calvert said: “I am personally concerned that raising the dam wall will mean it will shut debate down on the whole issue; people will be relaxed and say, ‘It’s all over now. We’ve raised the dam wall. You can relax.’ That will be a very dangerous position for us to be in.”

According to Pittock, the state government should be investing in a range of alternative long-term options to reduce the flood risk in the valley, such as upgrading major roads to ensure those currently living in high-risk areas can evacuate, and prohibiting more residential development in those same areas. Or, as Pittock puts it, “to stop putting people in harm’s way”.

The government could also choose to lower the water limit of Warragamba Dam by 12 metres to create extra space for floodwaters without damaging the environment. Research at the University of Technology Sydney has found that coupling this with the use of current and new desalination plants would not compromise Sydney’s water security and would be more cost-effective than raising the dam’s wall.

Another option is to relocate those residents living in the most high-risk areas to higher ground. This occurred after the 2011 Queensland floods: 100 residents in the town of Grantham were relocated, and were unaffected by further flooding two years later.

The environmental impact statement for the plan to raise the dam wall was scheduled to be publicly released in September last year but has still not been completed. Pittock hopes this delay is because the government is “taking their time to get it right … asking if there is a better way to spend $700 million”.

But then there’s the pessimistic side of him, which worries it is just because “they are busy trying to find ways to justify what’s indefensible”.

This article was first published in the print edition of The Saturday Paper on February 22, 2020 as "Flood ties".

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