For the fourth time in 18 months, floodwaters inundated homes and businesses in Sydney’s western Hawkesbury-Nepean Valley. The recent torrential rains are obviously the immediate cause. But poor decisions by successive New South Wales governments have exacerbated the damage.
The town of Windsor in the Hawkesbury region suffered a particularly high toll, with dramatic flood heights of 9.3 meters in February 2020, 12.9 meters in March 2021 and 13.7 meters in March this year. year.
At the time of writing, flood heights in Windsor have reached almost 14m. This is still significantly lower than the monster flood of 1867, which reached nearly 20 m. It is clear that standard flood risk reduction measures, such as raising the level of building floors, are not safe enough in this valley.
We have known for a long time about the risk of flooding in the region. Yet successive state governments have failed to properly mitigate its impact. Indeed, the recent urban development policies of the current New South Wales government will multiply the risks.
We knew this was coming
A 22,000 square kilometer watershed covering the Blue Mountains and western Sydney drains into the Hawkesbury-Nepean river system. The system faces a extreme flood risk because the gorges restrict the flow of the river to the sea, often causing rapid filling of water in the valley after heavy rains.
Governments have known about the risk of flooding in the valley for more than two centuries. Traditional owners have known them for millennia. In 1817 Governor Macquarie lamented:
it is impossible not to feel extremely unhappy and indignant at [colonists] Stubborn stubbornness to persist in continuing to reside with their families, herds, herds and grain on those flood-prone places, and whence they have often seen their prosperity swept away.
Macquarie’s was the first in a long line of governments to do nothing effective to reduce risk. The latest in this undistinguished string is NSW Planning Minister Anthony Roberts.
In March, Roberts would have been revoked the directive of his predecessor to better take into account the risks of flooding and other climatic risks in planning decisions, to favor housing development instead.
Roberts’ predecessor, Rob Stokes, had demanded that the Planning Department, local governments and developers consult with traditional owners, manage climate change risks and make information about natural disaster risks public. This could have helped limit development in the floodplains.
Why are we still building there?
The Hawkesbury-Nepean Valley is currently home to 134,000 people, a population projected at double by 2050.
In the Valley, for example, billionaire Kerry Stokes’ company Seven Group is would have been a co-owner of nearly 2,000 hectares in Penrith Lakes near the Nepean River, where a development of 5,000 homes has been considered.
Planning in Australia often uses the 1 in 100 year flood return interval as the safety standard. It’s not appropriate. The risk of flooding in the valley increases with climate change, and development in the watershed increases the runoff velocity from paved surfaces.
The historic 1 in 100 year safety standard is particularly unsuitable in the valley, due to the extreme risk of rising waters cutting off low roads and completely submerging cut off residents in the event of extreme flooding.
In addition, a “medium” climate change scenario will see a 14.6% increase rainfall by 2090 west of Sydney. This is expected to increase Windsor’s 1 in 100 year flood height from 17.3m to 18.4m.
The NSW government is expected to impose a much higher standard of flood safety before approving new residential development. In my view, it would be prudent to only allow developments capable of withstanding the 20m height of the 1867 flood.
No dam can control the biggest floods
The NSW Government’s main proposal to reduce flood risk is to raise the Warragamba dam by 14m.
There are several reasons for this the proposal must be questioned. They understand the potential flood not only of cultural sites of the Gundungarra Nation, but populations of endangered species, and part of the Blue Mountains World Heritage Area.
Perversely, flood control dams and levees often lead to higher flood risks. This is because none of these structures stop the largest floods, and they provide an illusion of security that justifies riskier floodplain development.
The current NSW Transport Minister suggested such a development in the valley last year. A similar development occurred with the construction of the Wivenhoe Dam in 1984, which failed to prevent major flooding in Brisbane in 2011 and 2022.
These are among the reasons why the NSW Parliament select committee on the proposal to raise the Warragamba dam wall advised last October that the state government:
not to continue the project to raise the wall of the Warragamba dam [and] instead pursue alternative floodplain management strategies.
What the government should do instead
The New South Wales government now has an opportunity to overcome two centuries of failed governance.
He could take substantial steps to keep homes out of the floodplain and out of harm’s way. We need big new measures including:
- prevent further development
- relocate flood-prone residents
- build better evacuation routes
- lowering the water storage level behind the Warragamba dam.
The NSW government should help residents move out of the most flood-prone places and restore floodplains. This has been undertaken for many Australian cities, such as GranthamBrisbane and along the great rivers of the world.
Relocating residents is not easyand all current Australian buyout and relocation schemes are voluntary.
I think it is in the public interest to go further and, for example, to force the acquisition or relocation of those whose houses have been destroyed, rather than letting them rebuild in the wild. This approach provides certainty to those affected by flooding and reduces longer-term community impacts.
It is obviously ridiculous to rebuild on sites that have been flooded several times in two years.
The NSW government says a buy-back program would be too expensive. Yet the cost would be comparable to the roughly $2 billion needed to raise the Warragamba dam, or the $5 billion WestInvest government fund.
An alternative measure to raising the dam is to lower the water storage level in the Warragamba dam by 12 m. This would reduce the amount of potable water stored to supply Sydney and provide flood control space.
The city’s water supply would then have to depend more on the existing desalination plant, a strategy considered profitable and with the added benefit of building resilience to drought.
The flood damage seen in New South Wales this week was entirely predictable. Measures that can significantly reduce the risk of flooding are costly and politically difficult. But as the risks of flooding worsen with climate change, they are worth it.