Cosmos Q&A: NSW flooding

NSW is currently experiencing severe flooding events, after a weekend of heavy rainfall saw flood and evacuation warnings and bulletins issued in several regions along the full length of the state’s east coast.

In the Sydney region, extreme rainfall caused the Hawkesbury and Nepean rivers to swell and Warragamba Dam, which contains the city’s main drinking water supply, to overflow.

Fast facts: NSW flooding

  • Approximately 65 km west of Sydney, Warragamba Dam provides 80% of the city’s drinking water.
  • The last time the dam overflowed significantly was in August 1990.
  • On Saturday 20 March, the dam was reported to be at 99.2% capacity – then was hit by over 150mm rain in the 24 hours from Saturday to Sunday, and the rains continue.
  • The dam is now overflowing, contributing to flooding of the river systems.
  • WaterNSW is reporting that Warragamba Dam is spilling around 500 gigalitres per day – about the volume of Sydney Harbour. It’s also twice the average daily flow of Niagara Falls and five times the average daily flow of Victoria Falls.

The greatest threat is to the Hawkesbury-Nepean Valley, which is seeing its worst flooding since 1961, with peaks expected to reach up to 15 metres. Thousands of people have been evacuated as homes and properties flood, some up to roof height; emergency crews have so far responded to more than 8,000 calls for help.

“Extensive outages of water, electricity, sewerage, telecommunications and gas are expected to last many weeks or months,” the NSW SES said on Monday.

“This is about a one-in-50-year flood,” says Stuart Khan, a water engineer at the University of NSW. “We expect them, we’ve had them before, and we certainly know that we’ll have them again.”

Cosmos spoke to Khan and several experts from Risk Frontiers to gain understanding of the current floods, whether they could have been prevented, and how to manage flood events in the face of a changing climate.

Why is this NSW region flooding?

Sydney is a low-lying area, surrounded by three rivers: the Hawkesbury to the north, the Nepean to the west and the Georges to the south (the Nepean is a Hawkesbury tributary).

Rainfall travels down the rivers into the oceans, but during big rain events the rivers struggle to cope with runoff volume. The area can fill up like a bathtub as water tries to escape through the relatively small plugholes provided by the rivers.

The Hawkesbury-Nepean Valley is particularly flood prone and has been for a long time.

“Extreme floods have been recorded in the valley since the earliest years of British occupation over two centuries ago,” says Jamie Pittock, from the Fenner School of Environment & Society at the Australian National University (ANU).

The intense rainfall over the weekend was prompted by several big, interacting weather systems. A low-pressure trough off the coast of NSW and a cloud band coming down from north-western Australia are interacting to dump a large amount of precipitation over NSW. Another system of stationary high pressure in the southern Tasman Sea has also been driving onshore winds onto the NSW coast for a week, holding the low-pressure trough in place.

Forecasters predict that the rain should ease by Tuesday. Compared to the weekend it has already lessened – but what’s still falling is landing on already-saturated ground, at the end of a wetter-than-average La Niña summer.

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State Emergency Service vehicles approach flooded residential areas in western Sydney on March 20, 2021. Credit: Saeed KHAN / AFP / Getty Images.

How rare are flooding events like this and how is climate change affecting their frequency and severity?

Andrew Gissing, an emergency management expert with the Bushfire and Natural Hazards CRC and General Manager of Resilience at Risk Frontiers, says that this is not a record flood.

“The flood of record occurred in 1867, peaking at 19.7 m at Windsor,” he says. “It is likely this flood is to be one of the worst floods, however, in living memory.”

Still, many are saying that this flood is a once-in-a-generation event.

Thomas Mortlock from Risk Frontiers says terms like “once in 100 years” can be misleading.

“It suggests this type of event can only occur once in 100 years, and that when it does, we will wait another 100 years until we experience this level of flooding again,” says Mortlock, who specialises in modelling coastal hazards. “Both of these are untrue.”

Flood events, he says, are in fact clustered in time due to oscillations like La Niña, which increase the seasonal probability of flooding.

It’s also worth nothing these floods are hitting a year after the devastating Black Summer bushfires. January 2020 was Australia’s hottest month on record, and in NSW, the average temperature soared to 6°C above the past century’s “normal”.

Though 2021 has been cooler and wetter due to the La Niña weather cycle, this does not mean a respite from natural disasters, as warming temperatures are increasing the likelihood of heavy rain and flooding.

“A warmer atmosphere also has the potential to hold more moisture,” Mortlock says. “Current projections are for a general drying of average rain conditions across Australia over the coming century, with a potential increase in extreme rain events.”

Australia’s latest State of the Climate report from 2020 confirms that we have seen an increase in the intensity of heavy rainfall events.

University of Melbourne School of Earth Sciences professor Todd Lane counsels a balanced view.

“This is a significant event, linked to the easterly wind flow transporting moisture from the Tasman Sea, which is interacting with inland troughs to produce large rainfall totals,” he says.

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This picture shows a street light pole under floodwaters along the overflowing Nepean river in Penrith suburb on March 21, 2021. Credit: Saeed KHAN / AFP / Getty Images.

“It is unusual in terms of rainfall accumulations, but we should be cautious about linking this event to climate change. There is still much uncertainty about future rainfall in the NSW region and climate models show limited agreement in projected rainfall and extreme rainfall trends.

“Importantly, little is known about how the frequency of weather events like this one will change with global warming.

“What we do know is that in a warmer world the atmosphere can hold more moisture, so as the climate continues to warm events like this have the potential to produce more rain when they occur.”

Was the NSW flooding preventable?

Concerns have been raised about the high-water levels of the Warragamba Dam and whether they should have been lowered to prevent spillover.

However, as Khan explains: “Warragamba Dam was built as a drinking-water supply. [It] doesn’t have any role in flood mitigation…and nobody has done anything wrong by not operating it that way.”

WaterNSW is not authorised to release water based on weather forecasts. In any case, to have enough capacity to capture the inflows from this weather event, Warragamba Dam would have had to reduce its levels to 25%. For comparison, the lowest it’s ever been was 38.8% in 2004, during the Millennium Drought.

Khan explains that other dams in Australia, such as the Wivenhoe Dam, about 80 km from central Brisbane, “have been designed and are operated partially as flood-mitigation dams, where they maintain a large volume of empty air space in the dam in order to be able to capture those big flood events when they come and then release them slowly”.

He has been warning the state government since at least 2015 that Warragamba should be operated in a similar way; that is, it should not be allowed to fill to capacity – it ought to retain space for flood mitigation in heavy, prolonged downpours.

However, it’s important to note that only about half of the water responsible for the flooding comes from the spillover at Warragamba. The other half comes from other rivers flowing into Hawkesbury-Nepean system, including the Colo, Grose and Macdonald rivers.

Mortlock raises the point that urbanisation may have also played a role in the severity of the flood: “The growth in impervious surfaces in urban areas can result in more run-off into creeks and rivers.”

But he adds that the true severity of the flood is due to the prevailing wetter-than-average conditions, spanning many days, not allowing the catchments to discharge.

As the world warms, what can we do to prevent and manage future floods?

Part of Warragamba Dam’s capacity could be reserved for flood mitigation, as Khan suggests – but then there are implications for the supply and security of Sydney’s drinking water.

“For drinking-water supply, we should be thinking about how we can incorporate as much climate-independent water supply as possible – things like recycling and desalination that are not significantly prone to impacts from drought,” Khan says.

If we don’t rely on one big dam but rather multiple sources, he says, “you’re in a much better position to be flexible and manage the water according to where difficulties might be incurred, and therefore have much greater resilience in being able to continue providing high-quality drinking water”.

One controversial proposed solution is to increase Warragamba’s capacity. The NSW government has ambitions to raise the dam wall by 14 m, which would hold an extra 1000 GL in the dam. But spillover would still occur: models predict that in this case, 500 GL would still spill downstream, Khan says.

“It all depends on the timing of whether or not that happened to coincide with a peak of water coming from the other tributaries,” he explains. “If it did, you’d still end up with the same flood peak, the same flood height – it would just be delayed by a couple of days.”

So raising the dam wall isn’t a silver-bullet solution – but could be one of several options to providing more flood security.

Another is to get people out of harm’s way. About 5,000 homes are built below the 1-in-100-year flood line in the Hawkesbury-Nepean Valley and some 90,000 people live on the floodplain – with the population set to increase.

“We can think about going the other way, moving people off the floodplain and reserving that land for agriculture, recreation, and forestry,” Khan says. “Then we wouldn’t have an emergency when the floods occur as they’ve always occurred.”

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A rescue worker helps residents cross a flooded road during heavy rain in western Sydney on March 20, 2021. Credit: Saeed KHAN / AFP / Getty Images.

Other options include building channels to give the water a better escape route – as current bottlenecks in the river limit its discharge capacity.

“We think about engineering solutions to that,” Khan says. “We could be talking about building a channel, perhaps across the worst part of the bottleneck down around Sackville, and…short-circuiting that water out to the Hawkesbury River.”

Gissing agrees, noting that “raising the dam would assist to mitigate flooding but would need to be part of an integrated approach consisting of enhanced risk-based land-use planning, maintenance of flood warning systems, community education campaigns, maintenance of evacuation route capacity and emergency management capability and planning”.

These are important questions we must face, he says, because “flooding in Australia and NSW is more deadly and damaging than bushfires”.

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