Thousands of Sydneysiders have been told to be ready to evacuate after Manly Dam threatened to overflow on Tuesday amid heavy rain and thunderstorms.
A red level alert was issued for the dam in Sydney's north, prompting a "get out now" evacuation order for 800 homes in the low-lying areas below. An hour later, after a drop in water levels, the warning was downgraded to amber and nearby residents were told to be ready to evacuate.
"The rain and thunderstorms have not finished yet," SES Commissioner Carlene York said. "Today there is a chance that [the dam] may go over that area and we may go back up into that evacuation order."
As of around 3:30pm, between 50 to 100mm of rain had been recorded in the northern suburbs, pushing up towards the Central Coast, since 9am, according to the Bureau of Meteorology (BOM).
A flood warning for homes and businesses near Narrabeen Lagoon has also been issued, with residents advised to evacuate via Pittwater Road by 7pm.
With heavy rain set to continue across the city, here's what you need to know about Sydney's dam system and what you can do to be prepared.
What happened at Manly?
Unlike the huge Warragamba Dam which supplies most of Sydney's water, Manly Dam and its surrounding bushland is now primarily used for recreation, including swimming, water skiing, kayaking and bushwalks.
The dam, located near Manly Vale and Allambie Heights, was built in 1892 to supply water to Manly and surrounding areas and was repeatedly expanded as demand in the area grew. By 1929, due to rapid population growth, its use as a water source was slowly phased out.
It's now heritage-listed as an example of dam building in the 19th century.
On Tuesday, with relentless thunderstorms and heavy rain, it started to fill up. The NSW SES received information that the dam had reached red level, prompting an evacuation order. But while the details were confirmed, SES spokesperson Greg Nash says they received word that "the data was not actually quite as high as anticipated".
The current amber level means the SES is continuing to monitor the inflow of water into that dam. "Should it reach a point again where it meets the criteria where it's going to overtop then we will issue a warning appropriately," Nash says.
"Overtopping" refers to a situation where the entirety of the dam starts to spill over, Nash explains, "when there is simply too much water behind the dam, it goes all the way over, and there's a lot of flow downstream."
Commissioner York told the Sydney Morning Herald on Tuesday there was no risk of the dam wall breaking.
What about Sydney's other dams?
There are 21 storage dams in Greater Sydney managed by WaterNSW, 11 of which are considered major.
All but two of the major dams in Sydney have reached 100 per cent capacity, according to WaterNSW, with Prospect Dam in Sydney's west and Blue Mountains Dams at 97.4 per cent and 99.9 per cent full respectively.
Capacity of Sydney's major dams
- Warragamba Dam: 100 per cent
- Woronora Dam: 100 per cent
- Avon Dam: 100 per cent
- Cataract Dam: 100 per cent
- Cordeaux Dam: 100 per cent
- Nepean Dam: 100 per cent
- Prospect Dam: 97.4 per cent
- Wingecarribee Reservoir: 100 per cent
- Fitzroy Falls Reservoir: 100 per cent
- Tallowa Dam: 100 per cent
- Blue Mountains Dams: 99.9 per cent
The biggest of all is Warragamba Dam in south-western Sydney, which supplies more than 80 per cent of Sydney's water. It's been flooding since last week and continues to spill.
Last week the dam was spilling at a rate of more than 300 gigalitres a day, with the March 2021 peak set at 440 gigalitres per day.
In the 24 hours to 9am on Tuesday, 48.3mm of rain has fallen on the catchment. Since then, rain has continued to batter Sydney with flash flooding across Sydney's west, south-west, and north-west as well as the northern beaches, with heavy rain likely to continue till early on Wednesday.
Will more dams 'overtop' in coming days?
All dams in New South Wales could be at risk of overtopping, Nash says, due to the highly localised nature of the storms.
"Because the ground is already soaked, even a small intense rain period if it's in the right catchment area for any dams, can cause a rapid inflow into the dam system or into the river system, which will then travel downstream and potentially cause issues," he says.
"There are procedures in place where we have certain trigger points in place for certain dams … if we get these triggers and notifications, then we have plans that we can instigate. "
He advises anyone in low-lying areas or near dams and rivers to have an evacuation plan ready to go. "Know where to go and what to take with you, that's half the battle," he says.
