Australia’s largest city is often described as being in the “Sydney basin” — particularly apt term right now, as it’s experiencing something known as the “bathtub effect”.
It’s what happens when a lot of water is tipped into a low-lying area (like Sydney) and it can’t escape quickly enough.
When it rains, the water that’s dumped on the ground eventually drains into rivers and ultimately the ocean.
If a lot of rain is trying to escape from somewhere at the same time, low-lying areas can fill up like a bathtub while the deluge makes its way through one small point (like a plug hole).
A quick geography lesson
Sydney is surrounded by three rivers: the Hawkesbury to the north, the Nepean to the west and the Georges to the south.
The abundance of water is one of the things that makes the Harbour City such a desirable place to live.
But when it rains a lot in a very short space of time, or steadily for several days, the rivers sometimes struggle to cope.
Authorities are particularly concerned about suburbs along the Nepean and Hawkesbury rivers (they actually connect to each other to form part of the same “system”) at the moment.
Tom Hubble from Sydney University, who has studied the area extensively, says the catchment that feeds the Hawkesbury-Nepean river system extends well beyond Greater Sydney.
“It encompasses a great deal of the Blue Mountains and much of the Southern highlands all the way out to Goulburn. There’s a large part of the catchment that feeds into the rivers around Sydney that people aren’t all that familiar with,” he said.
“Much of it is wild country, national parks and forests, with deep valleys and steep slopes which leads to rapid runoff.
“When a lot of water descends on this area, it makes its way to the ocean really quite quickly in two or three days.”
Professor Hubble said the Cumberland Plain area between the Blue Mountains and Paramatta was a natural depression in the landscape which had formed very slowly over the past 100 million years.
“It’s not like it can drop down a metre all of a sudden, but over geological time it has sunk a little lower than the surrounding areas,” he said.
“This has created a situation where the Nepean River Valley between Penrith and Sackville is a little bit like a bathtub.
“All of the water flowing out of the catchment has to exit to the sea through that depression.”
It’s happening right now.
According to modelling from the State Emergency Service (SES), there are three main “choke points” in this river system, which slow the water trying to escape to the ocean.
They’re at places where the rivers are particularly narrow around:
- 2.Emu Plains
While every flood is different, these points have historically been the first to be inundated when there’s too much water for the rivers to drain.
Eventually, the water levels go down, just like they do when you pull the plug out of a bath.
Since Friday, the Bureau of Meteorology has recorded more than 200mm of rain in Sydney, and with showers forecast until Wednesday, the “bath tap” will remain on for the moment at least.