Sky News Chief Meteorologist Tom Saunders says the “Bathtub Effect” often causes the devastating floods in the Hawkesbury-Nepean region.
“Five major tributaries act like taps, pouring water into the valley during a flood,” he says.
“While every flood is different, the Warragamba River contributes up to 70 per cent of the water in major floods that have happened in the past 60 years.
“All this water needs to get to the sea, but natural choke points – where the river narrows – slows the path in this valley.
“This is called the ‘Bathtub Effect’, where water backs up and starts filling the flood plain because there’s only one plug hole letting the water out.”