Towns in Victoria, New South Wales and Tasmania have been hit by major flooding in the last week, with flooding particularly severe along the Goulburn, Loddon and Campaspe rivers, and on parts of the Murray River.
This visual guide shows how the flooding happened.
Before the floods
The main climate driver of the recent rain in the south-east is the negative Indian Ocean dipole (IOD), according to Dr Milton Speer, a visiting fellow at the University of Technology Sydney.
In a negative IOD – which has occurred in two consecutive years for the first time since reliable records began in 1960 – warm water concentrates in the eastern Indian Ocean, leading to moisture-rich air flows towards Australia.
“Low pressure develops over south-east Australia because of favourable conditions in the upper atmosphere,” Speer said, which results in rain inland of the Great Dividing Range.
Last week, a low pressure system travelled east over the continent, bringing the rain which hit southern Australia from 12 October onwards:
Speer, who has worked as an operational meteorologist with the Bureau of Meteorology (BoM), said: “When you get these cloud bands that develop, they give rain to South Australia, western NSW and Victoria. Typically their source of moisture can either be from the Tasman Sea but also from the Indian Ocean.”
The La Niña event, which is under way for the third year in a row, is an important driver of rain in spring and summer. But Speer said “the Indian Ocean dipole is really the main cause of the [recent] inland drain.”
His research has found that global sea surface temperatures and Tasman Sea surface temperatures are also factors that affect rainfall over the northern Murray-Darling Basin.
During the rain
The resulting rain fell particularly heavily over central Victoria, with large amounts falling in the catchments of the Loddon, Goulburn, Campaspe and Avoca rivers, along with a number of creeks in the region.
Heavy rainfall also fell in a number of dams and reservoirs on these rivers.
While the amount of rain was less than the huge volumes seen in northern states earlier in the year, the situation in Victoria was made worse by the ground already being saturated. The state had experienced its wettest August since 2010, and September rainfall was above average across most of northern Victoria.
The following map shows the “root-zone soil moisture”, which is a measure of how wet the ground was on 13 October. When the soil is saturated it can’t hold as much water after rain which increases runoff into creeks and rivers, making flooding more likely.
Dr Margaret Cook is a lecturer at the University of the Sunshine Coast who specialises in the history of natural disasters. She said already high rivers, creeks, and sodden catchments had “very little capacity to absorb the rain that they’re receiving now, which means it becomes runoff pretty much straightaway”.
Flooding downstream
After the initial rainfall, it took some time for the large volume of water to make its way through the system of rivers and creeks that feeds into the Murray River.
The area north of the Great Dividing Range in Victoria is “a vast floodplain”, Cook said. “That area is unbelievably flat. It’s a very complicated system of rivers and creeks that all feed into the Murray-Darling system.
“The water is going to feed down and eventually end up at the Murray mouth at South Australia.”
Riverine flooding can be deceptive, Cook said, because it can occur from the flow of water upstream even after rain has stopped. “People think the disaster is over, but in fact it’s on its way,” she said. “You get your sunny day with the flood coming through.”
The next graphic is an animation based on river gauge data which illustrates how the flooding progressed downstream from the rivers’ sources in the mountains.
It shows a simplified view of three rivers leading into the Murray, and the towns which have been affected by flooding on these rivers. Each square indicates the level of water measured by a gauge in that location at that time, with the volume of particles also based on this value.
The result of the downstream flows illustrated above has been severe flooding in a number of locations. The figure below shows the river heights at four locations compared with the minor and major flooding thresholds for those areas:
Gauge data sourced from the Victorian Department of Environment.
Flood level data is from the Bureau of Meteorology.
River map is based on earlier work about the Murray-Darling Basin and benefited greatly from this tweet by Peter Bannan from the Swan Hill Guardian.
