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NSW floods break 120-year-old rain records during March rain event, BOM says

Monday April 19, 2021 - 22:56 EST
ABC image
The March floods were significant in the duration and extent of rainfall. - ABC

The entire New South Wales coast experienced its wettest week ever during last month's floods, according to a (BOM).

Extreme multi-day rainfall hit much of eastern Australia in late March, but it was NSW that bore the brunt.

BOM senior climatologist Blair Trewin said about 81,900 gigalitres of water fell on the state in a week.

Sydney Harbour, Dr Trewin said, held about 500GL.

"So the total amount of rain fallen on NSW equalled approximately 160 Sydney Harbours," he said.

Biggest floods on record

Flooding reached record heights on the Hastings, Manning and Camden Haven rivers on the Mid North Coast, while the Hawkesbury-Nepean catchment experienced its most significant flooding for more than 30 years.

A number of locations received more than 300 millimetres of rain in a day.

The highest total was 405.5mm on March 20 at Kendall, near Port Macquarie.

But that was not so rare, according to Dr Trewin, who said falls of more than 400mm occurred every couple of years in NSW.

What was unusual was how long the rain persisted.

"None of the individual seven days in the week were even in the top 50 wettest days on record for coastal NSW, but we just strung that long sequence of wet days together," Dr Trewin said.

Comboyne, also near Port Macquarie, received a record 943mm for the week ending March 24, one of a number of Mid North Coast locations that set weekly records.

Echoes of 1929 disaster

Multi-day records in the region were second only to February 1929, when the worst floods in recorded history struck the Mid North Coast.

"When we look at things like catchment-averaged rainfalls, the 2021 event lagged behind 1929 at all-time scales," Dr Trewin said.

"The biggest difference between 1929 and 2021 was the 1929 event was much more localised ? 1929 was much less significant to the north and to the south than 2021."

The March 2021 record rain was associated with a blocking high-pressure system in the Tasman Sea that drove a moist, easterly flow over the NSW coast.

Blocking highs move slowly and keep weather systems in place.

At the same time, a low-pressure system off north-west Australia fed a large volume of moist tropical air into the east.

"When you compare this with a lot of the most significant floods in coastal NSW, they typically come from east coast lows or easterly troughs," Dr Trewin said.

"Now, those can produce very intense rain, but they're usually pretty small and they generally don't hang around for a week like this system did."

Heavy rainfall extended from central Australia to northern inland NSW during that week, resulting in significant flooding on some inland rivers in the state's north and southern Queensland, including major flooding on the Gwydir, Mehi, Macintyre and Condamine rivers.

The floodwater moved slowly downstream into the Darling catchment during April.

The northern Murray-Darling Basin catchments saw two of their three largest one-day increases in water storage levels since 1993.


© ABC 2021

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