'Wettest since 1900': flood catastrophe sets rainfall records

A host of new rainfall records were set during the extreme flood event in Queensland and NSW from February 22 to March 9, with more than 50 sites recording over a metre of rainfall in a week, the Bureau of Meteorology (BOM) says.

The Bureau’s latest Special Climate Statement dramatically paints a picture in numbers of the devastating and deadly floods, which came with record insured losses bill of more than $3.4 billion.

The intense and sustained rainfall led to flooding from Maryborough in Queensland down to Grafton in NSW. Many areas received more than half their average annual rainfall total in just a week.

Across southeast Queensland and northeast NSW, rainfall in the last week of February was up to five times the monthly average to be “the wettest week since at least 1900” for large areas, BOM says.

The Mary River at Gympie recorded its highest flood peak since 1893, hitting a height of 22.96 metres, and there was major riverine flooding in the Sunshine Coast region, and Brisbane, Logan and Albert River catchments. In parts of northeast NSW, peak flood levels broke previous records, dating back more than 100 years in some locations, by “considerable margins”.

BOM says in recent decades, there has been a trend towards a greater proportion of high-intensity, short-duration rainfall events, especially across northern Australia.

The intensity of shorter, hourly extreme rainfall events is up 10% in some regions, especially in northern Australia.

“As the climate warms, heavy rainfall events are expected to continue to become more intense,” the statement said. “A warmer atmosphere can hold more water vapor than a cooler atmosphere, and this relationship alone can increase moisture in the atmosphere by 7% per 1 degree Celsius of global warming.”

Increased atmospheric moisture can also provide more energy for some processes that generate extreme rainfall events.

On February 28, NSW’s Lismore suffered devastating flooding as the Wilsons River hit a record 14.4 metres, exceeding the former record peak set in 1954 by over two metres, and Coraki and Woodburn on the Richmond River and Murwillumbah and Tumbulgum on the Tweed River were also inundated.

The Hawkesbury-Nepean catchment recorded its wettest two-week period since records started in 1900, while the lower Logan River saw the worst flooding since January 1974 – topping that caused by ex-Tropical Cyclone Debbie in March 2017.

Following two years of La Niña conditions, the rain fell on catchments that were already wet and quickly became saturated.

Many rainfall records were set across greater Brisbane, where the city gauge recorded 792.9 mm in six days – 78% of the annual average. The highest daily total set a record of 228.4 mm and there were three consecutive days with totals over 200mm.

“The Brisbane city gauge set records for all periods from three to seven days,” BOM said.

There were several floods along the Brisbane River and Australia’s highest daily rainfall total of 907mm was set at Crohamhurst. The Brisbane City gauge recorded 887 mm in the month of February, only behind 1025.9 mm in the same month in 1893.

“Averaged over the Brisbane River catchment, the rainfall in the last week of February was the highest on record (since 1900) for all periods from one to seven days, exceeding totals during the January 1974 floods,” BOM said.