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Toowoomba copped wrath of La Nina

A ONCE-IN-A-CENTURY storm and an already saturated and hilly landscape were the ingredients for the tragedy that engulfed Toowoomba.

On Monday, a series of storms, described by the Bureau of Meteorology yesterday as a super-storm, crossed the coast at a point just north of Noosa and headed in a line towards Toowoomba.

Roger Stone, the director of the Australian Centre for Sustainable Catchments and also a Toowoomba resident, said the line of storms was caused by the La Nina weather pattern in combination with a very warm Coral Sea.

As it travelled south, it hit the Great Dividing Range at Toowoomba and the elevation "enhanced the storm".

Professor Stone said his rain gauge showed 100mm fell in less than an hour, but there have been reports from Toowoomba of falls of more than 200mm in an hour.

Because Toowoomba, like most of southeast Queensland, had been primed by weeks of heavy rain, the soil could not take any more moisture and all of the deluge turned into runoff.

"All the streets and gullies in Toowoomba's suburbs effectively became creeks," Professor Stone said.

Toowoomba sits on top of the Great Dividing Range at a very steep point facing east.

From the top of the range, the city slopes back westwards, so that its central business district lies in a small valley.

There are two watercourses in the town: East Creek and West Creek. The place where they merge was the site of the original European settlement, which was chosen as a source of water. Both creeks are at the bottom of the landscape, and when the storm hit on Monday water ran straight towards them.

After the hour or so of heavy rain, both East Creek and West Creek were running rapidly. But the real danger came where the two run together, at a point near the central business district. It was this confluence of waters that led to the flash flooding that devastated the centre of the city.

Toowoomba is 700m above sea level. To the east, Withcott, at the foot of the range, is 270m above sea level, and Grantham, a further 30km further east is at 110m.

The vast majority of the water that flooded the Toowoomba CBD will flow west, and end up in the Murray-Darling system.

But some of the water that hit the northern part of Toowoomba flowed over the range and down Murphy's Creek, which was one of the worst-hit spots.

This water flowed into Lockyer Creek, then hit Grantham and Gatton.

Lockyer Creek flows directly into the Brisbane River just below Wivenhoe Dam, but many of the other watercourses in the Lockyer Valley flow into the headwaters of the Bremer River, which flows through Ipswich.

They were the foundations of the flood that hit Ipswich yesterday, although this situation was exacerbated by rainfall of more than 200mm in the Lockyer Valley yesterday.

That water in turn flows into the Brisbane River, and at a point that is below the Wivenhoe Dam, and this flows into Brisbane before finally going out to sea.

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Anatomy of a flash flood

Anatomy of a flash flood
Source: The Australian