Massive runoff too great for land that could bear no more

This satellite image shows more bad weather heading towards Queensland yesterday. Photo / Supplied

The raging floodwaters in Toowoomba and Brisbane were the product of a land that has had too much to bear.

The land can take no more water, and dams have reached their capacities: there is nowhere else for massive runoffs to go but across sodden catchments that hold much of Queensland's towns, cities and industries.

"You had all the worst scenarios, all the worst possible conditions." Hubert Chanson, professor of hydraulic engineering at the University of Queensland's School of Civil Engineering, said.

The first element has been the rain that has hammered Queensland. The Bureau of Meteorology said the state's wettest spring on record was followed by its wettest December.

Many catchments were soaked even before the flooding rain, drenched by the strongest La Nina since the 1970s.

Many rivers were at or near flood levels when the rain arrived this week.

"We need to keep in mind that we are in a situation where it has rained so much that our catchments are soaked and anything that is falling from the clouds is run off directly," Chanson said. "That is true of nearly all - 60 to 80 per cent - of Queensland."

Monday's disastrous rain fell at the worst possible spot: on the crest of the northern end on the Great Diving Range, where Toowoomba sits at about 700m above sea level, at the junction of the two streams that flow west to form Gowrie Creek.

To the east is the steep drop to the Lockyer Valley, a 2800sq km circular basin that lies between Toowoomba and the coast, draining into the Brisbane and Bremer River basin and out to sea.

Almost 300 floods have been recorded in the basin since the 1820s - 58 in the past 60 years, including the major floods of 1974 and 2008 that inundated Brisbane and other major centres.

On Monday the runoff from intense rain streamed west into Toowoomba, where streams flow through culverts under the CBD.

The volume and force was too great for the system to handle, and water burst into the streets without warning, building its own deadly momentum as it raged down the impervious surfaces of the city's streets.

"The streets are smooth," Chanson said. "You don't have grass, you don't have trees, to stop the water, and the water picks up speed."

At the same time, massive flows poured down the eastern slope of the range, smashing through communities at its foot, and raging out across the Locker Valley and into the Brisbane-Bremer basin.

Yesterday afternoon floodwaters broke into Brisbane.

"This is a very, very unusual, very, very deadly event," Chanson said.