Brimming Wivenhoe Dam has capital at its mercy

THE Wivenhoe Dam's ability to provide expansive flood protection to the city of Brisbane and surrounding districts was failing yesterday.

THE Wivenhoe Dam's ability to provide expansive flood protection to the city of Brisbane and surrounding districts was failing yesterday.

Waterflows had reached unprecedented highs and authorities drastically increased releases to ensure the dam could be safely managed.

The dam has so far been credited with extensively minimising flooding with releases being timed to avoid clashing with high tides that can back up the Brisbane River.

Late yesterday, South East Queensland Water revealed the dam was holding what was classed as "190 per cent" with its maximum capacity being 200 per cent, and that water releases had been increased to 490,000 megalitres a day -- a rate due to rise.

The announcement came shortly after Brisbane City Council Mayor Campbell Newman warned that the dam could no longer protect the Queensland capital.

"The dam is full. Every bit of rain that falls on the catchment can get to Brisbane, and there is not much more we can do about that," he said. "They now have to discharge that water . . . because more is on the way.

"Unfortunately, the big shock absorber that is that dam is now full.

"Whatever happens in that upper catchment is going to have a significant impact on Brisbane."

SEQW, however, declined to say at what point water would go over the top.

The dam has an emergency fuse plug that would allow water to pass over a part of the wall and ease pressure to ensure the dam doesn't burst but would be out of authorities' control.

The Wivenhoe Dam was originally a 1165GL water storage facility. The wall is an earth-rock embankment with a concrete gravity spillway controlled by five radial gates, which are 12m wide by 16m high.

A spokesman for SEQW late yesterday told The Australian that the fuse plug had not yet been activated but declined to say how much higher the water would have to go before that might occur.

A fuse plug is a structure located on top of a spillway that can gradually collapse under the force of rising dam water and ease pressure on the wall.

Earlier in the week, Premier Anna Bligh said: "We are seeing 1 million megalitres, or two Sydney Harbours, flow into the Wivenhoe catchment every day."

Meanwhile, government sources told The Australian that waterflows in the Brisbane River were already close to rates associated with the catastrophic 1974 floods, which the dam was built to alleviate.

"The Brisbane River was flowing at around 9500 cubic metres a second in the 1974 floods and it was at 7000 cubic metres a second today," said a government source.

The source said "we've got predictions of 12,000 cubic metres a second on Thursday". However, long-term Brisbane flood expert, water engineer Colin Apelt, yesterday said the dam would continue to alleviate the flooding but to what level was dependent on the manner in which it was controlled.

"The information I have heard is that the inflows coming into the Wivenhoe are certainly as large or larger than what occurred in 1974.

"If we hadn't had Wivenhoe in place we would certainly be facing a flood like 1974," said Professor Apelt, whose neighbours at Indooroopilly were in the process of evacuating to his property.

"It's protecting us from something worse and continuing to do that. As bad as it might get, it would have been a lot worse if Wivenhoe was not operating."

But he warned Brisbane was in unknown territory.

"We have had heavy rainfall in the catchment in the last 12 hours or so and that's coming down in the Brisbane River and that would be one reason why the release rate is increasing," he said.

SEQW said at 6pm last night that the controlled releases through Wivenhoe's five radial gates were expected to increase.

"Releases are being made in consultation with the Bureau of Meteorology and local councils and an effort to limit downstream impacts where possible," it said.

"Note these large releases are necessary for the safe management of the dam."