Wivenhoe dam release 'will be controlled and safe'

MORE than a million megalitres of water will be let out of the Wivenhoe dam into the Brisbane River over the next week.

MORE than a million megalitres of water will be let out of the Wivenhoe dam into the Brisbane River over the next week.

The equivalent of two Sydney Harbour catchments, will be let out in a controlled release that authorities said would not contribute to flooding.

SEQ Water Grid CEO Barry Dennien said that, over the next seven days in accordance with policy, Wivenhoe dam's level would be reduced from 190 per cent to 100 per cent.

"It's a precautionary release from the flood compartment of the vast dam which acts as a flood barrier to Brisbane," he said.

The release would be controlled and safe and would not contribute to flooding, Mr Dennien said.

Wivenhoe dam's ability to store floodwater has remained unchanged for more than 10 years, despite advances in research upgrading the size of potential floods, according to government reports and the water grid's own fact sheets.

And a dam engineer who helped upgrade the dam in 2004 and praised its performance said the authorities' ability to manage floods could be slightly improved if bridges below the wall were lifted to allow earlier release of greater volumes of water.

South East Queensland Water, which manages the giant dam, yesterday listed water flows over the wall at 228,000 megalitres per day, representing a dramatic fall from the peak of Tuesday night of 645,000 megalitres and said dam levels had fallen from 191 per cent to 186 per cent.

Despite the substantial drop in flow rates, vast quantities of water were still going into the Brisbane River, with the current daily releases equating to the amount that would fill some 91,200 Olympic swimming pools.

SEQW said after the downstream peak in the river, releases would be increased to 301,000 megalitres.

Meanwhile, SEQW's website reveals the dam's flood capacity is 1,450,000 megalitres, the same amount as listed for the dam in a 2001 paper that considered options for making it safer in response to new flood research that found a probable maximum flood was significantly larger than originally calculated.

Following the new calculations, the dam was then upgraded in 2004 to make it safer by augmenting a secondary spillway that featured fuse plugs that can gradually collapse under rising water pressure.

But Queensland flood engineer Greg Roads, who was part of the design team for the upgrade, confirmed that, while it had made the dam safer, it did not increase the capacity because the crest of the dam had not changed.

Mr Roads said one of the few things authorities could do to improve flood management involved lifting several key bridges downstream.

"They (authorities) do hold water back to prevent bridges from going under, and you would be able to let out water a bit earlier, but again it's small bikkies in a flood this size," he said.

Mr Roads said the management of the dam had been excellent. "We've been watching it the whole time and I think they have done a superb job on the way it occurred with two flood events in the upper catchment and then a massive flood in the Lockyer Creek and the Bremer River," he said.