Water releases before deluge too low: engineer

MORE serious questions about Wivenhoe Dam were raised yesterday by a senior engineer who claims the Brisbane River flooding was avoidable.

Volunteers remove mud and rubbish from St Lucia in Brisbane yesterday. Picture: Craig Greenhill

MORE serious questions about Wivenhoe Dam were raised yesterday by a senior engineer who claims the Brisbane River flooding was avoidable.

The engineer, Michael O'Brien, said the official data on water flows shows last week's Brisbane River flooding would have been largely avoided if the dam operators had raised their releases of water on the weekend before last Monday's deluge.

Mr O'Brien, and others not directly involved with the dam's operation, told The Australian that the river flood and the devastation of thousands of homes was inevitable after a decision to release relatively low volumes of water on Friday, January 7, and over the ensuing weekend.

The data shows that the dam went from a little over 100 per cent of its capacity on Friday, January 7, meaning it still had capacity for more than one million megalitres in its flood compartment, to about 150 per cent by the morning of Monday, January 10 - before the deluge hit.

Over that weekend and prior to the extreme rainfall event that would flood Toowoomba and the Lockyer Valley on Monday, the dam's operators released a total of about 200,000 megalitres.

Scrutiny of official water-release and dam volume data shows the flood would have been moderate at worst in Brisbane had there been larger releases in the days before the deluge. But it became extreme due to the sudden surge from the release of about 645,000 megalitres from Wivenhoe last Tuesday, which was about 30 per cent of the total capacity of a dam built to protect the city and surrounding suburbs.

This sudden release came because the in-flow of water from the dam's catchment meant its reservoir had risen to alarming levels of about 190 per cent and were closing in on the level that triggers an uncontrolled discharge.

Mr O'Brien, whose written review is based on publicly available data released by the Queensland government-owned dam's operators, SEQWater, said full disclosure was vital to reassure people that the dam was operated responsibly. But a more conservative approach over that crucial weekend would not have made any difference to the flooding that occurred in the Lockyer Valley and the city of Ipswich, both of which are in separate areas of the Brisbane River catchment.

SEQWater Grid chief executive Barry Dennien insisted last night that although the January 8-9 releases were relatively low compared with what occurred in the days afterwards, this was in accordance with the operating manual to mitigate flooding. He said that nobody had foreseen the extreme rainfall that ensued.

"We are very comfortable with the way the manual has been adopted and I would not be sitting here with this level of confidence if I did not think that everything happened the right way," Mr Dennien said.

"This is all about probability and what our people do is guided by a set of rules that they follow in the flood centre, and it is easy in hindsight to say they should have done things differently. We are proud of the way the dam and our people performed."

Mr Dennien said that if the dam's operators had pre-emptively released larger volumes from the flood compartment of the dam, which would have given the dam a much larger buffer for the rainfall that came, there would have been flooding in other parts of the catchment.

A senior hydrologist, Greg Roads, who helped design a recent  upgrade at the dam, and a Queensland disaster risk scientist, Ken Granger, said the experts operating Wivenhoe Dam were among the best in Australia and would have done everything by the book.

Mr Roads agreed that if the dam's operators had released more water on the weekend before the deluge there would have been significantly more capacity for flood storage in Wivenhoe and a lower river peak in Brisbane, but he said they had no way of knowing the severity of the weather that was coming.

"These questions about the dam's performance should be raised and answered and I support an inquiry," Mr Roads said.

He said he believed there would have been moderate inundation of some properties if the releases on the weekend had been directed at emptying the flood compartment as quickly as possible to increase its ability to store rainfall from an extreme event.

Dr Granger said that if the dam had been mismanaged "then that will come out in due course and procedures will be corrected, but these are very complex issues and the people making the decisions have all the information they need at that time as well as strict procedures".

Premier Anna Bligh, who has previously spoken about how "very, very close" the dam came to an uncontrolled discharge, said yesterday: "There's absolutely no doubt in my mind that without the Wivenhoe Dam this disaster would have been on a far larger and catastrophic level. But we will be looking into the full operation of the dam with a comprehensive and very public and transparent review. And we'll all get to understand a lot more about how the dam operated during the past week and what that meant for the way that this event was managed."

The Premier said on Saturday that current releases from the dam were being carefully calculated so that the maximum height of the water would stay within the river system.

Mr O'Brien, who spent his weekend completing a review that he has sent to one of the SEQWater board's members, said it appeared that the dam's operators were very slow to respond to the initial increase in levels at Wivenhoe and it took several days before there was any real increase in the rate of release from Wivenhoe to return the dam to proper flood management levels.

Mr O'Brien said another concern was that the policy for storing water for urban supply in the Wivenhoe Dam did not change to become more conservative despite the weather bureau warning from last year that the weather phases had changed from the drought-forming El Nino events to the wet La Nina.