- JoNova - http://joannenova.com.au -

Brisbane’s Man-made Flood Peak?

Posted By Joanne Nova On January 14, 2011 @ 1:28 am In Global Warming | Comments Disabled

In Australia we’re all watching the flood news unfold. Right now, two friends are trapped without electricity in an apartment building in inner Brisbane. The ground floor below them is inundated. Troy and Jan wrote on Tuesday night that they had little warning their exit route would be cut off, and by the time they knew it was, it was too late to leave. They were rushing to cook meals before the electricity went off and were expecting to lose the car. — My thoughts go out to them, and to those who are so much worse off. Which brings us to questions about what might have been.

The major dam above Brisbane, the Wivenhoe, may have missed the opportunity to release serious quantities of water in the week or two leading up to the major flood peak. Because the Wivenhoe was almost completely full, when the big danger-day came  they could do very little but eek out a small amount of water into what was a rising flood, with little capacity to absorb the massive flows. There are hard questions to be asked about water management.

It’s one of the severest La Nina seasons on record, and with above average rainfall already recorded across much of Queensland and parts of the state in flood, should the dam have been partially emptied when it was safe to release the water? Would it have made a significant difference to that wall of water if they had?  — JN

Guest Post by Ian Mott from  RegionalStates.

How SEQ Water failed “Flood Mitigation 101”. (13/01/11)

On the morning of 12th January, the day before the flood peak that inundated the Brisbane CBD and much of Ipswich, Brian Williams of Brisbane’s Courier Mail, in a masterpiece of misreporting by omission, reported that releases from Wivenhoe Dam were to be reduced from an overnight peak of 645,000 megalitres/day to 205,000 ML/day with the stated aim of “allowing the Bremer River and Lockyer River to subside, thereby easing floods on Brisbane downstream.”

“Wivenhoe Dam levels had dropped just 1 per cent from the previous night, reflecting the massive volumes of water flowing into the storage from its 7020 km2 catchment.” That 1% drop was from a dam capacity of 191% and is an oblique way of saying that the massive flood surge buffer had been pushed close to its limits and they now had no choice but to dump the same amount of water that was flowing into the dam.

What wasn’t mentioned was the fact that for more than a week prior to this large release, only 170,000 ML/day was being released as the storage capacity was allowed to rise to 191% from two weeks of heavy rains. And this meant the carefully designed flood buffer, having been taken to its limits, could no longer function as a buffer. The city was entirely at the mercy of the elements and it would only have taken another 37mm of rain in the catchment to hit the limits.

And as it takes 36 hours for water to flow from Wivenhoe to the CBD then it is absolutely clear that the flood peak of Wednesday night and Thursday morning was a direct result of the previous night’s forced release of the total inflow from the catchment. And this was only necessary because SEQ Water had spent two weeks releasing much less water than was being captured, into a river that was still well below minor flood level.

The article went on to report that releases would go back up to 301,000 ML/day in a few days to reduce the flood buffer volume and that this level of release was, “unlikely to cause a second significant rise in the river.”

What wasn’t mentioned in relation to the reduction from the overnight peak of 645,000 megalitres/day to 205,000 ML/day, with the stated aim of “allowing the Bremer River and Lockyer River to subside, thereby easing floods on Brisbane downstream,” was the fact that the earlier large forced release did the direct opposite. It prevented the Bremer and Lockyer Rivers from subsiding and exacerbated the flooding of Brisbane downstream.

By reducing releases to only 205,000 ML/Day after the peak discharge, SEQ Water is essentially admitting that the peak discharge impaired the flow from the Bremer and Lockyer Rivers by about 100,000 ML/day over that 36 hour period, which they then had to remedy with a lower Wivenhoe release.

At this point you might ask, “so why didn’t they release 300,000ML/day before the buffer was fully extended?” If they had done so there would not have been any need for a larger forced release at all.

Limited Wivenhoe releases on Monday and Tuesday were justified because the flash flooding in the Bremmer and Lockyer Valleys needed somewhere to go. But that doesn’t explain the low releases right through the previous week to Sunday the 9th January. Larger pre-releases in the order of 300,000 ML/day would have maintained sufficient buffer to ensure that no flood peak occurred at all. The river would have kept on flowing at minor flooding level right through this period.

What sort of people, in Queensland of all places, in a strong La Nina wet season, would not start serious dam releases when they were already at capacity, with saturated catchments, in the first week of December? Surely, pre-releases would be more prudent than post-releases in such circumstances?

We need a full inquiry into why this dam managed by SEQ Water, and others managed by Sunwater, were managed in a way that actually produced the kind of flood it was designed to prevent.

[Update: 13/01/11 4.53pm, The spin is on in full. Television reports are now wrongly reporting that the drop back from the temporary high release volume was instrumental in preventing a worse flood peak.]

UPDATE: From Jo

There is avid discussion in the comments, and much remains unknown. Thanks to many informed comments (a few copied here).

#10 David Cain: Wivenhoe has two functions – water supply and flood mitigation. “100% capacity” means 100% capacity of the water supply function. The dam is full at “225% capacity” – so the dam’s flood mitigation capacity is more than its water supply capacity. I am told that the dam operators are required by law to lower the dam level to “100%” within a week, although river heights downstream need to be taken into consideration.

The dam levels were repeatedly lowered to 100% before the present crisis http://www.seqwater.com.au/public/dam-levels

#31 Ian Mott replies:

David Cain#9 and John Watt#19 need to bear in mind that dams that do not have any flood surge capacity have routinely mitigated flood peaks by simply dropping their water level below full capacity just before the flood. The argument that flood volume couldn’t be released on Sat/Sun/Mon is bollocks. The rapid flow in the Lockyer didn’t take place till monday and it still needed a day to get to the confluence of the Wivenhoe flows. By wednesday the Wivenhoe flows were accounting for 80%, ie 8000/10000 m2/sec of lower river flows. This is all very much SEQ Water’s work.

They all took the weekend off and watched a 1 in 120 year flood event turn a simple task into a crisis they couldn’t deal with by Monday afternoon. All the folks who’s homes and businesses didn’t go under until Wednesday can rest assured that, despite their policies, they are actually fully insured, courtesy of the SEQ Water public liability policy. And if they SEQ Water doesn’t have a policy then the rate payers of the major shareholders, the State Government, Brisbane City Council, Ipswich Council and a number of others who are not anywhere near the flood zone, will eventually foot the entire bill. The meter is already ticking on the class action.

See Also Pat #17,  John from CA #20,  Treeman #24 #25 (The link to Hedley Thomas story is here) Craigo #47

Treeman #42:

Wyvenhoe was at 105% on January 7. But it was at 101.9 on January 4, 102.9 on January 6 and by January 10 it had crept up to 144.3. Conveniently there is no data shown at SEQ Water for January 8 or January 9 which are precisely the days when SEQ Water should have been releasing water at full bore! Not surprisingly these days were Saturday and Sunday when bureaucrats were fiddling, dam was rapidly filling and no-one could decide whether to open all the floodgates. By Friday night they would have known that the rate of increase was alarming. The dam level would have been over 130% by Saturday. It takes 36 hours for the flood release to get to Brisbane CBD. For the sake of more than minor flooding the gates could have been fully opened on Friday and closed on Sunday as the impacts of local heavy falls were starting to be seen in the river. This is what were told the Wyvenhoe was all about.

Robuk #26 Sums up the bigger issue.

It appears that your government have stopped development near the coast because of the non existent sea level rise but allowed development within a flood plain:

http://i446.photobucket.com/albums/qq187/bobclive/Australianfloods3.jpg
http://i446.photobucket.com/albums/qq187/bobclive/Ausralianfloodlevels2.jpg

VN:F [1.9.22_1171]
Rating: 5.5/10 (4 votes cast)

Article printed from JoNova: http://joannenova.com.au

URL to article: http://joannenova.com.au/2011/01/brisbanes-man-made-flood-peak/

Copyright © 2008 JoNova. All rights reserved.