Brisbane’s Man-made Flood Peak?

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.

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.]

Nice answer to the spin that Wivenhoe Dam ‘saved Brisbane’ - the damn Dam can’t function as a touted flood mitigation instrument when it’s FULL TO THE BRIM!

Now let’s see if they do it again. There is a possibility of a cyclone around the Australia Day weekend, January 26. Usually accompanied by King Tides.

If Wivenhoe remains full, woe betide Brisbane.

oh for heaven’s sake!

KNOWN FLOODS IN THE BRISBANE & BREMER RIVER BASIN
INCLUDING THE CITIES OF BRISBANE AND IPSWICH

http://www.bom.gov.au/hydro/flood/qld/fld_history/brisbane_history.shtml

1824 John Oxley, early explorer, mentioned evidence of an inundation which he discovered on 19 September 1824 in an area north of the junction of the Bremer with the Brisbane : “the starboard bank an elevated flat of rich land, declining to a point where had evidently by its sandy shore and pebbly surface, been at some time washed by an inundation; a flood would be too weak an expression to use for a collection of water rising to the full height (full fifty feet) which the appearance of the shore here renders possible.” (Ref 2)

Normally Brisbane is supposed to receive slightly above average rainfall during a La Nina Summer.

Looks like the normal expected weather patterns from La Nina are being amplified in Australia. We also have record rain fall amounts and snow amounts in areas that are not supposed to see above average rain and snow during a La Nina. As Bastardi stated a month ago, historically not all La Nina's and not all El Nino's behave the same. We also now have the new Cosmic Ray Theory about cloud formation. Low solar activity means more cloud formation.

At any rate, almost all ancient cultures have a story about a past massive flood. Was thinking that floods obviously occur when Ice Ages end, but it is also possible that we get massive floods as the Ice Age breaks out. There has to be a explanation for the mechanism of building mile high sheets of Ice over thousands of miles. Whatever the mechanism, it probably includes heavy snowfall in Arctic to Cold Temperate zones and incidental heavy rainfall in temperate to tropical climates.

May be a very good reason why almost every ancient culture has an old flood story. My 2 cents anyway.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.