Bureaucratic bungling behind the Brisbane flood?
How SEQ Water failed “Flood Mitigation 101”. Brisbane's huge flood mitigation dam (Wivenhoe) appears to have been negligently operated
by Ian Mott
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.