How SEQ Water failed “Flood Mitigation 101”.
It is surprising how some journalists couldn’t pick a story if it was humping their leg.
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 only 100,000 ML/day was released over the weekend 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 those who did not come down in the last shower 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. This is totally inconsistent with the 36 hour time lag for water to get from the dam to the city. For the record the major shareholders of SEQ Water are Anna Bligh (Qld), Campbell Newman (BCC) & Paul Pissale (Ipswich).
Wivenhoe Dam capacity is 1,165,000 ML normal (100%) and 1,140,000 ML flood surge (197.8%). It went to 191% or 2,225,000 ML with just 80,000 ML to spare.
Somerset Dam capacity is 380,000 ML normal (100%) and 524,000 ML flood surge (238%). It went to 190% or 722,000 ML with 182,000 ML to spare. This minimal buffer meant that only 37mm of additional rain in the 7020 KM2 catchment would have maxed the system out, giving them zero control over flows.
The original article by Williams with the same title has been rewritten for the CM archives with the volume data at the end rather than the beginning. You can still get the gist from this link. http://www.couriermail.com.au/ipad/wivenhoe-water-releases-wound-back/story-fn6ck45n-1225986185721 I still have a copy of the original if anyone wants to see it. Ian Mott.
See the SEQ Water site and scroll down to the graph. Note no data for the weekend. They all took the weekend off and came back to a crisis on Monday morning. http://www.seqwater.com.au/public/dam-levels