It is now long enough after the Brisbane floods for the vultures to start circling. With 20-20 hindsight they are certain that SEQ Water (the authority managing Wivenhoe Dam) should have done things diferently, and that if they had done so, there would have been no flood worth talking about.
Of course, it is probable that in the light of experience, the Wivenhoe Dam flood management strategy could be improved slightly. And it is almost certain that improvements to infrastructure and town planning could prevent a repeat of the disaster under similar circumstances. But this is a truism. It is true after every disaster, real or potential; and follows straight forwardly from the fact that humans are not perfect. There is always something more for us to learn.
But suggestions that SEQ Water should have done things differently given the knowledge they had available are simply wrong.
Here are the facts. In the early years of the millennium, Brisbane suffered from an extended drought. The result was that dam levels declined from 100% of normal capacity (1,165 Gigaliters or 2 Sydney Harbours) to 15% of capacity (175 Gigaliters) in August of 2007. Rainfall in 2008 was sufficient to lift that to 23% of capacity by midyear, but the dam did not reach full capacity until April of 2010. At the time of lowest capacity, the three largest dams from which Brisbane can draw water, representing 85% of Brisbane's water supply, had a capacity of 16.7% (293 Gigaliters). That represented enough water to supply Brisbane at then (much restricted) consumption rates for 1.5 years.
Since then it has been raining, on and of in Brisbane, and in the last few months, nearly always on. In early October, 2010, Wivenhoe's storage was lifted by 26% in just a few days. Controlled releases of 7.1% (80 Gigaliters) was predicted to cause localized flooding in Brisbane. Additional peaks followed just before (11% above normal capacity) and after (23% of normal capacity) Christmas. Releases following the former of 5.3% normal capacity (60 Gigaliters) in a day led to localized flooding in Brisbane.
The amount of water that can be safely released from Wivenhoe without flooding depends on rainfall in other parts of the catchment (Bremer River and Lockyer Creek), local rainfall at Brisbane, and the height of tides. These are difficult to juggle in that water from Wivenhoe takes approximately 36 hours to reach Brisbane, a time which varies depending on how smoothly the river is running. From recent experience, releases of as little as 60 Gigaliters per day can cause localized flooding, but under suitable conditions releases of up to 100 Gigaliters per day can be made without local flooding.
On Friday the 7th of January, 2011, the heavens opened up over the Brisbane River. In the four days that followed, 430 Gigaliters of water was released from Wivenhoe. The rate of release was twice that which had caused local flooding in Brisbane just three weeks before. It was managed because of lower tides, but it was pushing the limit. Higher than expected rainfall over Brisbane would have resulted in significant flooding. Despite this Wivenhoe increased its storage by 42% of normal storage capacity (490 Gigaliters, or about the volume of Sydney Harbour) over those four days.
On Monday 10th, Toowoomba got hit by its "instant inland tsunami". That water flowed west, but the part of the storm which hit the Lockyer valley devastated it, and flowed east to contribute around 5% of the total water that flooded Brisbane.
On that day, rainfalls north of Toowoomba in the Brisbane River catchment were higher than those in Toowoomba. As a result, water levels in Wivenhoe rose by 27.5% (320 Gigaliters) in a single day. To avoid them rising the extra 550 Gigaliters that would have caused the dam to overflow, destroying the wall, SEQ water released 645 Gigaliters over the spillway, a release that contributed 80% of the water to the Brisbane flood.
It is worthwhile thinking carefully about those figures. Had SEQ Water not increased the release of water, then the dam would potentially have filled to 99.8% of its maximum possible capacity without destruction. Even in that event, Brisbane would still have flooded because of the additional water coming down the Bremer River and Lockyer Creek. Any attempt to reduce the water coming from Wivenhoe to avoid flooding altogether would have guaranteed the destruction of Wivenhoe, with the consequent wall of water containing five times the volume of Sydney Harbour sweeping Brisbane flat.
As it happened, the rainfall in the Brisbane River catchment finished 12 hours earlier than predicted by the Bureau of Meteorology. Even so, additional water flowing down the creeks increased Wivenhoe's storage by and additional 10.6% (123 Gigaliters) on Wednesday.
So much for the 20:20 hindsight. Now for what was known at the time.
At the time of the initial releases (Friday through to Monday), it was not known that a super storm was on its way. BOM was predicting rain, but at no greater rates than in the days before. While releasing 100 Gigaliters a day over the weekend, SEQ Water did not know that a storm that would generate an "instant inland tsunami" when it fell over a few square kilometers in Toowoomba was about to hit their catchment of 7,000 square kilometers. But they allowed for it. They allowed for it by releasing water at the fastest rate they could without causing flooding.
When the clouds burst on Monday, SEQ Water did not know it was not going to rain just as heavily on Tuesday, but they allowed for the possibility that it would.
This is what the armchair critics are missing out - the limited knowledge available to decision makers. Having no knowledge of what was about to come, there was no reason for SEQ Water to cause flooding in Brisbane to keep the dam safe on the weekend (which is what the critics are retrospectively asking for). But, on Tuesday they faced a choice as to how much water to release, and how much to retain. Had they decided, for example, to reduce the flood waters at Brisbane by 40%, they would have pushed the dams storage level up over 200% of its normal capacity. (This would not have stopped the flood, only reduced its effect slightly.) They would then have been in a situation where a follow up storm would have given them no leeway but to release an even larger flood down the river.
In addition to the critics who have said that more water should have been released earlier, at least one well qualified critic has argued that the normal storage in Wivenhoe should have been reduced to around 60% of its current capacity. However, even with a reduction to 75% of capacity, a similar drought to that which preceded these floods would have seen Brisbane without water. A 50% reduction would have seen Brisbane without water for more than two years.
It is true that water recycling and a desalinisation plant have extended Brisbane's effective water supply since then, but this assumes that no drought worse than the 2000-2008 drought will occur in Brisbane. That's an unsafe assumption even given natural variability in the past. However, Global Warming predicts (in addition to more intensive floods) that droughts will become longer lasting and more severe in the future.
In all, the hydrologists at SEQ Water did a sterling job, and Brisbane ought to gratefully thank them for their efforts. It is only with 20:20 hindsight that some people imagine they could do better.
(I am, of course, open to being corrected on this. People who think that I am wrong need only point to the criticisms in print of the low levels of release from Wivenhoe before the storm hit Toowoomba. I know of no such criticism.)
(Photo courtesy of wikipedia, follow link for full size image)
As an addendum to the above discussion, I have just come across this call by the Liberal National Party that an additional 25% of Wivenhoe's capacity be used for storage rather than flood mitigation. SEQ Water's responce was that they had to carefully study the impact such a move would have on flood risk, to which the LNP's spokesman responded that the additional water should be stored before the study was complete. This was in March last year. Opinion by commentators is equally divided between those for and those against the proposal, including one by an Ipswich man who had lived through the 1974 floods and argued to keep the water because you can survive the floods, but running out of water is terminal. Not one person suggested the now popular alternative of lowering the storage capacity to increase mitigation. (Both ideas, are IMO bad ones.)