These calls represented the ideal time for anyone to argue for enhanced flood mitigation at Wivenhoe. Those such as Andrew Dragun, who now believes that water should have been released to prevent the risk of flooding, said nothing. He thought it was far more important to attack the Federal Governments Murray Darling Basin Scheme. Hedley Thomas, who has run so hard on this story for the Australian, ignored the story. And Michael O'Brien, the engineer who has so publicly condemned the dam operators was not to be heard. Their wisdom, it appears, is exclusively 20-20 hindsight.
Let's put this in context.
Late on Sunday the 9th, the engineers at Wivenhoe dam had no reason to suspect what was coming. They had already passed through three flood events (13th October, 21st December, and 29th December) without difficulty. Having lowered the dam levels to near 100% of the Full Supply Level (67 meters) after the last flood event, waters were on the rise again. They had increased flows over the spillway to a level that flooded all bar two of the cross river crossings above the Centenary Bridge to compensate, but had no reason to suspect what was coming. The weather forecast was for heavy rain, but they were experiencing heavy rain, and had not reason to suspect heavier.
The charge against them is that during this period they should have released more water. They should have flooded all the lower middle river crossings (between Wivenhoe and Jindalee) and low lying areas of Brisbane in anticipation of what was to come. How realistic is that?
According to GHD's report on the auxiliary spillway, a dam level of 71 meters represents 1,647 gigaliters of water (table 3.15.2). That is 141.4% of normal capacity. According to SEQ Water's report, on 9:00 am of Monday, the dam was at 148.4% of capacity, which is less than 71.25 meters of water. I consider 71 meters to be a reasonable approximation of the water levels of the dam at midnight of Sunday the 9th.
That is interesting because SEQ Water's report on different methods to increase Wivenhoe's storage capacity considers the effects of increasing the supply level to 71 meters with no other adjustments.
In particular, the consider the impact of a 1 in 200 rainfall event on the probability of flooding. A 1 in 200 48 hour rainfall event, according to this study, would result in peak inflows of 8,433 cumecs. It would necessitate peak releases of 6,037 cumecs. And it would result in dam levels rising to 74.82 meters.
In fact, over the 48 hours following Midnight of Sunday 9th, peak outflows reached 7,500 cumecs, average inflows over the 48 hours exceded 6,000 cumecs, and peak inflows certainly topped 8,000 cumecs (and may have topped 10,000 cumecs), and dam levels rose to 74.85 meters.
For comparison, a 1 in 500 48 hour rainfall event is expected to result in peak inflows of 10,543 cumecs, peak outflows of 7,649 cumecs, and a peak level of 75.664 meters (ie, 6 mm below the level of the first fuse plug).
Very clearly the events at Wivehoe lie somewhere between these two. They were significantly worse than a 1 in 200 year event, but not quite as bad as a 1 in 500 year event.
So when push comes to shove, what the engineers at Wivenhoe "did wrong" is that on the weekend preceding the flood, they did not anticipate an unpredicted 1 in 200 year (at minimum) event.
I am certainly open to suggestions that the operating manual needs to be reviewed in light of experience. With hindsight, it may have been better to allow the fist fuse plug to be overtopped and keep peak dishcharge below 6,000 cumecs. Or not. That is something only a full review can tell.
But suggestions that the engineers did the wrong thing, given what they knew are simply false. They are needless vilification of people who under trying circumstances saved Brisbane from a much larger disaster.
And what of the suggestions that Wivenhoe's dam level should be reduced before hand in anticipation of the flood? Had that advice been carried out during the last significant La Nina, Brisbane's water supply would have dropped to 5% of capacity in the recent drought. Had it been followed the last time the dam was full, it would have run dry a year before the end of the recent drought. I doubt Andrew Dagun or The Australian will indemnify the people of Brisbane against the risk of a similarly long drought following this La Nina.
Update (9:30 am, 8/2/11)
I have just come across this BOM graph of river levels at the Wivenhoe dam wall from 12 PM, Sunday 9/1/11 to approximately 9:00 AM, Friday 14/1/11:
Ryan Independant, Ryan being one of the federal electorates in Brisbane.)