That last post was written just before the floods hit.
The 6 largest floods recorded in Brisbane’s history since white settlement were in either January or February in the years 1841 1844 1893 (twice) 1974 and now 2011.
Pressure on Wivenhoe dam to run at 100% was reinforced 6 weeks ago when the Queensland Government announced new cost-saving policies including putting the new Tugun desalination plant on standby saving $10 million a year.
The policies meant that Wivenhoe Dam continued to run at 100% full supply (about 1.15 million megalitres) leaving its flood storage compartment of about 1.45 million megalitres to handle any extreme rainfall events.
Weeks after cabinet and disaster management groups were briefed by the Bureau of Meteorology to expect highly unusual rainfall in the 2011 wet season Queensland Treasurer Andrew Fraser and Natural Resources Minister Stephen Robertson announced “major reforms to the southeast Queensland water supply grid to help reduce rising household water bills”.
In Brisbane big events on the Brisbane River are associated with big wet periods. It only takes 40mm of rainfall runoff to fill 25 per cent of the storage so even if the dam was drawn down it would soon recover and most likely recov er before the beginning of a one-in-100-year event.
Wivenhoe has a catchment of 7000 square kilometres.
Engineer Ken Morris warned in an internal report “Brisbane River Flooding” that the existing and longstanding Queensland policy of operating the dam at full supply level meant its capacity to mitigate floods was significantly compromised.
He suggested that operating Wivenhoe at 75% and using the balance as additional flood storage would make a significant difference to the level in the Brisbane River during any flooding.
Engineers and hydrology experts who have been examining data on river height dam flow rates and weather systems have said Wivenhoe stored far too much water in its flood compartment over the weekend of January 8-9. They said the Brisbane flood was largely attributable to the dam’s operators releasing unprecedented and unnecessarily vast volumes of water on January 11 after they lost their flood buffer because the dam had not been drawn down significantly over the weekend before the huge rainfall dump.
Water Grid managers said they had at all times operated according to the manual and rigid rules.
The flooding and deaths in Toowoomba and Grantham were unrelated to Wivenhoe Dam. There was also flash flooding in Fernvale and in Samford-Dayboro Valley unrelated to Wivenhoe Dam.