Qld flood damage 'could have been cut'

WATER from Brisbane's Wivenhoe Dam during the January floods comprised more than half of the total flow in the lower Brisbane River.

WATER released from Brisbane's Wivenhoe Dam during the devastating January floods comprised more than half of the total flow in the lower Brisbane River, according to a report by the floods commission of inquiry's independent expert.

The report by hydrologist Mark Babister states that its findings support recommendations that "alternative (Wivenhoe Dam) gate operation strategies for flood mitigation should be reviewed for a full range of flood events".

Mr Babister has previously highlighted flaws in the official manual for the operation of the dam.

More than 22,000 homes in Brisbane and Ipswich were flooded early on January 13 as the Brisbane River rose to its highest level since the floods of 1974.

The commission of inquiry has heard that Wivenhoe Dam's outflow peaked at 7500 cubic metres a second on January 11 -- twice the flow that is known to cause damage to properties in Brisbane. The dam's operators made the higher releases when they became concerned about its structural integrity. The report states that complex modelling had revealed that earlier increases in releases from Wivenhoe Dam "could have reduced the eventual peak outflow and the resulting severity of flooding experienced downstream".

The model found that Wivenhoe Dam releases made up the greatest proportion of overall flow in the Brisbane River, at 59 per cent of total flood volume in the seven days from January 9.

Mr Babister tested several different scenarios using complex modelling and found that alternative operation of Wivenhoe Dam could have taken almost a metre off the flooding at Moggill and Jindalee, and about half a metre off the flooding in Brisbane.

The report, which does not criticise the dam operators, also stresses that, given the information available during the operations of the dam and using the strategies defined by the manual, the flood engineers "achieved close to the best possible mitigation result".

"If full foreknowledge of the dam inflows is available, the dam releases can be optimised to reduce peak discharge from the dam," the report says.

"Under this scenario, the peak outflow of Wivenhoe Dam is reduced (by about 40 per cent) from 7500 cubic metres a second to 4500cu m/s. This significant reduction in peak discharge accounts for the majority of the beneficial effect on peak flood levels estimated."

The report adds that, in reality, such a scenario would not have been plausible "as it relies on using discretion to increase discharge from Wivenhoe Dam above allowable thresholds" and "would have relied on foreknowledge of the large second inflow peak into Wivenhoe Dam".

In the weeks after the flood, Wivenhoe Dam's supply of water for drinking was drained to 75 per cent to give the dam more capacity in the event of a follow-up major rainfall event.

Mr Babister's report found that if Wivenhoe Dam had been at 75 per cent prior to the flood, there would have been "a reduction in total flood volume released from Wivenhoe Dam (about 11 per cent lower) and a reduction in the peak discharge from the dam from 7500cu m/s to 5200cu m/s (about a 30 per cent reduction)".

"If gate operations were revised to take advantage of the additional storage available under such a scenario, it is expected that the benefits on flood levels would improve further, although such scenarios have not been investigated here due to time constraints," the report states. But it adds: "Care must be taken with interpreting these findings, which are based on a single large flood event, in relation to the effectiveness of the strategies in the manual for dealing with future events, some of which will be larger.

"Any consideration of reducing water storage in the dam to improve flood mitigation should take into account the trade-off risks to water security."

In the drought years leading up to the flood, the Queensland government spent several billion dollars completing the construction of an integrated water grid, including desalination plants, to "drought-proof" the state and reduce the reliance on Wivenhoe Dam.

The report found that near the peak of the flood event at Jindalee, about 15km west of Brisbane, Wivenhoe Dam's flows comprised about half of the total flow measured by the gauging of about 10,000cu m/s.

He found that the Wivenhoe Dam peak flows, at the confluence of the Brisbane River and Ipswich's Bremer River, "occurred near simultaneously with Bremer River peak flows" and that, as a result, there was "significant backwatering of the Bremer River" to a distance of about 15km upstream.

Queensland Premier Anna Bligh said of the Babister report's findings: "I would hope that all of that would be taken into account by the (floods) commission of inquiry."

The floods inquiry's interim report is to be released next week.