Releases from Wivenhoe Dam raised water levels in the Brisbane River by up to 10 metres during January's flood, a panel of independent hydrologists has found.
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The hydrology report, commissioned by the Insurance Council of Australia and published yesterday, ruled the Brisbane flood to be a “dam release flood”.
It named the release of water from Wivenhoe Dam as being the “principal immediate cause” of the riverine flood, as rain ceased about 6pm on January 11, more than 24 hours before the Brisbane River peaked overnight on January 12-13.
The council commissioned the report, carried out by hydrologists from WorleyParsons, WRM and Water Matters International, on behalf of insurance companies for use in conjunction with their own hydrology assessments and local condition reports in processing claims.
Although the report acknowledges that flood waters from the Lockyer Creek and Bremer River contributed to the Brisbane flood, the hydrologists found the releases from Wivenhoe Dam created the primary “floodwave”.
“As this floodwave moved downstream, backwater flooding occurred along the lower reaches of all tributaries through the Brisbane City Local Government Area,” the report says.
The floodwave, according to the report, resulted in “backflow” in the piped drainage system, causing storm water drains to overflow in low-lying areas.
Although the hydrology report was completed before Wivenhoe Dam operator Seqwater this week published a log of the releases from the dam, the panel unequivocally linked the dam release to the flooding.
“It is clear that the dam release flood from Wivenhoe Dam was the immediate cause of flooding,” the report says.
“Flood events in lower Lockyer Creek, in the lower Bremer River and in the lower Brisbane River were dwarfed by the subsequent water level rise associated with releases from Wivenhoe Dam [between January 11 and January 12]
“The water level rise associated with the above dam releases increased water levels in the lower Lockyer Creek at O'Reilly's Weir by over five metres, in the lower Bremer River at One Mile Bridge by over seven metres, and in the Brisbane River at Savages Crossing and Mt Crosby by seven metres and 10 metres respectively.”
The Savages Crossing flood peak was 24.1 metres, while Mt Crosby's was 26.18 metres.
The report does not apportion any blame to dam operators, pointing out Wivenhoe was forced to collect massive inflows from surface runoff from the upstream catchment area, releases from Somerset Dam and direct rainfall.
“The greater the volume … of an incoming flood, the less effective are dams at mitigating flood flows, and the more constrained management options (releases) become for dam operators,” the report says.
“It may be more appropriate to assign flood causation to 'succeeding storm events over the catchment area of the dam, each characterized by exceptionally heavy [rain] and massive surface runoff volumes'.”
The report outlines the responsibility of insurers to assess flood-affected properties individually rather than declining claims based on the definition of the flood, as homes further away from creeks and streams may have been subject to initial flash flooding from heavy rain.
“Resolving this issue requires investigation of 'local' factors such as local topography and … the characteristics of overland flow,” the report says.
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