ACTEW Water has assured Queanbeyan residents the Googong Dam plays a positive role in mitigating floods and that a joint study with the city council and the National Capital Authority is being conducted in good faith.
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The assurance comes after rain caused about 1300 megalitres of water to spill from the dam last week, leading Queanbeyan council to once again stress the importance of reassessing the use of the dam.
A spokesman for ACTEW Water maintained the dam could not have prevented the 2010 floods which caused $1.3 million of damage in Queanbeyan and said the potential for flooding would be greater if the dam were not there.
"Googong Dam levels and potential spilling are dependent on a wide variety of factors including but not limited to rain fall density, concentration of the falls, [and] catchment inflow conditions," he said.
The latest spill, which came after 3275 megalitres washed over the spillway in early April and further spills reported in May and July, led the council to repeat calls for the dam to be capped at 80 to 85 per cent to create an "air reserve".
But in a statement, the ACTEW Water spokesman said capping the dam at this level would do little to avert any future flood in Queanbeyan.
"Googong Dam has a capacity of 119,000 megalitres. This volume is dwarfed by the amount of water that comes down the river in a flood," he said.
"For example, the peak inflow rate in the 2010 flood was around 98,000ML/d. If the dam were 85 per cent full before this flood started this flow rate would fill it in around four hours."
The spokesman also said it was hard to keep the dam partly empty as heavy rain led to large flows of water which entered the dam faster than water could be released.
The ACTEW Water spokesman said the Googong Dam rose to 100 per cent capacity on April 1 and stayed full until July 7, which resulted in a number of spills during that period.
The Bureau of Meteorology's Sean Carson, who was not surprised the dam had spilled given the rainfall throughout the area, said the dam had remained close to capacity since the end of 2010.
After last week's spill, ACT Liberals MLA Alistair Coe wrote a letter to Deputy Chief Minister Andrew Barr asking why "potential drinking water had been lost from the system while the Cotter Dam still had free capacity".
The ACTEW Water spokesman said the transfer of water from the treatment plant to the Cotter Dam could be dangerous and would test the limits of current infrastructure.
"The bulk water distribution system is designed to distribute water from the water treatment plants to town service reservoirs," he said.
"Transferring from Googong Water Treatment Plant to Cotter reservoir would require pipeline protection infrastructure to be bypassed and therefore operating assets beyond their rated capacity."
Queanbeyan City Council's infrastructure group manager Phil Hansen said little progress by made by the joint study established to consider how the dam could play a greater role in flood mitigation.
"We've been working with both ACTEW Water and the National Capital Authority to progress discussions of how the Googong Dam might be of benefit to the way Queanbeyan experiences floods," he said.
"At the moment, we're putting together a terms of reference for work to progress and all parties have been very co-operative."
The ACTEW Water spokesman "said the talks had been constructive, informative, and held in good faith".
ACTEW Water had no particular role in any flood event other than monitoring the safety and operation of the dam.
