Full to the brim, but is Googong a flood risk?

WEEKS of heavy rain has filled Googong Dam to the point it's now spilling over, but ACTEW Water says that the full dam poses no extra flood risk to Queanbeyan should there be further heavy rains this Autumn.

If anything, the dam helps reduce flooding simply by being there, ACTEW has said.

However, Queanbeyan Mayor Tim Overall says questions remain as to whether Queanbeyan is "unnecessarily exposed" to greater flood risk with the dam at capacity.

The comments come ahead of a meeting next month between the water authority and Queanbeyan City Council that will revisit a Council proposal from three years ago to reduce water volumes in the dam as a flood mitigation measure.

Mayor Overall initiated the discussion following the "once in 20-year" flood of December 2010, which caused an estimated $1.3 million damage to local grounds and infrastructure. He convened a meeting with ACTEW Water and the National Capital Authority shortly after that flood looking at ways to reduce the impact of future floods in Queanbeyan using the existing water infrastructure.

And with one flood warning already issued for Queanbeyan this month, Cr Overall told Fairfax Media that some residents "do get nervous" when the dam is at capacity.

"If Googong water levels can be managed at a level between 90 per cent and 80pc, this could reduce the flood impact and also provide businesses, residents and the council additional hours to relocate assets out of the floodway," he said.

"The Weather Bureau plot of storage levels shows that Googong has effectively been full since 2011 while other water storage dams have been operated and are well below capacity.

"The question that needs to be explored is ... is Queanbeyan being unnecessarily exposed to an increased flood risk with water supply being drawn from other storage reservoirs and not through the Googong Dam pipeline?

"This results in Googong continually being substantially full," the mayor said.

However when asked this week whether the full dam posed a greater flood risk, an ACTEW spokeswoman said "We don't believe so, no." Nor could it be easily operated as a flood mitigation dam, the spokesperson said.

"Googong Dam was not designed or operated as a flood mitigation dam ... [and] was not designed with specific features to prevent downstream flooding.

"Dams that are designed to help prevent downstream flooding have complicated gate structures on the spillway or very large storage spaces which remain empty until floods occur."

ACTEW Water also stated that simply by being a dam on the Queanbeyan River upstream of Queanbeyan, Googong does help mitigate downstream flooding, even at full capacity.

"In a flood event, when the peak flow comes down the Queanbeyan River, it has to 'squeeze' through the Googong Dam Spillway, and water can't get out as quickly as it comes in," the spokeswoman said.

The authority also noted the water level in Googong Dam prior to the major 2010 floods was around 75 per cent, which didn't prevent that flood.

Meanwhile, water was still overflowing the spillway this week, and some 3.275 gigalitres has spilled over the dam since March 29.

Council and ACTEW Water will meet to discuss the issue on Thursday, May 1.

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