Googong flood mitigation options considered

27 July 2015

The report jointly commissioned by the Queanbeyan City Council (QCC) and Icon Water to consider potential flood mitigation options for communities downstream of the Googong Dam has today been released.

The results support the opinion that it would be unsustainable to use Googong dam for flood mitigation and that the costs of operating Googong Dam at a lower level greatly outweigh any potential benefits.

The report, which was undertaken by Jacobs Group (Australia) Pty Ltd, considered a number of options which include the targeting of airspace in Googong Dam to provide a flood mitigation benefit; reduction in the operating levels of Lake Burley Griffin (LBG); and clearing of vegetation along the channel and floodplain of the Molonglo and the Queanbeyan Rivers between Googong Dam and LBG.

With regards to the 2010 flood event in Queanbeyan, the report findings indicate the following:

  • If Googong Dam was kept at 90% or 80% storage level, there would have been negligible reduction in flood damage to Queanbeyan;
  • Only by keeping the dam at 70% would there be significant reduction in the flood peak;
  • The increase in operating costs to Icon Water, and ultimately the community for the 70% scenario would far outweigh the damage costs by more than two-fold; and
  • Googong Dam reduced the peak of the flood in Queanbeyan by 47% just by existing.

Amanda Lewry, General Manager, Asset Management at Icon Water said that the report acknowledged much of what was already known, reaffirming that Googong Dam plays an important role in supporting Canberra and Queanbeyan’s drinking water security strategy.

“The report clarifies that even by choosing an optimised flood mitigation strategy for this particular flood, the cost would have been more than twice as much than the reported damage,” Ms Lewry said. “Of course, this assumes that all flood costs were recognised in calculating the impact of the 2010 flood – which we believe is highly unlikely—and does not acknowledge additional costs to Icon Water to amend its operational strategy for Googong Dam.”

In commenting on the report, Mayor Cr Tim Overall said that the stark reality is that the Googong Dam was not constructed for flood mitigation but for water storage. During periods of sustained rainfall, the extensive catchment area creates an extremely high flow rate into the dam and this occurs at a much greater rate than at which water can be released.

"We know that Googong Dam was just over 80% full 10 days before the 9 December 2010 flood peak. With the steady lead up rain, Googong Dam was 100% full and spilling from 3 December. Googong Dam was constructed solely as a water supply dam and does not have gate structures with the ability for high flow water release (unlike Wivenhoe Dam upstream of flood prone South East Queensland)," Cr Overall said.

"That being said, Googong Dam does have some flood mitigation effect at peak periods by moderating the outflow rate to a degree."

The study’s results endorse that Googong Dam should continue to be used primarily as a drinking water source. The only flood mitigation the dam should provide Queanbeyan is from its presence upstream; which can be quite substantial as in the case of the 2010 flood.

 The summary report can be found at www.iconwater.com.au