Googong Dam and flooding

What is the purpose of Googong Dam?

Googong is a water supply dam. It is major component of a network of other dams, reservoirs, treatment plants and pump stations providing secure drinking water for Canberra and Queanbeyan. In fact it represents around 50% of Canberra’s current water supply storage capacity. Googong Dam was not designed with specific features to prevent downstream flooding. Typically dams that are designed to help prevent downstream flooding have complicated gate structures on the spillway (e.g. Scrivener Dam on Lake Burley Griffin; and Wivenhoe Dam, upstream of Brisbane). 

Googong Dam does not have any flow control gates, but a spillway that cannot control the flow out of the dam. The only controlled flows that can be released from Googong are the relatively low flows (environmental flows) that are released to maintain river health at times when the dam is not overflowing. However, simply by being there, and being a dam, Googong does help mitigate flooding in Queanbeyan. Googong dam cannot prevent flooding in Queanbeyan, but the flooding would be worse if Googong wasn’t there.

What did Icon Water do to Googong Dam in 2010?

Recent construction work on Googong Dam was done to strengthen the spillway and increase the height of the walls on each side of the spillway. The work made the spillway better able to stand up to large floods. This work did not change Googong Dam’s performance during a normal flood, other than to make it less likely that a flood would damage the spillway. Click here to learn more about this project.

Will lowering the water level in the dam help in a flood?

No, probably not at all. The two main reasons are;

  1. Googong Dam has a capacity of 125,000 ML. This volume is dwarfed by the amount of water that comes down the river in a flood. For example the peak inflow rate in the 2010 flood was around 98,000 ML/day. (i.e. if that flow was maintained it would fill the dam, from empty, in about 30 hours). If the dam were 85% full before this flood started, this flow rate would fill it in around 4 hours. Peak flow is not generally maintained for 4 hours, but these numbers give a scale to the amount of water that comes down the river in a flood, compared to the volume of water stored behind Googong Dam. 
  2. It’s hard to keep Googong Dam partly empty. A flood usually comes from a period of wet weather that firstly wets the catchment and later intense rainfall runs off more readily, causing large flows.  Typically these high flows (i.e. those that occur before the peak flood flow) are much greater than the rate at which we can release water from Googong Dam (the maximum release rate is around 500ML/day, or 0.5% of the peak inflow rate of the 2010 flood.) So Googong Dam is typically full at the critical time, i.e. when the flood peak hits because in general, rain events in the lead up to a flood peak have created a situation whereby more water is coming into the dam than we can let out. 

Does Googong Dam help in a flood, or make a flood worse?

It helps. Although not designed to reduce flooding, Googong Dam does have some flood mitigation effect, even when it is full. 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. This squeezing of the flow reduces the peak outflow rate and hence the resultant flood level downstream of the dam. In the 2010 Queanbeyan River flood, for example, Icon Water records show that the peak outflow from Googong Dam was only about half the peak inflow. Consequently, the height of the flood downstream of the dam was significantly less than it would have been had Googong Dam not been there. This is despite there being no specific flood mitigation measures included in the dam design or construction.

What is Icon Water’s role during a flood event?

Icon Water is responsible for the safety and operation of Googong Dam, including preparing it to withstand flooding, and generally keeping it in good working order. Icon Water has no particular role in a flood event, other than monitoring how Googong Dam is handling the event. All flood related operations are undertaken by the Bureau of Metrology (rainfall and streamflow projections and warnings) and the NSW State Emergency Service (SES) for Queanbeyan.

Where do I get more information about Googong Dam and flooding?

If you are seeking information about:

  • Flood warnings - Contact the Bureau of Meteorology
  • Flood level forecast - Contact the Bureau of Meteorology
  • Flood damage and road closures, crowd control etc - Contact Queanbeyan City Council or the police
  • What to do in a flood - Contact the State Emergency Service (132 500)
  • If you need assistance - Contact the State Emergency Service (132 500)