As another big rain event threatens to drop as much as 100mm of rain over our region in the next four days and with all local dams at full capacity, the logical question which remains is: where will all this excess water go?
The answer: into the lake, beyond that, to the Lower Molonglo River and then to the Murrumbidgee. Hopefully, without any problems.
The previous big flood event in Canberra was 12 years ago when the Molonglo River burst its banks. It was the first major flood event in the local region since the 1970s and inundated parts of Queanbeyan and Captain's Flat, causing $1.3 million in damage.
In that flood event, more than 90 millimetres of rain fell in Tuggeranong in one night.
Queanbeyan was declared a natural disaster zone on December 9, 2010 after the river peaked at 8.4 metres, with 100 homes evacuated and the Riverside Caravan Park becoming not by the river, but in it.
The Queanbeyan and Molonglo Rivers are the two feeder rivers to Lake Burley Griffin and how high floodwaters climb upstream is partially determined by how fast Scrivener Dam can release it.
Scrivener Dam, named after master surveyor Charles Scrivener, was completed in 1964 and after delivering many years of solid service is due for a $38.5 million upgrade next year to strengthen the all-important structure - the dissipator - which absorbs the energy of the water flowing over the floodgates.
Around 700 new anchors will be installed into the foundation rock next year, increasing the thickness of the dissipator slab and improving its strength.
Scrivener Dam has five spillways but only very rarely - and the previous occasion was 2010 - does the speed of the rising water entering the lake require all five to open at once to release the pressure.
Designed for a 1-in-5000-year flood, with all five gates open, 2000 cubic metres of water per second can be expelled from the 58-year-old dam.
Logically, it could be assumed that given the flow data readily available from upstream, the gates could be opened to pre-release water early in anticipation of the incoming water. However, a 2015 report commissioned by Icon Water and the Queanbeyan council found otherwise.
"Even if a temporary reduction of 0.5 m in the operating level of Lake Burley Griffin was undertaken prior to a flood, there would be minimal reduction in peak flood levels upstream of the Majura Parkway and it is very unlikely to provide any flood mitigation benefit for Queanbeyan," the researchers found.
Googong Dam, which feeds the Queanbeyan River, was built with the express purpose of storing water and doesn't have any flood mitigation capability.
Its spillway is a concrete chute and the existing infrastructure, according to the same 2015 report, has limited capacity to release water quickly for flood control purposes.
In examining whether adding new gates or control measures to Googong Dam would be viable, the cost-benefit analysis was seen be unfavourable, viewing it - somewhat dispassionately - as cheaper to have flood insurance in the rare event that a flood occurs.
To add flood mitigation to Googong Dam would require the creation of so-called "airspace" to its storage capacity, dropping it to about 70 per cent.
With 30 per cent less water held during drought periods, water then has to be supplemented from elsewhere in the Icon system, where it's more expensive to source. And if Icon's costs go up, then that would inevitably get passed on to the consumer.
By far the worst flood disaster to hit Canberra was in 1971, when a severe storm caused flash flooding in the Woden Valley. Seven people died as a result of that weather event.
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