Wivenhoe Dam then and now
Wivenhoe Dam reached its highest-ever level on January 11, 2011 – a day like no other for the main supplier of Brisbane’s drinking water.
On that fateful day, Seqwater, the manager of Wivenhoe and Somerset dams, was urgently juggling the rate of dam water releases to prevent a major flooding problem for Ipswich and Brisbane.
Seqwater believed water was entering Wivenhoe Dam faster than it could be safely released and the integrity of the dam was seriously questioned.
A day earlier, water was being released at a rate of 2044 cubic metres of water per second from the five huge gates, according to the Queensland Floods Commission of Inquiry.
However, despite this, the dam reached 191 per cent of its water storage the next day – that is, 91 per cent of the flood storage capacity over the top of the 100 per cent water storage capacity.
In 2011, the dam’s management team faced the scenario of the dam receiving twice the volume of water received during south-east Queensland’s 1974 floods.
“On the morning of Tuesday 11 January 2011, water levels in Wivenhoe Dam began rising rapidly in response to very heavy localised rainfall on and close to the Wivenhoe Dam lake area,” the Seqwater report of March 2011 says.
“The volume of total inflow into Wivenhoe Dam during the Event was 2,650,000 megalitres.
“This volume is almost double [190 per cent] the comparable volume of inflow from the January 1974 flood event, and comparable with the flood of 1893.”
Overall there were two massive inflows to Wivenhoe Dam – 30 hours apart – close to January 11, the report says.
“The inflow into Wivenhoe Dam during the event was characterised by two distinct flood peaks, with each peak separated by about 30 hours,” it says.
In the aftermath, Justice Holmes found “even a large dam such as Wivenhoe has a limited flood mitigation capacity when the volume of water entering it is significantly larger than its storage capacity”.
“Its flood-mitigation effect for Brisbane was further limited by the fact that floodwaters from other parts of the Brisbane River catchment entered the river downstream of the dam, through the Bremer River and the Lockyer Creek.
“The flooding in Brisbane and Ipswich could have been reduced to some degree had the dam had its capacity reduced to 75 per cent prior to the December rains, but to appreciate what the magnitude of the rain would be and that it would fall in the dam area would have required a more than human capacity of prediction.”
Seqwater declined to comment.
Tony Moore is a senior reporter at the Brisbane Times