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Somerset and Wivenhoe dams,which provide water and flood protection to south-east Queensland,are due to be upgraded by 2035.Credit:Robert Rough

Both are required by law to be upgraded by their owner,the statutory agency Seqwater,by 2035,with neither having met national large dam safety standardsfor about two decades, when the true potential of extreme rain events was realised.

Full drinking supply levels were lowered in 2016 ahead of the planning and upgrade work. Wivenhoe levels are now being dropped further — to 80 per cent — until February amidconcerns about the looming wet season.

In 2017,after the dam owner commissioned a two-year,$2 million study to back “once-in-a-lifetime” upgrades to Wivenhoe,a spokesman said plans were to finish the work “well before 2030″ andinitial planning was already under way.

But five years later,the final business case for the first round of upgrade work — upstream at Somerset — is still not complete. A 2019 fact sheet had suggested this would be finished that year,with construction to start from 2021.

The cost of upgrading both dams,among some of the largest in the country,could easily surpass $1 billion. A Seqwater spokesperson told this masthead that upgrades of this scale were “extremely complex”.

“It’s important we get the planning right to ensure the dam remains operational throughout construction,and that this major investment delivers strategic outcomes and value for money for taxpayers,” they said.

“As Somerset and Wivenhoe Dams operate as one system for both water supply and flood mitigation,the outcomes of Somerset will inform considerations for Wivenhoe.”

The spokesperson said that the work so far on the business case had included “extensive technical investigations” to guide design choices,including hydraulic modelling on a scale lab model of the spillway,3D walls scans and aerial surveys.

While the spokesperson did not give detailed answers to questions about the expected timeframes,representatives of the state water department gave some insights to a parliamentary committee last week.

New rules in place from this month require dam operators to submit a report outlining when they were expecting upgrade work to be finished and justifying the timeframe.

“Best practice is,generally,dams take between five and 10 years to upgrade,” Jarrod Cowley-Grimmond,executive director of water operations and systems in the Department of Regional Development,Manufacturing and Water,told the committee.

The Seqwater website notes construction for Somerset alone is expected to take four-to-five years after the state government signs off on funding. Full drinking supply levels in both dams will remain lower until the work is complete.

Just last year,before the drenching received by large parts of the state over summer,the state government was weighing up options for south-east Queensland water as dam levels dwindled.

This included the need for new water sources such as desalination plants or arestart of the Western Corridor Recycled Water Scheme,revealed by this masthead,pending ministerial sign-off.

This month,the NSW government announced it wouldpush ahead with controversial plans to raise the wall of its major Sydney drinking supply dam by 14 metres,despite the impact on World Heritage listed bushland.

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