Government might need to move a highway to fix Wivenhoe Dam

The news

A highway might need to be relocated before work to upgrade south-east Queensland’s main water supply and flood-mitigation dam before 2035 can be started.

The state-owned operator of Wivenhoe Dam, Seqwater, is awaiting a business case from the Transport and Main Roads Department into the potential “significant realignment” of the Brisbane Valley Highway.

Wivenhoe Dam spilling water during the 2011 flood – with the Brisbane Valley Highway passing above.Credit: Dean Saffron

This work is said to be required as a “precursor” to efforts to bring the dam up to the national large dam safety standards it has not met for about two decades, and boost its “insufficient” flood capacity.

Seqwater’s annual upgrade program report, lodged with government last September and sought by Brisbane Times since, also notes ongoing work to determine if boosted flood capacity will come from raising the dam wall, an extra spillway, or both.

Why it matters

The previously labelled “once-in-a-lifetime” upgrades to Wivenhoe and upstream neighbour Somerset – with “very high” expected costs tipped years ago to nudge $1 billion – are insisted to still be tracking for their legal deadline.

Full drinking supply levels were lowered years ago and will remain lower until upgrades, likely to take between five and 10 years from a final government funding approval, are done.

While a preferred option for the Somerset upgrade has been found and is subject to a detailed business case as community consultation and early work begins, Wivenhoe options are still being considered.

What they said

The document states “robust option selection” is needed to consider “engineering feasibility limitations to raise the dam crest level”, along with the cost and scale of another spillway to boost flood capacity.

Consideration of “critical downstream flood safety issues” are also needed if flood capacity is lifted “without raising of [the] dam crest level”, which the highway crosses on its route from the Warrego Highway near Ipswich to the D’Aguilar Highway west of Kilcoy.

“Significant realignment” of the Brisbane Valley Highway is also to be considered, with key priorities for the year to September 2024 including a “TMR highway options analysis and detailed business case”.

What you need to know

A project timeline in the document puts the planned start date for the Wivenhoe tender process at February 2030. “Notable matters” also include a 2032 Games construction crunch.

Research commissioned by dam owners including Seqwater into the impact of climate change on extreme floods, expected by December, might force “revision of the preferred upgrade option”.

Reports have warned dam operators had not managed the risk that delays to the multibillion-dollar statewide upgrade bill could pose, with new water sources needed for a south-east Queensland population unable to be covered by current supply.

Behind our reporting

Seqwater unsuccessfully objected to the release of some detail left unredacted in the document given to Brisbane Times under right to information laws, as it had been created for cabinet – a point used to keep some of the previous report hidden.