Options for another Tweed dam on the table

Luis Feliu

A controversial longstanding plan to build a second dam in the Tweed Valley to secure the shire’s future water needs will be debated by councillors tonight (Thursday).

But three options which involve building a new dam at Byrrill Creek south of Uki are not preferred  by the shire’s engineering managers who have recommended raising the existing Clarrie Hall Dam wall to double its capacity.

Council’s chief engineer David Oxenham says a choice to augment the shire’s water supply needs to be made now so an additional water source is commissioned by 2026.

The five options considered by staff include: raising Clarrie Hall Dam, building a small Byrrill Creek dam, building a small Byrrill Creek dam and raising the Çlarrie Hall Dam wall at a later date, building a large Byrrill Creek dam, linking to South East Queensland (SEQ) Water, and linking to Gold Coast City Council’s water supply.

Mr Oxenham said in his report that, after analysing various impacts, ‘the raising of Clarrie Hall Dam is most advantageous to council and its community’.

‘If council continue to delay a decision on a preferred Tweed District Water Supply augmentation option, there may be insufficient time prior to 2026 to implement any of the dam options.

‘This will force Council to link to SEQ Water or Gold Coast City Council neither of which are preferred options,’ Mr Oxenham said.

He said that if the preferred option is to be a dam, ‘the planning, environmental assessment and construction of the dam, such that it can provide the increase in secure yield required by 2026, will take 10 years’.

Council has considered various options over the past five years to boost the water supply but is yet to decide on one.

The Byrill Creek dam option has been contentious from the start with protetst and parliamentarians taking it up over the years.

Mr Oxenham said a study on raising the wall at Clarrie Hall Dam by NSW Public Works had determined the optimum size of the dam was 43,000 megalitres (ML) based on raising the dam wall height by 8.5 metres to a dam wall height of 70 meters AHD (Australian Height Datum).

‘The secure yield of the raised Clarrie Hall Dam has been estimated by NSW Urban Water Services after undertaking stream flow estimation for both present flows and flows adjusted for climate change, and modelling the behaviour of the dam within licensed operating conditions. The modelling estimated the 2030 secure yield as 22,700ML/a.

‘The raised Clarrie Hall Dam would be able to provide adequate water supply to Tweed Shire until approximately 2046’.

He said the cost estimate by NSW Public Works for raising the wall was around  $43.44 million.

In comparison, Mr Oxenham said building a small Byrrill Creek Dam with capacity of 16,300 ML would cost around $54.35 million, while the option of building a small Byrrill Creek dam then raising the existing dam wall around nine years later would cost $105.17 million.

He said that to build a large Byrrill Creek dam with a capacity of 36,000 ML could cost around $81.86 million, but given the low degree of certainty with the cost estimate, this costing ‘could increase significantly’.

Linking to SEQ Water would involve building a pipeline capable of transferring up to 20ML/day, from the Tugun Desalination Plant to Piggabeen Road and a pump station, which would provide an average of 3,600 ML drawn from SEQ Water.

Mr Oxenham said the cost of that would be around $39.14 million but SEQ Water operating costs would be around $19,650 a year.

Linking to Gold Coast City Council, where the price of water is $3.77/kL, would also incur larger costs for pump station operations of $650,000 a year.

‘Because the cost of water from Gold Coast City Council is significantly greater than the cost of production of water by Tweed Shire Council increasing the pipeline capacity and drawing more water from Gold Coast City Council would cause the cost of water in Tweed to rise further’.

He said the link options were risky as they could impact on low-income residents such as pensioners.

‘Building Byrrill Creek Dam is a greenfield project and the risks associated with such a project are not as well understood as the risks of Raising Clarrie Hall Dam. Issues such as ground conditions and construction access are unknown,’ he said.

‘The risk of time delays due to adverse construction conditions are higher than other options. In general, the time risks for the building of Byrrill Creek Dam are significantly higher than the time risks associated with other options,’ Mr Oxenham concluded.