The NSW Government has released the Environmental Impact Statement for its controversial proposal to raise the height of the Warragamba Dam wall in order to reduce flood risks in the Hawkesbury-Nepean Valley.

The statement says the valley, which has the highest single flood risk exposure in Australia, will face increased threats due to climate change, while it also outlines upstream impacts on cultural heritage sites and the natural environment from raising the wall.

“Importantly, the upstream impacts of the proposal must be carefully measured against the social, economic and environmental impacts flooding can have on downstream communities in Western Sydney,” Jobs, Investment, Tourism and Western Sydney Minister Stuart Ayres said today.

The Insurance Council of Australia (ICA) earlier this year said it was “unable to support the proposal as it currently stands” due to concerns that evaluations had not properly taken into consideration cultural heritage values.

ICA will examine the Environmental Impact Statement and the feedback of all stakeholders, a spokeswoman said today.

“We remain in favour of exploring all options for flood mitigation, but in a process that means that all the impacts can be understood, costed, and done in agreement with the impacted stakeholders,” she told insuranceNEWS.com.au.

“[ICA] remains committed to working with the NSW Government and local government to find solutions to protect lives and property, and ensure that any impacts are fully assessed and addressed.”

The Environmental Impact Statement upstream study area includes national parks and state conservation areas, with about 300 hectares lying within the Greater Blue Mountains World Heritage Area.

The statement says some Aboriginal heritage sites would experience either increased or new temporary inundation, with rock art sites among those to be impacted.

“Aboriginal people generally consider all cultural and physical heritage sites to be of high cultural significance,” it says. “Impacts on Aboriginal heritage sites would be unavoidable and measures to record and disseminate information on these sites have been proposed."

The project would raise the central spillway crest by around 12 metres and the auxiliary spillway by around 14 metres, delaying downstream flooding and reducing peak levels.

Nearly all major floods in the Hawkesbury-Nepean are caused by east coast lows that happen on average several times a year and which are likely to be affected by climate change, the report says.

“The increase in rainfall due to climate change would result in an increase in downstream flooding with the existing dam and a deterioration in project flood mitigation capacity,” it says.

The report cites previous research that says if an event similar to the largest flood since European settlement were to occur in 2041 there would be an estimated $7 billion in damage and 158,000 to 171,000 people would need to be evacuated.

“Climate change may further increase this flood risk as it has the potential to increase the severity and frequency of the flood hazard in the Hawkesbury-Nepean Valley,” the impact statement says.

The current design proposal, one of nine measures that have been proposed to reduce flood risk in the valley, includes raising the dam side walls and roadway by 17 metres to enable adaptation to projected climate change.

WaterNSW will host a web portal and make available supporting information on the Environmental Impact Statement, which will be on exhibition until November 12. Details are available here.