Leaked documents reveal threat of releasing Warragamba Dam flood water

Raising the Warragamba Dam wall would force the NSW government to release a torrent of water after a flooding event.

Western Sydney Minister Stuart Ayres. Picture: Dylan Coker

Raising the Warragamba Dam wall would force the NSW government to release a torrent of water after a flooding event, which could cause bridges to close, sewage pump stations to fail and threaten commercial fishing activity in the area, leaked cabinet documents reveal.

Obtained by The Australian, the Impact Assessment of Infrastructure Flood Mitigation Options Final Report details the potential impact of releasing post-flood water into the Hawkesbury-Nepean rivers region in Sydney’s west.

The WaterNSW report from November 2015 presents two options: one for raising the dam wall by 14m; the other if the wall is raised by 20m. State Western Sydney Minister Stuart Ayres is considering raising it by up to 17m.

The report presents three avenues available for the 20m option: releasing 40GL/day for up to eight weeks; 100GL/day for up to four weeks; and 230GL/day for up to three weeks – the equivalent of releasing almost half the volume of Sydney Harbour each day.

Releasing over 120GL/day would have two significant consequences for potable water supply, the report warns. Primarily, the North Richmond water filtration plant would have “difficulty treating water to the required standard”, causing issues for meeting community demand for water.

Moreover, with the deluge potentially threatening the structural integrity of the Richmond and Windsor bridges, the report said, Sydney Water would be forced to isolate the water mains on the bridges to “protect the … water supply in the event that the bridge collapses”.

Closing both bridges would shut off water supply to the region, it said, but added mitigation strategies would likely be “highly effective” in reducing these impacts. The bridges would be unaffected under the 40GL scenario.

At around 80GL/day, three sewerage pump stations would “fail”, while the McGraths Hills sewage treatment facility would be inundated. “The WaterNSW report does not consider risks or impacts to the business of WaterNSW or the legal and liability implications from operating Warragamba Dam as a flood mitigation dam,” the report said.

Even with a 14m raise, commercial operations, such as fishing, eco-tourism and caravan and camping sites, would be impacted, while public areas adjacent to the river would likely be inundated by 100GL released daily.

The report also warns of damage to, and potential loss of, agricultural crops, pasture and livestock under longer inundation. And the commercial fishing industry of the Hawkesbury-Nepean rivers would also be threatened.

A NSW government spokeswoman said: “Floodwaters temporarily held behind the proposed raised dam wall would be released in a controlled way, improving the chances of thousands of people to safely evacuate and significantly reducing damages to homes and livelihoods. “