Experts say raising the Warragamba Dam would not have halted this week’s floods. 

Sydney’s western region has been inundated with rain this week, causing Warragamba Dam to spill, causing major flooding issues for Western Sydney suburbs like Penrith, downstream at Windsor and Richmond. Evacuations are taking place.

The NSW Government wants to raise the dam wall by 14 metres, with 17 metre abutments at either end.

Stuart Ayres, the Minister for Western Sydney, has previously told reporters that a higher wall would “create a 14-metre flood mitigation zone [that] would reduce average annual flood damages by about 75 per cent”.

But Dr Chas Keys, a former deputy director of the State Emergency Service and flood researcher, says raising the dam wall would probably have had little impact on the current floods.

“Building up the dam wall is seriously flawed and the government is exaggerating the mitigation benefits,” he told reporters.

“You can achieve a little bit of mitigation in a situation like what we are seeing today and lesser events by raising the dam, but in big events, because of the tributaries coming in below the dam, the mitigation is strictly limited.

“You might knock off a metre, for example, from a flood of 1867 proportions - instead of 19 metres you might only get 18 metres,” said Dr Keys.

He said the government would be better off investing in infrastructure that would give people an evacuation route even in a flood that peaked at 17 metres.

Dr Keys said many people see rivers as being locked to the channels in which they normally flow. But it is important to note that floodplains are part of river systems too, and that rivers will “reclaim them from time to time”.

“Floods are rivers’ way of reminding slow learners that they own the valleys,” he said.

Former NSW Emergency Services Minister Bob Debus - who opposes the dam-raising - says it is misleading to suggest the project could stop floods.

“It is very important to remember that it is not possible for Warragamba Dam, whatever its size, to stop the most extreme floods,” he said. 

“Raising the wall would have only a moderate effect even in a one-in-500-year flood event.

“I take it to be very significant that the Australian insurance industry has now withdrawn their support for the dam wall raising as the critical element in flood management and mitigation in the Hawkesbury-Nepean Valley.”