Former emergency services minister says raising dam wall won't stop flooding

"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.

"Leaked charts published in the Sydney Morning Herald have shown that raising the wall would have only a moderate effect even in a 1 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"I take it to be very significant that the Australian insurance industry has now withdrawn their support for the dam wall raising as the critical element in flood management and mitigation in the Hawkesbury-Nepean Valley."

The state government has proposed raising the dam wall in an effort to mitigate the risk of flooding in the Hawkesbury Valley region. However Indigenous residents, scientists, environmental action groups, local councils, politicians and the United Nations Educational, Scientific and Cultural Organization (UNESCO) have raised various concerns about the plan.

Mr Debus said 'other obvious and immediate' flood management actions were being overlooked.

"Very little has been done in recent years to improve evacuation routes for people on the floodplain despite requests from local government," he said. "Plenty of evacuation roads can be cut by quite minor flooding at present.

"But above all I can't help but agree with emergency services minister David Elliott when he pleaded with his colleagues Stuart Ayres and Melinda Pavey to preemptively release water from the dam last year. We all knew that big rain events were likely at this time in our seasonal cycle.

"Preemptive release before floods arrive is something that is done with other dams, but is ignored here because of domestic political agendas.

"There are no silver bullet solutions, but lowering the reservoir level by 10 meters has a similar effect to raising the wall by 10 metres and it doesn't cost $1.6 billion. Regardless, plenty of flooding in the Valley has nothing to do with Warragamba at all."

Warragamba was hit by more than 150mm of rain in the 24 hours to 9am on Sunday, and more than 250mm over the past four days.

A statement on the Water NSW website said current projections indicated this weather event could result in flooding similar to the 1961 flood.

"Warragamba Dam spillway is currently releasing water at a rate of 450 gigalitres per day (GL/day) and that rate could increase as inflows to the dam storage continue to rise (by comparison Sydney Harbour is estimated to hold 500 GL)," it said.

"Modelling indicates that approximately 1500 GL of water will flow into the dam in the seven days since the extreme weather event commenced, a figure that represents 75 per cent of the dam's storage capacity of 2000 GL.

"Flow data up to Sunday, March 21 indicates half of the floodwaters in the Hawkesbury-Nepean river system downstream of the dam were from tributary flow, not the dam.

"While WaterNSW is not authorised to lower the storage based on weather forecasts, significant pre-releases prior to a flood event could make flooding events more dangerous."

Mr Debus said Warragamba Dam was not on the Nepean or Grose Rivers or South Creek, "all of which would be flooding now irrespective of the water flowing over Warragamba Dam wall".

"In the prospectus for raising the Warragamba Dam wall, Infrastructure NSW has stated they plan to place an additional 134,000 people on the Hawkesbury-Nepean floodplain in the coming decades," he said. "But those people would not be safe from flood events, even if they were lulled into believing so.

"In this sense the raising of the dam wall would actually increase the risk to life and property into the future."

The Sydney Morning Herald reported a former deputy director of the States Emergency Service and flood researcher, Dr Chas Keys, and University of NSW global water institute's Professor Stuart Khan agreed the raising would probably have little impact on the current floods