‘Absurd’: Veteran environmentalist aims to stir Warragamba wall debate
Mr Debus said another problem was that the plan would flood almost 5000 hectares of vulnerable eco-systems within the Blue Mountains World Heritage area.
Infrastructure NSW, the state body managing the proposal, argues the higher wall is needed to protect residents and assets in greater Sydney, an area with annual economic output topping $100 billion.
The flood risk derives from the valley's bathtub effect. Instead of widening as the river approaches its mouth, the Hawkesbury-Nepean narrows to a single plug hole at Sackville Gorge, allowing floodwaters from five main tributaries to back up and rise rapidly.
Modelling shows that, with a 14-metre dam raising, "flood damages were reduced on average by 75 per cent per annum", a spokeswoman said. "This is as a result of the most damaging floods no longer impacting homes and other infrastructure downstream."
Damages from a one-in-a-500-year flood would now cost about $5 billion – let alone the human toll of evacuating residents, a bill that would be cut to $2 billion with the higher wall, Infrastructure NSW says. By 2041, such a flood's cost would drop from $7 billion to $2 billion.
Still, its prediction that the 134,000 now living and working in the flood plain – including 25,000 homes – will double in the next 30 years, is the sort of projection that worries Mr Debus.
“It would be normal practice to stop the further development of the flood plain in the north-west of Sydney in the Hawkesbury," he said. "Instead, the [wall-raising] proposal assumes the development will go ahead.”
As the NSW State Emergency Service noted last June at the 150th anniversary of the record 1867 Hawkesbury-Nepean flood that killed at least 13 people and inundated 16,000 homes, the valley has possibly the highest flood exposure in the country.
During that event, floodwaters reached 19.2 metres at Windsor, three metres above the majority of development in the area today. The so-called probable maximum flood is almost 10 metres higher again, Stuart Khan, an engineering professor at the University of NSW, noted in a 2012 flood inquiry submission.
Professor Khan, who take part in Monday's panel, said he favoured other measures than lifting the wall. These include lowering Warragamba's water level to create "airspace", giving the dam scope to fill in the case of flood. Currently, the dam's only role is for water storage.
For Mr Debus, the issues include the likelihood that hundreds of cultural heritage sites of the Gandangara nation, would be inundated by floodwaters behind a higher wall, as would vulnerable species, such as the Camden white gum.
Infrastructure NSW's spokeswoman said flood mitigation might result in temporary inundation "such as days or one to two weeks", adding that the area affected would depend on the size of the flood.
With an environmental impact statement and other approvals needed before a final business case is due to be submitted by 2020, Mr Debus argues "it’s not too late at all” to block the project.
The free event will be held at the Blue Mountains Community Hub in Springwood on Monday from 6.15 pm.
Peter Hannam is Environment Editor at The Sydney Morning Herald. He covers broad environmental issues ranging from climate change to renewable energy for Fairfax Media.
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