A significant spill at Warragamba Dam is not off the cards.Credit:
Nepean Dam,one of the “metropolitan” dams on the upper Nepean River system,has already filled and the others (Cordeaux,Cataract and Avon) might soon do so. The “accidental” flood mitigation capacity these dams and Warragamba have provided,thanks to the drought,might yet be exceeded.
Does any of this matter? Yes,because the state government intends to raise Warragamba Dam,ostensibly to provide protection against floods from Penrith to Windsor and further downstream. Raising the dam would achieve this to a degree.
But beyond a certain point,the benefit of storing floodwaters behind a raised dam wall will be exhausted. This benefit cannot be absolute,and it will be bought only by causing great damage to large areas of World Heritage bushland along the edges of Lake Burragorang.
The floodplain of the Hawkesbury already houses 130,000 people. The vast majority are untroubled by the flooding that is occurring now. But Infrastructure NSW estimates that this population will double over the next 30 years. They will be within reach of genuinely big floods.
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Raising Warragamba might cost something like $700 million,a lot of money to palliate the flood threat without getting remotely near to eliminating it. Should a genuinely big flood develop from rain over the Nepean and Grose rivers (from which today’s floodwaters have come,there having been no contribution from Warragamba Dam outflows),there will be a massive price to pay from the points at which the Nepean joins the Warragamba and where the Grose comes in at Yarramundi.
This point has largely been missed in public discussion about the government’s proposal. An already well-populated floodplain will be further settled. More people will be placed at risk come a rare,very big,but nonetheless inevitable extreme flood.