Swollen … Warragamba Dam spills over on Monday. Photo: Carlos Furtado
WESTERN Sydney councils have renewed calls for the government to change the operating rules of Warragamba Dam after automatic releases caused minor flooding and road closures in the Nepean-Hawkesbury valley.
Warragamba Dam, which accounts for about 80 per cent of Sydney's water reserves, began spilling over early on Monday after reaching full capacity, only the third such release since 1999.
The additional outflow was averaging 24,000 megalitres a day compared with the daily 1400 megalitres being piped to Sydney, said Debbie Low, a spokesperson for the Sydney Catchment Authority.
The dam's holdings jumped 6 per cent in the past week, boosted by heavy falls over the weekend that had already swollen the Nepean-Hawkesbury river systems and many other rivers across the state. The dam's spill will continue ''in the order of days, not weeks,'' Ms Low said.
The senior vice-president at the Western Sydney Regional Organisation of Councils, Tiffany Tree, said the Warragamba spill underscored the flood threat faced by the region.
Ms Tree, deputy mayor of Hawkesbury council, said the government should change the legislation that prevents the dam from releasing water until it is full. The laws were enacted when the dam was built as Sydney's main water source in 1960.
''If we had a situation with the dam at full capacity, already full rivers and more rain - that's the sort of combination you would expect to get large-scale floods,'' Ms Tree said. A legal change would allow operators to ''let the water out over a period of time''.
Several low-lying bridges and roads were already cut off and local ferry services are likely to be suspended for several days. The Minister for Primary Industries, Katrina Hodgkinson, said the government had started a detailed review of the flood management arrangements for the Hawkesbury-Nepean valley, with options including raising the Warragamba Dam wall 23 metres to help limit floods.
''The review will also consider whether the use of alternative operating procedures at Warragamba Dam might assist in mitigating flood impacts downstream,'' Ms Hodgkinson said.
The government expects to receive findings of the review by the end of the year.
Drew Bewsher, a flood mitigation consultant who completed a report for Hawkesbury council, said changing legislation would give the government the flexibility enjoyed by Queensland government. Wivenhoe Dam began releasing water over the weekend ahead of expected heavy falls over south-east Queensland.
''It makes enormous sense to pre-release water in those circumstance,'' Mr Bewsher said. ''The Hawkesbury region is one of, if not Australia's most, significant flood problems.''
Dam authorities and emergency service crews will be watching weather patterns closely with several days of rain expected over the catchment, including 15 to 25 millimetres on Thursday, Ms Low said.
A repeat of the 1867 record flood would inundate 21,000 residents and 143 hectares of industrial and commercial buildings, a study for the government found last year.
A flood of the rarity of the 2011 Brisbane floods would leave a damage bill of $8 billion or more.
''It would be a show-stopper for the area and the state and (federal governments) would have to pick up the tab,'' Ms Tree said.
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