Three days before Warragamba Dam could spill due to severe storms, the Baird government has committed to raising its wall to prevent a potential flood disaster in western Sydney, four years after being advised to review the options.
As a looming east coast low raised the possibility that the dam could spill on Sunday, the government said it will commit $58 million towards raising the wall by 14 metres to avoid a catastrophe that could place 43,000 western Sydney residents at risk.
Construction, at an estimated cost of about $690 million, is expected to be complete within three to four years after a business case is signed off in 2019, subject to planning approvals.
The Hawkesbury-Nepean Valley between Penrith and Sackville has the greatest flood risk of anywhere in NSW.
The government says up to 134,000 people who live and work there could require evacuation in the event of a large flood – a figure forecast to double in the next 30 years.
In 2012, Infrastructure NSW said a study it commissioned found that a one-in-1000-year flood in the area similar to the 2011 Queensland flood would cause damage estimated at $8 billion.
This included potentially putting 43,000 residents and 9000 local employees at risk, destroying 6500 homes and flooding 14,000 homes above floor level.
Premier Mike Baird told Fairfax Media that raising the dam wall 14 metres would "will offer significant extra protection for townships downstream, including Windsor, Richmond and parts of Penrith".
"This crucial investment is being made so we can reduce the risk to people, animals and property in the event of serious flooding," he said.
The $58 million to be included in Tuesday's NSW budget will go towards phase one.
Of that, $30 million will be used for concept designs, environmental assessments and the full business case to raise the dam wall.
The final $28 million will be used to "increase community flood risk awareness, create evacuation signage, improve flood forecasting and integrate the flood risk management with regional planning".
The government says raising Warragamba Dam's wall by 14 metres is expected to reduce potential economic impacts from flood risk "by about 75 per cent on average".
In its 2012 report to the state government, Infrastructure NSW recommended completing a review of the options for a flood mitigation plan, including raising the dam wall, by 2017.
While it did not specify an ideal dam wall increase, it referred to a 1995 report by NSW authorities which recommended raising it by 23 metres.
A spokesman for Mr Baird said the 23-metre "option" was to address both dam wall safety in the worst possible flood and flood mitigation.
The Carr Labor government decided to build a spill way to ensure dam wall safety in the worst possible flood event but did not fully address the mitigation issue.
"This dam wall raising will maintain dam safety and is assessed to be the most efficient and effective height for significant reduction to flood risk to life and property in the Valley," he said.
The spokesman said the government had met Infrastructure NSW's time frame and that "implementation of the preferred options will commence" by 2017.
"Finding the optimal cost effective recommended actions required detailed and cutting edge modelling of thousands of flooding, evacuation and infrastructure scenarios," he said.
Warragamba Dam is 97.7 per cent full after recent storms. WaterNSW is monitoring the dam in light of forecasts of "a significant rain event in coming days" and says if there is sufficient rain in its catchment, Warragamba could spill.
