Report: Raised dam wall would have saved homes in Hawkesbury-Nepean floods

A government report analysing the devastating floods across the Hawkesbury-Nepean has revealed how more homes could have been saved.

The devastating floods in the Hawkesbury-Nepean Valley could have been significantly less damaging if the wall of the Warragamba Dam was raised, a government review has found.

The largest flood to occur in the region in more than 30 years has been analysed by Infrastructure NSW with the agency assessing what impact various flood mitigation strategies would have had on the damage.

The organisation assessed the impact of pre-releasing water from the dam’s storage based on rain forecasts, permanently lowering the fully supply level and raising the dam wall.

The NSW government has proposed raising the wall of the dam but the idea has been met with staunch opposition from conservation groups who say it will damage cultural sites in the Blue Mountains.

The new Windsor Bridge was engulfed for days during the floods. Picture: Tim Hunter.

But the review’s findings support the argument to raise the wall, with the report stating the higher dam would have “spared the new Windsor Bridge form being overtopped, much reducing closure time”.

“The proposed raising of Warragamba Dam would have reduced the flood peak at Windsor by 3.4 metres, and delayed outflows from the dam by three to four days. The number of impacted dwellings and manufactured homes would have reduced by around 80 per cent and 60 per cent, respectively,” the report said.

“These benefits would be achieved by temporarily capturing water in the approximately 1000-gigalitre flood mitigation zone made available by the dam raising.”

The floods saw the new Windsor Bridge completely engulfed by flood water on March 22 and the structure was not visible again until March 26 when the water levels began to drop.

The report found 80 per cent of the houses could have been spared if the wall was raised. Picture: Dylan Coker

“The proposed dam raising would have reduced peak flood levels by 5.3 metres at Penrith and 3.4 metres at Windsor,” the report said.

“Compared to the option of permanently lowering FSL (full supply level) by 12 metres, the dam raising would have provided additional peak level reductions of 3.0 metres and 1.5 metres for Penrith and Windsor, respectively.”

Alternatively, pre-releasing water from the dam’s storage based on rain forecasts would have provided “minimal reduction of downstream flood peaks” and brought forward the closure of “downstream bridges” which would have made it potentially more difficult for emergency services to assist some communities.

The other option of pre-releasing water at an earlier date would put the region’s water supply at risk and “still have provided relatively small benefits”.

The inquiry into the proposal to raise the dam will have its next hearing on Monday. Picture: Toby Zerna

Another option considered was permanently lowering the full supply level by 12 metres, but the report stated that while this would have reduced the damage, there were still more benefits in raising the wall.

“Permanently lowering FSL by 12m would have reduced the flood peak at Windsor by 1.9 metres, and delayed outflows from the dam by two to three days. The number of impacted dwellings and manufactured homes would have reduced by around 65 per cent and 40 per cent respectively,” the report said.

The report will be discussed in Monday’s Upper House inquiry into the proposal to raise the Warragamba Dam Wall which will hear from senior officials from Infrastructure NSW among other witnesses.

Originally published as Report: Raised dam wall would have saved homes in Hawkesbury-Nepean floods