July 5th 2022 represented the worst day of flooding in the Hawkesbury in 44 years.
This flood exceeded 13.93m, beating both the March flood (13.71m) and the 2021 flood (12.91m). You would have to go back to 1978 (14.5m) to see a worse flood. The 1867 flood was 19.6m. A “one in a hundred” flood is 17.3m, each measured at the Windsor gauge.
I toured the district, commiserated with locals, and took note of what I saw. This video represents the waters as they peaked. Councillors are being briefed daily by Council staff and we’re proactive in guiding the response to the emergency.
I am reminded yet again how magnificently we stand by one another in times of trial. Our community is strong, and the only way we can get through this, is together.
There is one common misunderstanding that’s worth clearing up, because I hear it very frequently.
Warragamba is not a flood mitigation dam. It was built as a water storage dam in 1960 and legally cannot be deliberately operated at below 100% capacity in the sense of dumping water overboard in anticipation of a rain event, or permanently operating it at a lower capacity. This is unlike other city dams like the Wivenhoe Dam in Brisbane which has a permanent ‘air gaps’ to act as a buffer in times of flood. Wivenhoe’s water storage capacity is 1160 gigalitres but its air gap is a further 1967 gigalitres. IN other words, two thirds of its capacity is a flood buffer.
Warragamba’s current capacity is 2031 gigalitres, and the plan to raise it by 14m will add about 1000 gigalitres of emergency storage.
Some have argued that the water level could be drawn down from 100% as the result of a long range forecast for rain, anticipating an East Coast Low for example. Others argue the permanent level of the dam should be lowered and that this is equivalent to the kind of mitigation that raising the dam would confer.
However, this would not confer a sufficient degree of mitigation to be worthwhile. And here’s the analysis of flood hydrology that proves it:
If the dam level had been drawn down just before the 2021 flood, the resulting flood height would have been only 20cm lower.
If the dam was permanently kept lower by as much as 5m, it would have made a 60cm difference. Any more and Sydney’s drinking water would be imperilled. Had the water supply been lowered by 5 metres in 2016 for example, the dam storage would have dropped to around 26% in February 2020 at the end of that drought – a critically low level, even lower than during the Millennium drought when the storage reached 32%.
If a raised dam had already been in place, 500 of the 600 houses flooded along the Hawkesbury-Nepean floodplain would have been spared since that flood would have been 3.5m lower. And in the events of still-larger floods, the degree of protection grows further.
Only raising Warragamba dam will confer the necessary degree of mitigation. I commend this Infrastructure NSW FAQ and also the NSW Government’s post 2021 flood report for more information.