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Dangerous floods are inevitable, so stop putting people in their way

The inundation of homes and businesses in the Hawkesbury-Nepean Valley of western Sydney is tragic for the people affected, but it is no surprise. The losses are due to inept governance, not any act of God.

Dangerous floods in the valley are not new. On April 4, 1817, Governor Macquarie lamented “it is impossible not to feel extremely displeased and Indignant at [settlers] Infatuated Obstinacy in persisting to Continue to reside with their Families, Flocks, Herds, and Grain on those Spots Subject to the Floods, and from whence they have often had their prosperity swept away …”

The valley now presents a high risk to 70,000 residents as physical choke points along the river channel, such as the Sackville Gorge, bank up and slow the discharge of large floods to the sea. In modern Australia we expect our governments to apply expert knowledge in regulations and other programs to reduce excessive danger to citizens but such good governance has been lacking in the valley.

Unfortunately, there are now 5000 houses in the valley built below the one-in-100-year flood level, and a further 7000 lie under the one-in-500-year flood level. The Premier reported on Sunday that the current rain is predicted to be only a one-in-50-year event in the Hawkesbury-Nepean, although a one-in-100-year event on the mid-north coast.

The dangers of more frequent large floods are increasing with a changing climate, making government action to reduce risk more urgent.

The NSW government’s “silver bullet” solution to manage flood risk is to raise Warragamba Dam to hold flood peaks. The global experience is that flood control infrastructure increases danger and losses by encouraging more downstream development that is then inundated as design limits are inevitably breached by a large flood. Unbelievably, the state government plans to allow about 134,000 more people to settle in harm’s way on the floodplain by 2050.

Further, raising Warragamba Dam will not prevent the frequent flood damage from tributaries entering the valley below the dam, including the Grose River and South Creek. Raising the dam would also inundate cultural sites of the Gundangara nation and part of the Blue Mountains World Heritage Area.

In the 1990s, countries as diverse as China and the Netherlands realised that the best risk reduction is to restore floodplains to give rivers room to flood safely by moving people and sensitive infrastructure out of harm’s way. At places such as Gelderse Poort in the Netherlands, these restored floodplains are now the basis of thriving local economies based on agriculture, recreation and nature conservation.

Floodplain restoration could form a part of a better future for communities along the Hawkesbury-Nepean river corridor.

The NSW government should abandon its dam-raising plan and stop allowing new development in harm’s way. Regulators should not allow flood-destroyed infrastructure to be rebuilt as it was.

A long-term program is needed to help the most flood-prone residents and businesses to resettle on safe land. Such programs have been undertaken successfully elsewhere in Australia, most notably, the relocation of the town of Grantham following the 2011 Brisbane River flood.

The government should also reinvigorate stalled plans to rebuild evacuation roads that currently are cut by the smallest floods, leaving residents stranded. It could also increase floodwater holding capacity now by partly emptying the existing Warragamba Dam, with greater reliance on desalination for Sydney’s water supply.

There are no simple or cheap solutions to reducing the risk to life and property from floods in the Hawkesbury-Nepean Valley. Dangerous floods are inevitable and the safest option is to keep people off the floodplain, out of harm’s way.

Professor Jamie Pittock researches and teaches at the Fenner School of Environment and Society at the the Australian National University. He co-convenes ANU’s masters of climate change program.

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