Luke Grant speaks to Stewart Franks, a Hydro-climatologist and former professor of environmental engineering at the University of Tasmania & Newcastle, about these so called one in 100 year or one in three and a half thousand year floods and what that actually means.

Professor Franks says 1 in 100 (or more) flood levels are inaccurate and misleading and are only based on limited data of typically less than 100 yrs. He calls them an ‘Engineering convenience’ which provides a ‘consistent’ standard for design and planning but to get an accurate estimate of the yearly significance on an event, you should have many samples, not just one hundred year sample that we are relying on.

He points out that if you wanted a meaningful number based on 100 samples, then that’s 10,000 years of data, or back to the Ice Age when climate certainly was different. He says, the concept of a 100 year flood event assumes climate risk is static – it isn’t.

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