Lies, dam(ned) lies and flood statistics – Ian Fuller

Massey University’s Associate Professor Ian Fuller says we need to build up knowledge of past flood events so that we can better understand the risk of future events.

An excerpt (read in full):

So, another “500-year” flood event in the Bay of Plenty. The last was in 2005 at Matata.

Of course, this doesn’t mean that in either Edgecumbe or Matata we need to wait another 500 years before seeing a repeat of such a big event.

As ably put by Niwa hydrologist Roddy Henderson in Q&A: What is a one in 500-year flood, and does that really apply to Edgecumbe?, it’s about probabilities: a 500-year flood has a 0.2 per cent likelihood of happening in any one year.

These are rare events. No doubt the exact statistic assigned to Edgecumbe’s flood will be debated.

What matters most is that it exceeded stopbanks designed to protect Edgecumbe, and did so by 30 per cent, according to the Bay of Plenty Regional Council.

As noted by Henderson, the problem with flood statistics is that they are derived from short records; just 65 years in the Rangitaiki River.

We do not have data to reliably determine how often such large floods recur. The likelihood of the largest floods occurring during short periods of data measurement is inevitably small. Further complicating the issue is climate, which changes. If the climate shifts to a more extreme phase, frequent larger floods can be expected. Models cannot take this into account, because they use only existing data.

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