What "100 year flood" really means

How likely is a "Hundred Year Flood" this year? Does the likelihood change when you've just had one?

I have a subscription to an online new source. Many of the stories it publishes are open for comment. One of the reports was about a Turkish geologist, Naci Gorur, who was trying to raise earthquake awareness. I saved the following comment because it makes a crucial point about what the probabilities of "rare" events actually mean. The highlighted sentence sums up the math. Percentage odds are not intuitive. I've added the calculation below Repetto's comment. I used my computer's calculator to do the arithmetic.

[ by R.C. Repetto, Amherst, MA]

People can't deal with probabilities, such as "a hundred-year flood". If there was one ten years ago, they think they're safe for another 90 years. No, they face a one percent probability there will be one next year and more than a ten percent chance* there will be one in the next decade. That misunderstanding and shortsightedness is why people still move into disastrous locales, such as Florida or Phoenix or the mountainous regions of the West. It makes a mockery of the claim that "we" can adapt to climate change. We haven't and won't, until it's too late.

* If the odds of some event is 1 percent (one per hundred) per year, then the odds that it will happen within the next 10 years are (1.01^10*100)-100, or 10.4%

Footnote: If you knew there was a one percent chance of having an accident every time you drove your car, would you drive it?

A ramble through Stuart MacLean's Mind (The Vinyl Café Notebooks, 2010)

Stuart MacLean. The Vinyl Café Notebooks (2010) Just what it says. McLean sorted them according to themes. The tone is a mix of Welcome Home and the Vinyl Café stories.

An enjoyable read, even, I think, for people who aren’t fans. As in the stories, McLean sometimes pounds home the themes, which to me feels like he doesn’t trust his readers. Then I see an online post of some supposedly true-life story whose lessons are explained at (usually sentimental) length. And I recall the student who had trouble understanding anything more than the literal content of the stories. Which means, among many other things, that we tend to think that’s what’s easy for us must be easy for everybody. And so we come to so-called common sense, which is neither, most of the time. It’s just the notions that seem obvious to us, limited by our experience, and our brain’s depressing tendency to take a single example as proof of a generalisation.

OK, looks like I’ve committed Mclean-like ramble of thoughts.

Recommended. ***