Taking your chances with the “100-year” flood?

Why would Floridians knowingly put structures on a river bank or seashore right in the path of a “100-year” flood? For many people, the statistical odds of disaster might be acceptable. How so?

The hundred-year flood is estimated to occur, at least statistically, in 1% of years. Put differently, 99% of years are estimated to not have a flood of that magnitude. If a person’s time horizon of flooding concern is 50 years, the chances of not going over the 99% event for the next 50 years are 0.99 to the 50th power: 0.605. That means a 60.5% chance of not seeing a 100-year flood at an individual site in the next half-century. (Or, a 39.5% likelihood of experiencing the 100-year flood one or more times over that same period.)

Yes, estimates of flood frequency are not exact. Yes, rainfall and flood patterns may be changing. Yes, floods of less than 100-year severity can be quite damaging. Yes, the statistical likelihood for regional and state flooding is greater than for an individual. Yes, people are not good intuitive risk assessors.

Still, I suspect that many people feel that they can luck out. A 60.5% chance of a 100-year flood over 50 years doesn’t intimidate them. This is even more true if subsidized flood insurance is available or if they hope to receive emergency disaster assistance after the big flood.

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