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You may not be interested in math, but math is interested in you.

To determine a floodplain boundary, we first identify a “storm event” that concerns us. We use historical rainfall data and some statistical magic to calculate the worst storm event a place is likely to experience in a 100-year time span, probabilistically speaking, and we call that the “100-year storm.” There’s a push in the field to quit calling it that, because it confuses the muggles, so now we often say something like “the storm which has a 1% chance of happening in any given year.” Then we take that rainfall data, judiciously apply more math, and turn it into a flow rate in a river. Then we do hydraulics (more math) to determine how deep the river will have to be to carry that much water, and we draw a line on a map. […]

Let’s quickly walk through this. The chance of flooding, P(F), is 1%, or 0.01. The chance of not flooding, which we notate P(F’), is 100%-1%, or 99%, or 0.99. To see the chance you don’t flood two years in a row, you would have to “not-flood” the first year, and then “not-flood” the second year, so you multiply the two probabilities together, and get 0.9801. The chance of “not-flooding” 30 years in a row is calculated by multiplying the chance of not flooding with itself, over and over, 30 times, which is a power relationship. P(F’)³⁰. That’s 0.7397 chance of 30 consecutive years of no flood, which means a 26% chance of at least one flood.

And then your mortgage broker doesn’t give you your thirty-year fixed rate loan, because a 26% chance of a disaster is a big chance, when we’re talking about disasters.

Now let’s talk about a bigger, nastier disaster than a flood.