We can’t talk about 100-year floods

As much as 6 inches of additional rain across the Upper Midwest clearly threatens a reduplication of 1993 flooding.

So how can we talk about 100-year floods, if this is the second such occurrence in 15 years?

We can’t.

To borrow a page from the skeptical empiricism of David Hume, in much of the U.S., we only have complete weather data for a little over a century. Sure, tree rings can take us farther back, and in a general way, tell us about particular years being wet or dry.

But, more specific than that? We don’t have the information.

So, we don’t know if a “100-year flood” is anything close to that.

And, from that, it follows that we have no idea if our levees are built high enough. We have no idea of a “100-year flood plain” actually keeps residential housing out of major floods well enough or not.