Wednesday, April 21, 2010

USGS wants to stop calling them "100-year floods"

Why do we have 100-year floods more often than once every century?

The USGS has published a new poster explaining the origin of the idea of the "100-year flood," and why it's been so misunderstood.    They are now urging that instead, we use the annual exceedance probability (AEP) terminology which is defined as "the 1-percent AEP flood has a 1 in 100 chance of being equaled or
exceeded in any 1 year.”

They propose we should discuss the “1-percent AEP flood” as opposed to the “100-year flood.”

Frankly, I have a hard time expecting the public and news media making this change, even if the scientific and technical communities adopt it.


  1. What now no monsoon and no 100 year floods? Where will we go next, no hot temperatures?

  2. ...uh, so we just shouldn't build in flood plains?

  3. Anonymous3:25 PM

    A 500-year flood has a 1/500 chance of occurring. So we call it a 0.2% AEP flood? How does that make it conceptually less difficult to understand?

  4. Anonymous1:52 PM

    Floods don't happen over a year (more like on any given day) so the 500 year flood would have a 1/182500 chance of occurring. I prefer the easier and more media glorious 100 year religious atonement flood where all first born males are wiped out from the flood... people watch that with more vigor and attention on the news.