What, exactly, does “100-year-flood” mean?


Over the past two decades, Lehigh Valley residents have seen what seems like more than their fair share of flooding. Allentown, Bethlehem, Easton, Hellertown, and other small and large municipalities across the region have been affected.

What, exactly, defines a flood? According to the U.S. Geological Survey, “a flood is any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. Floods occur for many reasons, such as long-lasting rainfall over a broad area, locally intense thunderstorm- generated rainfall, or rapid melting of a large snow pack with or without accompanying rainfall.”

During these extreme weather events, phrases like “100-year flood” are often used, leading to some confusion: How can there be a 100-year-flood this year, when we had one last year?

To find the answer, we need to travel back to the 1960s, when the U.S. decided to define the probability of a flood for the National Flood Insurance Program. Officials decided to base the definition on something called the “1-percent annual exceedance probability (AEP) flood.” The 1-percent AEP means that there is a 1 in 100 chance of a particular flood being equaled or exceeded in any one-year period, and that type of flooding reoccurs, on average, every 100 years.

Clear as mud? We thought so. In short, the term “100-year-flood” does not mean that type of flooding happens only once every 100 years. It means there’s a fairly low statistical probability that a large flood will happen, but that it can (and does) happen. If our weather patterns continue to change, we can expect to see more “100-year-floods” in the near future.

If you’re interested in learning more about 100-year-floods, the USGS has a free poster available that explains the concept in detail. You can view it here.