What Is a ‘100-Year Flood’?

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Insurance Navy Auto Insurance What Is a 100-Year Flood

What’s a 100-year flood? It may sound self-explanatory but that’s not quite the case. In fact, it’s a topic that may seem complex and daunting at first but that’s why I’m here, to ease you into the subject. That way, very quickly, you’ll realize that this topic is equally important to know as it is easy to grasp with the proper guidance.

The Story of the “100-Year Flood”

A 100-year flood isn’t simply a flood somewhere in the world that occurs for 100-years at a time. It’s a method used to determine the probability of a flood of a certain magnitude occurring in a given area. The complexity of the true definition of a 100-year flood is why many hydrologists prefer describing the given area as having a “100-year recurrence interval” or a “1-percent AEP flood”.

Now, you’re probably asking “Alllright, so what the hell is a recurrence interval?”, to which I’ll tell you not to get too caught up on that right now. The more I explain the whole process, the more that phrase will begin to make sense. Let’s get back to the topic at hand, however. In fact, this quote from is a great summarization of that very topic: “Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100-years, it often is referred to as the ‘100-year flood'”.

Essentially, despite the designation “100-year flood”, it’s a term used to describe a flood’s magnitude, not duration.


While you’re letting that information sink in, you’re probably thinking “Ok, but why exactly is this even a thing?”. It’s a fair question because to folks who are new to the subject, it probably seems like an overly convoluted way to determine an area’s peak flood risk. For this explanation we’ll take a look at another wonderfully helpful quote from USGS:

“In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program.”

Evidently, this 1% AEP flood was then considered to be an appropriate balance between “protecting the public and overly stringent regulation”. Thus, the “100-year flood” was born. At the end of the day, the “100-year flood” data is simply used to determine the flood risk of that particular area. By studying the probability of a specific magnitude being either equaled or even exceeded, the risk of that area can be determined by experts.

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