Some people say they are not located in a 100-year flood zone because their property hasn’t flooded in 100. Even though it is a reasonable thing to say, it is unfortunately incorrect. The “100-year flood” is one of the most misunderstood terms in disaster preparedness.

What’s A 100-Year Flood And How Is It Determined?

In theory, a 100-year flood is something that has a 1-in-100 chance of happening in any given year. It is the kind of event that’s so unique that it might not make sense to plan around the possibility of it happening.

The issue is that 100-year floods are happening more and more often than probability predicts. So prevention planning has to evolve to acknowledge that a 100-year flood isn’t really a 1-in-100 chance anymore.

In the early 60s, the US government decided to use the 1% annual exceedance probability (AEP) as the basis of the National Flood Insurance Program. Scientists used statistical probability to put a context to floods and their occurrence. In order to determine these possibilities, they examine all the annual peak streamflow values measured at a stream gauge to estimate the AEP for various flood magnitudes.

FEMA Flood Maps

To state a bare minimum standard to create its flood maps, FEMA established the 100-year flood zone as the zone where having flood insurance is mandatory. However, being outside this zone does not mean you will not flood. With enough rainfall, any area can be affected by flooding.

It’s particularly hard to get the probability of an event perfect when climate keeps changing. Current weather trends could quickly increase the chance of severe flooding. A warmer atmosphere is able to hold more moisture, which raises the possibility of events such as massive thunderstorms and landfalling hurricanes to produce more massive rainfall totals than before. And that means more floods—much more often than once a century.

As a result, FEMA has to update its assessments of the floodplains. For example, the FEMA maps for Harris County (TX) had just been updated in 2016. However, the city of Houston itself is working off a Hazard Mitigation Plan that was developed in 2012, based on where FEMA established the city’s 100-year floodplains. In the meantime, the reality showed that the old models are out of date. Memorial City, an area of West Houston that was outside Houston’s 500-year floodplain, has already flooded three times in the past decade: in 2009, 2015, and 2016.