The Problem With Calling Them 100-Year Floods

The misleading term describes a devastating event that happens more often than you might think

A 100-year flood does not mean what many assume. The often misunderstood term simply means each year there is a 1 in 100 chance that a flood of a certain intensity will hit a specific area.

If you have a 30-year mortgage in a 100-year floodplain it has a 1 in 4 chance of flooding during that 30-year period

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The Takeaway

Building guidelines protect residents enough from floods because they are based on old data and the misunderstood 100-year flood standard. Preparing for the worst keeps property safe from both common and uncommon floods.

  • In a 100-year floodplain, homes have a 26% chance of severe flooding over the life of a typical 30-year mortgage. Since builders in most 100-year floodplains only need to protect structures from this low standard, lots of development is inadequately protected.

  • The big problem: America’s standard is shockingly easy for developers to clear compared to other countries. Both the Netherlands and Japan prepare for floods so severe they only have a 1 in 10,000 chance of occurring each year. Even cities like Houston, TX and Norfolk, VA prepare for severe floods that only have a 1 in 500 chance of happening each year.

  • Yes, and: Rainstorms and floods are getting worse. Events once considered “100-year floods” now happen more often than that yearly 1 in 100 chance. However, we don’t update our guidelines or floodplain maps to reflect the new risk, which allows development to continue where it should not.

The solution: Build smarter and don’t let new development make flooding worse.

  • Adjust guidelines to plan for bigger events. Preparing for a flood that has a 1 in 10,000 chance of happening each year will protect areas from rare but destructive floods, as well as those that are deceptively common (like 100-year floods).

  • Update floodplain maps to accurately reflect the changing levels of flood risk due to changing environmental conditions. 

  • Communicate flood risk in a way that is more easily understood.

The next step: Flood-prone communities should ask their elected officials to move to a higher development standard. Keeping outdated standards is unacceptable as flooding continues to worsen.

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A non-profit amplifying the power of our members' voices to demand and receive better flood protection

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