500-year floods aren’t what you think

Published: Sep. 3, 2021 at 12:17 AM EDT
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(WHSV) - The term “500-year flood” is used commonly when an area sees historic flooding. This does not mean the flood will occur every 500 years.

The definition of a 500-year flood is based on probability. There are 100-year floods, 500-year floods, 1000-year floods and so on. A 500-year flood is defined as a 0.2% chance of seeing a significant flood event. This probability averages out to seeing this kind of flood every 500 years but there’s a problem with that probability.

First off, it’s practically impossible to calculate 500-year floods in a given location. Records only date back approximately 150 years depending on location. We don’t have records that date back to 1521. We didn’t even have colonists at this time.

Secondly, 500-year floods have been proved to occur multiple times in a given location. For example, the flooding in New York City caused by the remnants of Ida is classified as a 500-year flood. Ida has drawn comparison to Superstorm Sandy as that event was also a 500-year flood. That was only nine years ago.

When Staunton dealt with flash flooding in August 2020, it was classified as a 500-year flood but that also has happened multiple times such as 1985, 1996, 2002 and 2003. In fact, southeast Texas had 500-year floods for 5 straight years starting in 2015.

The bottom line is a flood of the magnitude in New York City Wednesday or Staunton’s August 2020 flood can happen at anytime. Always have a plan ready in case a significant flood impacts you. In our area, remnants of tropical systems are one culprit of these kind of floods. With climate change, it is anticipated that this occurrence will become more common due to more violent storms.

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