What is a 1-in-5,000-year flood?
Helene caused historic flooding in East Tennessee.
NASHVILLE, Tenn. (WSMV) - The Tennessee Valley Authority (TVA) called the rainfall at the Nolichucky River watershed, associated with Helene, a one-in-5,000-year event
Scientists and engineers frequently use statistical probabilities to put into context floods and their occurrence. To understand these probabilities, the U.S. Geological Survey (USGS) annual peak streamflow values.
TVA is calling the flooding of the Nolichucky River watershed a one-in-5,000- year event. These terms are used to describe the extreme nature of the flood and help us to compare how common floods are annually.
USGS Hydrologists clarify that these terms are not connected to the number of years between floods. A 5,000-year flood means a flood of that size or greater has a 0.0002 chance (or 1 in 5,000 chance) of occurring in a given year.
The chart above shows where notable water levels stood in the Nolichucky River watershed. TVA says the waterway crested over the dam on Sunday morning (September 29), marking one of several rainfall records the East Tennessee region has seen from the remnants of Helene.
This was a statistically rare flood. That is what calling it a 1-in-5,000-year flood sheds light on.
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