“When you have two 500-year floods within two years of each other, it’s pretty clear it’s not a 500-year flood.” – Roy Cooper, North Carolina Governor
The idea of back-to-back 100-year floods sounds strange to most people. A 100-year flood should only happen every 100 years, right? Not quite.
The confusion comes from the description “100-year,” rather than the statistics behind the description. In this blog, we’ll shed some light on what hydrologists mean when they refer to an X-year flood.
Historical Data
The data used to predict precipitation events like 25-, 50-, 100-, and 500-year floods are gathered from streamflow-measurement sites on waterways. Some of these sites have been used to gather data for 100 years, but many locations have only been used more recently, so the resulting data sets are smaller.
Statistics guide a lot of the predictions that scientists make. The larger the data sample used to calculate the statistics, the higher the confidence in the prediction.
One challenge hydrologists face is making predictions based on smaller, more recent data sets.
So, What Is a 100-Year Flood?
The term “100-year flood” is used as an abbreviation to describe a flood that statistically has a one-percent chance of occurring in any given year based on historical data. Likewise, a 50-year flood has a two-percent probability of occurring in any given year. So, while the likelihood of an annual event with a one-percent chance of occurring in two consecutive years is low, it’s still possible.
Considering this a bit more technically, hydrologists use frequency analysis to predict the probability that any particular streamflow will occur during any year. For example, they might predict that there is a 1-in-50 chance that a streamflow of 7,500 cubic feet per second (ft3/s) will occur during any year at some particular streamflow-measurement site. This peak flow of 7,500 ft3/s site is described as having a 50-year recurrence interval. Hydrologists use the term “recurrence interval” to describe the period of years represented by the statistic.
If that level of streamflow causes the waterway to overflow its banks, then it will be considered a flood. And because there’s a two-percent probability of that level of streamflow occurring in any given year, people will call it a 50-year flood, and they may think they’re safe from another such flood for another 49 years. But that’s not the case. The following year, there will still be the same two-percent chance of a flood occurring.
The table below provides recurrence periods and the percentage probabilities of a flood event.
If you live on a floodplain (a flat area of land next to a river or stream that is prone to flooding) with a recurrence interval of two years, you know there’s a 50 percent chance of a flood occurring every year.
But just like flipping a coin, where you can have a run of heads before coming up tails, you might experience several “2-year” floods in consecutive years, or you might have be flood-free for several years.
Phrases like “100-year flood” tend to cause the public more confusion than hydrologists would like. They would prefer to just talk about the probability that a flood will happen.
Next Steps
If you have questions or concerns about this or other water-related issues, BAI Group can help. For over 30 years, BAI Group has assisted private industry and government agencies with flood plain management issues, flood insurance study revisions, stormwater management design and permitting, and other “storm of the century” issues. Contact us today to learn more about our groundwater services.