Hydrologist Explains Term ‘100-Year Flood’

July 13, 2021

By Robert Mace, Executive Director of the Meadows Center for Water and Environment at Texas State University

A 100-year flood, like a 100-year storm, is one so severe it has only a 1 percent chance of hitting in any given year.

Unfortunately, many people believe that if they experienced a 100-year flood this year, they will not see another one like it for 99 years.

It just doesn’t work that way. In reality, the chance of being flooded next year, and the year after that, is the same as it was when the house flooded the first time – 1 percent.

One percent is the same as a 1-in-100 chance. Hence, the shorthand: 100-year flood. The Federal Emergency Management Agency uses that measure when it draws flood plain maps – the maps that show which areas are most likely to be flooded and that insurers use when they set rates.

Because of the confusion, many flood plain managers want to do away with the term “100-year flood,” but that creates another problem. People generally do not have a good sense of risk as expressed as a probability, especially when that probability appears small. Look no farther than COVID-19, where about half the U.S. population was not concerned about a 1 percent chance of dying from infection while hundreds of people in the country were dying from it every day.

Why knowing flood risk matters

A better way to understand the risk is to think about a home with a 30-year mortgage.

What’s the minimum risk of a home being flooded over 30 years if it’s in a 100-year flood plain? At least 26 percent, since we’re looking over a longer period and there’s not a guarantee of seeing a 100-year storm. Given that homes tend to be the biggest investment most Americans make, that probability may cause people to think about buying flood insurance.

In some cases, the risks are even higher. Since some homes sit lower than their neighbors, risk in a 100-year flood plain isn’t consistent across the entire area. A homebuyer might consider their choice more carefully if the property actually has a 50 percent chance of flooding over 30 years. At some point, we’ll have better tools to easily assign risk home by home.