What Is A 100-Year Flood?

You may have heard the term “100-year flood” before, but what does it mean and why does it matter?

The first important point to make is that, contrary to popular belief, the 100-year flood is not indicative of how often a certain flood magnitude actually happens – it’s purely a product of probability, and how likely flooding of a certain magnitude is in any given year.

To break down how this works, it’s important to understand where our understanding of flood probability comes from. The U.S. Geological Survey maintains thousands of stream gauges on waterways across the nation, and these mainly measure stream flow. Stream flow is calculated by first measuring the depth of the stream at a given point using a gas-filled tube – the deeper the water is, the more pressure is exerted on gas being fed into the tube.

Using this well-established relationship to determine the height of the stream, hydrologists then compare the height to the expected streamflow (measured in cubic feet of water per second) on specialized charts for each site. Doing this with standardized curves (i.e., the same chart for each site) would lead to highly inaccurate data since waterways each have unique geometry which influences stream flow.

This data is very useful for contextualizing ongoing or forecast flood events. Hydrologists can gather each year’s maximum streamflow and plot each year from lowest to highest stream flow. The laws of mathematics show that there will be few occurrences of the lower stream flows (drought years) and of the highest stream flow years (floods).

From this information, the probability of a certain stream flow magnitude in a year can be found. This is, in essence, what a 100-year flood is: a stream flow or water depth that has a 1 in 100 (1 percent) chance of happening in a given year. In the same vein, a 10-year flood has a 1 in 10 (10 percent) chance of happening in a given year, and a 500-year flood has a 1 in 500 (0.2 percent) chance of happening in a given year.

The 100-year flood is not a schedule to forecast when these floods will happen. That’s the job of meteorologists, who create rainfall forecasts that can then be used by hydrologists to forecast future flooding risk. However, with knowledge about the yearly likelihood of a given water depth we can start to assess risk, and that’s exactly what the 100-year flood is designed to quantify.

Using elevation data and hydrological modeling, flood plains can be mapped out to answer a relatively simple question: “when a 100- or 500-year flood happens, what’s going to be under water?” These areas help decision makers plan for future disasters, and can inform evacuations in advance of long-duration rainfall events.

Flood plain data also informs who needs flood insurance and who doesn’t through the National Flood Insurance Program, though it’s also important to know that a lot of factors influence stream flow and by extension flood plains. Urbanization, flood control measures, local wildfires, and many other things can influence how much water gets absorbed by soil and therefore how much water rises on local waterways during a 100- or 500-year flood.

The 100-year flood is an important tool to gauge how severe a flood is, and hopefully knowing more about it will keep your family safe.

Source: USGS, NWS

Image Caption: Subway Avenue, Pomona, Queensland during the 2022 Queensland Floods.(Credit: Nford24 via Wikimedia Commons)