As water resource professionals, we use the term “100-year flood” quite often when determining the highest risk of flooding while working on flood protection systems such as levees and dams. We use the term “100-year storm” quite often while working on storm drainage projects. But flood and storm statistics can be confusing, and these terms may not mean what you think.
Is a 100-year flood the same as a 100-year storm?
The term “100-year flood” is defined as a flood that statistically has a 1% chance of occurring in a particular location in any given year. Similarly, the term “100-year storm” is used to define a rainfall event that has a 1% chance of occurring in a particular location in any given year.
Does a 100-year storm cause a 100-year flood?
The US Geological Survey defines a flood as “any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream.” A 100-year storm does not necessarily mean there will be a 100-year flood because runoff volume timing is based on several factors besides rainfall—the most important being how saturated the ground already is when a storm hits. The more saturated the ground, the less moisture it can accept, so more water will run off and accumulate in streams. Other factors influencing runoff include the type of ground cover and vegetation, terrain, storm duration, and precipitation intensity.
Flood maps developed by the Federal Emergency Management Agency (FEMA) are one tool communities use to know which areas have the highest flood risk. FEMA maintains and updates data through flood maps and risk assessments. A comprehensive analysis of precipitation return period climatology is provided in NOAA Atlas 14, an online precipitation frequency information resource.
What does the future hold?
Will we have more 100-year rainfalls and 100-year floods? Flood predictions, like all weather predictions, depend on a unique set of environmental variables that are almost never repeated. This means the one thing we can count on is that the results of any given storm will always be different. However, the likelihood of flooding is significantly increased due to the extreme weather patterns. Changes in the geography of the land also have a part to play.
As 100-year weather events happen with increasing frequency, signs of extreme weather risks to our aging infrastructure are everywhere. These risks are not going away any time soon. At Mead & Hunt, we are actively working with our clients to develop design options that address stormwater management and flooding problems in tandem with proposed improvements to their traditional drainage infrastructure systems. We will continue to do so as we move forward.