When it comes to flood preparedness, the term “100-year flood” can be misleading. While it suggests a rare event, it refers to the statistical probability that a flood of a certain magnitude has a 1% chance of occurring in any given year. This means that such floods can happen more frequently. Understanding this distinction is important for engineers and community planners as they assess risks, develop infrastructure, and protect people from the increasing unpredictability of extreme weather events.
Determining Flood Risk from Limited Records
Flood recurrence intervals refer to the average time period in which a specific flood event is anticipated to occur. To calculate a flood’s recurrence interval, hydrologists rank past flood events by size and use statistical methods to estimate the probability of future floods based on historical data. Detailed information on rainfall and other factors that affect flooding often has limited information, and its availability can vary by location. Therefore, a 100-year flood estimate may not be based on a full century of observed data.
Importance of Recurrence Intervals
Recurrence intervals play an important role in flood management by providing a standardized baseline for decision-making where the possibility of flooding is a concern. These intervals, like the 100-year flood, allow engineers, planners, and policymakers to assess the probability of flooding events and plan accordingly.
While preparing for future flood risks is inherently a matter of chance—since no one can predict precisely when and how severe the next flood will be—recurrence intervals are a standardized approach to this guesswork. They help verify that everyone involved in flood preparation and infrastructure design is working with the same data and risk assessment. This assures everyone is on the same page.
Related Reading: 100-Year Flood vs. 100-Year Storm: What’s the Difference? »
Formulas Versus the Future
Climate change presents a sizable challenge to the reliability of flood recurrence intervals. Changes in weather patterns and increased frequency of extreme weather events can alter the probabilities of flooding. The industry is adapting by incorporating climate change into rainfall frequency estimates which in turn will affect the probabilities of flooding. The National Weather Service (NWS) Office of Water Prediction received Federal funding to update the NOAA Atlas 14 precipitation frequency standards to account for climate change. These updated precipitation estimates will be referred to as NOAA Atlas 15. The final Atlas 15 for use and application will be available in 2026.
How People Shape Floodwaters
Our landscape is constantly changing. Rainfall today versus rainfall 50 years ago may have different impacts due to urban expansion and the resulting increased strain on stormwater systems. Changes in geology, river systems, and surrounding landscapes—whether driven by natural forces or human intervention—will inevitably impact where flooding occurs.
Our team of certified floodplain managers and hydrologists have a sound understanding of flooding principles and the science behind increased flood risk. Our team of professionals is fully prepared to help address concerns or issues related to flooding.
As we rely on yesterday’s data to shape tomorrow’s infrastructure, it’s important to recognize that the last 100 years may not accurately predict future flooding events. By understanding the limitations of historical data, we can design resilient, adaptable infrastructure that better prepares our communities for future uncertainties.
