Environment, Coasts, Oceans, & Infrastructure

A Civil Engineering Source article by Dr Ceyda Polatel on Aug 25, 2026: 'That will never happen again:' How are flood-risk language needs to evolve – discusses an issue that has long been understood as a misleading way of communicating to the public. If not seen – have a look, it's a good read article.

In the 1996 – USACE EM1110-2-1619 publication manual: Risk-Based Analysis For Flood Damage Reduction Studies – the issue is discussed in Chapter 3. It says: Long-term risk is a useful index for communicating plan of performance because it provides a measure of probability of exceedence with which the public can identify . . . Like Polatel – the manual recognized how a certain return-period event expressed in years can cause a communication blunder in public mind.

Long-term risk, is also known as Encounter Probability (further in Uncertainty and Risk and The World of Numbers and Chances). It says: It relates the return period (in years) of an event to the design life of a project. For example, the likelihood or probability that a 100-year event will be encountered during the 30-year design life is 26%. Note that return period is related to Annual Exceedance Probability (AEP) via the known sampling interval.

In the first graphic Figure – Polatel presented the long-term risk characterization or encounter probability as: Chance of Experiencing at least one "100-Year" Over Time.

Have a read – share your opinion and thoughts.

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Dr. Dilip K Barua, PhD

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Thanks for posting. I thought it was an excellent article, too, and posted it in the Risk Management Community of Practice. N.B. You and others might want to join this group.

https://collaborate.asce.org/riskmanagement/discussion/how-our-flood-risk-language-needs-to-evolve#bmc52a5311-2816-47f4-b3b5-03dd0bb2ce33

I asked my wife, a non-engineer, about communication of the 100-year flood risk. She found expressing it as a 26% chance of occurring over 30 years as much more compelling than speaking of it as a 100-year risk. Her point was that many people have a 30-year mortgage and can see themselves in their house for 30 years.