Some Floods Are So Severe They Rarely Strike More Than Once A Century, But Rising Seas Could Threaten Coastal Communities With Yearly Extreme Floods.
12 September 2023

WASHINGTON — Most coastal communities will encounter 100-year floods annually by the end of the century, even under a moderate scenario where carbon dioxide emissions peak by 2040, a new study finds. And as early as 2050, regions worldwide could experience 100-year floods every nine to fifteen years on average.

A 100-year flood is an extreme water level that has a 1% chance of being exceeded in any given year and is based on historical data. Despite the name, 100-year floods can strike the same area multiple years in a row or not at all within a century. But a new study finds that those historical trends will no longer provide an accurate outlook for future floods.

“The threshold that we expect to be exceeded once every hundred years on average is going to be exceeded much more frequently in a warmer climate until they are no longer considered 100-year events,” said Hamed Moftakhari, a civil engineer and professor at the University of Alabama who supervised the project. The study was published in Earth’s Future, AGU’s journal for interdisciplinary research on the past, present and future of our planet and its inhabitants.

On the coast, extreme floods can be caused by water pushed inland by storms, tides and waves, but this study focuses on a component that contributes to flooding over a much longer time scale — sea level rise. As higher seas creep up the shore, coastal infrastructure will be closer to the water, making storms, tides and waves more likely to impact communities.

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