First Street Foundation: Flood Risks Vastly Underestimated

According to a new report from First Street Foundation, a non-profit focused on weather risk research, the traditional “100 Year Flood” is now expected to happen every 25 years for 21 million Americans. In the most extreme cases – high risk counties that are home to over 1.3 million people – are expected to experience a “100 Year Flood” at least once every 8-10 years.

Intense rain events, like atmospheric rivers and torrential, training thunderstorms, are quickly making the idea of a “100 Year Flood event” obsolete, according to the 8th National Risk Assessment: The Precipitation Problem. As air temperatures increase, more water vapor may be held in the atmosphere and discharged during rainfall events. For every 1°C increase, 7% more water vapor is carried by the same air volume. As a result, increasing temperatures have created changes in the expectations of the intensity, duration, and frequency of rainfall events.

As the report notes, the consequences for not understanding these changes include overwhelmed stormwater systems that lead to residential and commercial flooding, impassable roads, critical infrastructure failure and even loss of life in unexpected high-intensity flash flood events.

How the map looks, according to First Street Foundation