More Than 17 Million Properties Face Substantial Flood Risk: Study

July 17, 2023 by

More than half of the U.S. population resides in a county that is likely to experience stormwater system failure due to flooding from heavy rainfall events, according to a new report published by First Street Foundation.

Another 17.7 million properties across the country are at substantial flood risk, which is more than two times greater than the amount included in FEMA’s 1-in-100 year Special Flood Hazard Area (SFHA), the report found.

FSF’s “The 8th National Risk Assessment: The Precipitation Problem” reveals an urgent need for accurate rainfall data at the federal level. The report states that National Oceanic Atmospheric Association (NOAA) and FEMA rely on outdated precipitation data.

NOAA’s current analyses of rainfall estimates don’t reflect present rainfall conditions or the changing climate and are inconsistent across regions, the report said. NOAA received appropriations from Congress in 2023 to update its analyses but such work will not be completed until 2027.

The FEMA Special Flood Hazard Area (SFHA) – the current authoritative flood risk information standard in the U.S. – doesn’t adequately account for precipitation, which leaves over half the country unaccounted for flood risk, according to the report.

FSF created a precipitation model (FSF-PM) to correct NOAA’s inaccuracies and provide a better understanding of heavy precipitation occurrence and flood risk. What the NOAA labels as 1-in-100 year precipitation events are estimated by FSF-PM to occur in many locations more frequently than previously estimated, the report said.

Fifty-one percent of the population is now more than twice as likely to experience what the local communities would consider a 1-in-100 year flood, according to FSM’s modeling.

Outdated characterization of flood risk is further exacerbated by the FEMA SFHA, the report said.

There are over 8 million properties not included in SFHA that are at risk of flooding due to precipitation, FSF’s flood model found. The states with the highest percentage of their properties not included in the SFHA due to precipitation risk include Washington D.C. (91%), Mississippi (90%), Iowa (83%), Kentucky (83%), and Rhode Island (83%), the report said.