Marvin Nauman/FEMA photo
Marvin Nauman/FEMA photo

More than 40 million Americans are at risk of flooding, with assets valued at $2.9 trillion exposed to flood loss—about triple the risk estimated by FEMA's official 100-year floodplain maps, according to a new study by experts at the University of Bristol in the United Kingdom (see: "Estimates of present and future flood risk in the conterminous United States," by Oliver Wing and colleagues). And that's just the risk from rainstorms and flooded streams, not including the risk of hurricane storm surge, study author Oliver Wing told JLC.

The Miami Herald reported on the study in March (see: "FEMA flood maps massively underestimate real risks, study finds. Florida’s a hot spot," by Jenny Staletovich). "With about $714 billion in property located in a 100-year floodplain, Florida is a national hotspot," the paper reported.

FEMA Deputy Public Affairs Director Eileen Lainez told the Herald that the agency is aware of the new models used in the Bristol study, but she said FEMA has to verify new modeling before the agency can integrate any results into its maps. “These maps are intended to inform flood insurance requirements and regulate development standards in high-risk areas—they are not intended to show absolute lines where flooding will and will not occur,” Lainez told the Herald. “Anywhere it can rain, it can flood.”

At an insurance conference in Miami Beach in March, Roy Wright, the chief executive of FEMA's Agency’s National Flood Insurance Program (NFIP), made it clear that FEMA doesn't say its flood zones are the only places where flooding is ever likely to happen (see: "America’s flood insurance chief has a message for all Floridians: You’re at risk," by Jenny Staletovich). If you live in Florida, rather than scrutinize NFIP maps, it would make sense to take a look at your driver's license, said Wright: "If it says Florida, you need flood insurance."

But most homeowners only carry flood insurance if their mortgage lender requires it—which only happens if the house is in FEMA's official NFIP flood zone. And even before the latest study from Bristol, it's well established that floods are quite common outside that FEMA-defined zone. One telling example: a study examining 10 years' worth of Houston, Texas, flood data—released just before Hurricane Harvey struck last summer—found that three-quarters of flood damage in the Houston area occurred outside the designated flood zone (see: "Decade of data shows FEMA flood maps missed 3-in-4 claims, by Jade Boyd).

The University of Bristol study is a leap forward from FEMA's modeling efforts, which have been hampered by lack of funding. Write the Bristol researchers: "This study is a first of its kind; utilising highly resolved, spatially comprehensive flood hazard information derived from a model that properly represents the physics of flood spreading in combination with high-resolution estimates of the present and future distribution of people and assets."

And it's worth noting that the Bristol study looks only at rainfall and fresh-water runoff and river flows, not at the equally dangerous threat of storm-surge flooding caused by hurricanes making landfall on the coast. Oliver Wing, one of the study authors, confirmed to JLC in an email: "We look exclusively at freshwater flooding in this study: there is no storm surge component to this analysis at all."

Adding storm surge to the picture would increase the risk, Wing observed: "I don't have any numbers to give you in terms of how much our stated figures would expand if we did incorporate storm surge - and in many cases, people will be at risk from both freshwater and coastal flooding, so 'double counting' exposed people wouldn't bring the total up - but I would expect the complete picture of riverine/rainfall/coastal flooding to produce much bigger numbers than ours."

And as a report in Scientific American notes, rainfall flooding and storm-surge flooding can combine in a single weather event, creating a risk that is greater than either factor on its own (see: "National Flood Insurance Is Underwater Because of Outdated Science," by Jen Schwartz).

"Thomas Wahl, a coastal engineer and oceanographer at the University of Central Florida ... published a paper in Nature Communications in 2015 on the effects of compound flooding, which is when increased river discharge, precipitation and storm surges happen at the same time," reported Scientific American. "This helped drown communities near Houston during Hurricane Harvey."

Said Wahl: "The biggest gap is the fact that FEMA’s maps do not connect inland flooding and coastal flooding. FEMA creates a flat map from the river side of things and a flat map from the storm surge side of things, and they just overlay them, which assumes that these two things are completely independent. But most tropical storms bring a lot of rain and storm surges. We understand why these events happen simultaneously, but what we haven’t done is include that information into risk assessments."