Now, a scientific study in the journal Environmental Research Letters pinpoints just how much: 41 million Americans live in a 100-year flood zone - three times as many as the Federal Emergency Management Agency estimates. That means a full 28 million are outside the boundaries of the 100-year flood zone on current FEMA maps, but would be in it if FEMA used what the study argues is better data.
"Producing maps the FEMA way essentially misses a lot of flood hazard," Oliver Wing, of the University of Bristol and lead author of the study, told City Lab. "And these maps are what inform risk management decisions in the U.S. at the moment."
One big problem is that FEMA maps tend to ignore smaller streams running through populated areas, Wing said.
In Harris County, for example, FEMA maps don't account for about half of the county's 2,400 miles of channel, according to the Harris County Flood Control District.
The study suggests that $5.5 trillion in assets are in the zones, with $400 billion of that in Texas. That figure will triple or quadruple by 2100, the study says.
It doesn't take into account sea level rise and heavier rainfall expected to come along with climate change, so it likely underestimates the exposure, the authors warned.
The 100-year floodplain is the area expected to flood in a storm that has a 1 percent chance of occurring in any year. Over the life of a 30-year mortgage, a home in that zone has a 26 percent chance of flooding.
Across the nation, cities including Houston use the FEMA-mapped 100-year floodplain to regulate what buildings are allowed and how high they must be elevated. But even people living outside of these zones are at risk of flooding, as Harvey demonstrated.
Mark Collette investigates flooding, government, chemicals, ice cream and everything in between. Follow him on Twitter and Facebook.
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