More than two million acres of floodplains have been developed in the United States over the past two decades, according to a study.

A nationwide analysis of community-level floodplain development found that more than two million acres of floodplain have been developed over the past two decades across the United States, with about half of all new floodplain housing built in Florida.

These findings, made by scientists at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Sciences, provide new insights into floodplain development patterns that pose a potential risk to people and communities in regions such as the southeastern United States, which are particularly prone to flooding.

In the new study, researchers combined geospatial data on land use, impervious surfaces and housing with information from digitized regulatory floodplain maps to measure the development of new floodplains for communities across the United States. The analysis, published in the journal The Future of the Earthfound that more than 840,000 new residential properties were built in floodplains across the United States, including about 398,000 in Florida, representing 21% of all new homes built in the state and the highest total of any U.S. state.

“Given the size of the floodplains and the amount of new housing growth overall, these numbers are actually much lower than we expected,” said the study’s lead author, Armen Agopian, a doctoral student in the Abess Graduate Program at the Rosenstiel School.

The researchers note that if new housing were distributed proportionally to the share of floodplain land in Florida, they would expect to see 40 percent of new housing built in the floodplain.

They also found that 74% of U.S. communities have restricted new development in floodplains, and 87% have restricted new housing construction in floodplains through regulations and local government practices. The analysis also found that coastal communities are more likely to concentrate new development and housing projects in floodplains than inland communities.

The study found that communities that participate in FEMA’s Community Assessment System, a voluntary incentive program that rewards communities that adopt certain practices with reduced flood insurance rates, had a higher likelihood of floodplain development.

“Communities facing flooding are signing up for the program, but their participation alone is not enough to foster safer development patterns. Instead, communities must both participate and improve their floodplain management practices – they are the ones who begin to limit floodplain development,” Agopian said.

Development in flood-prone areas is a major driver of increased flood damage, increasing both the likelihood that a flood will impact people and infrastructure and the severity of damage when it does occur.

This study is the first to measure the outcomes of floodplain management, community by community, at a national scale. To date, most research on flood damage and responses has focused on places that have experienced major flooding, with homes destroyed or lives lost.

“We found that many communities across the country took smart steps early on, avoiding development in their floodplains from the start. We can learn a lot from these communities that often quietly avoid flooding problems early on,” said Katharine Mach, lead author of the study and professor and chair of the Department of Environmental Science and Policy at the Rosenstiel School.