New Study Finds Flood Risk for Americans is Greatly Underestimated

Arlington, VA | February 28, 2018

A groundbreaking new study published today in Environmental Research Letters finds that 41 million Americans—some 13 percent of the population—are at risk from flooding rivers, which is more than three times the current estimate that is based on regulatory flood maps. The study used a new high-resolution model that maps flood risk from rivers across the entire continental United States, whereas the existing regulatory flood maps produced by the Federal Emergency Management Agency (FEMA) cover about 60 percent of the land in the continental U.S.

“We were all surprised by how many Americans we found are exposed to flooding from rivers,” said Oliver Wing, the lead researcher behind the study and a PhD student at the University of Bristol, United Kingdom. “It’s particularly worrisome considering that most of these people aren’t even aware of the risk they face. This research will help fill this critical information gap.”

The increase in numbers of those at risk is a result of the expanded coverage of the map combined with its ability to estimate flooding on small streams—something that wasn’t adequately captured in previous flood-risk models, according to the study’s researchers.

Wing said that not only is the model able to produce flood risk maps across the country, its level of accuracy on smaller scales—that is, along specific rivers—compares well with FEMA flood maps that presently serve as the primary source of flood hazard data in the U.S.

Another key concern, Wing added, is that people continue to build in harm’s way, often without realizing they are doing so. The study—which integrates new maps from the Environmental Protection Agency that more precisely estimate where people live now and where future population growth is expected—predicts that under potential population growth and development projections, more than 60 million Americans may be by 2050 vulnerable to “100-year,” or “one-percent,” floods, which are those mapped by FEMA.

“Because climate change may cause so-called 100-year floods to occur more frequently, even more people may be exposed to flooding in the future. All of this highlights the critical need for comprehensive floodplain and flood risk management planning,” Wing said.

Kris Johnson, a scientist with The Nature Conservancy and co-author of the study, said, “By knowing those places most at risk from floods, we could avoid development in these areas. Protecting floodplains or developing them in ways that can withstand flooding—such as flood-tolerant parks—can prevent unnecessary risk to people and help avoid expensive damage to property and infrastructure. Floodplains also store and convey floodwaters, which can help prevent flooding in nearby communities.”

The study calculated that in the continental U.S. there already exists $5.5 trillion worth of assets in areas subject to 100-year floods. Better planning and the protection of floodplains, Johnson said, is particularly important as average flood losses in the U.S. have steadily increased to at least $10 billion annually. One measure of this is the current debt of the National Flood Insurance Program, which now stands at some $24.6 billion. Congress also appropriated over $100 billion in response to hurricanes Harvey, Irma and Maria, much of which will aid areas impacted by flood damages.

“Floodplains provide high quality habitats for wildlife, they can offer outstanding recreational opportunities for people, and they help improve water quality,” Johnson said. “Protecting and restoring the natural infrastructure of floodplains provides an important complement to built infrastructure like dams and levees. We need comprehensive approaches to reduce the devastating effects that flooding has on so many communities.”

The new study analyzes and identifies only those at risk from flooding from rivers. The estimate of 41 million people does not include the millions of additional Americans that are at risk of coastal flooding.

The Nature Conservancy is a global conservation organization dedicated to conserving the lands and waters on which all life depends. Guided by science, we create innovative, on-the-ground solutions to our world's toughest challenges so that nature and people can thrive together. We are tackling climate change, conserving lands, waters and oceans at an unprecedented scale, providing food and water sustainably and helping make cities more sustainable. Working in 72 countries, we use a collaborative approach that engages local communities, governments, the private sector, and other partners. To learn more, visit or follow @nature_press on Twitter.

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