Maps that the US Federal Emergency Management Agency (FEMA) rely upon to determine flood risk underestimate the potentially affected population by a factor of three. A new model that covers the 48 contiguous states of the US (excluding Alaska and Hawaii) at 30-metre resolution places 41 million Americans in a 100-year floodplain, meaning the annual probability of flooding is 1%. FEMA’s maps put that population figure at just 13 million.
The reason for the discrepancy, according to Oliver Wing of the University of Bristol, UK, is that FEMA’s maps cover only about 60% of the land area, and “they tend to just focus on the primary large streams in those catchments”.
Wing, who presented his findings at the American Geophysical Union’s (AGU) Fall Meeting in New Orleans in December, said that FEMA “will tend to have not modeled the smaller headwater tributaries, and … when you add those up across the country, that’s a lot of risk that is therefore missed”.
Wing and his colleagues combined data from their expanded floodplain maps with population and land-use projections from the US Environmental Protection Agency (EPA), to estimate how the affected population might grow this century. Currently, 13.3% of the US population lives in a 100-year floodplain, they found. By 2050, that will increase to around 15.7%, and by 2100, it could reach as high as 16.8%.
The researchers also evaluated the amount of developed land within flood zones. Currently about 150,000 sq. km, this could increase by anywhere from 37% to 72% by 2100, depending on the growth scenarios. In addition, the value of assets located on floodplains, currently $5.5 trillion, will double during the rest of the century.
“We’re not even accounting for what climate change may do to flooding,” Wing said at AGU. “We are simply looking at the intensification of floodplain development, and due to that alone, we see that risk is increasing quite dramatically.”
Asked at a press conference whether FEMA would be making use of the data presented at the AGU meeting, Wing said his team would be happy to work with FEMA, if asked. He emphasized that the cutting-edge methodology employed in the study, involving “big data” and the enhanced computational power required to generate more detailed new maps, had only become available recently. He said he would be “very much surprised” if FEMA did not adopt the new methodology within the next five years.
The study included inputs from academia, the commercial world, the nonprofit sector, and government agencies, Wing said. At the time, the paper was under review for publication in Environmental Research Letters (ERL) – it has now gone live.