USA flood risk is vastly underestimated, study shows

01 Mar 2018 Simon Davies

Current flood risk models for the contiguous United States vastly underestimate the amount of people, land, and assets at risk from severe flooding, according to new research.

In the first study of its kind, a team led by researchers at the University of Bristol, UK, in collaboration with scientists at Fathom, The Nature Conservancy and the US Environmental Protection Agency, used high resolution population, asset and projected development maps of the entire conterminous United States, coupled to a flood hazard model with realistic flood physics, to produce new estimates of present and future flood risk.

Publishing their findings today in the journal Environmental Research Letters, the team show that around 41 million Americans live in areas at risk from serious (1% annual probability or 1 in 100 years) flooding – around 28 million more people than estimated using the Federal Emergency Management Agency (FEMA) flood maps.

The study also highlights that around 150,000 km2 of developed land, containing $5.5 trillion-worth of assets, is also within the so-called 100-year floodplain.

Lead author Oliver Wing, from the University of Bristol and flood mapping organisation Fathom, said: “The extreme flood events that recently occurred in the USA mean quantifications of flood risk are at the forefront of the public’s mind, as well as decision makers.

“FEMA flood maps do not cover the whole of the USA, and they are of varying age and quality. This means any nationwide risk or exposure calculations will result in underestimations. In fact, our data show that the total US population exposed to serious flooding is around 3 times higher than these FEMA maps suggest.

“Clearly, this has major consequences for flood risk management and policy in the US.”

Significantly, the study also found that population and GDP growth alone will lead to significant future increases in exposure – a change that may be exacerbated in the future by climate change.

Mr Wing said: “Currently, 13% of the US population is exposed to the 1 in 100-year flood. By 2100, this is expected to rise to over 16% (75 million people). Similarly, the value of assets on the floodplain is estimated to rise to just under $10 trillion by the end of the century.

“It is evident, therefore, that an increased share of future development is projected to take place within the floodplain.”

While the study identifies exposure missed by FEMA across the country, the researchers identified higher concentrations of exposure along the west coast, in urban centres around the Great Lakes, and across the inland western US. Louisiana, Florida, Arizona, and West Virginia were all identified as proportionally over-exposed states, with between a quarter and a third of all their assets being situated on the floodplain.

Mr Wing added: “Given the obvious need for policy makers and emergency planners to have accurate, detailed data, investment in updated, comprehensive flood mapping is essential. The commercial flood model used in our research generates the most comprehensive results currently available, and much more quickly and cheaply.