Hurricanes Milton and Helene expose the limits of US flood maps

Even before the second megastorm in as many weeks brings devastating flooding to the U.S. Southeast, it’s already clear that the federal flood risk maps that underpin the decisions of millions of American homeowners and businesses are seriously out of sync with a new era of climate-intensified disasters.

The best-known guide to flood risk in the US is the Federal Emergency Management Agency’s set of maps, which designate high-risk areas where homeowners with mortgages should purchase flood policies. Those maps — which are now widely and incorrectly used to understand the broader flood risk — are often outdated and do not focus on the danger of flooding from rain, even as storms are supercharged by rising temperatures.

That means far more Americans are at risk of flooding than maps show, and a look at flood insurance requirements might suggest otherwise. A Bloomberg Green analysis shows discrepancies between FEMA’s high-risk areas and four locations affected by flooding from Hurricane Helene: Tampa, Florida; Augusta and Valdosta in Georgia; and Greenville, South Carolina.

Bloomberg compared FEMA’s publicly available flood maps for the four cities with Helene’s peak flood estimates provided by Floodbase, a startup that conducts flood analysis and sells parametric flood insurance. The company applies an algorithmic model to satellite images to detect flooding in geographic areas. Floodbase did not provide data for Asheville, North Carolina, or other areas in the Blue Ridge Mountains because of the steep topography there and the resulting flash floods, which its algorithm can underestimate.

The Helene flooding in Tampa, as captured by Floodbase, mostly occurred in FEMA Special Flood Risk Areas. About 66% of the floods occurred in these areas, while 34% of them were observed outside them.

But in Augusta, less than half (49%) of the flooding observed by Floodbase occurred in FEMA high-risk areas. In Valdosta, it was just 17 percent, and in Greenville, 24 percent.

FEMA maps indicate which properties have at least a 1 percent chance of flooding each year—or, put another way, which properties have a one-in-four chance of flooding over the course of a 30-year mortgage. A FEMA spokesman said the maps are designed to focus on river and coastal flooding rather than extreme rainfall. The maps show the minimum standards for managing floodplains and the highest risk areas requiring flood insurance, the spokesman added, and “are not predictions of where it will flood”.

Paul Dow, Greenville’s city engineer, said local flooding “occurred in areas previously identified as being at risk.” The city is still doing its post-storm assessment, he said, but investigations so far “have not turned up any surprises.” He said the city has not yet determined the number of storm-damaged properties outside the SFHA, but within them, several dozen properties were affected, in some cases only by tree damage and not flooding.

Greenville recently updated its own floodplain map using climate data through 2019. “Therefore, updated FEMA floodplain maps would not help the city as much as it would help other communities,” Dow said.

Only a small portion of Americans overall—about 4 percent—carry flood insurance policies. In coastal areas where hurricanes visit often, such as along Florida’s Gulf Coast, the rate is much higher, sometimes exceeding 50 percent. But inside, it drops sharply.

Fewer than 2 percent of Georgia households have policies through the federal National Flood Insurance Program, by far the largest provider of flood insurance in the country, according to reinsurer Swiss Re AG. In North Carolina and South Carolina the rates are about 3% and 9%, respectively.

The US government does not expect homeowners to insure simply out of precaution (although it encourages it). Anyone buying a home in an area designated as high risk is required to purchase flood insurance. The idea is to ensure that vulnerable owners are protected.

However, the maps have received criticism for being incomplete and outdated. Although they are revised periodically, more than 3,000 counties still have flood maps that are more than 15 years old, and some have maps that are much older than that. Many places in rural America and on federal lands are not mapped at all.

“Not only are they often outdated, but they’re not very accurate,” Joel Scata, a senior attorney at the Natural Resources Defense Council, said of the maps. “They operate in the 50th percentile of confidence,” meaning they have a high degree of uncertainty.

As criticism of FEMA’s maps has grown, private companies have sprung up to offer their own flood risk maps. While they are more granular and account for climate impacts, they are not quality controlled and often disagree with each other, a Bloomberg Green investigation found.

The agency has a long-standing plan to modernize its mapping and updated the plan last year. The review will include technological improvements and the integration of climate change impacts into its models. However, the rollout was slow. FEMA says making sure it’s accurate and scalable will take time and collaboration at every level of government.

“If these flood maps greatly underestimate the risk — the actual flood risk that people face — then we’re not building to the standard we should be building to,” said Scata of the Natural Resources Defense Council. “And we don’t need insurance for the people who really need it.”

Photo: An aerial view shows people walking past a church destroyed by Hurricane Helene in Swannanoa, North Carolina. (Photo by Mario Tama/Getty Images)

Copyright 2024 Bloomberg.

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Catastrophe Natural Disasters USA Flood Hurricane