Which state has the highest hidden flood risk? Take a guess. Heck, take 49 guesses. You might not get it even then.
The answer? Arizona.
The Grand Canyon State has the highest percentage of homes that sit outside the Federal Emergency Management Agency’s designated Special Flood Hazard Areas (SFHAs) but are still at moderate or high risk of flooding, according to new research from financial analytics firm CoreLogic.
The report found that 68% of Arizona’s homes fit that criteria, easily outpacing more famously flood-prone spots like Florida (54%), Louisiana (49%) and Texas (31%).
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FEMA designates as SFHAs locations at especially high risk of flooding, and homeowners within these zones must have flood insurance to receive a mortgage backed by the federal government. Homes outside these SFHAs may still be at risk of flooding, however, and the CoreLogic report looks at this risk on a state-by-state basis.
The company compiled its risk numbers using an in-house database containing geospatial information on every residential and commercial property in the U.S. Combining elevation data, area flood histories, hydrological models and other information, it is able to predict flood risk down to a resolution of roughly 1,000 square feet, says Howard Botts, CoreLogic’s chief scientist.
"We can tell you the exact [flood] hazard at the front door of a property," he says. That level of precision is important given that "flooding is the most granular hazard event, where literally one property to another can have a different flood rating," he adds.
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In the case of Arizona, its surprisingly high flood risk stems from the fact that a large proportion of Phoenix, the state’s largest urban area, lies along the Salt River, which has a very wide floodplain, says Dr. Botts. Additionally, much of the Phoenix area is interlaced with irrigation canals, which, in the case of intense rainfall, may overspill their banks and flow into surrounding neighborhoods.
"So you have a very flat city built along a river and then also with these canals that can become conduits for flooding," he says.
Where’s the best place to live if you’re worried about hidden flood risk? Washington, D.C. According to CoreLogic’s numbers only 4% of District homes not included in a SFHA are at moderate or high risk of flooding. The swamp, it turns out, is already pretty drained.