The three-way collision of supercharged rainstorms, unchecked urban growth, and outdated infrastructure has produced a wave of urban flooding crises in recent years—both in the path of tropical storms (Houston) and in those places typically thought “safe” from climate change (Madison, Wisconsin).
It’s an anthropocene disaster, a man-made problem whose harms are decoupled from the Federal Emergency Management Agency’s flood plain maps, and from the risk awareness and preparation that goes along with that official warning. Many homeowners affected by flooding never thought they’d have to worry about it.
A new report by researchers at the University of Maryland and Texas A&M tries to assess the damage. “The Growing Threat of Urban Flooding,” released on Wednesday, surveys more than 300 stormwater officials and synthesizes the surprisingly scant body of knowledge. Rainwater is taking its toll on metropolitan America—but how do we measure it?
According to National Weather Service data, freshwater flood losses in the United States amount to nearly $8 billion a year over the past three decades—as if the damage from a very bad hurricane were spread throughout the country. Even during tropical cyclones, however, 2 in 3 flood insurance claims relate to freshwater flooding, not storm surge. Some flooding comes from swollen rivers or occurs on low-lying properties. But a surprisingly large degree of rainfall damage, based on local case studies, appears to be determined by man-made landscapes of asphalt, concrete, and iron. After scanning reports from local NWS field offices, the authors believe that the nation experienced some 3,600 urban flooding events in the past 25 years—about one every two to three days.
A Canadian data set illuminates the scale of the problem. There, severe rainfall has surpassed fire as the main cause of home damage—and the cost of basement flooding alone has been “rising at an unsustainable rate for more than 25 years” to more than $1.5 billion.
In the U.S., a few factors have come together to create a slow-moving crisis. First, urban sprawl consumed natural drainage areas and altered the way water flowed through cities. Second, climate change increased the frequency and severity of extreme rainfall events, especially in the Northeast and Midwest. According to the UMD-TAM survey of municipal officials in 48 states, 83 percent of respondents had experienced urban flooding—and 85 percent of those said it had occurred outside the FEMA flood plain, which encompasses low-lying areas along rivers and beaches. The risk area for urban flooding is unmapped, but we know it bears only partial resemblance to the federal maps that guide homeowners’ insurance purchases.
Third, sewer infrastructure settled into obsolescence or lay unmaintained, leaving older areas increasingly outmatched by rainfall. It’s as if we were still using early 20th-century traffic infrastructure for today’s bigger, faster cars. It doesn’t work.
But putting an exact figure on the damage is challenging. We know where National Flood Insurance Program policies pay out—but because urban flooding occurs mostly outside the flood plain, the NFIP isn’t a great indicator. Damage from rainfall or overflowing sewers tends to occur in pockets, and rarely garners the kind of federal disaster declarations that produce infusions of cash—if it generates any kind of payouts at all.
Unlike spectacular flooded rivers or surging waves that draw television cameras, flooded streets barely hold the focus of the local news. Still, for many Americans, they’re the first taste of climate change at home.
"How We Built Our Way Into an Urban Flooding Epidemic"; By HENRY GRABAR; Slate; November 29, 2018