New Report Assesses Threat From Urban Flooding

Slate staff writer Henry Grabar reports on a new study released on the threat posed by urban flooding:

It’s an anthropocene disaster, a manmade problem whose harms are decoupled from the Federal Emergency Management Agency’s floodplain 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, two in three 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 manmade 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.