Reuters

But the problem is not what you think.

It sounds like a pretty safe assumption that the people who live in flood plains, or own businesses there, face the highest chances of flooding. This was certainly true during Superstorm Sandy in low-lying coastal areas at the mercy of storm surges. And the Federal Emergency Management Agency calculates flood risk this way all over the country, plotting properties against the geography of flood plains.

And yet here is a map of the Chicago metropolitan area, its flood plains shown by squiggly blue lines:

The Prevalence and Cost of Urban Flooding: A Case Study of Cook County, Ill."

The zip codes shaded the darkest brown received the largest number of flood insurance claims between 2007 and 2011, from private insurance companies, the National Flood Insurance Program, or disaster relief assistance. Some of the zip codes with the most claims are in more densely populated parts of town. But they also sit nowhere near flood plains.

That map comes from a new report [PDF] by the Chicago-based Center for Neighborhood Technology that amassed an impressive database of 177,000 local flood insurance claims, worth a total of $660 million over the five-year span, covering 96 percent of the zip codes in Cook County (this represents a flood claim for one in every six properties in the county). The analysis found no correlation between damage payouts and floodplains, as this graph shows:

"It’s completely counter-intuitive," says Harriet Festing, CNT's water program director. "If you were to ask the average person, even the average expert in flooding, they would say the bulk of flooding happens in the flood plains."

So what's going on here? Consider this second map, which shows the percentage of "impervious" surface area across the county, by zip code. The dark brown patches are the places with the highest share of roads, roofs, parking lots and general asphalt cover through which rain fall or flood water cannot penetrate:

Chicago – and plenty of other cities just like it – has artificially created flood-prone places simply by paving over the region's natural ability to manage excess water. And, as Festing points out, most people are entirely unaware that the city has done this. In urban areas anywhere, when we focus instead on the risks posed by flood plains, we may be ignoring the even greater threat created by how we've designed cities to crowd out their essential green space.

This also means we need to think about flooding at the scale of whole cities instead of at the level of individually problematic properties.

"Some solutions for the individual make it worse for everybody else," Festing says. "Every time one individual finds a solution for their property which means shoving the water somewhere else, that makes the problem harder collectively for everyone else."

She is talking, for instance, about sump pumps.

"A lot of the problem at the moment," she says, "is that people aren’t realizing that this is a collective problem."

Top image of 2004 flooding in the Chicago suburb of Gurnee: John Gress/Reuters

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