In the months after Hurricane Harvey pummeled East Texas, Houston asked itself a hard question: How much blame does the city’s rapid growth deserve for the calamity?
As storms leave patterns of damage that bear little resemblance to FEMA’s flood plain maps, it’s clear that the topology of cities has become its own hydroscape, directing and displacing stormwater according to the layout of buildings, roads, parking lots, and storm drains. In Houston, the country’s fastest-growing city over the past few decades, rivers appear to be rising along with development as grasslands give way to pavement. That climate change is sending bigger, more frequent rainstorms is not helping. Even after hurricanes, about two in three residential flood-insurance claims are inland—the product of freshwater flooding.
A report this month in Nature raises a different, intriguing possibility about how urbanization might be making rainfall flooding worse, or at least how it did in Houston’s case: by literally slowing down storms. Researchers at the University of Iowa and Princeton built a model to estimate Harvey’s rainfall totals in the region. Then they took the city out of the model. They found the storm would have dropped less rain, and in different places, on a Harris County made of cropland.
Why? Basically, they think, the scale of the urban agglomeration in and around Harris County is creating atmospheric friction, putting a drag on storm winds. Climate change is supposed to produce more slow-moving storms like Harvey, which increases the scope of rainfall damage by leaving storm clouds to linger over one place—but cities may be slowing them down further, according to a co-efficient the authors call “roughness.” (Their models also conclude the growth of the city since 1950 is linked to a “clear increase in the magnitude and variability” of flooding.) In the case of Harvey, the storm’s extremely slow speed caused it to linger over East Texas for three days, concentrating rainfall damage.
It’s one more threatening way that “natural” disasters are evolving into our own creations.
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