Atmospheric modeling has revealed two reasons why Hurricane Harvey flooded Houston so badly. One of these was well known beforehand, but has now been quantified, but the other is much more surprising.
Hurricane Harvey killed 68 people when it struck Texas in August 2017 and caused an estimated $125 billion in damage, making it among the United States' worst disasters. Although the Gulf Coast has suffered from hurricanes throughout recorded history, Dr Gabriele Villarini of the University of Iowa set out to discover why this one was so extreme.
One answer is familiar to urban planners. Cities have a lot of hard surfaces, be they roofs, roads or parking lots, that can't absorb water. Where fields, forests or parklands reduce flooding by taking in the first few inches of rain, most human-built objects dump everything into drainage systems and rivers, greatly increasing flooding.
Less widely known is that modern cities also receive more rain. Hurricanes usually move quite rapidly over the ocean, but can be slowed down by complex terrain or “surface roughness”. Comparing the roughness of Houston with a model of the same geography, but covered in croplands and pastures, Villarini found the urban skyline caused Harvey to linger over the city. This gave it time to dump the astonishing 1,300 millimeters (52 inches) of rain that otherwise would have been dispersed much more widely.
"When Hurricane Harvey blew into Houston, it literally got snagged on the city's tall skyscrapers and towers," Villarini said in a statement. "The friction caused by high winds buffeting tall buildings created a drag effect that influenced air and heat movement and resulted in optimal conditions for precipitation."
Combined, the two urbanization effects increased the probability of extreme flood events by a factor of 21 times.
This, of course, is all in addition to the contribution of climate change. The theory global warming would increase the ferocity of hurricanes remains more controversial among climate scientists than most other predicted effects, but there is still evidence it was a contributing factor.
"While this research can't be extrapolated to every coastal urban area in the nation, it does highlight the unexpected costs of development," Villarini said. "For every new roadway poured and for every new high-rise erected, there is an increased risk for more adverse rainfall and flooding, and that's certainly something that city officials and residents should consider when they contemplate future growth."
It's not all bad news when it comes to the way humans affect hurricanes, however. A recent study found offshore wind farms can take a lot of the force out of cyclonic winds, and could have softened Harvey’s blow, for rain as well as wind. Although it would take an improbably large number of wind turbines to fully protect America, smaller numbers could still help. If offshore wind was subsidized to reflect the savings inland, it might make it far more economically competitive.