Where asphalt and concrete replace marshes and grasslands, heavy rain is more likely to cause flash flooding.
When Hurricane Harvey stalled for days over southern Texas in August 2017, it set US rainfall records — dumping more than 60 inches near the cities of Beaumont and Port Arthur.
“Harvey could be considered a rain bomb,” Eric Tate of the University of Iowa, who studies vulnerability to flooding, told BuzzFeed News.
The storm caused an estimated $125 billion worth of damage and killed 68 people — all but three of them victims of freshwater flooding. Thirty-six of those fatalities were in the Houston metro area, the fifth largest in the US with some 7 million people, which in the preceding two decades had seen a massive growth of paved surfaces, replacing natural prairies and wetlands that used to help soak up heavy rain.
“Houston is an area that has seen a lot of hardening of the watershed,” Mike Beck of The Nature Conservancy, who heads a research group at the University of California, Santa Cruz, studying the resilience of coastal cities to storms and floods, told BuzzFeed News in the wake of the storm.
A 2015 study from researchers at Texas A&M University found that Harris County, which includes central Houston and some of its suburbs, lost 29% of its wetlands between 1992 and 2010, largely to urban development.
“Loss of wetlands on this scale means a substantial loss in the ability of the landscape to detain and remove pollutants from stormwater,” the researchers wrote. “The results are increased flooding and degraded fishing grounds in downstream bayous and marshes.”
For Harvey’s victims, the sprawling city became an impermeable death trap, in which urban development compounded the effects of low-lying land and clay-based soils that are bad at soaking up water.
“These physical characteristics make the Houston metropolitan area prone to regular and sometimes catastrophic flooding,” noted a report on urban flooding from the National Academies of Sciences, Engineering, and Medicine, commissioned by the Federal Emergency Management Agency and published in March this year.
After visiting the city just before Harvey stuck, the committee that wrote the report concluded that the people most at risk included the “poor elderly, renters, minority, disabled, and non-native English speakers.”
2. New Orleans and Baton Rouge, Louisiana
Compared to Houston, the metro areas of New Orleans and Baton Rouge, with a combined population of more than 2 million, haven’t seen such explosive development.
New Orleans, hemmed in by its protective levees, has little room to expand — and the population, now at around 400,000 for the city proper, still hasn’t yet recovered to the level before those levees were breached in August 2005 by the storm surge from Hurricane Katrina. (The city’s population was almost halved in the immediate wake of the storm, with many of those who fled relocating to the Houston area.)
But as a separate downpour in the week before Barry came ashore showed, a dense city like New Orleans remains highly vulnerable to rain-triggered flooding even if levees hold. Hours of heavy rain on Jul. 10 left many streets flooded, causing Louisiana Governor John Bel Edwards to declare a state of emergency in advance of Barry’s arrival.
3. Jacksonville, Florida
In September 2018, Hurricane Irma roared in from the south up the Florida peninsula. Surging seas had been the main concern, but the Jacksonville metro area, home to about 1.5 million people on the Atlantic coast in the north of the state, wasn’t expected to be badly affected by that. Still, the city saw extensive flooding, largely from the St John’s River which had been swollen by torrential rain.
As this map shows, urban sprawl likely contributed to the extent of the flooding, as paved surfaces had grown significantly in the prior two decades.
4. Tampa Bay, Florida
With the heaviest rain falling to the east of the storm’s eye, Irma didn’t cause major flooding in the Tampa Bay area, home to more than 3 million people. But again, urban sprawl has increased the amount of impermeable surfaces, making the cities around the bay vulnerable to rain from future storms.
5. Raleigh, Durham, and Greensboro, North Carolina
Coastal cities aren’t the only ones at risk from flooding from major hurricanes.
In September and October 2018, parts of North Carolina twice experienced heavy flooding as first Hurricane Florence headed inland from the Atlantic coast, and then the remnants of Hurricane Michael, which came ashore in the Florida Panhandle, moved north.
Again, some of the state’s major cities, including the Raleigh-Durham-Chapel Hill metro area that is home to some 2 million people, have seen a big expansion of paved surfaces, making them more vulnerable to flash flooding.
6. New York City
New York City, part of the largest metro area in the nation housing more than 20 million people from New Jersey to Long Island, experienced severe flooding in October 2012 from Superstorm Sandy. That was mostly due to storm surge, rather than rain. But if a major hurricane were to stall over the area, the highly impermeable surfaces shown on this map in magenta would create a big risk of flash floods.
Tropical cyclones may present the main threat, but climate change is also causing heavier downpours elsewhere. “Across most of the United States, the heaviest rainfall events have become heavier and more frequent,” according to the National Climate Assessment, with the trend being most pronounced in the Northeast and Midwest.
7. Omaha and Lincoln, Nebraska
This year, much of the Midwest, including cities in Iowa and Nebraska, have experienced record flooding. Melting snow was one reason that rivers burst their banks, but heavy rains and the growth of paved surfaces in the region’s cities, shown here in Omaha and Lincoln, home to about 1.3 million people in their wider metro areas, made things worse. As climate change continues to increase the risk of major downpours, urban planners are going to have to think about how to allow excess water to drain away, experts say.
“If you neglect how you build cities, you’re going to be missing a large part of the problem,” Matei Georgescu of Arizona State University, who has studied the threats to the world’s cities posed by climate change, told BuzzFeed News.
When it comes to flooding from major storms, Georgescu said, there’s a simple prescription — albeit a hard one to achieve where space is limited: “More parks, more grass, more permeable surfaces.”
“It’s not just urbanization, it’s how we’re urbanizing and what kind of surfaces we’re putting down,” the University of Iowa’s Tate said.
- Climate Change Is Making Hurricanes Like Barry Wetter And More DangerousZahra Hirji · July 13, 2019
- Houston’s Urban Sprawl Meant Harvey Was A Disaster Waiting To HappenPeter Aldhous · Aug. 30, 2017
- Flooding Could Be The Worst Ever Recorded In Much Of The US This SpringClaudia Koerner · March 22, 2019
- Climate Change
Peter Aldhous is a Science Reporter for BuzzFeed News and is based in San Francisco.
Contact Peter Aldhous at [email protected]
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