Extreme rainfall events like Tropical Storm Cindy are becoming more common. But development on inland flood zones hasn’t slowed down.
Tropical storm Cindy made landfall near the Texas-Louisiana border early Thursday morning. Two states—Louisiana and Alabama—have declared a state of emergency. The storm has killed a 10-year-old boy in Alabama, while battering cities across a 500-mile stretch along the Gulf Coast with tornadoes, winds as high as 50 mph, and rainfall of up to 12 inches. Cindy has since been downgraded to a tropical depression, meaning sustained wind fell below 39 miles per hour. Up to 15 inches of rain could soak certain areas, according to the National Weather Service.
It isn’t just the coastal cities that are feeling that impact: Millions more residents as far north as the Ohio Valley are also at risk of inland flooding due to the heavy rainfall and swelling waterways as the storm moves northeast toward the Mid-Atlantic region.
As the National Oceanic and Atmospheric Administration explains, the majority of deaths during hurricanes (which differ from tropical storms only in the speed of maximum sustained winds) don’t happen along the coast. Between 1970 and 1999, roughly 60 percent of deaths happened in inland communities. Within cities, those floods can also cause billions of dollars of damages.
Such flooding will be a feature of our future, as Cindy-esque tropical storms and unnamed super-rains are symptoms of a warming world, meteorologist Eric Holthaus writes in Pacific Standard. “Since a warmer atmosphere can hold more water vapor (thanks to enhanced evaporation and other factors), rainfall rates during extreme events have become more intense.”
The good news, according to a new study backed by the National Science Foundation, is that overall, the U.S. has been making efforts to avoid development in flood zones, defined as an area that has a 1 percent chance of flooding in any given year. That’s particularly true in coastal areas, where urban development in these places has generally decreased. There have, however been prominent exceptions. Development in Miami has been continuing apace, despite the extreme risk presented by both storm-related flooding and rising sea levels. And in the New Jersey shore towns devastated by Hurricane Sandy, as the New York Times reported last week, there’s been a boom in new high-end housing steps from the beach.
But when the researchers overlaid national data on land cover, flood hazard, and population onto the U.S. as a whole, they found something alarming. "Within this general trend, a surprising result is that we found more [increase in] urban development in the flood zones in the inland counties than in coastal areas,” says Nina Lam, a professor of environmental studies at Louisiana State University and the study’s lead author.
Though it wasn’t part of the study, the researchers suspect that it could be a lack of awareness about the flooding risks within inland communities. Residents might misinterpret the term “100-year-flood” to mean that a flood happens once every century. But what the term really means is that an area has 1-in-100 chance of flood in any given year, based on the historic record. There’s another 1-percent chance of an equal (or greater) flood two years in a row. Lam adds that it’s not just named storms like Hurricane Katrina or Tropical Storm Cindy, which are more of a risk in coastal areas, that result in these floods. Heavy, nameless downpours can easily cause catastrophic localized flooding, as they did in South Carolina in 2015 and in West Virginia last year. But because they don’t get quite the same media coverage, people are less aware of the risks.
Case in point: In the aftermath of Hurricane Katrina, says Lam, many New Orleanians moved inland to East Baton Rouge Parish. But the water followed them: In 2016, continuous torrential downpours dumped as much as two feet of rain in parts of East Baton Rouge in two days, causing an estimated $8 billion in damages for the whole state. That was a “1,000-year rain,” meaning there was a 0.1 percent chance of it occurring in any given year.
But terms like 100-year, 500-year, and 1,000-year floods are increasingly outdated, as my colleague Laura Bliss wrote about Baton Rouge last year. Given the Earth’s warmer atmosphere, flood risk assessments need to be updated accordingly.
Flood risks also may not be high on the list of priorities when developers choose where to build, particularly in inland areas. That’s troublesome because that also means that they are less prepared than their coastal counterparts to mitigate the impact when a storm hits. The thing is that forecasting inland flooding is tricky, according to NOAA. It’s a balancing act between predicting the amount of rainfall—a challenge in itself—and where it will hit hardest. A small shift in a storm’s path is enough to decide which rivers will and won’t flood.
Lam does note, however, that the in the years not included in her study (2012 and beyond) intense rain events that affect inland communities have aroused more attention, which she imagines should decrease that knowledge gap. But the study, and the storms—whether named ones like Cindy or the many nameless ones that will follow in its wake—should serve as a “wake-up call.”