A car half-submerged in water on a roadway during a flash flood.
A car is seen in flood waters on Clara Barton Parkway near Washington, D.C. during four inches of rain that fell in just one hour. Reuters

One-hundred-year storms, of the kind that wreaked havoc on the nation’s capital Monday, are expected to become 1-in-25-year events by mid-century.

“It’s like the end of the world down in Alexandria,” a friend texted me from the Virginia suburb as I watched Monday’s record downpour drench the nation’s capital during the morning rush hour. Nearly four inches of rain—a month’s worth—fell in just one hour on the immediate area surrounding Washington, D.C. In Alexandria, neighborhoods and commercial areas flooded as river gauges reported an 11-foot rise in one of its major streams.

The rain lasted nearly five hours, triggering the region’s first-ever flash-flood emergency declaration from the National Weather Service. Photos and videos on social media showed drivers stranded on roadways that had become rivers, waterfalls cascading onto tracks inside Metro stations, sinkholes opening up, and standing water in the White House.

As one meteorologist described to the local news site DCist, the storm came with “exceptional intensity”—even more than a 100-year-flood. That is, it was a storm with less than 1 percent chance of happening in any given year.

Though those seem like long odds, extreme rainfalls are becoming more frequent as global temperatures rise. In fact, designations like 100-year and 500-year storms raise more confusion than answers. With climate change in effect, a 1-in-100-year storm in D.C. is expected to become a 1-in-25-year event by mid-century—four times more likely—and a 15-year storm by the 2080s, according to D.C.’s own analysis. (In the nearby town of Ellicott City, Maryland—which was also placed under a flash-flood warning on Monday—two back-to-back “1-in-1,000-year” storms hit just two years apart.)

In light of the changing probabilities, storm experts and officials are trying to use different language to clear up any lingering confusion. According to NPR, studies suggest it’s more effective to tell people their risk of experiencing intense floods over time, rather than in any given year:

For example, if there is a 1% chance that a home will flood each year, that means there’s a 26% chance it will flood over the course of a 30-year mortgage.

The hope is not only that residents will be better prepared, but cities as a whole will be. In D.C., which was built along two rivers (the Potomac and Anacostia) and the now-buried Tiber Creek, river and tidal flooding are common. But officials and residents can also expect the kind of urban and flash flooding we saw Monday—brought on quickly by intense rain—to become even more frequent.

And if Monday is any indication, the city and its larger metropolitan area aren’t prepared. Heavy rainfall overwhelmed stormwater systems in D.C., Virginia, and Maryland, leading to dozens of road closures due to flooding. Some roads took on as much as five feet of water, and more than 100 rescues were made. Buses and trains halted service, prompting Lyft to charge more than $50 for a three-mile trip, and according to the Washington Post, at least 8,000 households lost power.

D.C. is not alone: The first-ever national assessment of urban flooding, published in 2018, found this kind of flooding happens daily—despite being out of the national spotlight and under-studied. Seventy percent of officials told the researchers that aging and inadequate drainage systems were their main problems when it comes to flooding. Yet less than half of the cities that the researchers studied made the infrastructure improvements needed to adapt to heavier rainfall. (Rainfall levels across the country have risen 4 percent since 1901.)

It was only in 2016 that D.C. finally built a 2,700-foot-long storage tunnel capable of holding some 8 million gallons of water in the flood-prone neighborhood of Bloomingdale in the city’s northeast quadrant. Before that, 100-year-old pipes carrying both stormwater and raw sewage flooded neighborhoods with waste during heavy rainstorms. The tunnel will soon connect to others that run along eastern D.C.

Even with those tunnels, nearby neighborhoods could still flood with a storm as intense as the one on Monday. The tunnel was never a guarantee against flooding, and as the former head of the D.C. Water and Sewer Authority told a local radio station in 2017, “the most challenging issue is not total amount of rain, but its intensity. There’s nowhere in the United States that won’t flood if you get four inches an hour.”

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