News•February 4, 2015
What If Sandy’s Surge Swamped Washington, D.C.?
New Yorkers weren’t the only ones shocked and alarmed as Hurricane Sandy pushed a wall of water into the city, funneling the harbor into streets and sending torrents cascading into subways.
More than 200 miles to the south, officials in Washington, D.C., were watching and imagining their city in New York’s shoes. Officials with the Metro, jolted by the images of subway stations flooded floor to ceiling, wondered if such a storm were to hit D.C. “are we going to see those same images playing out on TV?” Jason Elliot, a senior service hydrologist at the National Weather Service office in Sterling, Va., said. “They were very, very concerned.”
Washington Harbor encroaches on the shore in September 2003 as storm surge from Hurricane Isabel raises water levels.
Credit: FEMA/Liz Roll
In the wake of Sandy’s historic strike on the most populous city in the country, local, state and federal agencies formed a group called Silver Jackets to help prepare the nation’s capital for just such a scenario. (The program has formed collaborations between agencies in several locations around the U.S. aimed broadly at reducing the risk of flooding and other natural disasters.)
The D.C. area had seen surge on a similar level when Hurricane Isabel hit in September 2003, raising water levels by more than 8 feet. That event is “etched in so many people’s memories,” Elliot said.
The impact from that storm, though, was concentrated more in Baltimore. The nation’s capital hadn’t yet seen anything along the lines of Sandy. But as Elliot said, “just because it hasn’t happened doesn’t mean it won’t.”
As part of the Silver Jackets team, Elliot looked at the question of how D.C. would fare if it got a Sandy-like surge. And the short answer is probably better than New York. More than 300,000 homes in New York City were damaged, and 53 people were killed in the state. Power was knocked out in the lower half of Manhattan and car and subway tunnels were completely flooded. The city sustained $19 billion of damage, including $5 billion in damage to the subway system. Elliot found that while the same amount of surge in D.C. would cause damage, the effects wouldn’t be nearly as catastrophic, in part because of differences in the way the two cities have been built.
Instead of trying to tweak computer models to make Sandy hit D.C. at just the right angle to bring the same amount of surge, Elliot found it was simpler to just take the amount of surge Sandy generated in New York Harbor — a little more than 9 feet — “and put it over us,” he said at the annual meeting of the American Meteorological Society in Phoenix, Ariz., in January, where he presented his work.
The resulting maps show a swollen Potomac River, with floodwaters surrounding the Jefferson Memorial, completely covering the FDR and Martin Luther King, Jr., memorials, as well as the World War II memorial and the area around the Reflecting Pool. The city’s northwest waterfront is also underwater and the flooding would cover runways at Reagan National Airport and impact the Waterfront Metro station.
Unlike New York City, which is heavily built up along its waterfront, both with businesses and homes, much of the D.C. riverside is covered by monuments and parks. So in a surge flooding event, “you don’t have to get people out,” Elliot said.
The areas of Washington, D.C., that would flood if the city were to receive the same storm surge that New York City
did during Hurricane Sandy, as seen on Climate Central's Surging Seas tool.
In fact, the World War II Memorial was designed to absorb floodwaters, and the city has levees and other structures to protect against storm surge. Elliot’s mapping looked at how the 17th Street levee closure — aimed at keeping waters from flowing north from the Tidal Basin — faired. In Elliot’s modeling, it did what it was supposed to and stopped the local subway station from flooding.
Overall, the Metro system fared surprisingly well in the scenario. “It wasn’t as bad as what we all thought,” Elliot said.
Of course, Elliot’s scenario didn’t take sea level rise directly into account (he superimposed Sandy’s surge on the highest possible high tide in D.C.). Sea level rise driven by global warming was a factor in Sandy’s ability to inundate New York. Water levels near New York City have risen by 1.5 feet since the mid-19th century, in part because of the expansion of warming ocean waters and ice melt. So any storm surge entering the area now is acting on top of those already higher waters, which will keep climbing as the planet’s temperature keeps rising.
The same is true of D.C., which is in an area that has seen 6 to 8 inches of sea level rise since 1960, according to the Environmental Protection Agency. Global sea level rise by 2100 is estimated to be between 10 inches and a several feet, according to the Intergovernmental Panel on Climate Change, and the Chesapeake Bay area has been rising faster than the global average.
Elliot's scenario also left out freshwater flooding from rains, which often combines with tidal surge in D.C. and makes large surges there less rare there than in New York.
Ultimately the Silver Jackets group is working on new flood maps for the D.C. area, both in terms of storm surge and rainwater flooding.
“I'm confident that, at least in the DC Monumental Core, those freshwater scenarios will exceed the tidal one we presented,” Elliot said in an email.
The maps are set to be released this fall and are aimed at helping resident visualize what flooding forecast numbers mean for them.
“They’ll actually see a picture,” Elliot said. But that picture could be a different one several decades from now.