Post-Hurricane Sandy flotsam and debris in the Potomac River flow under the Memorial Bridge in Washington October 31, 2012. (Gary Cameron/Reuters)

A surge of water 15 feet above normal races up the Potomac River. Ten inches of rain, swelling a watershed that spans four states, steams down the iconic river.

They converge at Washington. Parts of Georgetown and Alexandria disappear under 12 feet of water. The torrent pours onto the Mall, swamping the Smithsonian, threatening the National Archives and carrying raw sewage into the reflecting pool at the foot of the U.S. Capitol.

The water gushes into the Metro stations at L’Enfant Plaza and Metro Center, shutting down the subway system. Waves wash across the runway at Reagan National Airport. River bridges are impassable. Highways across the region are too dangerous for travel in the 100-mph winds. Millions of people are without power.

Dramatic? Perhaps. Could it happen? Yes. And the people who watch weather and who watch over the nation’s capital region know it.

In 1963, the Army Corps of Engineers reported that “the Washington Metropolitan area is vulnerable to severe damages from hurricanes.”

A powerful hurricane could inundate several areas of downtown with storm surge, according to a 2011 study.

That severe damage did not happen last week, and even had Hurricane Sandy landed squarely on Washington, this one probably did not have the power to devastate this area so thoroughly.

After the worst had passed — the 60-mph winds, rain that fell for hours on end, angry surf that ripped away ocean piers and flooding roiled by raw sewage — almost everyone here felt lucky.

New York and New Jersey got whacked hard. Washington just got slapped around.

And some were left wondering: Had the nation’s capital and its surrounding counties, home to 5.7 million people, taken the full force of last week’s freak storm, would it have fared any better than New York?

The bottom line: probably not.

“There are things that we can’t control,” said Ron Kirby, transportation planning director for the Metropolitan Washington Council of Governments. “We’ve got to get out of the mind-set that we can cope with everything.”

That same thought is echoed by Metro spokesman Dan Stessel:

“Even the best systems can be overcome by extraordinary events. What you’re seeing in New Jersey and New York — it’s not severe, it’s extreme.”

And David McKernan, head of Fairfax County’s Office of Emergency Management, agreed:

“It wasn’t that New Jersey and New York weren’t prepared. When you have a catastrophic event, no matter how well-prepared you are, residents will be hurting.”

‘One message, many voices’

Events of the past 15 years have helped crisis managers for the District, its two neighboring states and the federal government find a common sense of purpose that didn’t exist a decade ago.

After the Sept. 11, 2001, terrorist attacks, planning to evacuate the District became paramount. A year later, a pair of snipers terrorized the region. In 2003, an angry man drove his John Deere tractor into a pond on the Mall in a standoff that brought much of downtown Washington to a halt for two days. Six months later, Hurricane Isabel struck.

And then, in January 2011, a snowstorm struck harder and faster than anyone imagined at just the wrong time of day, trapping hundreds of thousands of people at work and stranding many thousands more in miles-long traffic jams. “That was a real wake-up call here,” Kirby said.

There were two key lessons learned: Local agencies needed to coordinate their preparations and actions; and those decisions needed to be explained to the public as if from a single megaphone.

“Who is our Mayor Bloomberg? Who is our Governor Christie? We don’t have one person in charge,” Kirby said. But now there are mechanisms so that two governors, multiple mayors and scores of county officials can coordinate.

“People talk about one message, many voices,” Kirby said.

As the storm approached, still looking as if it might cut inland to deliver its full force in Washington, public officials on every level said virtually the same thing. Notably, when Maryland Gov. Martin O’Malley heard that all federal offices were closing Monday, he gave the same command for his state’s offices.

The rest was common sense but seemed to be spoken with a singular voice: Evacuate lowlands, stay in the house, expect to lose power.

Taran Hutchinson, who heads a center created post-9/11 to collect and share transportation information with officials across the region, said the message got out to stay off the roads: “The stress wasn’t put on the transportation system like in years past. ”

Building a floodgate?

The perfect storm that could become Washington’s nightmare would be a powerful hurricane whose eye passed west of the Chesapeake Bay, pushing a huge storm surge from the Atlantic up the bay and all of its tributaries.

Should a multibillion-dollar floodgate be built across the Potomac at Mount Vernon?

“I think a floodgate would certainly be in the mix of things you’d want to consider if you’re trying to harden the capital against a future surge from the hurricane,” said Court Stevenson, professor of coastal ecology at the University of Maryland’s Center for Environmental Sciences. “You have to manage these very carefully, because when you have a big watershed up stream, like we do in the Potomac, you also have a secondary flood that occurs after a hurricane, so you have to make sure you can open those gates quick enough.”

Gates already are in use in London and Rotterdam, and Venice is investing $10 billion in a set of them.

“This is really pretty serious stuff, and I’d really like to see more action locally to get some plans underway,” Stevenson said.

The first steps to build that defense are being taken — efforts to build an eight-foot-high concrete wall across 17th Street to block floodwaters from the Mall. But when the cost runs into billions, officials have to weigh the same odds that Ben Barksdale considers when it comes to rescue boats and other hurricane equipment.

“Do you spend taxpayer money preparing for something that might only happen every 50 years?” said Barksdale, deputy chief of the Prince George’s County fire department. “You prepare for those things that happen more frequently.”

Kirby called it challenging to predict potential disasters but said regional officials need to do more “tabletop exercises,” where they plot how they would respond to different scenarios. “It’s a little like fighting the last war,” Kirby said. “You prepare for what happened [before] and then something happens that you didn’t prepare for. . . . I don’t think we spend enough time sitting down and going through hypothetical situations. We know how to deal with the January 2011 snowstorm now — we’re all over that — but the next situation will be different.”

McKernan, of Fairfax County, strikes a similar note.

“The definition of a catastrophe is not everything is going to be normal,” he said. “Property will be damaged, and people might die.”

Metro’s vulnerabilities

For a Washington resident watching the footage of flooded tunnels and waterlogged subway stations in New York, it’s easy to wonder: What if that much water surged into Metro’s tunnels? What if stations or entire rail lines were to close?

“We have a number of stations that are vulnerable to the same kind of storm surge that the New York subway system was vulnerable to,” said Tom Downs, vice chair of Metro’s board of directors.

Downs said past studies have suggested vulnerabilities in stations such as Anacostia and L’Enfant Plaza, and with past experiences of flooding in the Mall area, Washington is susceptible to similar damage sustained in the New York system.

Downs and other board members are urging Metro to study new data about storm surge vulnerability, to look at lessons from systems around the globe and to come up with a plan to lessen some of the region’s vulnerabilities.

Thanks to perpetual weekend track work, which regularly involves Metrobuses replacing trains, Metro and its riders actually have some experience on that front, according to the system’s top official. “We shut down parts of the system practically every weekend,” said Metro General Manager Richard Sarles.

There are 56 drainage pumping stations throughout the Metro system — each capable of pumping about 49,000 gallons of water per hour. Over the past 10 years, Metro officials have upgraded more than half of those stations with more powerful pumps.

“A few years ago, I wouldn’t have stood here and said, ‘Oh, the pumps are in good shape,’ ” Sarles said. “But we have spent a lot of time in the last couple of years cleaning up drains, basic stuff, making sure the pumps are ready, making sure the generators are working, making sure they are fueled.”

Mortimer Downey, chairman of the board’s safety and security committee, said he expects meetings on the issue and staff reports.

“There can be a level of which you are never prepared, as New York is experiencing, but you have to look at the system you have,” Downey said. “This is not an unknown. It should be known that this could be an issue here. I think we have to think through what are the risks, what are the potential remedies, what is the appropriate level of response.”

‘No time for coordination’

Although winds close to 100 mph have been recorded in Washington and extended power outages have been common after storms in recent years, hurricanes aren’t the biggest worry for power companies, said Brian L. Wolff, of the Edison Electric Institute, a nonprofit organization that represents Pepco and other public power companies in the Washington area and around the nation.

“Our city is not located directly on the ocean,” Wolff said. “We have the bay there; we have the barrier of the Eastern Shore. Proximity should not be ignored. If Sandy hit us with its full brunt, it just wouldn’t be the same for us.”

Advances in weather forecasting mean that hurricanes are never a surprise by the time they saunter up the coast and slowly turn inland. What sends power company executives reaching for their Sominex is the specter of a massive weather event or other cataclysm arriving with no warning — an unexpected disaster that sucker punches the region.

That is what happened June 29 — hurricane-force winds arriving with almost no warning, catching residents and emergency personnel flat-footed.

“We had no ability to stage prior to the derecho,” Wolfe said. “With any type of sneak attack like that, the restoration is going to take longer.”

The derecho storms plunged nearly 2 million area homes and businesses into the dark. It took Dominion Virginia Power, which was caught unprepared, nearly five days to get the lights back on in Northern Virginia.

Those unexpected catastrophes worry David F. Snyder, vice mayor of Falls Church, who has been active in regional emergency preparedness issues since the Sept. 11 attacks.

“I’m still concerned about the absence of very clear lines of communication and command,” Snyder said. “How do you determine who’s in charge when things are occurring very rapidly and there’s no time for conference calls?”

“You can deal with fragmented decision-making when you have time to talk and coordinate,” Snyder said. “But when something happens very quickly regionwide, there’s no time for coordination. That’s when you need to have a regional plan in place.”

Does the region have such a plan for unexpected events, such as an earthquake?

“Not really,” he said.

Lori Aratani, Mark Berman, Joe Stephens, Paul Duggan and Jason Samenow contributed to this report.