Floating towns and oyster beds: How US cities are preparing for rising seas

Most of America's urban infrastructure is coastal. Of the 25 most densely populated U.S. cities, 23 are along a coast. And two of the biggest threats from climate change are increasingly intense storms and rising sea levels.

The seas already rose some 7 inches during the 20th century, according to the Intergovernmental Panel on Climate Change. The U.S. Environmental Protection Agency says in this century they'll rise three to seven times that, or possibly more.

That could make for a very wet Financial District, and the rest of the Big Apple's famed neighborhoods may not fare much better—New York City's coastline, stretching 530 miles, is longer than that of Miami, Boston, Los Angeles and San Francisco combined.

Exiting Mayor Mike Bloomberg's panel on climate change warned of the twin-headed monster of extreme heat and heavy downpours. It forecast 90-plus-degree days to triple by 2050, while days with more than 2 inches of rain will increase by nearly 70 percent. The mayor's panel forecast seas rising 31 inches by mid-century; other estimates predict a sea-level rise of 2 meters in the 21st century.

Big, theoretical numbers don't necessarily sway the public. A quarter of Americans—and half of Republicans—still deny climate change. With the devastation wrought by Sandy still fiercely evident more than a year later and hundreds of residents and business owners still trying to recover from its damage, could this superstorm have been the tangible, physical wake-up call that Americans needed?

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Marco Marani, a professor of civil and environmental engineering at Duke University, said yes. And projects are under way to protect our coastal infrastructure from the resulting stormier rising seas: everything from "horizontal levees" and floating communities to new oyster beds and restored tidal marshes.

"It was a tragedy, of course, but these recent events have made people realize we have to worry about this," Marani said. "Otherwise, people tend to underestimate it—you don't worry every day about a once-in-50-year flood."

Marani knows a thing or two about protecting cities facing an immediate rather than hypothetical threat: He is an expert on the half-completed project to protect Venice, Italy, in the event of a superstorm. The plan is to close the three inlets from the Adriatic Sea using gates that will be raised from the seafloor.

In Venice it's been necessary to retreat vertically: No one lives on the ground floor. Marani said that the Big Apple isn't all that different: "In New York they must be thinking along those lines sooner or later. And things that cannot be moved up, particularly electrical infrastructure, will be made waterproof. … The adaptation will have to be radical."

On Oct. 29 Bloomberg's office announced that 73 percent of the city's short-term goals were complete or nearly so, with millions of cubic yards of sand replenishing beaches and building dunes higher than they were before Sandy. The superstorm inundated parts of Staten Island, for example, with 10 feet of water—and 6 feet as far as a mile inland.

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A new, "more resilient" $200 million boardwalk is planned for the Rockaway Peninsula, along with the possibility of a $150 million multipurpose levee with raised-edge elevations that would protect the East River shoreline south of the Brooklyn Bridge and create a new area for residential and commercial development. This new "Seaport City" would be modeled in part after Battery Park City, which Bloomberg believes acted as an enormous buffer against Sandy's powerful storm surge. These, among other proposals, tally up to around $20 billion.

Living shorelines

Meanwhile, in California, "the heart of the Silicon Valley economy is at risk because it's immediately adjacent to the bay," said John Bourgeois of the California Coastal Conservancy, naming such tech leaders as Google, Facebook, Yahoo and Intel as being in harm's way.

Airports and wastewater treatment plants are right there, too. "Some companies look at their own footprint to see if they're under water" on projection maps, he said. "But it's not as simple as that. What if your employees can't get to work or the grid goes down?"

Bourgeois is executive project manager of the South Bay Salt Pond Restoration Project, which aims to create and preserve wetlands. One new concept in coastal flood protection is the "horizontal levee," which uses the natural flood protection benefits of coastal tidal marshes to reduce the destructive forces of storms.

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Ecosystem restoration is also being used in combination with levees.

"When you put a marsh in front of a levee, you don't have to build it as high, it costs half as much, and then there are all the benefits to the ecosystem," Bourgeois said.

He also said the need to restore the tidal marshes is urgent: As much as 90 percent of them have been lost to agriculture, salt production and fill and development.

The size of the salt pond restoration is huge—it is the largest such undertaking west of the Mississippi, an area the size of Manhattan. And at just 10 years in now, it's projected to take five decades.

Also in the Bay Area is a much smaller pilot project: to grow oyster beds.

The Living Shorelines Project aims to determine whether naturally grown reefs can impede wave action while also providing habitat value. (New York is considering a similar project for Staten Island.)

Project manager Marilyn Latta said the 1-acre project, which began with the transplanting of eelgrass and bare oyster half shells in July 2012, has attracted some 2 million oysters to settle on the reef. It has further resulted in an increase in bird use.

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As for protecting infrastructure on shore, the Living Shorelines Project shows promise: It was found to reduce wave force by 30 percent .

Latta and Bourgeois said such nature-based solutions as oyster beds and restored tidal marshes are more effective, cheaper and more ecologically sound than highly engineered alternatives. "When you rely solely on a man-made structure," such as a seawall under the Golden Gate Bridge, as some have proposed, "when it fails, it does so all at once—and to devastating effect," said Bourgeois.

Future homes

New York officials say 400,000 people live in the existing 100-year floodplain, and thousands of homes are being lifted up 6 to 12 feet to meet new FEMA requirements. Several competitions have challenged architects and designers to reimagine coastline living spaces.

The winning entry for a competition created by Operation Resilient Long Island targeted the Brooklyn neighborhood of Red Hook, which was devastated by Sandy. Highlights included ground-floor structures that could be easily disassembled and moved to higher ground when storms approach, as well as a marsh located in the center courtyard of a city block that would "remediate local runoff and hint at the eventual reintroduction of wetlands to Red Hook."

Competition organizer Dan Horn, a recent graduate from the New York Institute of Technology, gave credit to the winner for looking forward as far as the year 2300, when sea levels may be 5 to 10 feet higher than today.

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Horn also admired an Honorable Mention that designed homes with sun shades that extended from rooftops, which could be lowered to "box up" the second floor against a squall.

The long-running Progressive Architecture Awards recognized a somewhat more radical concept, titled "Floatyard," an 87,000-square-foot floating multifamily community structure designed for Boston Harbor. "It's completely integrated in response to the environment and as a way of thinking," said one juror.

Retreat or else

Matthew Stutz, assistant professor of geosciences at Meredith College in Raleigh, N.C., worries that measures being taken are mostly localized and short-term.

When a disaster like Sandy comes along, he said, "The most common response is an immediate impulse to rebuild and replicate what has been damaged or lost. The Jersey Shore is an excellent example—people by nature take comfort in the familiar and gain the sense that they can overcome this problem. Within a few years a general sense of 'It won't ever happen here again' can creep back in."

Stutz said shoreline sand replenishment is an effective measure against threats from offshore, but it will become increasingly expensive and unsustainable in the long run. Furthermore, funding for beach renourishment began decreasing under the George W. Bush administration, and local governments could not or did not pick up the slack in many cases, leaving coasts increasingly vulnerable.

"We need to have more long-term management and thinking. Renourishment needs to be repeated every three to four years, and just 1 mile of beach requires 1 million cubic yards of sand."

Marani of Duke said that in many places, people simply need to give up the idea of coastline living. "On much of the U.S. Eastern Seaboard, the only solution is retreat—if not actively tearing down shoreline structures, at least not rebuilding there. But that's difficult in cities."

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Stutz agrees, saying a phased retreat from the shoreline is ultimately necessary. But in the meantime, he said, "we need to acknowledge it's going to become more and more difficult to maintain and protect this infrastructure."

But he admits the costs and the politics are tough.

As an example, he cited the Cape Hatteras lighthouse, which was moved from just over 100 feet from the water's edge to a half-mile inland.

It took 20 years of discussion and $10 million to get it done, he said.

—By Matt Twomey, Special to CNBC.com. Follow him on Twitter @Matt_Twomey