Ever since Hurricane Sandy’s 3.3-meter sea surge drowned parts of New York City on October 29, 2012, scientists and engineers have been scrambling to devise a plan to protect the city against future storms. Two reports are due within a week, as the 2013 hurricane season gets rolling. One report, to be released this week, is from the New York City Special Initiative for Rebuilding and Resiliency, created by Mayor Michael Bloomberg to figure out how the city should bolster itself against climate change. The other study, expected this month, is from the New York City Panel on Climate Change (NPCC), led by university researchers who in 2009 had published a long list of recommendations that would primarily fend off sea level rise, but not storm surges.

The most controversial and arguably most crucial piece of any plan to stop storm surges is a massive barrier built across New York Bay. The leading design, called the Outer Harbor Gateway, comes from Halcrow, an international engineering company based in Lower Manhattan (see image below). The eight-kilometer-long wall would connect Sandy Hook in New Jersey to the Rockaways in New York, separating all of New York City from the sea. The barrier would have three huge gates that would normally be open to allow ships to pass, and 11 other, smaller “sluice” gates within the wall that would also remain open to allow the mixing of seawater and freshwater necessary to keep the bay alive. All the gates would close as a surge approached.

Artist conception of the proposed Outer Harbor Gateway barrier that would protect New York City against storm surges. Courtesy of Halcrow, on GoogleEarth

According to Halcrow, the barrier could cost $6.5 billion. The structure may sound extreme, but engineers completed a very similar yet largely unknown barrier in 2010. It’s in Saint Petersburg, Russia, and it is three times as long. And in 2012 it held back a winter storm that generated the fourth-highest floodwaters ever recorded in Neva Bay, the innermost portion of the Gulf of Finland, on which the city lies.

Halcrow oversaw the second half of the barrier’s construction in Saint Petersburg, and its engineers say it is a model for New York City. The water depths, sea bottom and adjacent spits of land are similar in both places, and so are the potential surge heights. The Saint Petersburg barrier was designed in sections that could in effect be pieced together in a different order to fit New York’s span. “It’s almost like ‘plug and play,’” says Graeme Forsyth, a civil engineer and a director of Halcrow in Scotland. “You could take Saint Petersburg apart and plug in the bits you want to create a new configuration for New York.”

Even the strategic positioning of the two barriers would be the same. The primary risk in closing such a wall is that heavy rainfall and large, overflowing rivers could fill the bay from behind the wall like a bathtub, also causing bad flooding. But Neva Bay “is large enough of a reservoir to take heavy rain and river flow, and so is New York Bay,” says Jonathan Goldstick, an engineer and vice president at Halcrow.

Building a barrier is one thing but having it function during a real storm is another. On December 26, 2011, more than a year after the Saint Petersburg barrier was finished, winds in a heavy winter storm funneled a wall of water into Neva Bay. Engineers closed all the navigation and sluice gates for 12 hours. “That’s when we learned that the barrier really works,” Forsyth says. Engineers have also successfully closed and opened the gates during tests in the middle of winter, when the water leaning against the structures is capped with a half a meter of ice.

Graeme and Goldstick hope that the two systems will not be similar in one respect. The Saint Petersburg design sat on shelves for decades, and after an initial phase of construction the project was stalled for years before it was then revived and completed. If New York City wanted to build the Outer Harbor Gateway, the engineering studies, design and construction could all happen within eight to 10 years, Goldstick says. He notes, however, “I’ve never seen the federal government or local regulators move quickly enough to allow that kind of timing to happen.” He adds that environmental studies of how the barrier could impact water flow and therefore life in the bay could take on the order of a decade—as could raising the funds. Critics might also object to the plan, saying the money could be better spent to flood-proof all kinds of structures in the city itself against sea level rise and smaller storms, which they contend are much more likely in the future than another superstorm surge.

To fully protect New York, a second, smaller barrier would have to be built northeast of the city, across the Long Island Sound. (Halcrow has not proposed a design for that location.) Experts outside of Halcrow say that the two barriers together, allowing for inflation over such a long time span, could cost from $10 billion to $20 billion.

The gateway design could include a six-lane highway on top of the barrier (as in Saint Petersburg), which would connect central New Jersey to the city’s borough Queens and to Long Island, a potential boon to commuters and tourists. If the road were approved, Goldstick notes, part of the project’s cost could be covered with federal highway funds, or money from investors if the highway was a toll road.