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As seas rise, city mulls a massive sea barrier across Boston Harbor

After Hurricane Katrina, the federal government built a 1.8-mile barrier along Lake Borgne, a lagoon of the Gulf of Mexico. LEE CELANO/AFP/GETTY IMAGES/FILE

It would be a massive, highly controversial wall sure to cost billions of dollars. But this barrier would be much closer to home — and potentially more expensive — than the one President Trump has proposed along the Mexican border.

As rising sea levels pose a growing threat to Boston’s future, city officials are exploring the feasibility of building a vast sea barrier from Hull to Deer Island, forming a protective arc around Boston Harbor.

The idea, raised in a recent city report on the local risks of climate change, sounds like a pipe dream, a project that could rival the Big Dig in complexity and cost. It’s just one of several options, but the sea wall proposal is now under serious study by a team of some of the region’s top scientists and engineers, who recently received a major grant to pursue their research.

With forecasts indicating that Boston could experience routine flooding in the coming decades, threatening some 90,000 residents and $80 billion worth of real estate, city officials say it would be foolish not to consider aggressive action, no matter how daunting.

“There’s a sense of urgency about these issues,” said Austin Blackmon, the city’s environmental chief. “We need to evaluate the feasibility of options like this. If it’s the best solution to protect Boston, we shouldn’t hesitate.”

Other ideas being studied as ways to protect coastal areas include building berms around city neighborhoods, diverting flood waters into canals or other designated holding areas, and requiring coastal buildings to withstand flooding.

The vast majority of climate scientists attribute rising seas to man-made greenhouse gases being pumped into the atmosphere, which they say traps heat, causing glaciers to melt and oceans to expand.

A massive barrier that would extend across the 4 miles between Hull and Deer Island, and rise at least 20 feet above harbor waters at low tide, would rank among the largest of its kind, but wouldn’t be unprecedented. Similar barriers already exist, or are being built, off the coasts of New Orleans; Venice; and Rotterdam.

Like those barriers, Boston’s sea wall wouldn’t be a dam. It would have openings large enough for ships to pass through, but with gates that would close before significant storms.

In the city’s report, titled Climate Ready Boston, officials said a harborwide barrier would have two principal goals: holding back regular high tides and blunting the force of cresting waters during storms.

Narrowing the gaps between the harbor islands would reduce the amount of water that flows in and out of the harbor, effectively lowering high tides and increasing low tides. That, too, would reduce the impact of a storm surge, officials say.

At their seaside offices at the University of Massachusetts Boston, the professors who are spending the year studying the practicality of a barrier said they’re considering costs, potential environmental damage, effects on commercial shipping and fishing, and possible locations. They will also be looking at how a barrier might affect the ecology of harbor waters and marshes, the potential threat to the quality of its expensively cleaned waters, and the possible side effects of changes to natural currents.

“It’s a very complex project, with all kinds of economic, environmental, and social consequences,” said Paul Kirshen, a civil engineer and professor at the university’s School for the Environment.

Today, much of Boston’s waterfront is now only about a foot above high tide, he said. Within 30 years, large sections of the city could experience regular sunny-day flooding, when high tides inundate coastal areas.

“If we’re going to build it, we should have something in place by 2050,” Kirshen said. “That’s why we need to be considering this now.”

The report recommends that the city brace for sea levels to be at least 1½ feet higher by 2050 than they were in 2000, and 3 feet higher by 2070.

But a climate report released in January by the National Oceanic and Atmospheric Administration found that East Coast cities are likely to experience even higher seas than had been predicted. Without drastic reductions in greenhouse gases, the seas could rise as much as 8.2 feet by 2100, up from its previous estimate of 6.6 feet, researchers found.

“This definitely should motivate us to mitigate our emissions,” said Rebecca Herst, a senior project manager for UMass Boston’s Sustainable Solutions Lab.

The research team will also evaluate whether it makes more sense to build a significantly smaller barrier that would merely close off the inner harbor, from Castle Island to Logan Airport. That would be cheaper and easier to build but would leave much of South Boston, Dorchester, and Quincy vulnerable. They’re also considering a barrier that would loop around the Harbor Islands, ranging from Deer Island, around Long Island, to Moon Island, which juts into the harbor off Quincy.

Another option under consideration is an idea proposed by Bob Daylor, a private engineer in Boston, who has studied the issue and suggested an approach he calls the Sapphire Necklace, an homage to Frederick Law Olmsted’s ribbon of city parks called the Emerald Necklace. In his plan, the principal barrier would involve a series of dikes from Deer Island to Hull’s Telegraph Hill.

In a 2014 paper, he outlined how his barrier could be built in stages, allowing adjustments to changing conditions. The project might also provide perks, such as expanding Lovell Island, which he said would allow for additional campgrounds, hiking trails, and more pleasing views of the city.

Daylor estimated that a wall designed to seal the outer harbor during storms would cost in the “low billions of dollars” and would take about a decade to acquire all the necessary permits. His plan would in part involve dumping massive amounts of boulders into the harbor, which ranges from about 20 feet to 50 feet deep.

It would rely on submerged concrete walls and hydraulic gates that would open wide enough for shipping traffic to pass through most of the time. The wall would vary in size, depending on the depth of the water, but it would be built in a way that it could be made taller as sea levels rise.

He urged the city to take the idea seriously, noting that a 2013 World Bank report ranked Boston as the eighth most vulnerable major city in the world to property damage from rising seas, among 136 studied.

“Climate change is a big issue, and it will require big solutions,” said Daylor, a senior vice president and engineer at Tetra Tech, a California-based engineering company.

Estimating the cost of such a project is no easy feat, he and other engineers said. Hugh Roberts, an associate vice president at Arcadis, a Denver-based environmental consulting company, has worked on designing sea barriers in New Orleans and New York, and he estimated that a barrier along the outer harbor of Boston would cost in the “tens of billions of dollars, or more” and require federal as well as local funding.

“Each system requires unique infrastructure,” said Roberts, who is advising the team at UMass Boston. “It’s not like building a roadway.”

But the cost of not building a barrier could be even higher. If seas rise by 3 feet over 2000 levels, Boston would likely sustain an average of $1.4 billion a year in flooding damage, he said.

Somewhat similar projects have had relatively reasonable costs, especially compared with the Big Dig, which is estimated to have cost about $24 billion, with interest. After Hurricane Katrina pummeled New Orleans in 2005, the federal government spent about $1.1 billion and took five years to build a 1.8-mile barrier along Lake Borgne, a lagoon of the Gulf of Mexico.

In Venice, Italians have already spent nearly $5 billion — more than twice the original cost projections — to build a 1,200-foot barrier. The barrier is designed to remain submerged until it’s needed before storms, when it can rise like a leviathan to protect the historic city from excessive flooding.

Such projects are extraordinarily complex and often controversial. The Venetian barrier was first proposed in the 1970s, but construction didn’t begin until 2003. It is finally scheduled to be completed next year.

The higher costs were caused by delays in construction, rising costs of materials, and the complicated design of navigation locks, said Giovanni Cecconi, who has overseen the barrier’s installation as director of the Venice Resilience Laboratory.

“The costs might be high in Boston, but this would be an opportunity to solve a long-term problem,” Cecconi said.

In the Netherlands, where about 20 percent of the land is below mean sea level, residents are well acquainted with the dangers of flooding. In 1953, nearly 2,000 people died during a major winter storm that sent a surge of water rushing in from the North Sea.

Since then, the country has fortified its coast with all kinds of barriers. In the 1990s, the country spent more than $700 million to extend a 66-foot coastal barrier some 600 feet across the Nieuwe Waterweg waterway to protect Rotterdam, a metropolitan area where more than a million residents live.

“We haven’t had any flooding since then,” said Martien Beek, a deputy program manager at the Dutch Ministry of Infrastructure and the Environment. “This is a solution that has proven itself, and it could work for Boston as well. All major coastal cities that have big tidal movements should be considering this.”

At UMass Boston, which received a $360,000 grant from the Barr Foundation to study the possibility of building a barrier, scientists and engineers said they recognize there are many risks of such a major project. The foundation, started by cable magnate Amos Hostetter, is one of the region’s largest philanthropies and has funded a range of climate change studies.

“This is likely to be incredibly expensive and ecologically disruptive,” said David Cash, dean of the university’s McCormack Graduate School of Policy and Global Studies. “But if you look at the flood maps in 80 years, the danger is potentially catastrophic.”

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