A new report finds a giant Harbor Wall designed to protect Boston from devastating coastal storms would cost too much and take too long to build.

The wall would be massive, clearly visible from the Harbor Walk behind the Fan Pier Courthouse in South Boston. It would operate like a giant gate, perched on a pair of man-made islands. One would be built near Logan Airport, the other near the Seaport.

“And the gates would swing into the land, like dry docks when they’re open,” said Paul Kirshen, a professor at UMass Boston’s School for the Environment, “and then, when a storm comes, they’d swing closed.”

A gate in the water may sound like pie in the sky, but there are similar structures around the world. In the Netherlands, for instance, the city of Rotterdam constructed a floodgate twenty years ago called the Maeslantkering.

Kirshen led an eight-month study into the feasibility of two different barrier gates systems - one in Boston’s inner harbor and another that would be the largest in the world, stretching from Deer Island in Winthrop nearly four-miles south to the town of Hull.

The conclusion: “Right now, it doesn’t make sense for Boston to consider using barriers,” said Kirshen, “at least for a couple of decades.”

A big factor is the cost. Constructing a gated barrier in the harbor is a feat of engineering, the aquatic equivalent of Boston’s Big Dig with a price tag running from six to 12 billion dollars.

And then there’s the construction time – about 30 years.

Amid rising sea levels and the expectation of increasingly powerful storms, Boston may not be able to wait. Last winter storm surges sent water into places that haven’t previously flooded, making streets and MBTA stations inaccessible.

“I think the storms this winter were a very nice wake-up call because there wasn’t a whole lot of damage,” said Bud Ris, a senior advisor to Boston’s Green Ribbon Commission, the group of business and civic leaders which sponsored the Harbor-wide Barrier report. “Those floods verified the maps we’ve been using, they correlated pretty well with where we’re predicting there will be more flooding in the future,” said Ris.

With a barrier wall off the table, Ris says the city should now focus on shoreline resilience efforts including building elevated parks that drain water back into the sea and, on low lying streets, retractable floodgates.

“That’s the solution that we found would work in East Boston and parts of Charlestown, even along places like the Fort Point Channel, which are very low lying and vulnerable,” said Ris. “There’s a way to widen the harbor walk, maybe raise it a bit, add some berm-like structures, try to recreate nature a little bit along the water and you get a really good community benefit as well as protection from the floods.”

The Harbor-wide Barrier report highlights a big unknown: how much and how fast the sea level will rise. By the end of the century water levels in Boston Harbor could be at least seven feet higher than they are now, but the study points out that if greenhouse gas emissions go down, the rate of sea level rise could be limited to less than two-feet.