The walled city of New York… after the oceans rise.

Scientific American paints a peculiar picture of the Big Apple’s future, with the hustle and bustle taking place behind a series of levees, walls and other barriers to keep the ocean out:

“The city is on a good path … but it needs to be studying the barriers,” said Jeroen Aerts, a professor of risk management at the Institute for Environmental Studies in Amsterdam and lead author of the analysis, which also included researchers from the Massachusetts Institute of Technology, Princeton University and the University of Pennsylvania. “Risks increase very rapidly due to climate change.”

The analysis of future flood options for New York City does not capture all the psychological factors at play, such as concerns about huge floodgates being an eyesore or political factors, but aims to do a straight cost-benefit analysis of which measures can control flooding at an acceptable cost.

Assuming a “middle climate change” scenario of about a foot of sea-level rise by midcentury, the team further assessed the cost-effectiveness of each flood-control strategy by measuring whether its benefits, or avoided risk, would outweigh the investment costs.

Under that test, only one of the huge barrier solutions made the cut as an affordable strategy. Called “NJ-NY connect,” it envisions constructing two storm barriers—one in the East River and another connecting Sandy Hook in New Jersey with the tip of the Rockaways in Queens, New York.

However, the most cost-effective idea of all was the “hybrid” approach, using a combination of infrastructure improvements, and smaller flood barriers to keep water out of the city. This hybrid approach—while not as effective as keeping out water as a large floodgate—would keep damage to a minimum while keeping costs lower. “Houses are safe, as well as subways and critical infrastructure” under the hybrid approach, said Aerts.

However, if climate change becomes worse than expected, and sea levels rise faster than projected, other huge flood barriers will become a viable financial option very quickly, he said. And midcentury and beyond, there likely will need to be large flood barriers combined with a more modest approach, regardless of what is cost-effective immediately, according to Aerts.