America is building another big wall. This one will protect New York

By 2025, New York's Staten Island will be fortified by a towering seawall running 5.3 miles along the coast, an engineering feat designed to ward off a growing threat.

The climate crisis is predicted to create more powerful and extreme weather systems all over the world, and coastal engineers are racing to respond with structures to reduce their impact.

The first seawalls were built centuries ago, though there are now, arguably, greater assets to protect and more people living along vulnerable coastlines than ever before.

A recent report by the Center for Climate Integrity estimated it could cost the US more than $400 billion over the next 20 years to protect coastal communities.

That's a lot of money to invest in shorelines that naturally move and change with tidal flows.

"Where you have these public and private interests colliding in a contested space, like the coast, that faces ordinary weather events being compounded by climate change, people will look for a solution that gives them as much security as they can hope to achieve," said Tayanah O'Donnell, a senior lecturer at the Australian National University (ANU).

Seawalls are not only expensive to install but need regular maintenance if they are to withstand the prolonged barrage of pounding waves. But in many places they are considered vital to protect land and property that would otherwise be swept out to sea.

Techniques used centuries ago are being refined and reinvented, and the latest designs have found new ways of encouraging marine life to coexist with man-made structures.

Staten Island's new wall

When Hurricane Sandy smashed into the US East Coast in 2012, Staten Island was overwhelmed by massive waves that swept away properties and killed 24 of the dozens of people who eventually died in the storm.

With a population of almost half a million, low-lying Staten Island was no match for the waves whipped up in New York Harbor, one of which reached a record 32.5 feet high.

Seven years later, $615 million in funding has been secured for the ultimate defense -- a levee, buried seawall and vertical floodwall reaching 20 feet above sea level.

Topped with a public walkway, it's officially being called the "Staten Island Multi-Use Elevated Promenade." Graphic visualizations of the wall, released by New York Governor Andrew Cuomo's office, show happy cyclists riding along a wooden deck near an ice-cream stand and coin-operated telescopes pointed out to sea.

The boardwalk will be big enough to host concerts, carnivals, marathons and cultural events, the governor's office says. But the new promenade's true public value will only be measured by how well it succeeds in shielding people from natural disaster.

The seawall will be built to withstand a 300-year flood event -- a water height two feet above the highest levels recorded during Hurricane Sandy, said Frank Verga, a project manager at the New York District of the United States Army Corps of Engineers (USACE).

"The project is a proven engineering solution to withstand multiple storms, with adaptability to be modified in future to address sea level rise, if required," said Verga in an email.

According to the final feasibility study, the wall is estimated to reduce damages, including risk management, by $30 million each year.

But it won't prevent all flooding, and in the case of severe storms residents will still need to follow orders to evacuate.

Centuries-old technology

For centuries, humans have been barricading shorelines to prevent encroachment from the sea. Some of the earliest examples have stood the test of time, albeit only after regular reinforcement.

In India's Puducherry, formerly known as Pondicherry, a seawall constructed by the French in 1735 is credited with saving lives in 2004, when the Asian tsunami threw successive walls of water at the coast.

On the other side of the Indian subcontinent, villagers along the Kerala coast have been campaigning local authorities to erect new seawalls to stop their homes from being swept out to sea.