Huge coastal barriers could protect the world’s cities. But they’ll have unexpected costs.
Excerpt from this story from The New Yorker:
Pacifica embodies one of the central disagreements about rising seas. Fight or flight? Stay or go? Flight can seem unimaginable. But, if we try to fight the ocean with rock and concrete, it will cost us—and it may not work. Pacifica currently plans to borrow tens of millions of dollars to reinforce its seawall. Writing for a community blog, Gregg Dieguez, a critic of this plan, objected not just to its price but to the “moral hazard” it would create: by forestalling erosion, the seawall might only encourage more people to live in risky places. “Once sea level rise gets here, it’s never going away, at least not for thousands of years,” Dieguez wrote. Meanwhile, he noted, only one per cent of the homes in Pacifica were at risk. “You will have to decide, as a group, whether paying to hold back the tides is a good use of your precious money,” he concluded. It’s a question many more of us will soon be asking. When do seawalls make sense? And when is it better to give in to the tides?
The oldest known seawall was built around 5000 B.C., after a period of warming that melted glaciers and lifted the Mediterranean by a staggering twenty-six feet. A Stone Age community, living near a beach in present-day Israel, tried to ward off the sea with a wall of three-foot-tall boulders about the length of a football field. But, in the millennia that followed, the Mediterranean rose even more. Archeologists ultimately discovered the stones on the seafloor, under ten feet of water. The site, they wrote, was “ominously relevant” to our time. Other excavations have turned up ancient coastal fortifications in places like Lebanon and Egypt. Ancient Roman ports used a kind of concrete that grew stronger in contact with water.
In the war with the sea, the Dutch have probably spent the longest in the trenches. When Pliny the Elder visited the Low Countries in 47 A.D., he compared the people he met to marooned sailors living on artificial mounds of mud; by the early Middle Ages, locals started to build a seawall. A 1948 book by a Dutch engineer, “Dredge, Drain, Reclaim: The Art of a Nation,” describes the wall as a major victory in an existential struggle. “Formerly the terrible evils of the sea, the storm floods and the more terrible marine erosion, had to be endured, but now the fight began to throw the sea out of the country; a fight not yet ended and a fight for to be or not to be,” he wrote. But the walls had a side effect. They locked the land into place, trading in a dynamic shoreline for one that could not adapt as easily, and that would need to be defended forever.
There are many kinds of coastal protection. Some of the most effective are entirely natural. Marshes, mangroves, and even sandy beaches can absorb the destructive power of waves, helping to soak up water and energy that would otherwise wreak havoc. Engineers can fortify a shoreline by replenishing lost sand, or by adding rock, wood, or concrete. It’s also possible to augment the shore. A rock pile that parallels the coast, shielding the beach from waves, is called a breakwater. A pile that juts out to sea, trapping sand on one side, is called a groin. All of these measures are already widely used on coastlines around the world.
