Huge coastal barriers could protect the world’s cities. But they’ll have unexpected costs.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.Hard seawalls may be the bluntest instrument in coastal engineering. Typically, they are made from concrete, stone, wood, or metal, and rise vertically from the shore. But a wave that strikes a seawall never breaks and dissipates, as it would on a beach; instead, it bounces off like an echo, its destructive force intact. In the end, the flow of water and sediment is a zero-sum game. For a wave to spare one place, it has to strike another; for sand to accumulate somewhere, it has to wash away from somewhere else.When I ran these critiques of coastal protection by Rachel Gittman, a marine ecologist at East Carolina University, she offered another reason to worry about seawalls. Natural habitats already serve as powerful buffers against flooding, she said. They absorb water and energy; this is why marsh and mangrove restoration is often the best way to protect a coast. By contrast, when coastal communities wall off the shoreline, they tend to trap ecosystems between the water and the wall, causing a process called coastal squeeze. “It can be a slow drowning of those habitats,” she told me. When they disappear, we may be more vulnerable than when we started.Keep reading
