Seaside Debate

Sea walls are a potent symbol of the struggle between man and nature. Once considered triumphs of engineering know-how, these manmade barriers against the ocean have faced increasing controversy in recent years. Although sea walls are designed to prevent beach erosion, many experts claim they have detrimental side effects, sometimes causing more erosion than they prevent.

“Sea walls may or may not deserve the bad reputation they have,” says Ole Madsen, professor of civil and environmental engineering, who claims that attitudes about sea walls are seldom based on solid science. “We need a better general understanding of how sea walls work before we can make rational decisions about whether they provide any solution to coastal erosion,” he says, adding that this will take a great deal more research to accomplish.

The problem has grown in recent years as more people have moved to coastal areas. More than one-half of the national population now lives and works within 50 miles of the coastline, an area that accounts for only 11 percent of the nation’s land. More than a quarter million households are located in high-hazard coastal zones, according to the National Oceanic and Atmospheric Administration.

Protecting houses

Madsen, a world authority on beach erosion, says people build homes on the edge of the ocean because it’s a dramatic and beautiful place to live. “We’re attracted to the ocean and so we build a house there,” he says. “But when the ocean starts encroaching on us, endangering our property, then we want to exert control over it.”

According to Madsen, one of the biggest controversies with sea walls has come about because homeowners don’t act as a community against erosion. “If one person builds a sea wall and it does the job protecting his house, then the sea wall will still be there 10 years later,” he explains. “But the beaches on either side of it would still have retreated. So people see this sea wall-protected area sticking out like a sore thumb, and they blame the sea wall for all the erosion that probably would have happened anyway.”

This kind of biased thinking has caused many municipalities to prohibit all further sea wall construction, regardless of the potential benefits. Because every beach situation is different, Madsen says regulations must be carefully designed.

“You can’t have the same rule about sea walls for every piece of coastline,” he says. “For example, in North Carolina, they have one rule about what you can and cannot do on the barrier beach system that faces the Atlantic Ocean. They have a different rule for the mainland coastline that is mostly protected by the outlying barrier beach.”

Dutch success

Madsen says despite the controversy, there are some success stories in man’s centuries-old battle with the sea. “The Dutch have been building dikes for ages,” he points out. “Holland itself is evidence you can make sea walls work.”

The real key to making sea walls work, according to Madsen, is understanding the complex dynamics of beach erosion, such as wave direction, water turbulence, and sand transport. His research group has begun to unravel some of these issues by testing one of the most common criticisms of sea walls.

“One of the points of contention is whether a sea wall causes more erosion at the base of the side facing the ocean,” he says. “Is the erosion in front of the wall greater than what we would expect along an unprotected beach?” To answer that question, Madsen recently set up an erosion experiment using the Gunther Family Ocean Wave Facility at MIT’s Parsons Laboratory.

“We ran identical waves onto two beaches that were right next to each other,” he says. “One was protected with a sea wall, and the other one was just a sandy dune.” The researchers generated short, storm-type waves straight on to the beach and found that the waves dug into the sand in front of the sea wall and the dune to the same extent. “The beach profiles we got in front of the sea wall and in front of the dune were identical.”

Madsen says such large-scale experiments are an important first step in understanding how these natural processes work. “The ultimate goal would be a mathematical model that incorporates all the variables of a natural system–wave direction, sediment size, bottom profile–and makes an accurate prediction about whether a sea wall is a good idea or not,” he says. “And that capability is way out in the future.”

Until then the struggle between man and nature will likely continue. “I don’t know if we are control freaks, but we are one of the few species that has the ability to make drastic changes in our environment,” says Madsen. “There are just some things we can do and some things we can’t do.”

by Eve Downing

On Topic: civil engineering, environment