Disappearing beaches
Grim examples
Role of beaches
Solving beach erosion?
Global warming and sea levels
Sand flow at the Indian River inlet in Delaware was blocked by these jetties. Sand is now piped across the inlet from the wide beach to the narrow one, restoring the natural flow of sand and helping reverse erosion.
Courtesy of Robert Dalrymple, University of Delaware. Photo by U.S. Army Corps of Engineers.
Simple solutions boomerang
Having said this, we must point out that sea walls are contentious. Some experts, like Spencer Rogers of the North Carolina Sea Grant program, say they don't accelerate erosion, but rather prevent the landward migration of the beach. Nevertheless, he says, since the ocean side of the beach keeps moving, "What beach you do have will disappear" even if a sea wall is built along an eroding shore.
Landowners plagued by disappearing beaches quickly realized that building a rock wall perpendicular to the beach -- a groin -- would gather sand on the updrift side of the wall. The physics is simple: The structure slows the longshore currents that carry sand, and slow-moving water can carry less suspended sediment -- sand. The result is that sand is deposited on the updrift side, depriving the downdrift side of sand.
Groins were heavily built along the New Jersey coast, but they've also fallen into disfavor. "They work for the updrift property owner, but it's obvious that they remove sand from the longshore system," says Jim O'Connell, a coastal processes specialist at Woods Hole Oceanographic Institution, "resulting in less sand for the downdrift property."
Call it robbing Peter to pay Paul. Call it beggaring thy neighbor. Call it any cliché you like -- building groins is highly discouraged in many places. As O'Connell points out, the coastline is "all one linked system. If you alter one area, you will be causing an alteration in another."
Cities like Miami Beach that built right up to the bluffs above the beach soon noticed that the bluffs were eroding, bringing the ocean a bit too close for comfort. (Naming a city after a beach did not mean the founders wanted to do the bucket-and-shovel thing on Ocean Drive.) The city responded by reinforcing the bluffs with sea walls. But the walls reflected wave energy back to the sea, accelerating erosion, and depriving the beaches of sand that normally erodes from bluffs. For both reasons, sea walls have fallen from favor.
Faced with a kick to the groin, if we may phrase it thusly, beach restorers have resorted to pumping sand onto beaches, taking the sand from deep waters or dredging projects. This expensive solution seems to work -- for a while -- and it's the "method of choice these days," as Robert Dalrymple, a civil engineer at the University of Delaware Sea Grant program, puts it. So-called "beach nourishment" helped restore Miami Beach, to name one of many eroded beaches.
Eventually, however, the same forces that denuded the beach in the first place will remove sand, causing the problem to return. On the Middle Atlantic coast, you can figure to pump sand onto a beach about every five years, Dalrymple says.
Fine tuning
Dalrymple says even groins may have a place: "Robbing of sand will not happen when you fill the groin fields with sand before you use them. You don't make sand with these devices," he observes, but they can protect sand pumped in from elsewhere.
(Groins, incidentally, helped cause the Hatteras Lighthouse erosion, Rogers says. In the 1970s, the Navy built two groins just north of the lighthouse, to protect a building. Predictably, the groins caused erosion on the downdrift side, and, according to Rogers, "you'd have to say" it was a classic case of a groin field robbing sand from the downdrift side.)
If you're getting the picture that preventing beach erosion is either feckless or counterproductive, there is a bright side. Although coastal engineering devices are not perfect, "most of the solutions you've heard about will work in the appropriate places," Dalrymple says. Take sea walls, regarded just short of strychnine by many coastal experts. "You hear lots of bad things about sea walls on the open coast," he says, yet they may work "if you have lots of sand moving past."
Take a break, water!
More intriguing, he says, is a submerged breakwater, which offer many of the same benefits, without besmirching the horizon with rock piles. In essence, a submerged breakwater acts as a coral reef, causing the waves to break before reaching shore. However, Dalrymple says the details of how and where to build them have yet to be worked out, (and we imagine surfers would despise them).
Finally, Dalrymple points to the sand schlepping system shown in the photo above. The beach erosion at the top was caused by jetties built about 30 years ago to protect a channel used by pleasure boaters. The jetties interrupted the longshore drift, allowing the outgoing tidal current from the inlet to funnel sand to deep waters, where it becomes less useful than a bikini to a Victorian matron -- it will never be formed into a sand castle or make coral-colored asphalt from Turkish Taffy.
As the inlet project demonstrates, the young discipline of shoreline engineering is an area requiring lots of ingenuity and fine tuning, Dalrymple says. And it's just as well -- the beach is the destination of choice for millions of Americans each summer, and is worth $800-million per year in tiny Delaware alone.
Global warming is raising sea levels. That boosts erosion. How does global warming affect sea level?
Another possible solution is building offshore breakwaters to reduce wave energy before it reaches the beach. Breakwaters are long heaps of rocks dumped parallel to the shore to intercept waves, and 6,000 have been built in Japan. "Depending on how they are used, they will do fine," Dalrymple says, although he grants visible breakwaters can be "eyesores."
