Cape Hatteras Lighthouse is the 208-foot tall landmark was just hauled more than a quarter-mile back from its former perch, where it was threatened by the encroaching sea. Coastal erosion chewed away about 1,300 feet of beach, bringing the waves to within 150 feet of the 4,800-ton sentinel. When the light was erected in 1870, it stood about 1,500 feet back from the waves. The lighthouse, on the Outer Banks, North Carolina’s long barrier beach, was built to warn ships from waters called “the graveyard of the Atlantic.
Ironically, the move should serve as a warning about the growing problem of coastal erosion. Erosion is not just plaguing the Outer Banks. Coastal residents up and down the United States are worrying about undermined cliffs, disappearing beaches, and the occasional dwelling diving into the briny. Beaches are constantly moving, building up here and eroding there, in response to waves, winds, storms and relative sea level rise. Yet when commoners like you and me, and celebs like Steven Spielberg, build along the beach in places like Southampton, N. Y. , we don’t always consider erosion. After all, real-estate transactions are seldom closed during hurricanes or northeasters, which cause the most dramatic damage to beaches. Yet Southampton, like all the barrier beaches that protect land from the sea, is vulnerable to obliteration by the very factor that makes it so glamorous: the sea. And the problem is increasing because the sea is rising after centuries of relatively slow rise, and scientists anticipate that the rate of rise will continue to increase in the next century.
Land, in many places, is also slowly sinking. The result is a loss of sand that places the occasional beachside home inconveniently near — or in — the water. Still, erosion cuts in two directions. Without the process of erosion, we would not have the beaches, dunes, barrier beaches, and the highly productive bays and estuaries that owe their very existence to the presence of barrier beaches. Erosion of glacial landforms provides most of the beach sand in Massachusetts. A popular destination The beach-erosion problem has many causes.
Among them are: The ubiquitous desire to live near the sea. A historically rapid rise in average ocean levels, now estimated to be rising at about 25 to 30 centimeters per century in much of the United States. The gradual sinking of coastal land (since the height of the land and the sea are both changing, we use “relative sea level rise” to describe the rise of the ocean compared to the height of land in a particular location). Efforts to reduce erosion that have backfired and instead increased it.
Global warming, which is expected to accelerate the rise in sea level. The upshot is a threat to beaches and coastal communities around the world. At stake is far more than a movie mogul’s mansion. New Orleans, now several feet below sea level, would face a greater threat of annihilation. Island nations across the Pacific Ocean could disappear beneath the waves. Millions of Bangladeshis, already exposed to typhoons that drown hundreds of thousands at a time, would have to find new homes in one of the Earth’s most crowded nations.
The predictions growing out of global warming studies are unsettling. Much of Long Island’s extensive barrier beach, including not just the homes of the rich and famous in the Hamptons, but also public treasures like the vastly popular park at Jones Beach, would be submerged if sea levels rise by three feet, according to a projection by the National Environmental Trust, a Washington, DC, advocacy group. A three-foot rise over the next 50 to 100 years is possible, but extremely unlikely, according to current predictions. Coastal erosion is a knotty issue.
Slowing global warming — the ultimate cause for heightened concern about the future — is proving problematic, to put it charitably. And many localized cures for erosion are worse than the disease. Some are “beggar-thy-neighbor” solutions that steal sand from one location to save another. Others are expensive Band Aids that pump sand from deep waters to the beach, where it immediately begins washing away. . A widespread problem How extensive is the coastal erosion problem? Consider: During a 1992 storm, the Atlantic Ocean broke through a barrier island near Westhampton, N.
Y. destroying about 190 of the 246 homes on the island. The breakthrough was blamed on structures designed to build up beaches that blocked the flow of sand along the shore. These so-called groins build up some beaches while depriving others of their essential sand supply. Seventy-two percent of coastal towns in Massachusetts are “exhibiting a long-term erosive trend,” says O’Connell. Beaches in Southern California are losing vast amounts of sand, and some are down to bare rock. The beach sand came from river sediment, but damming and water removals have impeded that supply.
One drastic solution, the removal of Matilija dam on the Ventura River, is under consideration, with twin goals of restoring trout to the ocean, and sand to the beaches around Santa Barbara. In Britain, the Observer magazine described, under the headline “Incredible Shrinking Britain,” cliffside houses tumbling into the English Channel. In Galveston, Texas, more than 140 property owners entered legal limbo when beach erosion moved the public beach (defined as bare sand without vegetation) to their property. A thin line of protection All geology is about change.
Continents, as we know, drift gradually around the globe. The ocean floor is being created at the mid-ocean ridges and recycled beneath the crust at the margins. Mountains rise up and gradually erode back. These changes are slow, inexorable, and usually gradual. The changes on a beach, in contrast, can happen literally overnight, at least during a storm. Even without storms, sand may be lost to longshore drift (the currents that parallel coastlines). Or sand may be pulled to deeper water, essentially lost to the coastal system.
On the positive side, sand arrives from eroding uplands, river sediment, and longshore drift. Change — with a purpose All this change, however, is useful to those who live sheltered by the beach. Aside from providing recreation and wildlife habitat, beaches are protection for whatever lies behind. Like those foam-packed highway barriers that give way on impact, beaches absorb energy from the sea. Beaches are a very significant dissipater of wave energy, the wider, more gently sloping and permeable they are, the more energy will dissipate before it reaches landward development or natural resources.
Simple solutions boomerang 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. 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. 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.
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. ” 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 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. ” 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. ) Take a break, water! 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.
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. 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. . Warming ocean = rising ocean? After the last ice age, the rapid melting of glaciers rapidly raised sea level.
That melting tapered off about 6,000 years ago, and sea level — compared to land — became fairly stable. However, over the past century, sea level over much of the United States has risen by 25 to 30 centimeters relative to land, according to Jim Titus, the Environmental Protection Agency’s project manager on sea level rise. Even that figure is a guesstimate, Titus says. “We only know that sea level last century rose more than average over the last several thousand years. ” The warming of the atmosphere caused by increases in greenhouse gases is melting glaciers and causing ocean water to warm and expand thermally.
Both effects increase the volume of the ocean, raising its surface level. How much will these factors add to the existing trend toward sea-level rise? The Why Files asked Titus what to expect. He did his best to answer the question, but he started by throwing cold water on our desire to learn the absolute rise in the ocean surface. In other words, how far has the average ocean surface moved from the center of the Earth? Unfortunately, nobody has made that measurement consistently, so we must settle for records of relative sea level rise, which tells us what’s happened to sea level in comparison to a certain hunk of coastal real estate.
Windiness aside, what did he say? The short answer is that a 1995 EPA study projects about a foot — about 30 centimeters — of extra relative rise over the next century or so for the U. S. coast. Since the “background” rise is about 25 to 30 centimeters per century, the total relative rise comes to between 55 and 60 centimeters over a century. But remember that this depends on location — in areas where the land is actually rising, relative sea level might not rise at all. Now come the caveats. This calculation, Titus stresses, “makes no sense” because it amounts to adding a guesstimate of future rise to a range of historic rise.
Projections, he says, are “even less certain” than historic data. And remember that nobody knows the exact trend in carbon dioxide emissions, which drive the accelerated greenhouse effect. At any rate, EPA says there’s a 5 percent chance that global warming will not augment the existing trend toward a sea rise. But there’s also a 5 percent chance that the sea will rise an extra two feet, and a 1 percent chance that it will rise an extra three feet, above the existing trend. Stabilizing global carbon dioxide emissions would cut the rise in half, the EPA reported.
Those numbers don’t seem too intimidating, until you look at the consequences. Under the worst scenario, island nations, coastal cities and beaches alike would be threatened with obliteration, especially if the rise continued for more than one century. Groundwater aquifers could be polluted by salt water. Tunnels, harbors and coastal wetlands could be soaked with salt water. The 30-centimeter increase over the existing rate of rise, after all, is greater than the rise over the past century, which caused all the ruckus we’ve been describing. But nobody expects the prediction to be right on the money.
The obstacle to making accurate predictions, Titus says, is this: “We’ve never done this before — pumped greenhouse gases into the atmosphere at this rate. If we do this a couple of times, then we can have more confidence in our predictions. This is an experiment, and unexpected things can happen. ” The big dunk Eventually, if greenhouse warming continues, big changes could be in store for our planet. These so-called “doomsday scenarios” result from feedback effects — networks of cause and effect that amplify the warming caused by the original increase in carbon dioxide. We’ll mention just a couple of possibilities. Methane madness.
The continental shelves harbor huge amounts of stored methane, a potent greenhouse gas in its own right. If coastal waters warm enough, this methane could be released, causing greatly increased warming. Similarly, warming in the Arctic tundra could release vast stores of carbon dioxide, with similar effects. Ice-sheet insanity. Melting glaciers are but a trickle of water on the planetary scale. They’re small change compared to the West Antarctic Ice Sheet, which holds 3 million cubic kilometers of fresh water. Were it to melt, sea level would rise 20 feet, and coastal cities, not to mention beaches, would be in tough shape.
Let me make one thing perfectly obscure Ice shelves melt all the time, “calving” icebergs that, no matter how enormous, do not effect sea level. Because floating ice displaces the same volume of water as melted ice, iceberg that formed from floating ice on Antarctica’s vast ice shelves add no volume to the ocean. But when ice cascades off land, it does increase the volume of the oceans. Ten years ago, glaciologists were worried this might happen if the Ross Ice Shelf off the Antarctic coast continued melting, thus releasing massive ice flows from the West Antarctic Ice Sheet into the Southern Ocean.
The ice shelf, they thought, served as an obstacle holding back the enormous ice sheet. Calculations warned of a massive collapse of the ice sheet that could raise the sea level by 20 feet in a century. The current opinion among glaciologists is a lot more reassuring. Charles Bentley, a retired University of Wisconsin-Madison geologist who has spent 40 years studying the frozen continent, says the mathematics of the old calculations artificially accentuated the possibility of collapse, and also oversimplified the movement mechanism. To risk oversimplification ourselves, here’s the new view.
Most of the ice sheet rests on land that’s below sea level. At a point called the “grounding line” it starts floating, thus displacing its own weight in water, so the important question is to locate the grounding line. And as it turns out, the line may not move much because the flow of the ice streams seems to be restrained by friction against rocks at the bottom and sides rather than the ice shelf. The streams, says Bentley, “don’t particularly care whether there is an ice shelf there or not. ” So if the ice shelf melts, the flow of the streams should not change appreciably.
And since the volume added to the ocean depends on how much ice moves from land to water — as determined by the grounding line — the upshot seems to be relative stability. “The ice streams do not appear to be susceptible to the kind of unstable retreat once envisaged,” says Bentley. “Their flow is largely insensitive to the presence of the ice shelf so the grounding line would remain the same. ” Instead of possibly collapsing in 100 years, as was considered possible 10 years ago, Bentley says the West Antarctic Ice Sheet is more likely to collapse — if at all — in perhaps 5,000 years at the soonest.
