To the untrained eye, it looks like an obvious problem that calls for a straightforward solution. When ocean waves and winds are attacking your prized shorefront property, you defend it-preferably with something that will resist the erosive forces as long as possible. But look a little closer and things don’t appear to be so simple.
For one thing, armoring your shoreline to preserve your land, your home, your view, and the beach might actually create more problems for your neighbor or others farther down the coast. What’s more, even the experts don’t agree on the best way to deal with shoreline erosion. In fact, some question the desirability of doing anything at all to fortify seaside dunes, bluffs, and cliffs. In the end, they say, nature will win out, so the only sensible thing is to move inland and let the beaches be. Then again, there’s the pesky matter of paying for the costly protection. Should some of the public’s money be spent to help private homeowners and businesses stem their losses?
Consider efforts in the past few years to control shoreline erosion on the southwest coast of the state of Washington. For decades, the beaches had been widening, until the early 1990s, when erosion started to occur. Since then, some $60 million has been spent to beef up a jetty protecting a harbor and build riprap sea walls to defend a beach town, a hotel-casino, and a highway from wave attacks. Yet critics claim such armoring of the shoreline only increases erosion. What’s more, they say the 14 dams constructed over the past 70 years on the Columbia River, which borders Washington on the south, have trapped the sand that once flowed to the mouth of the river. From there, ocean currents used to sweep it north to replenish these beaches. If that’s true, do you go back and tear out the dams? Research also indicates that waves battering this coast have grown bigger over the past 25 years. Any ideas on how to shrink them? One more thing: As it’s been doing for thousands of years, the sea level continues to rise. What do you do about that? Drain off some of the ocean? Despite the obvious threats to beach dwellers and commercial interests in southwest Washington, is it a problem that can even be solved?
Shorelines are flexible, dynamic works of nature. The best way to save them, say opponents of armoring, is to leave them alone. In the long run, they contend, it’s also much less expensive to abandon roads and houses or to move them inland rather than try to save them from receding bluffs and beaches.
Shorelines are temporary geological features continually shaped and changed by wind, waves, and tides. The beaches and dunes absorb some of the impact of direct wave attack to help defend coastal bluffs and cliffs.
Coastal areas are subject to various types of erosion. It can occur on an annual cycle as beaches expand and retreat in response to seasonal changes in weather. In winter, waves generated by storms might scour out large amounts of sand, leaving steep, narrow beaches. Much of this sand is transported offshore across the beach profile to form sandbars. During summer’s calmer weather, gentler waves wash this sand back onto shore, expanding the beach and building gentle slopes. Because of many variables, including climatic changes, the amount of sand movement at any one location isn’t consistent from one year to the next. Overall, however, if the littoral sediment inputs are more or less in equilibrium with the erosive outputs, these seasonal changes tend to average out over the long run. In this way, for example, sand eroded from beaches along the Atlantic shoreline of Florida and other East Coast states is replaced by sand brought in from farther north via nearshore wave-driven currents flowing southward.
Shoreline erosion can also be caused by variations or gradients in the longshore transport if more sand leaves a reach of beach than enters it. This can be a natural process at inlets and headlands. It can also be caused by dredging or dams on streams or rivers flowing to the sea, which reduce the amount of sand available to the beach.
What’s more, the shoreline can erode when the areas upland of the beach itself-the dunes, low bluffs, or high cliffs-erode. Erosion can also occur when the shoreline moves landward because of rising sea levels-a worldwide phenomenon underway since the last Ice Age ended 18,000-20,000 years ago.
Of the 30 coastal states in the United States, those on the Gulf of Mexico experience the highest rates of erosion: an average of 6 ft. or more per year. In Maine, a little over half the state’s coastline is subject to erosion. That includes sandy beaches, which represent about 2% of the coastline. It also includes bluffs, some 50 ft. or more in height, along the coast, inner bays, and estuaries. These bluffs make up about half the coastline. The rest of the shoreline is rock, where little erosion occurs.
In general, the erosion rate among the Atlantic coastal states averages about 2-3 ft./yr. During a hurricane or other major storm, 100 ft. or more of shoreline might erode in just one day. In California, 86% (946 mi.) of the state’s 1,100 mi. of shoreline is actively eroding, and this will continue as long as the sea level continues to rise. Erosion rates range from little, if at all, along the granitic rocks of the Monterey Peninsula of central California to more than 6 ft./yr. in unconsolidated sand dunes just a few miles to the north in Monterey Bay.
“Coastal erosion or retreat is a natural ongoing process that has only become a problem because people have built permanent structures in areas that are prone to erosion or wave attack,” says Gary Griggs, director of the Institute of Marine Sciences at the University of California, Santa Cruz.
Over the years, the attraction of living and playing in the surf and sand has proven irresistible to ever-increasing numbers of people. Weekend shacks and cabins have been replaced by multimillion-dollar oceanfront homes, luxurious high-rise condominiums, and lavish resorts. That has heightened the economic and political pressures to fortify the shoreline in developed areas, even as the cost of doing so goes up. Last year, ABC News reported that in California alone, beaches generated about $12 billion in annual revenue from recreation and tourism.
Last year the Federal Emergency Management Agency issued results of a three-year study of the US coastline prepared by The Heinz Center for Science, Economics and the Environment. Among the findings:
- Each day, 3,600 people move to the coast.
- Development in several of the high-risk coastal areas studied increased by more than 60% over the preceding 20 years.
- About one of every four homes and other structures within 500 ft. of the US coastline-about 87,000 buildings in all-could be lost to erosion in the next 60 years. That’s about 1,450 structures a year.
- Annual erosion-related losses-ranging from failed septic systems to condemned or destroyed houses-are likely to exceed $500 million a year. Those living along the Atlantic and Gulf of Mexico coastlines are expected to bear about 60% of the costs.
“One of our biggest challenges is getting owners of beachfront property to realize that their beach will change over time and that sooner or later it will erode,” relates Janet Freedman, a coastal geologist with Rhode Island’s Coastal Resources Management Council.
One reason residents along Rhode Island’s shoreline, as well as those in other coastal areas, are seeing their property erode is that the sea level rises as the earth warms and glaciers dwindle in size. Since the last Ice Age, melting ice caps and glaciers have added an estimated 11 million mi.3 of water to the world’s oceans, reports Griggs. When the last Ice Age ended, the sea level was about 425 ft. lower than today, he notes. On the Atlantic Coast of the US-a relatively flat, depositional coastline-the shoreline might have been 50 mi. or more offshore of its current position. On the uplifted West Coast, with a much steeper continental shelf, the shoreline was probably no more than 10 mi. seaward of the present coast.
Currently, sea level is rising at an annual rate of about 2 mm. Many scientists believe, however, that burning of fossil fuels over the last century or so has accelerated global warming. That could increase the speed at which the sea rises. “The best estimate now is that sea level could rise as much as 3 feet by the end of this century,” Griggs says. Such a rise, of course, would have major implications around the world. Already some low-elevation islands in the South Pacific, as well Bangladesh and other low-lying areas of the world, are being threatened.
“Millions of people are living within a few feet of sea level, and that line is constantly changing,” notes Griggs. “Right now, we don’t know how fast and how high it will rise.”
Development inland is also adding to erosion of the US coastline. Dams and reservoirs can hold back sediment that would otherwise flow to the coast and drift along shorelines to build up beaches. A 1995 study found that dams in the San Gabriel River drainage of California had trapped more than 78 million yd.3 of sediment. As Griggs points out, that’s equivalent to more than 260,000 yd.3 of littoral drift per year for 300 years.
Dredging rivers to improve navigation and building levees to control floods can also add to the toll. Louisiana, for example, is losing more than 35 mi.2 of coastal land a year. At one time, sediment carried by the Mississippi River would be deposited in delta areas near the mouth of the river. As one area built up, the river would change course to build new delta areas and rebuild eroded ones. Erosion and delta formation remained in balance. However, this balance was upset with construction of levees in the early 1900s, which prevented the river from changing course, and dredging at the river’s mouth, which caused the river to deposit its load of sediment there.
Removal of oil, gas, and sulfur deposits along Louisiana’s coastline also led to subsidence of the coastal area as a result of fluid withdrawal from the subsurface. Development of these resources also led to construction of canals to transport crews, equipment, and materials through the marshlands. These canals allowed salt water to penetrate into the marshes, damaging and killing soil-holding trees and other vegetation and leading to increased erosion.
In response to erosion, many owners of coastal property have constructed a variety of sea walls and revetments to armor their shorelines and protect their homes and businesses against attack by ocean waves. The pace of construction has increased with the rise in development in recent years.
“It’s important to realize that both sea walls and revetments are constructed to protect upland structures, whether on a backbeach, a dune, a bluff, or a cliff,” says Griggs. “They are not designed or intended to protect the beach itself. Spending large amounts of money to install a coastal protection structure does not guarantee long-term protection for home and property. The degree of exposure to wave attack, the foundation materials, as well as the specific design, construction, and materials, will all influence the life span and effectiveness of a structure.
“On a high-energy and/or eroding shoreline, any protective structure built to withstand direct wave attack will probably eventually fail. Even a well-designed structure is likely to fail once its design life has been exceeded, especially if it has not been properly maintained. In general, engineers commonly think in terms of a 25- to 30-year life for a coastal protection structure.”
He describes several types of shoreline protective measures:
Sea Walls
Sea walls, usually vertical structures of concrete or timber, are constructed at the base of a cliff or on a beach to reflect wave energy, protecting dunes or cliffs and the structures built on them from direct wave attack. “Ideally any sea wall should be designed and built to avoid overtopping, undermining, or outflanking and to resist battering by waves and floating debris, like logs,” Griggs explains. “In California, a few well-engineered concrete sea walls that extend deep enough to prevent undermining or scour and are high enough to prevent significant overtopping have survived for 50 years or more, although many have not and have been repeatedly damaged and reconstructed or repaired.”
Revetments
Revetments are typically built of large rocks stacked in a wedge-shape configuration against a cliff, a bluff, or a dune to absorb most of the wave energy. To help prevent failure or collapse, Griggs maintains, they should include an excavated foundation and placement of filter cloth, followed by smaller core stones and then capstones weighing 4-6 tons or more, depending on wave conditions. Because they can extend 20-40 ft. seaward from the base of the bluff, they can completely cover the usable beach, he notes.
Groins
Groins are relatively short (about 100- to 300-ft.-long) rock, concrete, steel, or timber structures built perpendicular to the shoreline and designed to form, widen, or stabilize a beach by trapping littoral drift, Griggs describes. They’re often built in a series. “Because wide beaches form the best natural defense against shoreline erosion, groins can reduce shoreline erosion. They will temporarily trap sand destined for downcoast beaches until the groins are completely filled or charged. If groins are spaced too far apart, however, erosion can take place downcoast of individual groins.”
Jetties
Jetties are large structures built perpendicular to the shoreline and often in pairs to provide safe entry of vessels into a harbor. “Sediment accumulates updrift from the structure at a rate equal to the littoral drift,” Griggs says. “Meanwhile, erosion occurs downdrift at the same rate.”
Breakwaters
Breakwaters are also large structures that protect a shore area, a harbor, or an anchorage from direct wave attack. “They greatly reduce wave action within the protected area landward of the breakwater,” Griggs states. “This interrupts littoral drift and initially causes sediment accretion upcoast and shoreline erosion downcoast.”
Offshore Breakwaters
Typically these are built outside the breaker line parallel to the shoreline. Either submerged or piercing the water surface, they are usually built with a series of gaps, which allow water to circulate in the nearshore. These structures protect the shoreline by reducing the size of waves that reach the beach.
Various types of hard stabilization have been installed on US coastlines in such places as Atlantic City, NJ; Virginia Beach, VA; Myrtle Beach, SC; Daytona Beach, FL; and Galveston, TX. In California, 130 mi., or 12%, of the state’s coastline has been armored, Griggs reports. This includes 30% of the coastline of Santa Cruz County and 65% of the coast of Ventura County. Between 1971 and 1992, the amount of armoring along California’s coast quadrupled.
One criticism of sea walls is that, while they protect an individual structure or stretch of coastline, they might divert the ocean’s erosive energy to adjacent and other downcoast property. “Whenever a hard structure is built along a shoreline undergoing long-term net erosion, the shoreline will eventually migrate landward beyond the structure,” Griggs explains. “This migration causes a gradual loss of beach in front of the sea wall or revetment as the water deepens and the shoreface moves landward. This passive erosion seems to result from fixing the position of the shoreline on an otherwise eroding stretch of coast and rising sea levels. It’s independent of the type of sea wall constructed.”
This type of erosion has been documented in Oregon, Washington, and Hawaii. It’s also occurring on the Atlantic coast where offshore barrier islands are migrating landward as sea level rises, except where sea walls have stabilized the coastlines.
In Rhode Island, revetments have stopped erosion of headlands, which once supplied sand carried downcoast by littoral drift, reports coastal geologist Freedman. Also, until about 50 years ago, the largest coastal lagoons had narrow, shallow inlets that would plug up with sand during storms. This prevented large volumes of sand from entering the lagoons. In the 1950s, however, these breachways were stabilized, dredged, and protected with jetties. “Now,” she says, “sand is diverted into the lagoons, especially during large storms. Most of the sand that ends up in the lagoon stays there. As a result, sand that would normally travel downcoast is lost from the natural system.”
Freedman points out other drawbacks of sea walls, revetments, and other shoreline armoring. “Waves tend to reflect from these structures, eroding the beach on the sides and in front of the structures, sometimes to the point where they collapse. Public access along the coast is lost when the beach erodes in front of the structure. These structures can also prevent access to public beaches. In some areas, stone revetments that were originally built against a dune or bluff now extend so far from the shoreline that you can’t walk around them even at low tide.”
Armor structures have also been criticized for detracting from the natural look and aesthetics of shorelines. Cost is another concern. With price tags of $5 million-$15 million or more per mile, shoreline armoring projects can eat up a lot of money very quickly. That leads to the question of who pays the tab. “A private-property owner who can afford armor protection is one thing,” says Griggs. “But if construction of shoreline protective structures involves government disaster funds or other programs, is that the best use of public funds? The public can’t afford to protect the entire coast line of the United States from erosion. Where do you draw the line?”
The desirability of any given method of protecting shorelines can depend on perspective and time frame. Some experts tend to view the situation in terms of geologic time. Others, especially those with a personal stake in shorefront property, often measure time with a shorter scale-more like the next 25, 50, or 100 years. That’s where sea walls, revetments, and other shoreline protective structures can prove worthwhile in protecting increasingly valuable real estate, enhancing marine navigation, and protecting vessels.
William McDougal, a professor of civil engineering at Oregon State University, has extensive experience studying and designing coastal protection materials and structures. He is also involved with a project in California to develop state standards for beach nourishment, the adding of sand to eroding coastal areas.
“If beach nourishment provides adequate safety and is cost-competitive, I almost always favor this alternative over hard structures,” McDougal relates. He acknowledges that mistakes, such as interrupting longshore transport of sand, have been made in the use of hard structures. When designed and installed correctly, however, sea walls, revetments, and other hard armor can be an appropriate consideration for shoreline erosion problems, he adds.
“You need a realistic expectation of the purpose and function of hard structures. I am opposed to those that are not needed or not built and used correctly. For example, shoreline erosion caused by an infrequent or one-time event, like an unusually severe storm, doesn’t always require a structural response. But if erosion is a chronic problem, a hard structure might be a sound solution. It won’t protect a beach from eroding, but it will protect buildings on the backshore area. Where a great deal of money has been invested in development along the shoreline, hard structures may be a viable alternative to abandoning the site.”
Sometimes, McDougal says, installation of a revetment or sea wall will cause the beach to flatten, reducing the width of the dry beach area. “It causes the beach profile to readjust as sand from the higher part migrates offshore to the deeper, seaward portion of the profile. There is no net loss of sand from the beach profile, but there is a loss from the dry beach portion used by tourists.”
McDougal has monitored beaches at locations of seven hard structures along the coast of Oregon for 10 years, both before and after installation. These field measurements do not show increased erosion of the shoreline. “This is in part due to the fact that the structures are constructed high on the beach profile and are infrequently attacked by large waves,” he points out. “Several revetments protecting high, unstable bluffs actually improved the situation because they helped stabilize bluffs adjacent to the protected property.”
A variety of products are available for constructing temporary and permanent structures to control shoreline erosion. One is a system of individual concrete units, which resemble the small, six-legged metal toy children use in the game of jacks. Called A-Jacks and marketed by Armortec, they vary in size. The smallest unit weighs about 80 lb. and measures 2 ft. long. The largest unit installed to date is 10 ft. long and weighs about 10,000 lb. They can be installed either randomly or in a uniform pattern to armor a revetment. They are also used to control streambank erosion and bridge scour in rivers, protect coastal breakwaters and jetties, and develop habitat in river and coastal applications.
McDougal and Frank Atkinson developed the A-Jacks system as a more stable alternative to riprap structures. The system has environmentally friendly applications, McDougal notes. “The interstices formed within the A-Jacks matrix provide about 60% void space when placed in a uniform pattern. These voids can provide habitat for fish and other marine life when used as an artificial reef. In river applications, the voids can also be backfilled with suitable soils and planted with grasses, shrubs, and trees.”
Armortec also manufactures the Armorflex articulated concrete mattress system. These mats are composed of open-cell or closed-cell precast concrete blocks laced together with polyester or stainless steel cables. The individual blocks vary in thickness from 4.75 to 9 in. and weigh 31-243 lb. Although used primarily for inland channel lining, earthen dam rehabilitation, and lake shoreline protection, Armorflex also has a role in controlling coastal erosion where rigid structures are allowed, says Derek Dice, director of engineering for Armortec. “Unlike a rock armor revetment, Armorflex is a hydraulically stable, aesthetically pleasing system that does not require continuous maintenance throughout the lifetime of the structure.”
In 1997, Armorflex was used with a site-specific synthetic geotextile and native trees and other plants to control erosion of 1,800 ft. of shoreline along an abandoned landfill near Key West, FL. This combination of hard and soft armor costs about half the price of the next lowest bid-a conventional approach using granite riprap-and provides comparable erosion protection.
The design-build project involved Ocean Breeze Construction Company Inc. of Palm Beach Gardens, FL, in association with Delon Hampton & Associates of Atlanta, GA. It was designed to protect the shore from the erosive forces of 2.4-ft.-high breaking waves to meet the 50-year performance specification.
The existing embankment was laid back to a 3:1 slope, which crested about 10 ft. above the mean low waterline. The base subgrade was compacted and covered with a woven monofilament geotextile to provide separation and filtration. Bedding stone was placed atop the geotextile before the 7.5-in.-thick mat of articulated concrete blocks was installed. The openings within the blocks were filled with soil to provide a more natural appearance than riprap. Sea purslane, a ground cover, was planted in these blocks on the upper portion of the slope, while red mangrove seedlings were planted in parts of the lower slope. The roots of the mangroves provide habitat for marine life while tree foliage provides nesting habitat for the many shorebirds and restores a natural look to the site.
In deciding the merits of armoring shorelines, Griggs offers this perspective: “In the long run, any attempt to prevent erosion of shorelines is probably going to be futile. But if you’ve invested your life savings in a beachfront home, you’re much more likely to favor installing a hard structure that will protect your property for maybe another 25 or 30 years. In that case, you’re probably not so concerned about what happens 50 years or more down the road.”
Over the past two decades or so, most of the states on the Atlantic, Gulf of Mexico, and Pacific coasts have adopted plans designed to manage coastal erosion on a more long-term basis. Many encourage a more natural approach to defending the beaches.
“The closer we come to developing more natural solutions for shoreline erosion problems, the more self-sustaining any such system will be,” says Griggs.
“In Rhode Island, we prefer to let nature play out,” relates Freedman. “One of the goals of the Coastal Resources
