Defending the Coast From Attack – Part 3

Oct. 8, 2001

Taking a Hard Approach
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,” explains Griggs. “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 feet 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,” says Griggs. “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,” states Griggs. “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.

Concerns About Armor
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,” explains Griggs. “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?”

About the Author

Greg Northcutt

Greg Northcutt writes frequently on construction and business issues.

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