Defending the Coast From Attack – Part 4

Nov. 1, 2001

Armor’s Merits
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 feet long. The largest unit installed to date is 10 feet long and weighs about 10,000 pounds. 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 inches and weigh 31–243 pounds. 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 feet 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-foot-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 feet 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-inch-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.”

About the Author

Greg Northcutt

Greg Northcutt writes frequently on construction and business issues.