The fact that sea levels are rising does not come as a surprise to many scientific and engineering experts. For government agencies such as the US Army Corps of Engineers (USACE), whose responsibilities include protecting the public from flood damage along the coasts and inland waterways, rising sea levels had been a focus of concern for some time-decades, in fact-before the current urgent conversations on climate change gathered steam. Charley Chesnutt, a coastal engineer with the USACE Institute for Water Resources, says that his agency has “long been interested in the subject of sea level rise and has taken it into account preceding the topic of climate change.”

Back to the ’80s
Chesnutt says that the USACE, through its participation in a 1987 National Research Council study, was one of the first agencies to raise concerns about the potential acceleration of sea level rise due to the warming climate. The agency has gone a few steps further since that original report to incorporate accelerating rates of sea level rise into its plans. As part of its feasibility studies for each of its infrastructure projects, Chesnutt says, the USACE draws up an assessment of how each project will fare under the low-, medium-, and upper-range predictions for the rate of sea level rise under the influence of climate change.

But in terms of projects on the ground, Chesnutt says financial constraints are becoming an equally relevant component in the decision-making process, and the rationale for future projects hinges in a large degree upon their economic viability.

Regardless of the utility of enhancing society’s coastal buffers against rising sea levels, Chesnutt says, “I don’t think in the current economic situation that you’re going to see a lot of fixing it just for the sake of responding to projected acceleration of sea level rise-just because there isn’t money available to do very much. There isn’t enough money to do what we currently have on our plates to do. And that’s not a criticism; that’s just a statement of fact.

“I would say there isn’t money available in the federal treasury just to go around building upon and modifying the height of structures just for that purpose,” he adds.

Working within the current financial reality, Chesnutt predicts the short-term outcome. “What I think will happen more is, as projects do deteriorate and are considered for rehabilitation or reconstruction, we will then reconsider all the economics of it-is it a project that still has sufficient benefit to justify rehabilitating? And, if there are sufficient benefits to doing so, we will probably incorporate the new projections of potential sea level rise acceleration into the new design.”

To a certain degree, that process is already taking place, he says.

“We have a number of coastal structures that we have built-breakwaters, jetties, and seawalls-and as we have money for assessments, we are looking at what’s the state of these projects.”

A Dilemma of Solutions and Tradeoffs
Jeff Fiske, coastal and water industry manager for Tensar International Corp. Inc., says that while many people in the industry are familiar with the prospects for sea level rise, out in the field it’s the traditional erosion issues that often come to the fore in project discussions.

“For inland applications, you have a lot of different things that impact erosion on streams and channels. You might build a parking lot a mile away from the nearest stream, but the amount of additional runoff that it generates and dumps into that stream is going to increase volume, velocity, sediment load, and everything else.”

By the same token, Fiske says, on the ocean one of the biggest concerns is wave-generated erosion. “Scour is actually a pretty big issue as well, because we still have the currents that we deal with in the ocean as you would in any inland application, and sometimes they can even be more severe.” And added to the natural dynamics of the ocean, “a lot of the erosion problems may actually be exacerbated by some of the structures that are put in to protect pieces of property.”

For example, a groin structure that is designed to trap sand on one particular stretch of beach to prevent erosion at one location may prevent sand from flowing onto an area of beach immediately down drift from the protected property. In some cases, Fiske notes, “You’re likely to see higher extremes of erosion. Because of the way the system tends to feed itself, everything you’re trapping in one area is being kept away from another area. One of the big issues you see in the field is the need for proper planning and judicious use of structures.”

As a result of such considerations, he says, the decision-making process for designing structures and systems to address coastal erosion issues can be relatively contentious. “I’ve seen some instances where the local community gets very vocal and very active in the planning and preplanning stages because everybody wants to make sure that it’s not going to impact their property, their property values, or the environment.”

Scouring the Waves for Answers, or Residents Speak Up
The residents of North Wildwood, NJ, a summer resort area on the Atlantic Ocean adjacent to the Hereford Inlet, have a very good vantage point to observe the dynamics of waves. With high-value homes built right up to the edge of the sea, North Wildwood has enjoyed the protection of a massive seawall as long as anybody can remember.

Douglas Leatherman, senior civil engineer with the USACE Philadelphia District, says it’s not uncommon on the more developed areas of the New Jersey shore to see some significant lengths of protective seawall. It is also not uncommon to see some pretty substantial waves, sometimes rising to 12 or 13 feet.

But, Leatherman says, “Even if you have a specific design wave height, there is a certain point where you can’t make the structure large enough or high enough to protect against everything, so even with a 12- or 13-foot design wave you still may have some of that wave breaking over the structure. It’s a matter of comparing the cost of the structure with the economic benefits.”

While he says a 12-to-13-foot design wave is “fairly common along the New Jersey coast, in some areas they could be higher depending on the predominant wave direction and things like that. It’s pretty complex science.” And having a large structure in a place, such as the seawall protecting North Wildwood, is not always enough to keep the water out completely. “Sometimes, the waves would break over it,” Leatherman says.

But North Wildwood faced a worse problem than the occasional big wave; partly due to its age, the seawall was unraveling from the inside out.

“There was a church behind the wall that was losing material from its parking lot, and the whole existing seawall was unstable,” Leatherman says.

The culprit-scour.

According to Tensar’s Jeff Fiske, because the age of the community’s vintage seawall, there had never been a geotextile fabric installed underneath the revetment protecting the oceanfront side of the bulkhead. In the absence of a geotextile to contain the finer material over time, sand from beneath the seawall had been scoured out, carried away by the ebb and flow of the tide and the strong current from the inlet, causing the revetment to erode away and to dissipate, leaving the seawall in jeopardy.

“You could see that there was some piping of sand from behind the seawall going out underneath the seawall,” Fiske says. And, in the adjacent church parking lot, “you could see some small sinkholes that let you know that there was sand, and that whatever embedment there was beneath the seawall was going out under the seawall and washing away.”

To restore the protection for the community, the USACE developed a plan to reconstruct and refurbish the seawall using materials salvaged from the original structure. “A typical section of the project was basically a stone structure with various size stones,” explains Leatherman. “Starting from the bottom, we had stone-filled mats, then we underlayer stone, and then on top of that, large capstones that are 4 to 6 tons each.”

He explains the complexities and difficulties of getting the sizes and shapes needed to create the mammoth capstones. Quarries’ current reluctance to do controlled blasting was a deciding factor to salvage and use existing materials.

“We require a specific size and a blocky shape, almost like a cube. They [quarries] don’t want to produce that kind of a rock; they make more money by quick blasting of the rock and crushing it to use for road surfaces and highways. That, plus the distance we’d have to move the stone-most of it coming from southeastern Pennsylvania, some of it coming from Maryland-made it very costly to get the size and the shape we want, plus the cost of shipping it that far.”

Leatherman adds that it would have meant an order of about 50,000 tons of stone, given that each capstone weighs 4 to 6 tons.

To protect the investment in the stones needed for the job and to keep the valuable monumental stone blocks from sinking into the sands, ensuring the long-term integrity of the structure meant the structure would require a strong foundation. And that was going to be a particular challenge working on the bottom of the ocean.

“When you build a structure like that like on sand, because of the tides and currents most of the stone would disappear and keep going down. So, what you have to do is put in a separation layer to start with in order to put the stone on top of it, and that’s where Jeff Fiske came in with Tensar,” Leatherman says.

Fiske describes the dilemma the Corps of Engineers faced. “Knowing there was no geotextile under the original grade, they also realized that if they piled more stone out there and just beefed up the revetment, they would still have the problem of not having any geotextile,” which he says would be needed to serve as a separation layer to keep the fines from coming up and to keep the whole thing from sinking into the bottom. “That geotextile is what keeps the sand and subgrade material in place.”

To make the situation more challenging, the depth of the scour zone in need of this remediation along the deteriorating seawall plunged in some areas to greater than 60 feet. According to Fiske, trying to place panels of geotextile fabric by hand in anywhere from a few feet of water down to almost 30 feet of water along a mile stretch of seawall can be extremely difficult.

“It may have even been impractical on a project the size of North Wildwood.”

And, Leatherman says, because of safety concerns working in deep waters, using divers to place geotextile on the sea floor for a project of this type would be out of the question.

Leatherman says Tensar had developed a product called Polymeric Marine Mattress, composed of plastic grid polymeric mattresses that would be filled with small stone 1.5 to 3 inches in diameter, that would provide the answer. “We put geotextile on the bottom to keep the stone from coming through. Basically it’s a stone mattress that’s laid on the bottom before all of the remaining stone layers are put down on top of it.”

Unlike gabions, which often include steel structural components, Fiske says Tensar’s Polymeric Marine Mattress is made of 100% plastic material to resist corrosion. In addition, it is UV stabilized and made from nondegradable inert materials, “so there are very few chemical and biological degradation agents that are going to impact the mats.

“The other major difference from gabions is the manner in which they are filled and placed. In most cases, gabions are going to be filled in place. Our products are going to be filled prior to installation. They are filled and staged, and then lifted into place,” Fiske adds. “Certainly, if you are working under 55 feet of water, which we do a considerable amount of the time-or in situations such as an environmental restoration project we’re doing in Louisiana, where there is no construction equipment allowed on the shoreline-we’re able to fill all of these mats offsite, put them on a barge, and bring them in. The only thing that touches the shoreline other than the product itself might be a crew of three laborers onsite to spread the geotextile fabric and to help guide the filled mats into place.”

At the outset of the project, Leatherman notes, the USACE planned to line the Tensar mattresses from the inside with geotextile to keep fines in place on the floor of the seawall. However, as a cost-saving measure, the USACE discovered that it could modify that approach for the sections of the mats that would not be subject to strong deepwater currents. “We found we could save money by just fastening the geotextile on the bottom exterior of the basket. We didn’t need to have the whole baskets lined-only the bottom needed to have that separation layer.”

On the North Wildwood project, Fiske says, the marine contractor in charge of the installation brought yet another level of sophistication to the effort. If the mattresses were stitched together in sets of five, the combined weight of the stone fill would provide the anchoring for each set of mattresses to the prescribed position on the sea floor.

According to Fiske, the marine contractor had a lot of experience working on the Jersey shore. “The currents were no surprise to them. Rather than using divers to help position the mats and verify location, they began using a system of GPS and sonar and proposed that system to the USACE in lieu of divers. Even though it was still difficult to position the mats, it was a lot easier to verify, once things were on grade. We could see where they were without having to send people down into dangerous situations.”

The project was completed in 2007. Fiske says the cost of the marine mattresses in place and filled was $12 per square foot, including labor.

The local community is very happy with the project, Leatherman says. “I don’t know if we’ve had any really severe storms to test it, but so far everything’s been in good shape.”

Meanwhile, Down South, a Different Problem
While the beaches along the coast represent interesting and exciting destinations for those who can get there, the fuel they’ll need for making the trip likely comes via a different coastline, facing its own unique erosion threat.

Plaquemines Parish stretches 100 miles into the Gulf of Mexico from southern Louisiana. “The Mississippi River runs right down the middle of it. On the west bank there is one road in and out, and that’s Highway 23,” says Plaquemines Parish president Billy Nungesser.

“Five percent of all the natural gas in the USA runs through Plaquemines Parish,” Nungesser says. “We have several refineries, and we are the largest producer of oil and gas in the state of Louisiana-which probably makes us one of the top energy-producing counties in the country, so keeping that road open is vital to the oil industry,” he adds.

Although Highway 23 provides four-lane travel through Plaquemines to the city of Venice, LA, because of the low-lying terrain and proximity to the storm-churned Gulf of Mexico, it nonetheless represents a fragile lifeline.

A wind of 45 miles an hour, sustained over several hours, that beats that water up and against the levee on the south side of the islands can provide all the momentum needed to overtop the barrier and inundate the vital roadway, Nungesser says. “We’ve had Highway 23 underwater for several weeks as a result-then there was Katrina and Gustav.

“The economic damage is huge, because no one can get to south Plaquemines,” Nungesser adds. The flooding is damaging not only to energy production, but also to the area’s tourism industry. Venice has recently been touted as one of the top sport-fishing spots in the world, supported by numerous stores, restaurants, and small businesses tied to the area’s leisure industry. While driving along the highway on a typical day, Nungesser says, he might spot 50 boats hitched to trailers on their way to go fishing.

Although the levees protecting the highway have, up to recent times, been in private hands, they will soon be incorporated into the federal USACE system, with plans to provide upgrades raising them from a height of 4 feet to approximately 12 feet, which Nungesser says will provide permanent protection. In the interim, he says, Plaquemines Parish would need some sort of enhanced protection against storm surge for the 2012-2013 hurricane season.

The project that would accomplish this started as a temporary fix as Tropical Storm Debbie threatened Plaquemines Parish in the summer of 2012. Dennis Barkemeyer, senior technical representative for Hesco Bastion USA, arrived on the eve of the predicted storm. His company manufactures the Concertainer, a multicellular gabion used both for flood prevention and perimeter defense. He began coordinating with the Louisiana Department of Transportation and Development (LADOTD) to “allocate some Hesco Concertainer that they had in surplus, and some that was leftover or being reused from last year’s record-setting-high Mississippi River floods.” After the first day of work on storm preparations, however, Barkemeyer says the storm decided to take a turn back to Florida.

According to Barkemeyer, some of the weather models continued to forecast that the storm could turn back to Louisiana, prompting the LADOTD to conclude that the threat was an ongoing problem. “We already had materials staged and crews on the site, so we carried it out.”

Barkemeyer says Concertainer provides a relatively quick and easy interim flood prevention system. “Going back to hurricane Gustav a few years ago, we actually applied the product on that highway in two-and-a-half feet of standing flood water, so we could actually put pumps on the highway side and pump the water back out over the wall to reopen the highway about 10 days ahead of the schedule.”

With the cooperation of LADOTD, workers placed the Hesco cells 24 inches off the shoulder along a low-lying stretch of road through Louisiana cattle country. “We took a bulldozer and dragged it completely level, scraped the top grass off, and that’s where we set up the baskets,” Nungesser says.

He says that setting up the baskets and filling them with dredge material to complete the job on the 3-mile stretch of road took about a week and a half, in spite of the work being interrupted several times by thunderstorms.

Filled with locally accessible material, the Concertainer barriers can be set up with greater ease than sandbags Nungesser says. Furthermore, he says, it would have taken a huge volume of sandbags. “To get that height, you’ve got to go so wide” using sandbags, he notes, that it would have been difficult to avoid interfering with traffic.

The project used 19,500 linear feet of 4-foot-average Concertainers lining the right shoulder of the highway heading south.

According to Barkemeyer, the newest Hesco wall along Highway 23 will remain standing for the rest of the hurricane season. But he says Hesco walls can be turned into permanent flood protection by capping them. With a Hesco interim flood protection system in place, he says, a transportation department might decide to “come back down the line and cap it with clay-and now it’s permanent, because our geotextile will last 100 years if buried. The only thing that can damage the geotextile over a five-year period is UV light.” However, he notes that in the case of Highway 23, for reasons of traffic safety, LADOTD engineers have advised against that option.

Nonetheless, Nungesser says, “It’s a great asset; it gives us that extra height quickly. We’ve used them all over for flood fighting in the six years I have been parish president, and now we’re looking at using them for coastal erosion. It’s a great invention and has worked very well for us here in south Louisiana.”

About the Author

David C. Richardson

David C. Richardson is a frequent contributor to Forester Media publications.