A Combination of Gray and Green to Tame Nature

Hard armor and vegetation work together on slope repair and channel protection projects.

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A slope overlooking the Geist Reservoir in Indianapolis
To be green or to be gray, that is the question:
Whether ’tis nobler in the municipality to suffer the floods and perils of our fickle Mother Nature, or to take precaution against the sea and tides… To protect, to evacuate no more…

It’s likely Shakespeare might not approve of this borrowed and altered stanza, applying Hamlet‘s famous question to the current armoring of coastlines, but he did have a sense of humor, so the concept might well be appreciated, despite the mangling of prose.

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There is no question, however, that balancing the ­appropriate protective measures for coastlines is a ­demanding task. The discussion of possible solutions often raises more questions than it answers. Intervention must embrace current and future needs of humans and the ­natural environment—with particular sensitivity to wildlife—as well as address the effects of climate change, tides, sea level rise, and asset and infrastructure management as urbanization expands.

A slope overlooking the Geist Reservoir in Indianapolis

Taming Nature More Gently
Two years ago, residents of a neighborhood in Indianapolis, IN, faced with the consequences of throwing yard refuse “over the back fence,” had to fix the problem by reaching into their own pockets.

Eric Spangler, operations manager with Hoosier Aquatic Management, a company that specializes in soft-armor products, describes the dilemma: “For years, people had been throwing their lawn clippings and leaf rakings over the side of a slope that bordered the city’s Geist Reservoir.

“Then, about four years ago, we had a terrific downpour; the slope became liquefied, and trees slid down the slope into the reservoir.”

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A slope overlooking the Geist Reservoir in Indianapolis

What had happened, he says, is that the lawn clippings and other refuse covered the soil surface and prevented native plants from growing—and therefore eliminated the natural erosion protection groundcover would have provided. The downpour simply acted like a flame to wax on the soil—loosening it, creating excessive erosion, and causing root exposure.

“The neighborhood got together and found that grant monies from the Upper White River Watershed Alliance were available for amendment funding,” says Spangler. “Nobody really knew what to do though, as it was a very steep slope and any amendment would take years to literally take hold.”

He explains that the grant required an approved design and also required that the neighborhood front the money, but once the project was completed and met with the alliance’s approval, the alliance would reimburse 80% of the total cost.

“We were called in to assess the situation, and we decided to do terracing and use the Living Logs to fix the problem,” he says. “Terracing is not commonly used in the Midwest; it is in other areas, but not here. We devised a plan that would break up the slope, and allow some soil to hang onto the logs for vegetation to get established. The Living Logs are perfect because they come with plants already established in them.”

His company supplies the Living Logs. “The way this works is we have eight-inch-diameter, 10-foot-long poly net tubes that we fill with soil. Then we put in plant plugs that are each about two- by two- by four-inches, already with roots, spacing them every foot along the log inside the soil log. So, a 10-foot log has 10 plants already live and growing, implanted.”

Spangler describes the plants as a combination of wetland and upland plants, depending on where the logs were to be placed on the slope. They included black-eyed susan, echinacea, sedges, and swamp milkweed.

“We can source whatever native plants you might need, wherever your geographic location,” he says. “We’ll have those plants sent to us and then install them in the soil logs, and they’ll be ready for installation.”

Once the logs were placed onsite within the six terraces, crews hydroseeded the slope with grass seed and soil amendments to jump-start the growing process.

“In a short time, the plants in the logs start shooting up, and the grasses are holding it in place,” he notes. “The project was a big success. The Upper White River Watershed Alliance came in and said it was all good, and then reimbursed the neighbors 80% of their investment costs.”

Treating a sensitive area with low-impact intervention is his company’s stock in trade, Spangler says. He describes working on a 60-foot-long property where a 4.5-foot-deep creek that had been covered with English ivy for years disguised a huge problem.

“Here was an unusual scenario with this long creek property, owned by two Master Gardeners. They discovered that the once-dry creek bed was experiencing a continuous flow as urbanization [upstream] increased, causing erosion. And the problem was worsening; as the water rose, it was causing continuous undercutting of tree roots, creating a shelf and very serious erosion.”

The owners had looked at and priced gabions as an option to stem the erosion, “but they had very valuable plants on the property, and as the creek was 90 feet long, they were absolutely against any heavy equipment coming in to excavate and disturb this sensitive area,” he says.

Credit: IECS A stormwater channel in Hamilton, ON
Credit: IECS
A stormwater channel in Hamilton, ON

The owners eventually opted to use 5-foot-long, 18-inch-diameter soil bags from Hoosier Aquatic Management. “First, we had to dig out some of the roots vertically to create an appropriate slope space,” explains Spangler. “Then, using 220 soil bags, we stacked and battered them on that slope. There are grommets in the material, so we staked them to the soil on the back side, then backfilled the entire area, and planted over that surface.”

The property owners were very pleased with the results. Except for the bags on the face of the slope, which will eventually be covered with vegetation, the work is invisible.

“We had done flow and velocity analysis, and any water will pass through the bags into the creek, but the bags will keep the soil and slope intact,” he says.

Spangler is a firm believer in the durability of these systems. “Our living systems for erosion protection get stronger over time. Once these root systems get established, they are very tough. They get better, require no maintenance, and have tremendous longevity. You can’t always say that about hard armoring.”

Doing More by Doing Less
The city of Hamilton, ON, has built a number of weirs and spillways for stormwater management. “The consultant that was designing an emergency stormwater channel started looking at riprap for erosion protection,” says Greg Arvai of International Erosion Control Solutions (IECS). “Once he realized how much excavation they would have to do, and then bring in all that stone to bring the elevation back up to what it was when they excavated, they contacted us to come up with a better solution.”

Credit: CUMMINGS AND MCCRADY INC. The conjunction of two seawalls in Charleston’s harbor (foreground) is known as “The Turn.”
The conjunction of two seawalls in Charleston’s harbor (foreground) is known as “The Turn.”

Arvai explains that the company, which specializes in articulated interlocking systems, has evolved on the premise that “less land intervention could offer a better solution.”

IECS, headquartered in Rodney, ON, had done several analyses with university laboratories to show that its block mat compared favorably with loose rock erosion control, such as riprap. Dubbed Cable Concrete, the articulated concrete product, comes ready to install as a flexible mat and can be made in four different thicknesses, depending on what a site needs.

“This system is flexible—each concrete block is about 15.5 inches square, tapering to 11.5 inches at the top. They are a trapezoidal shape, and are interconnected with stainless aircraft cables. The mats are made of 72 blocks, and this covers about 12 square meters, or a little more than 13 square yards,” explains Arvai.

The blocks range in thickness, from 3 to 8 inches, with the smaller ones naturally weighing less.

An engineering analysis determines how much water will be flowing in the area where the mats are to be installed and, therefore, determines which size block to use.

“We look at how many cubic liters will be flowing at any given slope and how fast it will travel over the block, and that tells us the weight of the block needed to protect the subgrade from erosion,” says Arvai.

Credit: CUMMINGS AND MCCRADY INC. Pouring concrete at night on the Charleston seawall
Pouring concrete at night on the Charleston seawall

Managing More Rooftop Runoff
The challenge to create the right scenario for Hamilton arose from new residential developments that had been built in a hilly area that, Arvai says, added “a lot more surface runoff from rooftops.” The runoff was directed to a detention pond. “When those ponds overtop, the water has to go somewhere,” he says.

“Using these connected mats on the Hamilton project saved the contractor from having to excavate about 900 millimeters [about 3 feet] to put in rock, as they only needed to go down 5.5 inches for the concrete mats. Each mat of block is connected to its neighbor with cable. And, because the mats come with geotextile under each mat, you don’t need extra people to lay that down; it’s already installed.”

He adds that after a year of preparation and design approvals, “we manufactured all the material for them in two to three weeks, and then the installation took less than a week to put in the required 1,700 square meters. It’s a dry channel, about 20 feet wide and 4 feet deep, and it will stay dry unless there is storm, when it takes the water and channels it to the bottom of the hill to a water source, protecting the soil from washing away.”

The city of Hamilton wanted to conserve as much vegetation as possible. The Cable Concrete has 20% open surface, which allows vegetation to grow through the mat. After a while, “it won’t even be visible,” says Arvai. And beyond the erosion protection, there are other aspects that benefit the environment.

“Because we don’t have to excavate and the installation is so straightforward, we have a smaller impact. With few trucks traveling back and forth, we reduce the carbon footprint; we also have much less wear and tear on the local roads, and reduced scheduling for contractors.”

In addition, he notes, “The project will have little effect on the nearby hiking trail, as the flat surface—which eventually revegetates—allows for foot traffic of both local wildlife and people. You can’t easily do that with loose rock.”

He adds, “This system is a benefit to the land in managing erosion control, while, at the same time, conserving the natural landscape with less impact.”

A Century Later, Historic Seawall Gets a Facelift
In October 2013, and after 12 years of planning, the city of Charleston, SC, began the restoration of “The Turn,” the semicircle of seawall that conjoins two 100-year-old, 5,000-foot-long seawalls called the High and Low Battery. The Turn, which faces Charleston Harbor—the confluence of the Ashley and Cooper Rivers—is the most vulnerable portion of the historic structure. Pedestrians like to stand atop it for spectacular views across the water toward Fort Sumpter.

South Carolina history shows that the Battery, built originally as fortress at the tip of the city in 1726, offered an important view of any incoming “visitors” who might not be friendly. Today’s visitors who enjoy a stroll on the Promenade, the paved walkway on top the latest incarnation of the seawall, are treading the same pathways of former visitors—including pirates and patriots, both of which figure prominently in the defense and plunder of the city’s colorful more-than-three-century history.

After taking a beating, literally, from Union ships that shelled the city during the Civil War, the seawall was repaired, but it was later destroyed by hurricanes and rebuilt significantly in 1909. Over the next century, weather and deterioration mandated a complete rebuild with new technologies and materials that today’s city engineer, J. Frank Newham, says should last another few centuries.

“When we took out all the original wooden piles from the early 20th century, the ones that hadn’t been exposed to air were still in great shape,” says Newham. “However, because of erosion, the ends of many had been exposed to sun and oxygen, and had began to rot.”

As the aging structure settled, cracks had formed in the aggregate wall. “Then the rebar expands; the material expands and pops the concrete in spalling.”

Credit: CUMMINGS AND MCCRADY INC. On top of the finished seawall
On top of the finished seawall

Balancing History With Current Demands
“All the repairs were performed with the approval of the State Historic Preservation Office, which has a strong presence here in Charleston,” explains Newham. “We had to put everything back to look like it always did, which includes the walkways, handrails, seawall face—everything. However, over time, handrails had been replaced, and they were not uniform, having different profiles and looks, so we tried to establish one preferred post and rail, and then re-create and use that repeatedly.”

One of the biggest challenges, he adds, was to accommodate handicap access on the stone battery. “Before this project, since there is a slow grade rise from the Low to the High Battery, which were built at different time periods, people with strollers and wheelchairs had difficult access,” says Newham. “So, we had to remove the stairs and create a graded slope to provide better access.”

Project superintendent Pat Merli of Crowder Construction Company of Charlotte, NC, describes the work on the seawall—a project that won a Pinnacle Award from the American General Contractors Association—as “definitely not a typical job.”

“There were a lot of different entities to consider outside the actual construction of the seawall. First, there were the tourists, so we had to start later in the year—during October, when the season was winding down. Then there was working with the city, and the preservation aspects, and also managing complex logistics. We had to do things in the middle of the night, like take a 230-ton crane down through the city’s narrow streets, which took 12 to 13 hours.”

Newham adds that effective public relations work was key to keeping the community informed and happy. “We had citizen meetings to describe the project, and we had a website of our schedule so the citizens would know what’s going on. A lot of cities don’t have the tourism we do, and this is an important consideration. You can condition residents to your project and the potential inconveniences, but tourists are different every week, and it was important to maintain effective pedestrian and traffic control.”

Credit: CUMMINGS AND MCCRADY INC. Preparing to pour concrete on the Charleston seawall
Preparing to pour concrete on the Charleston seawall

A Surprise at the First Shovel
The old seawall had to be carefully dismantled, and the process revealed how the original structure had been built. Merli notes that in every job there’s always a surprise, but “this one was pretty spectacular.”

“When we first started, the very first bucket we dug had, unbelievably, a cannonball in it. So we sent it to an archaeologist. It was live. It was restored and then returned to the city. A lot of cannonballs were duds and didn’t go off, but they would still be loaded with gunpowder, which was the case here,” he says.

The team found how the wooden platform timber beams were bolted to the pilings and wooden decking on top, as well as to an angled layer sheathing of wood. A poured double wall of concrete had been backfilled with soil and rock.

After putting up barricades, the team dismantled the existing wall by cutting the structure on either side to take it down to water level.

“Then we drove a sheeting wall and created a coffer dam, and then dewatered within our limits,” says Merli. “Once we got to the old wall, we saw it was built on timber piling, so we removed all the old stone and concrete. Then we pulled the timber pilings out by using a straw puller, an implement, which essentially looks like a long straw and fits around the piling to literally create suction that pulls up the existing timber.”

The timbers removed were old pine trees about 35 to 40 feet long, sawed in half, and pointed on one end. Merli says some of these 100-year-old pines still had bark on them, “which is typical.” The original builders had probably used steam or a gravity drop hammer to install them in what he describes as “90% dredge sediment of pluff mud that had been brought in.”

Replacing the wooden pilings with steel, says Newham, required 70 new units, each 104 feet long, which were driven into the sediment with a diesel hammer. He says crews had to go through the sediment mud to drive them into subsurface marl, “a clay layer that is stable for any kind of pile-driving operation.”

Credit: CUMMINGS AND MCCRADY INC. Older wooden pilings were replaced with steel.
Older wooden pilings were replaced with steel.

To fill the space of the huge earth-filled double wall was a heroic task that Merli says required 25 separate ­concrete pours. “We had to do these at night, mainly

because of the heat. The project was 15 feet tall and 15 feet wide and 180 feet in length, and there was also provision for handrails, curbs, paving, extra drainage, and solid and impervious areas.”

Newham adds that crews used a low-porosity marine mix concrete, one that is used by the US Navy. It was poured first over the rebar, to prevent any saltwater intrusion.

Merli recalls there was much concern that the face of the concrete wouldn’t correctly replicate the appearance of the historic wall. “Aesthetics was very important, and we had to match the two sides, because the High Battery, built in the 1800s of random rock, was built earlier than the Low Battery, which was concrete with a lateral low pitch.”

To replicate the look of the wall, Newham says, “The formwork came in panels and mimicked the profile of the old Battery outside wall, and it was the same configuration as what was there before. This was assembled onsite, then braced.”

Newham says that following the pour, “We used rubbing stones to polish it out. It looked good and matched, and after a few weeks of exposure to the ocean, with barnacles and mildew beginning to form, no one could tell any difference.”

“Everyone was worried about how it would look,” says Merli, “but we poured about 300 yards of concrete and only about 10 yards actually shows.”

While all construction poses multiple issues beyond the work itself, this project had a unique set of challenges: it took place in one of the oldest cities in the country with narrow street access, surrounded by historic homes, and congested with tourists and all the amenities serving a tourist economy.

Credit: CUMMINGS AND MCCRADY INC. Closeup of the Turn
Closeup of the Turn

“We first thought about working from a barge, but at low tide there is very little water, and there’s an old sand spit about 35 feet out in the harbor, so it really wasn’t feasible,” recalls Newham. “But there was concern that the heavy construction so close to sensitive historic residential areas would cause damage.”

To ensure the work would not affect nearby neighborhoods, crews installed crack monitors and vibration monitors on the historic houses. “We learned in preconstruction monitoring that we really weren’t generating any vibration to these structures,” says Newham.

Nonetheless, the city contacted the homeowners to discuss the project and relieve their fears, “and most people were pretty happy,” he notes. “It’s essential to be in good communication with citizens and politicians and the local councilman, and brief them on the process so no one is blindsided. We also had to pre-qualify contractors, as we didn’t want to get a non-seawall-experienced company for this job, and this took a lot of burden away from worrying about this specific process. It was a low-bid procurement, but only coming from the ones that know what they are doing.”

The entire Battery will be replaced now that the Turn is done, but with 5,000 feet and 200 construction days required for every 500 feet, Newham says this is a long-term project that might take eight years or more.

A Basket for Every Need
JR Lucas, project engineer with Pillari Brothers Construction Company of Farmingdale, NJ, describes how gabions provided an ideal solution for the roadway extension at a prestigious East Coast university. “The institution was extending a roadway on their campus and putting in new traffic signals. However, a culvert needed to be built to take care of the stormwater issues arising from the increased road traffic. The first thing we had to do was build a culvert to handle the roadway on top, but maintain a natural bed underneath.”

Gabions from Maccaferri Inc. were used on the project. The Maryland-based company offers seven types of gabions for use—from the largest of civil engineering projects to modest streambank stabilization. The gabion baskets are manufactured as double-twisted, hexagonal woven steel wire mesh, and they can be delivered flat and filled with stones onsite, or filled and delivered as a ready-to-use product. High-quality steel wire is heavily galvanized, and a polymer coating adds longevity and additional protection for use in more aggressive environments.

A Walkway That Works and Looks Good, Too
Lucas explains that the area where the new roadway was built supports a number of wildlife populations, and, with the road cutting off natural trails, it was essential to maintain a route for access to the streambed underneath.

“This university area is very well known and maintains a high aesthetic in the community, so we were required to be creative and find an attractive, natural-looking solution—in other words, one that had functionally high-performance but also looked good.

“First we built a streambed out of the gabions so that water can still flow and percolate and accommodate wildlife. Then we extended the end gabion section in a flare, installing some of the baskets to create proper runoff to the existing stream.”

To work in the creek, crews built a cofferdam to provide a dry space to assemble the gabion baskets. The gabions would allow stormwater runoff from the new road to settle inside a sediment basin, and then this water would leave through a piping system for the nearby creek.

“We used three- to five-inch rock, which allows water to filter but keeps the soil where it should be,” explains Lucas.

He describes the Macaferri product as “a PVC-coated basket in dark gray to match the stone, so it’s not really intrusive. One reason we really like Maccaferri is they give us the ability to design our own gabion baskets and rolls, and they can give us very simple baskets in different sizes so we can mix and match them and make them custom to the job.”

He adds, “Because they’re packaged one at a time, the construction contractor can build a design and then fill them onsite with the gabion stone.”

Erosion control techniques have come a long way from a concrete cover up. With the benefits of new technologies and creative engineering designs, today’s products can employ the best of both green and gray solutions, alone or side by side, to benefit people, the landscape, and wildlife.

“Pluff mud,” also called plough mud, is a distinctive tidal mud found in the South Carolina Lowcountry. It’s soft, gooey, and very aromatic (not in a good way). This mud, however, supports the plants and wildlife of 400,000 acres of coastal marshes and 100,000 acres of tidal swamps. Its “rotten egg” smell results from hydrogen sulfide released when anaerobic bacteria consume organic material within the mud.


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