There was a time when Spring Creek, in Mobile, AL, was a rather unsightly, shallow stream suffering from erosion and frequent flooding. Its channel was clogged with an overgrowth of trees and shrubs, and any time a heavy rain appeared, neighbors shuddered.
There had been, and continues to be, quite a bit of development in the area, with a great deal of paving. Waterways upstream from Spring Creek are subject to erosion, with significant sediment carried downstream.
SJ&L Civil Contractors of Mobile was the general contractor tasked with repairing the severely damaged creek. Owner Mike Tew explains that the municipality directed him to construct a gabion system as part of the stream repair, and after examining a variety of bids, selected the gabion baskets produced by Modular Gabion Systems, a division of C.E. Shepherd Co.
George Ragazzo, general manager for Modular Gabion Systems, has worked in the gabion industry for 45 years. He explains why his company’s gabions were selected for the project. “We supplied the gabions semi-assembled into a given size. For Spring Creek, they utilized rolls of mesh, which come 300 feet long, so the contractor can build gabions 300 feet long, without joints. That saves a lot of material, because it saves all the side panels from the end of one gabion to the next-or, when you put gabions back to back, the rear panel can be eliminated. There is also the elimination of a large quantity of labor, possibly amounting to as much as 40%.
“Nobody supplies gabions fully preassembled, because only a very few can fit in a truck. So they are supplied semi-assembled, requiring less space for shipping purposes. Then the contractor individually finishes assembling each gabion separately, and then links them together. A lot of this effort can be eliminated by using the roll stock system.”
But before the gabions could be installed, SJ&L had a lot of preparatory work to do on the creek. The first phase involved removing the major vegetation from the channel. “The main problem with sedimentation and vegetation growth is that it restricts channel flow,” says Ragazzo.
Next, approximately 30,000 cubic yards of streambed was excavated from half a mile of the channel, widening Spring Creek by an average of about 50 feet.
“It was a challenging job,” says SJ&L’s Tew. “We were 6 and 8 feet below where the original creek was. We took it in sections and bypassed the water around us. If you were to have a big rain upstream, you could be sitting down there, and the next thing you know, you could be looking at water building up, and you’d have to crank up an additional pump. You might have gotten rain 2 miles away from the job, and that’s now starting to impact you, even though you never saw lightning or heard thunder at all.
“That was probably the biggest challenge-the amount of excavation and the bypass pumping on the job.”
As Ragazzo clarifies, “Both sides of the creek are lined with gabion walls, but we could only do one side at a time. So they built a cofferdam in the middle of the channel so they could work on one side. When they were done with that side, they reversed the cofferdam.”
The gabion baskets lining the edge of the channel and a portion of the banks were designed as gravity walls. Ragazzo describes their layout: “The rear face of the gabion wall is flush, while the front is stepped. These steps in front allow increased flow capacity, being set back 18 inches every 3 feet. There is a 6-foot-high wall that is 4½ feet wide at the base, the first grid. The next level is set back 18 inches, and the top tier is 3 feet by 3 feet.
“The project also called for a 9-foot-high wall. With this wall, first comes the gabion mattress as a base, then the bottom tier is 6 feet wide and 3 feet high. The second tier is 4½ feet wide and 3 feet high, set back 18 inches. The top tier is again 3 feet square.
“Along the slope is a 12-inch-thick gabion basket that extends 6 feet from the edge of the wall toward the center of the channel,” Ragazzo says, describing the gabion mattress lining the channel edge. “This is 12 feet wide, with 6 feet under the wall and 6 feet outside of the wall in the front. It is for scour prevention. If the scour is significant during heavy rain events, then this gabion mattress at the bottom is flexible enough to slide into the scour hole and stop the erosion from progressing. It lies flat and is only 12 inches thick, on the bottom of the creek. Again, some of it is under the wall, and some of it is outside of the wall.”
As it turned out, during the construction process, there were a number of severe rain events, and the gabion mattress indeed flexed to cover any scouring. As Tew comments, “Here in Mobile, there’s nothing that says you can’t get a 3½-inch rain in an hour.”
The gabion baskets and mattresses along the edge of Spring Creek, totaling 7,600 cubic yards of gabion walls, served their purpose in stabilizing the stream. Unfortunately, however, problems have developed beyond the control of the contractor and the gabion manufacturer.
“Sedimentation has been a problem with creeks in Mobile,” Ragazzo says. “Upstream of Spring Creek, there is still some erosion taking place. The city has thought about using gabions to stop the erosion along the banks upstream from here, but money has not become available so far. So sedimentation has continued to occur, and some vegetation has regrown inside the creek.”
Tew echoes his thoughts. “A lot of sediment has silted into it now, but as far as the gabion product itself, it’s still there. Now that you’re getting a lot of siltation, you’re getting a higher water level. Then, when heavy rains come, you’re going to have potential problems. I’m not sure if the creek has come out of its banks, but I’m sure it’s flooded several times since we put that project in.
“The city may have already gone in and removed some of that sediment. I’m almost sure that they have over the last several years. I think they’ve really gone out of their way to stop the intrusion of sand and sediment up the creek from where we are. It’s a shame to get that much sand and sediment down in it, but it’s all coming from upstream. They’ve gotten a lot more stringent with some of the homebuilders and with other projects, to keep the runoff from these jobs from getting into the creek.”
Nevertheless, it seems that further sediment and vegetation cleanout is called for in order to maximize the benefit of the gabion structures that were so carefully put into place.
Hard Armor Alongside Morningside Drive
Morningside Drive, in Fort Worth, TX, has its share of development, consisting of residential subdivisions, an apartment complex, a school, and a variety of commercial buildings. Concrete is a prominent element of each of these developments, contributing to increased stormwater runoff into the adjacent creek.
With steep slopes, the creek was suffering from serious erosion problems. “It was too much water for the existing culvert,” explains Jason Fernberg, area manager for Maccaferri Inc. “They had small, round culverts in originally to drain the water from the streets. What the city decided to do was to take those out and put in big concrete box culverts that could handle a higher volume of water in the creek.
“Once they decided to do that, they realized that if they are going to create more flow into the creek, they will need to armor the creek so that it won’t erode down, since it is fairly close to the road. The road is fairly significant in size, serving the school and homes and businesses in the area.”
Various hard-armor solutions were available to the city of Fort Worth, and Fernberg notes some of the elements that went into their decision-making process. “Both Dallas and Fort Worth are pretty gabion-friendly,” he says, “but they did look at doing a concrete channel lining. At the time, though, concrete was more expensive. There is also a lot of clay in the soil here, so you get a lot of expanding and contracting. As a result, the concrete has to be extremely inexpensive for them to go with that option, since they know that they’re going to have to repair it later due to the expansive properties of the soil. Concrete will crack, and they will have to go in and fix it.
“At the time they were designing this project, concrete was not cheap, so they decided to go with a gabion structure that is flexible and strong. But they did want to economize, so that’s why they went with the MacMat R up the slopes a little farther down. They figured the gabions would take the brunt of the water coming out of the culvert, and then they could use something that allowed vegetation but was still strong.”
The MacMat R is a turf reinforcement mat (TRM) manufactured by Maccaferri, but it differs from most turf reinforcement mats in that it includes a structural skeleton of double-twisted steel wire mesh within the polymer matrix of the TRM.
“This gives it the strength of the metal, but with the soil retention properties of the turf reinforcement mat,” Fernberg says.
Supplied in a rolled form, the MacMat R is typically anchored to the ground with sod pins or U-shaped staples. “But in this case,” according to Fernberg, “because the banks were so steep and they knew they would have a pretty good volume of water, they decided to use 6-foot earth anchors. They drove these into the slope, and then capped them on top of the MacMat R.”
The MacMat R was installed on both banks until the stream reached a more forested stretch. The channel was lined with standard gabion baskets, with 3-foot by 3-foot gabion baskets used to connect the gabion mattresses in the channel to the structures on the slopes.
Directly outside of the box culvert were concrete wingwalls, and originally the plan was to continue the concrete along the slopes. “But the city went with gabions instead,” Fernberg says, “where they needed the strength of the gabion structure. Where this stopped is where they started the MacMat R.”
Installation of the gabions and the MacMat R TRMs took about two months. As Fernberg explains, no other BMPs were required. With all the work taking place in the channel, the contractor wasn’t concerned about construction runoff entering the roadway.
One challenge that the contractor faced was the steepness of the slopes, particularly on one side where the workspace was limited by a fence.
“They only had a few feet to work there before the slope dropped off considerably,” says Fernberg. “So they had to anchor themselves to one of the trees, to protect against slipping. But most of the guys were climbing up and down like they were zoo animals. I didn’t hear about any accidents, but I know that one slope was pretty steep, so they had to take some precautions on that side.”
For the most part, the installation went according to plan and without major problems. There was one frightening moment, however, as Fernberg explains. “The workers were using some type of electric saw to cut the MacMat R to the size they needed, and when they got down to the metal, it created a spark, which then caught the polypropylene mat on fire. They were setting it up on the bank, though, so it wasn’t connected to any other mats. It looked like there was a burrito on fire, but it didn’t catch anything else on fire. It just went up really fast, and went out really fast, which was a good thing, because there wasn’t any water in the channel at the time.
“They were wondering how to put it out. Fortunately, they were working in an area where all the grass was already gone, but they pulled it even farther away from some adjacent grass, and it just burned out by itself. It went up quickly, because the polypropylene is an oil-based product, but it went out within minutes.
“After that, they decided to cut other mats with bolt cutters, and they didn’t have any further problems. It was a learning experience.”
To date, there has not been any additional erosion in the area. Fernberg notes that “Anywhere they have water running down on concrete, they have an inlet to divert it down into a pipe that goes down into the culvert, so there’s only rainfall entering the stream from the culvert, not heavy-duty water from over the top.”
There has been the occasional heavy rain, and water has reached a depth of 3 feet at times, but the gabion and MacMat R combination has worked well.
Containing Fountain Creek
“Pretty much any rain that falls between the Air Force Academy and Colorado Springs runs into Fountain Creek,” says Rob Hicks, field superintendent for Gracon Corp. “It’s a real active water system. Just during the time we’ve been there, we’ve seen water levels range from the normal foot-and-a-half all the way up to 12 feet, with a flow of 10,000 cubic feet per second.”
As a result, the creek has suffered significant erosion, which the US Army Corps of Engineers wanted to rectify. The corps selected a stretch of the channel about 70 feet wide and 300 feet long between the towns of Colorado Springs and Fountain, CO.
“It was originally requested that we use riprap-just regular riprap, not even concrete riprap,” Hicks explains. “But when our engineer did the calculations from the FEMA [Federal Emergency Management Agency] report, we would have had to go with 5-foot-size riprap and bigger. So it just wasn’t in the Corps of Engineers budget.
“So we proposed a solution using Cable Concrete, which really hasn’t been used in this particular aspect, although it’s been used for lining of channels before. Our engineer came up with a plan that would meet the requirements; it’s a C90 Cable Concrete mat from International Erosion Control Systems. The mat is 9½ inches thick, and each block is 3 feet by 3 feet; each mat contains 18 blocks.”
These articulated concrete-block mats line the entire width of the channel through this stretch of creek and are intended to reach as high as “normal” flood levels. The 12-foot-high water level experienced by Hicks was still below the outer stretches of the mats.
“The upper and the lower toes along the flow line are anchored into the bedrock,” he adds. “So it’s toed down to bedrock on the upper and lower end of the stream. Working to bedrock with water flowing can be a challenge. It’s a lot of wet work.”
In addition, Hicks notes that vegetation is growing through the Cable Concrete. “We’re trying to encourage more by planting willow cuttings during the season. The ones down in the water or closer to the water are doing better than the ones a little higher. Vegetation does come up through the blocks.”
An interesting sidelight to the project was that the mats were actually built on the job site. “Laying the mat only took about six weeks,” Hicks says. “Pouring the mat took the longest. They actually made the mat onsite. They don’t have to do this, but this was an unusual size. Because of the size and the shipping, they actually sent a crew out to make them onsite.
“We started making the mat in November 2011. In February 2012, three months later, we started placing the mat. The cure time is only seven days, but we had 136 mats made, and they were all made before we laid the first one.”
Both Hicks and FEMA inspectors were pleased with the results of the project. “They liked it,” says Hicks. “It doesn’t try to stand rigid against what Mother Nature throws at it. It’s actually flexible enough that it can change its position, controlled by the anchors that are drilled into bedrock. It can actually move.
“And it’s a place that people like to go to, down near the stream. It’s easy to walk on, with a trail right there.”
Big Plans for Little Wardensville
Wardensville, WV, population 271, has a total area of 0.3 square mile. But an important element in this small community is its wastewater treatment plant, consisting of two 3.4-acre lagoons. These lagoons have been battered by a series of major storms over the years, with only temporary fixes put into place.
Fearing the breakdown of these wastewater treatment lagoons, both FEMA and the state of West Virginia authorized a more comprehensive repair of the system, which occurred in the summer of 2010.
“The engineer called us before everything went out to bid, and he was looking at some options,” reports Alan Dinges, Maccaferri area manager for the region. “I believe riprap was an option, and they were looking at some turf reinforcement mats. It was a variety of hard-armor and soft-armor solutions.
“They actually went with a more aggressive solution with gabions underneath topsoil. They wanted to take away 6 inches of topsoil, put the gabions in, and then put the 6 inches of soil back on top.”
Instead of regular gabion baskets, however, the contractor opted to use 2,026 cubic yards of Reno mattresses from Maccaferri. These were massive, specially designed gabion baskets.
“We had three different sizes there-33 feet long, 60 feet, and 99 feet,” says Dinges. “And we had 6-foot and 9-foot-wide sizes. With a roll product such as this, it eliminates a lot of individual connections for the gabion baskets. So that’s a nice benefit.
“These Reno mattresses are essentially the same thing as gabion baskets,” he explains. “Instead of being 1 foot, 1-and-a-half foot, or 3 feet thick, they’re 6, 9, or 12 inches thick. The mesh opening size is slightly smaller than the traditional gabion basket, and the wire diameter is slightly smaller. It’s the same exact idea, but lighter wire, and smaller everything.
“We try to use a 3- or 6-inch stone for the mattresses. The idea is we want to have at least two layers of stone inside the baskets to help interlock everything.”
A special requirement for the project was that all elements had to be American made. “When it comes to gabions, it’s almost become a commodity now,” according to Dinges. “There are a lot of foreign manufacturers-in China, Peru, Mexico. So we had to certify that American-made materials would be on our plate.”
The project went according to plan. Six inches of topsoil was removed from the dikes of the lagoons, the mattresses were put into place, and the topsoil was then returned to the top of the gabion mattresses. Tall fescue seed was then applied to jumpstart vegetation growth along the dikes, with blown straw covering the seed for protection.
“The grass popped up pretty darn quick,” says Dinges. “I was surprised-it started within a week or two, and then really started to take off pretty quickly.”
He notes that there were a couple challenges encountered during installation of the gabion mattresses. “There was an issue with the proximity of the river alongside the project. We had a very narrow workspace. There was a silt fence up, and there was not a lot of room to work along that riverside. But storing the topsoil was probably the biggest problem. They had to remove the 6 inches of topsoil before they could put the baskets in and cover them back up.”
To date, more than two years post-construction, the lagoons are fully vegetated and no further erosion has occurred.