Erosion control methods have been around since the time of the Pharaoh. Seven thousand years ago, systems consisting of baskets of woven reeds were used along the Nile River in an effort to control erosion.
Today they’re called gabions. They are typically fashioned from wire mesh and filled with stone or dirt. Common in Europe for the past century, they now are being used throughout the world in a variety of applications. They are structurally sound, yet they are permeable and yield to earth movement.
“The wonderful thing about these new technologies is that they have expanded the ability to vegetate highly steeped sites,” says Robbin Sotir, president of Robbin B. Sotir & Associates in Marietta, GA. “You can do a reinforced soil system as steep as one-half-to-one and put vegetation in. This has increased the opportunity enormously for vegetative systems.”
A Foundation for Development in South Africa
Gabions have played a major role in a development project in South Africa, as outlined in a case study presented by Ronel Suthers, Chris Mulder, and Pieter Badenhorst last February at the International Erosion Control Association Annual Conference & Trade Expo in Las Vegas, NV.
R.T. Mabudahfhasi, deputy minister of environmental affairs and tourism, commended the developers “for the most responsible manner in which they conducted, supported, and financed the environmental research, which probably represents the most comprehensive, professional, and detailed environmental impact assessment ever undertaken in South Africa.”
Thesen Islands in Knysna, a community on the southeast coast of South Africa, is a 96-ha (237-ac.) development on an estate of 19 islands surrounded by wide tidal waterways linked by bridges. The challenge: Fifteen kilometers (9 mi.) of pristine waterfront property needed to be protected in the most environmentally friendly way possible against the erosive forces of fluctuating tide levels and wave action.
Obstacles to the marina development – initiated in August 2000 – were many. The soil profile includes fine sands with a friction angle of 13Â°. The sands are susceptible to liquefaction, meaning that under conditions of even moderate vibration they exhibit very low bearing capacity, and when saturated they flow and are virtually impossible to compact using vibratory equipment.
Another factor was water levels, which vary between 1 m (3.28 ft.) above and 1 m below mean sea level. The ground level on the islands had to be designed to be raised between 2.8 and 3.0 m (9-10 ft.) above mean sea level. The calculation was based on the highest astronomical spring tide with the extreme effects of river floods, bad sea conditions, atmospheric pressure, and strong winds and an allowance for potential effects of global warming and safety factors.
Geometric obstacles included canal width, high tide, and low tide. Environmental conditions of approval centered on a control that dictated for the first time in South Africa – and perhaps in the world – that all salt-marsh areas disturbed by the development be rehabilitated in such a manner as to ensure no net loss of salt marsh. Additionally, an ecobelt needed to be designated along all canal edges to protect the water edges from erosive forces of fluctuating tide levels and wave action, and development needed to be in harmony with the Knysna Estuary environment.
The only option found to achieve the standards set forth by the project specifications – environmental compliance, durability under aggressive marine environments, ease of installation and vegetation establishment and cost-effectiveness – for this largest marina development in South Africa included gabions, Reno mattresses, and a mesh panel. This combination approach protects each island against scour as a result of intertidal action and ensures the stability of the bank, especially under rapid drawdown conditions. Tidal flow is thus completely natural with excellent circulation and water quality and minimal silting.
Furthermore, the combination presented itself as the most appropriate solution in meeting the geometrical constraints of the canal embankments, providing durability under harsh marine environments, and playing a favorable role in environmental rehabilitation.
A bioengineering approach in the development of Thesen Islands comprises indigenous salt-marsh plants in conjunction with the inert materials in the intertidal zone. A diversity of birds and large fish are attracted to small fish and marine life that find food and shelter in the plants and natural rock along the edges of the waterways.
The basic design of the system consists of an underlying geotextile filter combined with a granular filter, overlain by a 230-mm- (9-in.-) deep Reno mattress, a 1-m2 (11-ft.2) gabion with a mesh-panel tail used as a lid, and a 170-mm- (7-in.-) deep Reno mattress lined with geofabric. The double-twisted mesh of the gabions, Reno mattresses, and mesh-panel tail is galvanized and PVC-coated.
The mesh-panel system comprises a front face and a reinforcement tail made from one continuous panel. In the Thesen Islands project, the lid of the traditional 1-m2 gabion was replaced with a mesh-panel tail, which closes the gabion and forms a reinforcement layer in the backfill.
Although gabions are traditionally manufactured with the double-twist mesh in a horizontal orientation, for this project all gabions were custom-made with the mesh in a vertical orientation along the front face. This orientation, along with a steel frame that supported the gabion during the rock-packing process, contributes to a desirable finish.
To make the system more pleasing to the eye, longitudinal tensioning was used. The process involves assembling an entire row of gabions and packing the end compartment to act as an anchor. A fencing wire tensioner is used to take up all of the slack in the mesh in the entire row. The opposite end is anchored in a similar fashion.
Preformed heavy-gauge braces were used to ensure timely and cost-effective construction. Bracing is the process of tensioning the front and back of the gabion to prevent bulging of the front face.
Specifications for the rock for the gabion and Reno mattress construction called for a limit on the minimum effective diameter and maximum dimension. The acceptable measurements are a minimum diameter smaller than the mesh diameter but a maximum dimension greater than 150 mm (6 in.) up to 300 mm (1 ft.). The lower 230-mm Reno mattress was prefilled before being placed mechanically in position, whereas the gabion and upper 170-mm Reno mattress were handpacked. The mechanical and labor-intensive construction allowed canal development to take place at a cost-effective rate of about 80 lin. m per day. The rock was mined in a local quarry.Protection against erosion of the fine estuarine sands of Thesen Islands was optimized by stabilizing the canal embankment. Reduced flow velocities combined with the rough surface area offered by the rock-filled gabions allow for the deposition of sediments. The rock-filled gabions provide surfaces for growth of algal-microbial films, which might contribute to nitrogen and phosphorus uptake from the tidal flow, thus reducing phytoplankton growth in the canals. Given the depth of the gabion walls, which are permeable to canal water, and the total surface area offered by the millions of stones packed into the gabions, it is believed the gabions might act as a macrobiofilter.
Chris Mulder Associates coordinated the overall design of the project. Mulder is one of the shareholders and owners in the development company. His own company does the land planning, master planning, urban design and architecture, and landscape architecture. Mulder has been involved with the project since its inception in 1990.
It took seven years to design and obtain all of the environmental permitting. When completed, there will be 540 home sites and a small harbor-town commercial village.
“The gabion work is going very well,” Mulder notes. “What has emerged thus far in the first phases where the waterways and canals have been open for two years and more is that rock in the gabions is a wonderful substrate for marine life and that the huge amount of rock in the gabions acts as a massive biofilter because millions of filter feeders settled on the gabions.
“The Reno mattresses and gabions are absolutely stable in the marine environment as we used them in the one-to-six slopes,” Mulder adds. “The top mattresses above the intertidal action are filled rock and topsoil mixtures, and there we planted salt-marsh vegetation, which is growing excellently.”
“We are verypleased with the outcome,” says Ronel Suthers, an environmental manager at Africa Gabions, a licensee of Maccaferri. She is experienced in biotechnical engineering and working with the vegetating of gabions. “The gabion work is fantastic, with a typical cross-section being constructed by local unskilled labor at a rate of 80 linear meters per day. Also, the vegetation has taken very nicely. We have been monitoring the performance of the gabions with a gabion management system, where sacrificial panels are installed and tested to monitor the performance of the mesh. The performance is excellent so far. Studies are also being conducted on the colonization of the gabions by marine organisms and the water quality within the canals. Results on this front are also promising.
“We did a rough calculation, and the approximately 10 miles [16 kilometers] of gabion and Reno mattress protection area provide a contact area of approximately 700,000 square meters [7.5 million square feet] of surface, more than any other embankment type would have provided,” Suthers reports.
Meanwhile, in San Angelo, TX, gabions have played an important role in water-quality issues with the North Concho River. The north fork of the Concho River winds through San Angelo, weaving through residential, recreational, industrial, and commercial land-use areas. The watershed for the North Concho River consists of one-third of the city limit area, which encompasses 22,000 ac.
For many years, the water quality had been poor. Fish kills were common. Studies confirmed that the primary cause was urban runoff and nonpoint-source water pollution.
Of special concern was a 6.75-mi. reach of the stream. After the O.C. Fisher Reservoir was completed in 1952, flow was seriously reduced in the stream, and scouring flood flows were completely eliminated. Low water levels in the O.C. Fisher Reservoir prevented significant downstream releases. What stream flows existed consisted of minor reservoir seepage, spring flow, and stormwater runoff.
An interior storm drain system is built around the river and many natural contributing surface drainage features. Constructed storm sewers in the downtown area comprise only a minor portion of the system. Field investigations showed that 70% of the stormwater flows entering the North Concho River originate within seven major subwatersheds.
Based on the impacts of the nonpoint-source pollution and on assessments identifying the North Concho River as one of the highest-priority water-quality problems in the Colorado River basin, work began in 1995 on a project funded from the Clean Water Act 319(h) grant program, administered by the United States Environmental Protection Agency through the Texas Natural Resources Conservation Commission. The project consisted of three phases:
- A master plan identifying and prioritizing projects within the watershed to include structural and nonstructural best management practices (BMPs)
- A public awareness program to educate the community
- Construction of a structural BMP as a demonstration project
Civic League Park was chosen as the site for construction of the city’s first BMP technology. A nearby pond, one of the most polluted water bodies in the state, had been too small in volume to accommodate the sediment loading from the watershed. The pond’s treatment effectiveness was severely diminished after a large volume of sludge had settled in it. When scouring flows were encountered, septic sludge would be discharged into the river, imposing a considerable oxygen demand.
In mid-1997, a complete redesign of the proposed BMP was drawn up, calling for the construction of a gabion retention structure along the riverbank and between the river and the existing wet pond.
A cut was made, and a concrete drainage culvert was installed through the embankment between the two structures to facilitate the diversion of stormwater into a new dry pond. A gabion structure was constructed to bisect the existing wet pond and divert a large portion of the storm flows into the new structure while allowing the normal water levels to be maintained within the wet pond. The stone fill came from local sources. The construction was completed by late August 1998.
Fred Teagarden of the Upper Colorado River Authority was with SK Engineering at the time the project was executed. He notes that while there are many alternatives for building structures to control stormwater – including wet ponds and vegetation filters – the gabions work well in dealing with high flow rates for large volumes. Using gabions was the only method that made sense in addressing the situation, Teagarden says.
It was acknowledged that construction of the BMP would not have been possible without the use of the gabion structure – constructed from Modular Gabion Systems (a division of C.E. Shepherd Company) products – for the primary retention wall next to the river. There was not enough space to allow for earthen embankments and associated slope distances. The top of the dike elevation of nearly 6 ft. above natural ground would have required 18 ft. of linear tow slope at 3:1 dike slopes. The only other alternative would have been the construction of concrete walls, an option deemed too costly.
While the gabion structure was cost-effective and blended attractively into the environment, the Upper Colorado River Authority believed its only disadvantage was the initial hydraulic characteristic that allowed stormwater to rapidly seep into the river before sufficient retention time for settling of solids. The initial porosity of the structure was close to 30%, and project designers had theorized the porosity would diminish with time as storm events occurred and debris and solids were captured within the gabion media.
“We did a lot of research in trying to come up with some technologies that we felt would work,” Teagarden says. “I realize this is probably not the norm for these kind of structures, but [first] we had a need for low cost, and [gabions] are much more economical than reinforced concrete.
“Second, being porous in nature, they’ll provide for a dry pond. Third, we theorized that through time if you filter water through these things, you’re going to build up what we call a thatch.”
A thatch is the result of small particles that get trapped in the stone; after a few storm events, they condition the structure so it becomes more retentive and water doesn’t pass through it nearly as fast, Teagarden explains.
“At the same time, it’s porous enough that we end up with a dry pond when the storm is over,” he adds. “We actually used the gabion construction technique in the construction of filter dams to provide a dry detention pond for stormwater control. These things will provide some detention time for settling. It also filters the floatables and provides time for settling for biological matter in suspended solids.”
Since completion of the gabion retention structure at Civic League Park and a retention structure at Santa Rita Park on another tributary to the river, the amounts of materials reaching the river have been noticeably reduced.
“One of the requirements of those [319(h)] grants is to monitor the efficiency of these structures,” says Teagarden. “We have a quality-assurance project plan in place to do that, and we’ve learned to wait until this structure conditions itself for maximum efficiency. So with the first storm, we don’t run down there and start sampling because we know that it’s not going to be nearly as efficient as it will be a little later.”
Yet results so far have been pleasing, Teagarden says. Large amounts of material that would have normally gone into the river are being captured.
“These things are very, very cost-effective,” he says, adding that he anticipates more being built under similar circumstances. “The river we’ve targeted to keep this stuff out of historically has experienced fish kill about the middle of June. This year, we had the thunderstorms, but we didn’t have the fish kill, so we’re pretty encouraged.”
At the site of another river in the US – the Santa Fe in New Mexico – a late winter storm in 2002 caused extensive erosion. The erosion exposed a 24-in.-diameter sewer main that runs east and west at an easement in a bend of the Santa Fe River and services the sewer needs of the city of Santa Fe.
“The rain caused a bank to wash out where it turned on an angle, and it exposed about half of the sewer pipe,” says Don Eker of Eker Brothers, a Santa Fe company that was chosen by low bid to provide the emergency solution through gabions. “It was in a bank that is 20 to 30 feet tall above the river. It exposed that sewer pipe about 12 feet from the bottom of the river. They were very concerned that if there was another quick rain, it could wash out the soil underneath the pipe completely, and [the pipe] would break, and all of the sewage would spill into the Santa Fe River bed.”
The City of Santa Fe Wastewater Management Division required a solution that could retain the riverbank, protect the pipe work, and provide long-term erosion protection for the future. The work had to be done rapidly, as the approaching rain season posed the possibility of additional problems.
A mechanically stabilized earth (MSE) structure with a gabion face proved the most effective choice. The gabions would provide long-term, high-capacity erosion control, and the geogrid would offer reinforcement to the reinstated embankment. Eker Brothers, which used Maccaferri products, was given 30 days to do the project.
“The first thing we did is build an earth dam to protect the sewer pipe,” Eker says. “We worked behind the dirt dam in case there was a rain. There was a small storm. The dam stayed in place, and we kept working.”
The MSE structure was 15 ft. high, of which 6 ft. was beneath the channel bed to allow for future scour. The wall was keyed into the existing slope at both the downstream and upstream ends to provide a smooth transition between the protected and unprotected riverbanks, limiting erosion.
Eker Brothers embedded the first layer of gabions at the level of the riverbed. There were four layers of gabions installed above that until the top of the top gabion was equal to the top of the sewer pipe. Polyester geogrids provided reinforcement to the reinstated embankment. Backfill was placed and compacted in 8-in. lifts.
“The weight of the dirt we put on each one of those layers held the fabric in place, and [the fabric] was tied to the gabions, so it held the gabions in place also,” Eker explains. “But it’s porous so that water can run through it and it still won’t destabilize the bank.” Eker’s company also drove down piles made of angle iron to hold the gabion baskets in place at the bottom of the river.
The Maccaferri gabion double-twist hexagonal mesh was PVC-coated to better withstand the water environment. The double-twist mesh is considered to be quite robust and can accommodate large differential settlement without rupturing or “unzipping.” The connection strength (or weave) between adjacent wires in the mesh is at least as strong as the wire itself. Stresses in the mesh can be dissipated in two dimensions throughout the mesh and can continue to be dissipated even if wires are cut or damaged, a vital characteristic in critical infrastructure applications where there is potential for differential settlement.
The city had mandated that the rock be blasted so that every surface would be rough. The rock came from a nearby plant and is greenish in color, making it aesthetically pleasing.
“When it’s all blasted and crushed, it interlaces together better and doesn’t move around inside the basket when water hits it,” Eker says.
Eker, who has utilized gabions quite a bit in the last 12 years, reports that the project went smoothly. “We wrapped it all up in 30 days. There were no hitches.”