As streambanks erode over time, erosion control specialists are called in to mitigate the problem with approaches that may include hard armor or a softer approach, such as bioengineering. The long-term goal—as well as costs, function, permeability, flexibility, and aesthetics—often dictate the approach.
Protecting Coral in Guánica Bay
One project in Puerto Rico called for soft techniques. In Guánica, the Maccaferri company has provided supplies and expertise on a riverbank protection project that, if proven successful, will be used as a template for similar projects throughout the United States.
The Río Loco Watershed Project is a multiagency endeavor that supports a portion of the Guánica Bay/Rio Loco Watershed as part of the efforts of the US Coral Reef Task Force.
Agencies involved include the National Oceanic and Atmospheric Administration, the Puerto Rico Department of Natural and Environmental Resources, the Puerto Rico Department of Agriculture, the Suroeste Soil and Water Conservation District (SWCD), the US Fish and Wildlife Service, the Center for Watershed Protection, the US Department of Agriculture (USDA), and the Natural Resources Conservation Service (NRCS).
Coral reefs have been impaired by sediment coming from the Loco River Basin. Since the beginning of the project, NRCS has committed to protect the coral reefs in the Guánica Bay, designing conservation practices to be applied in the agricultural lands of the Guánica Valley and in three segments of the Loco River to prevent erosion, according to Edwin Almodóvar, director of USDA NRCS Caribbean.
One of the three Las Latas segments involves a streambank stabilization project on the Loco River, Las Latas Sector, in Guánica.
The goal of the project is to stop erosion at the site and reduce sediment flowing into the Guánica Bay. The design includes structural protection of the riverbank with gabions and stream barbs. Bioengineering practices were implemented behind the gabions wall as part of the treatment.
The focus of the project is protecting a slope along the river that is affected by water volumes during heavy rain events.
“The river has a dam, and when the dam gets overflowed, which happens most likely three times a year, the community gets flooded,” notes Alberto Blasini, Maccaferri’s territory manager for the West Indies, Caribbean, Puerto Rico, and US Virgin Islands. “This wall will protect two things: the community and a USDA vegetable farm.”
Crews installed a gabion wall made from Green Gabions vegetated with the plants of the area to protect the area’s fauna, says Blasini.
The use of Green Gabions on the top row of the wall was a first for NRCS in Puerto Rico. The approach was designed to help secure the structural treatments and also provide for enhanced aesthetics as well as some habitat benefits, notes Almodóvar.
Because of the importance of this project for the agencies and the nearby community, Maccaferri provided training on how to install both regular gabions and Green Gabions to the contractor personnel, the Suroeste SWCD personnel, and NRCS personnel in the area.
More than 300 units of Maccaferri’s Green Gabions were combined with more than 500 PVC-coated gabions, five 500-square-yard rolls of nonwoven geotextiles, 80 rolls of coir erosion control blankets, more than 3,000 MacTies, 2,500 cubic meters of solid rock, and Reno Mattresses, notes Blasini. The total cost of materials was $108,000. They were sourced from Maccaferri’s Puerto Rico operations.
Maccaferri Green Gabions are modular gabion units used for streambank stabilization, restoration, and erosion protection. They are designed for use with soil bioengineering techniques such as live staking, brush layering, and rooted plants.
The Green Gabion is a basket manufactured from heavily galvanized and polymer-coated double-twisted hexagonal woven steel wire mesh lined with a 100% coir blanket. The unit is filled in place with gabion stone, and the voids between the stones are filled with quality topsoil before the unit is closed. The topsoil allows plant establishment, and the stone fill provides a tough skeleton to the system.
The project began in August 2015. USDA engineers have visited the site to view the installation process and the results on the riverbank, says Blasini.
The USDA favored the Green Gabions. “When they were looking for solutions, they didn’t want to work with concrete, as the appearance of the area would be severely affected,” says Blasini.
One challenge associated with the project was meeting government requirements for local employees to do the work, says Blasini, who also performed inspections. The project was constructed by V. Berrios Construction.
“They had no experience with gabion installation,” he says of the local crews. “We had to give them trainings and seminars on the job site to have them technically knowledgeable to install the gabions. We went there over three times. Finally, we had what we and the federal government were looking for.”
The project was the highlight of a US Coral Reef Task Force meeting in October 2015, which included a field trip to the sites in Guánica Bay, including Las Latas bioengineering site.
Restoring a Shoreline
A vegetated approach also was the preferred erosion control method on a project for the Indianapolis (IN) Department of Public Works. The department sought a solution for a lake edge restoration where a culvert had washed out, wanting to rebuild the area back to grade and install native plants.
The Indiana DPW chose Hoosier Aquatic Management to install vegetated soil tubes manufactured by its sister company, Living Log Aquatic Services. The companies focus on bioengineering erosion control techniques.
“We used a geotextile tube onto which we put two pieces of webbing on the back and grommets to secure that material back into the bank,” says Matt Kerkhof, company owner. “Then we filled it with the actual desired material.”
The company chose to use sand-based material at the base close to the water’s edge. Moving up toward the top of the slope, crews used a topsoil, layering in vegetation between each of the 10 courses. Some 60 soil tubes were used on a slope extending 8 feet in height. The project took one week to complete during summer 2014.
“We had to make an imprint in a geotextile tube that will allow us to put soil in that area as well as the plant plug,” says Kerkhof. “We used wetland species at the very bottom and then worked our way up to drier conditions as we moved up out of the channel.”
The driving factor behind using the sand-based material at the bottom was to avoid a heavy washout, says Kerkhof. “Sand will hold much better in those conditions, but as we come up out of the channel, we want to introduce organic material so that it helps the native plants survive.”
In its bioengineering approach, Hoosier Aquatic Management utilizes graminoids—the rushes and sedges that perform like a “workhorse that helps stabilize everything”—and forbs, the flowering plants, says Kerkhof.
“One of the flowering plants that we default to every time is swan milkweed—Asclepias physocarpa—which is a host plant for the monarch butterfly, so not only does it give you good color throughout the summer, but it also has benefits from the wildlife standpoint,” he adds.
Other plants used include black-eyed Susan (Rudbeckia hirta) and purple coneflower (Echinacea purpurea) “because people are very familiar with them and they lend themselves well to the native plantings,” says Kerkhof.
In contrast to a hard-surfaced erosion control method, bioengineering infiltrates stormwater through the roots as it transitions down the slope.
“The soil tubes will retain the soil and allow the plants to grow and still act as a filter,” points out Kerkhof. “It does not let hydrostatic pressure build up behind it because the water flows freely through it and it gets filtered at the same time.”
The approach does not retain water, but in the event that upstream construction creates more hard surfaces with additional runoff, the system grows and strengthens over time in such a way to handle the changes, in contrast to a riprap material that tends to fall apart and weaken as time goes on, says Kerkhof.
Bioengineering techniques are looked upon favorably by the US Army Corps of Engineers, the Indiana Department of Environmental Management, and the Indiana Department of Natural Resources, points out Kerkhof.
“We’re installing the native plant species with the mindset that infiltration and scrubbing is going on and now you’re creating a filter area rather than just a hard surface area that’s pushing the water off and creating problems for other people,” he says.
“We’re able to anchor the soil tubes back into the shoreline with earth anchors and hardwood stakes, which is really nice,” adds Kerkhof. “We will build that system up so the front of the system is very consistent and aesthetically pleasing as well as very functional.”
Traditional hard-armor approaches have their place as well. In Florence, AL, a drainage ditch that runs through a business district was not ample enough to contain the additional runoff created by the area’s development. Consequently, there was significant bank erosion that needed to be remedied, notes George Ragazzo, general manager and erosion and flood control specialist for Modular Gabion Systems, a division of C.E. Shepherd Co.
Upon review of the situation, civil engineering and land surveyor firm White, Lynn, Collins and Associates in Florence chose gabions for their economy, performance, flexibility, permeability, and aesthetics.
All gabion material was supplied by Modular Gabion Systems in Houston, TX, in roll-stock form to enable the contractor to field-form continuous gabions and gabion mattresses up to 300 feet long without joints, directly from job-specific engineered rolls of mesh, thus reducing labor and installation time, notes Ragazzo.
All gabions and gabion mattresses were fabricated from welded wire mesh meeting ASTM A974 standards. The wire was zinc-coated before welding. After welding, the wire mesh was primed and PVC-powder coated. The PVC coating was fuse-bonded onto the wire mesh to provide optimal corrosion resistance.
The first phase of the flood control gabion channel-drainage ditch improvement project called for lining the ditch with gabions. Shotcrete of America was contracted to line some 850 feet of the ditch with 9-inch-high gabion mattresses on the channel’s floor for stabilization and erosion control. Mattresses, shorter in height than the lateral dimension of a gabion, are generally used for channel linings. Gabion baskets were then installed along the channel’s sides to retain the runoff and protect the banks.
Paul Haralson, president of Shotcrete of America, notes this was the first project of this magnitude
for which his company installed gabions. His company used nearly 1,700 cubic yards of gabion material and 2,100 tons of stone.
A few challenges came up during installation. One was that the installation was quite labor intensive, says Haralson.
“A lot of times it seems like you’re not getting anywhere, but after you get going pretty good, it’s repetitious,” he adds. “You’ve got to have the same people doing the same thing to have any production at all.”
Weather during installation is another challenge. “You need to look ahead and get the mattresses in there before it rains, because if you don’t, it’s just sitting on soggy ground. Once you get the mattresses in, you can come back and build the walls. It was a challenge, but I always like a good challenge.”
Haralson—who credits Ragazzo for being onsite to offer installation pointers—says he is pleased with the results and would do another project with these materials. He notes that since the project bordered state-owned property at the low end, some state officials were on hand to see the installation. He says wouldn’t be surprised if the approach starts getting more widespread use elsewhere in the state.
In Redwood City, CA, at The Pauls Corp.’s Blu Harbor condominium development, B & B Hughes Construction, was hired for a stabilization and seawall project.
The development was being constructed on a peninsula with poor soil conditions, notes Mike Threde, sales engineer for Crane Materials International (CMI). The site is on a harbor and is slated to have 400 residential condo units and parking.
“As part of the soil enhancement techniques, the engineers on the project suggested a deep soil mixing (DSM) wall to stabilize soils,” he says. “This DSM wall was constructed along the perimeter of the project that fronts the water. As protection from scour erosion, CMI’s vinyl sheets were driven into the ground on the water side of the DSM wall as protection.”
CMI’s SG-625 is a standard cutoff wall vinyl sheet, boxed-shaped and 30 inches wide, designed to require fewer joints and drives for the contractor. The vinyl sheets do not rust or corrode and are designed for wet environments.
“The vinyl sheet piles offered the owner of the project a cost-effective and longer-lasting solution over steel sheet piles,” notes Threde, pointing out that steel sheets would have been subject to immediate rusting at the waterfront property.
During August and September 2015, some 1,500 of CMI’s SG-625 sheets, in lengths of 15 and 20 feet, were driven on the water side of the DSM wall. The wall is about 3,500 feet long.
Installation was performed by Hughes Construction. The sheets were driven into the ground using an ABI Mobilram rig and CMI’s PileClaw installation equipment. The five-minute cycle entailed picking a sheet off of the ground, loading it into the PileClaw, and driving the sheet to the desired depth.
“At a five-minute cycle, the contractor was able to install roughly 12 sheets per hour, equating to 30 linear feet using a 30-inch wide sheet,” says Threde.
The DSM wall that the sheets are protecting is completely buried. The vinyl sheets extend between 3–5 feet above ground, with the extra height acting as a barrier against high tides, says Threde. A 12-foot-wide sidewalk and incorporated concrete cap will cover all but 2 feet of the sheets.