Strong Streambanks: Beauty and Function

Dec. 12, 2013

Streambank stabilization projects are a large part of business for erosion control design companies and contractors. Municipalities around the country are undertaking such projects for a variety of reasons:

  • Replacement of old measures, such as concrete linings
  • Heavy rainfall or wave action causing undercutting
  • Increased runoff because of development upstream or conditions of damaged soil and destroyed vegetation after wildfire
  • Tighter restrictions for sediment loading of creeks and rivers
  • Desire for “greener” or more aesthetic solutions for streambank stabilization

Designers and construction firms must take into account what factors are the most important to their clients. Is the highest priority to control severe flooding and erosion? Do the stakeholders desire a project that will blend in and look natural while providing stable banks? Is the desire for a combination of both? Setting, either urban or a mixed use area, must also be considered. As always, erosion control specialists must think about the impact on sensitive water bodies and increasingly stringent regulations for water quality.

For many projects, a combination of hard and soft armor techniques works best. Making the right choices depends on knowing how the products can work most efficiently together, what is most cost effective, and what will lead to the desired result. Here are a few projects to stir your thinking about the possibilities.

Black Warrior River
The Black Warrior River in Tuscaloosa, AL, is a navigable river that experienced erosion from wave action caused by passing vessels. High river stages after heavy rainfall upstream created sediment runoff and undercutting of the bank. The US Army Corps of Engineers needed a plan to provide bank stabilization under these difficult conditions. Their design called for a gabion mattress slope revetment.

The contractor for the project, Misener Marine Construction Inc. of Tampa, FL, chose gabion mattress manufactured by Modular Gabion Systems, a division of C.E. Shepherd Co., of Houston, TX.

Circular sheet piling cells were built a few feet from the riverbank to provide support for a new ships’ mooring facility. The riverbank was graded to a 2:1 ratio and then lined with a geotextile filter fabric. This permeable material allows water to drain through the fabric and into the ground evenly, thus preventing hydrostatic pressure buildup and keeping the fine soil particles in place.

Gabion mattress units, supplied by Modular Gabion Systems, were installed over the geotextile fabric. The gabion mattress was made of galvanized, welded wire mesh and coated with PVC. The PVC coating is fuse bonded, leading to an even coat that protects the underlying wire from corrosion.

The gabion mattresses were placed on the geotextile fabric and connected by helical spiral binders into one monolithic structure. Each wire mesh section had a 1.5-inch by 3-inch opening. In this project, the gabion mattress rock fill ranged from 3 to 6 inches in diameter.

After the rock was installed, a continuous 6-foot-wide wire mesh lid was applied with the same spiral binders. The wire mesh lid is available in 6-foot-wide, 150-foot-long rolls. The gabion mattresses used on the Black Warrior River project were 9 inches thick, providing excellent stability for the streambank. To ensure protection from scour due to wave action and rapid flow, the lower part of each mattress was placed horizontally along the toe of the slope. In total, 9,000 square yards were covered by the gabion mattresses.

Ronald Schumaker, project engineer with Misener Marine Construction, says, “There was over a thousand lineal feet of riverbank exposed to runoff. Because of the ability to quickly and efficiently install the mattress system, we were able to get the bank protection in before any further erosion occurred. This was critical in maintaining water turbidity levels in the Black Warrior River and preventing rework on grading the riverbank.”

The US Army Corps of Engineers had estimated that the project would require three months to complete. However, the ease of installation of the Modular Gabion mattresses allowed crews to finish the work in just seven weeks.

Washington Washout
Streambank erosion not only causes damage to streams and their habitat; sometimes it can take out a house. A single-family residence in Seattle, WA, was threatened by an eroding stream. Extreme rainfall caused flooding along Thornton Creek, which eroded the soils under a house owned by the Kessler family. Several support piers were exposed, and a space of about 10 to 15 feet opened underneath the house. Immediate mitigation was needed to prevent collapse of the home.

The solution also had to be environmentally sound because Thornton Creek is a fish-bearing stream. The repair and restoration plan had to be approved by both the city of Seattle and the state Department of Fish and Wildlife.

Another challenge was that the home occupied most of the lot, leaving little room for erosion control and support measures. The streambank is about 10 feet high and has mature vegetation along the banks that had to be preserved as much as possible.

The entrance to the site included steep slopes with no access for equipment, so the installation had to be accomplished by hand. This limitation meant that the stabilization using large rock was ruled out. Regulations and costs also ruled out poured walls or bulkheads. The irregular shape of the exposed hole meant a flexible, strong solution was needed.

The choice was A-Jacks concrete armor units from Contech Engineered Solutions. A-Jacks are designed to interlock in a flexible structure that can be fitted to the shape needed. Five hundred 24-inch A-Jacks were installed and connected to provide the necessary strength and stability to support the house. The 24-inch units weigh 78 pounds apiece, so they can be placed without the use of large equipment. They were delivered stacked on pallets that could fit in the residential driveway and then hand-carried and placed. The complete installation took seven days. The Kessler home has the stability to withstand future flooding of the stream.

Post-Fire Flooding
In 2012, Whitewater Creek in southwestern New Mexico faced severe flooding and erosion. The area near the town of Glenwood had experienced the largest wildland fire in New Mexico history in the spring of that year. The area is arid, but the annual rainfall doesn’t come as soft, gentle rains over a period of days; most often, the rains come in heavy downpours during the summer monsoon season. When the yearly rains arrived, the creek would face heavy runoff and debris flows, which could cause severe channel erosion and flooding. The New Mexico Department of Transportation proposed a plan to install mitigation measures before the heavy rains could fall.

The production chosen was Contech’s ArmorFlex articulated concrete block (ACB). The ACB are laced with cables to form a mattress that can be quickly installed in a creek bed. Since speed was critical, the mattress format was deemed the most efficient application.

Local contractors installed 40,000 square feet of ACB mattresses in one month and beat the beginning of the rainy season. The strength of the ArmorFlex stabilized the creek bed. The mattresses have an open-cell design that allows vegetation to grow and still provides long-lasting erosion control.

William Hutchinson of the New Mexico DOT Environmental Design Division notes, “Contech was chosen for the design-appropriateness of its armoring solution and ability to rapidly deliver.” The performance of the mitigation measures has been outstanding during the storms of the monsoon season.

Development in Chicago
The Des Plaines River watershed drains a significant part of the western edge of Chicago. Increased land development over the past decades has caused a dramatic increase in water flow and velocity through the watershed. One of the river’s tributaries, Silver Creek, was the cause of great concern for the Village of Melrose Park.

The creek runs past industrial parks, shopping malls and stores, residences, and roadways. It has suffered intense erosion, including channel scour and loss of more than 1,200 feet of streambank. The village contracted with Ted Gray of Living Water Consultants Inc. in Oakbrook Terrace, IL, for an environmental design for a streambank in an urban setting. The project contractor was ENCAP Inc. of Sycamore, IL.

The project involved several steps and products from a variety of vendors, including applications of rock at the toe of the bank, bank slope grading, and use of geogrid, fiber rolls, rock riffles, and erosion control blankets.

There were several challenges for those working on the project. The Des Plaines River, only 2,500 feet downstream, experiences severe flooding every spring, which backs up into Silver Creek, causing massive sliding because of saturated soils. In addition, the dense urban environment allowed only limited access to the work area. Steep slopes made access difficult by foot and almost impassable for machinery. Also, permits required that no equipment be present in flowing water.

The growth of non-native trees along the banks had crowded out native vegetation. The first part of the project involved cutting and removing the trees and brush. Heavy-duty timber mats were used to keep the equipment out of the flowing water as required.

Rock riffles made of locally available stone were installed to slow the velocity of the runoff.

A bridge crossing was a main focus of the project. The outer curved bank had eroded and was nearly 6 feet in height, and further erosion was occurring with each storm. The bridge footing was threatened by undercutting. A rock toe was installed and, above that, courses of uniaxial geogrid reinforcement and turf reinforcement matting were put in place. The top of the bank and the courses were seeded, and a coir blanket was placed over the seeds. The entire area was seeded with native species. The length of this curve of the creek was about 200 feet.

This section quickly received a heavy test. Just 24 hours after construction, a severe thunderstorm deluged the area with nearly 6 inches of rain in a few hours. Flood depths rose to nearly 8 feet above base flow, including the work area. The project withstood the flooding, even without the establishment of the vegetation.

Increased Drainage Problems
The city of Newport Beach, CA, had problems with a drainage system known as Lower Buck Gully. The gully is a natural drainage passage from the foothills to the ocean, but more development in the area had increased the runoff in the gully, leading to destabilization. Exotic plants were also an issue in the gully. A system was needed to slow the water, reduce erosion, and provide an environment for the establishment of native plants.

A gabion system supplied by Maccaferri was chosen. The non-native plants were removed, and steps of gabions were constructed in the bed of the gully. The woven Maccaferri gabions are galvanized PVC-coated wire mesh. They are shipped in a flat format and are assembled by erecting the sides, ends, and centerpieces. Once the baskets are assembled, they are connected with high-tensile fasteners to form the shape required. Rock fill is loaded into the baskets and the tops are installed. For this project, the rocks were from 4 to 8 inches in diameter. Layers can be placed on top of a foundation layer to make the structure as high as needed.

The Maccaferri gabions stabilize the bed of the gully and slow water velocity. As sediment collects over the stone baskets, it provides a growing area for plants. Native plants are springing up in place of the exotics that were removed. Coir mat and straw wattles were used to finish the landscaping around the gully.

This project was finished on time and on budget and has enhanced the stability and the beauty of Lower Buck Gully.

Gabion Wall
Flooding of the Jacaguas River in Villalba, Puerto Rico, was endangering the city’s recreational facilities, some homes, and a street. The river has a steady flow and also suffers frequent flooding.

A plan was laid out by JLI Design Associates Inc., calling for three walls made from Maccaferri galvanized gabions. A local contractor, Santiago Electric, constructed the walls with the gabion material, filled them with local stone, and linked them into layered walls. The walls have a total height of 21 feet and together cover 250 linear feet. MacTex nonwoven geotextile fabric was placed between the layers of gabions. The first two courses of gabions were buried to provide protection against scouring. In contrast to the usual island gray/blue stone, the contractor used a white/rose colored stone from a nearby quarry.

The walls were barely completed when a week of continuous rainfall caused the river to rise within 7 feet of the top of the walls. The riverbank protected by the gabion walls suffered no damage. The stability of the walls has endured as Puerto Rico has experienced a record-breaking year of rainfall. The municipality of Villalba is planning to extend the walls and built other similar walls along the river to prevent erosion.

Softer Solution
Streambank stabilization does not always involve hard-armor solutions. Depending on the water velocity, slope of the channel, and other hydrologic factors, a softer approach might be chosen.

The town of Clyde, NC, received heavy amounts of rain from two hurricanes in 2004. By 2008, there was growing concern about the continuing erosion of a small tributary upstream of the Pigeon River. The erosion that was accelerated by the hurricanes was threatening the town’s fire station, a local bank, and a church. The streambank slopes were steeper than 1:1 and the erosion had caused vertical banks heights up to 11 feet. The sediment load from the deteriorating creek was threatening the river downstream.

Town officials requested a low-impact solution for this stream. They contracted with McGill Associates P.A. in Asheville, NC. Products chosen came from the line of erosion control applications manufactured by Tensar.

The design called for slightly relocating the stream channel, decreasing the slopes, and constructing a green retaining wall. After the grading and leveling down to a 3:1 slope was accomplished, EroNet C125 photodegradable double-net coir erosion control blanket was installed with biodegradable eco-stakes. Native seed was planted, and the EC blanket will protect the slopes for about 36 months, time for vegetation to become fully established. The blankets and the stakes will filter runoff and stabilized the slope until the native plants take over and the blankets and stakes eventually biodegrade. Coir wattles were placed out the outer corners to provide extra protection against water flow and to collect sediment; they are also biodegradable.

A mechanically stabilized earth wall was constructed next to the Clyde Fire Station, an area that experienced the most severe erosion. As a final touch, a hydraulic mulch mixture containing straw fibers, reclaimed cotton material, and tackifiers was applied to the entire area. A combination of seven native seeds as specified by the North Carolina Department of Environment and Natural Resources, Division of Water Quality, was also applied.

The native grasses began sprouting only two weeks after application; after two months, a healthy stand of the natives covered the restored streambank. The erosion control blankets and the vegetated wall have blended into the environment to provide a green, natural-looking solution to the erosion problem.

California Slope Repair
The Los Angeles Department of Water and Power (LADWP) had an ongoing erosion problem at a power plant, which was costing the department time and money. A road providing access to the power plant ran over an earthen dike alongside a canal that supplied the plant with water. Both sides of the road drop off to lower elevations. The canal is adjacent to the San Gabriel River, and the connection causes effects from stormwater runoff and from the daily tidal changes that influence the river. The channel had some rock lining that had deteriorated over the years, so the roadway was constantly being undercut and destabilized by the water of the canal. LADWP was faced with continually bringing in material to stabilize the roadway and replenish the slopes at a high cost for materials, labor, and time. The decision was to pursue a permanent solution.

John S. Meek Construction of Gardena, CA, was the contractor for the job; the company used ShoreGuard525 synthetic sheet pile from Crane Materials International (CMI). The rigid vinyl sections have a box profile that is 9 inches deep and 24 inches wide. Impact strength is measured at 13,750 inch-pounds per square inch. About 1,500 sheets of ShoreGuard 525 with an average length of 18 feet were connected for the sheet piling wall. The total made up 3,100 wall feet. Installation consisted of loading the sheet pile into the PileClaw Mandrel, part of CMI’s line of PileClaw installation equipment, and driving it into the ground with an HPSI vibratory hammer. Because of the narrow access road, a small crawler crane was used to set the sheet and drive them to grade.

Jim Jilk, general manager for John S. Meek Construction, says there were some early challenges with the installation of the sheet piling. The mandrel achieved the 18-foot depth that was needed and according to Jilk it “worked great;” however, when the mandrel was pulled out of the ground, the sheets would come, too. The solution was to strap the sheets down before removing the mandrel.

Once the sheet piling was in place, caps were placed to finish the installation. The sheet piling will stay exposed about 3 feet above the road level to provide more containment of the soil of the road.

Overall, the ShoreGuard 525 sheet pile system was very effective and is aesthetically pleasing, Jilk says, and it was a relatively inexpensive solution. The sheet piling adds safety because of its rigidity and will block any future washouts to support the roadway. In addition to the initial cost savings and low long-term maintenance costs, the vinyl sheet piling is manufactured from 90% or more recycled post-industrial PVC. The fact that LADWP will no longer have to constantly bring in fill material will lead to even great cost savings.

If you face a project of stream stabilization, you can find a number of products that will fit the conditions. You might need to consider a hard-armor solution to provide exceptional strength in the face of rapid water flows; a softer, green option; or a combination of the two. 

About the Author

Roberta Baxter

Author Roberta Baxter specializes in science and technology topics.