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Stabilizing Oyster Lake’s outfall

Case Studies, Erosion Control | February 1, 2021 | By:

The completed outfall with sea oats beginning to grow

Preserving a unique ecosystem with an environmentally friendly solution

By Drew Loizeaux

Coastal dune lakes are freshwater lakes located within 2 miles (3.2 km) of an ocean coastline. These rare ecosystems are only found in a handful of locations around the globe, including Madagascar, New Zealand, Australia and Walton County, Fla. Walton County is home to 15 coastal dune lakes, some of which are only a short distance from the Gulf of Mexico. These unique waterways create transitory interchanges between a natural stormwater lake and the gulf. When a coastal dune lake reaches a high water level, it will break through the dune system, forming a channel between the lake and the gulf. The channel, or outfall, is critical for regulating water levels and mitigating flooding

Oyster Lake is one of the coastal dune lakes located in Walton County. Major storms caused Oyster Lake’s outfall to become blocked, and urban growth along Highway 30A was causing strain on its overall health. If the outfall remained blocked, it would eventually release thousands of gallons of water, causing flooding to nearby vacation homes. 

The county was able to restore the outfall, but it and Walton County residents wanted to reinforce the outfall to protect from future blockages and flooding. In addition to stabilizing the outfall’s channel, another goal of the project was preserving the natural beauty of Oyster Lake. A solution that promotes vegetation was needed, so hard armor reinforcements like rock riprap and concrete block were not options. 

ARMORMAX Engineered Earth Armoring Solution was selected to reinforce the channel sides of the outfall. This system was chosen because it provides slope stabilization and erosion control while promoting vegetation. Another important concern for the project was the impact the reinforcement solution would have on the ecosystem, which is home to several endangered species including the Choctawhatchee beach mouse, snowy plover, and loggerhead and green sea turtles.

The engineered earth armoring solution is composed of PYRAMAT high-performance turf reinforcement mats (HPTRMs) and engineered earth anchors that work together to lock soil in place and protect against hydraulic stresses. The HPTRM’s carbon footprint is 5.6 pounds (2.5 kg) of CO2e per 11 square feet (1 m2) of material. This is about the same impact as driving for 7 miles (11 km). Comparatively, concrete-based alternatives for erosion control are up to 10 times or more higher, ranging from 44 to 77 pounds (20 to 35 kg) of CO2e per 11 square feet (1 m2). 

The HPTRM is an engineered plastic made from woven polypropylene. The growing concern over plastics and their impact on the environment, specifically the oceans, was a consideration. It is important to recognize that there are several differences between engineered plastics and single-use plastics, such as water bottles. Engineered plastics are stabilized to prevent degradation and designed to perform as long-term or even permanent solutions, whereas single-use plastics are not stabilized, are discarded quickly and begin to break down immediately. Additionally, single-use plastics are made from various unstable polymers that break down into microplastics and leach harmful chemicals. The HPTRM’s polypropylene has been found to contain no leachable components (The Blastic Project 2016–2018).

Of equal importance, the environmental benefits of the engineered earth armoring solution extend into the product’s useful life. Once installed, it helps to decrease sedimentation and pollutants, and encourages infiltration of water back into the groundwater table. These are two reasons why the U.S. Environmental Protection Agency (EPA) has identified systems that utilize HPTRMs as a best management practice (BMP) for improving water quality. 

The engineered earth armoring solution also features a patented trilobal design that locks seeds and soil in place to promote rapid root development for long-term vegetation. This vegetated outcome supports a living shoreline, whereas hard armoring can decrease streamside vegetation and adversely affect fish populations. A vegetated shoreline helps to maintain cooler water temperatures, which is healthier for aquatic life habitats, than traditional hard armoring. During installation, more than 2,000 sea oats were planted within the system. The solution’s patented X3 fiber technology helps hold roots in place, so they are not washed away during rain events. The vegetation establishment of the armoring solution is critical to long-term performance, providing a matrix of woven HPTRM trilobal fibers, roots, stems and soil to serve as a securing mechanism to the slope face. 

More than 2,000 square yards (1,672 m2) of engineered earth armoring solution with 6-foot (1.8-m) B2 anchors were installed along the channel to reinforce the outfall. The installation frequency or spacing of anchors is a result of the slope stability analysis. Anchors are typically installed in a staggered pattern, creating a checkerboard effect. HPTRM roll edge overlaps should be a minimum of 3 inches (8 cm), secured with anchors according to the prescribed pattern accompanied by securing pins on 12-inch (30-cm) centers.

After installation, Hurricane Michael, a category 4 hurricane, made landfall 60 miles (97 km) east of Oyster Lake. The effects at this project location were winds up to 80 mph (129 kph), storm surge, and significant rainfall and flooding. The vegetated slopes of the outfall withstood the extreme conditions, protecting several beachfront homes, a Highway 30A bridge and a portion of 30A’s 19-mile (31-km) bike path that runs along the famous Emerald Coast beach communities from Dune Allen to Seaside. 

References

The Blastic Project. (2016–2018). www.blastic.eu/knowledge-bank/impacts/toxicity-plastics. Accessed 11/30/20

Project Highlights

Stabilizing Oyster Lake’s outfall

OWNER
Walton County, Fla.

LOCATION
Santa Rosa Beach, Fla. 

CONTRACTOR
RBM Contracting

DESIGN ENGINEER
Dewberry

GEOSYNTHETICS PRODUCT
ARMORMAX 

GEOSYNTHETICS MANUFACTURER
Propex GeoSolutions

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