4 minute read
Bonner Bridge
Oregon Inlet, North Carolina, United States
Reducing wave energy for seagrass regeneration. Oregon Inlet is located in Pamlico Sound, the largest lagoon on the east coast and part of the Albemarle-Pamlico sound system, the second largest estuary in the United States. In this area, increased land use and development have in turn increased runoff, resulting in higher concentrations of nutrients, pollutants, and suspended solids in the sound. Overharvesting of oysters and the commercial removal of oyster reefs beginning in the late 1880s exacerbated the poor water quality in the sound, home to 90% of North Carolina’s commercial fishing industry. Therefore, mitigating the ecological impacts of the construction of Bonner Bridge was essential. In 2017, the North Carolina Department of Transportation hired CSA Ocean Sciences to find a mitigation opportunity for 0.52 hectares of affected submerged aquatic vegetation (SAV), but no locations near the project met the traditional mitigation requirements. Instead, they proposed an alternative mitigation strategy. The team installed a 150-meter wave-attenuating Atlantic Reefmaker structure, providing a wave-energy shadow that allowed the vegetation to grow undisturbed. Now, a dense and vibrant bed of seagrass lies at the bottom of the sound, creating habitat for fish and other aquatic species.
Article cover: Atop the reef structure facing the Bonner Bridge replacement. (Photo by CSA Ocean Sciences Inc.)
Producing Efficiencies
CSA’s extensive past research demonstrated that SAV habitat sheltered from wind waves developed continuous cover unlike those exposed. Therefore, the project team developed wave-forecasting models to precisely identify the size, location, and orientation of the wave attenuation system needed to aid SAV growth. The team installed the Atlantic Reefmaker system using pilings to support cast concrete disks at specified elevations above the seafloor. This design prevents the structure from settling into the seafloor over time and makes it readily adjustable to future sea level rise without requiring additional environmental permitting or compensatory mitigation.
Using Natural Processes
The reef installation method itself aimed to reduce environmental disturbances during placement, and the reef now acts as a natural impediment to waves. The wave shadow provided by the Reefmakers allows submerged vegetation to send runners along the seafloor, connecting existing patches and colonizing new ones. The result is a restored area of vegetation that will naturally increase habitat for other species. Furthermore, the Reefmaker’s piling-based design minimizes the footprint of the structure, allowing SAV to colonize more local seafloor.
Broadening Benefits
Installing the Reefmaker minimized impacts of the bridge construction. As the abundance of fish, shrimp, and crabs is directly proportional to SAV acreage, preserving and encouraging native SAV growth have had a positive effect on local species. Oysters have already heavily colonized the Reefmaker structure itself; and fish inhabit the spaces in, around, and under the suspended Reefmaker, further contributing to the development of a stable, complex ecosystem. The solution has provided ecological, commercial, and recreational benefits while allowing in-kind mitigation, protecting valuable resources.
Promoting Collaboration
The North Carolina Department of Transportation and CSA Ocean Services collaborated to conceptualize, plan, and implement the solution. By leveraging peer-reviewed science, the team was able to create a defensible and compelling strategy to mitigate environmental impacts of the Bonner Bridge project. The collaborators fostered a spirit of innovation and project ownership, garnering the support of other stakeholders and government agencies, such as the U.S. Army Corps of Engineers, the North Carolina Department of Environment and Natural Resources, and the Federal Highway Administration.
