Abstract: Session H  3:30 pm (Back to Session H)
Submerged Aquatic Vegetation Mitigation for the Bonner Bridge Replacement Project

Randy Boyd, PE
Atlantic Reefmaker
Wilmington, NC

Author(s):  Randy Boyd, Atlantic Reefmaker, Inc, Kathy Herring, NCDOT, Mark Fonseca, PhD, CSA Ocean Sciences, Inc

As part of the environmental permitting process for replacing the Bonner Bridge over Oregon Inlet in Dare County, the North Carolina Department of Transportation (NCDOT) agreed to mitigate for submerged aquatic vegetation (SAV) by installing the Atlantic Reefmaker (ARM), a wave attenuation structure. The ARM structure is designed to provide a wave shadow by allowing the existing beds of patchy SAV to coalesce.

The NCDOT hired CSA Ocean Services (CSA) to determine the best way possible to mitigate for 1.28 acre of SAV impacts associated with the bridge construction. CSA’s search of the Pamlico Sound determined that there were no existing SAV areas to ‘fix’, and that an alternative mitigation source was required.

Implementation of the ARM structure was completed in January 2017. The goals associated with ARM installation were 1) to provide SAV mitigation for Bonner Bridge replacement and 2) to contribute to the long-term effort to develop the best strategy and methodology for SAV restoration in North Carolina. The objective of this mitigation project was to reduce the amount of wave energy within the project site. By reducing the wave energy for the area, models estimated that there would be a more continuous cover of SAV (specifically for the seagrasses Halodule wrightii and Zostera marina to expand), thereby leading to an increase in seagrass acreage while providing other ecosystem services. These services included water quality improvement, aquatic habitat creation, reduced sediment movement and plant community establishment. The wave break proposed in this mitigation plan also created new linkages between intertidal and aquatic environments.

The NCDOT conducts monthly monitoring of the ARM structure and bi-annual monitoring to record changes of SAV behind the structure. The monthly monitoring studies the potential for scour around the structure, while the bi-annual reports note the health and coalescence of the SAV, wave attenuation, sediment changes around the structure and sessile life use of the structure.

This presentation will describe site selection, expand on the SAV mitigation goal and objectives, explain why the ARM structure was utilized in the mitigation and note the monitoring results to date.