NCDOT Research Project 2020-09 “Monitoring and modeling sound-side erosion near Oregon Inlet to support feasibility level transportation planning” used field observations, numerical modeling, fragility curves, and engineering analysis to (1) assess the hydrodynamics and erosion processes near Oregon Inlet, (2) estimate the potential for roadway failure based on marsh conditions, and (3) evaluate potential erosion mitigation options for NCDOT. Field measurements were collected over three years to inform the site conditions and the erosion processes at the marsh shoreline and the adjacent flood channel, and a classification system was developed to visually assess the erosion condition of the marsh shoreline. Bathymetric observations show that the flood channel is rapidly migrating landward. The locations where the channel is closest to the shoreline correspond to the highest short-term erosion (0.7 m to 2.7 m (2.3 ft to 8.9 ft) during storms) and longterm erosion rates (2 m/yr to 4 m/yr (6.6 ft/yr to 13 ft/yr)), as well as a severely eroded shoreline condition observed in the field. Observation data and numerical modeling results were used to develop a series of fragility curves (i.e., probability curves predicting damage, given environmental conditions), which quantify the conditions likely to lead to shoreline erosion or roadway flooding impacts. The fragility curve analysis was used to develop an online vulnerability indicator, which correlates water levels and wave heights measured at the Oregon Inlet Marina and Oregon Inlet Waverider to potential flooding conditions along N.C. 12.
A series of erosion mitigation alternatives, including structural, natural or nature-based, and hybrid options, were reviewed to determine the potential engineering, environmental, and regulatory considerations associated with each. Based on the initial screening, six of these options were simulated in a numerical model of the project area to determine the ability of each option to reduce erosive flow velocities. Three alternatives - terminal groin extension, channel relocation, and island restoration - displayed the most effectiveness at reducing flow velocities at the shoreline. The results of this effort provide a feasibility-level review of potential engineering alternatives to address the documented marsh erosion within the project study area as well as insight on the engineering, environmental, and regulatory constraints associated with these alternatives.
