Vulnerability Indicators for Coastal Roadways Based on Barrier Island Morphology and Shoreline Change <b>Predictions</b>

Abstract

Coastal roadways are vulnerable to changes in landscape that occur at variable spatiotemporal scales. In particular, highways on barrier islands suffer the consequences of the combined action of the ocean, the back-barrier lagoon, and the morphological changes in the island. Coastal dunes and beaches are typically the only barrier between the ocean and the island’s infrastructure, while low-lying marshes separate the infrastructure from back-barrier lagoons. This work addresses the spatiotemporal variability of these coastal features along a barrier island and proposes a set of vulnerability indicators that allow evaluating past, present, and future vulnerability of a coastal roadway. Systematically collected remotely sensed data were used to digitize dune elevations and oceanfront and estuarine shorelines in the northern portion of Hatteras Island, North Carolina, US. Based on these morphological data and their distance to the main roadway on the island, three vulnerability indicators were defined along shore-normal transects: (1)&nbsp;island width &lt;305 m, (2)&nbsp;dune crest elevation &lt;3 m above the highway, and (3)&nbsp;edge of pavement within 70 m of the ocean shoreline. In addition, potentially vulnerable areas of the coastal roadway have been predicted until 2030 based on historical records of shoreline positions (mid-1940s to present). Of the 20&nbsp;km of roadway analyzed, currently, nearly 6&nbsp;km meet at least one vulnerability criterion. By year 2030, 9&nbsp;km of roadway will potentially become vulnerable because of proximity to the shoreline. These results reveal five main regions of concern; of those regions, two breached during Hurricane Irene (2011) and the others have suffered major dune erosion, overwash, and flooding during winter storms and hurricanes. The vulnerability assessment presented here allowed identification of historical, present, and future vulnerable spots along the island and continues to inform the North Carolina Department of Transportation (NCDOT) for their planning and adaption strategies for future phases of highway improvements. The simplicity of the indicators makes them applicable to other coastal roadways and even other types of critical infrastructure in barrier islands and coastal regions. However, if used at different locations, the specific thresholds for the indicators can vary depending on local conditions that may differ from the ones analyzed here.