Decadal-scale evolution of a barrier island : Insights from storm overwash and shoreline change on Ocracoke Island, NC

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Ian W. Conery (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: Eastern North Carolina has over 300 km of barrier islands that comprise the Outer Banks and act as an important buffer from the Atlantic Ocean and boundary to the Albemarle-Pamlico Estuarine System. These islands also draw millions of visitors and dollars to the state every year. With sea-level rise and the persistence of extratropical and tropical storms, it is critical to examine the recent decadal response to storm events and geologic evolution in order to best prepare for future change. In this study, multiple methods were used to evaluate the recent decadal evolution of Ocracoke Island, NC. Shoreline change rates were calculated using a transect-based approach with imagery from 1949, 1974 and 2006. Other aerial imagery time steps were used to look at the spatial impact of historic storms and to select coring and trenching sites based on visible depositional history. The stratigraphic and sedimentological signature of recent and historic storm events was interpreted using seven vibracores and 32 trench excavations. Additionally, LiDAR data was used to assess morphologic change and to test a storm-impact scale based on storm surge, waves and maximum foredune height. The average long-term shoreline change rate for all of Ocracoke Island was determined to be - 0.54 m/yr. The majority of the island has been eroding (over 65% of transects), and the average erosion rate was greatest in the most recent period analyzed (1974-2006). The shoreline change rates highlight the narrowing of the island through time. In some regions island width has decreased by as much as 70% (180 m). Hurricane Isabel (2003) overwashed a total of 9% of the island area with an average thickness of 0.24 m. The storm-impact scale showed a quantitative relationship between overwash and pre-existing dune conditions along the coast. Sedimentation from Isabel represented up to 26% of total backbarrier subaerial volume and was comparable to dune volume loss. Isabel caused up to 40 m of sound-directed migration of the foredune and substantial oceanside erosion, representing more than 20% of long-term net change in some regions of the island. Four other distinct storm deposits were interpreted within the cores based on the sedimentological signatures of moderately to well-sorted fine to medium grained sand, coarse shell hash bases, and heavy mineral laminae. However, few cores had multiple deposits, indicating stacked overwash deposits are spatially and temporally variable. This collection of results shows the complexity of barrier island evolution and the necessity to examine soundward migration in three dimensions.

Additional Information

Publication

Thesis

Language: English

Date: 2023

Subjects

Geology;Coastal geomorphology;Overwash;Remote sensing;Shoreline change

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