Date of Award

Summer 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean/Earth/Atmos Sciences

Committee Director

John Klink

Committee Member

Tal Ezer

Committee Member

Richard Whittecar

Committee Member

Gangfeng Ma

Abstract

The Virginia barrier islands stretch from Assateague Island in the north to Fisherman’s Island in the south. This string of islands is subject to frequent North Atlantic extra-tropical storm and hurricane influence. In addition, sea level rise has the potential to cause continued shoreline loss and inlet widening for this island group. This dissertation describes the effect of sea level rise, changes in wave height, and teleconnection pattern (AO, NAO, and MEI) correlations to shoreline location changes to the islands and inlet location and width changes. Locations of island shorelines were determined from satellite and aircraft images using ArcGIS software and the USGS Digital Shoreline Analysis System (DSAS). Geographically registered images were digitized, and multiple transect locations on each island were measured from a common offshore baseline using DSAS. The resulting distances along with the remote sensing image dates were used to construct time series of shoreline location for each island. Data frequency was controlled by image availability, and ranged from weekly to semi-monthly. Approximately 250 shorelines per island from 1990 to 2014 were digitized. The results were then interpolated to give a monthly shoreline location for each transect. The combined observations were analyzed by empirical orthogonal function (EOF) and wavelet analysis. The analysis shows that the barrier islands are moving to the south as well as becoming narrower. As a group, the shoreline movement is primarily correlated with sea level change (r2 = 0.41). The islands also respond to storms and other episodic events. The response can be resilient or catastrophic depending on the extent of shoreline loss which precedes the event. If the island shoreline has narrowed to a width that cannot withstand wave and high water level induced loss, the island and associated shoreline is permanently lost. The rate of shoreline loss returns to the pre-storm rate within several weeks after the events unless island shore is permanently lost during the event. Increased losses of island length were concentrated in years 2004-2005 and 2009-2010. Wavelet analysis confirmed and explained the EOF results by showing the presence of shoreline change cycles which become more intense at times of shoreline loss. A large reversal of shoreline loss was caused by the placement of 3.5 million m3 of sand on the south end of Wallops Island in 2012 and the attachment of a large offshore shoal to the north end of the same island. Wreck and Ship Shoal Islands have had a net shoreline gain over the study period but with continued southward migration.

There are ten inlets between the Virginia Barrier islands. Five of these had width increases of more than 100% during the study period. Two of the five widened by more than 200%. As the barrier islands have become shorter, the inlets have become wider. Due to the episodic nature of the loss of some islands, the inlet width gains have also been episodic. The statistical correlation of sea level variability with inlet width change has a coefficient of determination (r2) of 0.77. The extent of this correlation is very large and may be due to a combination of sea level effects on both the islands and inlets. Inlet width change between 1999 and 2014 was analyzed by EOF and wavelet analysis. EOF analysis of all inlets indicates in addition to general widening, substantial inlet changes generally occur during high water level and high wave events associated with passing hurricanes and tropical storms. Ocean shoreline and inlet trends are for continued shoreline loss for most islands, island migration southward, and inlet widening. The findings of this study can be directly applied to adaptation or possible island protection measures where warranted.

Included in

Oceanography Commons

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