Shoreline analysis and barrier island dynamics: event to decadal scale patterns from Cedar and Parramore Island,Virginia

Nebel, Stephanie
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University of Delaware
Aerial photography, topographic maps, high-resolution satellite imagery, and GPS (global positioning system) data have been compiled in ArcMAPTM and analyzed using the Digital Shoreline Analysis System (DSAS) in order to determine both decadal and event scale trends of shoreline movement on Cedar Island, VA. Shoreline retreat rates for Cedar Island were found to have an alongshore average of -4.1 m/yr ± 8.9 m based on simple end-point analysis (1852-2007). The short-term (1994-2007) retreat rates for Cedar Island were calculated to be -12.6 m/yr ± 7.6 m. Retreat on Cedar Island, therefore, has accelerated over the past few decades. In addition to this, principal component analysis of the shoreline data confirms that the Cedar Island shoreline has been rotating through time. Barrier island response to storms over the short term (months to years) has been examined. GPS surveys conducted on Southern Cedar Island about 1.5 months before and 3 weeks following the passage of Hurricane Ernesto in 2006 chronicle the subsequent shoreline movement. On average, the southern section of the Cedar Island coast retreated between -25.4±1 meters as a result of the storm. The highest erosion (-54.8 m) was documented to the south of the ephemeral inlet. Recovery of South Cedar was examined in February and June 2007. Between September 2006 and February 2007, the shoreline accreted an average of +7.4 ± 2.1 meters. Between February 2007 and June 2007 the shoreline eroded an average of -0.2 ± 2.1 meters. A comparison of the July 2006 (pre- Ernesto) and June 2007 (nine months following Ernesto) shorelines revealed that the shoreline had not recovered to its post-storm position. Between July 2006 and June 2007, the shoreline eroded an average -18.3 ± 1 meters. Lastly, shoreline retreat patterns for Cedar Island are compared to the historical Atlantic hurricane tracks that have affected the Eastern Shore. The comparison of this storm frequency data with Cedar Island retreat rates showed that the acceleration in shoreline erosion that began in the 1980s occurs contemporaneously with an increase in tropical storm and hurricane frequency.