An empirical orthogonal function analysis of ocean shoreline location on
the Virginia barrier islands
Abstract
Shoreline change along the Eastern Atlantic shore of Virginia has been
studied for the individual barrier islands but not as an integrated
system. This study combines the Atlantic shoreline locations for eleven
barrier islands obtained from LANDSAT 5, 7, and 8 images. Approximately
250 shoreline locations over a 24-year period from Jan 1990 to Dec 2014
were extracted from the digitized shoreline data at 338 transects. The
resulting 338 by 250 matrix was analyzed by the empirical orthogonal
function (EOF) technique. The first four principal components (PC)
explained 86 percent of the sample variance. Since the data was not
detrended, the first PC was the overall trend of the data with a
discontinuity in 2004-2005. The 2004-2005 years included storm events
and large shoreline changes. PCs 2 to 4 reflect the effects of El Nino
events and tropical and non-tropical storms. Eigenvectors 1 to 4 all
show the effects of the nine inlets in the island group. Eigenvector
(EV) 1 explains 59 percent of the shoreline spatial variance and shows
the largest changes at the northern and southern island ends. EVs 2 to 4
reflect the pattern of EV1 but at sequentially smaller percentages of
the spatial variance. As a group, the eleven islands are losing ocean
side shoreline. The lone exception is Hog Island. Sea level had the
strongest correlation with the shoreline loss trend of PC1. The
coefficient of determination was 0.41. The NAO and MEI also correlated
with PC1 with correlations of determination of 0.05 and 0.12
respectively. These confidence level for the three factors was better
than 99 percent. Sea level also correlated with PC3 and PC4. The PCs as
a group show that the year intervals 2004-2005 and 2009-2010 had large
effects on the shoreline change pattern for the island group. EVs 1 to 4
had the highest range of shoreline change at the island ends indicating
the effect the changes of the inlets have on the adjacent islands. The
smaller islands as a group had a higher level of eigenvector variance
than the other eight islands. Sea level change is the major factor
affecting the ocean shorelines of these islands. Continued sea level
increase will facilitate loss of ocean shoreline for the barrier islands
as a group.