INVESTIGATING A MODEL OF BARRIER-ISLAND AND TIDAL-INLET EVOLUTION ALONG THE SOUTHERN DELMARVA PENINSULA WITH HISTORICAL SHORELINE AND BATHYMETRIC DATA (1852 TO 2009)
Change patterns portray shoreline retreat dominating both islands throughout the record with increasing retreat rates in the short-term. Cedar’s open-ocean shoreline experienced a retreat rate of -5.30 m/yr from 1852 to 2009 and a short-term rate of -4.22 m/yr from 2007 to 2009. However, the short-term rate increases to -10.18 m/yr when a breach-zone shoreline cell is excluded from the calculation. This bay-backed, open-ocean, breach zone has opened three times in the past 50 years and is currently characterized by rapid shoreline advance after breach closure in late 2006. The open-ocean shoreline of Parramore retreated -3.97 m/yr from 1852 to 2007 with a short-term rate of -9.72 m/yr from 1998 to 2007. Tidal prism at Wachapreague Inlet was 2.60 x 107 m3 in 1852, increased to 7.79 x 107 m3 by 1911, remained relatively stable through 1972 at 6.48 x 107 m3, and then jumped to 1.85 x 108 m3 by 1994. The steep increase from 1972 to 1994 is most likely caused by relative sea-level rise and an enlargement of open water in the bay in response to wetland degradation. Increased sediment volumes were deposited on the ebb-tidal delta in comparison to long-term trends.
Results indicate the barrier-island system has experienced retreat rates an order of magnitude greater than the average US mid-Atlantic background rate of -1.5 m/yr. These shorelines stand out as some of the highest long-term erosion rates along the entire US Atlantic coast. Parramore is a classic mixed energy, tide-dominated barrier island that is now experiencing severe erosion along its entire outer shoreline, whereas the morphology of Cedar has already evolved into a mixed energy, wave-dominated barrier island. This barrier-island complex may be progressing towards runaway transgression as per Fitzgerald model, and/or the southern extension of the large arc of erosion south of Fishing Point, VA has now reached Parramore.