INVESTIGATING A MODEL OF BARRIER-ISLAND AND TIDAL-INLET EVOLUTION ALONG THE SOUTHERN DELMARVA PENINSULA WITH HISTORICAL SHORELINE AND BATHYMETRIC DATA (1852 TO 2009)

The barrier-island and tidal-inlet evolution model of Fitzgerald (2004) is tested by examining changes in ebb-tidal delta volumes at Wachapreague Inlet in response to tidal-prism fluctuations and the impact on adjacent barrier-island shorelines of Cedar and Parramore Islands, VA. Shoreline datasets include T-sheets and GPS surveys (1852 to 2009), whereas bathymetric datasets include H-sheets (1852 to 1982) and cross-sectional areas of the inlet throat.

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.