GRAIN-SIZE TRENDS OF WASHOVER AND FLOOD-TIDAL-DELTA DEPOSITS ASSOCIATED WITH A RELICT ISLAND BREACH: CEDAR ISLAND, VIRGINIA, USA

Cedar Island, VA is a low-profile, washover- and wave-dominated, retrograding barrier island located along the outer shoreline of the southern Delmarva Peninsula, where mean tidal range is 1.7 m. The island has experienced an average retreat rate of 5.5 m/yr over the long term (1852-2010) and 15.4 m/yr over the short term (2007-2010). Cedar Island’s southern end has been breached by storm impacts on three occasions since 1950. Each time, the breach typically opens, migrates south, and closes within 10 years. When open, this temporary inlet is known as Cedar Island Breach. The study area is located within the final position of Cedar Island Breach before it closed in early 2007. Topographic surveying, surface sediment grab samples, and sediment vibracores provide dataset for addressing the horizontal and vertical grain-size trends of washover deposits.

Mean grain size of the surficial sediments ranges from 1.643 to 2.748Φ and shows a net landward fining trend from the lower foreshore (1.941Φ) to the distal washover fan/relict flood tidal delta (2.679Φ). This fining trend indicates that during overwash events, coarser particles are deposited first while finer particles are transported further landward. Sorting ranges from 0.408 to 0.784Φ. Sediments are moderately to moderately well sorted at the beach and on the distal portions of the washover fan/relict flood tidal delta but are well sorted to moderately well sorted along center portion of barrier island. Along island center, aeolian processes play larger role by reworking the surficial sediments, thus leading to better sorting. Skewness ranges from -0.434 to 0.196. Sediments are negatively skewed at the beach (-0.248 to -0.106) with positive skewness (0.157 to 0.010) dominating the island center then returning to negative skewness on the distal washover fan/relict flood tidal delta (-0.075 to -0.286). The negative skewness reflects waves reworking beach sand or washover deposits sourced from beach sand, whereas the positive skewness indicates exposure to aeolian processes and reworking along island center, which is slightly topographically higher where sands have an opportunity to dry out more often.