GRAIN SIZE DISTRIBUTION AND SEDIMENT TRANSPORT PATTERNS ALONG WALLOPS ISLAND, VIRGINIA: PRE- AND POST-BEACH REPLENISHMENT, AND POST-HURRICANE SANDY

Barrier Islands along the Mid-Atlantic coast are particularly susceptible to damage from nor’easters and hurricanes, as demonstrated recently by Hurricanes Irene (August 2011) and Sandy (October 2012). Wallops Island (WI), a barrier island off Virginia’s coast, has experienced severe erosion for more than a century. This erosion, combined with rising sea level, is causing the shoreline to retreat rapidly to within meters of NASA’s Wallops Flight Facility, a multi-billion dollar launch complex. WI is experiencing significant accretion to the north and severe erosion, averaging 3.7 m/yr since 1857, on the south end of the island. Storm surge, waves, and high winds often cause massive flooding and sand overwash and threaten existing infrastructure. In response to these threats, the beach was replenished in April-August 2012. Approximately 3.2 million cubic yards of sand were added, creating a 4.2-km long, 70-meter wide beach in front of the existing seawall. In order to understand how beach replenishment affects sediment transport and deposition on WI, we collected sediment samples from March 2011-April 2012 prior to replenishment activities, and during and after replenishment. Our results pre-replenishment show a south-north trend in grain size distribution, with coarser sand (-1-2 phi) accumulating to the south and finer grained sand (2-4 phi) to the north. This south-north sediment transport trend is reversed from regional mid-Atlantic transport. Our post-replenishment samples (April-October 2012) show a significant increase in mean grain size (1.75-0.5 phi), followed by evidence of replenished sediment transport and deposition to the north. Two months after the replenishment project, WI was hit indirectly by Hurricane Sandy storm waves. Initial beach surveys post-Sandy showed an overall retreat of the shoreline. More than 50 meters were lost in some areas of the natural beach, but significantly lower narrowing occurred on the replenished beach. Both areas showed evidence of beach flattening via overwash, and longshore beach-berm transport scoured erosional channels into both the new and natural beach. We collected sediment samples post-Sandy to determine its immediate effect on sediment distribution. Comparing post-Sandy data to prior data may help us predict how long the replenishment will last.