TRACKING TRENDS IN SAND GRAIN SIZE ALONG A BARRIER ISLAND AFTER A MAJOR BEACH REPLENISHMENT: WALLOPS ISLAND, EASTERN SHORE, VIRGINIA

Wallops Island (WI) VA, has experienced century-long shoreline erosion to within meters of NASA’s Wallops Flight Facility (WFF). A localized S-N reversal of longshore transport (LST) and overwash have caused erosion averaging 3.7 m/yr since 1857 on south WI (SWI). Due to decades of engineering, dredging, and LST, north WI (NWI) has prograded seaward ~1 km since 1930 (~11 m/yr). WI has thus rotated clockwise (CW) at a hinge-point near the N end of a seawall.

To protect WFF infrastructure, NASA performed a major beach replenishment in 2012, creating a 6 km long, 70 m wide beach seaward of the seawall. To track changes in grain size (GS) distribution along WI post-replenishment, we established baseline conditions in 2011. Samples were collected monthly from the beach face and dune (March 2011-Nov. 2012) at 31 sites. Pre-replenishment average GS (beach face) on SWI was 1.2-2.4 phi. Sand fined northward to 1.8-3 phi on NWI. Completed August 2012, replenishment added 3.2x106 yd3 of coarser sand (0.5-1 phi). Seasonal and post-event sampling took place Nov. 2012-Oct. 2018. Our dataset allows us to track replenished sand as it redistributed naturally. In Oct. 2012, Hurricane Sandy removed ~50% of the new sand. By 2014, GS on SWI beach fined as replenished sand was buried by sand deposited by LST. On NWI, replenishment caused immediate coarsening to 1.4-1.6 phi. Six years after replenishment, GS at the hinge-point is 1-2 phi, NWI beach widened by as much as ~100 m, whereas the new beach has eroded to <20 m and storm surge reaches the seawall.

Replenishment reduced the impact of Hurricane Sandy on WI infrastructure and accentuated the CW rotation of the island as sand moved both N and S of the hinge. NWI continues to prograde but is stabilizing, while SWI continues to erode. Other effects of replenishment include increased permeability of the beach system (infiltration: 4-9 ml/s), lower densities of coquina clams, a pronounced shallow freshwater lens, expanded freshwater wetlands, and vegetated dune-swale systems colonized by Phragmites.

This region is experiencing SLR at twice the global average and NASA is planning for a second round of nourishment that may include relocating sand from NWI back to the south. Our data highlight the ephemeral nature of beach replenishment and a few of the unintended consequences of this short-term management solution.