Analysis of barrier island sand grain size trends from 2011-2018 pre- and post- beach replenishment: Wallops Island, Virginia

Wallops Island (WI), a barrier island on the Eastern Shore of VA, has experienced century-long shoreline erosion to within meters of NASA's WI launch facilities. A localized S-N reversal of longshore transport and overwash have caused erosion averaging 3.7 m/yr since 1857 on the S end of WI (SWI). Due to shoreline engineering and a major 2012 beach replenishment, the N end of WI (NWI) has prograded seaward 1 km since 1930 ( 11 m/yr). Overall, WI has rotated clockwise (CW) at a hinge-point near the middle of the island, at the north edge of a seawall.

The goals of this research are to track changes in sand grain size (GS) distribution along WI before, during, and after replenishment. Samples were collected monthly from the beach face and dune (March 2011-Nov. 2012) at 31 sites. We conducted seasonal and event response sampling from Nov. 2012 to July 2018. Pre-replenishment average GS (beach face) on SWI was 1.2-2.4 phi. Sediment fined northward to 1.8-3 phi on NWI. Replenishment added 3.2 million cubic yards of coarser (0.5-1 phi) sand to the system. Our sampling resolution allows us to track the replenished sand as it was redistributed by natural processes. By 2014, GS on SWI beach fined as replenished sand was buried by sands deposited by longshore transport. On NWI, replenishment caused immediate coarsening to 1.4-1.6 phi. Five years after replenishment, GS at the hinge-point was 1-3 phi and the beach on NWI widened by as much as 100 m. Beach replenishment reduced the impact of Hurricane Sandy on WI infrastructure and accentuated the CW rotation of the island as replenished sand moved both N and S of the hinge-point. NWI is prograding and becoming more stabilized, while SWI continues to retrograde. Additional effects of replenishment include increased permeability of the replenished beach system, a shallower, more extensive freshwater lens, and expanded freshwater wetlands and vegetated dune-swale systems that are being colonized rapidly by Phragmites.

Analysis of GS data provide insight into sediment transport and groundwater flow, and allow for forecasts of future behavior of the island. This region is experiencing SLR at twice the global average. Based on these and other data, NASA is beginning the process of retrofitting and relocating necessary infrastructure for both higher water table levels and future erosional events on SWI.