SEA-LEVEL RISE AND UPDRIFT SEDIMENT TRAPPING DRIVE NET SAND LOSS ALONG THE VIRGINIA BARRIER-ISLANDS

Barrier-island evolution is broadly controlled by drivers such as sea-level rise, sediment availability and fluxes, and storminess. The Virginia Barrier Islands (VBI) provide an excellent natural laboratory to investigate controls on sediment trapping, sediment transport, and shoreline change, because they are largely undeveloped and their behavior is controlled by natural geomorphic and ecologic processes. Longshore transport of sediments from north to south is among the dominant controls on sediment fluxes to, and thus volume changes of, the VBI, which lack a major river or known significant offshore source of sand. We develop maps of subaerial island area through time for each of the 13 VBI using NOAA Historical Survey T-sheets and modern aerial and satellite imagery. We pair these areas with a representative island thickness derived from existing stratigraphic data and the lidar-derived average elevation of each island. The VBI lost 5.1% of total sand (2.01×10⁷ m³) from 1851 to 2013 C.E. The total loss from the southern VBI (all islands south of Assateague) was 67% (net loss of 9.7×10⁷ m³). Centrally located Parramore Island continuously decreased in sand volume over the 20^th and 21^st centuries. Assateague, through spit growth, had the largest net sand gain of 7.7 ×10⁷ m³. Taken together, these results indicate that while sediment trapping at Assateague Island functions as an updrift sediment sink, other multi-decadal to multi-centennial controls on downdrift coastal geomorphic behavior, such as the growth of flood and ebb tidal deltas, the offshore export of sand, and changing patterns of wave refraction, may also play key roles in the net loss of sand from the VBI. Moreover, changes within the morphology or sand volume of a single island is likely closely tied to that of adjacent, particularly updrift, islands with which it exchanges sand over annual to decadal scales.