THE INFLUENCE OF NEARSHORE GEOLOGY ON THE NORTHERN VIRGINIA BARRIER ISLANDS DURING SEA-LEVEL RISE

Geophysical surveys, consisting of 274 km of seismic reflection profiles and side-scan sonograms, were obtained from the nearshore of Virginia's northern parallel beach retreat islands during July 2009 in order to answer the following questions: 1) Does a source of sediment exist that can supply Virginia's barrier islands with sand during sea-level rise? 2) What are the seismic and sedimentological characteristics of those sediment sources and in what volumes do they occur? Finally, 3) How might the inherited geology, topography, and/or bathymetry have influenced or continue to influence barrier island morphodynamics during sea-level rise? Seismic reflection profile data indicate that the upper approximate 4 m of the nearshore, over a 400 km2 area, contains approximately 1.56 billion m³ of potential sediment for supplying the islands. Bedforms visible on side-scan sonograms (average ht < 1 m; wavelength ≈ 50 m) indicate sediment mobility and an onshore migration direction. However, sediment grab samples from the seafloor in this area indicate that the average barrier island grain size (0.25 mm ± 0.05 mm) does not match the average nearshore grain size (0.14 mm ± 0.05 mm). Consequently, surficial nearshore sediments are too fine to supply the islands given the existing energetic oceanographic conditions. Finally, a seismic facies analysis reveals the presence of older stratigraphic units (Miocene – Pleistocene), which likely control the location of the present-day tidal inlets and barrier islands.