WHAT GRAIN SIZE DISTRIBUTIONS REVEAL ABOUT SEDIMENT SOURCES AND SINKS FOR THE VIRGINIA BARRIER ISLANDS

The thirteen mixed-energy Virginia barrier islands (VBIs) comprise some of the most dynamic and pristine islands in the world. During the past two years, we have sought, in part, to establish a budget capable of determining sediment sources and sinks to this island chain and to answer questions regarding geologic and oceanographic controls on island morphodynamics. To this end, we developed an experimental design to collect sediment samples capable of statistical hypothesis testing and quantitative analyses. We used a systematic random sampling design to obtain 12 surficial sediment samples from the beachface of each Virginia barrier island (total length ≈ 104 km; total number samples = 155) as well as a stratified plan to sample the entire Assateague Island reach (total length ≈ 60 km; total number samples = 76). Additionally, we acquired 144 bottom sediment samples from the active shoreface (depth ≤ 10 m) of the Virginia barriers and southern Assateague Island. We processed the total 375 sediment samples using standard mechanical shaking methods and obtained statistical moment measures and statistical parameters. Results from these analyses have provided preliminary insight into the morphodynamics of the VBIs including the important role of large shoals (i.e., Fishing Point, Chincoteague Inlet, and Wachapreague Inlet) and the shoreface in controlling morphodynamics along the entire barrier island chain. In short, analyses of the offshore data and the onshore island data suggest that: (1) a distinctive southward fining in grain size exists along the barrier island and (2) statistically significant subsets or “pockets” of islands exist with a larger, regional trend produced by differences in sediment sources and influence of sediment sinks.