MORPHODYNAMICS AND SEDIMENT SOURCES AND SINKS ALONG THE VIRGINIA BARRIER ISLANDS

The thirteen mixed-energy Virginia barrier islands (VBIs) comprise some of the most dynamic and pristine islands in the world. The barriers migrate landward over backbarrier marsh deposits at rates that average 2.7 m/yr and reach as high as 32 m/yr on Cobb Island. Extensive duneless reaches, marshes and tidal creeks that outcrop on the foreshore, dead trees in standing position on the foreshore, and large boulder-sized marsh rip-up clasts originating from the nearshore attest to the highly erosional nature of the islands. This research sought to determine sediment sources and sinks to this island chain and to answer questions regarding geologic and oceanographic controls on island morphodynamics. To this end, we obtained surficial sediment samples from the beachface of each Virginia barrier island using a systematic random sampling design (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. Results from these grain size 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.