Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Christopher J Hein
Barrier islands provide critical habitat for shorebirds and buffer backbarrier and inland coastal habitats and communities from the direct effects of storms and wave energy. Yet, barrier systems are under threat from these very same processes. In particular, whereas long-term barrier behavior and landward migration rate is directly related to the rate of sea-level rise, decadal to centennial barrier-island dynamics are more complicated, and often highly localized. Barrier-island dynamics—including state changes between seaward growth and landward migration—are driven by a complex interplay of forcings and interactions, including changes in longshore and cross-shore sediment fluxes, underlying geology and slope, barrier-marsh and barrier-dune-vegetation couplings, and local hydrodynamics. The relative influence of these various processes is poorly constrained. This dissertation uses the Virginia barrier islands as a natural laboratory to explore the impact of three key factors (antecedent geology, sediment fluxes, and local to regional hydrodynamics) on barrier-island morphodynamics. First, Chapter 2—published in January 2021 in Sedimentology—quantifies the relative impact of time-varying antecedent slope and sediment supply as compared with sea-level rise by integrating field data and morphodynamic modeling. This approach reveals that antecedent topographic highs can “pin” barrier islands in place for multiple centuries; however, on the landward side of a high barrier, islands must fill with sediment vertical accommodation created by sea-level rise in order to migrate landward. If sediment supplies are limited, the island will likely experience severe erosion or drowning of the barrier. These effects hold measurably true for rates of relative sea-level rise up to 6 mm/yr, though slope changes continue to control barrier migration at even higher sea-level rise rates. Sediment stored through the growth of updrift beach and foredune ridges can similarly impact barrier-island changes on the multi-kilometer scale. Specifically, the results of Chapter 3—published in July 2021 in Quaternary Science Reviews—demonstrate that at least 60% of sand estimated to be delivered annually through longshore sediment fluxes to the northern Virginia coast is trapped in Assateague and Wallops islands, indicating the growth of these updrift landforms is a key control on sediment fluxes to downdrift barriers. In fact, this suggests that regional sediment budgets in Virginia and elsewhere must account for landform growth as a potential sediment sink. In addition to antecedent geology and longshore sediment supply, local to regional hydrodynamics influence the rate and style of coastal progradation. Chapter 4 demonstrates that highly local (kilometer scale) changes in sediment fluxes and wave energy can control the relative height and spacing of beach and foredune ridges. Specifically, wave-refraction patterns that vary through time and are forced by, for example, offshore and updrift sedimentary landforms, are reflected in ridge orientation and internal architecture. Additionally, beach-ridge sedimentology and internal architecture can, when paired with hydrodynamic modeling, reveal sediment-transport mechanisms and the formational processes of beach and foredune ridges. Finally, a key takeaway is that accurately deriving paleoenvironmental records from progradational coastal archives depends on a comprehensive understanding of ridge formational processes and local hydrodynamics that may overprint allogenic signals such as regional wave climate or storminess. This dissertation emphasizes the role of mesoscale processes in determining the pathway of sandy coastal landform response to sea-level rise. The trajectory and fate of associated clastic coastal landforms cannot be predicted using universal metrics; instead, local predictions and comprehensive coastal management must identify, quantify, and account for the key local factors that drive barrier change on annual to multi-decadal timescales.
© The Author
Shawler, Justin Lee, "Barrier-Island Dynamics: Transgression, Regression, And State Changes" (2022). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1673281476.
Available for download on Friday, December 08, 2023