HR: 16:45h
AN: H34B-04    [Abstracts]
TI: When Might Barrier Island Chains 'Collapse'? An Initial Model Investigation
AU: * Slott, J M
EM: jordan.slott@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Box 90229, Durham, NC 27708, United States
AU: Murray, A B
EM: abmurray@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Box 90229, Durham, NC 27708, United States
AB: There has been recent speculation that, in response to the accelerated sea-level rise and intensified storms expected over the coming century, barrier island chains such as those found on the US Atlantic and Gulf coastlines, could develop large (10-kilometer-scale) gaps in their most narrow stretches, or might disappear completely (Riggs, S. R., 2001). Such a collapse along the North Carolina Outer Banks barrier island chain, for example, would leave the mainland vulnerable to direct hits from Western Atlantic storm systems, and also would dramatically alter the estuarine system it encloses with potentially devastating effects to marine life. Concern for the future of the Outer Banks is also motivated by the decimation of the Chandeleur Islands in 2005 from Hurricane Katrina. We will present a series of initial numerical modeling experiments addressing how barrier island morphodynamics respond to the sudden creation of kilometer-scale gaps. Large-scale barrier island evolution is influenced by sea-level rise and barrier island overwash, alongshore sediment transport, tidal currents, and the availability of mobile sediment. Barrier islands transgress towards the mainland in response to sea-level rise through overwash: ocean-facing shorelines provide sediment that is transported onto the island to maintain its subaerial height and behind the island to maintain its width, while gradients in alongshore sediment transport typically dictate the large-scale shape of a coastline over long time frames (decades to millenia). Tidal currents also tend to scour inlet channels; the relative strength of this effect depends in part upon the width of the inlet channel. Our exploratory model includes both a one-line alongshore transport component and a cross-shore overwash component, as well as representations of underlying geology (weathering rates and material compositions). In our modeling experiments, we test the effects of perforating a 30 km barrier island chain with variable-sized gaps, ranging between 2.5-10 km. In preliminary model experiments, where we do not limit the availability of mobile sediment nor include tidal inlet dynamics, large gaps tend to close under all of the erosion rate scenarios and gap sizes. The ends of barrier islands extend to fill in the gaps and recurve landward. The rate of closure of gaps is unaffected by sea-level rise rates even under the most extreme cases; alongshore sediment fluxes exceed those associated with sea-level rise as highly curved isolated islands migrate rapidly landward before coalescing into an island chain again at a new location. In a natural setting, the overwash and spit-growth that maintain sub-aerial islands and tend to knit them back together (respectively) could be inhibited by a lack of mobile sediment. The shoreface of the Outer Banks, for example, consists of sometimes patchy Holocene sands perched atop a semi-lithified, sometimes more muddy Pleistocene substrate. Weathering of the Pleistocene substrate over long timescales generates mobile sediment consisting of both sands and muds. The fine-grained material, however, is typically lost to the nearshore system. The shoreface may not be able to weather fast enough to keep up with rapidly migrating islands. This effect, combined with that of substrate composition, will tend to limit the rate that sediment can be liberated, and, in turn, could prevent island-chain recovery. We conduct a series of model experiments to determine the combinations of geological parameters (weathering rates, composition) and forcing parameters (rate of sea-level rise, frequency of storms) that prevent barrier-island-chain recovery.
DE: 1630 Impacts of global change (1225)
DE: 1824 Geomorphology: general (1625)
DE: 4217 Coastal processes
DE: 4430 Complex systems
DE: 4546 Nearshore processes
SC: Hydrology [H]
MN: 2007 Fall Meeting