HR: 0800h
AN: H41C-0317    [Abstracts]
TI: Alongshore coastline instability: wave climate analysis and comparisons to nature
AU: * Ashton, A D
EM: andrew.ashton@duke.edu
AF: Duke University Division of Earth and Ocean Sciences Nicholas School of the Environment and Earth Sciences & Center for Nonlinear and Complex Systems, Box 90227, Durham, NC 27707 United States
AU: Murray, A B
EM: abmurray@duke.edu
AF: Duke University Division of Earth and Ocean Sciences Nicholas School of the Environment and Earth Sciences & Center for Nonlinear and Complex Systems, Box 90227, Durham, NC 27707 United States
AB: Deep-water, not breaking, wave angles best predict how gradients in wave-driven alongshore sediment transport shape a coastline. As relative wave angles (between wave crests and the shoreline trend) increase, the smoothing influence (diffusivity) of alongshore sediment transport decreases, with the diffusivity reaching zero for relative deep-water angles around 45 degrees (although breaking angles may be much less). For even greater relative wave angles, coastline evolution becomes anti-diffusive, where bumps grow rather than shrink. Although their details differ, different formulations for alongshore sediment transport all predict this instability for large deep-water wave angles. Numerical modeling suggests that wave climates dominated by anti-diffusive, `high-angle' waves can cause a coastline to self-organize into large-scale rhythmic or quasi-rhythmic configurations, resembling natural features such as cuspate forelands, cuspate spits, and alongshore sandwaves. Along a real coast, wave angles and heights typically change more rapidly than significant long-term evolution takes place. Some days the waves exert a smoothing influence, on other days waves tend to roughen the coastline. Summing over all of a wave climate's diffusivity contributions for a given shore orientation yields the long-term coastal stability. At natural locations with interesting features such as capes and sandwaves, measured and hindcast wave values suggest that high-angle waves dominate for the regional shoreline trend. However, along the capes of North Carolina, U.S.A., measured climates are locally low-angle dominant despite regional suggested instability. Numerical simulations also exhibit this phenomenon, where shadowing by neighboring protuberances and local coastline self-adjustment combine to evolve a coast predominantly low-angle despite regionally high-angle wave climates. These comparisons support the hypothesis the instability affects natural shoreline shapes, and further emphasize how long-range interactions and emergent behavior (such as shadowing by emergent coastline features) can affect shoreline evolution.
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
DE: 3020 Littoral processes
DE: 3220 Nonlinear dynamics
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting