HR: 0800h
AN: H41B-0506    [Abstracts]
TI: Exploring Morphological Process Indicators in Regional Patterns of Shoreline Change
AU: * Lazarus, E D
EM: eli.lazarus@duke.edu
AF: Duke University Earth and Ocean Sciences, 103 Old Chemistry Building, Box 90227, Durham, NC 27708, United States
AU: List, J H
EM: jlist@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543, United States
AU: Murray, A B
EM: abmurray@duke.edu
AF: Duke University Earth and Ocean Sciences, 103 Old Chemistry Building, Box 90227, Durham, NC 27708, United States
AB: Beach morphology changes over a continuum of time and spatial scales, from ephemeral patterns (10⁰ – 10² m alongshore) such as cusps and scarps that develop and dissipate over days to weeks, to persistent plan-view bumps in the shoreline (10³ - 10⁵ m), such as promontories, embayments, and capes, that tend to erode and accrete over years to millennia. Localized sediment cycling at smaller scales is strongly associated with cross-shore transport, while theoretical and data-analysis findings attribute large-scale shoreline evolution to alongshore sediment-transport processes; deep-water incident waves interacting with a sediment- covered shoreface are predicted to set up gradients in alongshore sediment transport that, depending on their approach angle, can exaggerate or smooth away shoreline perturbations. Patterns of alongshore sediment flux thus produce a correlation between shoreline-position change and shoreline curvature that depends on whether high- or low-angle waves dominated the wave climate over a given time span. Recent work on the northern North Carolina Outer Banks using lidar surveys to measure shoreline position shows a significant negative correlation, strongest at long time (decadal) and spatial (km) scales, between position change and shoreline curvature, suggestive of diffusion driven by a low-angle-dominated regional wave climate over the last decade. Correlations between position change and shoreline curvature, however, can arise from processes other than gradients in alongshore transport strictly related to shoreline curvature. Near the towns of Kitty Hawk, Kill Devil Hills, and Nags Head, a series of oblique sandy bars and gravel-floored troughs ~1 km wide define the nearshore bathymetry for approximately 5 - 8 km alongshore. Wave propagation over these features likely causes alongshore variations in breaking wave height and angle that affect alongshore sediment transport and patterns of shoreline change. From annual lidar surveys and vehicle-based shoreline measurements taken sub-annually for nine years, we find km-scale undulations in the shoreline that might arise from such bar-controlled modifications of nearshore processes. We identify patterns of shoreline change that in some areas suggest alongshore translation of these undulations, and less-regular changes in location and amplitude in others. Alongshore translation of features with a well-defined wavelength should give negative correlations between curvature and shoreline change, with a correlation magnitude that increases as the translation approaches half a wavelength (180° phase shift). Such behavior would also create a maximum correlation at zero-lag in a cross-correlation analysis, as we observe. We present data analyses and numerical modeling designed to diagnose the relative influences of the processes driving (over various time and spatial scales) the observed shoreline change in this enigmatic region. We also compare observations from the northern Outer Banks with two other internally continuous beach segments north and south of Cape Hatteras, introducing higher-angle wave climates and a greater range of known shoreface bathymetry to the investigation.
DE: 1824 Geomorphology: general (1625)
DE: 3020 Littoral processes
DE: 4430 Complex systems
SC: Hydrology [H]
MN: 2007 Fall Meeting