PROCESS SIGNATURES IN REGIONAL PATTERNS OF SHORELINE CHANGE ON ANNUAL TO SUB-ANNUAL TIME SCALES

The morphologies of sediment-covered shorelines evolve over a continuum of time and spatial scales. Ephemeral, small-scale patterns (10⁰ - 10² m alongshore) such as beach cusps and storm scarps are developed and reworked over days to weeks, typical of localized sediment cycling associated with cross-shore transport. Persistent, plan-view bumps in the shoreline (10³ - 10⁵ m), however, such as promontories, embayments, and capes tend to erode and accrete over years and decades—aggregate changes that recent theoretical and data-analysis findings attribute to alongshore sediment-transport processes. Shoreline perturbations set up gradients in alongshore sediment transport that vary with the approach-angle of deep-water incident waves; a high-angle (> ~45° in deep water) wave climate will tend to cause convex-seaward bumps to grow and concave-seaward embayments deepen, while a low-angle (> ~45°) wave climate will have an opposite, diffusive effect. Patterns of alongshore sediment flux thus affect a correlation between shoreline-position change and shoreline curvature that depends on whether high- or low-angle waves dominated the wave climate. Ongoing work on the northern North Carolina Outer Banks using lidar surveys demonstrates a significant negative correlation, strongest at large time (decadal) and spatial (km) scales, between position change and shoreline curvature, consistent with a low-angle-dominated regional incident wave climate over the last decade. With a set of vehicle-based alongshore transects taken monthly for three years along the same continuous, 60 km stretch of the North Carolina Outer Banks used in the lidar-based study, we document shoreline change, quantify alongshore patterns of erosion and deposition, and isolate signals diagnostic of cross-shore- and alongshore-transport processes evident at annual to sub-annual and storm time scales. Correlations between position change and shoreline curvature are consistent with the hypothesis that alongshore transport processes play a significant role in the observed shoreline change. However, on small space and time scales such correlations can arise for multiple reasons; cross-shore and swash-zone processes likely strongly influence shoreline shape on scales shorter than 12 months and smaller than 500 - 1000 m.