Nonlinear dynamics of alongshore shoreline position change: observations and modeling

The horizontal, shore-perpendicular change in shoreline position along the Outer Banks of North Carolina is a self-affine signal. We measure shoreline change by determining the horizontal change in position of the 0.8m contour at 20m intervals along the coast. The profiles are obtained from two LIDAR surveys performed in September 1997 and September 1998 as part of a collaborative USGS, NOAA, NASA project. For six selected sections of coast, wavelet analysis of the shoreline change signal indicates that the signal is self-affine with a scaling exponent that varies from 1.2 to 2.1. This self-affine behavior indicates that the shoreline change signal is non-stationary with long-range persistence. The scaling exponent is found to vary depending on the orientation of the shoreline, suggesting that these changes are driven chiefly by subtle gradients on alongshore transport associated with subtle deviations from a smooth shoreline. Recent modeling work has shown that when waves approach shore from deep water at relative angles greater than approximately 45° , shoreline perturbations grow, causing alongshore-heterogeneous shoreline changes on any scale at which perturbations exist. Waves approaching from deep-water angles closer to shore-normal tend to smooth out the shoreline. The patterns of change over some extended time period will result at least partly from the interactions between the roughening and smoothing influences, which will depend on the regional wave climate, including the relative proportions of high and low wave-approach angles. The observed trend in scaling exponents found for the Outer Banks, with scaling exponent varying as a function of shoreline orientation, is predicted by the alongshore transport model.