Collaborative Research: Coasts in Motion: Quantifying the patterns of coastal change using LIDAR

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ABSTRACTThe proposed research involves collecting and analyzing new observations of shoreline change. Four new LIDAR surveys of the North Carolina Outer Banks coastline, each separated by sixmonths, would be conducted. These surveys, combined with previous surveys of the same coastline segments in 1997, 1998, 1999, and 2000, would allow the analysis of patterns of shoreline change over 18 time intervals, ranging in length from six months to eight years. Analysis techniques previously applied to the changes over a single annual interval (Tebbens et al, 2002) quantify statistics including: 1) the amount of shoreline change as a function of alongshore length scale; 2) the distribution of the alongshore-lengths of contiguous zones of erosion and accretion; and 3) the distribution of the magnitudes of erosion and accretion occurring during a time interval. In the previous analysis, the statistics of the patterns of shoreline varied among the different coastline segments measured. These shoreline segments have different orientations, and therefore different effective wave climates. Repeating the analyses over many time intervals will test whether the statistics and the variations from one coastline segment to another are robust. The proposed work would also test a hypothesis and potential model for the main cause of the observed shoreline behaviors. The way the statistics describing the patterns of shoreline change vary as a function of regional wave climate suggests the hypothesis that these changes are driven chiefly by subtle gradients in alongshore transport associated with subtle deviations from a smooth shoreline. Recent 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 (Ashton et al., 2001). 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. A model treating alongshore transport (Ashton et al., 2001; Ashton et al., 2003a; Ashton et al., 2003b) predicts the observed trend with shoreline orientation (regional wave climate) in one of the statistics in the previous analysis (Tebbens et al, 2002). Broader Objectives. If the new data collection and analysis bears out the preliminary findings concerning different coastline segments, and if other model predictions are consistent with the observational results, the model be able to generalize the usefulness of the observations and analyses; the model will provide a way of extending, to any coastline for which a wave climate can be estimated, probabilistic forecasts including expected maximum magnitudes of erosion and accretion over a time interval, and alongshore extents of erosion and accretion zones. Along with the practical benefits such predictions could offer coastal managers, successful model tests would represent an advance in basic understanding of the processes that are important for shoreline changes on scales ranging from hundreds of meters to tens of kilometers. Models that will be able to elucidate the range of coastline changes to be expected in the next century as sea level rise accelerates rely on such improvements in basic understanding. In addition, undergraduate as well as graduate students would participate in the research and the presentation of the results.