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The longshore variability of the coastal response to hurricanes may be examined within the framework of a storm-impact scaling model that compares spatially-variable beach morphology and fluid forcing. The relative elevations of dune height and storm-induced water levels are used to define three impact regimes (swash, collision, and overwash), within which the magnitudes and processes of sediment transport are expected to be unique. Maximum total water-levels are modeled as the sum of astronomical tide, storm surge, and wave runup. The 2% exceedence level for runup, the sum of wave setup and swash, is calculated using a parameterization found to be accurate to 38 cm (rms error) based on comparisons to 491 data runs from ten field experiments. Techniques have been developed to extract accurate (15-cm rms) and detailed measures of large-scale coastal morphology and change from high-resolution topographic laser altimetry (lidar) surveys, allowing for quantification of relevant dune heights as well as the magnitudes and patterns of shoreline, dune, beach slope, and beach volume change in response to hurricanes.
Based on the relative elevations of modeled hurricane-induced water levels and lidar-derived measures of pre-storm (1997) dune morphology, the potential impact regimes for Hurricanes Bonnie (1998) and Floyd (1999) were defined at 20-m increments along a 70-km stretch of coast in Onslow Bay, North Carolina. Comparisons to the observed impact regime, quantified from calculations of dune erosion and overwash deposition, indicate that the predictive accuracy of the model was 55.4%, an improvement over the 33.3% accuracy associated with random chance. Regime-specific model sensitivity was highest within the overwash regime (86.9%), decreasing to 55.8% and 1.5% in the collision and swash regimes, respectively. Shoreline and beach volume change in response to the storms were spatially-variable: the standard deviation of change was the same order of magnitude as the mean. Magnitudes of coastal change scaled with the observed impact regime. Beach volume change within the overwash and collision regimes was over two times greater than that within the swash regime. Little recovery was observed in overwashed locations where sand was transported inland and removed from the nearshore system. Here, the volume of sand removed from the beach was balanced by that in the overwash deposits. |
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