TEMPORAL AND SPATIAL SCALES OF BEACH PROFILE CHANGE, DUCK, NORTH-CAROLINA

An 11-year time series of high-resolution beach profile surveys made on an Atlantic Ocean beach was analyzed for spatial and temporal characteristics of beach profile change. Approximately 300 profile surveys, most extending from the dune to 8-m depth, were available for the analysis on each of four cross-shore lines, together with electronically recorded and statistically processed wave time series from a nearshore gage located seaward of the survey site. The profile survey data set was analyzed for such quantities as depth change, frequency of depth change, general seasonal shape of the profile, seasonal depth change, and change in the profile produced by extreme storms. The morphodynamics of an inner and outer longshore bar were also examined, including depth to bar crest, bar height and length, and speed of bar movement onshore and offshore. Several properties of depth change were related to wave characteristics. Representative results were: average profile elevation change from +4 m to -4 m was symmetric about the mean sea-level shoreline; the average spring and autumn profile shapes were almost identical and occurred as transitional states between the summer and winter profile shapes; the depth of active profile movement (within survey accuracy of about 0.025 m) was 4 m for the summer and 6 m for the winter; and the 10-year frequencies of maximum absolute depth change in depths of 2, 4, 6, and 8 m were 1.64, 1.38, 0.22, and 0.12 m, respectively. A surprising result was that typical large storms transported sand into the nearshore from the seaward end of the profile (from a depth of about 6-8 m).