The morphology and migration of transverse bars off the west-central Florida coast

摘要

A series of migrating shore-normal sandbars with wavelengths of 75-120 m and heights up to 2 m have been identified off the northern tip of Anna Maria Island, a barrier island on the west-central Florida coast. Similar features have been described elsewhere since the 1930s and termed 'transverse bars'. The transverse bars identified off Anna Maria Island are found for about 3 km along the coast and extend 4 km offshore, well outside the normal surf-zone width. No cusps or any other associated beach expression is evident despite the fact that the bars come to within about 75 m of the beach. Sediments on the crests of the bars are a well-sorted fine quartz sand, whereas sediments in the troughs are a poorly sorted coarse carbonate shell hash. Historical aerial photographs and repeated high-resolution bathymetric surveys provide a means of quantifying the migration of the transverse bars. Analyses of orthorectified aerial photographs from the early 1940s through the mid 1990s clearly show movement or migration taking place in the bar field. In the 40-yr period from 1951 to 1991, the southern edge of the bar field moved 200-350 m to the south, with an average long-term migration rate of 8 m/yr. Repeated bathymetric surveys over an 8-month period give an average short-term migration rate of 21 m/yr to the south. Wave and current measurements suggest that southerly winds associated with the passage of cold fronts drive near-bed currents to the south that are strong enough to initiate sediment transport and cause the southerly migration of the transverse bars.