Long-term observations of migrating shore-normal bars

A series of migrating shore-normal sandbars with wavelengths of 50-200 m and heights of 0.5-2 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 1930's 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. 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. Historical aerial photographs from the early 1940's through the mid 1990's provide an excellent means of quantifying the migration of the bars for this time period. The historical photographs were orthorectified resulting in errors in geographic positions of 1-2 m. Analyses of the orthorectified photos clearly show movement or migration taking place in the bar field. In the forty year period from 1951 to 1991, the southern edge of the bar field moved 200-350 m to the south, with an average migration rate of 7.9 m/yr. A current-meter deployment suggests that southerly winds associated with the passage of cold fronts drives near-bed currents to the south that are strong enough to initiate sediment transport and cause the southerly migration of the transverse bars.