Modeling Transverse Coastal Bedforms at Anna Maria Island (Florida)

Sea bedforms with crests perpendicular to the coastline are observed at Anna Maria Island, close to the Passage Key inlet where the local velocity field is mainly driven by tidal currents and episodic longshore currents generated by the passage of cold fronts. An idealized model able to predict the formation and the geometrical characteristics of these bedforms is described. The depth-averaged oscillating flow generated around the inlet by the tide propagation and by the longshore currents is analyzed by assuming the flow Reynolds number large enough to determine the flow field as the superposition of an irrotational component plus a contribution due to the vorticity shed by the edges of the inlet. Qualitative comparisons of the dynamics of the vortices shed by the inlet edges with laboratory and field visualizations suggest that the approach can provide a reliable evaluation of the depth-averaged velocity. Then, the geometrical characteristics of the bedforms generated by the interaction of the local flow with the sea bed are determined by means of a linear stability analysis of the flat bed configuration. It is shown that the model fairly predicts the orientation and the wavelength of the observed bedforms. A southward migration of the bedforms is predicted only when the southward episodic currents observed in the area are taken into account.