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.

Conference Paper↗

USGS1997

Turbulent stresses in the surf-zone: Which way is up?

Velocity observations from a vertical stack of three-component Acoustic Doppler Velocimeters (ADVs) within the energetic surf-zone are presented. Rapid temporal sampling and small sampling volume provide observations suitable for investigation of the role of turbulent fluctuations in surf-zone dynamics. While sensor performance was good, failure to recover reliable measures of tilt from the vertical compromise the data value. We will present some cursory observations supporting the ADV performance, and examine the sensitivity of stress estimates to uncertainty in the sensor orientation. It is well known that turbulent stress estimates are highly sensitive to orientation relative to vertical when wave motions are dominant. Analyses presented examine the potential to use observed flow-field characteristics to constrain sensor orientation. Results show that such an approach may provide a consistent orientation to a fraction of a degree, but the inherent sensitivity of stress estimates requires a still more restrictive constraint. Regardless, the observations indicate the degree to which stress estimates are dependent on orientation, and provide some indication of the temporal variability in time-averaged stress estimates.

Conference Paper↗

USGS1991

Vertical structure of cross-shore currents from wind-induced setup

Most of the storm surge models presented in the literature are vertically averaged and calculate only the sea-surface elevation and mean flow. Whereas these models may be adequate for predicting storm surge heights for flooding purposes, they neglect the vertical structure of the flow and the boundary shear stress, which are both critical for predicting cross-shore sediment transport. The steady and horizontally uniform equations of motion are used here to compute the sea-surface slope, the vertical structure of the cross-shore currents, and the boundary shear stress in a shallow wind dominated environment. The steady state model developed here balances the pressure gradient and the stress divergence, resulting in sea-surface slope and associated pressure gradient in the opposite direction of the wind, thus inducing a reversal in the currents near the bed. The Reynolds stress is modeled with a depth-dependent turbulent diffusion coefficient so that both the boundary shear stress and the velocity field are calculated, avoiding the need to set a bottom drag coefficient. Input parameters for this model are simply the wind stress, the water depth, and z0, the bed roughness parameter. A sensitivity test of the model results to various values of z0 indicates that large changes in z0 cause only minor differences in the surface slope, and moderate differences in the velocity field and boundary shear stress. Given the sediment size distribution and the small scale morphology of the bed, a reasonable estimate of z0 may be obtained and the above uncertainty will be nearly eliminated.

Conference Paper↗