Inner Shelf Surface Dispersion in the Florida Panhandle

Abstract:

As part of the Surfzone Coastal Oil Pathways Experiment, 75 GPS-equipped drifters were deployed off the coast of Destin, FL for 5 days in December 2013 to examine surface material transport. The drifters were released as triplets from the edge of the surf zone to the outer edge of the inner shelf resulting in 382 drifter pairs. GPS data were obtained at 1 Hz for O(40 hrs). The experiment encompassed a synoptic storm event that provided differing wind and wave conditions. The nearshore region, composed of the surf zone and inner shelf, is also influenced diurnally by a freshwater, surface (<1m), buoyant plume that originates from the Choctawhatchee River that exits Destin Inlet.

Surprisingly throughout each daily deployment regardless of separation, the drifters moved in coherent patterns suggesting the surface flow is nearly homogeneous. Wind measurements from a tower located onsite and surface velocity estimates from bottom-mounted acoustic Doppler current profilers indicate that local wind forcing controls the flow at the surface. Pathlines derived from the surface currents match drifter trajectories. However, there are subtle differences in drifter trajectories, which were found related to their individual deployment location. The bulk, relative, surface dispersion (D²) using two-particle statistics, when viewed in a logarithmic frame of reference, shows that the growth of particle separation is ballistic (D² ~t²) with respect to time (t). This indicates that the region experiences local dispersion primarily driven by horizontal shear.

The goal here is to describe the evolution of dispersion and the corresponding controls on it using the linear frame of reference, which provides more detail often de-emphasized in the log-scale. This is important for understanding mixing in the nearshore environment owing to the rapidly changing conditions caused by the synoptic storm event, the location of the buoyant plume, and cross- and alongshore position of the drifters. At this higher-resolution scale, dispersion was found to vary temporally with changes in mean wind direction. In addition, the buoyant plume influenced dispersion, even though the drifter trajectories inside and outside of the plume were relatively similar. A discussion on these subtle changes in dispersion will be presented.